Protection relay and line break detection method for the same

Abstract

The present invention relates to a protection relay for detecting disconnection in a circuit connected with a converter, characterized by comprising: a disconnection sensing section including a plurality of nodes connected across outputs of the converter and a reference voltage generating section for forming a reference voltage, a plurality of resistors for forming a voltage dividing resistor and connected between the plurality of nodes and the reference voltage generating section; a sensing voltage generating section for applying a voltage for forming a prescribed voltage difference with respect to the reference voltage to the disconnection sensing section; and a disconnection judging section connected with a first node among the plurality of nodes and for detecting disconnection of at least one of the converter and a circuit connected with the converter based on a voltage signal detected from the first node, the magnitude of the voltage signal applied to the first node through the voltage dividing resistor being different depending on the disconnection of at least one of the converter and the circuit connected with the converter.

Description

Technical Field

[0001] This invention relates to a protective relay, and more specifically, to a protective relay for detecting whether a converter and the circuit connected to the converter are disconnected in a circuit connected to the converter. Background Technology

[0002] Generally, a converter is a device that converts the voltage or current input from a power system into a lower level of voltage or current, used to measure or relay the voltage and current of the power system.

[0003] These converters can be categorized into voltage transformers (PTs) that convert voltage signals and current transformers (CTs) that convert current signals. Voltage transformers (PTs) can be further divided into ordinary transformers using an iron core and transformers employing a voltage divider method using capacitors, resistors, etc. Current transformers (CTs) can be divided into ordinary converters using an iron core and converters using Rogowski coils with an air core.

[0004] First, in transformers or converters using iron cores, the iron core provides a stable path for magnetic flux. Therefore, it can output a waveform signal that is superior to the input signal. Consequently, the phases of the input and output signals can be the same, and because the input and output signals are electrically insulated, it offers excellent insulation performance between multiple input signals, i.e., excellent phase-to-phase insulation performance.

[0005] On the other hand, the converter may disconnect due to input voltage or current, defects in the internal circuitry of the converter, or external pressures such as high temperature or vibration. As mentioned above, in the event of a disconnection caused by the converter, relay elements such as low-voltage relays, grounding relays, low-current relays, and low-power relays may malfunction. Furthermore, such a disconnection in the converter may cause malfunctions in devices used to measure and convert input voltage or input current, or output voltage or output current, such as inverters.

[0006] Therefore, the protective relay determines whether the converter is disconnected to prevent such erroneous operation. Typically, this protective relay determines whether the converter is disconnected by detecting the magnitude, phase, or vector sum of the voltage or current input to the converter, and then makes a decision based on the detection result.

[0007] However, there is a problem that the measurement results for voltage, current, phase, or vector sums input through the converter are the same whether the converter is not applying a voltage or current signal (e.g., during normal operation of the converter without an applied voltage or current signal) or when the circuit is disconnected due to the converter. Alternatively, in the case of a circuit disconnection due to the converter, a high impedance (negation) state may occur, making it difficult to identify the current or voltage signal input from the converter. In this situation, it is difficult to distinguish whether the detected signal is from the high impedance state (i.e., the disconnection state) or the signal actually detected by the converter.

[0008] Furthermore, in the event of a short-circuit current or a starting current that occurs when the converter is activated, the large energy generated by the short-circuit current or starting current causes a temporary change in the voltage detected by the converter. However, it is usually difficult to distinguish between the change in current or voltage signal caused by the inflow of the short-circuit current or starting current and the change in current or voltage signal caused by a circuit break due to the converter when using voltage, current, or the sum of phase or vectors. Therefore, there is a problem that even if the circuit is normally connected, it is judged as a break when the short-circuit current or starting current occurs. Summary of the Invention

[0009] The problem the invention aims to solve

[0010] The present invention is proposed to solve the above-mentioned problems, and its purpose is to provide a protective relay and a method for detecting circuit breaks caused by converters that can more accurately detect circuit breaks.

[0011] Furthermore, the present invention aims to provide a protective relay capable of accurately distinguishing between a state where no voltage or current signal is applied to the converter and a state where the circuit is disconnected due to the converter, as well as a method for detecting the disconnection of the protective relay.

[0012] Furthermore, the present invention aims to provide a protective relay and a method for detecting the open circuit of the protective relay, which can distinguish whether the change in the signal detected by the converter is caused by an open circuit or by the short circuit current or the starting current of the motor or transformer, when the signal detected by the converter changes due to a short circuit current or the starting current of the motor or transformer.

[0013] means for solving problems

[0014] To achieve the above or other objectives, according to one aspect of the present invention, a protective relay according to an embodiment of the present invention is characterized by comprising: a disconnection sensing unit, including a plurality of nodes connected to the output terminals of the converter and a reference voltage generating unit for forming a reference voltage, including a plurality of resistors forming a voltage divider resistor and connecting the plurality of nodes and the reference voltage generating unit; a sensing voltage generating unit, applying a voltage for forming a predetermined voltage difference relative to the reference voltage to the disconnection sensing unit; and a disconnection determination unit, connected to a first node among the plurality of nodes, and detecting whether at least one of the converter and the circuit connected to the converter is disconnected based on a voltage signal detected from the first node, wherein the magnitude of the voltage signal applied to the first node through the voltage divider resistor varies depending on whether at least one of the converter and the circuit connected to the converter is disconnected.

[0015] In one embodiment, the disconnection determination unit extracts a DC offset value from the voltage signal detected at the first node, and if the result of comparing the extracted DC offset value with a preset threshold voltage is that the DC offset value is less than the threshold voltage, it detects the magnitude of the AC component from the voltage signal detected at the first node, and determines whether at least one of the converter and the circuit connected to the converter is disconnected based on the magnitude of the detected AC component.

[0016] In one embodiment, the disconnection determination unit determines whether at least one of the converter and the circuit connected to the converter is disconnected based on whether the detected AC component is greater than a preset value. The preset value is determined based on the magnitude of the AC component included in the voltage signal detected from the first node when a short-circuit current occurs or when the converter is started.

[0017] In one embodiment, in the disconnection determination unit, if at least one of the converter and the circuit connected to the converter is disconnected based on the result of comparing the extracted DC offset value with a preset critical voltage, the extracted DC offset value further reflects a preset design margin value. Based on the result of comparing the DC offset value that further reflects the design margin value with the critical voltage, a second determination is made as to whether at least one of the converter and the circuit connected to the converter is disconnected, and the disconnection of at least one of the converter and the circuit connected to the converter is detected based on the result of the second determination.

[0018] In one embodiment, the design margin includes a preset error value, which includes at least one of a measurement error occurring during the extraction of a DC offset value from a voltage signal detected at the first node and a design error arising from the circuit structure of the open circuit sensing unit.

[0019] In one embodiment, when the reference voltage is a ground voltage, the threshold voltage is the same as the voltage applied by the sensing voltage generation unit.

[0020] In one embodiment, the disconnection sensing unit includes: a first contact formed between the first node and the disconnection detection unit; a second node connected to a terminal of the output terminals of the converter that is different from the terminal connected to the first node; a third contact formed between the second node and the sensing voltage generating unit; a second contact formed on a circuit connecting the first contact and the third contact; a first resistor disposed between the first contact and the second contact; a second resistor connected between the second contact and the reference voltage generating unit, disposed between the reference voltage generating unit and the second contact; and a third resistor disposed between the second contact and the third contact.

[0021] In one embodiment, the disconnection sensing unit includes: a first contact formed between the first node and the disconnection detection unit; a second node connected to a terminal of the output terminals of the converter that is different from the terminal connected to the first node; a third contact formed between the second node and the sensing voltage generating unit; a second contact formed on a circuit connecting the first contact and the third contact; a second resistor connected between the second contact and the reference voltage generating unit, disposed between the reference voltage generating unit and the second contact; and a third resistor disposed between the second contact and the third contact.

[0022] In one embodiment, the disconnection sensing unit includes: a first contact formed between the first node and the disconnection detection unit; a second node connected to a terminal of the output terminals of the converter that is different from the terminal connected to the first node; a third contact formed between the second node and the sensing voltage generating unit; a second contact formed on the circuit connecting the first contact and the third contact and connected to the reference voltage generating unit; a first resistor disposed between the first contact and the second contact; and a third resistor disposed between the second contact and the third contact.

[0023] In one embodiment, when the converter and the circuit connected to the converter are in a normally connected state, the first node is applied a voltage signal of the same magnitude as the voltage applied by the sensing voltage generation unit. When at least one of the converter and the circuit connected to the converter is in a disconnected state, the DC offset value detected from the voltage signal applied to the first node is less than the DC offset value detected from the voltage signal applied to the first node when the converter and the circuit connected to the converter are in a normally connected state.

[0024] In one embodiment, if the voltage applied by the sensing voltage generating unit is greater than the reference voltage, and at least one of the converter and the circuit connected to the converter is in an open-circuit state, a voltage signal based on the difference between the reference voltage and the voltage applied by the sensing voltage generating unit and the magnitude of the resistance forming the voltage divider resistor is applied according to the following mathematical formula.

[0025] Mathematical expression

[0026] n1(V)=n2(V),

[0027] (If VS <VR)

[0028] Here, n1(v) is the magnitude of the voltage signal applied to the first contact, n2(v) is the magnitude of the voltage signal applied to the second contact, VS is the reference voltage, VR is the voltage applied by the sensing voltage generation unit, R2 is the resistance value of the second resistor, and R3 is the resistance value of the third resistor.

[0029] In one embodiment, if the extracted DC offset value is less than a preset critical voltage, the disconnection determination unit detects a disconnection in at least one of the converter and the circuit connected to the converter by adding a preset design margin value to the extracted DC offset value and comparing the DC offset value with the added design margin value with the critical voltage.

