Automatic Short-Circuiting of Individual Line Phases for Elimination of Arc Short Circuits

I. D. Kutyavin, N. V. Lisetskii, K. K. Ryazantsev

The broad application of high-speed relay protections in combination with automatic reclosing (AR) or single-phase automatic reclosing (SPAR) for the electric networks of modern power systems has proved to be the most effective means of increasing the reliability of power supply to consumers. However, this method of increasing the reliability of system operation can be applied by no means in all cases. Difficulties arise when it is used on lines connecting individual stations for parallel operation, owing to the need to monitor synchronism. Great difficulties also arise in cases where the minimally attainable cycle time of AR or SPAR is inadmissible under the conditions of stability of parallel operation of the stations in the system. These difficulties arise now and will arise even more often in the future with the growth of the capacity of electric power systems and with their interconnection. This will occur despite the fact that modern air circuit breakers, when AR or SPAR is used, make it possible to reduce the time required to eliminate accidents to 0.2–0.3 sec at 110 kV and to 0.3–0.4 sec at 220 kV [1 and 2]. In this connection, further work is needed to increase the speed of operation both of relay protection and of high-voltage circuit breakers.

However, further reduction of the operating time of individual devices participating in the elimination of accidents with the use of AR, for each subsequent hundredth of a second, will require ever greater efforts. It is therefore necessary to attempt to achieve a significant reduction in the time for eliminating accidents in electric power systems by means of new methods. One of these methods, in our opinion, may be the shunting of the arc that has formed during a single-phase or two-phase ground fault on an overhead line or on the busbars of a substation, by means of special devices that may be called short-circuiters. This method may find application in networks with solidly grounded neutrals (and with neutrals grounded through a low resistance) and consists in the following. At the supply points of the given voltage (for example, on the busbars of step-up substations), short-circuiters are installed, controlled by instantaneous undervoltage protection. When a single-pole or two-pole ground fault occurs in the network, the undervoltage protection switches on the short-circuiters at the supply points for the time required for the arc to extinguish. Protection of the network against two-pole short circuits not accompanied by a ground fault, and against three-pole faults, is assigned to the backup protection acting on the circuit breaker of the damaged section.

Shunting of the arc must be carried out no later than one to three half-periods after the occurrence of the short circuit and must reduce the current at the fault location to such a value at which the electric arc becomes unstable and is capable of being extinguished rapidly. Investigations by V. V. Burgsdorf [3] showed that an open arc becomes unstable and is extinguished rapidly at currents up to 150 A. This current, ve-

clear, and should be taken into account in selecting the degree of shunting of the arc, until more precise values have been obtained for the critical current in the arc, allowing for the very brief flow through it of the full short-circuit current.

It was indicated above that short-circuiters must begin shunting the electric arc within one to three half-periods after the occurrence of the short circuit. This requirement is based on the fact that, first, only with such speed of operation of the relay protection and the short-circuiter can the time for eliminating faults be substantially reduced and, consequently, the reliability of operation of electrical systems be increased; and, second, the shorter the time during which the full short-circuit current flows through the arc, the less stable it will be during shunting and the more quickly it will be extinguished. As short-circuiters it would be possible to use, for example, high-voltage air circuit breakers of the BB type, connecting them according to the circuit of Fig. 1,a. However, in this case the shunting-cycle time and the protection operating time amount to not less than 0.4–0.5 sec, i.e., more than the fault-clearing time when using APV with the same circuit breaker.

Fig. 1.

More advanced short-circuiters can be obtained by using ionic devices connected according to the circuit of Fig. 1,b, which at present cannot be implemented because powerful ionic devices for high voltage are not being manufactured.

Fig. 2.

Figure 2 shows one of the possible circuits for shunting an arc by means of an ionic short-circuiter, implemented on one phase with devices for medium voltage. In this circuit, the numbers 1 and 2 denote the main windings of the power transformer, consisting of three single-phase units. The ionic short-circuiter is connected to an auxiliary winding of the transformer, designed for brief current flow. Such a connection of the short-circuiter can be used when the protected network is supplied at the given point from a single step-up transformer, or when this network consists of one or two parallel lines connected in a block with generators and step-up transformers. However, ionic short-circuiters may be connected to any point of the network through special single-phase transformers or through auxiliary windings of compensating reactors.

In the laboratory of electric stations of the Tomsk Polytechnic Institute, a single-phase ionic short-circuiter was tested, constructed

according to Fig. 2 and supplied directly from a laboratory generator with a voltage of 450 V.

The short-circuiting device shown in Fig. 2 consists of two ignitrons 3 of type VG-100/1000, connected in inverse parallel. When both ignitrons operate, the lower phase is almost short-circuited to the neutral conductor or to ground, since the internal voltage drop of the ignitrons is only 12–15 V.

Ignition of the ignitrons is effected by thyratrons 4, controlled by means of an undervoltage relay 5, which is supplied from the secondary winding of a special voltage transformer 6.

Under normal operating conditions, the grids of the thyratrons are supplied from the voltage transformer (through auxiliary transformers 7) with the corresponding phase voltages, shifted by 180° relative to the voltages at their anodes, and therefore the thyratrons do not operate. In the event of a ground fault of any phase of the primary circuit, there is a sharp decrease in the corresponding phase voltage on the auxiliary winding feeding the short-circuiting device. As a result, the undervoltage relay 5 operates (if \(U_p < U_{\mathrm{cp}}\)), removing the blocking voltage from the grids of the thyratrons of the given phase and applying to them an unlocking voltage from the windings of the voltage transformer connected into the zero-sequence filter. This voltage is in phase with the voltage at the anodes of the thyratrons and immediately causes them to operate, and consequently the ignitrons to operate as well.

