Electrical Faults on Power System
Electrical power system have a dynamic and complex behavior. Different types of faults can interrupt the healty operation of the power system. Some of the major Electrical faults are phase faults include phase to phase faults and phase to ground faults and three phase faults. Other Electrical faults are of not major significance but still are considered, Open circuit faults occurs due to the parting of the overheadline or failure operation of the circuit breaker, Interturn fault occurs due to the overvoltage or insulation breakdown, Electrical Faults results in the overloads is due to the passing the current throught the conductor which is above the permissible value and faults due to real power deficit occurs due to mismatch in the power generated and consumed and results in the frequency deviation and collapse of grid.
Electrical Phase faults are characterised as:
- Phase to Ground Fault
- Phase to Phase Fault
- Phase - Phase to Ground Fault
- Three Phase Fault
Phase to Ground Fault:
In this type of Electrical fault all the three sequence components (positive, negative and zero sequence components ) are present and are equal to each other. In case of isolated neutral connection to the generator, there will be no return path for the current. So for such fault, fault current is zero.
Phase to Phase fault:
These are unsymmetrical faults as these faults give rise to unsymmetrical currents (Current differ in magnitude and phase in the three phases of power system).In case of Phase to Phase fault positive and negative sequence component of current are present, they are equal in magnitude but opposition in phase. zero sequence components are absent.
Phase - Phase to Ground Fault:
These faults are of unsymmetrical nature. In this type of faults negative and zero sequence faults are in opposition with positve sequence cmponents.
Three Phase Fault:
This type of faults are called symmetrical fault. This type of faults occur very rarely but more severe compared to other faults. In this faults negative and zero sequence component currents are absent and positive sequence currents are present.
- positive sequence currents are present in all types of faults
- Negative Sequence currents are present in all unsymmetrical faults
- Zero sequence currents are present when the neutral of the system is grounded and the fault also involves the ground, and magnitude of the neutral currents is equal to 3Io
Open Circuit Faults:
Open circuit faults occur either by overhead line parting or pole of the circuii breaker not fully closing. This results in load imbalance on generators and motors lead to negative phase sequence commponents in the stator current. This negative phase sequence component current s rotate at twice the supply frequency in the opposite direction in relation to the rotor and causes additional eddy current losses, results in temperature raise in the rotor.
Interturn faults occurs in machines i.e, Transformers, Motors and Generators. An Interturn fault occurs due to the insulation breakdown between the turns of the same phase or between the parallel windings belonging to the same phase of the machine. The cause of the interturn fault is usually an overvoltge or mechanical damage of the insulation.
Interturn Faults are more severe on large alternators (generators), High voltage motors and power transformers. Interturn fault is most ofen experienced in rotating machines where multiple windings are present in the same groove. For large generators generally single winding rod per groove is designed in such cases interturn fault can occur only in the winding head region.
Interturn Fault can occur at both stator and rotor for rotating machines like generators and motors.
When an interturn fault occurs on stator of a rotating machine there is a high probability that such fault can lead in to the ground fault.
When Interturn faults occur on the rotor winding following symptoms are observed:
- When such fault occur high excitation current is required and this is compensated by the voltage regulator.
- Machine runs less smoothly, because of the asymmetry of the excitation curve
- magnetization of the shaft due to asymmetrical flux
- Bearing damage due to current flowing in the bearings
Interturn faults on power transformers can be occured due to the overvoltages accompnying ground faults or deterioration of the insulation due to chemical influence of the transformer oil.
Interturn fault current depends on the number of the turms shorted and fault currents will be several times higher than the rated current of the windings and thus damages the windings.
Faults due to overload will occur due to exceeding the maximum permissible load current throught the windings, cables, or transmission lines or due to reduction in the cooling offered to the windings.
Electrical conductor is designed in such a manner that the conductor allows permissible amount of current without getting over heated. In this manner the current carrying rating of the conductor is decided. When the current passed through the conductor is above permissible level, no immediate damage occurs but over a period of time conductor insulation will be damaged due to the excess heat generated.
In large generators and power transformers of large MW ratings, the heat generated is enormous, so forced fooling is provided in such cases. For large generators hydrogen cooling is provided and for large transformers forced cooling is provided. This part is nicely presented in Transformer Cooling Methods. When this cooling methods fail then the damage to the equipment is certainly fast compared to the other case.
Real Power Deficit:
Under normal operation the power generated by the generators is equal to the load connected and the losses in the power system. real power is the part of the power which does useful work i.e, the power absorbed by the loads of the power system.
Real power deficit occur when the supply is less than the demand or loss of generating unit in the grid.
When real power deficit occurs frequency levels in the grid starts falling down. The rate of falling of the frequency depends on the magnitude of the deficit in the real power. In this case primary frequency control is carried by the generators connected to the grid. Governer mechanism connected to the turbine will try to drive the turbine with rated speed by accepting more fuel. In this manner little frequency deviations (Real Power Deficit) are managed. In case of still frequency falling down scenario spinning reserves available at the plant will start delivering power to the grid with in few seconds of frequency collapse (mainly Gas turbine plants and hydel palnts). If still the demand and supply gap is not taken care, then load shedding will be followed in the grid by shedding the load of the one part of the power system to mainatain the relation between the supply and demand of real power.
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