APPLICATION OF A LOW-FREQUENCY INDUCTIVE PARAMETRON WITHOUT DC BIASING IN EARTH-FAULT PROTECTION IN NETWORKS WITH COMPENSATED AND ISOLATED NEUTRALS*

R. A. VAINshTEIN, A. B. SHMOILOV

Recommended by the scientific seminar of the Department of Electrical Stations

At the present time, the problem of providing satisfactory protection against earth faults in networks with a compensated neutral still remains a pressing one [1].

A method of implementing protection that uses the direction of the active component of the fault current encounters the following difficulties. The use of wattmetric induction relays in this case is limited by the large power consumption of both the current circuits and the voltage circuits, and by the low multiples of torque at small active currents. Responding elements made with semiconductor devices have high sensitivity but have small output powers. Therefore, if a contact output is required, it is necessary to use highly sensitive polarized or magnetoelectric relays.

Earth-fault protection in networks with an isolated neutral, using the distribution of the zero-sequence current, is implemented simply and in most practical cases has the necessary selectivity and sensitivity. However, if the fault current of some line greatly exceeds the currents of the remaining lines, satisfactory protection on this line is obtained only when it is supplemented by an energy-direction element.

The use of energy-direction elements or the application of protections based on semiconductor devices encounters the same difficulties as in the protection of compensated networks. Usually, directional protection of such networks is implemented with the aid of the PNDF device, which has no contact output.

In this article it is proposed to use, as the responding element of earth-fault protections, a low-frequency inductive parametron without DC biasing.

Figure 1 shows the distribution of the active component of the zero-sequence current during an earth fault in a network with comp—

* The work was carried out under the supervision of Professor I. D. Kutyavin.

...with a compensated neutral (currents \(I_{a1}, I_{a2}\)). Figure 2 gives the distribution of the total zero-sequence current during an earth fault in a network with an isolated neutral (currents \(I_1, I_2\)). The distribution of these currents, as is evident from the figures, is such that their direction makes it possible to determine the damaged line.

Fig. 1.

A parametron \(\Pi\) is installed on each line; it must register either the direction of the active component or the direction of the total zero-sequence current when a fault occurs.

Figure 3 shows the schematic diagram of the parametron as the responding element of the protection.

On the cores of the parametron there are three windings each: \(W_{\mathrm{в}}\)—excitation windings, \(W_{\mathrm{к}}\)—tank-circuit windings, and \(W_{\mathrm{с}}\)—signal windings. Initial-signal windings \(W_{\mathrm{н}}\) may also be wound on the cores (shown by dashed lines in Fig. 3).

The windings \(W_{\mathrm{к}}\), \(W_{\mathrm{с}}\), \(W_{\mathrm{н}}\) are connected to one another in aiding polarity, and in opposition with respect to the windings \(W_{\mathrm{в}}\). The windings \(W_{\mathrm{к}}\), together with the capacitor \(C\), form an oscillatory circuit with a resonant frequency close to 50 cps.

The windings \(W_{\mathrm{с}}\) are connected to the zero-sequence current filter. The windings \(W_{\mathrm{в}}\) are supplied from the zero-sequence voltage filter.

Fig. 2.

According to [2, 3], the phase of oscillations in a parametrically excited circuit can have two mutually opposite values. If a small current is passed through the windings \(W_c\), whose phase is such that the phase of the signal induced in the windings \(W_k\) will coincide with one of the opposite phase values of the increasing orthogonal component of the random initial oscillations in the circuit, then the phase of the parametric oscillations will be predetermined.

A zero-sequence current is passed through the windings \(W_c\). The phase of the parametron oscillations must be predetermined either by the active component of this current, if protection of a compensated network is being implemented, or by the total zero-sequence current, if protection of a network with an isolated neutral is being implemented.

Consequently, in the case of protection of a compensated network, the phase of the zero-sequence current must be such that the phase of the signal in the circuit winding from the active component of the current coincides with one of the opposite phase values of the increasing orthogonal component of the fluctuation oscillations. The phase of the signal from the reactive component of the current in this case will coincide with the decaying orthogonal component and, consequently, when excitation is applied to the parametron, the signal from the reactive component will decay.

In the case of protection of a network with an isolated neutral, the phase of the signal from the total zero-sequence current must coincide with one ...

…one of the opposite values of the phase of the increasing orthogonal component of the fluctuation oscillations in the circuit.

Since a zero-sequence current is passed through windings \(W_c\), the phase of which is determined by the parameters of the network, the in-phase condition of the signal and the oscillations in the parametron in this case must be ensured by the appropriate selection of the phase of the excitation current. This selection is carried out by the corresponding adjustment of the resistance \(Z_{\mathrm{в}}\) (see Fig. 3) or by some other method.

Fig. 3.

When excitation is applied, the parametrons on healthy lines must have an oscillation phase opposite to that of the parametron installed on the damaged line, since the directions of the zero-sequence currents on the healthy and damaged lines are opposite.

As is known, in networks with an isolated or compensated neutral, high-frequency transient currents flow during a ground fault; their magnitude many times exceeds the steady-state currents. They may predetermine the phase of the oscillations and cause false operation of the protection. Therefore the excitation windings of all parametrons

are connected automatically to the zero-sequence voltage filter by contact \(K\) after a time delay deliberately greater than the damping time of the transient process. In this connection, the parametric oscillations will begin to build up after the transient process, and their phase will be determined by the steady-state current.

