On 100% Protection of Synchronous Generators Against Ground Faults

R. A. Vainshtein, A. V. Shmotilov

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

Of all damage to synchronous generators, the majority occurs in the stators, and the bulk of the damage is caused by manufacturing defects [1]. The statistical data cited there also show that the number of generator failures is not decreasing over time. Thus, for example, the total number of generator failures in 1964 increased in comparison with 1963. This is explained by the commissioning of new, unique generators for high-capacity thermal and hydroelectric power plants being built in the USSR. Most electrical failures caused by one reason or another are ground faults of the stator winding. Often a single-phase fault manages to develop into a multiphase short circuit before the resulting accident is cleared [2].

High-power generators (over 50 MW) operate in a unit connection with a transformer. In this case, with the exception of comparatively rare instances (several hydrogenerators operate in parallel on a single transformer winding), the generators have no electrical connection with one another. This circumstance in most cases facilitates the implementation of protection against ground faults of the stator winding, since it is not necessary to identify the damaged generator; however, it does not remove the problem of satisfying other modern requirements imposed on the ground-fault protection of synchronous generators. These requirements reduce to the following: 1) absence of a dead zone, 2) absence of the need to introduce a time delay in order to ensure selectivity.

The need to satisfy these requirements is explained by the following. The presence of a time delay in the protection increases the probability of a double ground fault occurring when the insulation of another phase is impaired.

If the given protection has a dead zone, very dangerous faults are possible—faults of a phase to the generator neutral. The steady-state current for this type of fault, for example in hydrogenerators, exceeds the steady-state current of a three-phase short circuit by more than a factor of two [3].

The existing protection against ground faults of synchronous generators operating in unit connection with transformers has, by its principle of operation, a dead zone near the generator neutral (15%). The need to ensure selectivity for ground faults on the high-voltage side

on the transformer side forces a time delay to be introduced into the operation of the protection.

Elimination of the time delay can be achieved only at the cost of coarsening the protection and, consequently, increasing the dead zone [6].

None of the known methods using natural electrical quantities in steady-state or transient conditions makes it possible to implement protection without a dead zone, since the neutral potential with respect to earth is close to 0.

In this connection, it appears advisable to develop protection that responds to artificially introduced electrical quantities which displace the electrical neutral of the network. Similar protection, as applied to a distribution cable network with compensated neutral, was developed at the Department of Electric Stations of the Tomsk Polytechnic Institute [4]. The protection is effected by connecting a source with a frequency of 25 Hz in series in the circuit of the arc-suppression coil.

Preliminary studies show that, with appropriate design of the responding element, this protection can be successfully applied to high-capacity generator-transformer units with grounding of the generator neutral through an arc-suppression coil. The protection can be implemented most simply for units with one generator or for enlarged units with a split primary winding of the unit transformer. The protection can also be implemented for enlarged units with one low-voltage winding of the transformer. It should be noted that the frequency of the auxiliary source and the method of connecting it into the neutral circuit are also of fundamental importance. At frequencies higher than the power frequency, the multiplicity of change of the auxiliary current, both in magnitude and in phase, is always smaller. The choice of a frequency of 25 Hz for the generator neutral-displacement source was determined by the possibility of using a contactless static device—an electromagnetic parametric frequency divider—and also by one year of operating experience with such a source under industrial conditions at Barnaul CHPP-2. It is shown below that the nature of the change in the current with a frequency of 25 Hz during a ground fault of one generator phase is very favorable. At the same time, the required characteristics of the responding element can be obtained by means of comparatively simple technical means.

In Fig. 1,a a schematic diagram for connecting the protection is shown, where the following designations are used: ASC—arc-suppression coil, CT—current transformer, TCS—test-current source at 25 Hz, RE—responding element. In Fig. 1,b an equivalent circuit is drawn for the generator-transformer section of the circuit. In it the following are denoted:

$U_k$ — voltage of the test-current source (TCS);

$Z_{kt}$ — internal impedance of the TCS;

$Z_d$ — total impedance of the arc-suppression coil;

$C_g$ — capacitance of the generator phase with respect to earth;

$C_t$ — capacitance of a phase of the low-voltage winding of the transformer with respect to earth;

$C_{t(mo)}$ — longitudinal capacitance of the transformer phase between the high- and low-voltage windings.

