Method for detecting power transformer magnetizing current surge intended for microprocessor-based relay protection device

Abstract

FIELD: electrical engineering.SUBSTANCE: invention relates to microprocessor relay protection of electric power systems, and is part of measures designed to adjust the protection of a three-phase power transformer against magnetization surge current. A method is proposed for detecting a current surge. It is proposed to analyse the harmonic composition of the differential current into the second harmonic and aperiodic component. In order to detect a current inrush, it is necessary that for at least two of the three phases a condition is met that for each phase the calculated value of the second harmonic content of the differential current is equal to or exceeds the first specified threshold value and the calculated value of the aperiodic component content of the differential current is equal to or exceeds the second specified threshold value.EFFECT: increasing the stability of non-operation of protection during a current surge.3 cl, 6 dwg

Description

[0001] Technical field

[0002] The invention relates to digital (microprocessor) relay protection of electric power systems and is part of measures designed to adjust the protection of a power transformer (hereinafter referred to as a transformer) from a surge in magnetization current.

[0003] Technical innovations in the protection device - MP URZA TEKON 300 are described.

[0004] It is known [1, pp. 105-106] that a significant part of the magnetization current (I) passes through the transformer winding located on the side of the electrical energy source. нам ) During normal transformer operation, the magnetizing current does not exceed a few percent of the rated current (approximately 5%). A sharp increase in magnetizing current when the transformer is turned on without a load or due to other causes is called a magnetizing current surge (hereinafter referred to as a current surge).

[0005] The following description of the operation of the power transformer is limited:

[0006] - normal operating mode;

[0007] - emergency short circuit mode (internal short circuit) that occurs in the windings and terminals of the power transformer, as well as on the busbars (hereinafter referred to as SC);

[0008] - electric surge.

[0009] In order to reduce the length of the description, the terms used in it within this description have the following meanings:

[0010] “Transformer” is a three-phase power transformer or autotransformer for which the protection detects an inrush current or short circuit.

[0011] “Primary winding of a transformer” is the winding of a transformer to which the energy of the converted alternating current is supplied (according to GOST 16110-82).

[0012] “Secondary winding of a transformer” is a transformer winding from which the energy of the converted alternating current is removed (according to GOST 16110-82).

[0013] "Inrush detection" is a synonym for the words "detection", "identification", "classification" of inrush current.

[0014] "Protection" - actions, operation, algorithm of the protection device.

[0015] "Protection trip" is a term used to describe the action of a protective device which causes a signal to be sent to a circuit breaker (hereinafter referred to as the circuit breaker) from the electrical power source, causing it to open its switching contacts, thereby disconnecting the transformer from the electrical power source.

[0016] "Blocking protection" is a term used in relation to the action of a protection device that results in blocking (stopping) the operation of the protection.

[0017] “Second harmonic content of differential current” - the value (in percent or relative units) of the ratio of the second harmonic (100±10 Hz) of the differential current to the fundamental harmonic (50±5 Hz) of the differential current or the value (in percent or relative units) of the ratio of the second harmonic differential current to the fundamental differential current.

[0018] “Content of the aperiodic component of the differential current” - the value (in percent or relative units) of the ratio of the aperiodic component of the differential current to the fundamental harmonic of the differential current or the value (in percent or relative units) of the ratio of the differential current of the aperiodic component to the differential current of the fundamental harmonic.

[0019] State of the art - general information on transformer differential protection and transformer protection features.

[0020] Longitudinal differential current protection is designed for fast-acting protection of a transformer from short circuit.

[0021] Fig. 1 shows a circuit diagram in which digital protection device 1 is used to protect transformer 2. Fig. 1 shows transformer 2 with two windings Y / D-11 (hereinafter Y / Δ-11). Fig. 1 shows one phase x, where x is phase A, or phase B, or phase C. Transformer 2 has a primary winding on the side of the electric power source connected in a "star" and a secondary winding connected in a "delta". Fig. 1 shows the transformer in normal operation, when the contacts of switches 3 and 4 are closed. At the terminals of the windings of transformer 2, measuring current transformers (hereinafter referred to as measuring CTs) 5 and 6 are installed, designed to transmit the measuring information signal to the digital protection device 1. Alternating primary currents I flow through the windings of measuring CTs 5 and 6 yx and I ΔxPrimary currents are transformed by measuring CTs into alternating secondary currents which enter the digital protection device 1. Further in the text, wherever there is no clarification about primary currents, currents are understood to mean secondary currents.

[0022] Traditionally for the positive direction of primary currents I yx and I Δx . the direction of currents is assumed to be towards the transformer. In normal operating mode of the transformer, the primary current I yx from the primary winding side has a positive direction, and the primary current 1 Δх from the secondary winding side of the transformer it has a negative direction.

[0023] In differential protection, instantaneous current values ​​can be compared (simultaneously the modules and phases of currents or only phases), as well as complex effective values ​​of currents [1, p. 295].

