JP2013013170A - Differential protective relay device and saturation determination method for current transformer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a differential protective relay device that eliminates the need for new construction, addition of facilities and so on by allowing the differential protective relay device itself from detecting a CT from getting saturated and preventing malfunction.SOLUTION: The differential protective relay device including an internal fault determination processing part which detects each terminal current to be applied to protection by a current transformer, and supplies an opening instruction signal for opening a breaker provided to each terminal to be applied to protection through differential operation of terminal currents includes: a CT saturation determination processing part which finds an exciting voltage and an induced voltage of the current transformer and compares them with each other to determine that the current transformer gets saturated; and an external fault determination processing part which detects a detected terminal current being a passing current to be applied to protection and determines an external fault. The opening instruction signal for opening the breaker which is generated by the internal fault determination processing part is stopped when the CT saturation determination processing part determines the saturation and the external fault determination processing part determines the external fault.

Description

  The present invention relates to a saturation determination method for a differential protection relay device and a current transformer, and more particularly, to determine the saturation of a differential protection relay device and a current transformer that can prevent malfunction due to excessive passing current at the time of an external accident. Regarding the method.

  As a typical protection principle of a power protection relay device, there is a differential protection relay device that realizes failure determination inside a section of a protection target device by a ratio differential principle.

  The differential protection relay device of this system is arranged so as to sandwich a current detection current transformer (hereinafter simply referred to as CT) with respect to the protection target device of the main circuit, and an internal failure occurs in the protection target section. The differential current obtained from the CT that is sometimes sandwiched is detected with high speed / high sensitivity, and a failure detection determination is performed to perform a protection operation.

  As a restriction for applying this method, it is necessary to match the rated specifications of the CT that is the current detection portion as much as possible. That is, not only the rated constant of CT but also the size of the connection cable, the distance of the cable, and the secondary burden of CT need to be rationalized. If there is a discrepancy between these specifications, multiple CTs will detect different current values for excessive passing current due to an external failure in the protection zone, and it will be the same situation as if an internal accident was determined. . For this reason, it is necessary to take various prior measures that do not generate detection errors as much as possible due to differences in the CT specification specifications.

  In other words, by making it possible to accurately obtain the secondary detection current of CT with respect to the excessive current generated when the main circuit fails, the protective relay device will output an error even if the fault current becomes excessive. Don't worry about it.

  In Patent Document 1, if a CT that is easily saturated exists in a part of a plurality of CTs, a difference current is generated and a malfunction occurs. Therefore, a second detector is additionally installed at the terminal of the CT that is easily saturated, and the measurement is performed. It proposes to prevent malfunction by using the tendency of values.

JP 2005-341770 A

  Patent Document 1 is premised on the fact that it is known in advance that there is a CT that is likely to be saturated in a part of a plurality of CTs.

  However, in recent years, the full turnkey system by ordering / construction from one manufacturer has been on the decline for the purpose of cost reduction even at the time of partial renewal of plants due to aging equipment and construction of new plants. In the protective relay system in these plants, it has been assumed that a protective relay system combining CTs manufactured by different manufacturers is constructed. That is, in these facilities, when the ratio differential protection relay system is adopted for the device to be protected, even if the CT rating matches, the CT excitation characteristics may differ greatly.

  Under the equipment installation environment under such conditions, it is difficult to grasp in advance that there is a CT that is likely to be saturated in some of the plurality of CTs, and the method of Patent Document 1 cannot be realized immediately. Even if the method of Patent Document 1 is implemented in advance, additional work and cost increase associated with newly installing a second detector is inevitable.

  Therefore, in general cases, there is a need to update the CT of the current detection unit at the same time to match the CT specifications when updating the protection target device, and there are inherent problems that the update cost increases and the update period becomes longer. Yes.

  In addition, with regard to the applied protective relay device, there may be cases where it is not possible to adequately take measures in advance that do not cause the CT detection error described above as much as possible. The possibility of unnecessary output due to the CT error is inherent to the excessive passing current that occurs due to the failure in the above.

  From the above, in the present invention, the protective relay device itself detects the saturation of the CT and prevents malfunction, so that the differential protective relay device and the converter that do not require any new construction or addition of equipment, etc. An object of the present invention is to provide a method for determining saturation of a fluency.

