G59 Mains Protection Relays – ComAp

Major updates to the G59 regulation have been announced effective from 1st February 2018. Learn more here. 


What is mains protection?

G59 mains protection relays ensure the safe operation of generators and the grid. They isolate renewable and local power generation facilities such as solar, wind, hydro-electric and CHP sites from grid fluctuations.

The need for G59 mains protection

The drive to use environmentally-friendly electricity has changed the power generation landscape. Instead of relying on a relatively small number of high-capacity power stations, we are turning towards a much larger number of low-capacity generators. Wind farms, photo-voltaic (PV) solar sites and hydro-electric plants are replacing coal and oil power stations.

Mains protection is essential when generating ‘parallel-to-mains’

The rapid expansion of lower-capacity generators can cause the grid to become less stable and this risks damaging both the generator and the grid. Systems that run ‘parallel-to-mains’ must include a relay to decouple the generator from the grid if a problem arises with either.


This 2-minute video summarises the main facts about G59.

The standard that governs mains protection in the UK and Ireland is G59. G59 mains protection is an important regulatory standard for both network operators (DNOs) and power generators. For the DNOs and utility companies G59:

  • Ensures safe balanced operation of the distribution system.
  • Guarantees reliable delivery of high quality electricity.
  • Allows smooth involvement of the distributed generation.
  • Enables remote control of power supply sources.

For generating companies, G59 offers:

  • reliable protection of their plant.
  • simple and standardised implementation across the whole of the UK.


What do G59 relays do?

A G59 mains protection relay continually monitors the quality and stability of the grid. It is programmed to react according to parameters set by the DNO including voltage, frequency, and ROCOF (rate of change of frequency). The standard has evolved through G59/2 and G59/3, and so parameters have been refined to improve the level of protection.

When the mains protection relay detects an event outside the programmed limits it trips a circuit breaker and decouples its generator from the grid.

The mains protection relay can also shut the generator down after an event. This is important if a generator (such as a wind turbine or solar panel farm) continues to supply power to its ‘island’ after the connection to the grid has been broken. This can prevent serious incidents. An engineer may arrive to repair the broken connection and, knowing there is no connection to the grid, start work on what he thinks is a dead cable. Without the mains protection relay, the cable could still be live.

What can IPU do for you?

IPU supply the most advanced and popular products in the market: ComAp’s MainsPro and InteliPro.

Panel-builders, consultants, contractors as well as distribution network operators (DNOs) all trust and know the MainsPro and InteliPro. There are 3,000 installations of the MainsPro in the UK (and over 10,000 around the world), making it one of the most popular.

IPU’s G59 product manager Ian Wassman has been an integral part of the recent G59 updates. Because of this, the expertise we can offer to customers is second to none.


What’s the latest G59 news?

There have been recent updates made to the G59 regulation.

An event recorded on 22nd May 2016 showed a significant number of embedded generation plants had tripped because of Vector Shift protection used for Loss of Mains (LoM). This event resulted in a loss of infeed and a frequency excursion bigger than anticipated.

Because of this, the following updates have been made:

  • Rate of Change of Frequency (RoCoF) settings must be changed to 1 Hz/s with a definite time delay of 500 ms for all new Non-Type Tested Generators.
  • All Non-Type Tested Generating Plants commissioned on or after 1st February 2018 will not be allowed to use Vector Shift (VS) as a means of Loss of Mains (LoM).

You can learn more here.

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