UDC 621.317.333 : 621.313.322

DEVICE FOR CONTINUOUS MONITORING OF GENERATOR STATOR WINDING INSULATION AT THE IRKUTSK HYDROELECTRIC POWER STATION

P. T. ANOKHIN, A. A. FINKEL

(Presented by the scientific seminar of the Department of Electric Power Stations)

The need to eliminate the dead zone in generator ground-fault protection at $3U_0$ required the use of more complex circuits. One of the possible approaches to solving this technical problem is the method of superimposing a rectified voltage on the stator circuit [1, 2].

The protection [2] has been in operation at the Irkutsk Hydroelectric Power Station since June 1968. One of its most important units—the logic section (LS)—was implemented on electromechanical relays. The development of the logic section using contactless elements, together with several other changes in the circuit, led to the creation of an improved version of the protection—RZG-100 [3, 4].

The power-supply unit (PSU) of the RZG-100 protection is a source of rectified stabilized voltage of 200 V, which in principle makes it possible to carry out continuous monitoring of the stator insulation resistance to ground on an operating generator.

According to GOST 183-66, the minimum permissible insulation resistance of the stator winding is determined by the formula

\[ R_{\text{ins}}=\frac{U_n}{1000+\frac{P}{100}},\ \text{M}\Omega, \]

where $U_n$ is the rated voltage of the generator (V);
$P$ is the generator power (kW).

Thus, for example, for the generators of the Irkutsk HPP, at $U_n = 13.8$ kV the permissible insulation resistance is $R_{\text{ins}} \approx 8 \cdot 10^6$ ohms.

Measuring insulation resistance with a megohmmeter makes it possible to detect various defects of the stator winding (foreign objects on current-carrying parts, moistening of the insulation, its contamination, breakdown, etc.). This is a very rough and not always reliable assessment of the insulation. In a number of cases, even when a serious defect is present, the latter cannot be detected with the aid of a megohmmeter. A more complete picture of the state of the insulation can be obtained only by testing it with an elevated voltage.

All tests of the stator winding insulation are carried out on a stopped generator. During operation of the machine, insulation defects may develop, leading to a reduction in its breakdown voltage. Therefore, monitoring ԥ

insulation under the natural conditions of its operation, that is, on an energized generator. This is especially important when the insulation resistance is at a level close to the minimum permissible value.

The voltage of the dc source in the RZG-100 protection (200 V), by itself, is of course not sufficient for evaluating the insulation resistance. However, under real conditions the electric field in the insulation is created by the generator operating voltage. In this case the superimposed dc voltage of 200 V performs only the monitoring function, providing a signal that depends on the equivalent insulation resistance.

For direct current, the equivalent insulation resistance of the stator winding to ground is shunted by the leakage resistances of capacitors \(C1\) and \(C2\) (Fig. 1). It has been established in practice that, with a supply-source voltage \(U_{\mathrm{p}} = 200\ V\), the total leakage current of the capacitors does not exceed \(2 \cdot 10^{-6}\ A\), i.e., their equivalent leakage resistance is not less than \(100 \cdot 10^{6}\ \Omega\). Consequently, monitoring the equivalent insulation resistance in the range from \(10 \cdot 10^{6}\) to \(1 \cdot 10^{6}\ \Omega\) is a quite feasible task.

To monitor the insulation resistance on an energized generator, the KIZ instrument was developed. The circuit of the KIZ instrument as applied to a generator operating with neutral grounding through an arc-suppression coil is shown in Fig. 1. In this implementation the instrument can be used only as an attachment to the RZG-100 protection.

The circuit of the KIZ instrument is based on a dc amplifier for small signals with conversion. It provides sufficient accuracy, is distinguished by high stability, and requires no adjustment during operation.

The block diagram of the KIZ instrument consists of the following principal units: a dc amplifier, an alarm device, and a power-supply unit. The dc amplifier amplifies the signal arriving at the input of the KIZ instrument. It consists of a modulator \(M\), which is an input converter for converting a dc signal into a proportional ac signal; a GPN generator—for forming rectangular pulses; an ac amplifier \(U\)—for amplifying the ac signal; and a demodulator DM with an integrator for extracting the dc component of the output signal.

From the amplifier the signal is fed to instrument \(P\) and to the alarm device.

A voltmeter, whose scale is graduated in megohms (\(M\Omega\)), is adopted as instrument \(P\) for visual determination of the insulation resistance.

The alarm device is a Schmitt trigger with output relay RP. It operates when the signal at the input of the KIZ instrument is proportional to a decrease in the equivalent insulation resistance of the stator winding to ground down to a level of about \(10\ M\Omega\).

The power-supply unit is a source of rectified stabilized voltage of 24 volts. The stabilizer maintains a constant voltage of \(24\ V\) when the supply voltage deviates within \(\pm 15\%\).

The KIZ instrument, like the RZG-100 protection, is supplied from the station-service voltage of 380 V. The serviceability of the KIZ instrument is checked by pressing button \(K_{\mathrm{n}}\), which, together with instrument \(P\), is located with consideration for ease of maintenance. The KIZ instrument is housed in a case with overall dimensions of \(175 \times 195 \times 200\) mm.

Fig. 1. Connection diagram of the RZG-100 protection with the KIZ instrument

Annotations in the diagram:

• Signal of voltage loss
• Signal of protection operation
• Signal of insulation decrease
• Signal of insulation decrease
• \(A\), \(B\), \(C\)
• RZG-100
• KIZ
• BP
• LI
• IO
• GPN-1
• M1
• U
• DM
• Trig. 2
• RP
• TN
• VM
• DGK
• NR
• \(R_y\), \(R_p\), \(R_v\), \(R_k\), \(R_n\)
• \(C_1\), \(C_2\), \(C_3\), \(C_p\)
• \(D_{12}\)
• \(D_r\)
• \(P\)
• \(K_n\)
• \(1\text{RN}\), \(2\text{RN}\)

Two sets of RZG-100 protection with attachments for insulation monitoring (KIZ) have been in trial industrial operation at the Irkutsk HPP since January 1972.

Conclusions

1. The use of RZG-100 ground-fault protection, based on the principle of superimposing a rectified voltage, with the KIZ attachment makes it possible to carry out continuous monitoring of the stator-winding insulation on an operating generator.

2. The use of the KIZ device will make it possible to accumulate operating experience with generators at reduced insulation resistance.

References

1. V. I. Novash, L. D. Rogov, V. K. Moroz. Sensitivity of ground-fault protections of generators operating in a unit with transformers. Elektricheskie stantsii, 1971, No. 3.

2. P. T. Anokhin. Protection against ground faults with 100% coverage of the stator winding of a unit generator. Elektricheskie stantsii, 1971, No. 6.

3. P. T. Anokhin. Investigation and improvement of protection of a generator–transformer unit against ground faults on the generator-voltage side. Dissertation, Tomsk, TPI, 1970.

4. P. T. Anokhin. Operating experience with 100% protection against ground faults of the stator winding of a unit generator. Izvestiya TPI, vol. 244, Tomsk, TSU Publishing House, 1972.