ON THE POSSIBILITY OF USING AN INDUCTIVE PARAMETRON FOR PROTECTION AGAINST GROUND FAULTS IN NETWORKS WITH ISOLATED NEUTRAL

A. Kh. MUSIN, A. V. SHMOILOV

(Presented by the scientific seminar of the Department of Electrical Stations)

In currently existing ground-fault protections, zero-sequence current transformers (ZSCT) are used. The power in the secondary circuit obtained from the ZSCT sometimes proves insufficient. When directional protections are used, it is necessary to take into account the angular error of the ZSCT. To increase the power in the secondary circuit, ZSCTs with bias magnetization are used. The power obtained in this case depends on the magnitude of the zero-sequence current and the phase of the bias magnetization.

The use of an inductive parametron makes it possible to obtain power in the secondary circuit that does not depend on the magnitude of the zero-sequence current.

Figure 1 shows the distribution of the total zero-sequence current during a ground fault in a network with isolated neutral. A parametron P is installed on each line; with its aid the direction of this current is recorded and the damaged line is determined.

Fig. 1. Distribution of the zero-sequence current during a ground fault in a network with isolated neutral.

The principle of the parametron device is shown in Fig. 2. Both ring magnetic cores A and B enclose the three phases of the protected electrical installation, for example a cable. The bias windings $w_{1A}$ and $w_{1B}$ are connected to each other in series opposition. A capacitor $C$ is connected to the secondary winding $w_2$, forming, with the inductance of the secondary winding, a parametrically excited oscillatory circuit tuned to a frequency of 50 Hz. A controlled contact $K$ and a resistance $z$ connected in parallel with it are included in the supply circuit of the bias windings.

The zero-sequence voltage $U_0$ that arises during a ground fault is applied to the bias windings through

resistance \(z\) (contact \(K\) is open). The ground-fault current \(I_0\) that exists in this case induces in the secondary winding an e.m.f. depending on the magnitude of the current \(I_0\). After some time contact \(K\) closes, for example after the insulation-monitoring relay has operated, shunting the resistance \(z\), and the full voltage \(U_0\) is applied to the bias windings. In this case stationary parametric oscillations always arise in the oscillatory circuit, with an amplitude independent of the magnitude of the current \(I_0\). This circumstance makes it possible to obtain an output power independent of the magnitude of the current \(I_0\). The amplitude of the oscillations that arise can be regulated, with the bias voltage \(U_0\) unchanged, by changing the design parameters of the parametron.

Fig. 2. Principle of the parametron device. A and B—two identical closed magnetic cores; \(w_{1A}\) and \(w_{1B}\)—bias windings with the same number of turns; \(w_2\)—secondary winding; \(C\)—capacitor; \(K\)—controlled contact; \(z\)—resistance; \(U_0\)—bias voltage; \(I_0\)—ground-fault current; BU—output device, which is a phase-selection element; \(U_{\text{op}}\)—reference voltage.

The phase of the stationary oscillations is rigidly connected with the phase of the bias voltage \(U_0\) and can have only one of two possible values: \(+90^\circ\) or \(-90^\circ\). This phase is controlled by the phase of the ground-fault current \(I_0\). In Fig. 3 the control characteristics for the phase of the parametron oscillations are shown. As is seen from the figure (curve 1), theoretically, in the region \(45^\circ < \varphi < 225^\circ\), the parametric oscillations that arise have phase \(+90^\circ\); in the region \(225^\circ < \varphi < 45^\circ\), they have phase \(-90^\circ\).

In practice (curves 2 and 3), the phase regions of unambiguous excitation are less than \(180^\circ\). Increasing the value of the current \(I_0\) causes the phase regions to approach \(180^\circ\). The centers of the phase regions have the values \(\varphi_0 = -45^\circ\) and \(\varphi_0 = 135^\circ\). The more the value of \(\varphi\) differs from \(\varphi_0\), the larger the value of \(I_0\) required for reliable control of the phase of the parametric oscillations. The minimum value of \(I_0\) for reliable control is obtained at \(\varphi = \varphi_0\). To ensure this it is necessary (Fig. 4) to bias the parametrons with a voltage \(U_0'\), shifted relative to \(U_0\) by an angle of \(-45^\circ\). Then the phase of the total fault current...

ground fault will differ from \(\varphi_0\) by an angle \(\delta\), due to the presence of the active component \(I_a\) in the ground-fault current. However, the magnitude of \(I_a\) is no more than 2–3% of the total ground-fault current [1]. Therefore the value of \(\delta\) may be neglected.

