Voltage stability at the design level on the other hand can be described as the systems deliberate measures to ensure that a steady voltage is maintained throughout the conditions as well as demand. Voltage control measures are the intervention strategies introduced for purposes ensuring that the power system remains stable throughout (Kundur, 1994, p17).
Voltage control is aimed at ensuring minimum disturbances by maintaining voltage within the acceptable limits and must satisfy the following. voltage utility at the terminals is always within the required limits, system stability issues are minimized and that reactive flow is maintained at a very low magnitude to facilitate low RI2 and XI2.
Voltage stability is maintained if for every system bus, V (voltage magnitude) increases by increases in the utilization at the other end of Q (reactive power injection). However, voltage instability occurs if as a minimum, one bus has V decreasing while Q increases. Otherwise put, voltage stability will be achieved if for all buses V-Q sensitivity is positive or instability will occur if negative for one bus as a minimum.
Transient stability is the capacity of a power system to sustain synchronism is exposed to strong transient instabilities or disturbances. The most defining characteristic of the instability that the system is meant to overcome is a system failure that requires a very short critical clearing time. This type of instability is however serious since the generator rotors experience a rapid kinetic energy accumulation so huge that the first power swing is not enough to release it.
High voltage power cables require high quality insulation due to the nature of the high electric strength of the currents they transmit. To ensure that the insulation system meets the standard for high voltage involved, several attempts have been made since a long time ago. One