[0024] Transformer protection 2 has its own unique features. For example, in real-world conditions, digital protection device 1 is designed to programmatically record:

[0025] - transformer transformation ratio;

[0026] - transformation ratios of measuring CTs;

[0027] - transformation ratios of auxiliary current transformers of the digital protection device itself, which transform alternating currents received by the digital protection device to a level suitable for processing in the ADC unit and the computing device. The user specifies the transformation ratios as settings in the digital protection device. These aspects should be taken into account in real-world transformer protection conditions; however, this aspect is intentionally omitted in this application to better convey the essence of the proposed new solutions.

[0028] However, it should be noted that the primary currents in the windings of transformer 2 are generally out of phase. Therefore, in digital protection device 1, the angular current shift compensation operation (phase compensation operation) is performed programmatically. Compensation for the angular current shift depends on the circuit and connection group of the transformer windings [2]. The user specifies the circuit and connection group of the transformer windings as settings in the digital protection device.

[0029] Compensation for the angular shift of the transformer currents Y / Δ-l1 can be carried out both from the side of the winding with a star connection and from the side of the winding with a delta connection.

[0030] If the compensation for the angular shift of currents is performed from the side of the winding with a star connection, then the phase compensation operation for instantaneous current values ​​is carried out according to the following formulas [3, pp. 106-109]:

[0031]

[0032] - instantaneous values ​​of currents from the side of the winding with a star connection diagram before the phase compensation operation in phases A, B and C;

[0033] - instantaneous values ​​of currents from the side of the winding with a star connection scheme after the phase compensation operation in phases A, B and C.

[0034] For the delta connection side, the phase compensation operation is not performed because the instantaneous current values ​​before the phase compensation operation correspond to the instantaneous current values ​​after the phase compensation operation:

[0035]

[0036] - instantaneous values ​​of currents from the winding side with a delta connection diagram before the phase compensation operation in phases A, B, and C;

[0037] - instantaneous values ​​of currents from the winding side with a delta connection diagram after the phase compensation operation in phases A, B, C.

[0038] Similarly, if the compensation for the angular shift of currents is performed on the side of the winding with a star connection, then the phase compensation operation for complex effective values ​​of currents is carried out according to the following formulas [3, pp. 106-109]:

[0039]

[0040] - complex effective values ​​of currents from the side of the winding with a star connection diagram before the phase compensation operation in phases A, B and C;

[0041] - complex effective values ​​of currents from the side of the winding with a star connection scheme after the phase compensation operation in phases A, B and C.

[0042] For the delta connection side, the phase compensation operation is not performed because the complex effective values ​​of the currents before the phase compensation operation are the same as the complex effective values ​​of the currents after the phase compensation operation:

[0043]

[0044] - complex effective values ​​of currents from the winding side with a delta connection diagram before the phase compensation operation in phases A, B, C;

[0045] - complex effective values ​​of currents from the winding side with a delta connection diagram after the phase compensation operation in phases A, B, C.

[0046] The currents of all sides of the transformer are used to calculate the differential current.

[0047] The instantaneous differential current is obtained from the instantaneous current values ​​(before filtering) taking into account the phase compensation operation. The instantaneous differential current in phase (x) for a Y / Δ-11 transformer is:

[0048] - instantaneous value of current from the star side after the phase compensation operation in phase (x).

[0049] - instantaneous value of current from the “triangle” side after the phase compensation operation in phase (x).

[0050] The vector (effective) differential current in phase (x) for the Y / Δ-11 transformer using complex effective values ​​of currents, taking into account the phase compensation operation, is equal to:

[0051] - complex effective value of current from the star side after the phase compensation operation in phase (x).

[0052] - complex effective value of current from the “triangle” side after the phase compensation operation in phase (x).

[0053] The operation of differential protection is based on the fact that in normal operating mode (Fig. 1), when the contacts of switches 3 and 4 are closed, the instantaneous differential current phase (x) is equal to zero and the protection device 1 does not operate.

[0054] If a short circuit occurs (Fig. 2), the current phase (x) of the secondary winding of the transformer will become equal to zero and the instantaneous differential current of this phase will be determined only by current 1 ухphases (x) of the primary winding of the transformer: i диф х = i´ ух In this case, a sharp increase in the amplitude of the differential current of the fundamental harmonic will lead to the activation of digital protection device 1, as a result of which a signal will be sent to open the contacts of switches 3 and 4.

[0055] A current surge can be caused by various reasons, but a current surge caused by a transformer being turned on without a load is the most unfavorable for the protection operation. In the transformer being turned on without a load (Fig. 3), when the contacts of switch 3 are closed and the contacts of switch 4 are open, the phase currents (x) on the secondary side of the transformer will be zero. Instantaneous differential current 1 ди f. х phase (x) will be determined by the current of phase (x) from the primary winding side of the transformer: The resulting inrush current will cause a sharp increase in the amplitude of the fundamental differential current, which the protection will interpret as a short circuit and erroneously trigger the protection, causing the contacts of switch 3 to open. This cannot be allowed to happen. Therefore, it is necessary to identify indicators other than the magnitude and phase of the current that can distinguish an inrush current from a short circuit. This will allow the inrush protection to be configured so that it only triggers in the event of a short circuit.

