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In the previous article we learnt about the importance of alternator on a ship and a method for alternator protection - Over current protection and about preferential trips. In this article we will learn about two more methods for protecting alternators on ship - reverse power protection system and under voltage protection.
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Reverse Power Protection
There is not much difference between an alternator and electric motors from the engineer's perspective. They are both based on similar principles. So just imagine what would happen if an alternator suddenly would act as a motor. This is only possible in systems where two or more generators are running in parallel.
Hence this type of protection system is used only if there is more than one alternator on board a ship. The system is designed in such a way that it will release the breaker and prevent motoring of alternator if a reversal of power occurs. This protection device is also used to prevent damage to the prime mover, which might be stopped due to some fault. Though it is extremely difficult to detect reverse current with an alternating current system, reverse power can be detected and protection can be provided by reverse power relay.
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Construction and Working
The system consists of a lightweight non magnetic aluminium disc, which is mounted on a spindle having low friction bearings. This disc is placed between two electromagnets made up of soft laminated iron core. The upper electromagnet contains a voltage coil which is connected through a transformer between one phase and an artificial neutral of alternator output. The lower electromagnet also contains a current coil which is supplied from the same phase with the help of a transformer.
The voltage coil is particularly designed with a high inductance so that it can lag current in the coil by an angle approaching 90 degrees. Due to this lag, the magnetic field produced by the current also lags the magnetic field in lower electromagnet. When both these field passes through the aluminium disc, production of eddy currents take place, which results in a torque that tries to rotate the disc.
Under normal power flow, the trip contacts on the disc spindle are open and the disc bears against a stop.When reverse power occurs , the disc rotates in other direction and moves away from the stop and moves towards the trip relay. A time delay of 5 seconds is also provided so as to prevent tripping during synchronizing process. The general reverse power settings are 2 to 6% for turbine power movers and 8 to 15% for diesel engines.
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Under Voltage Protection
This method is used to prevent closure of the breaker by mistake, or the generator that is coming on load during parallel operation. It also provides protection against loss of voltage while machinery is connected to the switchboard.
In this system tripping is generally delayed for discrimination purposes. This is done so that the voltage drop is caused by fault and time is allowed for the appropriate fuse or breaker to operate and the voltage to be recovered without the loss of power supply
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Marine electrical equipment and practice - H.D Mc George