RU2529684C2 - Method of selective protection from earth-faults in electrical network with small earth-fault currents - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention is used in electrical engineering. According to the method the zero-sequence currents of all outgoing connections are measured, the connection currents exceeding a minimum established level are selected, the phase shifts of zero-sequence currents of all outgoing connections with reference to the base signal are determined, the signal with minimum phase shift with reference to the base signal is determined, the decision is made regarding the feeder, where the current has minimum phase shift, is damaged, meanwhile the instantaneous values of zero-sequence currents of all connections are summarised, and the base signal is formed on the basis of this sum. In case of presence in the network of earthing inductor, resistor or their combination, the current in the grounding circuit is measured, and this current in the grounding circuit, is accepted as a base signal.

EFFECT: reliability improvement.

2 cl, 3 dwg

Description

The invention relates to the field of electrical engineering and electric power industry, in particular, to relay protection, providing detection of connections with a single-phase earth fault in an electrical network with isolated or grounded through inductive, active or active-inductive neutral 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 at the operating frequency of the network is measured, the measurement results 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 feeder. At the same time, there is no need to directly measure the magnitude of the currents and select the appropriate settings, as is customary in other protections. The essence of the algorithm is that the signs of the zero sequence current in the outgoing connections with respect to the sign of the current of the supply feeder are encoded as a logical unit when the signs of the compared currents coincide or as a logical zero if they do not match or the connection currents are low.

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 conditions of detuning from interference, for example, caused by network switching. A positive signal at the output indicates the coincidence of the signs of the currents, a negative signal indicates their mismatch. 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 detect a damaged connection when the power is lost on the input side, the current of which is used as the basic signal, or when the power is transferred to another source, the device loses its directivity function. In case of loss of direction, the protection may not work correctly when it is shorted to ground at one of the connections, because currents arising from earth faults on damaged and undamaged elements, especially in a compensated network, have insufficiently clear and stable differences.

The aim of the proposed method is to increase the reliability of directional protection against earth faults through the use of an independent base signal.

This goal is achieved by the fact that when determining the connection with a single-phase earth fault, a special basic signal is used to evaluate the phase shifts of the currents of the zero sequence of connections. If the network is made with isolated neutral, then the sum of instantaneous values of tones of the zero sequence of all outgoing connections of this section is taken as a special basic signal. If the neutral is grounded through active, inductive or active-inductive resistance, then the ground signal is taken as the basic signal.

This method allows you to do without introducing into the device an additional voltage signal or current of the supply feeder, relative to which the current phase of each feeder is determined, which improves the reliability of the device.

Figure 1 presents a functional diagram of a method for determining a damaged connection, consisting of unit 1 of an analog-to-digital converter that converts the secondary currents (i _{0_1} - i _{0_n} ) of current transformers {TA1-TAn) of all feeders into a digital signal, unit 2 for determining the effective values (l _{0_1} - l _{0_ n} ) and phases of signals (φ _{0_1} -φ _{0_ n} ), block 3 of the starting element, block 4 of the selection of signals with a value higher than the minimum specified level, block 5 of the formation of the virtual base signal, block 6 of the signal definition with the minimum phase shear relative to the base signal.

Figure 2 shows a vector diagram of the location of the base signal.

Figure 3 shows a vector diagram of determining the phase shifts of the currents of the feeders relative to the virtual base signal.

The currents of all feeders in block 1 are converted to digital signals. In block 2, the effective values of the currents and phases of the signals of all feeders are determined.

Block 3 is the starting body. If one of the currents exceeds the setting level, commensurate with the current of a single-phase circuit, a signal is sent to the ground to determine the damaged connection.

Next, in block 4, signals are selected whose level is higher than a specified minimum level. From these signals, by adding their instantaneous values in block 5, a virtual basic signal I _{Σ is formed} .

In block 6, a signal is determined whose phase shift has a minimum value relative to the virtual base signal. A feeder with this current is considered damaged. If the value of the phase shift exceeds 180 °, then it is not the minimum and its value is replaced by the value (360-φ) °. The feeder whose phase shift is the smallest relative to the base signal is damaged.

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 to voltage circuits and the transfer of power of controlled connections to another source are possible. In addition, there is no need for a zero-sequence current sensor and a signal cable for the feeder.

Information sources

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

2. Copyright certificate, USSR, No. 792421, H02H 3/16, 1980.

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

Claims (2)

1. A method for determining a damaged feeder during a single-phase earth fault in a network with low earth fault currents, in which zero sequence currents of all outgoing connections are measured, connection currents exceeding the minimum set level are selected, phase shifts of zero sequence currents of all outgoing connections relative to the base are determined signal, a signal with a minimum phase shift relative to the base signal is detected, a decision is made that the feeder, in which the current has t the minimum phase shift is damaged, characterized in that the instantaneous values of the zero sequence currents of all outgoing connections are summed up and a basic signal is generated from this sum. 2. A method for determining a damaged feeder during a single-phase earth fault in a network with low earth fault currents, characterized in that in the presence of a grounding reactor, resistor or combination thereof, the current in the grounding circuit is measured and this current in the grounding circuit is taken as the base signal.

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