[0056] When the transformer is turned on at no-load from the star-connected primary winding side, the entire primary current is magnetizing current

[0057] Next, we discuss the known magnetizing current characteristics during inrush current, which are used to distinguish inrush current from short circuits. These magnetizing current characteristics are also used to detect inrush current, regardless of the methods used to detect short circuits.

[0058] Prior art - harmonic composition of magnetization currents during current surges until the moment of saturation of measuring CTs.

[0059] It is known [1, p. 442] (in the general case) that the magnetizing current during a current surge can contain a large aperiodic component (component), as well as a significant percentage of higher harmonics (primarily the second). This is the fundamental difference between the magnetizing current during a current surge and the short-circuit current. The short-circuit current at the moment of its occurrence contains only the aperiodic component.

[0060] It is known [4] (in general) that during a current surge, conditions may arise where the magnetizing current may contain an aperiodic component (unipolar or aperiodic current surge) and may not contain an aperiodic component (bipolar or periodic current surge). The amplitude of a unipolar current surge with an aperiodic component can reach 6-8 times the amplitude of the transformer's rated current. A bipolar current surge, however, can only be observed in one phase, and its amplitude can reach 1-2 times the amplitude of the rated current.

[0061] It is known [1, p. 105] (in the general case) that the initial value of the magnetization current amplitude during a current surge depends on the magnitude of the residual magnetic flux in the transformer winding and on the voltage angle of the electrical energy source at the moment the transformer is turned on (hereinafter the voltage angle).

[0062] It is known [5] that the type (unipolar or bipolar) and amplitude of the magnetizing current are highly dependent on the voltage angle. The maximum value of the magnetizing current, subjected to phase compensation, is observed at voltage angles of 0° and 180°, while bipolar magnetizing current is obtained at voltage angles of 90° and 270°.

[0063] All the above-mentioned known data regarding the harmonic composition of the magnetizing current during a current surge are given in general terms and do not detail the composition of the magnetizing currents by phase.

[0064] It is also known [4] that the inrush current curve without an aperiodic component is characterized by a second harmonic content of at least 40%. And the inrush current curve with an aperiodic component is also characterized by a second harmonic, but its content is significantly less and can be approximately 15%.

[0065] TekonGroup conducted a more detailed analysis of differential current curves During a current inrush for each of the three phases (x), depending on the voltage angle. Research was conducted on the harmonic content of magnetizing currents during a current inrush. Specifically, the second-harmonic content of the differential current and the aperiodic component content of the differential current were analyzed for each of the three transformer phases at 0°, 90°, and other voltage angles. In contrast to existing data, it was found that at zero voltage angle in the phase with the maximum unipolar current inrush (i.e., with the maximum aperiodic component), the second-harmonic content of the differential current can be below 10%. In the other two phases with moderate aperiodic components, the second-harmonic content of the differential current will be greater than 10%.

[0066] In conclusion of the above, it can be concluded that during a current surge, regardless of the value of the voltage angle, at least in two phases, the differential current will contain a sufficient share of the second harmonic and a sufficient share of the aperiodic component (the values ​​of the shares (contents) are indicated in the section “Essence of the invention”).

[0067] Prior art - saturation of measuring CTs during current surge or internal short circuit of the transformer.

[0068] Saturation is a common problem with electromagnetic measuring CTs. After an inrush current or short circuit, the measuring CT correctly transforms the primary current into secondary current only for a few time periods. The measuring CTs then saturate with the aperiodic component of the inrush current or the aperiodic component of the short circuit current, which is present at the time of their occurrence. After the measuring CTs saturate, the aperiodic component disappears in the magnetizing current, short circuit current, or short circuit current during the inrush current. At the same time, a second harmonic appears in the short circuit current, which is also characteristic of the magnetizing current. Consequently, after the measuring CTs saturate, there is no qualitative difference in the harmonic composition of the short circuit current and magnetizing current during the inrush current.The errors of measuring current transformers when saturated can be so large that they can cause a significant delay in the protection device's response during a short circuit (loss of response speed) or erroneous (unscheduled) operation of the protection device intended to disconnect the transformer from the power source during a current surge. Loss of response speed leads to failure of the power transformer. Erroneous disconnection of a transformer is undesirable for its long service life.

[0069] The proposed technical solution for detecting current surges is carried out during a time interval in which saturation of the measuring CTs does not occur, and an undistorted signal from the measuring CTs is sent to the computing processor.

[0070] The level of technology - known methods of tuning out current surges.

[0071] There are known methods for adjusting relay protection against inrush current, which were based on electromechanical components and which are not used in digital protection due to their low sensitivity to short circuits.

[0072] Such methods include:

[0073] - Using differential current cutoff (SU 936164 A1).

[0074] - Use of intermediate fast-saturating current transformers of differential protection circuit (SU 660139 A2, SU 256032 Al, SU 545036 A1).

