JPH0640697B2 - Excitation inrush current detection method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exciting inrush current detection method for detecting an exciting inrush current by distinguishing it from a fault current in a transformer differential differential relay method or the like.

(Prior Art) Conventionally, as a protection relay method of a transformer, a current ratio differential relay method is usually adopted for main protection. This method
As is well known, the current passing through each terminal of the transformer to be protected is taken out from the current transformer as a current equivalent to the transformation ratio, and a differential current circuit is built for each terminal to protect against external accidents in the protected area. Does not generate a differential current, but energizes the operating coil by the differential current generated in the case of an internal accident to operate the circuit breaker.

However, this difference current may occur in cases other than internal accidents. That is, when the transformer is excited with no load,
As shown in Fig. 12, when the fault section is removed by the trip of the circuit breaker CB due to an accident outside the protection section of the ratio differential relay Ry, the excitation inrush current (inrush current) Ir 'is returned when the voltage is restored. There is a case where the differential relay Ry malfunctions by flowing into the protection section and generating a differential current. In the figure, MT is the main transformer, G is the system power supply,
CT ₁ and CT ₂ are current transformers, respectively.

Therefore, conventionally, various measures have been proposed for discriminating the inrush current of the excitation and the internal fault current to prevent malfunction of the ratio differential relay Ry, and one of them is a so-called second harmonic suppression method. It has been known. This method focuses on the fact that the second harmonic component of the normal internal fault current is smaller than the large amount of the second harmonic component contained in the excitation inrush current. When the ratio of the components exceeds a certain value, it is determined as the exciting inrush current, and the ratio differential relay Ry does not operate (lock) with the second harmonic component as the suppression force.
The state is set, and in other cases, the relay is operated by judging the internal fault current.

(Problems to be solved by the invention) However, in recent years, the electrostatic capacitance to ground has increased due to the increase in the voltage of the power system, the increase in the capacity and the increase in the distance, and the increase in the cable system. The frequency tends to decrease, and particularly in an ultrahigh voltage system, it is expected that a large amount of low-order harmonic current near the second harmonic will be generated due to system sway during an internal transformer accident. Therefore, as described above, the second
The condition that the harmonic component is always smaller than the second harmonic component in the exciting inrush current is no longer satisfied. That is, according to the conventional second harmonic suppression method, the second harmonic in the accident current is mistakenly recognized as being due to the inrush current of the current despite the occurrence of the internal accident, and the operation delay or the worst case is caused. There is a risk that the relay lock may not work (misoperation).

The present invention has been proposed to solve the above problems, and its object is to provide a second harmonic current (hereinafter referred to as a reference second harmonic current) synchronized with the fundamental current of the system.
And the phase of the second harmonic current extracted from the system (hereinafter referred to as the extracted second harmonic current) are compared, the excitation inrush current is discriminated as the internal fault current and detected reliably, and the relay malfunctions. Another object of the present invention is to provide an exciting inrush current detection method capable of completely eliminating erroneous malfunction and operation delay.

(Means for Solving Problems) In order to achieve the above object, the present invention provides a reference second harmonic current created based on a fundamental current extracted from a system by a fundamental extraction digital filter, and a second Compare the phase difference with the extracted second harmonic current extracted from the system by the harmonic extraction digital filter, and excite the case where the extracted second harmonic current is in the phase opposite to the reference second harmonic current. The feature is that the determination is made as the occurrence of an inrush current, and a relay lock command or the like is obtained as necessary based on the determination result.

(Operation) First, as is well known, the exciting inrush current has a half-wave rectified waveform with respect to the fundamental wave. That is, even-numbered harmonics such as the second, fourth, and sixth harmonics are superimposed in opposite phases. This means that the main transformer side is superposed as a harmonic generation source. Further, the superposition of the second harmonic current included in a system failure such as an internal accident in the protection section is determined by the failure occurrence phase (due to the three-phase voltage phase), but it can be said that it is almost in phase with the fundamental wave. .

The present invention focuses on the fact that the second harmonic current generated during the excitation inrush phenomenon and the system failure in this way has a different phase region with respect to the fundamental wave. That is, first, the reference second harmonic current is created from data in which only the fundamental wave is extracted by the fundamental wave extraction type digital filter. On the other hand, the second harmonic current to be compared with the reference second harmonic current is the one extracted from the system by the second harmonic extraction filter, and the phases of these second harmonic currents are compared to determine the area. Do.

