Selective Protection Against Ground Faults in Networks with Isolated Neutral

A. I. Zaitsev, L. A. Zaspanov

(Presented by the scientific seminar of the Electromechanical Faculty)

Networks with an isolated neutral belong to installations with a small ground-fault current. The magnitude of these currents is determined only by the capacitive current, which depends on the extent of the network and the value of the operating voltage. Numerically, capacitive currents may amount to several amperes and fractions of an ampere, respectively, for high- and low-voltage installations. The sparking that arises at the point of a short circuit to ground may, under the conditions of the chemical, coal, and other industries, lead to an explosion of the surrounding medium or to a fire. Proceeding from the requirements of safety, installations in an explosive or fire-hazardous environment must have improved high-speed protection against ground faults.

At present, general sensitive protection against ground faults has been satisfactorily solved through the use of leakage relays in the coal industry of the RUV type and of sensitive current circuits reacting to zero-sequence current [1, 2, 3]. However, when several consumers are supplied from the same busbars, the existing circuits do not possess selective action, unless one allows at least a brief grounding of the neutral through a current-limiting ground. Selective protection against ground faults in networks with an isolated neutral can be obtained if, for this purpose, a phase-sensitive circuit is used which responds to the voltage and current of the zero sequence. The proposed selective-protection circuit consists of zero-sequence current and voltage filters, an electronic tube, and a relay.

The schematic diagrams for high- and low-voltage installations are shown respectively in Figs. 1 and 2. The zero-sequence current filter is made in the form of a cable transformer with a large number of turns in the secondary winding, which is included in the grid circuit of the electronic tube. The required power of the current transformer is insignificant because of the small currents in the load circuit \(R\) and \(C\), which have a large value. The voltage filter for low-voltage installations is made in the form of a three-phase choke, to whose neutral point the anode circuit of the electronic tube is connected. In high-voltage installations, the zero-sequence voltage filter, as is known, is obtained by connecting the secondary windings in an open delta in five-limb or three single-phase transfor-

voltage transformers. The electronic tube is connected to the zero-sequence voltage with the relay winding in the anode circuit. This tube performs the functions of the phase-sensitive element of the circuit and of the zero-sequence power amplifier. In short circuits to ground in systems with an isolated neutral, no distortion of the line-voltage system occurs. In the case of a metallic short circuit, the neutral point of the phase-voltage system will in this case be at a voltage equal to the phase value. The zero-sequence voltage

Fig. 1.

Fig. 2.

with respect to ground will be equal to the zero-sequence voltage. The magnitude and direction of the capacitive current are determined relative to the line voltages of phases \(A\) and \(B\) when phase \(C\) is faulted. The resultant current, equal to the zero-sequence current, will be equal to the geometric sum of the capacitive currents \(I_{AC}\) and \(I_{BC}\) and will be directed with respect to the zero-sequence voltage vector at an angle of \(90^\circ\). The vector diagram of currents and voltages for a metallic ground fault in networks with an ungrounded neutral is shown in Fig. 3. Since the zero-sequence current and voltage are shifted by \(90^\circ\), it is necessary to connect active and capacitive resistances in the secondary winding of the transformer so that the voltage in the grid circuit, which is taken from the active resistance, is shifted relative to the zero-sequence current by an angle close to \(90^\circ\). If this condition is fulfilled, the anode voltage (the zero-sequence voltage) will coincide in phase with the grid voltage, and the current flowing in the anode circuit will be maximum if the short circuit occurs in the protection zone—the point \(k_2\) in Fig. 1. In the case of a fault outside the protection zone at point \(k_1\), the anode voltage and the grid voltage will be in phase opposition, and the current in the anode circuit will be minimal—

Fig. 3.

…determined. By an appropriate choice of \(R\) and \(C\) the required sensitivity can be obtained. In Fig. 4 the sensitivity curves of the protection are given, equal to the ratio of the relay current when a fault occurs in the protection zone \(I_{к2}\) to the relay current when a fault occurs outside the protection zone \(I_{к1}\), for an experimental setup as a function of the resistance \(R\) in the grid circuit. The experiments were carried out with a 6N8S tube with the triodes connected in parallel. The capacitance of the capacitor in the grid circuit was taken as \(0.125\ \mu\mathrm{F}\). The curves were obtained for the initial data given in Table 1. In this table the value \(C_2\) corresponds to the capacitance of the protected line, and \(C_1\) to the capacitance of the remaining lines. The curves shown in Fig. 4 indicate that, with appropriate circuit parameters, it is possible to create selective protection against ground faults in networks with an isolated neutral with any sensitivity. If the relay winding is shunted by a capacitor, then by selecting the capacitance it is possible to create the required time delays. Under this condition, selective protection can be created not only for radial lines but also for trunk lines by setting the protections according to time delays, as is done, for example, for overcurrent protection of such lines. In those cases where, because of several sets of selective protections, the total resistance of the zero-sequence point relative to ground may decrease, and where under operating conditions such a decrease is inadmissible, and also where a very sensitive circuit is required, two-stage and higher-stage protection amplification circuits may be used. In this case the first zero-sequence stage will operate in the voltage-amplification mode, having as the load in the anode circuit of the tube any large active resistance. The subsequent stages will operate in the current- or power-amplification mode. These stages can already be fed from the operating network. At the output of such amplifiers it is practically possible to obtain any power at any value of the ground-fault current.

Fig. 4.

Table 1

The use of a protection scheme responding to the zero-sequence voltage and current makes it possible to be completely insulated from the influence of transient processes, as a result of which considerable unbalance currents may appear. When starting a motor and other transient—

processes, when no change occurs in the insulation resistance with respect to ground, the appearance of an unbalance current in the current filter will not cause the protection to operate, since there will be no voltage in the anode circuit of the electron tube. The protection will operate only when current and voltage of the zero sequence appear simultaneously, thereby eliminating false operation of the protection.

Selective ground-fault protection schemes in networks with an isolated neutral can also be used as elements of directional overcurrent protection.

An analogous protection scheme can be obtained using semiconductors. A semiconductor scheme will have greater reliability and will always be ready for operation.

Conclusions

1. A ground-fault protection scheme that responds simultaneously to zero-sequence current and voltage has selective action and very high sensitivity.

2. The proposed scheme can be used as directional overcurrent protection.

3. The transient resistance at the point of short circuit causes an automatic phase shift of the voltage on the grid. With appropriate adjustment of the relay operating current, any value of the minimum permissible insulation resistance with respect to ground can be set.

4. When several protection sets are installed, one common zero-sequence voltage filter may be used.

Literature

1. Leibov R. M. Leakage in mine electrical networks. Ugletekhizdat, 1952.

2. Burguev S. A. Electrical stations and substations for agricultural purposes. Selkhozgiz, 1958.

3. Trofimov R. F. Selective ground-fault protection for mine section networks. Mining Electrical Engineering, Ugletekhizdat, 1957.