JPH0360323A - Malfunction preventing equipment for ground overcurrent relay - Google Patents

Abstract

PURPOSE:To prevent malfunction due to the excitation surge current of a transformer by providing a circuit for preventing polarized DC magnetization having a current generating circuit for applying a reversed DC current corresponding to the excitation surge current of the transformer to the secondary circuit of a CT. CONSTITUTION:The primary conductor of the main circuit in a transformer 1 is provided with a CT2 and the secondary circuit of the CT2 converted to handle a current suitable for an overcurrent relay 7 and a ground overcurrent relay 3 is provided with a circuit 5 for preventing polarized DC magnetization. The circuit 5 is operated when the amplitude of current is considerably shifted only in one direction, limits the DC current reversed according to the amplitude in one direction for a predetermined time and sets the total sum of three-phase components to zero. Thus, unbalanced current is prevented from flowing through the zero-phase circuit of the CT2 and malfunction of the ground overcurrent relay 3 is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a malfunction prevention device for a ground fault overcurrent relay.

[Conventional technology]

Conventionally, in order to prevent malfunctions of ground fault overcurrent relays due to excitation inrush current of transformers, protective relay systems [Denki Shoin] have been used.
(1974), pages 202 to 207, there are harmonic suppression methods.

[Problem to be solved by the invention]

When the second harmonic suppression type relay described above is used, there are the following disadvantages. For example, 500 of 22kV system
The primary rated current of a 0 kVA three-phase transformer is approximately 130 A, and the excitation inrush current is approximately 100 OA, of which the second harmonic is over 50%, or 500 A, and IOA
In a grounding system, when a ground fault occurs at the same time as the breaker is closed, the ground fault current is IOA. However, in the second harmonic suppression type relay, if the second harmonic content exceeds about 15%, the operation will be locked, making it impossible to detect an accident.

The present invention has been made in view of the above points, and it is an object of the present invention to provide a malfunction prevention device for a ground fault overcurrent relay that can prevent malfunctions caused by magnetizing inrush current of a transformer.

[Means to solve the problem]

The above purpose is to form a device with a DC bias prevention circuit that is installed in the secondary circuit of a CT and has a current generating circuit that applies a DC current whose polarity is reversed corresponding to the excitation inrush current of the transformer to the secondary circuit of the CT. By doing so, they become distant relatives.

[Effect]

Since the above means is provided, DC bias magnetization of the iron core of the CT is prevented, and no unbalanced current flows in the zero-phase circuit of the CT.

In other words, we focused on the fact that the current generated in the primary of the CT due to the excitation inrush current of the transformer is only in one direction, either positive or negative, and this causes saturation in the core of the CT. We investigated ways to prevent malfunctions.

In a normal load current, a current with a waveform similar to a sinusoidal wave with balanced positive and negative values appears in the CT1st order, but in the case of an excitation inrush current, the current appearing in the CT1st order is shown in Figure 3. As shown in the figure, if you look at the l phase of the three phases R, S, and T, it appears as a current in only one direction, positive or negative. For example, the R-phase magnetizing inrush current is a positive-direction current, and the S-phase magnetizing inrush current is a negative-direction current.

On the other hand, when the magnetic flux generated by the current in the primary winding of the CT is in only one direction, such as an excitation inrush current, the iron core is magnetized in only one direction, and the magnetic flux generated by the current in the primary winding of the CT is magnetized in only one direction. The magnetization curve no longer follows, and the magnetization is biased in one direction. If this phenomenon continues for a long time, the iron core of the CT may reach saturation. In other words, as shown in Figure 4, which shows the magnetization of the CT iron core when a current with an amplitude in only one direction flows, such as the R phase described above, the magnetization of the CT iron core is shown in the same figure. As shown in (B), it depends on the area integral of the current i, so if a current i with a unidirectional amplitude flows, the magnetic flux density B of the iron core will increase by one amplitude, as shown in (B) of the same figure. If the magnetic flux density B increases significantly, saturation of the iron core will be caused. When this happens, CT
Normal current no longer flows in the secondary winding of the CT, and as shown in Figure 5, an unbalanced current flows in the zero-phase circuit of the sum of the secondary currents of the three CTs, causing an earth fault overcurrent. It was discovered that the relay was malfunctioning.

