ON THE POSSIBILITY OF USING AN INDUCTIVE PARAMETRON FOR IMPLEMENTING GROUND-FAULT PROTECTION IN COMPENSATED NETWORKS

A. Kh. Musin, A. V. Shmoilov

(Presented by the scientific seminar of the Department of Electrical Systems and Networks)

The development of ground-fault protection (signaling) devices in compensated networks continues to proceed in two directions. On the one hand, these are devices that respond to artificially created quantities, i.e., those based on the superposition on the protected network of a control current with a frequency different from the industrial frequency and having the character of the zero sequence [1, 2]. On the other hand, they are devices that respond to natural quantities.

Among the latter, devices that respond to the level of higher harmonics in the ground-fault current have recently become most widespread [3].

The use of devices employing traveling-wave transients is also known. There is positive operating experience with such devices [4].

The wide application of both these and other protection devices encounters obstacles caused by their inherent shortcomings. Thus, the use of devices that respond to artificially imposed quantities in networks having two or more arc-suppression coils (ASC), geographically distributed at different points of the network, requires the creation of a complex system for blocking the protection, starting, and signaling the operating condition of the control-current sources installed at all points of the network where ASCs are present.

If alternating control-current sources are used, it is necessary to ensure the phasing of the sources during starting. Therefore, at present, devices that respond to artificially imposed quantities are used mainly for radial networks.

A disadvantage of devices that respond to the level of higher harmonics is the instability of the harmonic character in the ground-fault current. The harmonic composition of the fault current changes with changes in the operating mode and configuration of the network, the nature of the load, the number of generators and transformers in operation, the location of the fault, the faulted phase, and the transient resistance at the fault point. Instability of the harmonics can lead to false operation of the protection on undamaged lines. The protection may fail to operate when the fault occurs through a transient resistance, owing to an insufficient level of harmonics.

The presence of higher harmonics in the normal operating mode of the network leads to the need to detune the protection from the normal mode.

According to the selectivity condition, the protection can operate only in networks with a large number of connections.

To eliminate the indicated shortcomings, the authors of this protection proposed switching to a comparison of the levels of harmonics in the faulted and healthy feeders, the ratio of which is determined by the parameters of the network and of the individual feeders. However, constructing a simple and reliable comparator for a large number of inputs is far from an easy task. Therefore,

Fig. 1. A and B are two identical closed magnetic cores encompassing the three phases of the protected installation (cable); \(w_{1A}\), \(w_{1B}\) are bias windings with the same number of turns; \(i_1\) is the bias current; \(w_2\) is the secondary winding; \(w_3\) is the additional-signal winding; \(R\) is an additional active resistance; \(K\) is the control contact; \(C\) is a capacitor whose value is chosen so as to ensure the occurrence of parametric resonance at a frequency of \(50\ \mathrm{Hz}\) in the secondary circuit when the magnetic cores are biased by current \(i_1\) and contact \(K\) is open; \(D\) is a choke which, together with capacitor \(C\), forms a filter trap at a frequency of \(50\ \mathrm{Hz}\); \(OU\) is an output device, representing a phase-selection element; \(I_0\) is the ground-fault current; \(U_2\) is the secondary voltage arising under parametric resonance; \(U_{\mathrm{ref}}\) is the reference voltage, for which the zero-sequence voltage is used.

this device for relative measurement of the level of higher harmonics has been introduced in the form of visual indication of the level of higher harmonics in all feeders of the network during a ground fault.

Finally, the indicated protection can be used only in radial networks. But even for radial networks, in connection with the intensified research and development now being carried out on methods and means of compensating higher harmonics in networks, very sensitive measuring elements may be required.

Protection that responds to the traveling-wave transient process can be used both for radial and ring networks. A disadvantage of this protection is the lack of repeatability of operation.

The most stable behavior during a ground fault is exhibited by the active component of the fault current, since one of its components, condi-

...caused by losses in the arc-suppression coil, is practically unchanged. This circumstance, in combination with a highly sensitive device for the direction of the active component, makes it possible to create protection that eliminates the shortcomings of existing protections. The inductive parametron makes it possible to create such a device.

The application of the parametron is illustrated in Fig. 1.

