Sensitive Directional Ground-Fault Protection for Networks with Isolated Neutral

L. A. Zaspanov, A. I. Zaitsev

(Presented by the scientific seminar of the Electromechanical Faculty)

Existing protections against ground faults used in networks with isolated neutral do not meet all the requirements imposed on them. They either lack selectivity of operation, or are too complex and expensive, or are insufficiently sensitive.

The conference on issues of protection against ground faults in systems with isolated or compensated neutral, held in May 1959 in Moscow, set the task of creating new, more advanced protective devices. Therefore, in recent years various organizations have created and are testing a whole series of new protections that more fully satisfy the requirements imposed on them. Nevertheless, it has so far not been possible to select the most universal and fully valuable ground-fault protection device for industrial manufacture and mass introduction [1].

The Department of Electrification of Industrial Enterprises of TPI also joined the work on creating more advanced ground-fault protection. We have developed a simple and sufficiently sensitive relay for selective ground-fault protection (RSGP), intended for use in cable networks of 0.23, 0.4, 3, 6, and 10 kV with isolated neutral [2, 3].

The RSGP relay belongs to directional-action devices operating in the steady-state ground-fault condition and responding to the direction of the reactive component of the zero-sequence power.

The directivity of the relay is achieved by using a switching phase-sensitive circuit assembled on semiconductor elements.

The relay circuit (Fig. 1) consists of one stage of a phase-sensitive power amplifier, an active-capacitive phase-shifting bridge, and a polarized relay connected at the amplifier output.

A cable current transformer is used as the zero-sequence current filter. In low-voltage networks a three-phase choke serves as the voltage filter; in high-voltage networks, a five-leg voltage transformer is used.

When a phase-to-ground fault occurs on the protected feeder, a rectified current arises in the collector circuit of triode $P$; its magnitude depends on the phase shift between the zero-sequence current and voltage applied to the triode. To attain the maximum current, it is necessary, by varying the resistance of the phase-shifting bridge, to rotate the zero-sequence voltage vector until it coincides in phase with the zero-sequence current.

When a fault occurs outside the protection zone (on any of the other feeders), the direction of the zero-sequence current changes by $180^\circ$; the triode “closes,” and no current will flow in the collector circuit.

The operating element of the protection is a polarized relay, whose winding is connected into the collector circuit of the triode. To smooth the pulsations, the relay winding is shunted by capacitance $C_2$.

To protect the triode against the flow of large zero-sequence currents during double ground faults, a stabilivolt $D_2$ with current-limiting resistance $R_2$ is introduced into the circuit.

Fig. 1.

Parameters of the relay elements

$P$ — planar triode P-26,

$P$ — polarized relay RP-4 U172 2048,

$D_1$ — semiconductor diode D7B,

$D_2$ — semiconductor stabilivolt D-808,

$R_1$ — variable resistance SP 1000 ohms,

$R_2$ — resistance 100 ohms,

$C_1 = C_2$ — capacitor with capacitance 4 μF,

$C_3$ — capacitor with capacitance — 10 μF,

$T$ — transformer of the phase-shifting bridge, iron cross-section 8.8 cm², PEV wire $\varnothing 0.18$, primary winding $w_1 = 1000$ turns, secondary winding $w_2 = w_3 = 315$ turns.

The sensitivity of the protection depends on the type of cable current transformer; moreover, for each type of current transformer the sensitivity can be sharply increased if the number of turns of the secondary winding is increased (Fig. 2).

The maximum value of the current flowing through the relay is limited by the selected voltage of the secondary winding of the phase-shifting-bridge transformer and by the resistance of the relay winding.

The minimum operating currents of the protection when used as the actuating relay of a polarized relay RP-4 with a winding resistance of 4500 ohms and with the protection supplied from various current transformers are given in the table.

In the winter of 1962–1963, tests were carried out on the operation of the protection in the 6 kV networks of the Tomskenergo system. The operation of the protection was checked by creating artificial ground faults in the range of ground-fault currents of 1.1–18 amperes. The protection relays were supplied from current transformers of the TZ type. At a ground current of 1.1 amperes, the secondary winding of the TZ current transformer was wound up to 180 turns. In all cases the protection operated, correctly indicating the damaged feeder.

Fig. 2.

LITERATURE

1. ORGRES. Signaling of ground faults in compensated networks. GEI, 1962.
2. A. I. Zaitsev, L. A. Zaspanov. Selective protection against ground faults in networks with isolated neutral. Izvestiya TPI, vol. 97, 1959.
3. A. I. Zaitsev, L. A. Zaspanov. Selective protection against ground faults on semiconductors. Izvestiya TPI, vol. 115, 1960.

Received by the editors
in May 1962.