SU64905A1 - Device for preventing malfunctioning of fast acting distance protection with between phase faults in the voltage circuit - Google Patents

Description

In case of damage in the voltage circuit and at the same time blown fuses on the low side of the voltage transformer, the distance protection may not work properly. In order to avoid this, special output protection from blocking is applied, based on the fact that when the circuit is broken, the voltage is transformed and thus turns into an asymmetrical circuit, the voltage is zero (successive.

In the case of a one- or two-phase short circuit to the neutral wire or to earth (with the grounding point of the secondary side of the voltage transformer), the voltage of the sequence appears simultaneously with the occurrence of a short circuit. A different position is created during the interphase short circuit in the voltage circuit, where the blocking can take effect only when the fuse blows, while the protection itself begins to work only after a short circuit has occurred.

According to the data of the inventor, the corresponding engineers showed that there was no fuse in the coil of the fuse, during which the zero sequence voltage was absent, depending on the magnitude of the fault current and the fuse's nominal current, can reach a considerable value.

In (EVERY way, during this period of time, the high-speed remote protections fail to work properly and turn off the oil switch.

According to the present invention, in order to prevent the protection from malfunctioning, such an asymmetry of the resistances in the voltage transformer circuit is artificially created, so that during interfacial 10m short circuit, a zero sequence voltage should appear in the circuit for a blocking relay.

It is perfectly obvious that under normal conditions of operation of a voltage transformer, i.e., in the absence of short circuits in it, the asymmetry should

be such that it does not cause unacceptable distortion of the phase or linear voltages applied to the protection. The limit of 3Uo in the normal operating mode of the voltage circuit should be considered as such a value that does not exceed the natural unbalance voltage equal to 2-2.5 volts.

Determining the asymmetry of the voltage transformer circuit required to & e sufficient to operate a blocking relay of a value of voltage of zero sequence is made on the basis of the following considerations.

The power of the transformer caps is negligible compared to the power of the entire energy system in which they operate, it does not exceed 5 kV per phase and in relation to voltage transformers this system can be considered an infinitely large power system. Therefore, in any mode of operation of a voltage transformer, including short circuits on its low side, it can be considered that the voltage at the terminals of its high-voltage winding does not change and is completely consumed in the transformer and its V11Or: h1NO, th purpose as a voltage drop .

If one voltage is added to one of the voltage transformer phases, then the phase voltage vectors, with a short circuit between this phase and any other, will be asymmetric and the sum of all three phase voltages will not be zero.

As can be seen from the layout of the drawing (diagram is given for the damage site), the sum of the phase voltages, different in LO, will be equal to the voltage drop from the short circuit current in the additional resistance, i.e.

3Uo lfe-Az,

where UZi is the additional resistor JieAie, which is included in one of the phases.

Since, IB turn, for a two-phase short circuit:

Pl

If 2ZTp- | -2Znp-b 2 /

Il Azi

behind

2 (zTp + Znp) -fAzi

from where

 - 2 (z.p + Znp)

Az, - ° il-z.

Here:

UL is the nominal linear voltage of the secondary winding of the transformer, Uo is the desired value of the zero sequence voltage in a two-phase short circuit, ZTP is the impedance of each winding

transformer phase, Znp- resistance of wires per phase to the self and remote point of the circuit.

The additional resistance for the other phase of the voltage transformer is calculated in the same way, given that the resistance of each phase of the c-api transformer has increased by liiZb.

D2, Dggn G.r-Ts pr + Dg,)

or ZO,

+ -U.

Az, (2)

-zts.

There is practically no need to EXECUTE the additional resistances AZi and D 2 In the form of impedance, defined by equations (I) and (2) - it is quite possible to perform - and pure ohmic, which greatly simplifies and facilitates manufacture.

The additional resistances are included in the two phases of the secondary circuit of the voltage transformer in its immediate vicinity. It is obvious that there is a short circuit between the phase, in which the additional resistance is not included, and

the phase in which the larger of them is included is accompanied by a greater incompleteness of the circuit and, consequently, a larger value of 3Uo.

From the point of view, the values of 3Uo, generally speaking, it does not matter what phases of the voltage transformer include the pre-capacitive resistances. However, in order to reduce voltage imbalance in a normal mode of operation, it is extremely desirable by successive circular permutation of the additional resistances in the phases to achieve the minimum unbalance voltage.

Calculations and experimental data show that the value of 3Uo and three-phase short circuits x is much larger than with two-phase ones. Therefore, a biphasic short circuit should be adopted for the design case to determine the additional resistances.

The practical application of the proposed device for improving the blocking operation is limited to the following: a) significant resistance of the voltage transformer circuit to the farthest point at which interphase short circuits can occur, and b) the load value of the voltage transformer. The first causes a relatively large amount of additional resistances D Zi and DE2, which, with a significant load on the voltage transformer, causes an unacceptable voltage unbalance during normal operation of the circuit. Under these conditions, the correct solution can sometimes be found by dispersing the additional resistance and connecting most of it into separate branches of a voltage transformer circuit, which naturally flows with less current than the voltage transformer itself.

The issue is also resolved by reducing the resistance of the voltage transformer circuit by increasing the cable cross section.

It should be borne in mind that the inclusion of two additional SBS1X TI | HS NY

The voltage transformer circuit somewhat reduces the short circuit current, the value of which should be verified from the point of view of fuse melting.

As can be seen from equation (2), the magnitude of the additional resistance AZa is more than twice the AZi. It follows that it is Za that has the greatest effect on voltage asymmetry in the normal mode of operation of the circuit.

Therefore, if an additional resistance AZb is included in one of the phases of the transformer voltage, it is advisable to disconnect the asymmetry in the other two phases not by including AZ2 in one of them, but by decreasing the resistance of one of the phases to the other.

The latter is easily accomplished if there are loose conductors in the trunk cable. When one of the transformer voltage phases is switched on to two parallel-connected cable cores, it is easy to ensure circuit asymmetry for two-phase short circuits that occur outside the cable.

A well-known disadvantage of this method is that, in one of the three possible cases of a two-phase short circuit in a magnetic cable, the asymmetry of the circuit may not be sufficient for the blocking action until the fuses blow.

However, this kind of short circuit in the cable is extremely rare, and this case can hardly be considered.

Subject invention

A device for preventing the malfunctioning of the high-speed distance protection for interphase faults in the voltage circuits, consisting of a set of high-speed remote relays and an intermediate. interlocking relays that provide protection interlocking in case of blown fuses during

Auxiliary voltage transformer circuit, characterized in that additional two-phase voltage transformers have additional resistances connected with two phases in order to create a zero-sequence voltage during interphase closures x in the voltage circuits, which are sufficient to block the protection until the fuses blown .

Znp