Voltage Studies in Power Systems Explained
Voltage studies form a core part of transmission system planning. They assess how voltage behaves across the network under different operating conditions, ensuring the system remains within safe limits during both normal operation and following credible faults. This page explains what voltage studies involve, the limits they must comply with, and how non-compliance is resolved. If you are not yet familiar with how voltage is controlled on the network, it is worth reading the Voltage Control page first.
What Are Voltage Studies?
The majority of voltage studies assess the steady-state voltage performance of the National Grid transmission system. This is done by carrying out AC load-flow studies, which model the flow of active and reactive power through the network and calculate the resulting voltages at every busbar. These studies can be applied nationally across the whole transmission system, or locally to investigate a specific area or connection.
In essence, voltage studies check whether the system operates within acceptable voltage limits under both peak and off-peak demand conditions and confirm there is sufficient reactive power margin to avoid voltage instability.
Why Is Voltage Control Important?
Voltage control serves three key purposes on the transmission system:
1. Compliance with the GB SQSS
The transmission licence requires National Grid to plan and develop the system in accordance with the Security and Quality of Supply Standard (SQSS). This means ensuring that steady-state voltages remain within defined limits both before and after a fault, that voltage step changes do not exceed permitted levels, and that sufficient reactive power margin exists to avoid voltage collapse.
2. Meeting Customers’ Needs
Adequate voltage is essential for both generators and demand customers. Insufficient voltage can cause generators to trip or prevent motors from starting — and if this happens on a widespread basis, it can trigger a cascade of further trips leading to system collapse. For demand customers, maintaining correct voltage levels is fundamental to the quality of their electricity supply.
3. Efficient Power Transmission
Voltage control is directly linked to the reactive power balance on the system. A deficit of reactive power causes voltages to fall, which can lead to voltage instability. Reactive power is supplied naturally by the network itself and traditionally by synchronous generators. Converter-based generators can also provide reactive support, with their reactive power output typically set proportional to their active power output.
The Three Categories of Voltage Performance
The SQSS and Grid Code define voltage compliance requirements in three categories:
- Pre- and post-fault planning voltage limits — the steady-state voltage must remain within defined bounds both before and after credible network outages.
- Voltage step change limits — switching operations, faults, and changes in generation or demand must not cause sudden voltage changes exceeding permitted levels.
- Voltage performance margins — sufficient reactive power margin must be maintained to prevent system instability or voltage collapse.
Operational and Planning Voltage Limits
Two distinct types of voltage limit apply to the transmission system:
Operational limits are the voltage boundaries within which the transmission system must be operated securely under normal and credible network conditions. Voltages outside these limits are considered non-compliant.
Planning limits are tighter targets used when designing and assessing the network. They ensure that sufficient margin remains before operational limits are reached, even following specified contingencies.
Pre-Fault Steady-State Voltage Limits
| Nominal Voltage | Operational Min | Operational Max | SQSS Planning Min | SQSS Planning Max |
|---|---|---|---|---|
| 400 kV | 0.90 pu | 1.05 pu | 0.975 pu | 1.025 pu |
| 275 kV | 0.90 pu | 1.10 pu | 0.95 pu | 1.05 pu |
| 132 kV | 0.90 pu | 1.10 pu | 0.90 pu | 1.05 pu |
| <132 kV | Network-specific | Network-specific | Typically 0.94 pu | Typically 1.06 pu |
Post-Fault Steady-State Voltage Limits
| Nominal Voltage | Operational Min | Operational Max | SQSS Planning Min | SQSS Planning Max |
|---|---|---|---|---|
| 400 kV | 0.90 pu | 1.05 pu | 0.95 pu | 1.025 pu |
| 275 kV | 0.90 pu | 1.10 pu | 0.90 pu | 1.05 pu |
| 132 kV | 0.90 pu | 1.10 pu | 0.90 pu | 1.05 pu |
| <132 kV | Network-specific | Network-specific | Typically 0.94 pu | Typically 1.06 pu |
Voltage Step Change Limits
The SQSS treats voltage step change as a power quality issue. A voltage step change is a sudden change in RMS voltage magnitude between two steady-state conditions. Step changes must be assessed at every interface point between the national transmission system and users’ plant.
The limits depend on the nature and frequency of the event:
| Event | Max Voltage Fall | Max Voltage Rise |
|---|---|---|
| Normal operational switching occurring more frequently than every 8 minutes | −3% | +3% |
| Infrequent operational switching | −6% | +6% |
| Following a fault outage of a double-circuit supergrid overhead line (planning studies) | −6% | +6% |
| Following any other secured event | −6% | +6% |
| Secured event — fault outage of a section busbar or mesh corner (users connected above 132 kV) | −12% | +6% |
| Secured event — fault outage of a double-circuit overhead line (users connected above 132 kV) | −12% | +6% |
| Loss of a double-circuit transmission overhead line and associated supergrid transformers supplying a 132 kV substation | −12% | +6% |
| Loss of a single transmission circuit and associated supergrid transformers supplying a 132 kV substation | −12% | +6% |
| Loss of a double-circuit transmission overhead line operating at 132 kV | −12% | +6% |
| Loss of one or more Grid Supply Transformers supplying systems below 132 kV | −12% | +6% |
Quick Reference: General Rule
Operational switching: ±3%
Secured event (normal N-1): ±6%
Severe transmission outages (specific cases): −12% / +6%
The ±6% limit applies to infrequent operational switching where:
- Switching occurs no more than once every 8 hours.
- Intervals of less than 2 hours are permitted only under abnormal conditions.
- Activities such as routine maintenance or plant access switching that are expected to occur only occasionally.
How Voltage Studies Are Carried Out
To accurately assess voltage performance, a full generation dispatch should be modelled rather than simply scaling background generation. This better represents how reactive power moves between network regions and captures realistic voltage behaviour. Generator reactive capability is typically derated to 90% of its theoretical value to account for equipment failures and known long-term reductions in reactive plant availability.
The assessment follows a structured sequence:
- Begin with peak demand conditions (Average Cold Spell demand) on an intact network.
- Test the network under secured outages — circuit outages, transformer outages, and fault trip scenarios.
- If the system is compliant at peak demand, repeat the studies at minimum demand to check that excessive voltages do not arise during off-peak periods.
- If high voltages are identified during off-peak conditions, consider operational measures and existing reactive plant before installing new equipment.
Voltage studies are generally performed after thermal and fault level studies, because those studies determine the network configurations and running arrangements that need to be assessed for voltage performance.
Solutions When Voltage Non-Compliance Is Identified
When a voltage study identifies non-compliance, the preferred approach is to consider low-cost operational solutions before progressing to more expensive network reinforcements. Options are typically assessed in the following order:
| Option | Action |
|---|---|
| 1 | Change network running arrangements |
| 2 | Install additional circuits, such as transmission lines or Supergrid Transformers (SGTs) |
| 3 | Switch existing reactive equipment such as shunt reactors or capacitor banks |
| 4 | Adjust voltage targets to increase reactive support from generators |
| 5 | Change target voltages on Static VAR Compensators (SVCs) |
| 6 | Contract with generators to provide additional reactive power support |
| 7 | Install new reactive compensation equipment — capacitor banks, reactors, SVCs, STATCOMs, or series compensation |
Key Takeaway
Voltage studies are an iterative process. Engineers work through operational solutions first, progressing to physical reinforcements only when lower-cost options have been exhausted. This approach keeps costs to a minimum while ensuring the network meets all SQSS and Grid Code requirements.