G59/3 mains protection is here – are you ready?

G59/3 defines the latest engineering requirements for connecting renewable energy sources to the UK’s power grid. Before any new generating source can go live the local Distribution Network Operator (DNO) is obliged to check the installation meets G59/3 specifications.

Launched in November 2014, G59/3 advances G59/2’s security standard by specifying extra stability checks that enhance system protection.

Leading mains protection relays such as ComAp’s MainsPro and the InteliPro provide the system stability and protection required to comply.

G59/3’s scope

G59/3 testing assesses the performance of the mains protection relay that sits between the generator and the grid. It’s not a test of the generator itself. The relay’s purpose is to disconnect (“decouple”) the generator from the grid if it detects instability on either side of the connection.

The scope of the regulation is extensive:

  • all generator capacities and voltages
  • all generator technologies
  • commercial and domestic generators
  • short-term (less than 5 minutes per week) and long-term (more than 5 minutes per week) connections to the grid
  • renewable energy sources and STOR sites

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

G59/3’s improvements

One of the new requirements is the inclusion of stability testing. Testing is conducted using multiple frequency and voltage settings, evaluating both to determine the system’s overall stability.

Stability testing assesses the mains protection relay’s ability to monitor power quality over specified time periods.
For example, power is meant to be supplied to the UK grid at 50Hz. A G59/3-compliant relay should decouple the generator in 0.5 seconds if it detects a large anomaly; smaller anomalies are monitored for 90 seconds before the relay has to decouple the generator.

The same principle applies to G59/3’s voltage testing. Large fluctuations prompt the relay to trip after 0.5 seconds. Smaller fluctuations can persist for up to 1.0 or 2.5 seconds before the relay trips.

This stability testing seems complex but it exists for good reasons. Not all fluctuations are severe enough to warrant decoupling generators. Decoupling without good cause is both disruptive and expensive – circuit breakers have a finite life before they need expensive maintenance.

G59/3’s sophisticated stability testing ensures that generators are only decoupled for events that are likely to cause serious danger to the grid, the generator or the public. The previous regulation only measured the system’s overall setting and time delay. This resulted with an inaccurate voltage reading which didn’t take into account voltage unbalance which occurs during voltage surges.

DNOs carry out the following test to assess an installation’s compatibility with the regulation:

  • Under Voltage (Stage 1 & 2)
  • Under Voltage (Stage 1 & 2)
  • Over Frequency (Stage 1 & 2)
  • Under Frequency (Stage 1 & 2)
  • Vector shift
  • ROCOF (Rate Of Change Of Frequency)
  • LOM (Loss Of Mains)


Development of G59/3

The standard is developed and defined by the ENA, the Energy Networks Association, in conjunction with other stakeholders such as Ofgem, the UK regulator for electricity and gas. The ENA represents the transmission and distribution networks for electricity and gas in the UK and Ireland.


Providing G59/3 mains protection

IPU supplies Europe’s leading G59/3 mains protection relays, the MainsPro and InteliPro from ComAp.

The MainsPro is a best-selling relay that’s well understood, easily deployed and readily accepted by the industry. It provides robust protection for generating sites.

The InteliPro supports the advanced features needed by higher capacity or more complex sites: pole-shift protection, export limitation, power factor evaluation, AC reclosing and a heartbeat function.