A CT secondary circuit open-circuit detection device and method

Abstract

An embodiment of the present invention relates to a CT secondary circuit open-circuit detection device and method. The device includes a voltage conversion module, a resistance matching module, an ADC input module, and an ADC. The voltage conversion module is connected to the CT secondary circuit to be measured to convert the current signal of the secondary circuit into a voltage signal. The resistance matching module is connected between the voltage conversion module and the ADC input module. The ADC input module is connected to the input end of the ADC. The ADC collects the voltage value of the CT secondary circuit through the ADC input module to perform open-circuit detection. The CT secondary circuit open-circuit detection method provided by the embodiment of the present invention determines whether an open-circuit may occur by the change of the voltage value sampled by the ADC, and further determines whether an open-circuit occurs by the change of the ADC sampling value after increasing and decreasing the PGA common-mode voltage.

Description

Technical Field

[0001] The embodiments of the present invention relate to the technical field of microcomputer relay protection, and in particular to a CT secondary circuit open-circuit detection device and method. Background Art

[0002] Microcomputer relay protection devices are an important part of the power system. To ensure the safe and stable operation of the power system, it is necessary to monitor power equipment in real time. When a fault occurs in the power system, the microcomputer relay protection device must send out an alarm message and initiate a protection action to cut off the fault and ensure the reliable operation of the power system. The internal current transformer (Current Transformer, hereinafter referred to as "CT") of the relay protection device is large in volume and high in weight. During the device transportation process or several times of board card plugging and unplugging, it is very easy to cause the fracture of the secondary pins. The open circuit of the CT secondary circuit will cause the sampling circuit to be unable to sample correctly. The high voltage at the CT open circuit point will threaten the safety of equipment and personnel, and in severe cases, it will also cause misoperation of the protection; on the other hand, when the CT secondary circuit is open-circuited, if there is a fault current in the primary circuit, the device cannot give an early warning, which will cause accident risks. Therefore, how to detect the open circuit of the CT secondary circuit has become an urgent problem to be solved. Summary of the Invention

[0003] Based on the above situation of the prior art, the purpose of the embodiments of the present invention is to provide a CT secondary circuit open-circuit detection device and method. Based on the ADC of the adjustable programmable gain amplifier (Programmable Gain Amplifier, hereinafter referred to as "PGA"), it judges the change of the ADC source impedance and then detects whether the CT secondary circuit is open-circuited. This method is simple and easy to implement, with high accuracy, and can quickly detect whether the CT secondary circuit is open-circuited without adding other detection circuits.

[0004] To achieve the above object, according to one aspect of the embodiments of the present invention, a CT secondary circuit open-circuit detection device is provided, including a voltage conversion module, a resistance matching module, an ADC input module and an ADC; wherein,

[0005] The voltage conversion module is connected to the CT secondary circuit to be measured to convert the current signal of the secondary circuit into a voltage signal;

[0006] The resistance matching module is connected between the voltage conversion module and the ADC input module;

[0007] The ADC input module is connected to the input end of the ADC;

[0008] The ADC collects the voltage value of the CT secondary circuit through the ADC input module to perform open-circuit detection.

[0009] Further, it further includes a filtering module;

[0010] The filtering module includes a filtering capacitor and a filtering resistor, and the filtering module is connected between the resistor matching module and the ADC input module.

[0011] Further, the ADC includes an ADC with a programmable gain amplifier inside and adjustable common-mode voltage.

[0012] Further, the resistor matching module includes a matching resistor.

[0013] Further, the ADC input module includes a first input resistor and a second input resistor;

[0014] The ADC collects differential voltage signals through the first input resistor and the second input resistor.

[0015] According to another aspect of the present invention, there is provided a method of detecting by using the CT secondary circuit open-circuit detection device as described in the first aspect of the present invention, including the steps:

[0016] Obtain the voltage value collected by the ADC;

[0017] When the collected voltage value meets the open-circuit detection condition, perform the open-circuit detection step:

[0018] Raise the common-mode voltage of the programmable gain amplifier inside the ADC, and read the ADC sampling voltage value U H ;

[0019] If U H ―U 0 >20 LSB, then lower the common-mode voltage of the programmable gain amplifier inside the ADC, and read the ADC sampling voltage value U L ;

[0020] If U L =U 0 , then it is determined that a CT secondary circuit open-circuit has occurred;

[0021] Among them, U 0 is the voltage value collected when meeting the open-circuit detection condition.

