Question

A distribution board distributes energy to various items of plant via an installation which has a high voltage (H.V.) panel, a low voltage (L.V) panel and a transformer. The details are as follows: The H.V. panel obtains a 3-phase supply at 11 kV. The transformer circuit-breaker is equipped with 75/5A current transformers which feed a very inverse 3-poles overcurrent relay of 5-A rating. The transformer has a rating of 1000 kVA and voltage ratio of 11000/380 V The L.V. panel has a circuit-breaker on incoming supply from the transformer and feeds the following three-phase balanced loads via switchgears, Lighting and general power- 100 kW at a power factor of 0.8 lagging Motive power-180 kW at 90% efficiency and a power factor of 0.8 lagging Welding-250 kVA Power factor correction capacitor-60 kVAr (a) Select suitable type and current rating for the switchgears of the four loading circuits. (b) Calculate the suitable plug setting of the overcurrent relay (c) Calculate the overcurrent relay operating time if there is a 3-phase short circuit at the 380V busbar. The p reactance of the transformer is 0.08 р.u. The fault level at the 1 1 kV busbar is 50 MVA. The time multiplier of the relay is set at 0.4. System resistance can be neglected The operating time of relays with very inverse time characteristic between 2 to 20 times of its current setting is: 13.5 -1 Is I-fault current; Is relay current setting Т.М.-relay time multiplier setting where

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Answer 1

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