  In the present invention, in order to solve the above problem, each terminal current to be protected is detected by a current transformer, and the circuit breaker provided at each terminal to be protected is opened by differential calculation of the terminal current. CT saturation determining current transformer saturation by obtaining excitation voltage and induced voltage of current transformer and comparing them in differential protection relay device having internal failure judgment processing unit for providing open command signal for A determination processing unit, and an external failure determination processing unit that detects that the detected terminal current is a passing current to be protected and determines an external failure, the CT saturation determination processing unit performs saturation determination, and external failure determination When the processing unit makes an external determination, an open command signal for opening the circuit breaker by the internal failure determination processing unit is blocked.

  Further, the CT constant and impedance for obtaining the excitation voltage and the induced voltage of the current transformer are given as the set values of the differential protection relay device.

  Further, when comparing the excitation voltage and the induced voltage of the current transformer, correction is made so that the saturation can be easily determined when a DC component is superimposed on the detected current.

  Further, it is determined from the asymmetry rate of the positive / negative waveform of the detected current that a DC component is superimposed on the detected current.

  In addition, when the external failure determination processing unit makes an external determination, the operation sensitivity for differential calculation by the internal failure determination processing unit is reduced.

  Also, as the CT constant for obtaining the excitation voltage of the current transformer, the CT rated load, the CT secondary rating and the CT overcurrent constant are used, and the product of the CT rated load and the CT overcurrent constant is used as the CT secondary load. Calculated as the value divided by the rating.

  The CT primary rating and CT secondary rating are used as the CT constants for determining the induced voltage of the current transformer, and the product of the primary current, CT secondary rating and impedance measured by the current transformer is the CT primary rating. Calculated as the divided value.

  In the present invention, in order to solve the above-described problem, in a current transformer saturation determination method for detecting a terminal current to be applied to protection of a power system, the current transformer is compared by comparing the excitation voltage and the induced voltage of the current transformer. And determining the excitation voltage as a value obtained by dividing the product of the CT rated load and the CT overcurrent constant by the CT secondary rating, and the induced voltage measured by the current transformer, the primary current and the CT secondary rating Calculated as the product of the impedance divided by the CT primary rating.

  In addition, when comparing the excitation voltage and the induced voltage of the current transformer, the correction comparison is performed so that the saturation determination can be easily performed when a DC component is superimposed on the detected current.

  Further, it is determined from the asymmetry rate of the positive / negative waveform of the detected current that a DC component is superimposed on the detected current.

  In a differential protection relay device with different CT characteristics, even when the CT specifications and CT secondary impedance are different, an internal failure is accurately detected and unnecessary operation at the time of an external failure in the protection zone is prevented. Things will be possible.

The figure which shows the detailed structure of the CT saturation determination process part. The figure which shows the equivalent circuit and external connection circuit of CT. The figure which shows the whole structure of the differential protection relay apparatus of this invention. The figure explaining the operation | movement aspect of each process part 5, 6, and 7 of FIG.

  FIG. 2 shows an CT equivalent circuit and an external connection circuit. In this figure, the CT equivalent circuit 2 can be expressed as a T-type circuit between the current source 1 and the external output terminals 2a and 2b. This is because a series circuit of the CT primary winding and secondary winding impedance (primary impedance Zp and winding impedance Zs) is arranged in the portion corresponding to the upper side of the T-type circuit, and excitation is applied to the legs of the T-type circuit. In this configuration, the excitation impedance Zo of the winding is arranged.

  When a protective relay device is connected to the CT, a series circuit of the cable impedance Zl and the impedance Zry of the protective relay panel is connected as an external connection circuit between the external output terminals 2a and 2b.

  In the CT equivalent circuit, the CT saturation phenomenon generally means that the CT secondary excitation voltage Vs (terminal voltage of the excitation winding) is saturated and the excitation voltage Vs does not change any more. This shows a phenomenon in which current to the secondary does not flow. This excitation voltage is inherent in the basic characteristics of CT and includes an eigenvalue generated in the CT manufacturing process. The higher the excitation voltage, the less likely it is to saturate.

  The CT excitation voltage Vs includes the CT rated load (load connected to the secondary circuit of the current transformer), the CT secondary rating (rated voltage and rated current of the current transformer secondary circuit), and the CT overcurrent constant (primary It is known that it can be expressed by equation (1) using a constant representing the performance when the current is larger than the rated primary current.