The logic part of the protection must contain a phase-selection device that distinguishes mutually opposite phases of the parametric oscillations. This device can be made relatively simply, since the amplitude of the oscillations in the parametron is always constant, regardless of the magnitude of the signal determined by the zero-sequence current. Owing to this, the fixation of one of the possible phase values can be accomplished by comparing the voltage on the oscillatory circuit with some reference voltage.

We shall not dwell here on the description of the phase-selection device.

The active components of the zero-sequence currents of individual lines are always very small; the total zero-sequence currents of individual lines in some cases may also prove to be very small, so that the magnitudes of these currents may be insufficient for reliable control of the phase of the oscillations in parametrons installed on healthy lines.

To increase the reliability of the protection operation, an artificial initial signal is created in all parametrons (currents \(I_{p1}\), \(I_{p2}\), \(I_p\); see Fig. 1), which coincides in phase with the signal from the active or total zero-sequence current of the healthy lines. The magnitude of the initial signal must be chosen such that, in the parametrons, even in the absence of a natural signal, oscillations with the required phase are established. However, the artificial signal must be smaller than the signal caused by the total zero-sequence current of the network (active or total) flowing through the faulted line.

If the indicated conditions are observed, the phases of the oscillations of the parametrons on the healthy and faulted lines will be uniquely predetermined: in a network with a compensated neutral, by the active current; and in a network with an isolated neutral, by the total zero-sequence current.

The initial signal can be created by connecting winding \(W_{\mathrm{n}}\) to the zero-sequence voltage filter through resistance \(Z_{\mathrm{n}}\). The latter is selected so as to ensure the necessary magnitude and phase of the initial signal.

If the design of the parametron does not provide for an initial-signal winding, the initial signal can be obtained by connecting windings \(W_c\) or \(W_k\) to the zero-sequence voltage filter through resistance \(Z\), the angle of which must be such as to ensure the required phase of the initial signal.

At the Department of Electric Power Stations of the Tomsk Polytechnic Institute, a prototype of a low-frequency inductive parametron without biasing was tested.

The signal windings of the parametron were connected to a type T.Z. zero-sequence current transformer.

A small voltage was applied to the initial-signal windings, the signal from which coincided in phase with the signal from the current of the T.Z. transformer. A current was passed through the primary winding of the transformer whose reactive component was equal to 14 A.

A current was fed to the excitation windings of the parametron, by means of a phase-shifting device, with such a phase as to ensure an optimal increase of the signal from the active component of the current and attenuation of the signal from the reactive component.

With the active component of the current equal to 0, the parametron was excited with a phase determined by the initial signal.

With the same reactive component of the current (14a) and an active component equal to 3a, the phase of the signal, which is opposite to the initial signal, excited the parametron in the opposite phase. Changing the direction of the reactive component by 180°, which corresponds to the transition from the overcompensation mode of a network with a compensated neutral to the undercompensation mode, has no effect on the capture of the phase of the parametron oscillations by the active component of the current.

The reactive zero-sequence current of one line rarely exceeds 10–12a. Consequently, the tested parametron can, for example, be used to create responsive protection elements for a compensated cable network with an active fault current of not less than 3a.

The following test was also carried out with the above-described prototype parametron.

A current was fed to the excitation windings whose phase was such as to predetermine the phase of the parametron oscillations by the signal from the total current, which was supplied to the signal windings \(W_c\) from transformer T. Z. The primary current of transformer T. Z., sufficient for reliable control of the phase of the parametron oscillations, proved to be 2.5a.

On parametrons with such characteristics, protection of a radial cable network with an isolated neutral can be implemented if the differences between the total zero-sequence current and the zero-sequence currents of each of the lines exceed 2.5a.

During the test, an external active load equal to 0.3 W was connected to the parametron. The critical load, at which the parametron is no longer excited, exceeds 1 W.

A power of about 0.3 W is sufficient to use coarse relays as the output devices of the protection. The power consumed by the parametron excitation circuit does not exceed 1.5–2 VA. The tested parametron is made on two cores of E-41 transformer steel (Ш-12 stamping, stack 11 mm). The parametron has a simple design, is made from widely used materials, and is easily adjusted.

CONCLUSIONS

1. The possibility has been shown of applying a low-frequency inductive parametron without DC biasing as a responding element in protections against earth faults in compensated networks and in networks with an isolated neutral.

2. Experimental verification of the proposed device showed that protection with a parametron is selective and has a sensitivity no lower than that of the protections proposed in [1].

3. The output voltage of the parametron does not depend on the zero-sequence current, i.e., the protection is equally sensitive.

This also makes it possible to implement the logical part of the protection simply.

1. The output power of parametrons, with small overall dimensions, makes it possible to use low-sensitivity relays as the output elements of the protection.

2. The power consumed by the parametron tested does not exceed 1.5 W.

REFERENCES

1. Signaling of earth faults in compensated networks. Collection of articles edited by V. I. Idelson. GEI M–L, 1962.

2. Parametrons. Collection of articles. Translated from Japanese and English by I. L. M. 1962.

3. Weinstein R. L., Shmoilov A. V. Low-frequency inductive parametron without dc biasing. Article in this collection.