High-voltage and extra-high-voltage (EHV) substations are the nodes of subtransmission and transmission networks; in North American usage EHV starts above 230 kV, with 345, 500 and 765 kV as typical values. At those levels a fault affects many customers and the interconnected system, so substations use bus arrangements with more breakers (double bus, breaker-and-a-half, ring bus), duplicated main protection, insulation coordination against switching overvoltages and a substation automation system operated from a remote control center.
What counts as high and extra-high voltage
There is no single worldwide cut-off. A common classification in North American usage, based on ANSI C84.1, is:
| Level | Range | Typical voltages |
|---|---|---|
| Extra-high voltage (EHV) | Above 230 kV | 345, 500 and 765 kV |
| High voltage (HV) | Above 100 kV up to 230 kV | 115, 138, 161 and 230 kV |
| Medium voltage (MV) | Above 1 kV up to 100 kV | 4.16, 13.8, 34.5 and 69 kV |
Other countries draw the lines elsewhere, and the local code and the utility define them for each project. In IEC terms, insulation design changes criteria above 245 kV of highest voltage for equipment, and many texts call extra-high voltage everything from that point up. In most grids, high voltage corresponds to regional subtransmission and extra-high voltage to bulk power transfer between regions.
What changes compared with medium voltage
At medium voltage, a fault takes out a feeder. At high and extra-high voltage, a substation can be the node through which the power of a whole region flows. The differences come from there:
- Reliability: bus arrangements with more breakers, so that a fault or maintenance does not leave circuits out.
- Selectivity and speed: fast main protection on every element, duplicated, and local breaker failure backup. Clearing times are set by the system operator’s requirements and the stability studies.
- Insulation: as voltage rises, switching overvoltages come to dominate the design along with lightning.
- Remote operation: most are operated from a control center, without permanent staff.
Usual bus arrangements
The choice of bus arrangement determines what is lost after a fault and what can be maintained without interrupting service.
| Arrangement | Typical use | What it offers |
|---|---|---|
| Main and transfer bus | High voltage, regional substations | Maintenance of a breaker without taking the circuit out |
| Double bus | High voltage | Splitting circuits and maintaining one bus without an outage |
| Breaker-and-a-half | Extra-high voltage and important HV nodes | No circuit lost for a bus fault or for maintenance of a breaker |
| Double bus, double breaker | Extra-high voltage, maximum availability | Each circuit with two breakers of its own |
| Ring bus | Few bays, first stage of a substation that will grow later | One breaker per circuit without buses |
The interlocking logic of each arrangement is covered in interlocking in breaker-and-a-half, interlocking in double bus and interlocking in double bus, double breaker.
Switchyard equipment
The equipment is the same as in any substation, but with tougher requirements:
- Circuit breakers with SF6 or other gases, with two trip coils and, at extra-high voltage, multiple interrupters, pre-insertion resistors or controlled switching to limit overvoltages.
- Large power transformers and autotransformers; between 500 and 230 kV networks, the autotransformer is common.
- Shunt reactors on long extra-high voltage lines, to control the no-load voltage.
- Capacitive voltage transformers (CVTs), which also serve as coupling for power line carrier.
- Surge arresters at line entrances and at the transformers.
Technology: AIS, GIS or hybrid
The conventional air-insulated (AIS) substation takes more land but is easier to inspect and expand. The gas-insulated (GIS) substation cuts the area to a fraction and is used in cities, in polluted sites or where there is no space; the hybrid substation combines enclosed modules with air-insulated buses. The decision depends on the land, the environment, future expansion and life-cycle cost. The parts common to every voltage level are in electrical substation.
Insulation, clearances and grounding
IEC 60071-1 splits insulation coordination into two ranges according to the highest voltage for equipment: up to 245 kV, the lightning impulse withstand voltage and the power-frequency withstand voltage are specified; above it, the switching impulse withstand becomes decisive. IEEE 1313.1 and IEEE 1313.2 follow the same principles. Electrical and safety clearances, insulator pollution and corona grow with voltage.
The grounding grid is designed for the highest fault currents in the system, following IEEE 80. Step, touch and transferred voltages are usually verified by measurement once the installation is built, including the fence, and the operation and maintenance manuals must leave no room for mistakes. You can make a first estimate of a grid’s resistance with our free grounding resistance tool.
Protection and control
At these levels the protection philosophy usually includes:
- Distance protection 21 with teleprotection schemes, or line differential 87L over fiber.
- Bus differential 87B and breaker failure 50BF.
- Transformer differential and the transformer’s own mechanical protection.
- Single-pole or three-pole reclosing with synchronism check.
Two independent main protection systems, with separate DC supplies and trip coils, are common practice at extra-high voltage; see line protection: main 1 and main 2. Control is done with bay controllers and an IEC 61850 substation automation system (SAS), operated from the transmission owner’s control center. The specific requirements are set by the system operator and the grid code for each project.
Further reading
Electric Power Substations Engineering, edited by J. D. McDonald, covers design, automation and testing, and is a common reference in substation design courses.
We can help with your substation
Our team works on medium, high and extra-high voltage substations: primary and secondary engineering, protection coordination, relay configuration and commissioning. Design and studies are done remotely, worldwide, adapted to the local code and the utility’s requirements. See our engineering design and studies service or contact us with the scope of your project.
Related guides
- Interlocking in breaker-and-a-half
- Interlocking by voltage level
- Line protection: main 1 and main 2
- Electrical substation: what it is, parts and how it works
- Substation control building: what is inside
- Substation commissioning: stages and tests
References
- IEC 61936-1: power installations exceeding 1 kV AC.
- IEC 60071-1: insulation coordination – definitions, principles and rules.
- IEEE 1313.1 and IEEE 1313.2: insulation coordination.
- IEC 62271-1, IEC 62271-100 and IEC 62271-203: high-voltage switchgear and controlgear, circuit breakers and gas-insulated switchgear.
- IEEE 80: guide for safety in AC substation grounding.
- ANSI C84.1, Electric Power Systems and Equipment – Voltage Ratings (60 Hz).
- J. D. McDonald (ed.), Electric Power Substations Engineering, CRC Press.
Frequently asked questions
From what voltage is a substation considered extra-high voltage?
It depends on the country and the standard. In North American usage (ANSI C84.1), extra-high voltage means above 230 kV, with 345, 500 and 765 kV as common values. IEC 60071-1 changes the insulation design criteria above 245 kV of highest voltage for equipment, and many texts call that range extra-high voltage.
Why is breaker-and-a-half used at extra-high voltage?
Because each circuit is connected to both buses through two breakers. A bus fault or the maintenance of a breaker does not take any circuit out of service, which justifies the additional breaker in a transmission network, where one line out can limit system operation.
Which references are used to design high and extra-high voltage substations?
The main standards are IEC 61936-1 or the local code for installation, IEC 60071 for insulation coordination, IEC 62271 or IEEE C37 for switchgear and IEEE 80 for grounding, together with the requirements of the utility and the system operator. As a textbook, Electric Power Substations Engineering, edited by J. D. McDonald, covers design, automation and testing.