RU2565060C2 - Method of centralised selective earth-fault protection in electrical distribution system - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention is used in the field of electric engineering and power engineering. According to the method the zero sequence currents of all outgoing connections are measured, the currents of connections exceeding the minimum pre-set level are selected, the basic signal is formed. The measured zero sequence currents of all outgoing connections are compared in the line of the basic signal if the direction of current of one connection is close to the direction of the basic signal, then the signal of damage of this connection is formed.. If directions of currents of all connections are close to the direction of the basic signal, the signal of damage on buses or in a winding of the feeding transformer is formed.

EFFECT: improvement of reliability and increase level of selectivity of earth-fault protection.

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Description

The invention relates to the field of electrical engineering and electric power industry, in particular to relay protection, which provides for the identification of elements with a single-phase earth fault in an electrical network with isolated or grounded through inductive, active or combined resistance.

In relay protection, a method is used to determine the damaged connection during single-phase earth faults in networks with isolated neutral or neutral, grounded through an arcing reactor or resistor, based on a comparison of the values of zero sequence currents in each feeder [1]. But the currents that occur when an earth fault occurs on damaged and undamaged elements, especially in a compensated network, do not have sufficiently clear and stable differences.

Also known is a method of directional protection against single-phase earth faults in AC distribution networks, which is implemented in a device for directional protection of a zero sequence from single-phase earth faults [2]. In accordance with the method, the phase difference between the current and voltage signals of the zero sequence is measured at the operating frequency of the network, the results of measurements are compared with a given interval of phase angle angles. The specified method has such a disadvantage as low reliability of voltage circuits, in case of malfunction of which the method loses its directivity property.

The closest technical solution is the method of high-speed selective protection against single-phase earth faults [3], using only the directions of the currents of the zero sequence of connections relative to the current of the supply feeder, while there is no need to directly measure the currents and select the appropriate settings, as is the case in other protections . The essence of the algorithm is that the directions of the zero sequence current in the outgoing connections with respect to the direction of the current of the supply feeder are encoded as a logical unit when the compared currents are in phase or as a logical zero when they are out of phase or have a low level of connection currents. The instantaneous values of the signals from the zero-sequence current sensors of the outgoing lines are multiplied with the instantaneous value from the zero-sequence current sensor supplying the line node. The integration time is selected from the condition of detuning from interference, for example, caused by network switching. A positive signal at the output indicates the out-of-phase currents, a negative signal indicates antiphase. The coding of signals is carried out by comparing each with a zero sequence current unbalance setting selected depending on the type of sensors and the parameters of the protected distribution network. If the signal value is less than or equal to the setting, then it is assumed to be zero, if it is more, then the signal is normalized to a value of 1. Damaged is considered to be an outgoing feeder, the encoding of the result of which gives one, provided that the encoding of the others gives zero.

However, the known method does not allow to identify a damaged element during a single-phase circuit on the tires or in the winding of the supply transformer connected to the bus. In this case, the conditions for choosing a damaged connection are not met for any of the outgoing connections, and the protection may not work correctly, because uncertainty arises: there is a single-phase earth fault in the network, and a damaged connection is not defined.

The aim of the proposed method is to increase the reliability and selectivity of protection against earth faults.

This goal is achieved by the fact that in the known method of centralized selective protection against earth faults in a distribution electric network, in which zero-sequence currents of all outgoing connections are measured, connection currents exceeding the minimum set level are selected, a basic signal is generated, measured zero-sequence currents of all outgoing connections the connections are compared with the base signal, the damaged outgoing connection is determined when determining the element mains with a single-phase earth fault, a new operation is performed: if the conditions for choosing a damaged connection are not met for any of the outgoing connections, it is decided that the earth fault occurred on the busbars or in the winding of the supply transformer.

A comparative analysis of the proposed solution with the prototype showed that the claimed method differs from the known one in that in case of an earth fault on the tires or in the winding of the supply transformer, the conditions for selecting a damaged connection are not fulfilled for any of the outgoing connections, and it is decided that the circuit is closed ground has occurred on the busbars or in the winding of the supply transformer. Thus, the claimed method meets the criterion of "novelty."

Figure 1 presents a functional diagram of a method for determining a damaged connection.

Figure 2 shows a vector diagram of the currents of the zero sequence of connections in case of damage at the first outgoing connection. The following notation is used on vector diagrams: I _w1 , I _w2 , I _wn are the current vectors of the zero sequence of the first, second and nth connections, respectively, I _B is the vector of the base signal.

Figure 3 shows the vector diagram of the currents of the zero sequence when damaged on the tires.

The method of centralized selective protection against earth faults in a distribution electric network can be implemented by a digital device in accordance with the functional diagram shown in figure 1. Zero-sequence currents of all feeders are measured by a digital transmitter (block 1). In this case, the digital measuring transducer can be implemented both with individual measuring modules located in the feeder cells, and with measuring modules located in the block 1 itself.

In block 2, connection currents exceeding the set level are selected. Block 2 is the starting body. If at least one of the monitored currents exceeds the set level, operations to determine the damaged connection begin to be performed.

In block 3, a basic signal is generated.

In block 4, the measured zero sequence currents of all outgoing connections are compared with the base signal, and the number of the damaged outgoing connection is determined.

If a single-phase earth fault occurred in one of the outgoing connections, for example, on line W1 (point K1), then in block 4 the number of the damaged outgoing connection is determined and a signal corresponding to this number is generated. In this case, for example, the direction of the zero-sequence current vector of the damaged connection W1 I _w1 will be close to the direction of the vector of the basic signal I _B (Fig.2).

If a single-phase earth fault occurred on the substation's tires (at point K2) or in the transformer winding T (point K3) connected to this bus, then the number of the damaged outgoing connection will not be determined in block 4, and no signal will be generated at its output . The direction of the zero-sequence current vectors of all the connections in this case will be close to the direction of the base signal vector I _B (Fig. 3). In this case, in block 5, a decision is made that a single-phase earth fault has occurred on the buses or in the transformer winding, and the corresponding signal is generated.

Using the proposed method for determining a damaged feeder during a single-phase earth fault allows to increase the reliability of the protection in conditions where damage is possible on the buses of the power substation and in the winding of the power transformer connected to the bus.

Information sources

1. Andreev V.A. Relay protection and automation of power supply systems: Textbook. For universities - 3rd ed. - M .: Higher. Shk., 1991, p. 213.

2. USSR author's certificate No. 792421, Н02Н 3/16, 1980.

3. RF patent No. 2410812, Н02Н 3/16, 03/10/2009.

Claims (1)

A method of centralized selective protection against earth faults in a distribution electric network, in which zero-sequence currents of all outgoing connections are measured, connection currents exceeding the minimum set level are selected, a basic signal is generated, measured zero-sequence currents of all outgoing connections are compared in direction with the basic signal, if the direction of the current of one connection is close to the direction of the base signal, a damage signal is generated this exhaust connection, characterized in that the directions of the currents if all electrical connections close to the direction of the base signal, the signal formed of the damage to the tires or in the winding of the supply transformer.

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