[0075] There are known methods for detecting inrush current, which were initially based on the electromechanical element base of the relay, and then their basic principles migrated to digital protection.

[0076] Such methods include:

[0077] - Method of analyzing the "curve shape" of differential current (first method).

[0078] - Method of "harmonic analysis" of differential current (second method).

[0079] The first method of analyzing the current "waveform" uses the differences in the inrush current magnetization curves and the short-circuit current curve (SU 365766 Al, SU 535644 Al, SU 943970 Al, SU 170100 Al, SU 1775788 A1, EP 2891216 B1, EP 4415195 A1). The inrush current magnetization curve, unlike the short-circuit current curve, has a unipolar (non-sinusoidal) shape with current-free pauses. These features of the inrush current curve are used to detect it. However, it should be taken into account that at the moment of switching on (as already noted above), a bipolar inrush current may be observed in one of the phases, which the protection system may mistake for a short circuit. To eliminate false triggering of the protection, the “curve shape” method is carried out simultaneously in at least two phases [5, pp. 200-202].

[0080] The second method of "harmonic analysis" of current evaluates the content of the second harmonic in the differential current. It utilizes a well-known property of magnetizing current - the presence of the second harmonic. During a current inrush, the content of the second harmonic will be significant. The method of "harmonic analysis" for the content of the second harmonic of differential current is used by almost all domestic and foreign manufacturers of protective devices, namely Siemens, ABB, General Electric, Alstom, Schneider Electric, RADIUS Avtomatika [3, pp. 78, 89, 194, 210] and TekonGroup, since this feature clearly distinguishes the magnetizing current during a current inrush from the short-circuit current, which practically does not contain higher harmonics (does not contain it until the saturation of the measuring CTs). The generally accepted threshold value for the content of the second harmonic of differential current is 15%.If the calculated value exceeds the set threshold, the condition is recognized as an inrush current, and protection operation is blocked for a period of 60 to 500 ms. A short blocking time does not ensure acceptable protection operation reliability when the transformer is turned on without load, while prolonged blocking may result in protection failure when a short circuit occurs during an inrush current. Therefore, while blocking is in effect, monitoring of the detected inrush current continues using the same method used to detect the inrush current or another method. It is known that if inrush current monitoring is continued using harmonic analysis for the content of the second harmonic of the differential current (US 2025246894 Al), the content of the second harmonic of the differential current will increase in the decaying inrush current. However, if the content of the second harmonic of the differential current begins to decrease, this will indicate the occurrence of a short circuit.

[0081] It should be noted that each of the above-described inrush current detection methods has drawbacks that prevent reliable detection. Therefore, to overcome the drawbacks of each of the above-mentioned methods, manufacturers of protective devices employ two well-known methods in parallel [6]. For example, they use the "harmonic analysis" method of current and the "waveform analysis" method. It is possible to temporarily use both known methods, and after the inrush current is detected, only one of the methods, such as the "waveform analysis" method, can be used to continue monitoring the detected inrush current [6].

[0082] A device for differential protection of a transformer converter unit (RU 2785823 C1) is known. It consists of three units, one for each phase, to monitor for the presence of an aperiodic component in the magnetizing current surge. Therefore, a known property of magnetizing current—the presence of an aperiodic component—is exploited. These units can be implemented using microcontrollers. These same units are designed to detect short circuits, which, like inrush currents, contain an aperiodic component. The operation of the units is not described, and it is unclear how the unit distinguishes between inrush currents and short circuits, both of which are characterized by an aperiodic component at the moment of occurrence.

[0083] Description of analogs and prototype

[0084] A method for detecting the inrush current of a three-phase power transformer is known, designed for the RET 521 microprocessor-based transformer protection terminal from ABB Automation Technology Products [6]. The method for detecting the inrush current consists in the fact that in each phase with alternating current on each side of the power transformer there is a measuring CT designed to transmit a signal of measuring information to the protection terminal, in which the following operations are performed:

[0085] (a) calculating the instantaneous differential current for each phase from its instantaneous current values ​​of each side of the power transformer, using the instantaneous current values ​​after the phase compensation operation;

[0086] (b) Calculate the fundamental harmonic of the differential current for each phase;

[0087] (c) Calculate the second harmonic of the differential current for each phase;

[0088] (d) calculating the value of the second harmonic content of the differential current for each phase as the ratio of the second harmonic of the differential current to the fundamental harmonic of the differential current;

[0089] (e) detecting a magnetizing current inrush if, in at least one phase, the calculated value of the second harmonic content of the differential current is equal to or greater than a first preset threshold value of 10-25%.

[0090] The manufacturer of the RET 521 terminal indicates in the technical manual [6] that this method may lead to an increase in the response time during severe short circuits with saturation of the measuring CTs, for which, just like with an inrush current, the second harmonic is characteristic, the size of which is less than 15%. Since the range of threshold values ​​​​is set for the protection terminal of 10-25%, the manufacturer of the RET 521 supplements the "harmonic analysis" method with the "curve shape" analysis method, which was already discussed above in the section "Prior Art - Known Methods of Inrush Current Detection". Only the detection of an inrush current for the same phase in each of the parallel methods provides grounds for blocking the protection.

