ON 100% SELECTIVE PROTECTION AGAINST GROUND FAULTS OF THE STATOR WINDING OF SYNCHRONOUS GENERATORS OF AN ENLARGED UNIT

R. A. VAINSTEIN, V. T. GETMANOV

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

At present, enlarged units are used at electric power stations, in which several generators are directly connected at generator voltage and operate on one common secondary winding of a transformer. The need for 100% coverage of the turns of the stator winding at present is beyond doubt, since such units, as a rule, employ high-power generators (more than 50 MW). The presence of a dead zone in such protections leads to very dangerous damage—double ground faults—which, naturally, requires large expenditures for post-accident repair. In the case of enlarged units, in addition to ensuring complete coverage of the turns of the stator winding, the problem arises of determining the damaged generator.

Known protections that use natural electrical quantities in the steady-state or transient condition do not make it possible to solve this problem. In this connection, it is expedient to develop a protection that responds to electrical quantities artificially imposed on the generator-voltage circuits.

Such a protection was developed at Tomsk Polytechnic Institute as applied to the enlarged unit of the Saratov HPP (four generators operate in the unit). To impose a control current on the generator circuits, sources of this current, whose voltages are identical, are connected in series into the circuits of the arc-suppression coils of each generator. To implement the protection, it is necessary to compare the phase of the voltage of the control frequency on the secondary winding of the arc-suppression coil and the phase of the current of the control frequency in the zero-sequence current at the generator terminals with respect to the voltage of the control-current source (reference voltage). For further analysis of the operation of the protection, it is necessary to know the nature of the variation of these electrical quantities as a function of the transient resistance at the point of the ground fault and of the control-current frequency. It should be noted that, for selecting the control-current frequency, it is sufficient to analyze the nature of the variation of the voltage on the secondary winding of the arc-suppression coil, since the starting element of the protection responds to this electrical quantity.

The zero-sequence equivalent circuit for determining the electrical quantities used in the protection is shown in Fig. 1. In the diagram the following are denoted: \(L\)—the inductance of the arc-suppression coil; \(C_{\text{г}}\)—the total capacitance of all three phases of the generator with respect to ground; \(R_n\)—

transient resistance at the fault point; \(U\) is the voltage of the control-current source; \(I_{\mathrm{g}}\) is the auxiliary-frequency current at the terminals of the damaged generator; \(I_{\mathrm{z}}\) is the auxiliary-frequency current at the terminals of the healthy generator.

The voltage at the frequency of the control-current source on the arc-suppression coil, when it is tuned to resonance, is equal to

Fig. 1. Zero-sequence equivalent circuit of an enlarged unit for determining the electrical quantities of the injected current in steady-state operation

\[ U_L = \frac{ R_{n*}^{2} n^{2}(\kappa^{2}-1)+1+jR_{n*}\dfrac{n}{\kappa} }{ R_{n*}^{2}n^{2}\dfrac{(\kappa^{2}-1)^{2}}{\kappa^{2}}+1 }, \tag{1} \]

where

\[ R_{n*}=\frac{R_n}{X_L}; \]

\(X_L=\omega L\) is the inductive reactance of the arc-suppression coil at a frequency of 50 Hz; \(n\) is the number of generators operating in the unit;

\[ \kappa=\frac{\omega_{\mathrm{k}}}{\omega}, \]

where \(\omega_{\mathrm{k}}\) is the angular frequency of the control current,
\(\omega\) is the angular frequency of the power-frequency current.

The angle of the vector \(U_L\) with respect to the vector \(U\) is determined by the expression

\[ \varphi_{u_L}=\operatorname{arctg} \frac{ R_{n*}\dfrac{n}{\kappa} }{ R_{n*}^{2}n^{2}(\kappa^{2}-1)+1 }. \tag{2} \]

To determine the effect of the control-current frequency, let us plot the curves of the dependences \(\varphi_{u_L}(R_{n*})\) for various values of \(\kappa\) and for \(n=4\) (Fig. 2).