[0022] Further, the open-circuit detection condition includes:

[0023] The same analog input channel of the ADC outputs the same sampling value U 0 continuously for N times;

[0024] Among them, N is the open-circuit detection threshold.

[0025] Further, the continuous output of the same sampling value for N times includes that the error between N sampling values is less than or equal to 10 LSB.

[0026] Further, without oversampling, the minimum value of N is:

[0027] N = 10 × f sample × (R PD + 2 × R FILTER ) × C FILTER

[0028] where f sample is the sampling frequency; R PD is the resistance value of the matching resistor; R FILTER is the resistance value of the filter resistor; C FILTER is the capacitance value of the filter capacitor.

[0029] Further, when the common-mode voltage increases, U H ―U 0 ≤ 20 LSB; or when the common-mode voltage decreases, U L ≠ U 0 ; then return to the first step to re-obtain the voltage value collected by the ADC.

[0030] In summary, the embodiments of the present invention provide a CT secondary circuit open-circuit detection device and method. The device includes a voltage conversion module, a resistance matching module, an ADC input module, and an ADC; the voltage conversion module is connected to the CT secondary circuit to be measured to convert the current signal of the secondary circuit into a voltage signal; the resistance matching module is connected between the voltage conversion module and the ADC input module; the ADC input module is connected to the input end of the ADC; the ADC collects the voltage value of the CT secondary circuit through the ADC input module for open-circuit detection. The CT secondary circuit open-circuit detection method provided by the embodiments of the present invention determines whether an open-circuit may occur by the change of the voltage value sampled by the ADC, and further determines whether an open-circuit occurs by the change of the ADC sampling value after increasing and decreasing the PGA common-mode voltage, and has the following beneficial technical effects: using the circuit elements of the sampling loop itself, quickly detecting whether the CT secondary circuit is open-circuited without adding other detection circuits, and simplifying the circuit design. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a block diagram of the CT secondary circuit open-circuit detection device according to the embodiment of the present invention;

[0032] Figure 2 is a schematic circuit structure diagram of the CT secondary circuit open-circuit detection device according to the embodiment of the present invention;

[0033] Figure 3 is a simplified schematic circuit diagram of the bipolar analog input loop before entering the ADC;

[0034] Figure 4 It is a flowchart of the CT secondary circuit open-circuit detection method according to an embodiment of the present invention. Specific embodiments

[0035] To make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present invention. In addition, in the following descriptions, the descriptions of well-known structures and technologies are omitted to avoid unnecessarily confusing the concepts of the present invention.

[0036] The technical solutions of the embodiments of the present invention will be described in detail below with reference to the accompanying drawings. According to an embodiment of the present invention, a CT secondary circuit open-circuit detection device is provided. Figure 1 The block diagram of the open-circuit detection device is shown in. It includes a voltage conversion module, a resistance matching module, an ADC input module, and an ADC. The voltage conversion module is connected to the CT secondary circuit to be measured to convert the current signal of the secondary circuit into a voltage signal; the resistance matching module is connected between the voltage conversion module and the ADC input module; the ADC input module is connected to the input end of the ADC; the ADC collects the voltage value of the CT secondary circuit through the ADC input module to perform open-circuit detection. It further includes a filtering module; the filtering module includes a filtering capacitor and a filtering resistor, and the filtering module is connected between the resistance matching module and the ADC input module.

[0037] Figure 2 The schematic circuit structure of the open-circuit detection device is shown in Figure 2 As shown, the CT secondary circuit open-circuit detection device is connected to the secondary circuit of the CT. After the alternating current is transformed by the CT, the output current signal is converted into a voltage signal by the voltage conversion module. According to some embodiments, the voltage conversion module is a resistor R1, which is a parallel resistor on the secondary side of the CT. The resistance values of the parallel resistors for different models of CTs are different, but the order of magnitude is about 100 Ω; other circuit structures capable of performing current-voltage conversion can also be used to implement the voltage conversion module. The output of the voltage conversion module is connected to the filtering module through the resistance matching module. According to some embodiments, the resistance matching module includes a matching resistor Rpd, and the converted voltage signal is grounded through the matching resistor Rpd to prevent the ADC input end from being floating, and then enters the ADC through the filtering module. The filtering module may include filtering resistors R3 and R4, the resistance values of the filtering resistors R3 and R4 are the same, and a filtering capacitor C1. The ADC input module includes a first input resistor R5 and a second input resistor R6, and the ADC collects the differential voltage signal through the first input resistor R5 and the second input resistor R6 of the ADC input module.

