A New Scheme for Protecting Units Against Ground Faults
Abstract Generated abstract
This paper proposes a relay protection scheme for generator units against ground faults that addresses false zero-sequence currents dominated by higher harmonics, especially the third harmonic. Instead of grounding through a reactor or using a frequency filter, the author argues for a wattmeter relay with a 30 degree internal phase shift, connected to a zero-sequence current transformer and a generator line-to-line voltage so that harmonic components in the false current do not produce relay operation. The scheme is analyzed for faults on different phases, showing unequal but usable sensitivity, and an example for a 100 MW unit estimates pickup current, dead zones, and relay power. The paper concludes that the arrangement allows lower pickup currents and higher sensitivity while remaining relatively simple.
Full Text
A NEW SCHEME FOR PROTECTING UNITS AGAINST GROUND FAULTS
I. D. Kutyavin
As is known, for the protection of units against ground faults, the Guidelines on Relay Protection (vol. II, section A—2) recommend the use of maximum-current zero-sequence protection.
The studies of engineer A. B. Krikunchik1 and other authors have shown that, under the normal operating conditions of a unit, and also under various faults beyond the step-up transformer (i.e., outside the protection zone), considerable false currents flow through the resistance connecting the generator neutral to ground; these currents consist chiefly of higher-harmonic currents. The fundamental-frequency current does not exceed 10–15%, while the third-harmonic current amounts to 80–100% of the total false current.
The fault current in the protection zone will consist chiefly of the fundamental-frequency current; therefore, the problem of reducing the false current may be reduced to limiting the higher-harmonic currents in it.
In his work, A. B. Krikunchik proposes two methods for limiting the magnitude of the higher-harmonic currents in the false current. One method consists in grounding the neutral point of the generator through a reactor. In this case, the inductance of the reactor must not come into resonance with the capacitance of the generator-voltage circuit for any third harmonic, since this may lead to the opposite effect, i.e., to an increase in the current of this harmonic.
The second method, recommended by A. B. Krikunchik, consists in applying a frequency filter to supply the current relay of the protection. This method requires the use of a nonstandard current transformer and bulky capacitors.
The influence of higher-harmonic currents on the operation of the relay can easily be eliminated if, for carrying out the protection, a wattmeter relay with an internal phase shift of \(30^\circ\) is used. The current winding of the relay must be connected to the zero-sequence current transformer, and the voltage winding to any line-to-line voltage of the generator, as shown in Fig. 1a. Since third harmonics are completely absent in the line-to-line voltage, while the remaining harmonics are present only in very small amounts, this relay will not respond to the higher-harmonic currents contained in the false current.
In the event of a ground fault in the protection zone, the fundamental harmonic of the fault current will produce on the moving system of the relay a torque proportional to the power:
\[ P = U_b I_p \cos(\varphi_p + 30^\circ). \]
If the voltage \(U_{ac}\) is applied to the relay (Fig. 1b), then, in the event of insulation damage of phase \(A\), the power at the relay will be positive and
is equal to \(U_p I_p\), and if the fault occurs on phase \(B\) or \(C\), then the power will be negative and equal to \(-0.5 U_p I_p\).
The power relay must be provided with double-throw contacts, as shown in Fig. 1; then the protection will operate when any phase is damaged, but it will have different sensitivity: the size of the dead zone on one of the phases will be two times smaller than on the other two.
Reworking the relay contact system, as well as selecting an additional resistance for the relay voltage winding in order to obtain an internal shift of \(30^\circ\), can be carried out in the station relay laboratory.
Example. The unit rating is \(100\) MW, the generator voltage \(U_n = 15.75\) kV, the maximum ground-fault current \(I_{z m} = 10\) A, the transformation ratio of the measuring voltage transformers \(n_H = 20000/100\), and the transformation ratio of the zero-sequence current transformer \(n_T = 5/5\).
On the basis of A. V. Krikunchik’s tests, for a \(100\) MW unit with a solidly grounded transformer neutral, the maximum false current of the first harmonic is \(110\) mA. The protection pickup current may be taken as \(0.2\) A.
The dead zone of the protection for phase \(A\):
\[ z_0=\frac{I_{pT} n_T}{I_{z m}}\,100 =\frac{0.2\cdot 1}{10}\,100 =2\%. \]
The same for phases \(B\) and \(C\):
\[ z_0=\frac{I_{pT} n_T}{0.5 I_{z m}}\,100 =\frac{0.2\cdot 1}{0.5\cdot 10}\,100 =4\%. \]
Relay pickup power:
\[ P_{pT}=U_{p m} I_{pT} =\frac{U_n}{n_H} I_{pT} =0.2\frac{15750}{200} =15.75\ \text{W}. \]
To implement the protection, an IM-41 relay may be used.
Conclusions. The proposed scheme for protecting units against ground faults does not respond to higher-harmonic currents contained in the false current, and therefore permits the use of small pickup currents, as a result of which the sensitivity of the protection proves higher than that of other schemes.
In its simplicity, the scheme is second only to maximum current protection without a frequency filter.
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“Electrical Stations,” 1940, Nos. 10–11, 12. ↩