JPH0522455B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a ground fault directional relay used for ground fault protection of ungrounded power distribution lines.

Figure 1 shows an example of a bank configuration of a normal non-grounded power distribution substation. In Figure 1, T is a transformer connected to the power supply, with a feeder on the secondary side.
F ₁ to F ₃ are connected, and a ground voltage potential transformer CPT is also connected. Each feeder F ₁ ~ _{F 3}
Zero-phase current transformers ZCT ₁ to _{ZCT 3} are inserted, and ground fault direction relays DG ₁ to DG ₃ are connected thereto. In addition, a limiting resistor CLR is connected to the open delta circuit constituted by the tertiary winding of the ground voltage instrument transformer GPT, and the zero-sequence voltage Vo generated between the terminals in the event of a ground fault is Earth fault directional relay
It is designed to be input to DG ₁ to _{DG 3} .

In such a bank configuration, if a ground fault occurs in feeder E ₁ , for example, a zero-sequence voltage Vo will occur across the limiting resistor CLR connected to the open delta circuit on the tertiary side of the grounded instrument transformer GPT. , and zero-phase current transformers for each feeder F ₁ to F ₃
Zero-sequence current Io flows through ZCT ₁ to ZCT ₃ . This zero-sequence voltage Vo and zero-sequence current Io are introduced into the ground fault direction relays DG ₁ to _{DG 3} of each feeder, and the ground fault feeder is selected based on the phase relationship.

In this case, the phase characteristics of the earth-fault directional relay are such that the phase of the zero-sequence current Io with respect to the zero-sequence voltage Vo advances from 0° and changes to about 90° depending on the bank configuration and the ground capacitance C ₁ to C ₃ of the line. , it is normal for the characteristics to be such that the maximum sensitivity is achieved at a 45° advance.

By the way, recently there has been a tendency for cables to be used frequently as power distribution lines, and intermittent ground faults are becoming more common in the distribution line. The ground fault current is in a pulse form as shown in Figure 2a, and the zero currents Io ₁ and Io ₂ of the faulty feeder and healthy feeder detected by the zero-phase current transformer are pulse-like as shown in Figures 2b and c. The waveform of the zero-phase voltage Vo often becomes a rectangular wave as shown in FIG. 2d.

For this reason, ground fault direction relays are configured to be sensitive to such waveforms and to accurately select the fault feeder.
As shown in the figure. In Figure 3, TA is an input transformer connected to the secondary side of the zero-sequence current transformer, and a current Ioa proportional to the zero-sequence current Io flows through the secondary side, and a voltage VIo corresponding to this flows. . BPF ₁ is an active bandpass filter for the fundamental wave that inputs this voltage VIo, and TV is an input transformer connected to the tertiary side of the ground voltage instrument transformer, which is used to take in the zero-sequence voltage. be. BPF ₂ is a band pass filter provided at the zero-phase voltage input section, and like the filter BPF ₁ , a filter with a high Q is used to extract only the fundamental wave component. PS is a phase circuit, and PC is a phase comparison circuit, which is used to compare the phases of each basic board. ID is the zero-sequence current detection circuit, AND is the output of this zero-sequence current detection circuit ID and the phase comparison circuit PC
An AND circuit whose input is the output of the AND circuit, and X is an output relay driven by the output of the AND circuit.

In this way, high-Q active bandpass filters BPF ₁ and BPF ₂ are provided at the Io input section and Vo input section, extracting only the fundamental components of each waveform, comparing the phases of their outputs, and performing Io detection. If relay X is driven by AND output, feeder
If a cable ground fault occurs in F ₁ , the zero-sequence current I′o ₁ of feeder F ₁ and the zero-sequence current of healthy feeder F ₂
The fundamental wave component of I′o ₂ has a phase relationship as shown in FIG. 4 with respect to the fundamental wave component of the zero-phase voltage Vo, and the device operates normally. That is, the zero sending current I′o ₁ falls within the operating range (the shaded area side).

Let us now consider the influence of zero-phase current transformers. The equivalent circuit of a zero-phase current transformer is as shown in FIG. 5, if the resistance of the winding is ignored. In the figure, Xo
is the excitation impedance of the zero-phase current transformer, and R _L is the load resistance of the zero-phase current transformer. The load of a zero-phase current transformer is a combination of the impedance of the lead wire, auxiliary current transformer, and relay, but the inductance component is small compared to the resistance component, so it is ignored and treated as pure resistance.