In what follows, when determining the steady-state test current, we make the following assumptions:

1) we neglect the longitudinal impedances of the zero sequence;

2) we neglect all active losses;

3) We consider the IKT to be a voltage source because \(Z_{\mathrm{kt}}\) is negligibly small in comparison with the resistance of the arc-suppression coil. Taking into account these assumptions, which, generally speaking, are commonly accepted in determining zero-sequence currents of frequency 50 Hz in networks with an isolated or compensated neutral (at a control-current frequency of 25 Hz, these assumptions can be accepted all the more), we obtain the equivalent circuit shown in Fig. 2.

Fig. 1

Labels in Fig. 1: Transformer; Generator; \(C_T(\mathrm{mo})\); \(C_T\); \(Z_g\); \(Z_{\mathrm{kt}}\); \(U_k\); \(a\); \(b\); \(TT\); \(DGK\); \(DO\); \(IKT\).

Fig. 2

In the equivalent circuit the following are denoted:

\(L\) — inductance of the arc-suppression coil;

\(C\) — total capacitance of the generator-voltage network;

\(R\) — transient resistance at the point of the ground fault;

\(I_k\) — current of 25 Hz through the protection current transformer;

\(U\) — IKT voltage.

When the fault occurs through a transition resistance, the test current is equal to

\[ \dot I_k=\frac{\dot U(1+jR\omega C)}{(R-\omega^2 RLC)+j\omega L}, \tag{1} \]

where \(\omega=\dfrac{1}{2}\omega_p\) is the frequency of the test current;

\[ \omega_p=\frac{1}{\sqrt{LC}} \]

is the resonant frequency of the circuit formed by the arc-suppression coil and the total capacitance of the generator network.

Since an attempt is made to set the tuning of the arc-suppression coil as close as possible to resonance, we shall assume that \(\omega_p=\omega_{\text{network}}\).

Introducing the notation \(\omega_p L=X_L\) and putting \(\dot U=1\), after transformations of expression (1) we obtain

\[ \dot I_k=\frac{16R}{9R^2+4X_L^2} +j\,\frac{2(3R^2-4X_L^2)}{9R^2X_L+4X_L^3}. \tag{2} \]

When the transition resistance \(R\) changes within the limits \([\infty \div 0]\), the end of the vector \(\dot I_k\) will describe a semicircle with its center lying on the perpendicular to the direction of the vector \(\dot U=1\) (Fig. 3). The center of the circle is displaced from the origin of the vector \(\dot U\) by a distance \(\dfrac{2}{3X_L}\) toward lag.

Fig. 3

It is seen from the diagram that the ratio of the magnitude of the current \(\dot I_k\) during a fault without transition resistance to the current in the normal mode is equal to 3; the current phase changes by \(180^\circ\) in this case. Figure 4 gives the curves: 1) the dependence of the modulus of the test current on the value of the transition resistance, \(I_k=f_1(R)\); 2) the dependence of the phase of the test current on the value of the transition resistance, \(\varphi=f_2(R)\). The nature of the variation of the curves \(I_k=f_1(R)\) and \(\varphi=f_2(R)\) makes it possible to conclude the following.

1. The responding element of the protection can be implemented on the current principle. The sensitivity and selectivity of such an element will be satisfactory provided that it has increased sensitivity to current of frequency 25 Hz. This will facilitate detuning from the higher harmonics and the industrial frequency in the event of a fault on the high-voltage side of the unit. A disadvantage of such an element is its low sensitivity in the case of a fault through a transition resistance.

Fig. 4

1. The responding element can be made phase-sensitive. A change of phase by \(180^\circ\) in the case of a metallic fault makes it possible to implement—

there may be braking in normal operation. In this case the responding element will be very well detuned from higher harmonics and from faults on the high-voltage side of the unit.