[0030] In one embodiment, the reference voltage is the ground voltage.

[0031] In one embodiment, when the converter and the circuit connected to the converter are in a normally connected state, the first node is applied a voltage signal of the same magnitude as the voltage applied by the sensing voltage generation unit. When at least one of the converter and the circuit connected to the converter is in a disconnected state, the DC offset value detected from the voltage signal applied to the first node is greater than the DC offset value detected from the voltage signal applied to the first node when the converter and the circuit connected to the converter are in a normally connected state.

[0032] In one embodiment, if the reference voltage is greater than the voltage applied by the sensing voltage generation unit, and at least one of the converter and the circuit connected to the converter is in an open-circuit state, a voltage signal is applied based on the voltage applied by the sensing voltage generation unit, the difference between the voltage applied by the sensing voltage generation unit and the reference voltage, and the magnitude of the resistance forming the voltage divider resistor, as shown in the following mathematical formula.

[0033] Mathematical expression

[0034] n1(V)=n2(V),

[0035] (If VS>VR)

[0036] Here, n1(v) is the magnitude of the voltage signal applied to the first contact, n2(v) is the magnitude of the voltage signal applied to the second contact, VS is the reference voltage, VR is the voltage applied by the sensing voltage generation unit, R2 is the resistance value of the second resistor, and R3 is the resistance value of the third resistor.

[0037] In one embodiment, if the extracted DC offset value is greater than a preset threshold voltage, the disconnection determination unit subtracts a preset design margin value from the extracted DC offset value, and detects a disconnection in at least one of the converter and the circuit connected to the converter by comparing the DC offset value minus the design margin value with the threshold voltage.

[0038] In one embodiment, the voltage of the sensing voltage generator is the ground voltage.

[0039] In one embodiment, when the converter is a converter using a Rogowski coil, an integration circuit for restoring the delayed waveform generated according to the characteristics of the Rogowski coil is further included between the disconnection sensing unit and the disconnection determination unit. Alternatively, the disconnection determination unit integrates the DC offset value and determines whether at least one of the converter and the circuit connected to the converter is disconnected based on the comparison result of the integrated DC offset value and the threshold voltage.

[0040] In one embodiment, the disconnection detection unit further includes an output unit, which is used to output a reminder signal indicating the disconnection status of at least one of the converter and the circuit connected to the converter, and the output unit has a communication function for transmitting the reminder signal to other pre-set devices.

[0041] To achieve the above or other objectives, according to one aspect of the present invention, a method for detecting open circuit of a protective relay includes: a step of detecting a voltage signal from an open circuit sensing unit and obtaining a DC offset value from the detected voltage signal, wherein the open circuit sensing unit includes a plurality of nodes connected to the output terminals of the converter and includes a plurality of resistors forming voltage divider resistors, and the voltage signals applied to the plurality of nodes are different from each other depending on whether at least one of the converter and the circuit connected to the converter is open; a step of determining whether at least one of the converter and the circuit connected to the converter is open by comparing the obtained DC offset value with a preset threshold voltage; a step of calculating the magnitude of an AC component from the detected voltage signal based on the result of the first determination; a step of determining whether a short circuit current or a starting current of the converter has been generated by comparing the magnitude of the calculated AC component with a preset AC current signal magnitude; and a step of determining whether at least one of the converter and the circuit connected to the converter is open a second time based on the determination result of whether the short circuit current or the starting current has been generated.

[0042] In one embodiment, the step of determining whether the circuit is disconnected includes: comparing an acquired DC offset value with a preset threshold voltage; determining whether the DC offset value reflects a preset design margin value based on the comparison result of the DC offset value and the preset threshold voltage; and determining whether at least one of the converter and the circuit connected to the converter is disconnected by comparing the DC offset value reflecting the design margin value with the threshold voltage.

[0043] In one embodiment, the step of determining whether the circuit is disconnected for the second time further includes a step of outputting a reminder message indicating the disconnection status if the result of the second determination is that at least one of the converter and the circuit connected to the converter is disconnected. It also includes a step of re-executing the step of obtaining the DC offset value up to the step of determining whether the circuit is disconnected for the second time, based on whether a preset period has ended, if the result of the second determination is that the converter and the circuit connected to the converter are normally connected.

[0044] In one embodiment, the preset period is determined based on the time it takes for the device connected to the protection relay to complete its active operation.

[0045] The effects of the invention

[0046] According to at least one embodiment of the present invention, the present invention is provided with a disconnection sensing unit that detects different voltages when the converter is in a normally connected state and when the converter is disconnected, and determines whether the converter is disconnected based on the voltage detected by the disconnection sensing unit, thereby having the effect of accurately distinguishing between a state where no voltage or current signal is applied to the converter and a state where the circuit is disconnected due to the converter.

[0047] Furthermore, according to at least one embodiment of the present invention, when the detection result of the disconnection sensing unit is that a disconnection caused by the converter has occurred, the present invention detects the magnitude of the AC component from the voltage signal input by the converter, and detects whether a short-circuit current or a starting current has occurred based on the detected magnitude of the AC component, thereby having the effect of being able to distinguish whether the detection result of the disconnection sensing unit is caused by the short-circuit current or the starting current. Attached Figure Description

[0048] Figure 1 This is a block diagram illustrating the configuration of a protective relay of an embodiment of the present invention connected to a converter.

[0049] Figure 2 This is a circuit diagram showing the circuit structure of the open circuit sensing unit and the sensing voltage generation unit in the protective relay of an embodiment of the present invention.

[0050] Figure 3 This is a block diagram illustrating the configuration of the open circuit detection unit in the protective relay of an embodiment of the present invention.

[0051] Figure 4 This is a flowchart illustrating the operation process of the disconnection judgment unit in the protective relay of the present invention, which determines whether the converter is disconnected based on the voltage signal detected by the disconnection sensing unit.

[0052] Figure 5 This is a diagram showing the magnitude of the voltage signal detected by the disconnection sensing unit in the protection relay of an embodiment of the present invention when the converter is normally connected.

[0053] Figure 6 This is a diagram showing the magnitude of the voltage signal detected by the disconnection sensing unit in the protective relay of an embodiment of the present invention when the converter is disconnected.

[0054] Figure 7 This is an example diagram showing the DC offset value detected by the disconnection sensing unit in the protection relay of an embodiment of the present invention, under the condition that the converter is normally connected and under the condition of disconnection.

[0055] Figure 8 This is an example diagram illustrating a converter startup current or short-circuit current with a negative voltage flowing into the converter.

[0056] Figures 9 to 12 This is an example diagram illustrating how a protective relay of an embodiment of the present invention is connected to a converter of a different type.

[0057] Figures 13 to 14 This is an example diagram showing a protective relay according to an embodiment of the present invention, which has a structure that omits any one of the resistors constituting the voltage divider resistors, and has a disconnection sensing section.

[0058] Figure 15 This is a circuit diagram showing the structure of a disconnection sensing unit in a protective relay according to an embodiment of the present invention, which has a structure of a voltage generating unit that applies a preset voltage instead of a ground voltage.

[0059] Figure 16 It is in having Figure 15 The flowchart illustrates the operation process of the disconnection detection unit in the protective relay shown, which determines whether the converter is disconnected based on the voltage signal detected by the disconnection sensing unit.

[0060] Figures 17 to 18 It is shown in having Figure 15 The diagram shows an example of a protective relay with a disconnection sensing section that omits any one of the resistors constituting the voltage divider resistors.

[0061] Figure 19 This is a circuit diagram showing the structure of a sensing voltage generating unit in a protective relay according to an embodiment of the present invention, which has a structure that applies a ground voltage instead of a preset voltage. Detailed Implementation

[0062] It should be noted that the technical terms used in this specification are for illustrative purposes only and are not intended to limit the invention. Furthermore, unless the context clearly indicates otherwise, singular expressions include plural expressions. The suffixes "module" and "part" used in the following description for structural elements are assigned or used interchangeably for ease of writing and do not inherently carry a distinguishing meaning or function.

[0063] In this specification, terms such as “constituting” or “including” should not be construed as including all the constituent elements or steps described in the specification, but should be construed as excluding some constituent elements or steps, or including additional constituent elements or steps.

[0064] In addition, in the process of describing the technology disclosed in this specification, when it is determined that a detailed description of the relevant known technology would obscure the essence of the technology disclosed in this specification, a detailed description thereof is omitted.

[0065] Furthermore, it should be understood that the accompanying drawings are provided to facilitate understanding of the embodiments disclosed in this specification. The technical concepts disclosed in this specification are not limited to the drawings. This invention includes all modifications, equivalents, and substitutions made within the technical concept and scope of this invention. In addition, not only the individual embodiments described below, but also combinations of embodiments can naturally be considered as modifications, equivalents, and substitutions included within the technical concept and scope of this invention, and thus fall within the scope of this invention.

[0066] Figure 1 This is a block diagram showing the configuration of a protective relay 10 of an embodiment of the present invention connected to a converter 110. Figure 2 This is a circuit diagram showing the circuit structure of the open circuit sensing unit 120 and the sensing voltage generating unit 130 in the protective relay of an embodiment of the present invention. Figure 3 This is a block diagram illustrating the configuration of the disconnection detection unit 150 in the protective relay of an embodiment of the present invention.

[0067] First, refer to Figure 1 The protective relay 10 of this embodiment may include: a disconnection sensing unit 120 configured to be connected to a converter 110; a sensing voltage generating unit 130 that applies a predetermined voltage to the disconnection sensing unit 120; and a disconnection determination unit 150 that can determine the circuit connection status with the converter 110 based on the voltage signal detected by the disconnection sensing unit 120.