また一般的に、ＣＴにおいては一次インピーダンスＺｐが小さいことからこれを無視すると、ＣＴ定格負担及びＣＴ過電流定数が大きくなる程、励磁電圧Ｖｓが大きくなり、結果として一次側の大電流に対して飽和しにくくなる関係にある。

In general, if the primary impedance Zp is small in CT and ignored, the excitation voltage Vs increases as the CT rated load and the CT overcurrent constant increase. It is in a relationship that becomes difficult to be saturated.

  On the other hand, the induced voltage Vr of the CT with respect to the current is related not only to the CT eigenvalue but also to the magnitude of the total burden constituting the protective relay system including the protective relay device and the connection cable. Can be represented by

  Further, the relationship between the excitation voltage Vs in the expression (1) and the induced voltage Vr in the expression (2) will be examined. When a failure occurs, a direct current component is superimposed. Assuming that 100% of the maximum superimposition is taken into consideration as the direct current component, it is assumed that twice as the fault current flows into the CT. The relationship for preventing the CT from being saturated with the current under this condition is derived from the equations (1) and (2).

  In the present invention, focusing on the relationship of equation (3), the excitation voltage point at the current value is always calculated from the excitation voltage unique to CT in real time and the magnitude of the current value detected from the CT secondary. -CT saturation determination is achieved by comparison.

  More specifically, the CT rating burden, CT secondary rating, and CT overcurrent constant used to determine the excitation voltage Vs in equation (1) are determined for each CT and described on the nameplate of the CT. These are values that can be known in advance, and here they are referred to as CT constants. In equation (2), the CT primary rating, CT secondary rating, winding impedance Zs, and impedance (Zl, Zry) of the external connection circuit used for obtaining the induced voltage Vr of CT are values that can be known in advance. is there. Of these, the CT primary rating and the CT secondary rating are also CT constants.

  In addition, in Equations (1) and (2), there are the winding impedance Zs and the impedance (Zl, Zry) of the external connection circuit other than the CT constant, which are measured when connecting the protective relay device. That's fine. Other numerical values are CT constants, and these are publicly disclosed numerical information even if they are CTs manufactured by other manufacturers. Therefore, these values are treated as set values, and these values are set in advance at the time when the protective relay device is installed. As a result, the excitation voltage Vs in the equation (1) can be treated as a CT-specific value, and only the primary current in the equation (2) remains as an unknown value.

  FIG. 3 shows the overall configuration of the differential protection relay device of the present invention. First, the differential protection relay device 3 is applied to, for example, a transformer as a protection application target 4, and current transformers CT1 and CT2 provided at all terminals (shown here as examples of primary terminals and secondary terminals). Currents If1 and If2. In addition, although an electric current is detected about all the phases of alternating current three phases, only the single phase is described here.

  The differential protection relay device 3 includes an internal failure determination processing unit 5 and an external failure determination processing 6. Although the illustration of the specific circuit configuration is omitted, the internal failure determination processing unit 5 executes, for example, a vector operation in which the current flowing in the protection application target 4 is positive and the current flowing out is negative. When the vector sum of currents obtained from the current transformers CT1 and CT2 exceeds a predetermined value, an internal fault is determined and the circuit breakers CB1 and CB2 of all terminals are opened.

  For example, the external failure determination processing unit 6 performs current input in which the current flowing in the protection application target 4 is positive and the current flowing out is negative, and there is an outflow current sum having the same value as the inflowing current sum. After confirming this, the external accident is judged and the function of preventing the circuit breakers CB1 and CB2 from being opened is achieved. Specifically, the open circuit operation signal O1 of the circuit breakers CB1 and CB2 given by the internal failure determination processing unit 5 is blocked by the AND circuit 29 based on the output O2 of the external failure determination processing unit 6.

  As the determination logic in the external failure determination processing unit 6, several types of logic using the above concept are known. In the example of FIG. 3, the phase difference processing unit 61 that uses the phase difference and the instantaneous value sum are used. An example of a different type duplex system that includes an instantaneous value sum processing unit 62 that performs external determination when both are externally determined will be described.

