Protection of Generators Against Interturn Short Circuits
Abstract Generated abstract
The paper proposes a protection method for detecting interturn short circuits and related asymmetrical stator winding faults in generators operating in parallel on generator-voltage buses with grounded neutral points. It derives the zero-sequence current that arises between generator neutrals during such faults and uses its direction to justify a wattmeter-type maximum power relay connected through a neutral current transformer and a zero-sequence voltage transformer group. Formulae are given for selecting the relay operating power, including allowance for grounding resistances, transformer ratios, the number of stator turns, and reliability factors, followed by a numerical example. The proposed scheme is presented as selective, simple, and useful both for interturn fault protection and as backup for differential protection, but it is limited to parallel generator operation and does not function for a single generator on the network.
Full Text
PROTECTION OF GENERATORS AGAINST TURN-TO-TURN SHORT CIRCUITS
I. D. Kutyavin
Candidate of Technical Sciences
The protection of generators against turn-to-turn short circuits of the stator winding proposed by the author is applicable only to generators operating in parallel on generator-voltage buses.
Figure 1 presents a diagram of the distribution of fault currents for generators operating in parallel. As can be seen from Fig. 1, in the case of a turn-to-turn short circuit in the stator winding of generator No. 1, between the neutral point of the damaged generator and the neutral point of the healthy one there will appear a potential difference equal to
\[ U_0 = xU_{\phi}, \tag{1} \]
where \(U_{\phi}\) is the phase voltage of the generator and \(x\) is the number of short-circuited turns in the phase of the stator winding, in fractions of unity.
Fig. 1
Under the action of this potential difference, between the neutral points of the generators, through the grounding resistances and the earth, a current will flow equal to
\[ I_{з} = \frac{xU_{\phi}}{R_1 + R_2} K_{\text{п}} . \tag{2} \]
In the general case, with several generators having different grounding resistances,
\[ I_{з} = \frac{xU_{\phi}K_{\text{п}}} {R_1 + \dfrac{1}{\dfrac{1}{R_2} + \dfrac{1}{R_3} + \dfrac{1}{R_4} + \ldots}} = \frac{xU_{\phi}K_{\text{п}}} {R_1 + \dfrac{1}{\sum \dfrac{1}{R_n}}}, \tag{3} \]
where \(R_1, R_2, R_3\) are the grounding resistances of the neutral points of the generators, and \(K_{\text{п}} \le 1\) is the coefficient of incompleteness of the short circuit between turns.
In the grounding resistances of the healthy generators this current will be directed toward the neutral point (as also in the case of ground faults in the generator-voltage network), and in the resistance of the damaged generator—from the neutral point into the earth. This circumstance is used for estab-
devices for protection against turn-to-turn short circuits, the diagram of which is shown in Fig. 2. The protection is implemented by means of a maximum-power relay responding to zero-sequence power directed from the neutral point to ground. The current coil of the wattmeter relay is fed from a current transformer connected into the neutral bus of the generator, and the voltage coil—from a group of three single-phase voltage transformers connected in a zero-sequence filter and connected to the buses of the generator voltage.
Fig. 2
This protection will also operate for other kinds of asymmetrical damage to the stator winding not accompanied by a ground fault, since in these cases a zero-sequence current sent by the other generators will flow through the stator winding and the grounding resistance of the damaged generator, and a zero-sequence voltage will appear on the secondary winding of the voltage transformers.
Selection of the principal protection parameters
The power at the terminals of the wattmeter relay during a short circuit of \(x\) turns of a phase of the generator stator winding, according to equations (1) and (3), will have the following expression:
\[ P = \frac{x^{2}U_{\phi}^{2}}{n_{\mathrm{н}} n_{\mathrm{т}}} \frac{K_{\mathrm{п}}R_{1}} {\left(R_{1}+\dfrac{1}{\sum \dfrac{1}{R_{n}}}\right)^{2}} . \tag{4} \]
For reliable operation of the protection when the minimum permissible number of turns of a phase is short-circuited, it is necessary to provide at the relay terminals a power sufficient for the operation of the protection. Then the operating power of the relay will have the expression
\[ P_{\mathrm{т}} = K_{\mathrm{н}}P_{\min} = \frac{x_{\min}^{2}U_{\phi}^{2}}{n_{\mathrm{н}} n_{\mathrm{т}}} \cdot \frac{K_{\mathrm{н}}K_{\mathrm{п}}R_{1}} {\left(R_{1}+\dfrac{1}{\sum \dfrac{1}{R_{n}}}\right)^{2}}, \tag{5} \]
where \(K_{\mathrm{н}}\simeq 0.8\) is the reliability coefficient of protection operation.
In practice it may be assumed that the number of short-circuited turns in an interturn short circuit without a ground fault will be an integer; therefore it is desirable to take the smallest number of short-circuited turns for which the protection must operate as equal to unity, then
\[ x_{\min}=\frac{1}{W_{\phi}}, \]
where \(W_{\phi}\) is the number of series-connected turns in a phase of the stator winding.
The final expression for the relay operating power will have the form
\[ P_{\tau} = \frac{ U_{\phi}^{2} K_{\mathrm{n}} K_{\mathrm{p}} R_{1} }{ n_{\mathrm{n}} \cdot n_{\mathrm{t}} W_{\phi}^{2} \left( R_{1} + \frac{1}{\sum \frac{1}{R_{n}}} \right)^{2} } \ \text{W}. \tag{6} \]
Example:
The power of the generators operating in parallel is \(P_{1}=P_{2}=55.5\) MW. The generator voltage is 10.5 kV. The number of turns in a phase of the stator winding is \(W_{\phi}=18\). The grounding resistances of the neutral points of the stator windings of the generators are \(R_{1}=R_{2}=120\ \Omega\). The transformation ratio of the voltage transformers is \(10000/100\) volts.
The maximum current flowing through the grounding resistance when a generator terminal is short-circuited to the frame is
\[ I_{\mathrm{g}} = \frac{U_{\phi}}{R_{\mathrm{g}}} \cdot \frac{10500}{\sqrt{3}\cdot 120} \simeq 50\ \text{A}. \]
The transformation ratio of the neutral current transformer may be taken as \(50/5\) amperes.
The relay operating power at \(K_{\mathrm{n}}=0.8\) and \(K_{\mathrm{p}}=0.8\) is
\[ P_{\tau} = \left( \frac{10500}{\sqrt{8}\cdot 18} \right)^{2} \frac{0.8\cdot 0.8\cdot 120}{10\cdot 100\cdot (120+120)^{2}} = 0.15\ \text{W}. \]
However, in order to increase the relay operating power, the transformation ratio of the neutral current transformer could have been taken considerably smaller, since only a short-time fault current flows through it.
Conclusion
It is necessary to note the following positive aspects of the proposed protection.
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The protection will operate for all asymmetrical faults of the stator winding (including turn-to-turn short circuits) that are not accompanied by a ground fault.
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In the cases indicated in item 1, it will operate independently for turn-to-turn faults and will serve as backup for differential protection in the event of other faults.
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The protection will operate selectively, since positive power at the relay terminals will be absent in all cases except those indicated in item 1.
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The protection is simple and does not require large expenditures.
Disadvantages of the protection: 1. The protection is applicable only to generators operating on generator-voltage buses and having grounded neutral points. 2. The protection does not operate when a single generator is operating on the network.