[0068] First, converter 110 is a device that converts the voltage or current input from the power system into a lower level of voltage or current, which may include a transformer (PT, Potential Transformer) or a current transformer (Current Transformer).

[0069] Depending on the method of voltage conversion, the transformer can be a transformer that uses resistive voltage division or a transformer that uses an iron core. Similarly, depending on the method of current conversion, the converter can be a converter that uses an iron core or a converter using a Rogowski coil with an air core.

[0070] Furthermore, the disconnection sensing unit 120 may be provided with nodes that are respectively connected to the two output ends of the converter 110. Therefore, when the nodes are connected to the two output ends of the converter 110, a circuit can be formed, and the circuit between the nodes can be disconnected when the converter 110 is disconnected.

[0071] On the other hand, any one of the nodes 111 (hereinafter referred to as the first node) can be connected to the disconnection judgment unit 150, while the other node 112 (hereinafter referred to as the second node) can be connected to the sensing voltage generation unit 130.

[0072] Referring to the circuit structure of the wire breakage sensing unit 120 shown, Figure 2 The disconnection sensing unit 120 may include a plurality of resistors R1 (125), R2 (126), and R3 (127) forming a voltage divider resistor.

[0073] The first resistor R1 (125) of the plurality of resistors R1 (125), R2 (126), and R3 (127) can be connected to the first contact n1 (121) formed between the first node 111 and the disconnection detection unit 150. The third resistor R3 (127) can be connected to the third contact n3 (123) formed between the second node 112 and the sensing voltage generation unit 130.

[0074] The first contact n1 (121) and the third contact n3 (123) can be connected to each other. Furthermore, the circuit connecting the first contact n1 (121) and the third contact n3 (123) can include a second contact n2 (122). Here, the first resistor R1 (125) can be positioned between the first contact n1 (121) and the second contact n2 (122), and the second resistor R2 (126) can be positioned between the second contact n2 (122) and the third contact n3 (123).

[0075] Furthermore, the second contact n2 (122) can be connected to the reference voltage generating unit 129 that provides a reference voltage to the disconnection sensing unit 120. In addition, another resistor (second resistor R2 (126)) forming the voltage divider resistor can be disposed between the second contact n2 (122) and the reference voltage generating unit 129.

[0076] Here, the reference voltage generation unit 129 can be configured to be grounded, and the ground voltage can be provided as the reference voltage.

[0077] Therefore, with the circuit between the first node 111 and the second node 112 normally connected, a circuit connecting the first contact n1 (121) and the third contact n3 (123) can be formed. Therefore, a predetermined voltage applied by the sensing voltage generation unit 130 can be applied to the first contact n1 (121).

[0078] Conversely, if the circuit between the first node 111 and the second node 112 is broken, i.e., if the converter 110 is disconnected, the first contact n1 (121) and the third contact n3 (123) are disconnected. Therefore, the first contact n1 (121) can be given a voltage signal having a voltage that is distributed by the difference between the voltage applied from the sensing voltage generation unit 130 and the voltage applied from the reference voltage generation unit 129 and the plurality of resistors forming the voltage divider resistor.

[0079] On the other hand, since the disconnection judgment unit 150 detects the voltage signal of the first contact n1 (121) connected to the first node 111, the detected voltage signal can change according to the circuit connection state between the first node 111 and the second node 112, i.e. whether the converter 110 is disconnected.

[0080] Furthermore, the sensing voltage generation unit 130 can apply a voltage to the disconnection sensing unit 120 to form a predetermined voltage difference relative to the reference voltage. For this purpose, the sensing voltage generation unit 130 can be configured to provide a voltage with a different magnitude than the reference voltage generation unit 129 of the disconnection sensing unit 120.

[0081] As an example, such as Figure 2 As shown, when the reference voltage generation unit 129 of the disconnection sensing unit 120 is formed in a grounded configuration to provide a ground voltage, the sensing voltage generation unit 130 can be configured to provide a voltage different from the ground voltage. Therefore, in addition to the ground 132, the sensing voltage generation unit 130 may also include a voltage generation unit 131 for applying a predetermined direct current (DC) voltage to the disconnection sensing unit 120. Hereinafter, the voltage applied from the sensing voltage generation unit 130 may also be referred to as "VR".

[0082] Conversely, if the reference voltage generation unit 129 of the disconnection sensing unit 120 is configured to apply a predetermined DC voltage instead of a ground voltage, then the sensing voltage generation unit 130 can of course also be configured in a grounded form to provide a ground voltage.

[0083] Furthermore, the reference voltage generating unit 129 and the sensing voltage generating unit 130 can each apply a predetermined voltage greater than the ground voltage. In this case, the reference voltage generating unit 129 and the sensing voltage generating unit 130 can be configured to apply different DC voltages to each other.

[0084] On the other hand, the disconnection detection unit 150 can determine whether a circuit disconnection caused by the converter 110 has occurred based on the voltage signal sensed by the disconnection sensing unit 120. For this purpose, as... Figure 3As shown, the disconnection determination unit 150 may include: a control unit 155; a conversion unit 156 connected to the control unit 155; a plurality of determination units (a first determination unit 151, a second determination unit 152, and a third determination unit 153); and a memory 158. Additionally, the disconnection determination unit 150 may also include an output unit 157 for outputting the result of determining whether the circuit is disconnected.

[0085] First, the conversion unit 156 can convert the analog voltage signal sensed by the disconnection sensing unit 120 into a signal that can be recognized by the disconnection determination unit 150. As an example, the conversion unit 156 can be an analog-to-digital converter (ADC).

[0086] On the other hand, the conversion unit 156 can be limited to a range of voltage signals that it can process. Therefore, the disconnection sensing unit 120 can be configured to input a voltage signal within the range that the conversion unit 156 can process to the conversion unit 156. As an example, when the signal range that the conversion unit 156 can process is 0V to 3.3V, the reference voltage generation unit 129 and the sensing voltage generation unit 130 can be configured to input different voltage signals from 0V to below 3.3V to the converter 110 when the circuit is normally connected and when it is disconnected.

[0087] That is, the reference voltage generation unit 129 and the sensing voltage generation unit 130 can be configured to input a voltage signal with a center voltage of 1.65V to the conversion unit 156 when the circuit is normally connected. However, when the circuit is disconnected, the reference voltage generation unit 129 and the sensing voltage generation unit 130 of the disconnection sensing unit 120 can be configured to input a voltage signal with a voltage less than 1.65V (when the voltage of the reference voltage generation unit 129 is less than the voltage of the sensing voltage generation unit 130) or greater than 1.65V (when the voltage of the reference voltage generation unit 129 is greater than the voltage of the sensing voltage generation unit 130) to the conversion unit 156.

[0088] In addition, the memory 158 may store data that supports the functions of the disconnection detection unit 150. More specifically, the memory 158 may store data and instructions related to the application programs driven in the first detection unit 151, the second detection unit 152, and the third detection unit 153 provided in the disconnection detection unit 150.

[0089] As a series, the memory 158 may store: information about a preset threshold voltage, as data for determining the first determination unit 151; the resistance of the second resistor R2 (126) and the resistance of the third resistor R3 (127), the voltage of the reference voltage generation unit 129 forming the voltage divider resistor, and information about the magnitude of the voltage according to the design margin, as data for determining the second determination unit 152. Additionally, as data for determining the third determination unit 153, it may store the smallest AC current component detectable when a fault current such as a short-circuit current or the start-up current of the converter 110 occurs, i.e., information about the magnitude of the fault current signal.

[0090] On the other hand, the control unit 155 is configured to control other components connected thereto, and to control the overall operation of the disconnection judgment unit 150.

[0091] First, if the voltage signal is converted into a digital signal by the conversion unit 156, the control unit 155 can obtain a DC offset value from the converted voltage signal. Furthermore, the first determination unit 151 can be controlled to compare the obtained DC offset value with a preset threshold voltage.

[0092] As described above, compared to the case where the circuit connected to the converter 110 is functioning normally, in the case of a circuit disconnection caused by the converter 110, the voltage applied to the first contact 121 may become smaller (in the case where the voltage of the reference voltage generation unit 129 is less than the voltage of the sensing voltage generation unit 130) or larger (in the case where the voltage of the reference voltage generation unit 129 is greater than the voltage of the sensing voltage generation unit 130). Therefore, if the preset threshold voltage and the DC offset value of the voltage signal detectable when the circuit connected to the converter 110 is functioning normally are the same, the control unit 155 can determine whether the circuit connected to the converter 110 is disconnected based on the comparison result of the first determination unit 151.

[0093] On the other hand, if the disconnection determination result (hereinafter also referred to as the first disconnection determination result) based on the comparison result of the first determination unit 151 is a disconnection state, the control unit 155 can determine whether the difference between the obtained DC offset value and the critical voltage is within a preset error range.

[0094] Therefore, the control unit 155 can reflect a preset margin value in the obtained DC offset value. Furthermore, the second determination unit 152 can again compare the magnitude of the obtained DC offset value reflecting the margin value with the critical voltage.

[0095] Furthermore, based on the determination result of the second determination unit 152, if the magnitude of the DC offset value reflecting the margin value is less than the critical voltage (when the voltage of the reference voltage generation unit 129 is less than the voltage of the sensing voltage generation unit 130) or greater than the critical voltage (when the voltage of the reference voltage generation unit 129 is greater than the voltage of the sensing voltage generation unit 130), the control unit 155 can determine that the difference between the DC offset value and the critical voltage has deviated from the preset error range.

