OPERATING EXPERIENCE WITH 100% PROTECTION AGAINST GROUND FAULTS IN THE STATOR WINDING OF A UNIT-CONNECTED GENERATOR

P. T. ANOKHIN

(Presented by the scientific seminar of the Department of Electric Power Stations and Electrical Systems and Networks)

At present, regardless of the type and power of a generator operating in a unit connection with a transformer, protection against ground faults is implemented as zero-sequence voltage protection [1, 2]. This protection has a dead zone located at the generator neutral.

In the operation of generators, the most frequent faults are ground faults of the stator winding. The presence of an uncontrolled ground fault can lead to a severe accident in the event of a repeated fault in the generator-voltage circuit. Modern large generators practically always operate in unit connection. Owing to their high cost, the requirement imposed on ground-fault protection is the complete absence of a dead zone [3].

In developing a protection system without a dead zone, the author used the well-known principle of applying a rectified voltage to the protected primary circuits [4]. The protection is installed on generator No. 6 of the Irkutsk Hydroelectric Power Station. For its technical implementation, the arc-suppression coil (ASC) is grounded through capacitor \(C_1\), of the IMZ-100 type with a capacitance of \(100\ \mu\text{F}\), and the neutral point of the primary winding of the voltage transformer (VT) is grounded through capacitor \(C_2\), of the KBG-P type with a capacitance of \(10\ \mu\text{F}\), rated at \(2\ \text{kV}\). The protection device, containing a power-supply unit and a measuring element, is connected to capacitor \(C_1\). Detuning of the protection from the charging current of capacitor \(C_1\) when power is applied is achieved through the use of a logic section implemented with electromechanical relays.

Since 1968 the protection [4] has been in pilot industrial operation with action for signaling. Two cases of stator-winding ground fault were recorded, in which the protection operated. No replacement of any elements of the circuit was carried out. However, implementation of the logic section with electromechanical relays does not allow the protection to be made compact. In addition, the possibility of a signal indicating operation of the protection is excluded if the fault in the protected zone occurred before the supply voltage was applied. Detection of such a fault by means of a light signal at the place where the protection is installed does not exclude subjective errors. These shortcomings are completely eliminated in the improved version of the protection—the RZG-100, Fig. 1.

The logic section (LS) of the RZG-100 protection is implemented with contactless elements. The main element of the LS is transistor 17, connected according to

in a common-emitter circuit. The transistor is controlled through a circuit consisting of resistor 13 and capacitor 14.

The transistor operates in switching mode and, depending on its state, either shunts or unshunts the measuring element (ME). To turn off a transistor of the \(p\)—\(n\)—\(p\) type, the base—emitter junction voltage must be \(U_{\text{be}}\ge 0\). This occurs when capacitor 14 is charged, when no current flows in the emitter—base circuit (\(U_{\text{be}}=0\)), or when it is discharged (\(U_{\text{be}}>0\)).

Fig. 1. Schematic diagram of the RZG-100 protection

When power is applied, capacitors \(C_1\) and 14 are charged. The flow of the charging current of capacitor 14 \((I_{\text{ch.14}})\) produces a voltage \(U_{\text{be}}<0\); therefore the transistor opens. The resistance of the emitter—collector junction of the open transistor is very small, and practically the entire charging current of capacitor \(C_1\) \((I_{\text{ch.}C1})\) flows through it.

For reliable blocking of the protection against the current \(I_{\text{ch.}C1}\), it is necessary that, during the charging of capacitor \(C_1\), the transistor be in the saturated state. This is ensured by appropriate matching of the currents \(I_{\text{ch.}C1}\) and \(I_{\text{ch.14}}\) in magnitude, and also of the charging time constants of capacitors \(C_1\) and 14 \((\tau_{\text{ch.}C1}\) and \(\tau_{\text{ch.14}})\). Such matching is possible if the ratio of the collector current, which we take to be equal to the current \(I_{\text{ch.}C1}\), to the base current does not exceed the value of the transistor static current gain \(B_{\text{st}}\). Neglecting the shunting action of Zener diode 15 and resistor 16, we obtain:

\[ \frac{I_{\text{ch.}C1}}{I_{\text{ch.14}}}<B_{\text{st}}\approx \frac{I_{\text{cn}}}{I_{\text{bn}}}, \]

where

\[ I_{\text{cn}}\ \text{and}\ I_{\text{bn}} \]

are the collector and base currents at the boundary of the saturated state of the transistor [5].

