Semiconductor-Based Selective Protection Against Ground Faults
Abstract Generated abstract
This paper addresses selective protection against phase to ground faults in electrical networks with isolated neutral, where established selective schemes were lacking despite serious safety and fire hazards. It proposes a semiconductor protection circuit that responds to the direction and magnitude of zero sequence power in a feeder, using zero sequence current and voltage filters, a phase shifting bridge, and a phase sensitive amplifier with a relay output. Laboratory measurements and control tests at the Tomsk Rubber Footwear Plant indicated selective operation, adjustable sensitivity governed chiefly by relay pickup current, and reliable performance under varied network capacitance and transition resistance. The authors argue that the circuit is simple, sensitive, suitable for low voltage and comparable high voltage isolated neutral networks, and potentially useful in peat enterprise and coal mine section networks.
Full Text
Selective Semiconductor-Based Ground Fault Protection
A. I. Zaitsev, L. A. Zaspanov
(Presented by the scientific seminar of the Electromechanical Faculty)
Ensuring high rates of electrification of the national economy is, as is known, the basis of technical progress and of the technical equipment of labor. The ever wider use of electric power sets before us the task of ensuring maximally safe conditions for its application.
One of the dangerous operating conditions of an electrical network is the breakdown of the insulation of one of the phases of electric motors and electrical apparatus to the frame, as a result of which the service personnel may come under a voltage whose magnitude, in networks with an isolated neutral, reaches the phase value. In addition, when insulation breakdown occurs, sparking may arise, which, in the presence of a suitable medium, may lead to an explosion or fire.
In networks with a grounded neutral, advanced selective protections against such faults have been developed. However, for networks with an isolated neutral, such selective protections have not yet been created. Below is a description of a circuit for selective protection against ground faults for networks with an isolated neutral. The circuit was tested at the Tomsk Rubber Footwear Plant. The tests gave good results and fully confirmed the possibility of creating such circuits.
The proposed selective-protection circuit responds to the direction and magnitude of the zero-sequence power flowing through the protected feeder when a phase is faulted to ground. The circuit (Fig. 1) consists of zero-sequence current and voltage filters, a phase-shifting bridge, and one stage of a phase-sensitive power amplifier assembled on semiconductor elements.
The circuit is designed for use, as the zero-sequence current filter, of a cable current transformer. As the voltage filter, as has already been indicated [1], in low-voltage networks a three-phase choke can serve; in high-voltage networks—a five-limb voltage transformer, or three single-phase transformers whose secondary windings are connected in open delta.
The phase-shifting bridge is intended to eliminate the 90-degree phase angle between the zero-sequence current and voltage, which makes it possible to increase the gain factor.
power and, accordingly, the sensitivity of the protection. The bridge makes it possible to adjust the voltage phase shift within the range of \(180^\circ\). Connecting the winding of the phase-shifting-bridge transformer, which has a high resistance, between ground and the “neutral” of the zero-sequence voltage filter makes it possible to limit the magnitude of the current flowing to ground through the choke to any small values, i.e., practically to eliminate the influence of the choke on the magnitude of the ground-fault current.
Fig. 1.
The amplification of the power taken from the zero-sequence current and voltage filters is carried out by a planar semiconductor triode connected in a common-emitter circuit.
The actuating element of the protection is a sensitive relay, whose winding, shunted by a diode, is connected through a rectifier bridge into the collector circuit of the triode. When a phase is short-circuited to ground, a current \(I_p\) arises in the collector circuit supplying the relay; its magnitude is determined by the values of the zero-sequence voltage and current, as well as by the phase-shift angle between them. To achieve the maximum value of the current \(I_p\), it is necessary to eliminate the angular shift, which is achieved by adjusting the value of the resistance \(R\) (Fig. 1). The value of the optimum resistance \(R\) depends on the parameters of the protection circuit and is to be determined only once—during its adjustment. The direction of the current flowing through the relay changes by \(180^\circ\) when a fault occurs within the protection zone (point \(k_1\)) and outside the protection zone (point \(k_2\)). For selectivity of operation, the relay winding is shunted by a diode so that the relay operates when a fault occurs within the protection zone.