[0091] It should be noted that for the "harmonic analysis" method, protection device manufacturers use both the instantaneous differential current, from which harmonics are filtered, and the vector differential current calculated for each of the filtered harmonics. For example, in the "Sirius-T" device from RADIUS Avtomatika [3, p. 210], the ratio of the second-harmonic differential current to the fundamental differential current is used to evaluate the second-harmonic content of the differential current. Regardless of how the second-harmonic content of the differential current is calculated, the result will be the same.

[0092] The method for detecting inrush current in a three-phase power transformer described in application GB 2635991 A of Mitsubishi Electric Corporation is adopted as a prototype. The application provides descriptions of various methods for detecting inrush current, based on the analysis of the second harmonic content of the differential current (see the table in Fig. 5). One such method is the "D condition for detecting inrush current," which consists of a measuring CT located in each AC phase of each side of the power transformer, designed to transmit a measuring information signal to a digital protection device, in which the following operations are performed:

[0093] (a) calculating the value of the second harmonic content of the differential current for each phase as the ratio of the second harmonic of the differential current to the fundamental harmonic of the differential current or as the ratio of the second harmonic differential current to the fundamental differential current;

[0094] (b) Detect a magnetizing current inrush if the values ​​calculated in operation (a) in at least two phases are equal to or greater than a preset threshold value of 0.15 p.u. (or 15%).

[0095] The authors of application GB 2635991 A consider "condition D for determining inrush current" to be one of the least reliable of all those described in this information source, because, in their opinion, the calculated value of the second harmonic content of the differential current can exceed the specified threshold value of 15% in only one phase. Therefore, the authors propose their own method for detecting inrush current, in which two specified threshold values ​​for the second harmonic of the differential current are introduced. In this case, the second specified threshold value is significantly less than the first specified threshold value. Each phase is checked for compliance with the conditions for exceeding each of the two specified threshold values. An inrush current is detected if the calculated value of the second harmonic content in one phase exceeds the first specified threshold value, and the calculated values ​​of the second harmonic content in the other two phases exceed the second specified threshold value.The application description proposes using 15% for the first specified threshold value and 5% for the second specified threshold value. However, these values ​​are not claimed in the patent claims. It should be noted that in modern transformers, the second harmonic component in the magnetizing current may be lower than 15%, so it can be assumed that the threshold values ​​claimed by the authors may be subject to change.

[0096] Our search did not reveal a single source of information that combined the inrush current detection features, namely the aperiodic component and the second harmonic in phase differential currents. These features of the magnetizing current during an inrush current are reliable up to the moment of saturation of the measuring CTs. It should be noted that in one of the three phases during an inrush current, the aperiodic component may be too low or completely absent, or the second harmonic content may be low, for example, less than 10% (see the section "Prior Art - Harmonic Content of Magnetizing Currents during an Inrush Current Up to the Moment of Saturation of the Measuring CTs").

[0097] Essence of the invention

[0098] The task is to improve the efficiency of the digital protection device by increasing the stability of the protection failure during a current surge.

[0099] The technical result is achieved by proposing a reliable method for detecting inrush currents, which is used at the moment a potential inrush current occurs. Inrush current detection occurs within 40-60 milliseconds. During this time, the inrush current does not cause saturation of the measuring CTs, and an undistorted signal from the measuring CTs is sent to the computing processor. Faster saturation of the measuring CTs during a short circuit will not lead to false inrush current detection.

[0100] A method for detecting a surge current of a three-phase power transformer is proposed, which consists in the fact that in each phase with alternating current on each side of the power transformer there is a measuring current transformer designed to transmit a signal of measuring information to a digital protection device, in which the following operations are carried out:

[0101] (a) calculating the value of the second harmonic content of the differential current for each phase as the ratio of the second harmonic of the differential current to the fundamental harmonic of the differential current or as the ratio of the second harmonic differential current to the fundamental differential current;

[0102] (b) Selecting the phases for which the calculated value of the second harmonic content of the differential current is equal to or greater than the first set threshold value;

[0103] (c) calculating the value of the aperiodic component content of the differential current for each phase as the ratio of the aperiodic component of the differential current to the fundamental harmonic of the differential current or as the ratio of the differential current of the aperiodic component to the differential current of the fundamental harmonic;

[0104] (d) selection of phases for which the calculated value of the aperiodic component content of the differential current is equal to or greater than the second specified threshold value;

[0105] (e) detecting a magnetizing inrush current in the case where at least two selected phases for which the calculated value of the second harmonic content of the differential current is equal to or exceeds a first preset threshold value coincide with at least two selected phases for which the calculated value of the aperiodic component content of the differential current is equal to or exceeds a second preset threshold value.

[0106] The first threshold value can be set to 0.1 pu (10%), and the second threshold value can be set to 0.2 pu (20%).