Introducing into the biasing circuit of the parametron a controlled contact \(K\) and a resistance \(z\) makes it possible to eliminate the influence of the residual magnetization of the cores on the phase controllability of the parametric oscillations, thereby increasing the sensitivity of the device to the magnitude of the ground-fault current \(I_0\). The value of the resistance \(z\) must be chosen so that the voltage applied directly to the bias windings with contact \(K\) open is insufficient to excite parametric oscillations.

The influence of higher harmonics in the current \(I_0\) affects the phase controllability of the parametron at the following values: the third harmonic—when the current \(I_0\) is exceeded fivefold; the fifth—eightfold, and so on. In existing networks such a level of higher harmonics in the ground-fault current is not encountered [2].

Fig. 3. Characteristics of phase controllability of parametron oscillations. \(P\)—probability of occurrence of parametric oscillations with phase \(+90^\circ\); \(\varphi\)—phase angle between the bias voltage and the fault current; 1—theoretical curve; 2, 3—experimental curves respectively for \(I_{02} > I_{03}\).

Fig. 4. Vector diagram of currents and voltages during a ground fault in the network of Fig. 1.

The parametric oscillations arising in the oscillatory circuit carry information both about the fact that a ground fault has appeared and about the phase of the fault. The occurrence of parametric oscillations indicates the presence of a ground fault, while the phase of the oscillations that arise makes it possible to judge the phase (direction) of the fault current. The latter makes it possible to identify the damaged feeder.

As the output element for phase selection, a phase-controlled average-current amplifier may be used, whose load is an RES-9 relay.

Above it was assumed that the parametrons are supplied from an additional winding of the NTMI voltage transformer. If a large number of parametrons is installed, the power of the NTMI may prove insufficient to supply all the parametrons. In that case it is possible to supply the parametrons with unipolar current pulses at a frequency of 100 Hz, formed from an external DC source. The formation may be carried out by means of a thyristor. Biasing the parametron with pulses at a frequency of 100 Hz makes it possible to eliminate the influence of the unbalance caused by the nonidentity of cores A and B on the phase controllability of the parametrons. The control-

the leading pulses are formed from the zero-sequence voltage \(U_0\).

The results of an experimental verification of the parametron as a responding element of protection against ground faults are given in the Appendix.

Conclusions

1. The possibility has been shown of using an inductive parametron as a responding element in protection against ground faults 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 of \(0.3 A\) to the zero-sequence current.

3. The output voltage of the parametron does not depend on the zero-sequence current; i.e., the protection is equally sensitive. This makes it possible to implement the logical part of the protection simply, using low-sensitivity relays.

Appendix

Under laboratory conditions an inductive parametron was tested (Fig. 1). The magnetic cores A, B were assembled from transformer steel E41 of type G-25. The stack thickness of each magnetic core was \(8 mm\). The winding parameters were \(w_{1A} = w_{1B} = 1100\) turns, PEL wire \(\varnothing 0.51\); \(w_2 = 1900\) turns, PEL wire \(\varnothing 0.41\). The clear opening was \(75 \times 75 mm\).

The capacitance was \(C = 2 \mu F\) (MBGCh-1; \(2 \mu F \pm 10\%\), \(U_n = 250 V\)).

An active resistance was connected as resistance \(z\). The current \(I_0\) was passed through a conductor running inside the window of the parametron. The bias voltage was \(92 V\) (\(50 Hz\)). The value of \(z\) was chosen so that the voltage applied directly to the bias windings with contact \(K\) open was \(32 V\).

The voltage \(U_2\) of the resulting parametric oscillations was \(180 V\) (\(50 Hz\)). The minimum value of the current \(I_0\) reliably controlling the phase of the voltage \(U_2\) was \(0.3 A\).

The power consumed by the bias circuit was \(30 VA\), and that taken from the output circuit was \(3 VA\).

A voltage relay RN with two windings was used as the output device. A reference voltage \(U_{\mathrm{op}}\), in phase with \(U_0\), of such magnitude that the relay did not operate was applied to one winding. The other winding was connected to the output of the parametron.

References

1. F. A. Likhachev. Selection, Installation, and Operation of Arc-Suppression Coils. Moscow, GEI, 1955.

2. A. A. Grikmanis, Ya. K. Rozenkrons. “Detection of Single-Phase Ground Faults.” Elektricheskie stantsii [Electric Power Stations], No. 7, 1965.