RU2667064C1 - Method for protecting three-phase network against overloading and short circuit - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: way to protect a three-phase network from overload and short circuit is that the input signals proportional to the instantaneous phase currents are differentiated, after which it is squared, the received signals are summed, obtaining a generalized signal that is compared with the set voltage value, and in case of exceeding by the generalized signal of the set value of voltage by means of the comparator, a signal is issued to disconnect the overloaded feeder.

EFFECT: increased speed of response when determining the time of the overload or short-circuit of a three-phase source.

1 cl, 3 dwg

Description

The invention relates to the field of protection of electrical networks from overload and short circuit, namely three-phase networks, and can be used in centralized control systems of parameters.

The closest analogue of the claimed invention is a technical solution from patent RU 2163423. The method consists in measuring the instantaneous values of the derivative of the electrode current with respect to time di _j (t) / dt and the instantaneous values of phase voltages u _j (t) from the low voltage side of each transformer phases of the electric furnace during the entire period of current change; signals are proportional to the first and third harmonics of the derivative of the electrode current and phase voltage for each phase; the in-phase components of these signals are determined with respect to each of the harmonics of the derivative of the electrode current for each electrode individually, and the magnitude of the parameters of the inductive interaction between the phases of the three-electrode electric furnace is calculated using these in-phase components by solving a system of algebraic equations.

However, the disadvantage of the proposed method is the need to separate the first and third harmonics, both for the derivative of the current signal and for the phase voltage signal, which reduces the speed of the proposed solution, negatively affects the speed and requires the need to solve a system of algebraic equations.

The technical problem to be solved by the claimed invention is directed is the insufficient speed of the known methods for protecting a three-phase network from overload and short circuit.

The technical result of the claimed invention is to improve performance when determining the time point of an overload or short circuit of a three-phase source.

The specified technical result is achieved due to the fact that in the method of protecting a three-phase network from overload and short circuit, the input signals proportional to the instantaneous phase currents are differentiated, then squared, the received signals are summed, obtaining a generalized signal that is compared with the set voltage value, and if the generalized signal exceeds the set voltage value by means of a comparator, a signal to turn off the overloaded feeder is issued.

A generalized signal during overload or short circuit unambiguously determines the time instant of a short circuit or overload and the magnitude of the current overload.

The disclosure of the claimed invention is shown using FIG. 1-3, which depict:

in FIG. 1 - design diagram of the inclusion of a three-phase load;

in FIG. 2 is a block diagram of the implementation of the proposed method;

in FIG. 3 - waveforms.

In FIG. 1-3 items 1-16 show:

1 key;

2 - complex impedance of the load to the circuit of the key;

3 - complex load impedance after circuit lock;

4 - the first differentiating block;

5 - second differentiating block;

6 - the third differentiating block;

7 - the first block of multiplication;

8 - the second block of multiplication;

9 - the third block of multiplication;

10 - adder;

11 - a comparator;

12 - non-inverting input of the comparator;

13 - inverting input of the comparator, which serves the set voltage value;

14 - phase current;

15 - zero voltage value;

при возникновении трехфазного К3.16 - generalized parameter

when a three-phase K3 occurs.

при допущении, что сеть большой мощности.For clarity and understanding of the proposed method, the process of switching on a three-phase load by closing the key 1 (Fig. 1) at a time

under the assumption that the network is high power.

The load equation has the form:

where U _S is the module representing the network voltage vector;

Z _H = R _H + jwL _H - complex 3 of the load impedance after closing key 1;

R _H - the active phase resistance of the load after closing the key 1;

L _H - inductance of the load phase after the closure of key 1;

Z ₀ = (R ₀ + R _H ) + jw (L ₀ + L _H ) - complex 2 of the load impedance to the closure of key 1;

γ ₀ - random phase of the inclusion of the load;

,

,

- производная от мгновенного фазного тока.

,

,

- derivative of instantaneous phase current.

The solution of the differential equations presented above has the form:

where ϕ ₀ = arctan [w (L ₀ + L _H ) / [R ₀ + R _H )] is the phase of the current relative to the voltage until the key 1 closes;

i _a , i _b , i _c - signals proportional to instantaneous phase currents;

ϕ _H = arctan (wL _H / R _H ) is the phase of the current relative to the voltage after closing key 1;

T _a = L _H / R _H - time constant of the load circuit after the closure of key 1;

γ = wt is the current angle.

To diagnose the occurrence of the moment of current overload or short circuit (short circuit), as well as the magnitude of the overload, a generalized value equal to the sum of the squares of the derivatives of the phase currents is used

,

,

- производные по времени от мгновенных фазных токов.Where

,

,

- time derivatives of instantaneous phase currents.

Using the substitution into this expression of the derivatives of the instantaneous phase currents presented above, the following expression holds:

At time (0-), preceding the inclusion of key 1, the proposed generalized value (i ') ² is

Since at this moment in time Z _H = Z ₀ , ϕ _H = ϕ ₀ .

Given that w ² = (2πƒ) ² = const, the expression holds:

where I _S - module depicting the current vector of the three-phase system

At time (0+), immediately after turning on key 1, the proposed generalized value (i ') ² is

до величины

.Thus, when the key 1 is turned on, the generalized quantity (i ') ² changes “stepwise” from the quantity

up to

.

It is this property that allows you to determine the moment and magnitude of the current overload.

The method is as follows.

Input signals proportional to the instantaneous phase currents i _a , i _b , i _c , are differentiated using differentiating blocks 4-6, and then squared using multiplication blocks 7-9. The received squares signals from the derivatives of the instantaneous phase currents are added to the adder 10. The signal with the value

where i _a , i _b , i _c are signals proportional to instantaneous phase currents; arrives at the non-inverting input 12 of the comparator 11, the inverting input 13 of which serves the set voltage value U _set .

When the voltage of the signal supplied to the input 12 of the comparator 11 is exceeded, the value of the set voltage value U _{mouth is} exceeded, the output signal of the comparator 11 abruptly changes from 0 V to +15 V. The operation of the current protection depends on the output signal of the comparator 11, which disconnects the overloaded feeder. Changing the value of the set voltage value U _mouth , change the value of the set value of the voltage operating current protection.

при возникновении трехфазного КЗ. До КЗ цепь была нагружена током примерно в 3 раза меньше, чем во время КЗ.The proposed waveforms (Fig. 3) are obtained by recording the output signal of the adder 10 at short circuit. Phase current i _a is represented by curve 14, “0” - zero voltage value - direct line 15. Curve 16 shows a generalized parameter

when a three-phase short circuit occurs. Before the short circuit, the circuit was loaded with current about 3 times less than during the short circuit.

The above examples are special cases and do not exhaust all possible implementations of the claimed invention.

The specialist in the art should understand that various variations of the proposed method do not change the essence of the invention, but only determine its specific embodiment.

Claims (1)

A method of protecting a three-phase network from overload and short circuit, characterized in that the input signals proportional to the instantaneous phase currents are differentiated, then squared, the received signals are summed, obtaining a generalized signal that is compared with the set voltage value, and if the generalized value is exceeded the signal of the set voltage value through the comparator gives a signal to turn off the overloaded feeder.

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