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Selecting the right protection for your generator

  The choice of electrical protection for your generator will depend on a number of things such as
  • The size of the generator
  • The type of network it will be connected to (i.e. 3 or 4 wire system)
  • The number of generators that will be connected in parallel
  • The importance of the function of your generator i.e. is it going to act just as a standby generator or is it going to be permanently running supplying important feeds to motors, essential services etc etc
  • What is the cost impact of down time
  • The cost of the protection itself


Take for example a generator whose sole purpose is to power an electric fire pump. In a real emergency, the electrical protection would be inhibited to prevent  any trips. You wouldn't want the pump to stop pumping water even if there was an electrical fault on the generator. The only time that the protection  would be in service is during weekly or monthy routine testing. So in this instance, to be cost effective, you would use the bare minimum of protection.  Perhaps just machine differential, voltage restrained overcurrent, over voltage, earth fault and motor protection.

On the other hand, three 20 Mw generators connected together supplying the power for an offshore installation would probaby require the full suite of   protection comprising -
  • Machine differential
  • Voltage restrained over current
  • Field failure
  • Under/over Frequency
  • Under/over voltage
  • Negative phase sequence
  • Restricted earth fault
  • Standby earth fault
  • Reverse power
  • Voltage zero sequence
Nowadays, with the developement of multi function relays such as the CEE NPG800, or the Schneider Micom P34x suite, most of the above functions are contained  in one relay. And in instances where not all of the functions are required it may still be cheaper to buy a multi function relay and simply set the non  required functions out of service.

So as you can see there are a number of things to consider. There are a number of PRAG's (Protection relay application guides) freely available on the net.  Click here to view the CEE Relay PRAG.

New website feature! Calculate your relay trip times

Why not use our new calculator feature to calculate the trip times of your overcurrent or thermal relays. Tripping curves based on the IEC formulas for all types of inverse curves.

Check out the latest range of ICE products from CEE Relays here

Check out the latest range of ISA manual and automatic test equipment here

Check out the latest Micom range from Schneider

Check out the latest REF range from ABB

Check out the latest NP800 range from CEE

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