Now, when a pulsed ground fault current as shown in FIG. 6a flows as an input to the zero-phase current transformer, what kind of current flows in the secondary side of the zero-phase current transformer will be explained below.

In FIG. 6, when time t is in the range t ₀ ≦t<t ₁ , the pulsed input current Io is divided into a component I _pL flowing through the load resistance R _L and a component I _px flowing through the excitation impedance Xo. Since the excitation impedance Xo is several tens of times as large as the load resistance R _L , t ₀ ≦
In the range t<t ₁ , the current Io, Io _L , I _px and the voltage V
The following relationship holds true between them.

Io=L _pL +I _px ≒L _pL ……(1) V=R _L・I _pL ……(2) Next, at time t=t ₁ , the current I _px (t ₁ ) flowing through the excitation impedance X _p is , using equations (1) and (2), I _px (t ₁ )=1/L _p ∫ ^t1 _p Vdt=R _L /L _p ∫ ^t
1 _p I _pL dt≒R _L /L _p ∫ ^t1 _p I _p dt……(3) However, L _p is the reactance of the impedance X _p , and I _px (t ₁ ) is the integral value of the input current I _p , that is,
It is proportional to the area of the pulsed waveform. this current
Since the input current I _p has already become 0, I _px (t ₁ ) will be discharged through the load resistor R _L.

That is, when the time t is in the range of t ₁ ≦t<t ₂ , the circuit becomes a closed circuit of X _p and R _L , and the currents I _pL and I _px are expressed by equations (4) and (5).

I _px = I _px (t ₁ )ε ^-t/ 〓 ……(4) I _pL = −I _px = −I _px (t ₂ )ε ^-t/ 〓=R _L /−L _p
∫ ^t1 _p I _p dt・ε ^-t/ 〓……(5) However, time constant τ=L _p /R _L In this way, when t ₁ ≦t<t ₂ , the load resistance R _L
The current flowing in the input current I _p flows in the opposite direction to that when the input current I p is applied, and its magnitude attenuates with a time constant τ=L _p /R _L . In a zero-phase current transformer, this time constant τ is about 0.1 seconds, so there is almost no attenuation during the half cycle of the commercial frequency, which is about 10 milliseconds.
The current waveform becomes a rectangular wave like I _pc in Figure b,
According to equation (5), its magnitude is proportional to the integral value of the input current, that is, the area of the pulsed current.

This current I _pc has a small ground capacity of the distribution line,
This is hardly a problem when the pulse current is small, but as the number of cable distribution lines increases and the ground capacity increases, it becomes a big problem.

This is because the bandpass filter shown in Figure 3
Since BPF ₁ is an active filter, its output cannot exceed the control voltage, and the dynamic range cannot exceed this control voltage. If an input higher than the control voltage is input, the active filter function will be lost, so in order to limit the input within this range, varistor V ₁ is connected between the input terminals of bandpass filter BPF ₁ as shown in Figure 3. Normally, the upper limit value of the input is clipped to obtain the phase characteristics shown in FIG.

However, since ground fault directional relays require high sensitivity such as 0.2A at the primary of the zero-phase current transformer,
The upper limit of the dynamic range cannot be set extremely high. However, in recent cable ground faults, large pulse currents with peak values of several hundred amperes have started to flow, although the pulse width remains the same. This tendency is particularly noticeable at the beginning of a ground fault when the cable's ground capacity is large.

For example, the peak value of the pulse current can be calculated using an equivalent circuit as shown in FIG. 1st
In Figure 1, E is the voltage value when a ground fault occurs, L is the reactance of the power source impedance, C is the sum of the ground capacity of healthy feeders, and R is the limiting resistance of the ground voltage instrument transformer GPT.

FIG. 12 shows the calculated values using the equivalent circuit of FIG. 11. As can be seen from FIG. 12, the peak value of the pulse current increases as the ground capacity of the cable increases. For example, if the ground capacity of the cable is 5 μF, the peak value of the pulse current will be about 400 A, and the secondary current I _pc of the zero-phase current transformer will be several A according to equation (5).