[0038] According to some embodiments, the ADC is a common-mode voltage adjustable ADC including an internal programmable gain amplifier, and is configured to detect disconnection of the secondary circuit by increasing and decreasing the common-mode voltage. Figure 2 Among them, R7 and R8 are the feedback resistors of the internal PGA of the ADC. In this detection device, the matching resistor Rpd is connected in parallel to the secondary circuit of the CT. The larger the resistance value of the matching resistor Rpd, the smaller the influence on the sampling accuracy of the circuit itself. However, when the secondary circuit of the current transformer is disconnected, due to the existence of the input bias current of the internal PGA of the ADC, a sampling voltage will be generated on the matching resistor Rpd. When the sampling value exceeds the threshold value for protection startup, it will cause misoperation of the protection.

[0039] According to another embodiment of the present invention, there is provided a method for detection using the CT secondary circuit disconnection detection device as described in the above embodiments of the present invention. Simplifying the circuit before the ADC for the bipolar analog input loop shown in Figure 2 results in the circuit shown in Figure 3 The bipolar input passes through the internal front-end PGA of the ADC and outputs in a fully differential manner. Defining AF as the open-loop gain of the fully differential operational amplifier, the output voltage is:

[0040] V OD =(V out )+ - (V out- ) (1)

[0041]

[0042] (V out )+ - (V out- )=A F (V p- V n ) (3)

[0043] The common-mode voltage of the differential output at both ends, that is, the average value of the two differential output signals, is equal to VOCM. It can be understood that the two differential output terminals will fluctuate around VOCM. According to the characteristics of the deeply negative feedback operational amplifier, when in deep negative feedback, AF>>1,

[0044] Assume

[0045] The relationship between the input and the output can be obtained as:

[0046]

[0047] When the secondary circuit of the CT is disconnected, that is, Figure 2 the switch S is opened. At this time:

[0048] V in + = O, Vin- = 0, β 1 ≠ β 2

[0049] It can be obtained that:

[0050]

[0051] At this time, the output voltage changes with the common-mode voltage of the operational amplifier. When the CT secondary circuit is not open-circuited, that is Figure 2 The switch S is closed. At this time, R3 = R4, R5 = R6, R7 = R8, and the parallel resistance of Rpd and R1 can be ignored compared with R3, R5, R7 (Rpd << R1). Therefore:

[0052] β 1 ≈ β 2 ≈ β

[0053] This output voltage can be expressed as:

[0054]

[0055] Based on the above analysis, the CT secondary circuit open-circuit detection method provided in this embodiment Figure 4 The flowchart of the method is shown, including the following steps:

[0056] S1. Obtain the voltage value collected by the ADC. For example, use the ADC in the open-circuit detection device in the above embodiment to collect the voltage value. In this embodiment, for example, use AD7606B to collect the voltage value, and based on the software automatic mode of AD7606B, start the acquisition conversion.

[0057] S2. When the collected voltage value meets the open-circuit detection condition, execute the open-circuit detection steps S3 - S5. Among them, the open-circuit detection condition includes: the same analog input channel of the ADC outputs the same sampling value U continuously for N times 0 ; where N is the open-circuit detection threshold, and the value of N is to output the same sampling value continuously for N times, including that the error between N sampling values is less than or equal to 10 LSB, and the error can be within 10 LSB.

[0058] In the case of not using oversampling, the minimum value of N is:

[0059] N = 10 × f sample × (R PD + 2 × R FILTER ) × C FILTER

[0060] Among them, f sample is the sampling frequency; R PD is the resistance value of the matching resistor; R FILTER is the resistance value of the filtering resistor; CFILTER is the capacitance value of the filter capacitor.

[0061] In the detection step,

[0062] S3. Raise the common-mode voltage of the programmable gain amplifier inside the ADC, and read the ADC sampling voltage value U H .

[0063] S4. If U H ―U 0 > 20 LSB, then lower the common-mode voltage of the programmable gain amplifier inside the ADC, and read the ADC sampling voltage value U L .

[0064] S5. If U L =U 0 , then it is determined that a CT secondary circuit disconnection has occurred; where U 0 is the voltage value collected when the disconnection detection condition is met.

