Even though most protective system designs are built around individual components, system-wide disturbances in power systems are rather a frequent occurrence and a challenging issue for electric utility companies. When major disturbances occur in power systems, it is imperative to implement coordinated protection and control actions to stop the system degradation, restore the normal state and minimize the effect of the disturbance. Local protection systems are often not capable of protecting the overall system, which may be affected by the disturbance.
Incidents that create power system disturbances can be categorized into the following groups:
These phenomena are mitigated using a variety of protective relaying and emergency control measures.
Each time when a fault or a topological change affects the power balance in the system, the instantaneous power imbalance creates oscillations between the machines. Stable oscillations lead to transition from one (pre-fault) to another (post-fault) equilibrium point, whilst unstable oscillations allow machines to oscillate beyond the acceptable range. If the oscillations are large, then the station’s auxiliary supplies may undergo severe voltage fluctuations and eventually trip. If this happens, the subsequent resynchronization of the machines might take a long time. Thus, it is desirable to trip the machine exposed to transient unstable oscillations whilst maintaining the plant auxiliaries energized.
The purpose of out-of-step protection is to remove the possibility of damage to generators as a result of an out-of-step condition. In case where power system separation is imminent, it should separate the system along the boundaries, which will form islands with balanced load and generation. Distance relays are usually used to provide an out-step protection function, whereby they are called upon to provide blocking or tripping signals upon detecting an out-of-step condition.
Voltage stability is the ability of the system to maintain voltage such that when the load admittance is enhanced, load power will increase, so that both power and voltage are controllable. On the other hand, voltage collapse is defined as the process by which voltage instability leads to a very low voltage profile in a significant part of the system.
Voltage instabilities in power systems arise from heavy loading, insufficient reactive support resources, and unforeseen phenomena and/mis-coordinated action of the tap-changing transformers. Such phenomena can lead to system-wide blackouts. The risk of voltage instability increases as the transmission system becomes more heavily loaded.
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Voltage stability problems are manifested by several features such as:
Voltage instability can be alleviated by combination of the following corrective actions:
Overload of one or a few power system elements may lead to a cascading overload of many more elements, mostly transmission lines, and may ultimately lead to a complete power system blackout. A simple, swift and reliable technique to reestablish active power balance is to shed load by under-frequency relays.
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