From the curves in Fig. 2 it is seen that, at a control-current frequency of 25 Hz \(\left(\kappa=\dfrac{1}{2}\right)\)

the phase of the voltage on the arc-suppression coil, when \(R_{n*}=0\div 10\), changes by \(\approx 175^\circ\), while at a control-current frequency of 100 Hz it changes only slightly and, judging by the nature of the change, implementation of the method is very difficult. As follows from expression (2), analogous dependences for \(k=3, 4\ldots\) have the same character of variation as for \(k=2\), while the maximum of the curve occurs at still smaller values of the angle and of the relative transient resistance.

This simple analysis shows that it is desirable to have the frequency of the control current below the industrial frequency and different from the frequencies of the higher harmonics present in the EMF of generators and in the nonlinear elements of the zero-sequence circuit.

As sources of the control current for the proposed protection, electromagnetic parametric frequency dividers by two of special design [1] are used. Similar sources of control current are being successfully operated on 12 hydro-generators of the Krasnoyarsk HPP, in the compensated networks of the Barnaul and Balkhash thermal power plants.

Fig. 2. Dependences of the phase of the voltage on the arc-suppression coil on the ratio of the transient resistance to the resistance of the arc-suppression coil for various values of \(k\)

In Fig. 3 are given curves of the dependences of the moduli and phases of \(U_L\), \(I_n\), \(I_z\) with variation of the transient resistance within the limits \(10 \ge R_{n*} \ge 0\), at a control-current frequency of 25 Hz for \(n=4\). From the curves of Fig. 3 it is seen that, for \(10 \ge R_{n*} \ge 0\), the modulus of the voltage on the arc-suppression coils changes by a factor of 3, and its phase by \(\approx 175^\circ\); while the component with a frequency of 25 Hz in the zero-sequence current at the terminals of the faulty generator (\(I_n\)) is directed toward the generator, and at the terminals of the healthy generator (\(I_z\)), conversely, away from the generator. Moreover, \(I_n=3I_z\).

Thus, the nature of the change of the electrical quantities with a frequency of 25 Hz during a ground fault of a generator phase, as compared with the normal condition, makes it possible to implement selective one-hundred-percent protection against ground faults.

The responding element of the protection is made on a phase-sensitive principle, since the phases of the named quantities change more sharply with variation of the transient resistance than do their moduli. In developing the schematic diagram it was taken into account that the operation of the responding element must not be substantially affected by the voltage with a frequency of 50 Hz arising on the secondary winding of the arc-suppression coil during a ground fault in the stator winding, and it must not operate for a fault on the high-voltage side of the unit transformer, when, because of the presence of interwinding capacitances of the power transformer, a considerable neutral displacement is observed.

The responding element of each generator of the unit contains a starting element, a selective element, and a restraining element. Two electrical quantities are supplied to the starting element: one proportional to the output voltage of the control-current source (reference voltage), and the second—to the control current flowing through the arc-suppression-

… coil. The latter is taken from the measuring winding of the arc-suppression coil. Since, in the absence of a ground fault, the measuring winding of the arc-suppression coil has a voltage at a frequency of 25 Hz that is opposite in phase to the voltage when a fault is present, the starting element provides restraint of the protection in the normal mode, whereby high noise immunity is achieved during short circuits on the high-voltage side of the unit.

When a ground fault appears in the stator winding of a generator of the enlarged unit, the starting elements of the responding protection elements of all generators operate; at the same time, the selective element of the responding element of the generator on which the fault has occurred operates. The selective element, which makes it possible to determine the damaged generator, compares the phase of the zero-sequence current at a frequency of 25 Hz at the generator terminals with respect to the reference voltage. The zero-sequence current at the generator terminals is picked up by connecting the protection element into the neutral conductor of one of the groups of current transformers installed at the generator terminals.

Fig. 3. Dependences of the electrical quantities used in the protection on the ratio of the transient resistance to the resistance of the arc-suppression coil at a control-current frequency of 25 Hz

The outputs of the starting and selective elements are connected to a coincidence logic circuit.