Fig. 3.

As is evident from the circuit of Fig. 2, the undervoltage relay also sends a control pulse to the single-pole contactor 8, which is switched off and interrupts the operation of the short-circuiting device for a time somewhat exceeding the time delay of the line’s backup relay protection, and then is switched on again, as a result of which the entire circuit returns to its initial state.

Fig. 3 shows an oscillogram of the operation of the short-circuiting device constructed according to the circuit of Fig. 2. Curve 1 of this oscillogram represents the shunted short-circuit current, curve 2 the current flowing through the short-circuiting device, and curve 3 the total current of the circuit. This oscillogram was taken with a steady short circuit in the circuit and with artificially periodic operation of the short-circuiting device. As can be seen from Fig. 3, the short-circuiting device almost completely shunts the current in the short-circuit circuit.

When protecting a line or network by means of the circuit considered, the full short-circuit current will flow through the arc formed at the point of insulation breakdown during the time required for the undervoltage relay to operate. This time does not exceed 0.01–0.02 s. However, if the blocking voltages applied to the thyratron grids are rectified, it is possible to eliminate this undervoltage relay from the circuit. Then operation of the short-circuiting device will begin simultaneously with the occurrence of the short circuit. As a result, the arc channel will not have time to form, and therefore the arc can probably be extinguished within several half-cycles after the occurrence of the short circuit.

Supplying the short-circuiting device from the auxiliary winding of transformers or compensating reactors, and finally from special transformers—

...makes it possible to use ignitrons at any voltage. The power of the auxiliary winding (or of a special transformer) may be small, since under normal operation they are not traversed by current, and during the operation of the short-circuiter they are loaded by the short-circuit current for several half-periods.

Ignitrons were used as the principal devices of the short-circuiter because, according to published data, they allow a 20–100-fold short-time overload current. Thus, the rated power of the ionic devices of short-circuiters for a network of a given voltage may be 20–100 times less than the power of the short circuit flowing in the given circuit.

However, the use of ionic devices at medium voltage makes the short-circuiters bulky and of little practical suitability. Of great interest in this respect are Marx arc valves, which can be manufactured for the rated line voltage and for the full short-circuit power. The rapid wear of the anodes of these rectifiers will not hinder their use in short-circuiters, since they will operate for a very short time.

Fig. 4.

A preliminary test was carried out using saturating reactors as short-circuiters. This test showed that reactor-type short-circuiters may have a somewhat higher resistance than ionic ones, but they may prove cheaper and more readily available. The principal characteristics of reactor-type current short-circuiters have not yet been determined, but it has been established that compensating reactors of transmission lines having an air gap cannot be used as short-circuiters.

Then the possibility was established of using biasing of the main power transformers, which proved to be very effective and may compete with the ionic short-circuiter.

Fig. 5.

For carrying out this experiment, a model of a single-phase transformer with a shell-type split core was made; in the event of a short circuit behind the transformer, the halves of the core were biased by direct current in opposite directions. As a result, the greater part of the short-circuit current flowed through the magnetizing branch of the transformer.

The experimental circuit with transformer biasing is shown in Fig. 4, and the corresponding oscillogram in Fig. 5.

The resistance $Z_1$ in the circuit of Fig. 4 simulated the resistance of the generator circuit and of the transformer primary winding, while the resistance $Z_2$ simulated the resistance of the secondary circuit up to the point of short circuit.

Curve 1 of the oscillogram in Fig. 5 represents the short-circuit current, curve 2 the biasing current, and curve 3 the total per-

direct current. The premagnetization current was obtained from a shunt dynamo by connecting it to the premagnetization winding with low resistance; therefore, in the dynamo there occurred a “field suppression.” This also explains the shape of the premagnetization-current curve.

The use of field suppression of the dynamo to stop the flow of premagnetization current eliminated the need to disconnect this current.

It is interesting to note here that the premagnetization current rises rather rapidly; as a result, the shunting of the short-circuit current occurs practically simultaneously with the energizing of the premagnetization circuit. The high rate of rise of the premagnetization current obtained in the experiment is not accidental. The rise of the premagnetization current is forced by the magnetizing force of the alternating current; therefore one may expect that the premagnetization process will have an effect just as rapidly in practical circuits as well.

The curve of the premagnetization current proved to be somewhat unsuccessful. It would be desirable for this curve to have a rectangular form and for the premagnetization current to continue for no more than 5–10 periods.

Premagnetization of the main transformers may find application primarily in unit transmission systems consisting of series-connected generators, step-up transformers, transmission lines, and step-down transformers. Single-pole and two-pole ground faults on the transmission line of such a unit will be eliminated at a speed of up to 0.1–0.2 sec by premagnetization of the main transformers, while any other faults will be cleared by disconnecting the unit at the receiving end and reclosing it by the self-synchronization method. In such units it will not be necessary to install circuit breakers at the ends of the high-voltage line.

A major drawback of using premagnetized reactor short-circuiters or main transformers is that a large amount of direct-current power may be required for deep saturation of the core steel. However, at the present stage of development of the subject, this question is still completely unclear.

References

1. Tseirov E. M., “Air Circuit Breakers for 110- and 154-kV VZI Designs,” Bulletin of the Electrical Industry, 1950, No. 10.

2. Afanas'ev V. V. and Makarova N. A., “Air Circuit Breaker for 220 kV, 1000 A, 5000 MVA Type VV-220,” Bulletin of the Electrical Industry, 1954, No. 6.

3. Burgsdorf V. V., “Open Electric Arcs of High Power,” Electricity, 1948, No. 10.