In this determination, if it is found that the extracted second harmonic current is in the opposite phase region with respect to the reference second harmonic current, it is determined that it is due to the exciting inrush current, and a relay lock command is output if necessary. And make the relay inoperative. vice versa,
If not, it is due to the normal fault current and the relay is operated without locking.

In the present invention, only the second harmonic current is extracted to prevent the delay of the relay operation by suppressing the suppression by the higher harmonics in the event of a system fault.

(Example) An example of the present invention will be described below with reference to the drawings. First,
In FIG. 1, reference numeral 1 is an analog filter to which a system current having a frequency of 50 Hz is input, and is composed of, for example, a bandpass filter. This analog filter 1 is for removing DC components and harmonics and for avoiding aliasing error due to sampling. The system current that has passed through this filter is the fundamental wave extraction filter 2 and the second harmonic extraction filter as digital filters. Input to 3.
Of these, the fundamental wave extraction filter 2 extracts the fundamental wave current of the system and uses it as the reference second harmonic vector (hereinafter,
The reference second harmonic _{2 a} ) is created, and the second harmonic extraction filter 3 obtains only the second harmonic vector (hereinafter, extracted second harmonic _{2 b} ) included in the system current. It is a thing.

The output of each of these digital filters 2 and 3 are applied to the phase comparator 4, this by calculation described later in the phase comparator 4, as shown in FIG. 2 extracts the second harmonic _{2 b} the reference second harmonic _{2 When there is a} phase difference of ± 120 ° or more with respect to a, that is, when the phase difference is in the substantially opposite phase region, it is determined that the current is an inrush current, and a relay lock command is output. The phase comparison of the second harmonic current is performed by r, s,
t For each phase, the relay lock command for each phase is issued to the AND circuits 5 to 7 and the OR circuit 8 as shown in FIG.
Thus, the main relays of all three phases are finally locked by the relay lock command of at least two phases.

Next, the reference second harmonic 2 in the fundamental wave extraction filter ₂
It will be described in detail _a creation method. First, the fundamental wave from the analog filter 1 is sampled every 30 ° at a sampling frequency of 600 Hz and A / D converted into digital data. To create a reference second harmonic with the same instantaneous value from these data, input data every 30 ° is 60
It is necessary to convert to each data and make the time the same.
In other words, as shown in Figs. 4 and 5, every 30 ° (1.66
File location _i for each _{7ms), i 1, i 2} , the input data _i t of the time series corresponding to _{......, i t-1, i} t-2, ......
_{(I t-1} is only one sampling period showing the historical data. Than _{i t)} from the file location _i n for each 60 °, i
_{n1, i n2,} input data _i t of the time series corresponding to _{......, i t-2, i} t-4, and not get ..., the algorithm is shown in Figure 6.

That is, the flowchart shown in FIG. 6 is executed by executing the program every 30 ° (every 1.667 ms) corresponding to the input data. First, in step S1, it is determined whether or not the content of the counter CNT is equal to the sampling interval φ. At the start of the program CNT ≠
the routine proceeds to the next step S2 for a phi, transfers the reference file position i _n the input data i _t from the file location i of the input data file of the second harmonic data file. For example, the data i at an electrical angle of 60 ° is now in the file position i.
if _t-2 has been filed, the _{i t-2} is transferred to the file location _{i n.} At the same time, counter CN
The contents of T is changed to phi, also it until the data i _t-4 which has been filed in the file location i _n is n = n + 1
_{Is filed} in a new file position _in1 .

Then, when one sampling period has elapsed, the program is started again. At this time, since CNT = φ in the previous step S2, the process proceeds from step S1 to step S3, and the content of the counter becomes 1. During this time,
Data _{1 t-1} of 30 ° is newly filed at the file position i due to the lapse of one sampling period.
Such data program is not transferred to the file location i _n order not through step S2.