The operation of such a device is based on the following. The phase of the oscillations \(U_2\) arising in the parametrically excited oscillatory circuit, formed by the capacitance \(C\) and the inductance of the secondary winding \(W_2\), assumes one of two possible mutually opposite values [5]. This phase can be predetermined if, before the occurrence of parametric oscillations, a current (hereinafter called the signal) of small magnitude with a certain “control” phase is introduced into the oscillatory circuit. The value of the control phase depends on the magnitude of the relative detuning of the parametric oscillatory circuit and can be determined by calculation [6]. Since the oscillatory circuit is parametric, the presence in the circuit of a signal orthogonal to the signal with the control phase has no influence on the process of controlling the phase of the parametric oscillations; i.e., the parametron has selectivity with respect to orthogonal control signals. In the device of Fig. 1, the active and reactive components of the ground-fault current \(I_0\) are orthogonal control signals. Thus, if the system is adjusted so that the phase of the active component of the ground-fault current (the active signal) coincides with the control phase, then the phase of the arising parametric oscillations \(U_2\) will be predetermined by the phase of the active component. The reactive component of the ground-fault current (the reactive signal) will have no influence on the phase-control process.

It is known that the active components of the ground-fault current of faulted and unfaulted feeders have mutually opposite phases (for radial lines). By installing parametrons on all feeders, we obtain mutually opposite phases of the secondary voltages of the faulted and unfaulted lines.

By comparing the phase of the voltages with the phase of some reference voltage, for example the zero-sequence voltage, the faulted feeder can be identified.

It should be particularly noted that the magnitude of the secondary voltage does not depend on the magnitude of the signal, i.e., it does not depend on the magnitude of the ground-fault current. This ensures a guaranteed reliability factor for the operation of the output relay, irrespective of the magnitude of the ground-fault current. Any preassigned magnitude of the secondary voltage \(U_2\) can be provided by adjusting the design parameters of the parametron (the cross section of the magnetic core, the number of turns of the windings, etc.).

The selectivity of the parametron with respect to orthogonal control signals is not unlimited. In order to increase the selectivity, it is expedient to premagnetize the parametron with unipolar current pulses at a frequency of 100 Hz and a duty factor of 2. Such a premagnetization frequency (different from the parametric-resonance frequency of 50 Hz) makes it possible to exclude the influence of the unbalance caused by the nonidentity of cores A and B on the selectivity of the parametron. The premagnetization pulses can be formed from a direct-current source by means of a thyristor. The thyristor control pulses should expediently be formed from the zero-sequence voltage. The power supply is common to all parametrons.

Adjustment of the system so that the phase of the parametric oscillations is predetermined by the phase of the active component of the ground-fault current is carried out by adjusting the phase of the bias current. The latter is accomplished by adjusting the phase of the pulses controlling the thyristor. In the absence of detuning of the oscillatory circuit, the leading edge of the bias current must coincide with the instant of the maximum of the zero-sequence voltage.

For reliable control of the phase of the arising parametric oscillations, the magnitude of the control signal must be no less than a certain minimum value. For the specimen we investigated this value is \(0.01\ A\) (see the Appendix). The active component of the ground-fault current of some short, undamaged feeders may prove to be less than this value. In that case the control signal is increased by introducing an in-phase additional signal into winding \(W_3\) (Fig. 1). The additional signal flows under the action of the zero-sequence voltage.

To achieve in-phase signals, it is necessary that the resistance of the additional-signal circuit be active; this is achieved by connecting into this circuit an active resistance \(R\) (Fig. 1) of such a value that the inductive resistance of the circuit is much smaller than \(R\). If necessary, the inductive resistance of the circuit can be compensated by connecting a capacitive resistance in series with it.

For the faulted feeder, the additional signal is in antiphase with its active signal. Taking this into account, the magnitude of the additional signal is chosen so that the difference between the active signal of the faulted feeder and the additional signal is not less than the minimum permissible value of the control signal. For existing networks this requirement can always be met (with the minimum permissible value of the control signal equal to \(0.01\ A\)).

The ground-fault current contains higher harmonics, which, depending on their relative weight, may impair the selectivity of the parametron with respect to orthogonal signals of the fundamental frequency. To reduce this influence, a choke \(D\) (Fig. 1) is included in the circuit, forming, together with capacitor \(C\), a 50 Hz trap filter. The trap filter forces the current of the fundamental-harmonic signal to flow only through the magnetizing branch of the parametron, thereby increasing the content of the fundamental harmonic in the control signal. Parametric oscillations arise after contact \(K\) opens. The time delay for opening contact \(K\) serves to eliminate the influence of residual magnetization of the cores on the selectivity of the parametron.