In this case, as discussed earlier, the pulse area of the current I _p increases, so the secondary current of the zero-phase current transformer
Since the clipping voltage of the varistor V ₁ is constant while I _pc (Io ₁ in Figure 6b) increases, the input of the bandpass filter BPF ₁ has a waveform like V _Ip in Figure 6d. I'm getting old. When the fundamental wave is extracted from this waveform using a bandpass filter, it becomes a waveform as shown in Figure 6e (the dotted line is when there is no varistor), and on the phase characteristics, the zero-sequence current Io ₁ of the faulty feeder and the zero-sequence current of the healthy feeder. The apparent phase of Io ₂ is as shown in Figure 4, and the ground fault direction relay of the faulty feeder malfunctions, and the ground fault direction relay of the healthy feeder malfunctions, completely opposite to the originally expected operation. This becomes a big problem.

It should be noted that the current Igpt discharged through the neutral point of the ground voltage voltage transformer GPT is superimposed on the Ioc ₂ on the healthy side (Io ₂ in FIG. 2) on the previous Ioc.

The present invention was made to solve the above-mentioned problems, and by using a saturable active filter as the fundamental wave bandpass filter of the Io input section, it is possible to accurately prevent ground faults even when the ground capacity is large. The purpose of the present invention is to provide a ground fault directional relay that can provide protection.

Hereinafter, the present invention will be explained in detail based on illustrated embodiments.

FIG. 7 shows an embodiment of the present invention.
and TV is the input transformer, BPF ₂ is the Vo bandpass filter, PS is the phase circuit, PC is the phase comparison circuit,
ID is zero-phase current detection circuit, AND is AND circuit,
is an output relay, HBPF is a saturable active bandpass filter, and this filter HBPF is Io
The difference from the conventional method (FIG. 3) is that a fundamental wave bandpass filter is used in the input section.

Here, the saturable active bandpass filter HBPF has input-output characteristics and input-output phase characteristics as shown in FIGS. 13A and 13B. This refers to an active bandpass filter in which the input limit voltage is set to a value greater than the input voltage at this time, and a specific circuit example thereof is shown in FIG.

In Fig. 8, R ₂ , R ₃ , R ₁₁ and R ₁₂ are resistances,
C is a capacitor, A is an operational amplifier, and V is a clipper (varistor) for input limiting.

This kind of configuration is usually called a multiple feed pack type bandpass filter, and R ₃ = R ₁ + R ₂ / R ₁ R ₂ O ² ωo ² ...(6) Here Then, by adjusting R ₁ =R ₁₁ +R ₁₂ ω _p :the center angular frequency of the filter, a bandpass filter centered at fo (=ωo/2π) can be obtained.

When f=fo, the input Vi of the bandpass filter
and the output Vo, that is, the gain Ao is Ao=Vo/Vi=R ₃ /2R ₁ (7). Also, the Q of the filter is Q=ωoCR ₃ /2 (8).

Next, the operation will be described. When the input is within the range where operational amplifier A is not saturated, the output Vo of the bandpass filter is equal to the input Vi as shown by the solid line in Figure 10.
The output is just the opposite phase. Input Vi
As becomes even larger and the output of operational amplifier A becomes saturated, the above equations (6), (7), and (8) no longer hold, and from the characteristics shown in Figure 13A and B, As shown by the dotted line in Figure 10, the output Vo saturates and the phase lags.

In this way, when the output of the saturable active bandpass filter HBPF lags behind the input,
As shown in FIG. 9, the phase characteristics of the earth fault direction relay are bent toward the advancing side. This first turning point _A1 is the point at which operational amplifier A begins to saturate.

Furthermore, when the input becomes large enough that the input limiting clipper (varistor) V limits the input, the input is limited at that level, so the output
The phase delay of Vo with respect to the input Vi also becomes slower, and a second bending point _A2 is created as shown in FIG. 9 as a phase characteristic of the earth fault direction relay.

The first bending point _A1 is the gain Ao of equation (7)
The second bending point _A2 can be arbitrarily set by adjusting the clipping voltage of the clipper V.

Note that the upper limit value of the clip voltage is set to a level that does not destroy the operational amplifier A.