If one more sampling period has passed, the file position i
The data i _t of 0 ° is newly filed in. In this case, the process proceeds from a CNT ≠ phi in step S1 to the next step S2, the data i _t As with before last are transferred to the file location i _n. At the same time, CNT = φ, and the data of the reference second harmonic data file it _-2 , i
_t-4 is _{filed in} new file positions _in1 and _in2 respectively with n = n + 1.

Thereafter, by transferring a manner the data i _t file position i to a file location i _n once to 60 ° Similarly, it is possible to create a reference second harmonic synchronized with the fundamental wave.

On the other hand, the second harmonic extraction filter 3 shown in FIG. 1 is realized by combining various basic digital filters of addition type and difference type. That is, a filter for removing a fourth harmonic and its multiples harmonics (for convenience, X ₃ filter and referred to the finger. Hereinafter the same.), 3, 9, X ₂ filter to remove the first 15 harmonics, sixth, It consists of a combination of an X ₁ filter that removes the 18th harmonic and an X ₆ filter that removes the fundamental and odd harmonics, of which the X ₂ filter is used in two stages in series. The principle formula is _{X 3 X 2 (2) X} 1 X 6, also arithmetic expression _{I n + I n-1 +} I n-3 -I
_{n-5 -I n-6 +} I n-8 + I n-9 -I n-11 -
In _{-13 -In-14} + 2 (In _{-2 -In-12} )
Becomes The gain characteristic of the second harmonic extraction filter 3 is shown in FIG.

Then, in the phase comparison means 4, the reference second harmonic
While comparing _{2 a} and the extraction second harmonic _{2 b} and the phase,
In this phase comparison, it is conceivable that the reference second harmonic _{2 a} may be deteriorated in accuracy because the sampling interval is 60 °. However, if the error is allowed to some extent, the following arithmetic expression can be applied.

Here, the left side of the equation means the inner product of _{2 a} and _{2 b} , and the right side means that the data at every 60 ° in each half wave of the second harmonic is added and multiplied by K, and K is ψ ( _Two
phase difference between _a and _{2 b)} = indicates the setting value corresponding to ± 120 °. From this equation, the following equation holds.

│ _{2 a} ｜ ・ ｜ _{2 b} ｜ ・ cos ψ ≧ K ・ ｜ _{2 a
| · | 2 b | ...... cosψ} ≧ K ...... i.e., the phase comparison is intended to take place at the final formula, it can be determined that ψ ≧ ± 120 ° if the expression is satisfied. In addition, in this calculation, a quantization error occurs in addition to the sampling error due to the data at intervals of 60 ° as described above. It is possible.

The formulas for the respective values used in the above calculation are shown below.

I. _{2 a} ………… _{2 an} b. _{2 b} ………… _{2 bn} c. _{2 a} · _{2 b} ... _{2 an} · _{2 bn} (the fourth harmonic) d. (Instantaneous data) e. F. cos ψ ... (Averaging data of E. data; 15-point average) A calculation list of each value calculated according to these is shown in Table 1, and typical waveforms are shown in FIGS. 8 (I) to (III). The eighth
Figure (I) has ψ = 0 °, (II) has ψ = 90 °, and (III) has ψ = 18 °.
This is the case of 0 °.

In Table 1, the phase difference of the second harmonic is calculated at 10 ° intervals, and cos ψn beats by the sampling phase. Further, for example, when ψ = 0 °, _{2
If a} = sin ωt and _{2 b} = sin (ωt + ψ), then _{2 a} · _{2 b} = sin ² ωt = (1-cos2ωt) /
It becomes 2. Furthermore, Causes a sampling error of about ± 8%.

Therefore, when the phase comparison of the second harmonic is performed at the sampling frequency of 600 Hz with respect to the fundamental wave, it is necessary to set the settling value K in consideration of the correction amount of the sampling error (here, ψ = Angle error at ± 120 ° is ± 15
° is about).

Next, FIG. 9 shows the result of a simulation in which the present invention is applied to simulate the response when an exciting inrush current is generated. A main transformer with a primary side rating of 275 KV was used, and the inrush analysis conditions are as shown in Table 2 below. Cases I to IV are shown in (I) to (IV) of FIG. 9, respectively. Correspond. Note that the decay time of the exciting inrush current was temporarily set to 500 ms, and the phase region determination of the extracted second harmonic was set to ψ ≧ 120 ° as described above.

