Longitudinal Differential Protection Scheme for a Generator with Blocking of False Trips in the Event of Faults in the Secondary Circuits of Current Transformers

K. K. Ryazantsev

(Presented by Professor, Doctor of Technical Sciences I. D. Kutyazin)

The schematic diagram of the longitudinal differential protection of a hydrogenerator is shown in Fig. 1. The operating current of the protection, according to [1], may be taken as equal to:

\[ I_{c3} = (0.5 \div 0.6) I_{n.g}. \]

When faults occur in the secondary circuits of the protection, a false tripping of the generator is possible if its load is 50–60% of the rated value, since the protection has no blocking that would prevent it from operating in such cases. Such blocking may be accomplished by means of a current relay connected into the neutral conductor of the scheme, similarly to how this is done for turbogenerators. But in the latter case this relay only monitors the serviceability of the secondary circuits of the current transformers and acts only on a signal (Fig. 2).

In the proposed scheme, the operating circuit of the differential protection passes through the normally closed contacts of relay 2 (Fig. 3). At an operating current

\[ I_{cp2} \simeq 0.2 \frac{I_{n.g}}{k_{mn}} \simeq 1\ a \]

relay 2 has a margin with respect to the unbalance current under normal conditions and during generator overloads, since in these cases only the unbalance current from two three-transformer zero-sequence current filters flows through this relay (Fig. 4). If each filter is composed of ordinary (not specially selected) transformers, then its unbalance current does not exceed \(0.1\ a\) at rated primary current [1].

Thus, it may be expected that the unbalance current of the scheme flowing through relay 2 under normal conditions will be on the order of several tenths of an ampere, and that relay 2 will also not operate during generator overloads.

In the event of faults in the secondary circuits of the protection associated with the flow through relays 1 and 2 of currents exceeding their operating currents, relay 2 will break the operating circuit before relay 1 closes its contacts. This will occur because relay 2 operates to open; its operating current is smaller and, moreover, it is connected without a current-saturating transformer, which introduces some delay into the action of the main relays.

In the case of multiphase short circuits inside and outside the protection zone, the unbalance currents may exceed the operating current of relay 2, and it will break the operating-current circuit, which is inadmissible. To avoid failure

of the protection, the operating circuit of relay 2 is supplied in such cases from relay 3 of the overcurrent protection, which has a pickup current

\[ I_{cp3} = (1.3 \div 1.5) I_{n.g}. \]

As a result, an operating positive is applied to the contacts of relay 1 even when the generator current increases by 30–50%, i.e., in those cases when operation of relay 1 is not yet to be expected.

In the event of faults in the stator winding near the neutral, two variants of short circuit should be assumed—a solid metallic short circuit and a short circuit through transition resistance.

Fig. 1.

Fig. 2.

As is known from [2, 3], in the case of a metallic short circuit near the neutral, the current on the neutral side has a value considerably exceeding the pickup current of relay 3; the latter will close its contacts and ensure operation of the differential protection. In the case of a short circuit through transition resistance, the current on the neutral side may prove insufficient for operation of relay 3, but then the current on the bus side is also small. The point is that in both variants of short circuit the current on the bus side, as the point of the short circuit approaches the neutral, tends to zero, since the difference between the e.m.f. of the damaged and undamaged generators decreases and, in the limit, is equal to zero.

Therefore, in the case of short circuits near the neutral through transition resistance, the unbalance current should not reach the value \(I_{cp2}\), and relay 2 will not open the operating circuit.

If a fault occurs in the secondary circuits of the protection and relay 2 operates, the generator will not be left without high-speed protection. Relay 3 of the overcurrent protection, through the closed operating contacts of relay 1 of the differential protection, will send a trip impulse if the current is equal to or greater than \((1.3—1.5) I_{n.g}\).

On the basis of all that has been set forth above, it may be assumed that the proposed method of blocking differential protection may also find application in turbogenerators, especially since its implementation does not require any major complication of the standard scheme, while at the same time making it possible to reduce the pickup current, which will lead to an increase in the sensitivity of the protection.

In the figure: “For tripping”; “For signal.”

Fig. 3.

Fig. 4.

Literature

1. A. M. Fedoseev, Relay Protection of Electrical Systems, Gosenergoizdat, 1952.

2. N. F. Margolin and A. B. Chernin, Method for Calculating Currents in Internal Faults in Synchronous Generators, ONTI, 1937.

3. G. V. Podkovyrov, On longitudinal differential protection of generators, Electric Stations, 1953, No. 11.