[0107] List of figures

[0108] Fig. 1 - a schematic diagram in which the protection device is applied to a transformer in normal operation mode (for the section “Background of the art - general information on differential protection of a transformer and features of transformer protection”).

[0109] Fig. 2 - a schematic diagram in which a protection device is used for a transformer during a short circuit (for the section “Prior art - general information on differential protection of a transformer and features of transformer protection”).

[0110] Fig. 3 - a schematic diagram in which the protection device is applied to a transformer in the no-load switching mode (for the section “Background of the art - general information on differential protection of a transformer and features of transformer protection”).

[0111] Fig. 4 - a schematic diagram in which the protection device is applied to a transformer (for the section “Essence of the invention”).

[0112] Fig. 5 is a block diagram illustrating a possible sequence of operations for detecting an inrush current.

[0113] Fig. 6 is a block diagram illustrating a possible sequence of operations for detecting an inrush current.

[0114] Fig. 4 shows a three-phase Y / Δ-11 transformer 2 and phase x, where x is phase A, or phase B, or phase C. The proposed technical solution is not limited to the Y / Δ-11 transformer and can also be used with transformers of other circuits and winding connection groups, including those with three windings. The method is further described using the Y / Δ-11 transformer as an example. Transformer 2 has a primary winding on the side of the electric power source, connected in a "star", and a secondary winding on the side of the load, connected in a "delta". The electric power source is connected to the primary winding of the transformer. Between them, per-phase measuring CTs 5 and a switch 3 are installed. The secondary winding of the transformer is connected to the load (not shown). Between them, phase-by-phase measuring CTs 6 and switch 4 are installed. Alternating primary currents flow through the primary windings of measuring CTs 5 and 6. which are transformed by measuring CTs into alternating secondary currents which are fed to the input of the digital differential protection device 9 (hereinafter referred to as protection device 9).

[0115] Further in the text, wherever there is no clarification about primary currents, currents are understood to mean secondary currents.

[0116] Protection device 9 is designed to protect transformer 2 and includes:

[0117] - analog input block (not shown) with auxiliary current transformers designed to convert alternating currents into analog signals, the level of which is suitable for processing in the ADC block and in the computing processor 10;

[0118] - analog-to-digital conversion (ADC) block (not shown), configured to measure converted analog signals at equally spaced discrete moments in time to convert them into instantaneous current values

[0119] - computing processor (CPU) 10, designed for mathematical and logical operations described below;

[0120] - digital output block 11, designed to output a signal to open the contacts of the corresponding switch in accordance with the instruction received from CPU 10.

[0121] The CPU 10, at least some of its functions, may be implemented as a microcontroller or an electronic circuit such as an application-specific integrated circuit (ASIC) or a field-programmable gate array (FPGA), or may be implemented by a combination of two or more of the above devices.

[0122] CPU 10, configured for computing and logical operations required for:

[0123] - Inrush current detection (software module 12 responsible for inrush current detection).

[0124] - Short-circuit detection (software module 13 responsible for short-circuit detection). Software short-circuit detection method using the tripping characteristic (brake characteristic) is widely known and used by all manufacturers of digital security devices, so it is not described in this application.

[0125] - Issue of protection activation signal (operator “I” 14).

[0126] When a short circuit is detected, module 13 outputs a trip signal to the AND operator 14. The AND operator 14 calculates the logical AND between the protection trip signal output by module 13 and the blocking signal 15 output by module 12. If the blocking signal is present, the output of the AND operator 14 is inactive, and the protection trip output is blocked.

[0127] When the transformer 2 is turned on without a load (Fig. 4) (the contacts of the switch 3 are closed, the contacts of the switch 4 are open), alternating primary currents I flow through the windings of the measuring CTs 5 yx- The currents in the phases (x) of the secondary winding are equal to zero

[0128] It should be noted that the transformer can be connected to the power source from the delta-connected side of its winding (switch 4 contacts closed, and switch 3 contacts open). In this case, the currents in the phases of the star-connected winding will be zero. The winding connected in a triangle in this case becomes the primary winding.

[0129] When an inrush current occurs due to reasons other than the transformer being turned on without load, the residual current i диф.х (or I диф.х ) is calculated using formulas (13) and (14) of this description.

[0130] Detection of current surge is carried out over an interval T0 of the order of two to three periods of industrial frequency ƒ0(50±5 Hz), i.e. for 256-384 measurements.

[0131] In order to detect the inrush current, it is necessary to analyze the harmonic composition of the differential currents iдиф.х (or I диф.х ) for each of the three phases, namely to analyze the content of the second harmonic of the differential current and the aperiodic component of the differential current for each phase.

[0132] Initially, the instantaneous differential current i is calculated in CPU 10 диф.х for each phase. The instantaneous differential current is then filtered for its harmonic analysis.

