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An alternator is an important aspect of a ship's electrical system. Any kind of obstacle in its performance can mar the working of the ship's overall electrical system. It is for this reason that it requires adequate protection systems to prevent any kind of hindrance to the ship's functionality.
Several protection systems are installed to protect the ship's alternator and the main distribution system against various faults. This protection system is of utmost important, for without it, the ship might have to face black out and even system failure. So let us study these mechanisms and find out how does an alternator protection system work.
The three main type protection systems are:
- over current protection
- reverse power trip
- under voltage trip
The over current protection works by removing power supplies to non essential services on a preferential basis. This is done with the help of preferential trips which decides the load that is to be removed in case of overloading of the distribution system.
Reverse power trip and under voltage trip are required only if there is more than one alternator on board which requires working of two alternators on load.
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Over Current Protection
Every alternator has an over current protection. With the help of this trip,the alternator and distribution system can be protected from various faults but the main thing to be considered in this method is to maintain power to the distribution system till the time the alternator trips on any other protection devices.
For this reason,the protection device has been designed in such a way that in case the over current is not high enough, a time delay provided by an inverse definite minimum time (IDMT) relay occurs, which prevents the alternator from tripping in case the over current values reduces back to normal within the IDMT characteristics
But in case of a major fault such as short circuit, the alternator will trip instantaneously without any delay, protecting all devices on the distribution system.Overload of alternator is caused either due to increased switchboard load or serious fault causing very high current flow.
If sudden over load occurs then, the load is reduced with the help of preferential trips which removes non essential load such as of air conditioning, ventilation fans etc., from the switchboard.These preferential trips are operated by relays which are set to about 110% of the normal full load of alternator.
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Inverse Definite Minimum Time (IDMT) Relay
The construction of this type of relay is similar to that of a household watt meter.The actual inverse time delay characteristics are obtained with the help of an induction type relay.
The system consists of a lightweight non magnetic aluminium disc mounted on a spindle which has helical spring and low friction bearings. This disc is placed in between two electromagnets which are made up of soft laminated iron core. The purpose of the helical spring is to prevent rotation of the disc. The upper electromagnet is supplied with current from the main winding through a current transformer. The main winding is tapped and the taps are brought to a bridge which is used to select different current settings. The lower electromagnet is wounded using a closed coil.
The alternating current in the coil of the main winding located on the center leg of the upper laminated core produces a magnetic field which induces a current in the closed winding. The magnetic field associated with the closed winding is displaced from the magnetic field of the main winding and aluminium disc to produce changing eddy currents in the closed winding. These eddy currents makes the disc to rotate. The rotation is generally prevented by the helical spring when the normal current is flowing through it. Excessive and very large current causes rotation of the disc, making the disc come in contact with the trip circuit.
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Marine Electrical Equipment and Practice - H.D Mc George