  The phase difference processing unit 61 in FIG. 3 detects the phase difference from the reference phase with respect to the same phase current of each CT, and calculates the phase difference between the outputs of each CT. Specifically, the reference point is obtained from the positive / negative wave boundary data by high-speed sampling, and the difference from the given reference phase in the positive / negative wave is calculated. The obtained reference phase of each phase is added for each positive wave / negative wave by a phase synthesis unit that performs synthesis processing of all phases.

  In the normal passing current in the protection target, the phase difference for both positive and negative waves conforms to the phase error characteristic of each CT, and the phase difference is in the vicinity of 0 degree in the steady state. Here, in consideration of the CT inherent error, it is determined that the combined phase is within the setting (Kθ), and it is determined as an external failure.

  The instantaneous value sum processing unit 62 has a function of taking an instantaneous value sum in the same phase of each CT and performing effective value processing on the obtained result. The instantaneous value sum processing unit 62 determines that the combined current is within the setting (KId) in consideration of the CT inherent error because the combined current is close to 0 in the normal passing current in the protection target, and the external failure And

  The determination results of the phase difference processing unit 61 and the instantaneous value sum processing unit 62 are determined by the AND circuit 21, and when the processing results of the phase difference processing unit 61 and the instantaneous value sum processing unit 62 are satisfied, it is determined that an external failure has occurred. Then, the flip-flop 23 is set. Since the reset condition of the flip-flop 23 needs to reset the external failure determination at high speed when the external failure progresses to the internal failure, the determination results of the phase difference processing unit 61 and the instantaneous value sum processing unit 62 are both If not established, the flip-flop 23 is reset with the determination of the OR circuit 22.

  As described above, according to the differential protection relay device 3 configured by the internal failure determination processing unit 5 and the external failure determination processing unit 6 that have explained the concept, the internal failure determination processing unit 5 internally determines the inflow current, The external failure determination processing unit 6 should externally determine the outflow current so that the internal / external failure can be correctly determined.

  However, when the value detected by CT includes an error due to CT saturation or the like, the internal failure determination processing unit 5 detects an excessive passing current detection error at the time of an external failure and gives a circuit breaker open signal, On the other hand, even if the external failure determination processing unit 6 includes an error in the input, since it is a passing current, the external failure is correctly detected, and therefore it is not possible to give a circuit breaker open prevention signal. As a result, the circuit breaker is opened despite the external failure.

  The CT saturation determination processing unit 7 of the present invention detects that the value detected by CT includes an error due to CT saturation or the like, and prevents the internal failure determination processing unit 5 from opening the circuit breaker. FIG. 1 shows a detailed configuration of the CT saturation determination processing unit 7.

  In FIG. 1, a CT saturation determination processing unit 7 is installed for each CT phase. Since each phase may have the same circuit configuration, the a-phase circuit configuration is described in the example of the drawing, and the description of the configuration of the other phases is omitted.

  The saturation determination unit 73 in FIG. 1 executes the expression (3). Moreover, the induced voltage calculating part 71 performs (2) Formula. The excitation voltage calculation unit 72 executes equation (1).

  Among these, in order to execute the expression (1) in the excitation voltage calculation unit 72, since all the values in the expression are known in advance as described above, these are input in advance as settling values. That is, the excitation voltage calculation unit 72 takes in the CT constant (secondary rating, rated burden, overcurrent constant) as a set value, and obtains the induced voltage Vs as a constant unique to CT.

  Further, in order to execute the expression (2) in the induced voltage calculation unit 71, as described above, since many values in the expression are known in advance, these are input in advance as settling values. That is, the induced voltage calculation unit 71 takes in the CT constants (primary rating, secondary rating, winding impedance) and the device side load (cable impedance Zl, relay panel load Zry) as settling values. The induced voltage Vr is calculated from the equation (2) using the current CT secondary current.

  In calculating the induced voltage Vr, the current CT secondary current is obtained as follows. The CT secondary detection current is input to the filter unit 78 to extract the fundamental wave component. At this time, a phase lag of the detected current is caused by the filter unit 78, so the phase lag compensation unit 79 compensates for the phase lag. The phase compensated data is integrated by the integration processing unit 70 to obtain an effective value. In the induced voltage calculation unit 71, the current CT secondary current subjected to the effective value processing in the integration processing unit 70 is subjected to the equation (2) using the value input as the settling value, and is induced in the CT. The induced voltage Vr is calculated.