As is apparent from FIG. 9, at least two phases are in the anti-phase region (ψ ≧ ± 120 °) when the exciting inrush current occurs, and the relay lock command can be reliably obtained from the three-phase determination output. In this case, the conventional second harmonic suppression method is used in combination, and this is taken as a countermeasure when the inrush is released. That is,
Since the second harmonic content rate decreases when the excitation inrush current is extinguished, the phase discrimination element becomes unstable. Therefore, the lock is released after a fixed time period because the main detection relay has become inoperative.

Next, the results of a simulation in which the present invention is applied to simulate the response when a system failure such as a short circuit accident occurs will be described with reference to FIGS. 10 and 11. The system model used is
As shown in Fig. 10, where G is the system power supply and R is
R is a load resistance, Ca and Cb are system capacitances, Rb is back impedance, Lb, Lla and Llb are inductances, SW ₁ and SW _{2 are} switches, Ry ₁ and Ry ₂ are relays, and r is a fault point resistance. Show. In this simulation, the switch SW ₁ is closed to activate the power system, and the switch SW ₂ is closed to cause a short circuit accident. Then, the switch SW ₂ is opened to simulate the recovery of the accident. The waveform in Fig. 11 shows the current i on the power supply side.
₁ shows the response of the relay Ry ₁ by ₁ .

In this simulation, the harmonics contained in the system current are the second harmonic (frequency 100 [Hz]), the phase in which the accident occurred relative to the r phase is 90 °, and the system inductance is 0.
02533 [H], system capacitance 0.001 [F], back impedance 0.05 [Ω], fault point resistance 0.01 [Ω], load resistance 100 [Ω].

From Fig. 11, in the worst case, one phase is locked when a system failure including the second harmonic occurs (because of the phase of the failure, the three-phase voltages are 120 ° out of phase with each other). Since the other two phases are not locked, the relay can be reliably operated and the trip command can be obtained by the three-phase determination output. It should be noted that other than the 2nd harmonic, the 2nd to 2.8th harmonics, the 3rd, 4th, 5th harmonics, etc. are the 2nd harmonics shown in FIG.
It can be attenuated or removed by the harmonic extraction filter 3.

(Effects of the Invention) As described in detail above, according to the present invention, the exciting inrush current can be discriminated from the fault current and detected reliably only by the current input element from the system, and the relay malfunction or internal fault can be detected. It is possible to completely prevent erroneous malfunctions when they occur.

Also, since it uses a filter that extracts only the second harmonic, it does not respond to the occurrence of a large amount of low-order harmonics such as a system fault, which has been attracting attention in recent years, and prevents misidentification as an inrush current. A stable relay operation without delay can be realized.

Furthermore, the present invention can be simultaneously applied to various relays such as a ratio differential relay and a distance relay that may malfunction due to an exciting inrush current by common software, thereby improving reliability and economical efficiency of transformer protection. It can be significantly increased.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is an explanatory diagram of a phase region, FIG. 3 is a block diagram for obtaining a relay lock command from each phase determination output, and FIG. FIG. 5 is an explanatory diagram for producing a second harmonic current, FIG. 5 is a similar waveform explanatory diagram, FIG. 6 is a same flowchart, FIG. 7 is a gain characteristic diagram of the second harmonic extraction filter, and FIG. (III) is a waveform chart of each value based on Table 1, FIGS. 9 (I) to (IV) are waveform charts showing the results of simulation of the inrush current generation, and FIG.
FIG. 11 is a circuit diagram of a system model used for the system failure simulation, FIG. 11 is a waveform diagram showing the result of the simulation, and FIG. 12 is an explanatory diagram showing the generation of the magnetizing inrush current in the ratio differential relay system. 1 ... Analog filter 2 ... Fundamental wave extraction filter 3 ... 2nd harmonic extraction filter 4 ... Phase comparison means 5-7 ... AND circuit, 8 ... OR circuit

Claims (1)

[Claims] 1. A phase difference between a reference second harmonic current created from a fundamental current extracted from a grid and an extracted second harmonic current extracted from the grid is compared to determine that the extracted second harmonic current is An exciting inrush current detection method, characterized in that it is determined that an exciting inrush current has occurred when it is in a phase substantially opposite to the reference second harmonic current.