[0096] Therefore, the control unit 155 can again determine that the circuit connected to the converter 110 is broken. Hereinafter, the result of the breakage determination made by the second determination unit 152 will be referred to as the secondary breakage determination result.

[0097] On the other hand, in the event of an inrush current generated when the converter 110 is started or a short-circuit current generated due to a short circuit, although the inrush current and short-circuit current are AC components, the magnitude of the DC component voltage signal sensed by the disconnection sensing unit 120 may change. Therefore, even in the case of no circuit disconnection, if the inrush current or short-circuit current occurs, the magnitude of the voltage signal detected at the first contact 121 may decrease or increase by a predetermined level.

[0098] Therefore, if the result of the secondary disconnection determination is that the circuit is disconnected, the control unit 155 can determine that the determination result is caused by the occurrence of the starting current or short-circuit current.

[0099] For this purpose, the control unit 155 can detect the magnitude of the alternating current (AC) component signal, for example, by calculating the RMS (Root Mean Square) value from the voltage signal converted by the conversion unit 156. Furthermore, the control unit 153 can determine whether the magnitude of the detected AC signal component is greater than a preset fault current signal magnitude.

[0100] Here, the fault current can be the short-circuit current or the converter's startup current. Therefore, the magnitude of the preset fault current signal can be determined as the smallest AC signal component that can be detected by the disconnection sensing unit 120 when the short-circuit current or the converter's startup current occurs. The magnitude of the fault current signal can be preset by the user or determined by the results of repeated experiments performed according to the present invention.

[0101] On the other hand, if the judgment result of the third judgment unit 153 is that the magnitude of the AC signal component detected from the converted voltage signal is less than the preset fault current signal magnitude, the control unit 155 can determine that the short-circuit current or starting current has not been generated. Therefore, the control unit 155 can determine that the current circuit disconnection is not caused by the short-circuit current or starting current, and ultimately determine that the circuit connected to the converter 110 is disconnected.

[0102] Thus, if the circuit is ultimately determined to be disconnected, the control unit 155 can transmit a reminder signal indicating the disconnection status of the circuit to the user through the control output unit 157.

[0103] As an example, the control unit 155 can output an alert signal as an auditory or visual signal via the output unit 157. Alternatively, the control unit 155 can control the output unit 157 to transmit the alert signal to other pre-set devices, such as a user's mobile terminal, and cause the mobile terminal to output an alert signal, thereby transmitting the alert signal to the user. For this purpose, the output unit 157 can have a communication function for transmitting the alert signal.

[0104] The operation of the protective relay 10 according to this embodiment of the present invention will now be described in detail with reference to the flowchart and several example diagrams. Furthermore, in the following embodiment, it is assumed that the reference voltage generation unit 129 is configured to have a ground voltage, and the sensing voltage generation unit 130 has an additional voltage generation unit 131 to have a voltage higher than the ground voltage, which will be explained for ease of description.

[0105] Figures 4 to 8 This diagram illustrates the operation of the disconnection determination unit 150 in the protective relay 10 of this embodiment of the invention, which determines whether the circuit is disconnected due to the converter 110 based on the voltage signal detected from the disconnection sensing unit 120.

[0106] Figure 4 This is a flowchart illustrating the operation process of the wire breakage detection unit 150. Figure 5 This is a diagram showing the magnitude of the voltage signal detected by the disconnection sensing unit 120 in the protection relay 10 of the present invention when the converter 110 is normally connected. Figure 6 This is a diagram showing the magnitude of the voltage signal detected by the disconnection sensing unit 120 in the protection relay 10 of the present invention when the converter 110 is disconnected.

[0107] in addition, Figure 7This is an example diagram illustrating the difference in DC offset value detected by the disconnection sensing unit in the protection relay 10 according to an embodiment of the present invention, under the condition of normal converter connection and disconnection. Figure 8 This is an example diagram showing an example of a converter 110 startup current or short-circuit current with a negative voltage flowing into the converter.

[0108] First, the control unit 155 of the disconnection judgment unit 150 of the protection relay 10 provided in this embodiment of the invention can repeatedly perform the following procedure for judging whether the converter 110 is disconnected according to a preset cycle. Figure 4 The operation process. Here, the preset period can be the period corresponding to the period of the signal input from the line to the converter 110. In this case, it can be executed at the time point when the main operation of the protection relay 10 is completed. Figure 4 The process of the action.

[0109] In this case, the main operation of the protection relay 10 can vary depending on the device connected to it. For example, if the device connected to the protection relay 10 is a digital relay, the main operation can be a relay operation; if the connected device is a digital measuring instrument, the main operation can be a measuring operation. Additionally, if the connected device is a power conversion device, the main operation can be a power conversion operation.

[0110] Reference Figure 4 If a voltage signal is detected from the disconnection sensing unit 120 when the preset cycle ends, that is, a voltage signal applied to the first contact 121 connected to the disconnection sensing unit 120, the control unit 155 can convert the voltage signal through the conversion unit 156 and extract the value of the DC component, that is, the DC offset value, from the sampled digital data of one cycle that has been converted (S400).

[0111] Furthermore, the control unit 155 can control the first judgment unit 151 to compare the extracted DC offset value with a preset threshold voltage, thereby determining for the first time whether the circuit is disconnected (S402).

[0112] like Figure 5 As shown, when the circuit connected to converter 110 is in normal operation, a circuit can be formed that connects the third contact 123 and the first contact 121 via converter 110. Therefore, as Figure 5 As shown, the reference voltage VR of the sensing voltage generation unit 130 connected to the third contact 123 can be applied to the first contact 121 through the converter 110.

[0113] In this case, since converter 110 is in normal operation and the impedance of conversion unit 156 is much larger than the line resistance of converter 110, the voltage at the first contact n1 (121) can be the same as the voltage at the third contact n3 (123). Therefore, when the circuit is normally connected to converter 110, if a reference voltage VR is applied to the third contact 123, a voltage VR that is the same as the voltage applied to the third contact n3 (123) can be applied to the first contact n1 (121). Therefore, the DC offset value of the voltage signal corresponding to the reference voltage VR can be extracted by conversion unit 156.

[0114] On the contrary, such as Figure 6 As shown, if the circuit is disconnected due to the converter 110, there may be no connection between the first contact 121 and the third contact 123. In this case, the disconnection detection unit 150 will be in a high impedance state, and therefore, according to the following mathematical formula 1, the same voltage signal as the second contact 122 can be applied to the first contact 121.

[0115] Mathematical Formula 1

[0116] n1(V)=n2(V)

[0117] Here, n1(v) is the magnitude of the voltage signal applied to the first contact 121, and n2(v) is the magnitude of the voltage signal applied to the second contact 122.

[0118] On the other hand, a voltage signal can be applied to the second contact n2 (122) depending on the voltage divider resistor formed by the second resistor R2 (1260) and the third resistor R3 (127) and the reference voltage VR applied from the sensing voltage generation unit 130. The following mathematical formula 2 shows the magnitude of the voltage signal applied to the second contact n2 (122) according to the voltage divider resistor and the reference voltage VR.

[0119] Mathematical formula 2

[0120]

[0121] Here, n2(v) is the magnitude of the voltage signal applied to the second contact 122, VR is the reference voltage, R2 is the resistance value of the second resistor R2(126), and R3 is the resistance value of the third resistor R3(127).

[0122] On the other hand, according to the aforementioned mathematical formula 2, since n2(v) has a value less than 1 and the product of this value and the reference voltage VR, the value of n2(v) is always less than the reference voltage VR. Therefore, as... Figure 7As shown, the magnitude of the DC offset value 700 extracted when the circuit is connected normally can always be greater than the magnitude of the DC offset value 702 extracted when the circuit is disconnected.

[0123] Therefore, when the critical voltage is the reference voltage VR, if the circuit is in an open state in step S402, the DC offset value extracted in step S400 can be less than the critical voltage. Therefore, if the determination result of step S402 is that the magnitude of the currently extracted DC offset value is less than the critical voltage, the control unit 155 can determine that the circuit currently connected to the converter 110 is open.

[0124] Conversely, in step S402, if the DC offset value extracted in step S400 is not less than the threshold value, the control unit 155 can determine that the circuit is in an unbroken state. Furthermore, if the determination result of step S402 is that the circuit is in an unbroken state, the control unit 155 can end the unbroken state determination process in the current cycle.

[0125] On the other hand, if the disconnection determination result in step S402 indicates that the circuit connected to converter 110 is in a disconnected state, control unit 155 can add a voltage value based on the design margin to the currently extracted DC offset value (S404). Here, the voltage value based on the design margin may also include a preset error value. This error value may be a measurement error, and may also include errors that may occur during the conversion process in conversion unit 156.

[0126] In step S404, if the design margin including the error value is added to the DC offset value, the control unit 155 can control the second determination unit 152 to compare the added voltage value with the critical voltage again (S406).

[0127] Subsequently, if the comparison result of step S406 shows that the voltage value obtained by adding the DC offset value and the design margin is above the critical voltage, then the control unit 155 can determine that the circuit is in an unbroken state. Furthermore, if the determination result of step S406 is that the circuit is in an unbroken state, the control unit 155 can end the unbroken state determination process in the current cycle.

[0128] However, if the comparison result in step S406 is that although a design margin has been added to the DC offset value, the summed voltage value is still less than the critical voltage, then the control unit 155 can again determine that the circuit currently connected to the converter 110 is disconnected (secondary disconnection judgment).

[0129] On the other hand, if the secondary determination indicates that the circuit currently connected to the converter 110 is disconnected, the control unit 155 can determine whether the difference between the DC offset value and the threshold voltage is caused by short-circuit current or starting current generated when the converter 110 is started.

