Voltage collapse

Voltage collapse is a serious accident in power systems caused by a severe drop in voltage. When the power system is operating normally, the reactive power output of the power source is balanced with the reactive power consumption by the load and the reactive power losses in the network. When there is a shortage in power supply or reactive power compensation capacity, the voltage at the load side is forced to drop. Once the voltage falls to a certain critical value, it continues to decline without being able to recover, which is known as voltage collapse. A voltage collapse will force all loads in the region to lose power, and it may even lead to a loss of synchronization between various parts of the system, resulting in asynchronous oscillations and an accident across the entire system, with further loss of loads. The relationship curves between reactive power and node voltage for the generator and loads at a given node, QG-U and QLD-U (see figure). They have two intersection points, a and b. Point A is a stable point that can withstand small perturbations ； Point b is an unstable point. When operating at point a, and when operating at point b, the critical condition is that the voltage corresponding to that point is the critical voltage Ucr. When operating at point a, if the reactive power QLD > QG, it will restore the voltage to Ua ； Operating at point b, where QLD＞QG, will cause the voltage to drop further, leading to a vicious cycle and voltage collapse. At the critical point, if QLD < QG, the voltage must rise to point a in order to reach equilibrium. Measures to prevent voltage collapse include: ① Providing sufficient reactive power compensation sources during planning and design to achieve a basic balance at different levels and zones ； ②In the event of a reactive power deficit, the accident overload capacity of generators and synchronous condensers should be fully utilized, along with the use of strong excitation devices and automatic excitation regulators ； ③Put the switchable electrostatic capacitors and controllable static compensators into operation ； ④If necessary, part of the load should be cut off, or the load should be automatically disconnected at low voltage.

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