G59-3.4 and G99 Mains Protection Relays – ComAp

There have been big changes to the world of mains protections. These changes bring consistency and stability to the grid.

G99 came into force on 27th April 2019. Sites will no longer be able to make new connections using the G59-3.4 standard. Sites connected after February 2018 with G59-3.4 will need to make the upgrade to G99. Learn more about the new recommendation here.

Sites connected before February 2018 will remain on G59-3.4, but will need to make upgrades to ensure they are up-to-date and fully compliant. The National Grid have launched the Accelerated Loss of Mains Change Programme with funding available to encourage a quick and smooth change over.

The Accelerated Loss of Mains Change Programme refers to the retrospective upgrades (to either settings or firmware) to ensure compliance to G59-3.4. Read more here about how ComAp can help you through the process.


What is mains protection?

G59-3.4 and G99 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 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.

The standard that governs mains protection in the UK and Ireland is (as of April 2019), G99 (and G59-3.4 for site connected before Feb 2018). G59-3.4 and G99 mains protection is an important regulatory standard for both network operators (DNOs) and power generators. For the DNOs and utility companies, G59-3.4 and G99:

  • 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, G59-3.4 and G99 offers:

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


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.


Mains protection video

Although G59 has now been replaced with the new G99, the basic principles of mains protection show in this video still apply.


What can ComAp do for you?

ComAp 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.

ComAp’s mains protection product manager Ian Wassman has been an integral part of the recent G59/G99 updates. Because of this, the expertise we can offer to customers is second to none.



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