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Customers who are seeking to increase the amount of generation or demand installed but have been advised an increase in capacity will require costly or time bound upstream reinforcement, may choose to restrict the net export or import of their connection rather than wait for or contribute to the reinforcement cost.
A Customer Limitation Scheme (CLS) measures the current flow (in Amps) at the exit point, typically adjacent to the meter, of the installation and then uses this information to either restrict the total installed generation capacity output or increase/decrease the customer demand to prevent the Maximum Export Capacity or Maximum Import Capacity from being exceeded.
These schemes allow network operators to continue to maintain the security and quality of existing customers’ supply within network capacity limits, whilst also enabling new generation and demand onto the network. The schemes would be expected to operate infrequently and for minimal duration and therefore have no real adverse or known effect on the customer experience.
Once installed and commissioned, the CLS settings cannot be altered by the Customer and may only be changed with the written agreement of the DNO. Therefore, the settings will be protected by either password, PIN or physical access that is capable of being sealed.
Customer Limitation Schemes must comply with the relevant power quality standards and with EREC G100. Generation, including energy storage systems, also needs to adhere to EREC G98 and G99 as applicable.
The customer limitation scheme must reduce the imported or exported current to a value that is equal to, or less than, the Maximum Import/Export Capacity, typically within 15s with a maximum reaction time of 60s.
The system must be fail-safe. If any of the components or signalling systems that comprise the curtailment scheme fail or lose power the scheme must reduce the exported or imported current to a value that is equal to, or less than, the Maximum Export/Import Capacity typically within 15s, with a maximum reaction time of 60s.
For connections at High Voltage and above, a reverse power flow relay must also be installed
For ease, the Energy Network Association (ENA) has provided a database of compliant G100 devices called Connect Direct.
The installer must fill in and submit a Customer Limitation Scheme Application form to National Grid Electricity Distribution (NGED) in addition to the relevant generation application forms.
Once NGED has approved the connection, the equipment may be installed and commissioned. In some cases, NGED will need to witness some of these commissioning tests.
The installer must fill in a Customer Limitation Scheme Installation and Commissioning form and submit this to NGED within 28 days.
A G99 application process has been developed for smaller installations that comprise EREC G98 compliant generation (e.g. solar PV) rated up to 16A and EREC G98 compliant energy storage rated up to 16A fitted with an EREC G100 compliant Export Limitation Scheme that restricts the export to 16A per phase or less. Get further information on G99 applications.
There are alternative schemes available for commercial, High Voltage, Connections. See the examples given in EREC G100.
G100 installed capacity is not unlimited, there are a series of tests run by our engineers when studying our network to ensure our system is not adversely effected during the time of operation of the curtailment scheme, e.g. 15s.
The thermal capacity of our network must not be over the individual asset short term rating.
The protection must not be operated by the short term flow of excess current during the operation time of the curtailment scheme.
The voltage rise caused by the export must not exceed the upper statutory volt limit by more than 2% i.e. 253V +2% of nominal (4.6V)
The installation must not cause any "jumps" in voltage seen by other customers, this is called voltage step change.
The maximum installed generation capacity of a single phase installation must not exceed 17kW for compliance with G99.
For larger installations, the reduced export capacity does not affect the fault level contribution of the devices, the installed capacity of the installation must be taken into account.
If you're ready to apply for the installation of your generation device(s), or notify us of an installation, you can do so using our online form. You can indicate the presence of an ELS during your application.
If you are considering installing an Export Limitation Scheme (ELS), an ELS application form must be submitted alongside your generation application.
Alongside the generation and ELS application forms, installers must also complete a commissioning form to confirm all requirements have been met.
Download the Engineering Recommendation G100 Summary Guide from the Energy Network Association (ENA) document catalogue.
