This loss of load is undesirable except as an alternative to the total shutdown of the power pool. 2.2.3 Protection Devices The use of circuit breakers to protect lines against faults is common practice even though their use may cause generator instabilities. This section describes the nature of the instability that can be caused by the normal operation of circuit breakers. When a line is faulted, generators connected electrically close to the fault experience a sharp decrease in their load (since the voltage at the fault is zero, no real power can flow in the faulted line except for line loss) while other units in the system are required to pick-up the fraction of load isolated from the generators on the other side of the fault. This means that during the faulted condition, some generator rotors are accelerated while others are decelerated. Consequently, when the fault is cleared, the system is in a configuration in which some generators are advanced and some are retarded from their previous equilibrium values. There is a maximum angular displacement from which a generator can recover a stable equilibrium. To illustrate this point, consider the simplified case of a generator supplying an infinite bus through a series of transmission lines. Under such circumstances, the power balance of the system is described by the following differential equation.
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