Friday, June 28, 2013

Safeties of generators: Aux & Emer.: Learn their safeties!

Protection of generators: Aux & Emer.

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Part C Machinery, Electricity, Automation and Fire Protection
  Chapter 2  Electrical Installations

7.8 Protection of generators

7.8.1 Generators are to be protected against short-circuits and overloads by multipole circuit-breakers.
For generators not arranged to operate in parallel with a rated output equal to or less than 50 kVA, a multipole switch with a fuse in each insulated phase on the generator side may be accepted.
7.8.2 When multipole switch and fuses are used, the fuse rating is to be maximum 110% of the generator rated current.
7.8.3 Where a circuit-breaker is used:
the overload protection is to trip the generator circuit-breaker at an overload between 10% and 50%; for an overload of 50% of the rated current of the generator the time delay is not to exceed 2 minutes; however, the figure of 50% or the time delay of 2 minutes may be exceeded if the construction of the generator permits this
the setting of the short-circuit protection is to instantaneously trip the generator circuit-breaker at an overcurrent less than the steady short-circuit current of the generator. Short time delays (e.g. from 0,5 s to 1 s) may be introduced for discrimination requirements in "instantaneous" tripping devices.
7.8.4 For emergency generators the overload protection may, instead of disconnecting the generator automatically, give a visual and audible alarm in a permanently attended space.
7.8.5 After disconnection of a generator due to overload, the circuit-breaker is to be ready for immediate reclosure.
7.8.6 Generator circuit-breakers are to be provided with a reclosing inhibitor which prevents their automatic reclosure after tripping due to a short-circuit.
7.8.7 Generators having a capacity of 1500 kVA or above are to be equipped with a suitable protective device or system which, in the event of a short-circuit in the generator or in the supply cable between the generator and its circuit-breaker, will de-excite the generator and open the circuit-breaker (e.g. by means of differential protection).
7.8.8 Where the main source of electrical power is necessary for the propulsion of the ship, load shedding or other equivalent arrangements are to be provided to protect the generators against sustained overload.
7.8.9 Arrangements are to be made to disconnect or reduce automatically the excess load when the generators are overloaded in such a way as to prevent a sustained loss of speed and/or voltage (see Ch 2, Sec 2, Tab 6). The operation of such device is to activate a visual and audible alarm. A time delay of 5-20 s is considered acceptable.
7.8.10 When an overload is detected the load shedding system is to disconnect automatically, after an appropriate time delay, the circuits supplying the non-essential services and, if necessary, the secondary essential services in a second stage.
7.8.11 Alternating current generators arranged to operate in parallel are to be provided with reverse-power protection.
The protection is to be selected in accordance with the characteristics of the prime mover.
The following values are recommended:
2-6% of the rated power for turbogenerators
8-15% of the rated power for diesel generators.
The reverse-power protection may be replaced by other devices ensuring adequate protection of the prime movers.
7.8.12 Generators are to be provided with an undervoltage protection which trips the breaker if the voltage falls to 70%-35% of the rated voltage.
The undervoltage release also prevents the closing of the circuit-breaker if the generator voltage does not reach a minimum of 85% of the rated voltage.
The operation of the undervoltage release is to be instantaneous when preventing closure of the breaker, but it is to be delayed for selectivity purposes when tripping the breaker.


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