The influence of higher harmonics in the signal on the selectivity of the parametron with respect to orthogonal signals, as well as an increase in the selectivity of the parametron in the absence of higher harmonics, is promoted by connecting an active load into the secondary circuit.

As an output device performing phase selection, a phase-controlled average-current amplifier [7] may be used, whose load is an RES-9 type relay.

The principle underlying the operation of the proposed protection—recording the direction of the active component in the ground-fault current—makes it possible to provide protection not only for radial, but also for ring cable networks.

For the latter, it is necessary to install inductive parametrons at both ends of each protected section of the network and to provide for the exchange of information between them.

The results of an experimental test of an inductive parametron as an operating element of ground-fault protection for compensated networks are given in the appendix.

Appendix

Under laboratory conditions an inductive parametron was tested (Fig. 1). Magnetic cores A and B were made of transformer steel E41, type size G-25. The stack thickness of each magnetic core was 8 mm. The winding parameters were \(w_{1A}=w_{1B}=110\) turns, PEL Ø 0.8, \(w_2=1900\) turns, PEL Ø 0.41. The clear window was \(75 \times 75\) mm. The capacitance was \(C=2\,\mu\text{F}\) (MBGCh-1, \(2\,\mu\text{F}\pm10\%\), \(U_n=250\) V).

The power source is a rectifier using Larionov’s circuit, connected through a three-phase transformer to a 0.4 kV network. The formation of magnetizing-current pulses with a frequency of 100 Hz and a duty factor of 2 is carried out by means of a thyristor, for the control of which the Jones circuit [8] is adopted. The thyristor control pulses are formed from an alternating voltage of 50 Hz, modeling the zero-sequence voltage. The average value of the magnetizing current is 1 A.

The voltage \(U_2\) of the resulting parametric oscillations is 120 V. The signal \(I_0\) contains active and reactive components. The phase of the parametric oscillations is reliably predetermined by the phase of the active signal when the reactive signal exceeds the active one by up to 30 times and when the active signal is at least 0.01 A.

The influence of higher harmonics in the signal on the selectivity of the parametron begins to become apparent at the following magnitudes: the third harmonic—when the active signal is exceeded by 12 times, the fifth harmonic—by 20 times, etc.

Changing the magnitude and direction of the reactive signal, which simulates the network compensation mode, has no effect on the operation of the device.

As the output device for phase selection, a mean-current amplifier with phase control [7], loaded by a relay of type RES-9, was used.

Conclusions

1. The possibility has been demonstrated of using an inductive parametron as an operating element of protection against steady ground faults in networks with a compensated neutral.

2. The output voltage of the parametron does not depend on the magnitude of the ground-fault current; i.e., the protection is equally sensitive. This makes it possible to implement the logical part of the protection simply.

3. Operation of the device does not depend on the network compensation mode.

4. By its operating principle, the device has repeatability of action.

5. The device can be used for the protection of both radial and ring networks.

6. The tested device operates reliably when the active component of the ground-fault current is 0.01 A and when the reactive component of the ground-fault current exceeds the active component by no more than 30 times.

1. The influence of higher harmonics in the ground-fault current on the operation of the tested device appears at the following magnitudes: for the third harmonic—when the active component of the fault current is exceeded by a factor of 12; for the fifth—by a factor of 20, etc.

REFERENCES

1. I. M. Sirota. Signaling of ground faults in a compensated network based on the use of the second-harmonic current. Moscow, GEI, 1962.

2. R. A. Vainshtein. Application of low-frequency parametric systems for protection against ground faults in compensated networks. Dissertation. Tomsk, 1965.

3. V. M. Kiskachi et al. Devices for signaling ground faults. Elektricheskie stantsii, 1972, No. 4.

4. G. V. Sokolova. Development of ground-fault protection using wave transient processes. In: Problems of the optimal development of power systems and new technical means of protection. Moscow, Nauka, 1970.

5. Parametrons. Collection of articles, IL, 1962.

6. A. E. Kaplan et al. Parametric generators and frequency dividers. Sovetskoe radio, 1966.

7. V. T. Doroguntsev et al. Elements of automation devices for power systems. Energiya, 1970.

8. Thyristors (technical handbook). Translated from English. Energiya, 1971.