[0133] The sequence of operations in the software module 12 for detecting a current surge is shown in Fig. 5 where:

[0134] - instantaneous value of instantaneous differential current for phase (x);

[0135] 16 - Fundamental Fourier Digital Filter differential current for phase (x);

[0136] 17 - Second Harmonic Fourier Digital Filter differential current for phase (x);

[0137] 18 - the ratio of the second harmonic of the differential current to the fundamental harmonic of the differential current (the content of the second harmonic differential current) for phase (x);

[0138] 19 - moving average filter of the aperiodic component of the differential current I а.диф.х for phase;

[0139] 20 - the ratio of the aperiodic component of the differential current to the fundamental harmonic of the differential current (the content of the aperiodic component I а.диф.отн.х differential current) for phase (x);

[0140] 21 - the logic module receives the calculated values ​​of the second harmonic content for phases A, B and C differential current. These calculated values ​​are compared with the first setting of 0.1 p.u., and those phases for which the condition is true are selected.

[0141] 22 - the calculated values ​​of the aperiodic component content for phases A, B and C are sent to the logic module differential current. These calculated values ​​are compared with the second setting of 0.2 p.u., and those phases for which the condition is true are selected.

[0142] 23 - those phases (x) are selected for which two conditions are true:

[0143] 24 - checking the number of phases selected in the logic module 23 (x) for equality or exceeding the number "2" (x>2). This is due to the fact that during a current surge in the differential current one of the three phases may have too little or no aperiodic component. And during a current surge in the differential current i диф.х (or I диф.х ) one of the three phases, the content of the second harmonic may be below 10% (see the description section “Prior art - harmonic composition of magnetizing currents during a current surge before the saturation of the measuring CTs”). To detect a current surge (position 25)

[0144] At least two phases are required, differential current i диф.х (or I диф.х) each of which contains enough of the second harmonic and aperiodic component.

[0145] It is clear to a specialist that the order of operations of the software module 12 may be different from that shown in Fig. 5. Thus, it is possible to first select the phases (x) with the content of the second harmonic of the differential current There should be two or three such phases during a current surge. Then, for the selected phases, check the content of the aperiodic component of the differential current. which must be greater than or equal to 0.2 p.u. During a current surge in at least two phases (x ≥ 2), an increased content of the second harmonic of the differential current and the aperiodic component of the differential current must be observed.

[0146] A specialist understands that harmonics can be filtered from instantaneous current values each side of the transformer after their phase compensation operation (Fig. 6). From the filtered harmonics, the vector differential current of the fundamental harmonic is calculated (position 26) and the vector differential current of the second harmonic (position 27). Then the value of the second harmonic content is calculated differential current (position 28) as the ratio of the differential current to the second harmonic to the differential current of the fundamental harmonic Calculated value of the second harmonic content of the differential current for phase(x) (position 28 in Fig. 6) is equal to the calculated value of the content of the second harmonic of the differential current (position 18 in Fig. 5) for the same phase.

[0147] Content of aperiodic component can be calculated from the instantaneous values ​​of currents each side of the transformer after the operation of their phase compensation (not shown in the figures), and from the instantaneous differential current (position 29). The calculated values ​​of the content of the aperiodic component of the differential current for each of these calculation options will be equal.

[0148] Calculated Relationships (Fig. 6) are compared with the first specified threshold value, on the basis of which phases with a sufficient content of the second harmonic of the differential current are selected (position 21). The calculated ratios are compared with the second specified threshold value, on the basis of which phases with a sufficient content of the aperiodic component of the differential current are selected (position 22). Then, phases (x) with a sufficient content of the second harmonic of the differential current and the aperiodic component of the differential current are selected (position 23), followed by a check of the number of phases (x) selected in the logic module 23 for equality to or exceedance of the number "2" (x ≥ 2).

[0149] Regardless of the selected sequence of computational and logical operations (Fig. 5 or Fig. 6 or the sequence of operations not shown), the results will be the same.

[0150] Amplitude of fundamental harmonic of differential current

[0151] Ampere, for phase (x) is equal to:

[0152]

[0153] - the real component of the complex value of the fundamental harmonic of the signal;

[0154] - imaginary component of the complex value of the fundamental harmonic of the signal.

[0155]

[0156]

[0157] i диф.х (n⋅ΔT) - instantaneous value of instantaneous differential current for phase (x);

[0158] ΔT - current signal sampling step, s;

[0159] n - number of the current current signal sample;

[0160] N- number of current signal samples;

[0161] ω - angular frequency of the fundamental harmonic, rad / s;

[0162] T0- interval, s;

[0163] ƒ0 - fundamental harmonic frequency, Hz.

[0164] Amplitude of the second harmonic of the differential current A 2.диф x: , Ampere, for phase (x) is equal to:

[0165]

[0166] Re 2.диф.х - real components of the complex value of the second harmonic of the current signal;

[0167] Im 2.диф.х- the imaginary component of the complex value of the second harmonic of the current signal.

[0168]

[0169] Calculating the second harmonic content value differential current as the ratio of the second harmonic of the differential current to the fundamental harmonic of the differential current for phase (x):

[0170]

[0171] Sets the first threshold value (first setting) for the content of the second harmonic of the differential current to 0.1 p.u.

