If it is assumed that the power network is operating in steady state and that a sudden change takes place due to a faulty condition, then the network will enter a dynamic state. Faults have a variable impact over time, with the highest values of current being present during the first few cycles after the disturbance has occurred. This can be appreciated from Figure 1.19, where a three-phase, short-circuit at the terminals of a synchronous generator give rise to currents that clearly show the transient and steady (sustained) states. The figure shows the currents in phases a, b and c, as well as the field current. The source of this oscillogram is (Kimbark, 1995).
Faults are unpredictable events that may occur anywhere in the power network. Given that faults are unforeseen events, strategies for dealing with them must be decided well in advance (Anderson, 1973). Faults can be divided into those involving a single (nodal) point in the network, i.e. shunt faults, and those involving two points in one or more phases in a given plant component, i.e. series faults. Simultaneous faults involve any combination of the above two kinds of faults in one or more locations in the network. The following are examples of shunt faults:
Fig. 1.19 Short-circuit currents of a synchronous generator (© 1995 IEEE).- three-phase-to-ground short-circuit
- one-phase-to-ground short-circuit
- two-phase short-circuit
- two-phase-to-ground short-circuit.
- one-phase conductor open
- two-phase conductors open
- three-phase conductors open.
In addition to the large currents flowing from the generators to the point in fault following the occurrence of a three-phase short-circuit, the voltage drops to extremely low values for the duration of the fault. The greatest voltage drop takes place at the point in fault, i.e. zero, but neighbouring locations will also be affected to varying degrees. In general, the reduction in root mean square (rms) voltage is determined by the electrical distance to the short-circuit, the type of short-circuit and its duration.
The reduction in runs voltage is termed voltage sag or voltage dip. Incidents of this are quite widespread in power networks and are caused by short-circuit faults, large motors starting and fast circuit breaker reclosures. Voltage sags are responsible for spurious tripping of variable speed motor drives, process control systems and computers. It is reported that large production plants have been brought to a halt by sags of 100 ms duration or less, leading to losses of hundreds of thousands of pounds (McHattie, 1998). These kinds of problems provided the motivation for the development of Custom Power equipment (Hingorani, 1995).
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