IN Brief:
- GSR036 raises the upper operational voltage limit for the 200kV to 300kV transmission network from 9% above nominal voltage to 10%.
- The previous limit had become a significant cause of outage rejections when voltage-control equipment or large generators were unavailable.
- The SQSS change takes effect ten working days after Ofgem’s 2 October decision.
Ofgem has approved an increase in the upper operational voltage limit across Britain’s 200kV to 300kV transmission network, restoring the permitted tolerance from 9% to 10% above nominal voltage.
The GSR036 modification changes the National Electricity Transmission System Security and Quality of Supply Standard, or SQSS, which sets planning and operating criteria for the transmission system. The revised limit applies under both pre-fault and post-fault conditions and will take effect ten working days after Ofgem’s 2 October decision.
The practical effect falls mainly on the 275kV network, where a +9% ceiling has applied since 2017. Voltages up to 10% above nominal will become compliant with the SQSS once the modification takes effect, while anything beyond that limit will remain subject to existing Grid Code and SQSS controls.
Transmission voltage changes continuously with network configuration, reactive power flows, generator output and demand. Lightly loaded circuits can raise voltage through their reactive characteristics, while shunt reactors, generators and other controllable equipment are used to absorb reactive power and hold the system within its operating range.
Those controls become more restricted when parts of the network are taken out of service. Reinforcement, maintenance and equipment replacement can require transmission circuits, transformers or reactive compensation equipment to be removed temporarily, altering the way power and reactive current move through the remaining system.
A planned outage may therefore be technically necessary for construction work but impossible to accept if the surviving network would breach the prescribed voltage range following a credible fault. Ofgem says the +9% limit has become a significant cause of rejected outage requests, particularly when large generators or voltage-control equipment are already unavailable.
That problem has grown as transmission owners undertake larger reinforcement programmes. New substations, reconductoring, overhead line works and transformer replacements all require coordinated system access, often across areas already carrying significant power flows. Refusing an outage because of a marginal voltage exceedance can delay the work intended to increase network capacity.
The additional one percentage point does not change equipment ratings or allow voltage to rise without control. Transmission owners and the National Energy System Operator must still operate assets within their technical limits and maintain secure conditions following defined faults. GSR036 instead changes the point at which operation becomes non-compliant solely because the system has exceeded the previous +9% SQSS threshold.
The limit had originally been +10% before the 2017 Fundamental SQSS Review reduced it to +9% for the 275kV system, with the same approach subsequently applied across the broader 200kV to 300kV range. System access work carried out since then has identified operating cases where voltage remained within asset capability but exceeded the tighter standard.
Those cases could require derogations or changes to outage plans even where the underlying equipment was capable of operating safely. Restoring the +10% threshold removes that additional process where system voltage remains within the revised standard.
The change was developed through the National Energy System Operator’s System Access Reform programme, which is examining how existing access rules interact with the volume of transmission construction planned across Great Britain. The programme includes changes to operational standards and outage processes intended to allow more work to proceed without reducing network security.
Managing voltage during an outage can require a different combination of generators, shunt reactors and network switching from normal operation. A circuit removed for reinforcement changes both the available transmission route and the system’s reactive characteristics, while the loss of a large generator can simultaneously remove a source of dynamic voltage control.
Under the revised limit, planners gain a slightly wider operating range before those conditions trigger an SQSS breach. That can allow an outage to proceed where the post-fault voltage would have exceeded +9% but remained below +10%, avoiding the need either to cancel the work or seek exceptional treatment.
The modification will not resolve all access constraints. Thermal limits, stability, fault levels, protection arrangements and lower-voltage boundaries continue to determine whether individual outages can be accepted. Network reinforcement can also require several linked outages to be sequenced over months or years, making one parameter only part of a wider planning problem.
GSR036 nevertheless removes a constraint NESO identified repeatedly during reinforcement planning. It is the first SQSS modification delivered through System Access Reform, alongside wider work examining operational requirements on the onshore transmission system.
Once the decision takes effect later in October, system planners will be able to treat voltages up to 10% above nominal across the 200kV to 300kV network as compliant with the SQSS, giving transmission owners a wider operating margin when arranging construction and maintenance outages.
