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• • • The reverse power relay is prevents power from flowing in the reverse direction. 

• • • The relay is used in installations where a generator runs in parallel with another generator

• • • to prevent power from the bus bar or another generator from flowing back to the active generator when its output fails.

• • • If generator voltage falls, it quickly disconnects the generator coil to avoid power from flowing into the stator coil.

• • • When the prime mover fails, the generator stops producing power and may instead start drawing power from the other parallel sources and start motoring. The reverse power relay senses any reverse direction of power flow and disconnects the generator to avoid any possible damage.

 

 

• Reverse power relay Construction and operation

 

• The relay is made of a lightweight non-magnetic aluminum disc between two soft laminated iron core electromagnets, and fixed on a spindle running on low friction bearings.
•  The upper electromagnet is wound with a voltage coil which is then supplied from one phase and an artificial neutral of the generator output.
•  The other magnet has a current coil from supplied from a current transformer connected to the same phase as the voltage in the upper electromagnet.
• The voltage coil has a high inductance, designed in a way that the voltage lags the current in the coil by about 90 degrees. This lag ensures that the magnetic field generated from the current in the upper coil lags the magnetic field produced by the current in the lower electromagnet.
• The two magnetic fields which are out of phase, produces eddy current in the aluminum disc, and this creates a torque that tries to rotate the disc.
• Under normal condition when power is flowing as expected, the trip contacts of the relay are open, and the disc is against a stop. If a reverse power starts to flow, the disc rotates in the opposite direction, moves away from the stop and towards the trip contacts that activates the trip circuit.

[cat60y]

One of the PSC / Class checks is checking if the reverse power reley is working.

How to check is… to run 2 generator on synchronise.

Changed the switch to manual, reduce the governer on the switchboard to reduce the load until negative. The generator should trip.

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