  Finally, the final CT saturation determination is performed by the saturation determination unit 73 according to the expression (3) using the outputs of the excitation voltage calculation unit 72 and the induced voltage calculation unit 71. In the present invention, correction by the correction coefficient K is further performed. By taking into consideration, it is determined with higher accuracy. The correction coefficient K includes a direct current component, and the positive value and the negative value are significantly different. Therefore, the content ratio of the direct current component is grasped in the form of a positive / negative non-target rate, and (3) This is reflected in the calculation of the formula.

  For this purpose, the asymmetry determination unit 76 first calculates the positive wave peak value and the negative wave peak value from the instantaneous value of the CT secondary current in order to determine the asymmetry rate of the CT detection current, and calculates the asymmetry rate from the ratio. . The asymmetry rate is calculated, for example, as 0% when the DC component is not included, and therefore when the absolute value of the positive wave peak value and the negative wave peak value are the same, and 100% when the DC component of the same amount as AC is included.

  In the correction coefficient K selection table 77, the constant K applied by the saturation determination unit 73 is selected from the result of the asymmetry determination unit 76. K = 0.5 when the asymmetry rate is 0%, and K = 1 when the asymmetry rate is 100%. As a result, even if the peak value of alternating current is the same, the more the direct current component is contained, the more easily it can be determined that it is saturated.

  The above saturation determination is performed for each phase. Finally, the OR circuit 74 determines that the saturation is obtained when any of the determination results of each phase of the CT of the terminal is saturated. . Similarly, the same processing is performed on all CTs of other terminals that are taking current into the protective relay device, thereby having a function of detecting a CT saturation phenomenon in the configured system.

  Returning to FIG. 3 again, the differential protection relay device with an external failure determination function in this embodiment will be described. First, as shown in FIG. 3, the output of the CT saturation determination processing unit 7 in FIG. 1 is matched with the output of the external failure determination processing unit 6 in the AND circuit 28. When the CT saturation determination processing unit 7 determines that the output is saturated, and the external failure determination processing unit 6 determines that an external failure has occurred and outputs the result, and both determination results are satisfied, the AND circuit 28 causes the protection target device 4 to Judged as an external failure.

  The output O2 of the AND circuit 28 is given to the AND circuit 29, whereby the output of the internal failure determination processing unit 5 is locked during the determination period. In the case of a single determination output from the external failure determination processing unit 6, it is determined that a CT error has occurred due to an external failure, and in this case, the signal is used to reduce the detection sensitivity of the differential protection relay device applied. Apply.

  An operation mode by providing the processing units 5, 6, and 7 of each unit of FIG. 3 described above will be described. FIG. 4 shows a first case where an internal failure occurs and no CT saturation, a second case where an internal failure occurs and CT saturation, a third case where an external failure occurs and CT saturation does not occur, an external failure occurs and CT saturation occurs Each part state of FIG. 3 in the fourth case is shown. The operation modes of the respective processing units 5, 6, and 7 in FIG. 3 will be described for each case.

  First, in the case of an internal failure occurrence, in the case of an internal failure in which current flows from each terminal, the internal failure determination processing unit 5 gives “1” as the output O1. On the other hand, the external failure determination processing unit 6 does not output and is “0” because it is not externally determined. Therefore, in this state, even if the CT saturation determination processing unit 7 determines that the saturation is performed and outputs “1” as in the second case, or the CT saturation determination processing unit 7 is not in the state as in the second case. Even if it is determined to be saturated and “0” is output, the output O2 of the AND circuit 28 is “0”. Therefore, the AND circuit 29 correctly issues a breaker opening instruction based on the output of the internal failure determination processing unit 5.

  Next, in the case of the passing current due to an external failure, the third case is CT non-saturated, and the internal failure determination processing unit 5 does not detect the difference current and gives “0” as the output O1. However, in the fourth case, due to CT saturation, the internal failure determination processing unit 5 may detect the difference current and give “1” as the output O1.

  On the other hand, the external failure determination processing unit 6 performs external determination regardless of CT saturation or CT non-saturation and outputs “1”. In contrast, the CT saturation determination processing unit 7 gives “1” when CT is saturated (case 4), and gives “0” when CT is not saturated (case 3). For this reason, the output O2 of the AND circuit 28 gives “1” when CT is saturated (case 4) and gives “0” when CT is not saturated (case 3). It is possible to prevent the internal output determination result (O1) from being supplied to the circuit breaker as a circuit breaker circuit breaker signal.