[0130] The short-circuit current or starting current generated when the converter 110 is short-circuited or started refers to a temporarily generated current that is several times larger than the normal operating current of the converter 110. This short-circuit current or starting current has a pattern of momentarily reaching a large current and then gradually decreasing in magnitude.

[0131] On the other hand, this short-circuit current or starting current can not only have a positive voltage, but also, as Figure 8 The voltage curve shown (AC component 810, DC component 820) has a negative value compared to the normal condition 800. In this case, because the short-circuit current or starting current has very large energy (a voltage signal with a negative voltage), therefore... Figure 8 As shown, the value of the DC component applied by the sensing voltage generation unit 130, i.e., the reference voltage VR, can be reduced. As a result, the magnitude of the voltage signal applied to the first contact n1 (121) may be reduced, so that even in the absence of a circuit disconnection, the magnitude of the voltage signal detected at the first contact 121 may be lower than the critical voltage VR.

[0132] Thus, to prevent misjudging whether the circuit is open due to short-circuit current or starting current, the control unit 155 of the protection relay 10 in this embodiment of the invention can detect the magnitude of the AC component from the sampled digital data of one cycle converted by the conversion unit 156 (S408) even if the secondary open circuit judgment result in step S406 is a circuit open circuit. For example, the control unit 155 can detect the magnitude of the AC component by calculating the RMS (Root Mean Square). Furthermore, the detected magnitude of the AC component can be compared with the magnitude of a preset fault current signal (S410).

[0133] On the other hand, if the comparison result of step S410 shows that the magnitude of the detected AC component is greater than or equal to the preset fault current signal magnitude, the control unit 155 can determine that a short-circuit current or a startup current caused by the start-up of the converter 110 has occurred. Furthermore, it can be determined that the circuit disconnection state based on the determination (secondary disconnection determination) result of step S406 is an erroneous judgment caused by the generation of the short-circuit current or the startup current.

[0134] That is, if the control unit 155 determines that a short-circuit current or a starting current has occurred based on the judgment result of step S410, even though it is already the result of the secondary disconnection judgment, it can still be determined that the circuit is in a normal connection state. Therefore, the control unit 155 can directly end the circuit disconnection judgment process in the current cycle without outputting a reminder signal or the like.

[0135] Conversely, if the comparison result in step S410 shows that the detected AC component is less than the preset fault current signal magnitude, the control unit 155 can determine that no short-circuit current or startup current generated when the converter 110 is started has occurred. Therefore, the control unit 155 can ultimately determine that the circuit currently connected to the converter 110 is disconnected and output a reminder signal (S412) to transmit the disconnection of the circuit to the user.

[0136] As described above, when a circuit is determined to be open based on the voltage signal sensed by the open circuit sensing unit 120, the protection relay 10 of this embodiment can use a preset margin voltage to re-determine whether the voltage signal is within the error range. This prevents misjudgment of the circuit's open circuit state due to measurement or design errors.

[0137] Furthermore, the protection relay 10 of this embodiment, when determining that the circuit is open based on the voltage signal sensed by the open circuit sensing unit 120, detects whether a short-circuit current or a starting current has occurred based on the magnitude of the AC component signal detected from the voltage signal. This allows it to determine whether the voltage signal is affected by the short-circuit current or the starting current. Therefore, it is possible to prevent misjudgment of the circuit's open circuit state due to the influence of the short-circuit current or the starting current.

[0138] On the other hand, as mentioned in the above description, examples of converter 110 can be transformers using resistive voltage division, transformers using iron cores (PT), converters using iron cores (CT), converters using Rogowski coils, etc. Figures 9 to 12 Examples of the various converters 110 described above being connected to the protection relay 10 of an embodiment of the present invention are shown.

[0139] first, Figure 9 An example of the protection relay 10 of the present invention being connected to a transformer in a resistor-divided manner as a converter 110 is shown.

[0140] Reference Figure 9In the case where the converter 110 is a transformer based on a resistive voltage divider, the resistor used for voltage conversion can perform the function of the converter 110. Therefore, either the first node 111 or the second node 112 of the disconnection sensing unit can be connected to the resistor 110 used for voltage conversion, while the other node can be directly connected to the line. Thus, the circuit of the disconnection sensing unit 120 can be formed by the lines L and N and the resistor 110.

[0141] in addition, Figure 10 An example of a protective relay 10 of the present invention being connected to a transformer (PT) using an iron core as a converter 110 is shown.

[0142] Reference Figure 10 In the case where the converter 110 is a transformer (PT) using an iron core, the first node 111 and the second node 112 of the disconnection sensing unit can be connected to the two ends of the transformer 110 (PT) respectively, and form a circuit of the disconnection sensing unit 120 connected through the transformer 110 (PT).

[0143] and, Figure 11 An example of a protective relay 10 of the present invention being connected to a converter (CT) using an iron core as a converter 110 is shown.

[0144] Reference Figure 11 When the converter 110 is a converter (CT) using an iron core, the first node 111 and the second node 112 of the disconnection sensing unit can be connected to the two ends of the converter 110 (CT) respectively, and form a circuit of the disconnection sensing unit 120 connected through the converter 110 (CT).

[0145] on the other hand, Figure 12 An example of a protective relay 10 of the present invention being connected to a converter using a Rogowski coil as a converter 110 is shown.

[0146] Reference Figure 12 In the case where the converter 110 is a converter using a Rogowski coil, the first node 111 and the second node 112 of the disconnection sensing unit can be connected to the two ends of the Rogowski coil respectively, forming a circuit of the disconnection sensing unit 120 connected through the Rogowski coil.

[0147] Without using a Rogowski coil with an iron core, since air is used as the magnetic flux path, there is no need to worry about output waveform distortion due to saturation, as is common in conventional converters. However, there are several problems: the output voltage waveform includes more noise, and there is a 90-degree phase delay between the output voltage waveform and the input current waveform. Additionally, the output voltage varies depending on the position and angle of the cable passing through the Rogowski coil.

[0148] Due to the problems described above, when using a Rogowski coil, an integrating circuit or integrating program is required to recover the waveform delayed by 90 degrees. Therefore, although not shown, when using a Rogowski coil, the integrating circuit can also be included between the disconnection sensing unit 120 and the disconnection determination unit 150. Alternatively, the disconnection determination unit 150 can include an integrating unit (not shown) instead of the integrating circuit, which performs integration according to a preset integrating algorithm in order to recover the delayed waveform.

[0149] Additionally, the disconnection detection unit 150 may be equipped with a noise filter circuit or digital filter for noise removal, and the converter 110 and the disconnection sensing unit 120 may be designed with a structure in which the Rogowski coil and the cable passing through the Rogowski coil remain constant. In this case, if it is necessary to adjust the magnitude of the sensed voltage, the disconnection sensing unit 120 may also include a load resistor.

[0150] On the other hand, in the above description, the circuit structure of the disconnection sensing unit 120 is described as having a voltage divider resistor formed by the first resistor R1 (125), the second resistor R2 (126) and the third resistor R3 (127). However, any one of the first resistor to the third resistor 125, 126 and 127 can be omitted.

[0151] Figures 13 to 14 This is an example diagram showing an example of a disconnection sensing unit 120 in a protective relay 10 according to an embodiment of the present invention, which has a structure in which any one of the resistors constituting the voltage divider resistors is omitted.

[0152] First, refer to Figure 13 , Figure 13 An example is shown where the first resistor R1 (125) is omitted or the resistance value of the first resistor R1 (125) is 0 ohms.

[0153] The first resistor R1 (125) is used to prevent the voltage applied to the second contact n2 (122) from being applied to the first contact n1 (121). In the event that an excessive voltage is applied to the second contact n2 (122), the resistor is connected to the first contact n1 (121) to prevent the excessive voltage from being applied to the disconnection judgment unit 150, i.e., the switching unit 156.

[0154] On the other hand, when the circuit is in a normally connected state, a signal with a reference voltage VR can be applied to the first contact n1 (121). Furthermore, when the circuit is disconnected, a signal lower than the reference voltage VR can be applied. Therefore, if the reference voltage VR is of a suitable value (its value will not cause damage to the switching unit 156), even without the first resistor R1 (125), excessive voltage will not be applied to the switching unit 156. Therefore, as... Figure 13 As shown, the disconnection sensing unit 120 may also have a structure that omits the first resistor R1 (125).

[0155] Additionally, refer to Figure 14 , Figure 14 An example is shown where the second resistor R2(126) is omitted or the resistance value of the second resistor R2(126) is 0 ohms.

[0156] Reference Figure 14 Since the state of the second resistor R2 (126) is omitted, if the voltage signal applied to the second contact n2 (122) is calculated (as shown in the above mathematical formula 2) when the circuit is in an open state, the voltage signal applied to the second contact n2 (122) can be 0V. Furthermore, even if the voltage signal applied to the second contact n2 (122) is 0V, the voltage signal applied to the first contact n1 (121) can still be 0V when the circuit is in an open state.

[0157] Conversely, when the circuit is in a normally connected state, as described above, the magnitude of the voltage signal applied to the first contact n1 (121) can be the same as the magnitude of the voltage signal applied to the third contact n3 (123) as the reference voltage VR. Therefore, without omitting the second resistor R2 (126), the difference between the DC offset value of the circuit in the normal state and the DC offset value of the circuit in the disconnected state detected by the conversion unit 156 can have a large drop from the reference voltage VR to 0V.

[0158] However, in this case, the conversion unit 156 needs to be able to receive voltage signals from the reference voltage VR to 0V. That is, when the input signal range of the conversion unit 156 includes the range from the reference voltage VR to 0V, such as... Figure 14As shown, the disconnection sensing unit 120 may have a structure that omits the second resistor R2 (126).