Of particular note is the very abrupt change in the phase of the control current in the event of a fault through a large transient resistance. This circumstance makes it possible to implement protection that operates at significant transient resistances. In this case its sensitivity can be adjusted by changing the angle of inclination of the line of sign change of the output quantity of the responding element. The protection may also be equipped with a more sensitive stage, acting for an alarm in the braking region of the main element. Since, in addition to the current with a frequency of 25 Hz, a power-frequency current flows through the current transformer of the ground-fault protection, it is necessary to give the responding element frequency-selective properties or to compensate the power-frequency current in the secondary circuits by means of a zero-sequence voltage, which is always proportional to this current. It is proposed to implement the required properties of the responding element on the basis of a ring modulator using semiconductor diodes or on the basis of a phase-sensitive amplifier using transistors. The latter is, of course, preferable, since, having the same phase characteristic as the ring phase-sensitive circuit, it has a considerably greater output power. The reference voltage for the phase-sensitive devices will be supplied from the source of the control current. It should be noted that these circuits, at least in part, by their principle of operation will simultaneously solve the problem of detuning from power-frequency currents. This is explained by the fact that, when the ratio of the frequencies of the reference voltage and the signal is an even number, the average value of the voltage at the output of both phase-sensitive circuits is equal to 0.

If there are several units at the station (provided the generators are not electrically connected), the protection can be provided for each generator separately (Fig. 1).

Fig. 5

To implement the protection according to this variant, an ICT must be installed in the neutral of each generator.

However, another variant is also possible, shown in Fig. 5, a, b. In this variant, one ICT is used to protect a group of generators. In such a case the power rating of the ICT may have to be greater than the power ...

...of the individual ICTs in order to provide, under normal operating conditions, the necessary braking current in the responding element. The sensitivity of the protection to ground faults is not reduced. When using the protection variant under consideration, it is advisable to install a backup ICT, which will increase the reliability of the protection.

When implementing the protection according to this variant, it will be necessary to connect the grounded terminals of the arc-suppression coils of the generators.

Figure 6, a, b shows variants of implementing protection against ground faults for generators operating on busbars: a) with several ICTs according to the number of generators having compensated neutrals; b) with one ICT. To implement selective protection in these variants, it is necessary to use a differential circuit. In the normal operating mode of the generator, a capacitive braking current will flow through the responding element, and in the event of a ground fault—the current of the opposite phase, determined by the inductance of the arc-suppression coils.

Fig. 6.

At present, when protection using the natural zero-sequence current is implemented, a busbar zero-sequence current transformer is installed at the generator terminals [6]. This transformer can also be used in the present case without any modification for connection into the differential circuit. The current transformer installed in the neutral circuit will have to be matched with it. It should be noted that, owing to the possibility of frequency detuning of the zero-sequence current filter from unbalance currents, the requirements for the transformer installed at the generator terminals may be reduced; furthermore, the fundamental possibility appears of using a three-transformer zero-sequence filter.

Conclusions

1. Major damage to generators occurs in connection with damage to, or weakening of, the insulation with respect to ground.

2. Existing insulation monitoring and ground-fault protection devices that respond to natural electrical quantities have, by their principle of operation, a dead zone, operate with a time delay, and are weakly sensitive to transition resistances at the fault location.

1. The protection under consideration, which responds to a change in the phase of the control current \(25\,\text{Hz}\), introduced into the generator circuits by means of a parametric frequency divider, has no dead zone, is more sensitive to transient resistances, can operate without time delay, and signals insulation weakening.

REFERENCES

1. Analysis of accidents and cases of defects in operation at power plants and networks of power systems in the electrical and hydraulic-engineering parts. “Energiya,” 1965.

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

3. M. P. Kostenko, L. M. Piotrovsky. Electrical Machines, Part II, “Energiya,” 1965.

4. R. A. Vainshtein. Application of low-frequency parametric systems for protection against ground faults in compensated networks. Dissertation, Tomsk, 1965.

5. K. T. Mityushkin. Magnetic frequency divider. “Elektrichestvo,” No. 8, 1957.

6. Guidelines for Relay Protection. Issue 5. Protection of generator–transformer and generator–autotransformer units, “Energiya,” 1963.