RU2321126C1 - Line backup protective gear for grounded-neutral mains - Google Patents

Abstract

FIELD: relay protection; line backup protective gear.

SUBSTANCE: proposed protective gear depends for its operation on detection of maximal and minimal phase currents, evaluation of difference between maximal and minimal values, and comparison of this difference with double value of minimal current. To this end protective gear has three current-to-voltage converters for phases A, B, C made in the form of transformer-reactor with first and second secondary windings, electrical quantity comparison unit, time relay, and subtracter. Newly introduced in protective gear are maximum selector, minimum selector, and reference value generator.

EFFECT: enhanced sensitivity of backup protective gear.

1 cl, 1 dwg

Description

The invention relates to electrical engineering, namely to relay protection, and can be used for backup protection of lines in networks with a grounded neutral.

A device for backup protection of lines against short circuits, containing a three-transformer filter and a current relay connected to its output (Fedoseyev A.M., Fedoseyev MA Relay protection of electric power systems. M .: 1992, 528 S.).

However, it has low sensitivity for long high voltage lines, since the zero sequence current flows through all the grounded neutrals of the network.

The closest to the proposed technical essence and the achieved effect is a device for backup line protection, containing a subtractor, a comparison unit, a time relay connected to the output of the comparison unit, and for each phase a current to voltage converter connected to the current transformer of this phase (Kletsel M .I., Nikitin KI Backup line protection, reacting to the difference between the modules of the phase currents and their increments. Electricity, 1993, No. 10, pp. 23-26).

The disadvantage of this device is the need to isolate itself from currents caused by close asymmetrical faults at adjacent connections, which significantly reduces the sensitivity of the protection to earth faults.

The technical result of the invention is to increase the sensitivity of backup line protection to short circuits to earth.

The technical result is achieved by the fact that in the line backup protection device comprising a subtracter, a comparison unit, a time relay connected to the output of the comparison unit, and for each phase, a current to voltage converter connected to a current transformer of this phase, an additional maxi selector, mini -selector and driver of the reference voltage, and the current to voltage converter is made in the form of a transreactor with the first and second secondary windings, the maxi-selector inputs are connected to the outputs of the first secondary windings t of the rereactor, and the mini-selector is connected to the second secondary windings, the subtraction unit is connected to the outputs of the maxi-selector and mini-selector, the reference voltage unit is connected to the output of the mini-selector, the comparison unit is connected to the outputs of the subtractor and the reference voltage driver.

The drawing shows a structural diagram of a device backup line protection.

The device contains current to voltage converters 1, 2, 3, connected respectively to phases A, B, C of the line, maxi-selector 4, inputs connected to the first outputs of converters 1, 2, 3, mini-selector 5, inputs connected to second outputs converters 1, 2, 3, subtractor 6, inputs connected to the outputs of maxi-selector 4 and mini-selector 5, driver 7 of the reference voltage, input connected to the output of mini-selector 5, block 8 comparison, inputs connected to the outputs of elements 6 and 7 and output to time relay 9.

The device operates as follows.

In any mode, converters 1, 2, 3 convert current to voltage. From the signals received from the outputs of the converters 1, 2, 3, using the maxi-selector 4 and mini-selector 5, the minimum and maximum values are selected. They are proportional to the phase currents I _max and I _min. Further, in the shaper 7 of the reference value, the value of the minimum value increases by one and a half times, and in the subtractor 6, the minimum value is subtracted from the maximum. Then the output of the subtractor 6 will receive a voltage proportional to the current I _V :

Then, in comparison unit 8, this voltage is compared with a reference value proportional to 1.5 · I _min . If

then a signal is input to the time relay 9.

In the symmetric load mode, the current values in the phases are equal to each other. Therefore, there are no signals at the outputs of the maxi-selector and mini-selector, and the device does not work.

In unbalanced load mode, the difference between the phase currents can reach 50% (with a close two-phase fault on an adjacent line). Therefore, signals corresponding to the maximum and minimum currents in phases appear at the outputs of the maxi-selector and mini-selector. For example, if I _A = I _nom , I _B = 0.5 · I _nom and I _C = 0.5 · I _nom (here I _nom is the rated load current), then the values I _max = I _nom and I _min = 0 5 · I _nom . At the output of the subtractor 6, we have a voltage proportional to the current I _V :

_The I = I _max -I _min = I -0,5I _{SG SG SG} = 0,5I.

In the driver 7 of the reference voltage, the value of this voltage at the output becomes proportional to the value of 0.75 · I _nom . Since the value of I _B <0.75 · I _nom , then the output of block 8 of the comparison signal will be absent. Therefore, the device will not work.

The single-phase short-circuit mode, for example phase A, the output selector value maxi voltage proportional to I _max = I _K, and the output selector mini - I _min = 0. At the output of the subtractor 6, we have a voltage proportional to the current:

I _B = I _max -I _min = I _K -0 = I _K.

Since I _min = 0, then at the output of the shaper 7, the voltage value proportional to the current will be zero. The voltage value proportional to I _V becomes larger than the value received from the shaper 7, therefore, a signal will appear at the output of the comparison unit 8, which will be fed to the input of the time relay 9, and the latter will signal to turn off the line switch.

Technical and economic efficiency lies in the fact that the proposed device has greater sensitivity than known, and therefore eliminates the possibility of destruction of the lines, switching elements and transformers supplying the line, with distant single-phase short circuit on high voltage lines in case of failure of the main protection.

Claims (1)

A line backup protection device for networks with a grounded neutral, comprising a subtractor, a comparison unit, a time relay connected to the output of the comparison unit, and for each phase, a current to voltage converter connected to a current transformer of this phase, characterized in that it is additionally introduced with maxi -selector, mini-selector and driver of the reference voltage, and the current to voltage converter is made in the form of a transreactor with the first and second secondary windings, the maxi-selector inputs are connected to the outputs of the first volt of the transreactor’s original windings, and the mini-selector is connected to the second secondary windings, the subtractor is connected to the outputs of the maxi-selector and mini-selector, the voltage shaper is connected to the output of the mini-selector, and the comparison unit is connected to the outputs of the subtractor and the voltage reference shaper.