Voltage
Holding voltage within statutory limits at every tier.
The GB grid has six voltage levels, each with a nominal and a statutory operating range. Voltage must stay within its range during normal operation and recover within defined times after a fault. NESO procures reactive power to keep transmission voltages in band. DNOs manage distribution voltage. Voltage constraints, not thermal limits, are now often the binding factor on new connections in parts of Scotland and the South East.
11Route 11 of 12 · Operations and networksAfter this route you will be able to
- Name the six GB nominal voltage levels and their statutory tolerance bands.
- Explain what reactive power does and why it matters for voltage.
- Describe how voltage and thermal constraints differ as connection-limiting factors.
- Identify the NESO reactive power products (Obligatory, Enhanced, Voltage-only).
- Make a reasoned call on voltage-constrained connection treatment.
14 July 2022Scotland voltage constraint · £200m in constraint payments
Scotland's 400 kV network could not voltage-support all its generation at once. NESO paid wind farms to turn off.
On a breezy summer afternoon in July 2022, Scottish wind output exceeded the ability of the Scottish transmission system to export the power south while holding voltage in statutory limits. NESO paid Scottish wind farms to reduce output and paid gas generators in England to increase output. The week's constraint cost reached
The physics is subtle. Thermally, the lines had capacity. Voltage-wise, long 400 kV circuits carrying high power develop voltage rise at the receiving end and voltage droop at the sending end. Without reactive power compensation, voltage drifts out of its statutory band and the lines must be de-rated.
The 2022 constraint pattern became routine in 2023 and 2024. Scottish wind now regularly outstrips the ability of the transmission system to deliver it to England within voltage bounds. The response is a queue of reinforcement projects (ASTI programme, £58 bn to 2035) plus tactical fixes: synchronous condensers, SVCs, STATCOMs.
The grid has enough thermal capacity. Why is voltage the binding constraint, and what does that tell us about where to put the next billion pounds of reinforcement?
The answer starts with the voltage tiers and their statutory limits. Each tier carries a different tolerance; each tolerance is set in a specific standard.
Section 01 · Voltage tiers and limits
Six nominals, six operating bands.
GB voltage levels follow IEC 60038. The statutory operating ranges are set in the Grid Code (transmission) and the Electricity Safety, Quality and Continuity Regulations (distribution).
LV asymmetric range (+10 %, -6 %) reflects the practical reality of residential installations. BS EN 50160 adds 10-minute average and rapid voltage change limits that apply alongside ESQCR.
Section 02 · Reactive power
Reactive power is the lever. Three NESO products procure it.
Reactive power (measured in VAr or var, MVAr, GVAr) is the component of electrical power that maintains voltage. It does no useful work at the load but is essential for the system to hold voltage within its band.
Generators, capacitor banks, STATCOMs, SVCs and synchronous condensers all produce or absorb reactive power. NESO procures three related products on the transmission system.
Obligatory Reactive Power Service (ORPS) is what every transmission-connected generator above a threshold must provide as part of its connection contract. It is paid at a regulated tariff.
Enhanced Reactive Power Service (ERPS) is additional capability beyond ORPS, tendered for in specific locations where NESO needs more voltage support than obligatory contributions deliver.
Voltage-only services (including synchronous condensers) provide voltage support without active power capability. Since 2020 NESO has contracted synchronous condensers at three sites to replace some of the inertia and voltage support that departing thermal plants used to provide.
Each Large Power Station and Embedded Large Power Station connected to the Transmission System shall be capable of continuously supplying full rated reactive power at any voltage between 95 % and 105 % of Nominal System Voltage.
Grid Code CC.6.3.2 (reactive capability)
Section 03 · Voltage-constrained connections
Reinforce, install STATCOMs, or flex the connection?
When a proposed connection is blocked by voltage rather than thermal capacity, there are three responses. Each has different cost and timeline.
Check your understanding
Three questions on what you have just read.
Key takeaways
- Six voltage tiers, each with a statutory steady-state band and fault-recovery requirement.
- Reactive power holds voltage. NESO procures it through Obligatory, Enhanced, and Voltage-only services.
- Voltage, not thermal, is often the binding constraint on new connections in Scotland and South East.
- STATCOMs and synchronous condensers are tactical fixes; ASTI reinforcement is the long-run answer.
- 2024 constraint costs were ~£2 bn, mostly paying Scottish wind to reduce output while southern gas filled in.
References
- NESO: Grid Code CC.6 (voltage)
Statutory voltage ranges and reactive capability.
Primary statutory reference.
- Electricity Safety, Quality and Continuity Regulations 2002
Distribution voltage tolerance bands.
Statutory distribution reference.
- NESO: Reactive power services
ORPS, ERPS, voltage-only service specifications.
Operational procurement reference.
- Ofgem: Accelerated Strategic Transmission Investment (ASTI)
£58 bn programme for transmission reinforcement to 2035.
Long-run reinforcement programme source.
The next route covers connections. Where voltage, thermal and queue reform combine into the practical question of how and when a new generator gets connected.