ABSTRACT
An overcurrent is when the maximum continuous load current permissible for an item of electrical plant is exceeded. Overcurrents are characteristic of phase and ground faults, but can also occur during normal operation, e.g. when energizing power transformers, induction motors etc. Impedance is the criterion measured to detect faults on transmission systems or underexcitation or out-of-step conditions on generators. A directional unit is used in combination with an overcurrent unit at locations on the power system where the overcurrent criterion on its own is insufficient to preserve discrimination. In an AC system a reference is needed to determine power direction and for this purpose the busbar voltage is used. The deviation of the frequency of an electrical power system from its rated value is an indication of an imbalance between real power generation and load demand, too little power causing the frequency to fall and too much power causing it to rise.