[0065] When the common-mode voltage increases, U H ―U 0 ≤ 20 LSB; or when the common-mode voltage decreases, U L ≠U 0 ; then return to the first step to re-obtain the voltage value collected by the ADC. Generally speaking, the common-mode voltage of the PGA inside the ADC is set to two levels. When the collected voltage value meets the disconnection detection condition, first raise the common-mode voltage to the high level, read the sampling value of the ADC, and then lower the common-mode voltage to the low level to determine whether the ADC output value changes with the change of the common-mode voltage of its internal PGA. When the ADC output value of this analog channel changes with the change of the common-mode voltage of its internal PGA, it is determined that the CT sampling secondary circuit is disconnected. At this time, the disconnection signal can be output to notify the operation and maintenance personnel to deal with the fault in time, minimize the impact of the CT secondary circuit disconnection on the power system, and ensure the reliable and stable operation of the power grid.

[0066] In summary, the embodiment of the present invention relates to a CT secondary circuit disconnection detection device and method. The device includes a voltage conversion module, a resistance matching module, an ADC input module, and an ADC; the voltage conversion module is connected to the CT secondary circuit to be measured to convert the current signal of the secondary circuit into a voltage signal; the resistance matching module is connected between the voltage conversion module and the ADC input module; the ADC input module is connected to the input end of the ADC; the ADC collects the voltage value of the CT secondary circuit through the ADC input module to perform disconnection detection. The CT secondary circuit disconnection detection method provided by the embodiment of the present invention determines whether a disconnection may occur by the change of the voltage value sampled by the ADC, and further determines whether a disconnection has occurred by the change of the ADC sampling value after raising and lowering the PGA common-mode voltage.

[0067] It should be understood that the above specific embodiments of the present invention are only for illustrative explanation or interpretation of the principles of the present invention, and do not constitute a limitation to the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made without departing from the spirit and scope of the present invention shall be included within the protection scope of the present invention. In addition, the appended claims of the present invention are intended to cover all variations and modifications that fall within the scope and boundaries of the appended claims, or equivalent forms of such scope and boundaries.

Claims

1. A method of detection using a CT secondary circuit open - circuit detection device, characterized in that, the CT secondary circuit open - circuit detection device includes a voltage conversion module, a resistance matching module, an ADC input module and an ADC; wherein, the voltage conversion module is connected to the CT secondary circuit to be measured to convert the current signal of the secondary circuit into a voltage signal; the resistance matching module is connected between the voltage conversion module and the ADC input module; the ADC input module is connected to the input end of the ADC; the ADC collects the voltage value of the CT secondary circuit through the ADC input module for open - circuit detection; the method includes the steps: Obtain the voltage value collected by the ADC; When the collected voltage value meets the open - circuit detection condition, execute the open - circuit detection step: Raise the common-mode voltage of the programmable gain amplifier inside the ADC and read the ADC sampling voltage value U at this time H ; If U H -U 0 > 20 LSB, then reduce the common-mode voltage of the internal programmable gain amplifier of this ADC and read the ADC sampling voltage value U L ; If U L = U 0 , it is determined that a CT secondary circuit break has occurred; Among them, U 0 is the voltage value collected when the disconnection detection condition is met.

2. The method according to claim 1, characterized in that, the open - circuit detection condition includes: The same analog input channel of the ADC outputs the same sampled value U continuously for N times 0 ; where N is the open - circuit detection threshold.

3. The method according to claim 2, characterized in that, the continuous output of the same sampling value for N times includes that the error between N sampling values is less than or equal to 10 LSB.

4. The method according to claim 3, characterized in that, in the case of not using oversampling, the minimum value of N is: N = 10 × f sample × (R PD + 2 × R FILTER ) × C FILTER Among them, f sample is the sampling frequency; R PD is the resistance value of the matching resistor; R FILTER is the resistance value of the filtering resistor; C FILTER is the capacitance value of the filtering capacitor.

5. The method according to claim 4, characterized in that, When the common-mode voltage increases, U H -U 0 ≤ 20 LSB; or when the common-mode voltage decreases, U L ≠ U 0 ; then return to the first step to re-acquire the voltage value collected by the ADC.

6. The method according to claim 1, characterized in that, the device further includes a filtering module; the filtering module includes a filtering capacitor and a filtering resistor, and the filtering module is connected between the resistance matching module and the ADC input module.

7. The method according to claim 6, characterized in that, the ADC includes an ADC with a programmable gain amplifier inside and adjustable common - mode voltage.

8. The method according to claim 7, characterized in that, the resistance matching module includes a matching resistor.

9. The method according to claim 8, characterized in that, the ADC input module includes a first input resistor and a second input resistor; the ADC collects the differential voltage signal through the first input resistor and the second input resistor.

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