  As described above in detail, in the present invention, the asymmetry rate is calculated using the instantaneous value of the CT secondary current, and the CT excitation voltage Vs and the induced voltage Vr at the current current value are processed in real time. Ratio differential protection is realized by combining a CT saturation determination function, a difference current detection by instantaneous value synthesis, and an external failure determination function by phase synthesis error determination with respect to the current flowing into the current.

1: Current source 2: CT equivalent circuit 2a, 2b: External output terminal 3: Differential protection relay device 4: Protection application target 5: Internal failure determination processing unit 6: External failure determination processing unit 7: CT saturation determination processing unit 21, 28: AND circuit 22, 74: OR circuit 23: flip-flop 29: AND circuit 61: phase difference processing unit 62: instantaneous value sum processing unit 70: integration processing unit 71: induced voltage calculation unit 72: excitation voltage calculation unit 73: Saturation determination unit 76: Asymmetry determination unit 77: Correction coefficient K selection table 78: Filter unit 79: Phase lag compensation unit CT: Current transformer CB: Circuit breaker Vs: Excitation voltage Vr: CT induced voltage Zp: Primary impedance Zs: Winding impedance Zo: Excitation impedance Zl: Cable impedance Zry: Relay panel impedance

Claims (10)

An internal failure determination processing unit that detects each terminal current to be protected by a current transformer and provides an open command signal for opening a circuit breaker provided to each terminal to be protected by differential calculation of the terminal current In the differential protection relay device comprising:
A CT saturation determination processing unit that determines an excitation voltage and an induced voltage of the current transformer and compares them to determine saturation of the current transformer, and the detected terminal current is a passing current of the protection application target. An external failure determination processing unit that detects the occurrence of an external failure and blocks the internal failure determination processing unit when the CT saturation determination processing unit makes a saturation determination and the external failure determination processing unit makes an external determination A differential protection relay device characterized in that an open command signal for opening the device is blocked. The differential protection relay device according to claim 1,
A differential protection relay device, wherein a CT constant and an impedance for obtaining an excitation voltage and an induced voltage of the current transformer are given as a set value of the differential protection relay device. In the differential protection relay device according to claim 1 or 2,
The differential protection relay device, wherein when comparing the excitation voltage and the induced voltage of the current transformer, correction is made so that saturation can be easily determined when a DC component is superimposed on the detected current. The differential protection relay device according to claim 3,
A differential protection relay device, wherein a DC component is superimposed on a detected current is determined from an asymmetry rate of a positive / negative waveform of the detected current. The differential protection relay device according to claim 1,
A differential protection relay device characterized in that, when an external failure determination processing unit makes an external determination, operation sensitivity for differential calculation by the internal failure determination processing unit is reduced. The differential protection relay device according to claim 2,
The CT rating load, CT secondary rating, and CT overcurrent constant are used as the CT constant for obtaining the excitation voltage of the current transformer, and the product of the CT rating load and the CT overcurrent constant is used as the CT secondary rating. A differential protection relay device characterized by being obtained as a value divided by. In the differential protection relay device according to claim 2 or 6,
CT primary rating and CT secondary rating are used as CT constants for obtaining the induced voltage of the current transformer, and the product of the primary current, CT secondary rating and impedance measured by the current transformer is used as the induced voltage. A differential protection relay device characterized by being obtained as a value divided by a primary rating. In the method for determining the saturation of a current transformer that detects the terminal current of the power system protection application target,
Saturation of the current transformer is determined by comparing the excitation voltage and the induced voltage of the current transformer, and the product of the CT rated load and CT overcurrent constant is divided by the CT secondary rating. A method for determining saturation of a current transformer, characterized in that the induced voltage is obtained as a value obtained by dividing a product of a primary current measured by a current transformer, a CT secondary rating, and the impedance by a CT primary rating. The current transformer saturation determination method according to claim 8,
When comparing the exciting voltage and the induced voltage of the current transformer, the saturation judgment of the current transformer is performed so that the saturation judgment is facilitated when a DC component is superimposed on the detected current. Method. The current transformer saturation determination method according to claim 9,
A method for determining the saturation of a current transformer, comprising: determining that a DC component is superimposed on a detected current from an asymmetry rate of a positive / negative waveform of the detected current.

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