By the way, when the number of feeders in a distribution substation bank is small and the ground capacity is small, as is well known, the zero-sequence current flowing through the grounded instrument transformer and having the same phase as the zero-sequence voltage Vo will undergo a zero-phase change. Because much of the current is supplied to the ground fault direction relay through the current transformer, the phase relationship between the zero-phase current transformer secondary output I″o ₁ of the faulty feeder and the zero-phase current transformer secondary output I″o ₂ of the healthy feeder is as follows. As shown in Figure 9, they are almost in phase or out of phase. In this way, the area where the current flowing while being limited by the limiting resistor CLR of the grounded instrument transformer has a large effect is as follows:
This is a region where the absolute value of the zero-phase current transformer secondary current is small because the amount of lead flowing through the ground capacitance is small.

Therefore, in the region where the zero-sequence current Io is small as shown in Figure 9, the maximum sensitivity phase angle of the phase characteristic is set to about 45°,
In the region where the zero-sequence current Io is large, the lead current due to the ground capacitance is large, so rotating the maximum sensitivity phase angle to the lead side is useful for substations where the ground capacitance of the bank changes greatly. The present invention can also be applied to ground faults, providing accurate ground fault protection.

It also operates normally even in the case of cable ground faults with large peak values. This is because when the cable ground fault current input has a large peak value, the effective value of the I ₀ input also becomes large, and when the zero-sequence current Io input is large (the phenomenon that Io ₁ and Io ₂ rotate to the leading side appears) ), the phase characteristic curves toward the leading side as shown in Figure 9.
This is because, as shown in Io ₁ and Io ₂ , normally malfunctioning feeders operate while healthy feeders do not operate.

Note that the saturable active bandpass filter
The HBPF is not limited to the circuit shown in FIG. 8, but may be any active bandpass filter that causes a phase delay when saturated.

As described above, according to the present invention, the fundamental wave bandpass filter of the Io input section is a saturable filter that actively saturates the operational amplifier when a large zero-sequence current Io is input by increasing the limiting voltage of the input limiting clipper. We used a type active bandpass filter, and intentionally used the phase change of the input and output of the bandpass filter to obtain a phase characteristic with a break point.
Even when a cable ground fault occurs in a bank with a large ground capacity, it is possible to accurately determine which feeders are faulty and which are healthy. Further, even in the case of a system configuration in which the ground capacity is small and the zero-sequence current Io is almost in phase with the zero-sequence voltage Vo, a faulty feeder can be accurately identified.
Another advantage is that the breaking point of the phase characteristics can be easily set at any point.

[Brief explanation of the drawing]

Figure 1 is a bank configuration diagram of a distribution substation, Figures 2 a to d are current and voltage waveform diagrams at the time of a ground fault, and Figure 3
The figure is a block diagram showing an example of a conventional ground fault directional relay, Figure 4 is a phase characteristic diagram of the same ground fault directional relay, Figure 5 is an equivalent circuit diagram of a zero-phase current transformer, and Figures 6 a to e are Waveform diagram of bandpass filter of ground fault direction relay,
FIG. 7 is a block diagram showing an embodiment of a ground fault directional relay according to the present invention, and FIG. 8 is a circuit diagram of a saturable active bandpass filter in the same embodiment.
Figure 9 is a phase characteristic diagram, Figure 10 is a bandpass filter input and output waveform diagram, Figure 11 is an equivalent circuit diagram of the system when a ground fault occurs, and Figure 12 is the cable ground capacity and pulse current peak. Graphs 13A and 13B showing the relationship with values are characteristic diagrams of HBPF. ZCT...Zero-phase current transformer, GPT...Grounded instrument transformer, HBPF...Saturable active bandpass filter, BPF ₂ ...Bandpass filter for Vo, ID...Io detection circuit, PC... phase comparison circuit,
AND...And circuit, X...Output relay, A...
... operational amplifier, R ₂ , R ₃ , R ₁₁ and R ₁₂ ... resistance,
C...Capacitor, V...Input limiting clipper (varistor).

Claims (1)

[Claims] 1 Extract the fundamental waves of zero-sequence current and zero-sequence voltage using a bandpass filter, compare their phases, and set a ground fault that activates the output relay when there is a specified phase relationship and the zero-sequence current is above a specified level. In a directional relay, an active band filter is used as a fundamental wave bandpass filter in the zero-phase current input section, which has a phase characteristic break point at the saturation point of the operational amplifier, and whose input limit voltage is set to a value greater than the input voltage at this point. A ground fault directional relay characterized by using a pass filter.