[0172] If Then, for phase (x), a sufficient second harmonic content in the differential current is recorded, and this phase (x) is selected for further inrush detection operations. There should be two or three such phases during an inrush current.

[0173] If , then for phase (x) the absence of the second harmonic is recorded.

[0174] Aperiodic component of differential current in Amperes (A) for phase (x) is equal to:

[0175]

[0176] Calculating the content value of the aperiodic component differential current as a ratio of the aperiodic component differential current to the effective value of the fundamental harmonic of the differential current, which in times smaller than the amplitude of the fundamental harmonic differential current:

[0177]

[0178] Sets the second threshold value (second setting) for the aperiodic component of the differential current to 0.2 p.u.

[0179] If Then, for phase (x), a sufficient content of the aperiodic component in the differential current is recorded, and this phase (x) is selected for further inrush current detection operations. There should be two or three such phases.

[0180] If then for phase (x) a low content or absence of the aperiodic component is recorded.

[0181] Inrush current detection occurs when at least two selected phases for which the calculated second-harmonic content of the differential current is equal to or greater than the first set threshold coincide with two selected phases for which the calculated aperiodic component content of the differential current is equal to or greater than the second set threshold. If the mode is classified as an inrush current, this is one of the indicators for blocking protection operation in at least one phase.

[0182] It should be noted that the claimed inrush current detection method can be temporarily combined with an additional inrush current detection method, which is also implemented in inrush current detection module 12 (not shown in the figures). The additional method, similar to the claimed method, can also employ the "harmonic analysis" method of current. However, in this case, the additional method only evaluates the second harmonic content of the differential current. The additional method has its own threshold value (possibly the generally accepted 0.15 p.u.); exceeding this threshold in one phase leads to inrush current detection in the additional method, which is also one of the indicators for blocking protection operation. Only combined inrush current detection in both the claimed and additional methods activates blocking signal 15.After this, monitoring of the detected inrush current over time continues, but only the additional method for the second harmonic content of the differential current is used, while the claimed method becomes inactive. The additional method continues to operate until the detected inrush current attenuates or until a short circuit occurs against the background of the detected inrush current. In this case, blocking signal 15 is removed. However, these measures are the subject of a different technical solution.

[0183] Bibliography

[0184] 1. Fedoseev AM, Relay protection of electric power systems: Textbook for universities. - 2nd ed., revised and enlarged. - M .: Energoatomizdat, 1990, pp. 105-106, 295,442.

[0185] 2. GOST 3484.1-88, Power transformers. Electromagnetic testing methods, p. 10.

[0186] 3. Aleksandrov AM, Differential protection of transformers: a tutorial. - St. Petersburg: PEIPK, 2011., pp. 106-109, table 7-1 and table 7-2, pp. 78, 89, 194, 210.

[0187] 4. Golanzov E.B., Molchanov V.V., Differential protection of transformers with relays of the DZT-21 (DZT-23) type. - M.: Energoatomizdat, 1990, pp. 19-20.

[0188] 5. Electronic devices for relay protection and automation in traction power supply systems, Bykov V.A. and others, M., "Transport", 1974, Section 8 "Protection of traction substation transformers", § 39. Adjustment of differential protection of transformers from magnetization currents when switched on in no-load mode, pp. 195-199, 200-203.

[0189] 6. Protect Technical Reference Manual. RET 521*2.5 Transformer Protection Terminal, Identification Number: 1MRK 504 036-UEN, Issue Date: December 2003, pp. 110-112, 125 (Settings).

Claims

1. A method for detecting a surge current of a three-phase power transformer, which consists in the fact that in each phase with alternating current on each side of the power transformer there is a measuring current transformer, designed to transmit a signal of measuring information to a digital protection device, in which the following operations are carried out: (a) calculating the value of the second harmonic content of the differential current for each phase as the ratio of the second harmonic of the differential current to the fundamental of the differential current or as the ratio of the second harmonic differential current to the fundamental differential current; (b) selection of phases for which the calculated value of the second harmonic content of the differential current is equal to or greater than the first specified threshold value; characterized in that additional operations are performed in the digital protection device, namely: (c) calculation of the value of the content of the aperiodic component of the differential current for each phase as the ratio of the aperiodic component of the differential current to the fundamental harmonic of the differential current or as the ratio of the differential current of the aperiodic component to the differential current of the fundamental harmonic; (d) selection of phases for which the calculated value of the aperiodic component content of the differential current is equal to or exceeds the second specified threshold value; (e) detecting a magnetising current inrush in the event that at least two selected phases for which the calculated value of the second harmonic content of the differential current is equal to or exceeds a first predetermined threshold value coincide with at least two selected phases for which the calculated value of the aperiodic component content of the differential current is equal to or exceeds a second predetermined threshold value.

2. The method according to claim 1, characterized in that the first threshold value is set to 0.1 relative units.

3. The method according to paragraph 1, characterized in that the second threshold value is set to 0.2 relative units.

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