Therefore, in the present invention, since both the primary winding and the secondary winding have the same effect on the CT iron core, magnetic flux is generated by passing DC current through the secondary winding so as to cancel the magnetic flux caused by the CT's primary current. It was designed to act to prevent saturation of the CT core. That is, a malfunction prevention device is formed so as to apply a direct current whose polarity is reversed to match the excitation inrush current of the transformer to the secondary circuit (secondary winding) of the CT. By doing this, the CT does not become saturated and unbalanced current no longer flows in the zero-phase circuit of the CT, making it possible to prevent malfunctions of the earth fault overcurrent relay due to the transformer's excitation inrush current. It is possible to obtain.

〔Example〕

The present invention will be explained below based on the illustrated embodiments. An embodiment of the invention is shown in FIGS. 1 and 2. FIG. As shown in the figure, the ground fault overcurrent relay 3 installed in the zero-phase circuit of the three-phase CT2 installed in the primary on the three-phase transformer is overloaded by the excitation inrush current of the transformer. In this embodiment, a malfunction prevention device for the ground fault overcurrent relay 3 that prevents malfunction is formed as follows. A DC unbiased magnetization prevention gap M5 is provided in the secondary circuit of CT2 and has a current generation circuit Is4 that applies a DC current whose polarity is reversed to match the excitation inrush current of transformer 1 to the secondary circuit of CT2. By doing this, DC unbalanced magnetization of the iron core of CT2 is prevented, and unbalanced current no longer flows in the zero-phase circuit of CT2. A malfunction prevention device for the overcurrent relay 3 can be obtained.

In other words, CT is connected to the primary conductor of the main circuit of circuit breaker 6 and transformer 1.
2 was provided, and a DC bias prevention circuit 5 was provided in the secondary circuit (secondary winding) of the CT 2, which was converted into a current suitable for the overcurrent relay 7 and the ground fault overcurrent relay 3.

This DC bias prevention circuit 5 operates only when the current amplitude is significantly deviated in only one direction, and flows a DC current that is inverted according to the amplitude in one direction for a certain period of time.

Also, the settings are made so that the sum of the three phases becomes zero.

As shown in FIG. 2, the DC bias prevention circuit 5
The current flowing through the secondary winding 2c facing the primary winding IIA2b of T2 is converted into a voltage by the I/V converter 8 in the current generating circuit 4, and the current is introduced into the current generating circuit 4. The polarity determining circuit 9 detects the total amount of the positive and negative sums, determines whether it is zero or not, and outputs an output to the switching circuit 10 if the current is not zero or if the current is biased in one direction. Adjust so that it comes out. Load current and fault current have waveforms in both positive and negative directions, and the total amount is zero, so output permission is not issued.

When a positive excitation inrush current occurs as in the R phase shown in FIG. , only the positive direction is detected, and a permission signal is given to the positive output of the switching circuit 10, and no output permission signal is given to the negative output. At this time, the positive current selection amplification circuit 11 is a circuit that amplifies only the positive voltage given by the I/V converter 8 and outputs DC, and works effectively against the excitation inrush current.

The positive level is amplified and an output is given to the switching circuit 10. On the other hand, since the output of the negative current selection amplifier circuit 12 is zero for a voltage in the positive direction, it does not output an output for an excitation inrush current in the negative direction such as the S phase in FIG. As described above, since the voltage level output from the switching circuit 10 is in the positive direction, the V/I conversion circuit 13 is set to output a negative current commensurate with this.

This creates a magnetic flux φ to cancel the magnetic flux φ□ generated by the excitation inrush current (positive direction) in the secondary winding 12c of CT2.
By superimposing the negative DC current 2 in order to generate 2, saturation of the iron core 2a of the CT 2 can be prevented. Similarly, in the case of excitation inrush current (negative current), the positive and negative currents are reversed and work effectively.

In addition, the load current level detection circuit 14 supplies the current level superimposed on the secondary winding 2c of the CT2 to the three-phase total current adjustment circuit 15, and the three-phase total current adjustment circuit 15 monitors the DC current superimposed on each phase. , adjust the total current of the three phases to zero.

