As you may be aware, Engineering Recommendation G59/3 places an obligation on customers connecting generation to electricity networks to provide Loss of Mains (LoM) protection to ensure that generating plant is disconnected from the system should connection to the Distribution Network Operator (DNO) network be lost. For small schemes (<50MW), Loss of Mains protection has generally been provided either by Rate of Change of Frequency (RoCoF) or Vector Shift methods. Inter-tripping is also used but this is normally only needed for larger schemes (>50MW).
Under the provisions of the Distribution Code distributed generation customers are required to liaise with the relevant DNO to establish suitable protection arrangements. These protection arrangements will depend upon the generator’s installation and the requirements of the DNO’s distribution system.
An investigation last year into the loss of a significant volume of distributed generation in the South West of England following a fault on the transmission system attributed the loss to the widespread operation of vector shift protection. More recently similar events have occurred across UK Power Networks’ three license areas (SPN, LPN and EPN). Whilst these events were relatively contained, a number of our customers were adversely affected. Investigations have identified a clear linkage to vector shift operation. Investigations are still ongoing to fully understand the extent of the disruption caused. However, it is clear that the issue continues to increase in both magnitude and severity. If unaddressed, in the future a larger loss of infeed to the transmission system could have a major impact on system stability, ultimately affecting the continuity and quality of supply to a far greater number of customers.
In light of the above and as a result of the findings from the latest studies conducted on the Southern transmission network, from 15 September 2017 all generation connection quotations for connections in UK Power Networks’ three license areas issued pursuant to section 16 of the Electricity Act 1989 (as amended) will expressly prohibit the use of vector shift protection in relays. Moreover, to further mitigate the risk to system stability, where RoCoF protection is used as Loss of Mains protection the applied setting should be 1Hzs-1 with a definite time delay of 500ms.
Please also note that, irrespective of when a generation connection quotation was issued and accepted, UK Power Networks will not allow any distributed generation connecting and energising after 30 December 2017 to use vector shift protection in relays. For such connections Loss of Mains protection in the form of RoCoF or inter-tripping (where applicable) must be used instead.
Customers wishing to connect before 30 December 2017 are encouraged in the strongest possible terms to utilise RoCoF or inter-tripping (where applicable) instead of vector shift protection. However, where it is uneconomic or not practicable to do so, where agreed with UK Power Networks, vector shift protection may still be employed for connections energised before 30 December 2017.
Whilst acknowledging that work in this area is ongoing and being consulted upon within the industry, we have concluded that the prohibition of the use of vector shift protection and harmonization of RoCoF settings is an appropriate and proportionate response to the potential risk to both existing and future customers’ continuity of supply.