[0159] On the other hand, the above description uses the case where the reference voltage generation unit 129 has a ground voltage as an example. However, the reference voltage generation unit 129 can also be configured to have an additional voltage generation unit 1500 to have a voltage higher than that of ground.

[0160] Figure 15 This is a circuit diagram illustrating an example of the structure of the protection relay 10 of the present invention, as described above, in which the disconnection sensing unit 120 has a reference voltage generation unit 129 and an additional voltage generation unit that applies a preset voltage instead of the ground voltage.

[0161] like Figure 15 As shown, when an additional voltage generation unit 1500 is included to give the reference voltage generation unit 129 a voltage higher than ground, the reference voltage generated by the reference voltage generation unit 129 can be determined based on the additionally connected voltage generation unit 1500. Therefore, in the following description, the voltage of the reference voltage generation unit 129 is assumed to be the voltage VS of the additional voltage generation unit 1500.

[0162] Hereinafter, the voltage VR applied from the sensing voltage generation unit 130 will be referred to as the first reference voltage, and the voltage VS of the reference voltage generation unit will be referred to as the second reference voltage, in order to distinguish between the voltage VS of the reference voltage generation unit and the voltage VR applied from the sensing voltage generation unit 130.

[0163] On the other hand, in having Figure 15 In the case of the disconnection sensing unit 120 with the structure shown, if the circuit connection is normal, the first contact n1 (121) connected to the disconnection judgment unit 150 can be applied with the same voltage signal as the voltage signal applied to the third contact n3 (123). Therefore, the first contact n1 (121) can be applied with a voltage signal having a first reference voltage VR as shown in the following mathematical formula 3.

[0164] Mathematical Formula 3

[0165] n1(V)=n3(V)=VR

[0166] Here, n1(v) is the magnitude of the voltage signal applied to the first contact 121, n3(v) is the magnitude of the voltage signal applied to the third contact 123, and VR is the first reference voltage.

[0167] Conversely, in the event of a circuit break caused by converter 110, the first contact n1 (121) connected to the break detection unit 150 can be applied with the same voltage signal as the voltage signal applied to the second contact n2 (122). Therefore, the second contact n2 (122) can be applied with a voltage signal having a voltage difference between the second reference voltage VS and the first reference voltage VR divided by the second resistor R2 (126) and the third resistor R3 (127).

[0168] In this case, if the value of the second reference voltage VS is less than the value of the first reference voltage VR, then the second contact n2 (122) can be applied with a voltage signal having the voltage difference between the second reference voltage VS and the first reference voltage VR divided by the second resistor R2 (126) and the third resistor R3 (127). The following mathematical formula 4 represents the magnitude of the voltage signal applied to the second contact n2 (122) in this case.

[0169] Mathematical expression 4

[0170] (If VS <VR)

[0171] Here, n2(v) is the magnitude of the voltage signal applied to the second contact 122, VR is the first reference voltage, VS is the second reference voltage, R2 is the resistance value of the second resistor R2(126), and R3 is the resistance value of the third resistor R3(127).

[0172] Therefore, as stated in mathematical formula 4, the DC offset value (voltage) detected from the voltage signal sensed at the first contact n1 (121) when a circuit break occurs can be less than the DC offset value extracted from the voltage signal VR detected at the first contact n1 (121) when the circuit is normal.

[0173] On the other hand, when the value of the second reference voltage VS is greater than the value of the first reference voltage VR, as in mathematical formula 5, the voltage signal applied to the second contact n2 (122) can have a voltage greater than the first reference voltage VR.

[0174] Mathematical formula 5

[0175] (If VS>VR)

[0176] Here, n2(v) is the magnitude of the voltage signal applied to the second contact 122, VR is the first reference voltage, VS is the second reference voltage, R2 is the resistance value of the second resistor R2(126), and R3 is the resistance value of the third resistor R3(127).

[0177] That is, when a power supply with a voltage VS greater than the first reference voltage VR is connected to the reference voltage generation unit 129, when a circuit disconnection occurs, the DC offset value detected from the voltage signal sensed from the first contact n1 (121) can be greater than the DC offset value extracted from the voltage signal VR sensed from the first contact n1 (121) when the circuit is normal.

[0178] Therefore, if the DC offset value extracted from the voltage signal sensed by the disconnection sensing unit 120 is greater than the value of the critical voltage VR, the control unit 155 can determine that the circuit is disconnected, rather than determining that the circuit is disconnected when the DC offset value is less than the critical voltage VR.

[0179] Figure 16 This is a flowchart illustrating the operation of the disconnection determination unit 150 in the protection relay 10 of this embodiment of the invention, which determines whether the converter 110 is disconnected based on the voltage signal detected by the disconnection sensing unit 120. In the following description, it is assumed that the second reference voltage VS has a larger value than the first reference voltage VR.

[0180] Reference Figure 16 When the preset cycle ends, if the disconnection sensing unit 120 senses a voltage signal, that is, a voltage signal applied to the first contact 121 connected to the disconnection sensing unit 120, the control unit 155 can extract the value of the DC component, that is, the DC offset value, from the sampled digital data of one cycle converted by the conversion unit 156 (S1600).

[0181] Furthermore, the control unit 155 can control the first judgment unit 151 to compare the extracted DC offset value with a preset threshold voltage, thereby determining whether the circuit is disconnected (S1602).

[0182] As described above, in the normal state of the circuit connected to converter 110, a circuit can be formed that connects the third contact 123 and the first contact 121 via converter 110. Therefore, the reference voltage VR of the sensing voltage generation unit 130 connected to the third contact 123 can be applied to the first contact 121 via converter 110. Therefore, a DC offset value of the voltage signal corresponding to the reference voltage VR can be obtained by converter 156.

[0183] Conversely, in the event of a circuit disconnection due to converter 110, the first contact 121 and the third contact 121 may not be connected. Therefore, as stated in Equation 3, the first contact 121 can be subjected to the same voltage signal as the second contact 122.

[0184] On the other hand, the second contact n2 (122) can be an applied voltage signal, which is a voltage signal having the voltage difference between the second reference voltage VS and the first reference voltage VR divided by the second resistor R2 (126) and the third resistor R3 (127). In this case, as described above, since the value of the second reference voltage VS is greater than the value of the first reference voltage VR, the magnitude of the voltage signal applied to the second contact n2 (122) can be calculated based on the voltage difference between the second reference voltage VS and the first reference voltage VR, the second resistor R2 (126), and the third resistor R3 (127), as described in Equation 5.

[0185] Furthermore, in this case, since the value of the second reference voltage VS is greater than the value of the first reference voltage VR, the second contact n2 (122) can be supplied with a voltage signal having a voltage greater than that of the first reference voltage VR. Therefore, the magnitude of the DC offset value extracted when the circuit connection is disconnected can be greater than the magnitude of the DC offset value extracted when the circuit connection is normal.

[0186] Therefore, when the critical voltage is the first reference voltage VR, if the circuit is disconnected in step S1602, the DC offset value extracted in step S1600 can be greater than the critical voltage (first reference voltage VR). Therefore, if the determination result in step S1602 is that the magnitude of the currently extracted DC offset value is greater than the critical voltage, the control unit 155 can determine that the circuit currently connected to the converter 110 is disconnected.

[0187] Conversely, in step S1602, if the DC offset value extracted in step S1600 is not greater than the threshold voltage, the control unit 155 can determine that the circuit is in an unbroken state. Furthermore, if the determination result of step S1602 is that the circuit is in an unbroken state, the control unit 155 can end the unbroken state determination process in the current cycle.

[0188] On the other hand, if the disconnection determination result of step S1602 is that the circuit connected to converter 110 is in a disconnected state, then control unit 155 can subtract a voltage value based on the design margin from the currently extracted DC offset value (S1604). Here, the voltage value based on the design margin may also include a preset error value. Here, the error value may be a measurement error, and may also include errors that may occur during the conversion process of conversion unit 156.

[0189] In step S1604, if the design margin including the error value is subtracted from the DC offset value, the control unit 155 can control the second determination unit 152 to compare the voltage value after subtracting the design margin with the critical voltage again (S1606).

[0190] Subsequently, if the comparison result of step S1606 is that the voltage value obtained by subtracting the design margin from the DC offset value is not greater than the critical voltage, then the control unit 155 can determine that the circuit is in an unbroken state. Furthermore, if the determination result of step S1606 is that the circuit is in an unbroken state, the control unit 155 can end the unbroken state determination process in the current cycle.

[0191] However, if the comparison result of step S1606 is that even after subtracting the design margin from the DC offset value, the calculated voltage value is still greater than the critical voltage, then the control unit 155 can again determine (secondary disconnection judgment) that the circuit currently connected to the converter 110 is disconnected.

[0192] On the other hand, if the secondary determination indicates that the circuit currently connected to the converter 110 is disconnected, the control unit 155 can also detect the magnitude of the alternating current (AC) component from the sampled digital data of one cycle converted by the conversion unit 156 (S1608). Furthermore, the detected magnitude of the AC component can be compared with the magnitude of a preset fault current signal (S1610).

[0193] On the other hand, if the comparison result of step S1610 shows that the magnitude of the detected AC component is greater than or equal to the preset fault current signal magnitude, the control unit 155 can determine that a short-circuit current or a startup current caused by the start-up of the converter 110 has occurred. Furthermore, it can be determined that the circuit disconnection state based on the determination (secondary disconnection determination) result of step S1606 is a misjudgment caused by the occurrence of the short-circuit current or the startup current.