The restraining element is introduced to restrain the protection during external phase-to-phase short circuits, when a very large increase in the unbalance current of the three-transformer zero-sequence filter is possible, and also during double ground faults. The restraining element is made responsive to the operating value of the current in the neutral conductor of the current transformers and is set away from the maximum unbalance current of the normal mode.

The protection also operates correctly when some of the generators in the unit are disconnected. When only one generator remains in operation, the output of the selective element is shunted and the signal indicating generator damage is issued upon operation of the starting element alone.

Since the generators of the enlarged unit are directly connected at generator voltage, it turns out that the control-current sources are electrically connected. Therefore, correct operation of the protection is assumed only for in-phase operation of the control-current sources. It is known that parametric frequency dividers can be excited and sustain oscillations at half frequency with equal probability in either of two opposite phases; therefore, such a mode of operation of the control-current sources is possible in which one of them operates in phase opposition to the others. The possibility of such an operating mode is due to…

also on the peculiarity of the conditions under which the control-current sources operate. During the transient process, when a generator phase is shorted to ground at the moment when the generator phase voltage passes through a value close to zero, the aperiodic component of the arc-suppression-coil current, branching into the magnetizing branch of the divider, causes a temporary disappearance of the parametric oscillations. The latter are restored after the aperiodic component decays in either of two opposite phases.

Fig. 4. Oscillograms of the voltage on the arc-suppression coils of the unit generators in the in-phase operating mode of the control-current sources

In Fig. 4 are shown oscillograms of the voltage with a frequency of 25 Hz on the arc-suppression coils of the unit generators in the in-phase operating mode of the control-current sources. When a generator phase is shorted to ground, the voltage phase on the arc-suppression coils (curves 2, 3, 4, 5) changes by 180° compared with the normal mode, which corresponds to the operation of the starting elements of the protection. Curve 1 illustrates the voltage of the control-current source (reference voltage). In Fig. 5 are placed the indicated electrical quantities in the mode when

Fig. 5. Oscillograms of the voltage on the arc-suppression coils of the unit generators in the mode when one of the control-current sources operates in antiphase

one of the control-current sources operates in phase opposition to the others. Curve 1 is the reference voltage of the generator protection sensing element whose frequency divider operates in phase opposition; 2 is the voltage on the arc-suppression coil of the same generator. Curve 3 is the reference voltage of the protection sensing elements of the generators whose frequency dividers operate in phase. Curves 4, 5, and 6 are, respectively, the voltages on the arc-suppression coils of these generators. From the oscillograms in Fig. 5 it is clear that, in the normal operating mode of the generators, the voltage on the arc-suppression coils corresponds to the tripped state of the starting elements. When a phase is grounded on any of the generators, the voltage on the arc-suppression coils in phase does not change in comparison with the normal mode; only the amplitude of these voltages changes. It is quite obvious that such a mode of operation of the control-current sources is highly undesirable, since it may cause incorrect operation of the protection.

To ensure in-phase operation of the control-current sources, a device based on electromagnetic relays is provided, making it possible to synchronize and automatically switch on the control-current sources in the event of an operational loss of the supply voltage.

Oscillographic recording was carried out on an electrical model simulating an enlarged power unit with control-current sources.

The protection sensing element is implemented using semiconductor elements and is structurally housed in the casing of a KRS-132 relay.

At present the protection has been tested under laboratory conditions and has shown satisfactory results. The protection has been developed and will be installed for trial operation on four generators of the enlarged unit of the Saratov Hydroelectric Power Station.

Conclusions

1. The protection considered, based on an injected current, makes it possible to protect 100% of the turns of the stator winding of the generators of an enlarged unit and to selectively identify the damaged generator.

2. The frequency of the injected control current must be lower than the industrial frequency and must differ from the frequencies of the higher harmonics.

3. It is necessary to provide a device to ensure in-phase operation of the control-current sources.

REFERENCES

1. R. A. Vainshtein et al. Use of an electromagnetic parametric frequency divider as a source of control current for protection against ground faults in compensated networks. Izvestiya TPI, vol. 179, 1969.