The resistance of resistor 13 is selected from the condition of transistor saturation at the minimum possible static current gain for transistors of the given type. The circuit of Fig. 1 uses an MP-26B transistor.

The capacitance of capacitor 14 is selected from the condition of matching the charging time constants \(\tau_{\text{ch.}C1}\) and \(\tau_{\text{ch.14}}\).

To limit the current in the circuits of the RZ device in the event of a ground fault on the generator-voltage side, resistors 24 and 26 are provided. In order to reduce the time constant \(\tau_{\mathrm{z.s1}}\), resistor 24 is shunted by valve 25.

The power-supply unit (PSU) is a source of stabilized rectified voltage of about 200 V. Transformer 1 can be supplied either from the 380 V station-service network or from the 100 V secondary winding of the voltage transformer of the protected generator.

The voltage of the power source is monitored by means of a minimum-voltage relay 4 of type RN 54/160, in which the rectifier and the additional resistances are not used.

Voltage stabilization is necessary in order to prevent delayed operation of the protection in the event of insulation damage in the stator winding at the moment when generator field forcing is in operation.

Reliable detuning of the protection from ground faults on the high-voltage side of the unit transformer is achieved by using a blocking filter consisting of inductance coil 22 and capacitor 23. The filter parameters are selected on the basis of blocking the fundamental-frequency voltage on the side of capacitor \(C_1\) when there is no ground fault in the protected zone.

The main element of the measuring element (ME) is relay 21 of type RP-7 with a winding resistance of 6300 ohms. The operation of the protection is based on monitoring the magnitude of the direct current \(I_n\) flowing in the measuring element and depending mainly on the transition resistance at the point of the ground fault. The sensitivity of the protection can be adjusted by the resistance of resistor 20.

The RZG-100 protection is installed on generator No. 5 of the Irkutsk Hydroelectric Power Station.

Since the protection is expected to operate with tripping of the generator, its sensitivity to ground faults through transition resistances is limited to a level of about 80 kOhm.

The power consumed by the protection circuit from the AC side in normal operation is about 6 VA. In the event of a metallic ground fault in the protected zone, the protection consumes no more than 12–15 VA.

Structurally, the RZ device is made as a complete relay and is housed in an enclosure with overall dimensions of \(195 \times 175 \times 180\) mm.

The RZG-100 protection underwent factory tests on a generator excited up to rated voltage and, since October 1970, has been in pilot industrial operation. A ground fault of the stator winding was recorded, during which the protection operated.

Conclusions

1. Implementation of the logic part of the RZG-100 protection using contactless elements ensures high noise immunity under any changes in the supply voltage. In addition, the protection as a whole has turned out to be simpler, more reliable, and more compact.

2. The existing operating experience with the RZG-100 protection at the Irkutsk Hydroelectric Power Station makes it possible to conclude that it satisfies all modern requirements and can be adopted as the main protection of the stator winding against ground faults.

References

1. Guidelines on relay protection, issue 5, protection of generator–transformer and generator–autotransformer units. Energiya Publishing House, Moscow, 1963.

2. A. M. Fedoseev. Fundamentals of Relay Protection. Gosenergoizdat, 1961.

3. Ya. S. Gelfand. Issues in the implementation of relay protection in power systems, TsNTI, ORGRES, 1969.

4. P. T. Anokhin. Protection against ground faults in the stator winding of a unit-connected generator using a direct-current source. TsNTI information sheet, Irkutsk, 1970.

5. N. N. Goryunov, A. Yu. Kleiman, Ya. A. Tolkacheva. Handbook of Semiconductor Diodes and Transistors. Energiya Publishing House, Moscow, 1968.