[0194] That is, if a short-circuit current or starting current is determined to have occurred based on the judgment result of step S1610, the control unit 155 can determine that the circuit is in a normal connected state even if it is the result of the secondary disconnection judgment. Therefore, the circuit disconnection judgment process in the current cycle can be ended without outputting a reminder signal.

[0195] Conversely, if the comparison result of step S1610 shows that the detected AC component is less than the preset fault current signal magnitude, the control unit 155 can determine that no short-circuit current or startup current caused by the start-up of the converter 110 has occurred. Therefore, the control unit 155 can ultimately determine that the circuit currently connected to the converter 110 is disconnected, and output a reminder signal for conveying the circuit disconnection to the user (S1612).

[0196] On the other hand, the Figure 16 The operation process can be repeatedly executed according to a preset cycle. Here, the preset cycle can correspond to the cycle of the signal input from the line to the converter 110. In this case, the operation can be performed during each main action of the protection relay 10. Figure 16 The operation process is described. In this case, the main action of the protection relay 10 can vary depending on the device connected to it. For example, if the device connected to the protection relay 10 is a digital relay, the main action can be a relay operation; if the connected device is a digital measuring instrument, the main action can be a measuring operation. Additionally, if the connected device is a power conversion device, the main action can be a power conversion operation.

[0197] on the other hand, Figures 17 to 18 It is shown in having Figure 15 The circuit diagram of the disconnection sensing unit 120 is shown in the protective relay 10 with any one of the resistors constituting the voltage divider resistors omitted.

[0198] First, refer to Figure 17 , Figure 17 The case where the first resistor R1 (125) is omitted or the resistance value of the first resistor R1 (125) is 0 ohms is shown.

[0199] The first resistor R1 (125) is used to prevent the voltage applied to the second contact n2 (122) from being applied to the first contact n1 (121). In the event that an excessive voltage is applied to the second contact n2 (122), it is connected to the first contact n1 (121) to prevent the excessive voltage from being applied to the disconnection judgment unit 150, i.e., the switching unit 156.

[0200] On the other hand, Figure 15In the case where the voltage difference between the first reference voltage VS and the second reference voltage VR can be applied by voltage division using the second resistor R2 (126) and the third resistor R3 (127). Furthermore, when the first reference voltage VS is less than the second reference voltage VR, since a signal with a voltage lower than the first reference voltage VR is applied to the first contact n1 (121) when the circuit is disconnected, the maximum voltage that can be applied to the first contact n1 (121) can correspond to the first reference voltage VR. Therefore, when the magnitude of the first reference voltage VR is suitable, the disconnection sensing unit 120 can also have a structure that omits the first resistor R1 (125).

[0201] Conversely, when the first reference voltage VS is greater than the second reference voltage VR, the first contact n1 (121) may be subjected to a signal with a voltage greater than the first reference voltage VR when the circuit is disconnected. Therefore, as Figure 17 As shown, in order to have a structure that omits the first resistor R1 (125), not only does the first reference voltage VR need to be of a suitable size, but the second reference voltage VS also needs to be of a suitable size, that is, it needs to have a voltage that can prevent damage to the conversion unit 156.

[0202] On the other hand, refer to Figure 18 , Figure 18 It is shown in the Figure 15 In the circuit of the disconnection sensing unit 120 shown, examples of the second resistor R2 (126) or the case where the resistance value of the second resistor R2 (126) is 0 ohms are omitted.

[0203] Reference Figure 18 Since the second resistor R2 (126) is omitted, when the circuit is in an open state, if the magnitude of the voltage signal applied to the second contact n2 (122) is calculated according to the mathematical formula 4 or mathematical formula 5, the magnitude of the voltage signal applied to the second contact n2 (122) can be 0V. Furthermore, if the magnitude of the voltage signal applied to the second contact n2 (122) is 0V, the magnitude of the voltage signal applied to the first contact n1 (121) when the circuit is in an open state can also be 0V.

[0204] Conversely, as described above, when the circuit is in a normally connected state, the magnitude of the voltage signal applied to the first contact n1 (121) can be the first reference voltage VR. Therefore, without omitting the second resistor R2 (126), the difference between the DC offset value in the normal circuit state and the DC offset value in the disconnected state detected by the conversion unit 156 may have a large drop from the first reference voltage VR to 0V.

[0205] However, in this case, the conversion unit 156 needs to be able to receive voltage signals from the first reference voltage VR to 0V. That is, when the input signal range of the conversion unit 156 includes the range from the first reference voltage VR to 0V, such as... Figure 18 As shown, the disconnection sensing unit 120, which also includes a second reference voltage VS, may have a structure in which the second resistor R2 (126) is omitted.

[0206] on the other hand, Figure 19 This is a circuit diagram showing the structure of a sensing voltage generation unit 130 in the protection relay 10 of this invention, which has a structure that applies a ground voltage as a first reference voltage instead of a preset voltage.

[0207] like Figure 19 As shown, when the sensing voltage generation unit 130 applies the ground voltage as the first reference voltage, the reference voltage generation unit 129 needs to further include an additional voltage generation unit 1500 for providing a second reference voltage different from the ground voltage. In this case, the first reference voltage VR applied by the sensing voltage generation unit 130 can be the ground voltage (0V), and the voltage VS applied by the voltage generation unit 1500 can be the second reference voltage.

[0208] In this case, with the circuit in a normally connected state, the first contact n1 (121) can be supplied with a voltage signal of the same magnitude as the voltage signal supplied to the third contact n3 (123). In this case, since the third contact n3 (123) is supplied with the first reference voltage, i.e., the ground voltage signal, the first contact n1 (121) can also be supplied with the ground voltage, i.e., a 0V voltage signal.

[0209] Conversely, since the second reference voltage VS is greater than the first reference voltage VR, in the event of a circuit disconnection due to the converter 110, the magnitude of the voltage signal applied to the first contact n1 (121), as described in equations 4 and 5, can be determined based on the magnitude of the voltage signal applied to the second contact n2 (122). Therefore, in the event of a circuit disconnection, the first contact n1 (121) can be supplied with the first reference voltage, i.e., a voltage signal having a voltage greater than the ground voltage.

[0210] Therefore, the control unit 155 of the disconnection judgment unit 150 can determine whether the circuit is disconnected based on whether the DC offset value obtained by the conversion unit 156 from the voltage signal of the first contact n1 (121) sensed by the disconnection sensing unit 120 is greater than the ground voltage. Furthermore, it can make a second determination of whether the circuit is disconnected based on the result of the first disconnection judgment, taking into account the design margin, and finally determine whether the circuit is disconnected based on the result of the second disconnection judgment and the magnitude of the AC component obtained by the conversion unit 156 from the voltage signal of the first contact n1 (121).

[0211] On the other hand, although specific embodiments have been described in the above description of the present invention, various modifications can be made without departing from the scope of the present invention. In particular, although various embodiments without either the second resistor R2 (126) or the third resistor R3 (127), or with an additional reference voltage applied, have been mentioned in the above description, the present invention is not limited to such embodiments. For those skilled in the art, various modifications and applications not described above can be made without departing from the essential characteristics of the present invention.

[0212] For example, the first determination unit 151 and the second determination unit 152 are determination units that compare the DC offset value obtained by the conversion unit 156 and the value reflecting the design margin in the DC offset value with a preset critical voltage, respectively. However, in this case, the control unit 150 may also calculate the value reflecting the design margin in the preset critical voltage instead of the second determination unit 152.

[0213] Therefore, the first judgment unit 151 and the second judgment unit 152 can also be integrated into a single judgment unit that compares the value specified by the control unit 155 (the DC offset value obtained by the conversion unit 156, or the value in which the obtained DC offset value reflects the design margin) with a preset critical voltage. In this case, the integrated judgment unit can sequentially compare the DC offset value obtained by the conversion unit 156 and the value in which the DC offset value reflects the design margin with the preset critical voltage according to the control of the control unit 155, in order to perform the first disconnection determination and the second disconnection determination.

[0214] The aforementioned invention can be implemented by computer-readable code stored in a medium containing a program. Computer-readable media include all types of recording devices storing data readable by a computer system. Examples of computer-readable media include hard disk drives (HDDs), solid-state drives (SSDs), silicon disk drives (SDDs), ROM, RAM, CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, etc., and can also be implemented in carrier wave (e.g., Internet-based transmission) form. Furthermore, the computer may also include a control unit 155 of the disconnection detection unit 150.

[0215] Therefore, the detailed description described above should not be construed as restrictive in all respects, but rather as exemplary. The scope of the invention should be determined by a reasonable interpretation of the appended claims, and all modifications within the equivalent scope of the invention should fall within its scope.

Claims

1. A protection relay for detecting whether a line connected to a converter and an electric circuit connected to said converter is broken, characterized in that comprises: a disconnection sensing section including a plurality of nodes connected across an output of the converter and a reference voltage generating section that forms a reference voltage, and including a plurality of resistors that form a voltage dividing resistor and are connected between the plurality of nodes and the reference voltage generating section; a sensing voltage generating section that applies a voltage for forming a prescribed voltage difference with respect to the reference voltage to the disconnection sensing section; and a disconnection determining section that is connected to a first node among the plurality of nodes, and that detects whether or not a disconnection has occurred in at least one of the converter and a circuit connected to the converter, based on a voltage signal detected from the first node, the magnitude of the voltage signal applied to the first node by the voltage dividing resistor being different depending on whether or not a disconnection has occurred in at least one of the converter and the circuit connected to the converter; the plurality of resistors including a plurality of resistors connected in series between the plurality of nodes, and a resistor connected in parallel with the plurality of resistors between the plurality of resistors connected in series, the reference voltage generating section being connected to the resistor connected in parallel, and applying a voltage different from the sensing voltage applied from the sensing voltage generating section via the resistor connected in parallel.

