RU1771031C - Device for current protection of electric motor from phase-to-phase short-circuits - Google Patents

Abstract

Usage: in the electric power industry. Summary of the invention: the introduction of an AND element with one direct and two inverse inputs, an OR element and a third current relay connected to the sum of the current transformers of phases A and C from the input side of the power supply and phase B from the side of the zero motor leads reduces metal intensity of the protection device. 5 ill.

Description

The invention relates to the electric power industry, in particular to a relay protection technique, and can be used as a protection of an electric motor against phase-to-phase short circuits.

A device for overcurrent protection of a motor is known, comprising current transformers on the power supply side of the motor and a current relay. Its disadvantage is insufficient sensitivity to phase-to-phase fault in the stator winding of the electric motor.

The closest to the proposed technical essence and the achieved effect is 2 device for current protection of the electric motor against interphase short circuits, containing the first and second current transformers installed on the input side of the power supply in phases A and C, respectively, and the third current transformer installed on the zero side conclusions, the first current relay with a make contact, included in the secondary

a circuit of a first current transformer, a second current relay with a make contact, and an actuator. In the practice of operation, protection made in a two-phase design, i.e. its implementation requires four current transformers and two differential relays. The installation of current transformers in phases A and C from the side of the zero terminals complicates the design of the electric motor.

The purpose of the invention is to reduce metal consumption.

This goal is achieved by the fact that in the device for current protection of the motor against interphase short circuits, containing the first and second current transformers installed on the power input side in phases A and C, respectively, and a third current transformer installed on the zero-side side, the first current relay with make contact included in the secondary circuit

cl

h xj

about

s

„“ L

a first current transformer, a second current relay with a make contact, an actuator, an additional third current relay with a make contact, and AND and OR elements are added, the third current transformer designed to be installed in phase B, the second current relay connected to the secondary circuit of the second current transformer , the third current relay is switched on for the sum of currents of current transformers, the closing contacts of the first and second current relays are connected to the direct and first inverse inputs of the element And, the output of which is connected to th input of the OR gate, make contact third current switch connected to the second inverted input AND gate and a second input of the OR gate, the output of the last connected to the input of the actuator.

Comparative analysis with the prototype shows that the claimed device, in contrast to the prototype, additionally contains a third current relay, an And element with one direct and two inverse inputs and an OR element, and the third current relay is switched on by the sum of the current transformers of phases A and C from the input side of the power supply and phase B from the side of the neutral terminals of the electric motor. Thus, the claimed device meets the criteria of the invention of novelty.

Comparison of the claimed solution with other technical solutions shows that current transformers, relays, AND element with one direct and two inverse inputs, OR element are widely known. However, when introduced into this connection, the proposed device exhibits new properties, which leads to a saving in the number of current transformers and simplification of the design of the electric motor. This allows us to conclude that the technical solution meets the criterion of significant differences,

In FIG. 1 shows a diagram of a device for protecting an electric motor from interphase short circuits, where 1 is an electric motor; 2-4 - current transformers; 5, b, 7 - relays, moreover, relays 5, 7 are connected in series in the circuit of phase A current transformers from the input side and phase B from the zero output side, respectively, relay 6 is connected to the sum of the current transformers of phase A and C currents from the input and phase side In from the side of the zero terminals of the electric motor 1.

In FIG. 2 shows the logical part of the proposed protection device, where 5, 6, 7 are relay contacts; 8 - AND element with one

direct and two inverse inputs, 9 - OR element.

In FIG. Figure 3 shows a vector diagram of the currents during short-circuit inside the electric motor between phases A and B; on fig, A is the same, between phases A and C; in FIG. 5 - the same, between phases A, B, C.

Consider the operation of the device. In normal operation of the electric motor 1, the geometrical sum of the secondary currents of current transformers 2, 3,4 flows through relay 6, which are equal in magnitude and phase-shifted by an angle of 120 °:

fe h + s + U.

In the case of the same magnetization characteristics and secondary loads of current transformers, this sum is zero. However, due to the non-identical nature of current transformers, unbalance current flows through relay b, therefore, to prevent false operation, the response current of relay 6:

Wed 6 Kots1n6,

where Kote is the offset coefficient; 1nb - unbalance current.

Secondary currents of current transformers 2 and 3 flow through relay 5 and 7, respectively (secondary load currents of phases A and B), but they do not work, because their operating currents are tuned from double rated

current (maximum current in the intact phase when the electric motor feeds an external two-phase short circuit)

lcp.5 - Kote 2 1N, 1 av. 7 Kote 2 1N

Therefore, protection in normal mode does not work.

During start-up, self-start, and an external three-phase short circuit, the currents in phases A and B. C of electric motor 1 are equal in magnitude and are shifted by an angle of 120 °. However, these currents will exceed the charge currents by several times in magnitude, therefore, relays 5 and 7 will operate and their contacts will be closed, and relay 6 will not work. Therefore, the protection will not work, because none of the inputs of element 9 OR

no signal appears. When the currents in the phases are reduced to a value of 2 IH, the protection does not work falsely, because contacts 5 and 7 are opened, and contacts 6 will still be open (see, Fig. 2).

With an external two-phase short circuit, for example between phases A and B, a current flows in relay 6:

b 12 + C s.

1b 0, since (s z + A (according to the first Kirchhoff law), and currents will flow in relays 5 and 7

 Ch, I 14 2 1H

and the protection does not work (because relay 5 and 7 have tripped). The protection is similar in case of other external two-phase faults.

With a two-phase short circuit inside the electric motor 1, for example between phases A and B, a current flows through relay 6:

la- l2 + l3-k

which is larger than the nb (see Fig. 3). The protection will trip and turn off the motor 1. Similarly, protection will occur when the phases B and C are closed.

When the phases A and C are closed, a current flows in relay b

16 12 + h + k

which will be zero. However, currents will flow through relays 5 and 7

Is

  1H

(see Fig. 4). Relay 5 will operate and close its contacts, at the output of OR element 9 a signal appears to turn off the motor 1.

With a three-phase short circuit inside the electric motor 1, a current flows through relay 6 (see Fig. 5)

b 12+ s.

The magnitude of the current and at the first moment of the short circuit will be approximately equal to the starting current, which will quickly decay. But even if «О 0, then 1c flows through the relay b - Pa I From I.

Consequently, the protection will trip and shut off motor 1.

Thus, the proposed device in all operating modes of the electric motor behaves correctly. Moreover, for his

the implementation requires fewer current transformers on the side of the terminal leads, and this, in turn, simplifies the design of the electric motor.

Formulas of the invention

Device for current protection of an electric motor against phase-to-phase short circuits, comprising first and second current transformers installed with

power input side in phases A and C, respectively, and a third current transformer installed on the side of the zero terminals, the first current relay with a make contact, included in the secondary circuit

the first current transformer, the second current relay with a make contact, an actuator, characterized in that, in order to reduce the metal consumption, a third current relay with a make contact and I and IDI elements are additionally introduced, the third current transformer being designed for installation in phase B. the second the current relay is connected to the secondary circuit of the second current transformer, the third

the current relay is switched on for the sum of the current transformer currents, the closing contacts of the first and second current relays are connected to the direct and first inverse inputs of the I element, the output of which is connected to the first input of the OR element, the closing contact of the third current relay is connected to the second inverse input of the And element and the second input element OR, the output of the latter is connected to the input of the Executive element.

Figure 1

5

I

n

6

FIG. 2

FIG. 5