Subsynchronous resonance is a condition that can exist on a power system where the network has natural frequencies that fall below the fundamental frequency of the generated voltages. Transient currents flowing in the ac network have two components; one component at the frequency of the driving voltages and another component at a frequency that depends entirely on the elements of the network. For a network with only series resistance and inductance, an isolated transient, such as switching a load, will consist of a fundamental component and a de component that decays with a time constant that depends on the LIR ratio of the equivalent impedance between source and load.
Since loads are frequently switched on and off, the transient currents usually appear as random noise, superimposed on the fundamental frequency currents. The addition of shunt capacitors to the network result in new natural frequencies of oscillation that are always greater than the fundamental frequency. In networks containing series capacitors, the currents will include oscillatory components with frequencies that depend on the relative magnitude of the transmission line Land C elements, but have frequencies that are below the system fundamental frequency.
An insulator or dielectric is a substance within which there are no mobile electrons necessary for electric conduction. However, when the voltage applied to such an insulator exceeds a certain value, then it breaks down and allows a heavy electric current (much larger than the usual leakage current) to flow through it. If the insulator is a solid medium, it gets punctured or cracked. The disruptive or breakdown voltage of an insulator is the minimum voltage required to break it down. Dielectric strength of an insulator or dielectric medium is given by the maximum potential difference which a unit thickness of the medium can withstand without breaking down. In other words, the dielectric strength is given by the potential gradient necessary to cause breakdown of an insulator. Its unit is volt/meter (V/m) although it is usually expressed in KV/mm. For example, when we say that the dielectric strength of air is 3 KV/mm, then it means that the maximum PD which one mm thickness of ...
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