This prevents the unbalanced current of the three-phase total current from flowing into the zero-phase circuit, which is the sum of the secondary circuits of the CT 2, and prevents the ground fault overcurrent relay 3 from malfunctioning. That is, even if a magnetizing inrush current occurs, the CT2 is not saturated, and as a result, the magnetizing inrush current does not cause the ground fault overcurrent relay 3 to malfunction.

In this way, according to this embodiment, when the excitation inrush current occurs, C
By suppressing the zero-phase unbalanced current caused by saturation due to DC bias magnetization of T, it is possible to prevent malfunction of the ground fault overcurrent relay and detect ground wire faults, which is effective for the primary protection circuit of the transformer. There is.

〔Effect of the invention〕

As described above, the present invention provides a malfunction prevention device for a ground fault overcurrent relay, which prevents malfunction of the ground fault overcurrent relay due to the magnetizing inrush current of the transformer. can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the device configuration of an embodiment of the malfunction prevention device for a ground fault overcurrent relay of the present invention, FIG. 2 is a front view of a CT showing prevention of DC bias magnetization according to the embodiment, The figure shows the 071st order current waveform when the excitation inrush current is generated. Figures 4 (a) and (b) show the magnetization of the CT core when the excitation inrush current occurs, and (a) shows the time t and the current i. (b) is a characteristic diagram showing the relationship between time t and magnetic flux density B, and FIG. 5 is a CT secondary current waveform diagram when an excitation inrush current is generated. 1...Transformer, 2...CT, 2c...Secondary winding! (
Secondary circuit), 3... Earth fault overcurrent relay, 4... Current generation circuit, 5... DC bias prevention circuit, 8... r/V
converter. 9...Polarity discrimination circuit, 10...Switching circuit, 11...
- Positive current selection amplifier circuit, 12... Negative current selection amplifier circuit, 13... V/I conversion circuit, 14... Additional current level detection circuit, 15... Three-phase total current adjustment circuit. Fig. 1 1...Transformer 2...CT 3...Ground fault overcurrent R9 relay 4...Current generating circuit 5...Direct R911 + anti-magnetic am 8...1/■ converter 9... Polarity discrimination circuit 10...Switching circuit 11...Positive current selection amplifier circuit 12...Negative current selection amplifier circuit 13...V/■ conversion circuit 14...Additional current level detection circuit 15...3 Phase total i current adjustment circuit Fig. 3z Fig. (a) Fig. Zero-sequence current generation due to unbalance

Claims (1)

[Claims] A ground fault overcurrent relay provided in a zero-phase circuit of a three-phase CT provided in the primary of a one- and three-phase transformer is prevented from malfunctioning due to the excitation inrush current of the transformer. In the device for preventing malfunction of a ground fault overcurrent relay, the device
It is formed of a DC unbalanced magnetization prevention circuit that is provided in the secondary circuit of the CT and has a current generation circuit that applies a DC current whose polarity is reversed to match the excitation inrush current of the transformer to the secondary circuit of the CT. A malfunction prevention device for a ground fault overcurrent relay, characterized by: 2. The current generation circuit includes an I/V converter connected to the secondary circuit of the CT, a positive current selection amplifier circuit, a negative current selection amplifier circuit, and a polarity discrimination circuit connected to the I/V converter. , a switching circuit connected to these amplifier circuits and polarity determination circuits, a V/I conversion circuit connected to this switching circuit, and a V/I conversion circuit connected between this V/I conversion circuit and the I/V converter. 2. A malfunction prevention device for a ground fault overcurrent relay according to claim 1, which comprises an additional current level detection circuit. 3. The DC unbalanced magnetization prevention circuit is composed of the current generation circuit and a three-phase total current adjustment circuit connected between the V/I conversion circuit and the additional current level detection circuit of this circuit. A malfunction prevention device for a ground fault overcurrent relay according to claim 1. 4. The malfunction prevention device for a ground fault overcurrent relay according to claim 1, wherein the DC bias prevention circuit is built in the CT.