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lecture18 - EE 740 Professor Ali Keyhani Lecture#18 Short...

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EE 740 Professor Ali Keyhani Lecture #18 Short Circuit Studies
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Short Circuit Studies 1) Short circuit study as an engineering tool: The main objective of the short circuit study is to determine the interrupting capability required for circuit breakers at each switching location. For this type of application, the maximum currents the breaker may be exposed to during the three phase balanced faults on each side of the breaker need to be determined. For protective relaying design it is necessary to determine voltages and currents for balanced and unbalanced faults at many locations in the system. 2)Short Circuit Problem Formulation: Assumptions: 1) Normally, all shunt elements including loads and line charging are neglected. Loads may be represented with constant load impedance models. 2) Each generator is represented by a constant voltage behind its subtransient reactance. 3) All tap changing transformers are assumed to be at their nominal tap settings. 4) Balanced transmission lines are assumed. For a balanced three phase fault only the positive sequence network is excited. For an unbalanced fault, all three sequence networks may be excited. 5) The negative and positive sequence networks are normally assumed equal and coupling between adjacent circuits is taken into account in the zero sequence network only.
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3) All voltages behind the generator reactances are assumed equal in magnitude and phase angle (normally one per unit). Therefore no current is flowing in any portion of the network prior to the fault.
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