The sensitivity of the protection is determined by the relay operating current. Tables 1 and 2 give experimental data from a study of the described protection under laboratory conditions.
Table 1
Influence of network capacitance on the current \(I_p\)
| Network capacitance outside the protection zone, \(C\), \(\mu\mathrm{F}\) | Network capacitance outside the protection zone, \(C\), \(\mu\mathrm{F}\) | Network capacitance outside the protection zone, \(C\), \(\mu\mathrm{F}\) | Network capacitance outside the protection zone, \(C\), \(\mu\mathrm{F}\) | Network capacitance outside the protection zone, \(C\), \(\mu\mathrm{F}\) | Network capacitance outside the protection zone, \(C\), \(\mu\mathrm{F}\) | Network capacitance outside the protection zone, \(C\), \(\mu\mathrm{F}\) | Note | |
|---|---|---|---|---|---|---|---|---|
| 1 | 2 | 4 | 6 | 8 | 10 | 12 | ||
| Current in the relay winding when short-circuited in the protection zone, \(I_p\) [mA] | 0.1 | 0.41 | 2.5 | 5.1 | 6.0 | 6.5 | 7.0 | Capacitance in the protection zone \(C_z = 2\,\mu\mathrm{F}\) |
Table 2
Effect of transition resistance on the current \(I_p\)
| Current in the relay winding \(I_p\) [mA] | Capacitance of the network outside the protection zone, \(\mu F\) | Transition resistances, \(\Omega\): 0 | 50 | 75 | 150 | 250 | 500 | 1000 | Note |
|---|---|---|---|---|---|---|---|---|---|
| Current in the relay winding \(I_p\) [mA] | 6 | 6.00 | 5.80 | 5.30 | 3.80 | 1.60 | 0.27 | 0.04 | Capacitance in the protection zone \(C_1 = 0.5\,\mu F\) |
| Current in the relay winding \(I_p\) [mA] | 2 | 0.45 | 0.43 | 0.41 | 0.35 | 0.28 | 0.12 | 0.03 | Capacitance in the protection zone \(C_1 = 0.5\,\mu F\) |
| Current in the relay winding \(I_p\) [mA] | 0.5 | 0.08 | 0.08 | 0.08 | 0.08 | 0.07 | 0.06 | 0.04 | Capacitance in the protection zone \(C_1 = 0.5\,\mu F\) |
The measurements were carried out at a network voltage \(U_c = 220\) V. As the voltage filter, a choke from an RUV-2 leakage relay was used. As the current filter, a model current transformer of type UTT-5 was used. The number of primary turns was 1.
Subsequent control tests at the Tomsk Rubber Footwear Plant confirmed the results of the laboratory investigations and the reliability of the protection operation.
Conclusions
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The protection circuit possesses selectivity of operation that does not depend on the ratio of the magnitudes of the currents in the protection zone and outside the protection zone.
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The sensitivity of the protection is determined only by the magnitude of the pickup current of the output relay and, when polarized relays are used, can be very high. If necessary, the sensitivity can be increased by using a two-stage power-amplification circuit.
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In terms of its parameters and simplicity of implementation, the circuit described has major advantages over the most sensitive selective-protection circuit presently used in the networks of peat enterprises.
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The circuit may be recommended for use both in low-voltage networks with an isolated neutral and in similar high-voltage networks.
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The circuit described should be of particular interest for the networks of peat enterprises and for the section networks of coal mines. In the latter case, the circuit may be used for joint operation with a leakage relay.
REFERENCES
A. I. Zaitsev, L. A. Zaspanov, “Selective protection against ground faults in networks with an isolated neutral.” Izvestiya TPI, vol. 97, 1959.