SU1339725A1 - Method of grounding protection in three-phase electric mains with isolated neutral wire - Google Patents

Abstract

The invention relates to electrical engineering and is intended to protect the network from earth fault. The aim of the invention is to increase the speed and noise immunity of protection during transients in the network. In the event of a leakage in one of the network phases, the outputs of the voltage sensors 1 cause an increase in the phase voltages and the neutral bias voltage, which are allocated in blocks 2-5 of the selection discrete increments. o :) co wore

Description

In Comparators 9-16, these signals are compared with setpoints, when they are exceeded, at the outputs of the comparators logical signals are generated, which are supplied for analysis on the elements 18 and 19 of the coincidence, Depending on the sign of the increments on

one

The invention relates to electrical engineering, in particular to fast-acting earth fault protection in three-phase electrical networks with insulated neutral, made with shielded cable, and can be used to ensure the safe use of electrical energy in mines, dangerous by sudden emissions of coal and gas.

The purpose of the invention is to increase the speed and noise immunity of protection during transients in the network.

Figures 1 and 2 show voltage distribution diagrams in a three-phase network with an insulated neutral, illustrating the formation of phase voltage increments and a neutral displacement voltage during a single-phase earth fault through a finite conduction leak; on fig.Z - the same, with a short circuit between the two phases b and C; Fig, 4 is a functional diagram of the device that implements the proposed method.

Figs. 1 and 2 show the voltage diagram of the phases U, Ug, Uj relative to the earth with the same resistance, phase isolation, voltage U Ug, U (- and the neutral voltage Uo when the phase A closes to ground through the leakage of the final conductivity. The corresponding instantaneous values of the phase voltages are also shown here), Uj, (to), Uc (to) until the occurrence of the leakage, as well as the phase voltages U (t), Uj (t), Uj, ( t,) and neutral displacement voltage Up (t) after the occurrence of leakage in phase A. On the basis of these data, the increments of all of the specified output of one of the elements Ntov 18 or 19, a signal of a logical unit is generated, which through the OR 20 circuit arrives at the output of the device, forms a command to turn off the monitored section of the network. 4 il.

Occurrences occurring after the left of the leakage with respect to the moment preceding the occurrence of leakage.

A. (kt) and, (-U. (K-t),

where i A, B, C, O are the network and neutral phases. From here

dU.Ctgk) - uUbCt -k) k)

CD -dU, (t, .k).

The time interval t between two successive moments of voltage measurements does not exceed 0.001 s.

Using an arrester, it is possible to continuously determine the increments of the instantaneous values of the phase voltages and the neutral displacement during time t and, comparing them to each other, generate a trip signal according to the condition of the fulfillment of relation (1), if the increments of the voltages exceed the response values of the corresponding elements .

Fig. 3 shows voltage diagrams in the event of a short circuit between phases B and C, from which it follows that condition (1) does not hold — protection does not work,

Functional device diagram

(FIG. 4) contains three phase voltage divider sensors and a neutral voltage voltage divider sensor (block 1) connected to the controlled network,

voltages from which through blocks 2–5 of the increment of voltages, controlled by a generator of 6 pulses.

arrive at the inputs of the comparator blocks of positive 7 and negative 8 increments consisting of comparators 9-16, the outputs of which are st - heads with the inputs of logic unit 17 containing two AI elements (18 and 19) to the inputs of each of which the signals of the phase increment comparator are fed the voltage of one character and the signal of the comparator increment the voltage of the neutral of another character, and their outputs are connected to the inputs of the element OR 20,

The device works as follows.

With the help of voltage sensors 1 connected to a three-phase network, instantaneous values of phase voltages U (t), Ug (t), U (t) and the bias voltage of the neutral are measured. These voltages are applied to discrete increment blocks 2-5.

voltage of LUd (ta), dUg (ta), 4U | (tg)

and AUjCtn). The increment signals from the outputs of blocks 2-5 are fed to the inputs of comparators 9-16, which perform

threshold element function - comparison with trigger settings,

From the outputs of comparators 10–13, signals arrive at the coincidence circuit (element 18), and from the outputs of the co mars of tori 9 and 14–16, to circuit 19 coincidence.

When a leakage occurs in one of the phases of the network, the phase voltages and the bias voltage of the neutral change their values dramatically, respectively, the increment of these voltages increases. When these voltages exceed the thresholds of the comparators, which are dependent on the setpoint, the logic signals are generated at their outputs, which are fed to the logical block 17 for analysis (elements 18-20). If condition (1) is satisfied, the output of one of. elements 18 or 19 matches, depending on the sign of the increments, forms

with a signal of a logical unit, which through the scheme OR 20 goes to the output of the device - the command to turn off the controlled part of the network.

The use of the proposed method provides, in comparison with the known ones, the following advantages: there is no dead band, which occurs when a leakage occurs during the passage of a phase voltage across zero; increases the speed of the devices built by the proposed method; ensures robust protection during transients in the network.

Claims (1)

Invention Formula A method of protecting against earth faults in a three-phase electrical network with an insulated neutral, including automatic control of the instantaneous value of the neutral bias voltage and generating a signal to turn off the monitored network while increasing the controllable parameter of a given value, which is increase the speed and noise immunity of protection during transients in the network, additionally control the instantaneous values of the voltages of the phases of the protected network relative to the ground, the values of the increments of the phase voltages and the neutral bias voltage for the same time intervals are compared, the values of the increments are compared with each other by the sign and form a signal to turn off the monitored network under the condition that the signs of the increments of the phase voltages coincide with the ground and the increment sign does not match them displacement voltage of the neutral, as well as under the condition that the absolute values of these increments exceed the specified value. 1 and V (t,} 0 41  a, if, n),) Editor A. Kozoriz Fig.Z Compiled by S. Khrenov Tehred M. Khodanych Order 4235/47 Circulation 617 Subscription VNIIPI USSR State Committee for inventions and discoveries 113035, Moscow, Zh-35, Raushsk nab., 4/5 Production and printing company, Uzhgorod, st. Project, 4 Proofreader V.Girn to