2. The protective relay according to claim 1, wherein the disconnection determining section extracts a DC offset value from the voltage signal detected at the first node, and in a case where a result of comparing the extracted DC offset value and a predetermined threshold voltage is that the DC offset value is smaller than the threshold voltage, detects the magnitude of an AC component from the voltage signal detected at the first node, and determines whether or not a disconnection has occurred in at least one of the converter and the circuit connected to the converter, based on the magnitude of the detected AC component.

3. The protective relay according to claim 2, wherein the disconnection determining section determines whether or not a disconnection has occurred in at least one of the converter and the circuit connected to the converter, based on whether or not the magnitude of the detected AC component is greater than a predetermined magnitude, the predetermined magnitude being determined based on the magnitude of an AC component included in the voltage signal detected at the first node at the time of a start-up current generated at the time of a short-circuit current or a start-up of the converter.

4. The protective relay according to claim 2, wherein in the disconnection determining section, in a case where a disconnection has occurred in at least one of the converter and the circuit connected to the converter is determined once based on a result of comparing the extracted DC offset value and a predetermined threshold voltage, a predetermined design margin value is further reflected in the extracted DC offset value, whether or not a disconnection has occurred in at least one of the converter and the circuit connected to the converter is determined a second time based on a result of comparing the DC offset value further reflecting the design margin value and the threshold voltage, and a disconnection in at least one of the converter and the circuit connected to the converter is detected based on a result of the second determination.

5. The protective relay according to claim 4, wherein the design margin includes a predetermined error value, The preset error value includes at least one of a measurement error occurring in a process of extracting a DC offset value from a voltage signal detected at the first node and a design error generated due to a circuit structure of the disconnection sensing section.

6. The protection relay according to claim 2, wherein In a case where the reference voltage is a ground voltage, the critical voltage is the same as a voltage applied by the sensing voltage generation section.

7. The protection relay according to claim 1, wherein The disconnection sensing section includes: a first junction formed between the first node and the disconnection judging section; a second node connected to a side different from a side connected to the first node among both ends of an output of the converter; a third junction formed between the second node and the sensing voltage generation section; a second junction formed on a circuit connecting the first junction and the third junction; a first resistor arranged between the first junction and the second junction; a second resistor arranged between the reference voltage generation section and the second junction, connecting between the second junction and the reference voltage generation section; and a third resistor arranged between the second junction and the third junction.

8. The protection relay according to claim 1, wherein The disconnection sensing section includes: a first junction formed between the first node and the disconnection judging section; a second node connected to a side different from a side connected to the first node among both ends of an output of the converter; a third junction formed between the second node and the sensing voltage generation section; a second junction formed on a circuit connecting the first junction and the third junction; a second resistor arranged between the reference voltage generation section and the second junction, connecting between the second junction and the reference voltage generation section; and a third resistor arranged between the second junction and the third junction.

9. The protection relay according to claim 1, wherein The disconnection sensing section includes: a first junction formed between the first node and the disconnection judging section; a second node connected to a side different from a side connected to the first node among both ends of an output of the converter; a third junction formed between the second node and the sensing voltage generation section; a second junction formed on a circuit connecting the first junction and the third junction, connected to the reference voltage generation section; a first resistor arranged between the first junction and the second junction; and a third resistor arranged between the second junction and the third junction.

10. The protection relay according to any one of claims 7 to 9, wherein in a case where the converter and a circuit connected to the converter are in a normally connected state, the first node is applied with a voltage signal of the same magnitude as a voltage applied by the sensing voltage generation section, The DC offset value detected from the voltage signal applied to the first node when at least one of the converter and the circuit connected to the converter is in the open-line state is smaller than the DC offset value detected from the voltage signal applied to the first node when the converter and the circuit connected to the converter are in the normally connected state.

11. The protective relay according to claim 10, wherein If the voltage applied by the sensing voltage generation section is greater than the reference voltage, and if at least one of the converter and the circuit connected to the converter is in the open-line state, a voltage signal is applied in accordance with the difference between the reference voltage and the voltage applied by the sensing voltage generation section and the magnitude of the resistance forming the voltage dividing resistor, as shown in the following mathematical expression. Mathematical Expression ， Here, n1(v) is the magnitude of the voltage signal applied to the first junction, n2(v) is the magnitude of the voltage signal applied to the second junction, Vs is the reference voltage, VR is the voltage applied by the sensing voltage generation section, Vs < VR, R2 is the resistance value of the second resistor, and R3 is the resistance value of the third resistor.

12. The protective relay according to claim 11, wherein If the extracted DC offset value is smaller than the preset threshold voltage, the open-line determination section detects the open line of at least one of the converter and the circuit connected to the converter by adding a preset design margin value to the extracted DC offset value and comparing the DC offset value to which the design margin value has been added and the threshold voltage.

13. The protective relay according to claim 10, wherein The reference voltage is a ground voltage.

14. The protective relay according to any one of claims 7 to 9, wherein In the case where the converter and the circuit connected to the converter are in the normally connected state, the first node is applied with a voltage signal of the same magnitude as the voltage applied by the sensing voltage generation section, The DC offset value detected from the voltage signal applied to the first node when at least one of the converter and the circuit connected to the converter is in the open-line state is greater than the DC offset value detected from the voltage signal applied to the first node when the converter and the circuit connected to the converter are in the normally connected state.

15. The protective relay according to claim 14, wherein If at least one of the converter and the circuit connected to the converter is in the open-line state, a voltage signal is applied in accordance with the voltage applied by the sensing voltage generation section, the difference between the voltage applied by the sensing voltage generation section and the reference voltage, and the magnitude of the resistance forming the voltage dividing resistor, as shown in the following mathematical expression, in the case where the reference voltage is greater than the voltage applied by the sensing voltage generation section. Mathematical Expression ， Here, n1(v) is the magnitude of the voltage signal applied to the first contact, n2(v) is the magnitude of the voltage signal applied to the second contact, VS is a reference voltage, VR is a voltage applied by the sensing voltage generation section, VS > VR, R2 is the resistance value of the second resistor, and R3 is the resistance value of the third resistor.

16. The protection relay according to claim 15, wherein In a case where the extracted DC offset value is greater than a predetermined threshold voltage, the disconnection judging section subtracts a predetermined design margin value from the extracted DC offset value, and detects disconnection of at least one of the converter and the circuit connected to the converter by comparing the DC offset value from which the design margin value is subtracted with the threshold voltage.

17. The protection relay according to claim 14, wherein The voltage of the sensing voltage generation section is a ground voltage.

18. The protection relay according to claim 2, wherein In a case where the converter is a converter using a Rogowski coil, an integrating circuit for restoring a delayed waveform generated in accordance with a characteristic of the Rogowski coil is further included between the disconnection sensing section and the disconnection judging section, or The disconnection judging section integrates the DC offset value and judges disconnection of at least one of the converter and the circuit connected to the converter in accordance with a comparison result of the integrated DC offset value and the threshold voltage.

19. The protection relay according to claim 1, wherein The disconnection judging section further includes an output section for outputting an alarm signal indicating a disconnection state of at least one of the converter and the circuit connected to the converter, The output section has a communication function for transmitting the alarm signal to another device set in advance.

20. A method of protecting a relay from a line break detection method of detecting whether a line break has occurred in a converter and a circuit connected to the converter, characterized by, The disconnection detection method includes: a step of detecting a voltage signal from a disconnection sensing section and obtaining a DC offset value from the detected voltage signal, wherein the disconnection sensing section includes a plurality of nodes connected to both ends of an output of the converter and a plurality of resistors forming a voltage dividing resistor, and voltage signals applied to the plurality of nodes differ from each other in accordance with disconnection of at least one of the converter and the circuit connected to the converter; a step of determining disconnection of at least one of the converter and the circuit connected to the converter once by comparing the obtained DC offset value with a predetermined threshold voltage; a step of calculating a magnitude of an AC component from the detected voltage signal in accordance with a result of the once determination; a step of judging whether a short-circuit current or a starting current of the converter is generated by comparing the calculated magnitude of the AC component with a predetermined magnitude of an AC current signal; and a step of determining disconnection of at least one of the converter and the circuit connected to the converter twice in accordance with a result of the judgment of whether the short-circuit current or the starting current is generated; The plurality of resistors includes a plurality of resistors connected in series between the plurality of nodes, and a resistor connected in parallel with the plurality of resistors between the plurality of resistors connected in series, a sensing voltage is applied to any of the plurality of resistors connected in series, a reference voltage having a prescribed voltage difference from the sensing voltage is applied to the resistor connected in parallel.

21. The method of claim 20, wherein the step of determining the disconnection includes: a step of comparing the obtained DC offset value with a predetermined threshold voltage; a step of determining that the DC offset value reflects a predetermined design margin value, based on a result of the comparison between the DC offset value and the threshold voltage; and a step of determining the disconnection of at least one of the converter and the circuit connected to the converter, by comparing the DC offset value reflecting the design margin value with the threshold voltage.

22. The method of claim 20, wherein the step of determining the disconnection again, further includes a step of outputting a warning indicating a disconnection state, in a case where the result of the determination again indicates that at least one of the converter and the circuit connected to the converter is disconnected, and further includes a step of executing the steps of obtaining the DC offset value to the step of determining the disconnection again, again, based on whether a predetermined period ends, in a case where the result of the determination again indicates that at least one of the converter and the circuit connected to the converter is normally connected.

23. The method of claim 22, wherein the predetermined period is determined based on a time at which a device connected to the protective relay completes a main operation.

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