Physical
The voltage cascade and the operator boundary at each step.
The GB electricity grid is built around six nominal voltages, from 400 kV on the super-grid backbone down to 230 V at a socket. Each transformer is also an operator boundary. This route is the detailed physical-layer reference: voltages, tolerances, operators, typical assets at each tier.
16Route 16 · FoundationsAfter this route you will be able to
- Name the six GB voltage tiers, their nominals, and their statutory tolerance bands.
- Identify which operator runs each tier.
- Describe the typical assets at a primary substation and at a secondary substation.
- Explain why transformer ratings and circuit thermal limits differ in function.
- Know where to go for connection-level detail (network, voltage, connections routes).
29 September 2023ASTI programme launched · £58 bn of transmission investment to 2035
Twenty-six projects. Seven years. The largest single transmission build commitment since privatisation.
The
The rationale is unambiguous: Scottish wind cannot reach English demand in volumes the Clean Power 2030 pathway requires. The physical layer must be rebuilt to carry the flows the digital and governance layers are trying to coordinate. ASTI is the largest single piece of physical-layer reform of the 2020s.
The test is delivery. Planning, consent and supply chain all have to align across a dozen years. By 2026, early projects are in construction; mid-programme projects are still in consent. Each year of slip pushes a portion of the 2030 pathway into the 2030s.
£58 bn rebuilds the physical layer. But a cable is not a policy. What is the physical asset, where does it sit in the cascade, and what does each tier do?
The answer is the six-tier voltage cascade below. Each tier has a nominal, a tolerance, and a specific operator.
Section 01 · The voltage cascade
Six nominals. Each step is a transformer and an operator boundary.
The GB network steps voltage down in discrete stages rather than continuously. Each transformer is the place a new operator takes over responsibility.
Super grid (400 kV)
NGET England & Wales. SPEN-T and SSEN-T in Scotland. NESO balances the whole.
±5 %
↓ super grid transformer
Sub-transmission (275 kV)
Scottish backbone and some interconnection in E&W.
±10 %
↓ grid supply point
Regional transmission (132 kV)
TO-to-DNO hand-off. GSP is where distribution takes over.
±10 %
↓ bulk supply point
Primary distribution (33 kV)
DNO-operated. Feeds primary substations, large industrial sites, grid-scale batteries.
±6 %
↓ primary substation
Secondary distribution (11 kV)
Radial feeders to secondary substations that serve streets and estates.
±6 %
↓ local transformer
At the meter (230 V / 400 V three-phase)
Residential, small commercial, and three-phase industrial.
+10 / -6 %
Voltage ratio from 400 kV to 230 V is about 1,740 to 1. Operator changes at each step mean a connection at 33 kV is governed by the DNO; a connection at 132 kV needs NESO involvement.
Section 02 · Typical assets at each tier
Substation types, transformer ratings, protection scheme.
A primary substation at 33 kV to 11 kV looks different from a secondary substation at 11 kV to 400 V. Knowing the common configurations is the first step to reading an LTDS model or interpreting a connection study.
Grid Supply Point (GSP): The 400 kV or 275 kV to 132 kV or 33 kV hand-off substation. Two or more transformers, each typically 60-120 MVA. Connects a DNO region to the transmission system. Backup switchgear ensures one failed transformer does not lose the supply.
Bulk Supply Point (BSP): 132 kV to 33 kV. Transformers 45-90 MVA. Feeds primary distribution. A DNO may have 20-50 BSPs across its licence area.
Primary substation: 33 kV to 11 kV. Transformers 5-30 MVA. Houses feeder circuit breakers for each outgoing 11 kV feeder. One DNO might operate 500 of these.
Secondary substation: 11 kV to 400 V / 230 V. Ground-mounted or pole-mounted transformers 100-1,000 kVA. Serves a local area, typically a few hundred to a few thousand homes.
Protection equipment at each tier limits fault propagation: circuit breakers trip circuits that carry faults. Protection relay settings are tuned to clear faults in 80-200 ms at transmission level, slower (500 ms to 5 s) at distribution.
Every User connected to the Transmission System shall operate and maintain its equipment to ensure that fault infeed to the System does not exceed the Short Circuit Rating of the System at the Connection Point. Connection studies shall verify compliance at design stage.
Grid Code CC.6.3 (fault ride-through and short-circuit capability)
Section 03 · Thermal or voltage
When a connection is blocked, which constraint binds?
Two constraint families dominate connection decisions: thermal (how much current a circuit can carry without overheating) and voltage (how high or low the voltage can drift while staying in its statutory band).
Check your understanding
Three questions on what you have just read.
Key takeaways
- Six voltage tiers, each with a nominal and a statutory tolerance band.
- Each transformer is an operator boundary. Who connects where is determined by voltage.
- Typical transformer ratings: GSP 60-120 MVA, BSP 45-90, primary 5-30, secondary 100-1,000 kVA.
- Thermal limits and voltage limits both constrain connections. Different fixes.
- ASTI is £58 bn of physical-layer reform, 26 projects to 2035.
References
- NESO: Grid Code
CC.6 voltage bands, fault ride-through requirements.
Primary code reference.
- Ofgem: ASTI programme
£58 bn transmission investment.
Primary programme source.
- NGET: National Grid Electricity Transmission
E&W 400/275 kV infrastructure.
Transmission owner reference.
- ENA: Energy Networks Association
DNO network statistics.
Distribution-network trade body.
Continue with the digital layer. How the physical assets above are observed, modelled and controlled.