What Are Thermal Studies in Power Systems?

Thermal studies are an essential part of power system analysis and planning. They determine whether the equipment within an electrical network can safely operate under normal and abnormal conditions without exceeding its thermal limits. Understanding thermal constraints is fundamental to network planning — when equipment overheats, the consequences range from accelerated ageing to outright failure.

Why Does Current Cause Heating?

A fundamental characteristic of any electrical power system is that when current flows through a conductor with electrical resistance, power is lost in the form of heat. This relationship is described by the equation:

The heating effect is proportional to the square of the current. This means even a relatively modest increase in current produces a disproportionately large increase in heat. In transmission and distribution systems, operating voltage is generally maintained within a narrow range by generators, transformers, and voltage control equipment. When the demand for power transfer increases, it is largely achieved through an increase in current, making thermal capability one of the primary constraints on how much power can be transferred through the network.

Purpose of Thermal Studies

The primary objective of a thermal study is to ensure that all network assets remain within their thermal ratings during both normal operation and credible outage conditions. Exceeding thermal limits can lead to:

• Accelerated equipment ageing.
• Damage to insulation systems.
• Excessive conductor sag on overhead lines.
• Reduced asset life.
• In severe cases, equipment failure.

Thermal studies are performed using power system modelling software. A load flow analysis calculates the voltage profile, power transfers, and current flows throughout the network. These current flows are then compared against the thermal ratings of the equipment being assessed. The study identifies assets that may become overloaded, allowing engineers to determine whether reinforcements or operational measures are required.

Thermal Ratings of Power System Equipment

All power system equipment has a thermal rating that defines the maximum current or power it can carry without exceeding its allowable operating temperature. Equipment subject to thermal limits includes overhead lines, underground cables, transformers, circuit breakers, busbars, and reactors.

Thermal ratings are provided for different operating durations:

Continuous (Pre-Fault) Rating

The continuous rating is the maximum loading that equipment can carry indefinitely while remaining within its design temperature limits. This is the rating used during standard network operation.

Post-Fault or Emergency Rating

The post-fault rating is an enhanced rating that can be used for a limited duration following a network outage or fault. Because these conditions are temporary, equipment can safely operate at higher temperatures for defined periods, such as 10 minutes, 20 minutes, or 6 hours. These higher ratings provide operational flexibility, allowing the network to remain secure while corrective actions are taken.

Assessing Network Conditions

When undertaking a thermal assessment, engineers must identify the operating conditions that produce the highest loading on the asset being studied. The most onerous scenario is typically one that maximises power flow through the equipment, this generally occurs when generation is increased on one side of the asset and reduced on the other, creating the largest feasible power transfer across a network boundary.

For example, consider an overhead transmission line where power normally flows from north to south. To maximise loading during the study:

• Generation north of the line would be increased.
• Generation south of the line would be reduced.
• Demand conditions would be adjusted to create the largest feasible north-to-south transfer.

This approach ensures the study reflects the worst credible operating condition the asset may experience in practice.

Security Standards and Contingency Analysis

Thermal studies must assess not only normal operating conditions but also credible outage scenarios. The Great Britain transmission system is designed and operated in accordance with the Security and Quality of Supply Standard (SQSS), which defines the contingencies the network must be able to withstand:

• N-1 conditions – loss of a single network element.
• N-2 conditions – loss of two network elements.
• N-D conditions – outage combinations associated with maintenance or planned outages.

For each contingency, load flow studies determine how power redistributes across the network. The resulting asset loadings are then compared against the applicable thermal rating. As a general rule, circuits loaded to approximately 95% of their thermal capability or above are flagged for further assessment, as they have limited remaining capacity and may become overloaded following a secured event.

Solutions to Thermal Constraints

When thermal studies identify overloaded assets, a range of operational and reinforcement solutions can be considered. The most appropriate option depends on the severity of the overload, project timescales, cost, and wider network requirements. Solutions are typically considered in order of increasing cost and complexity.

1. Changes to Running Arrangements and Additional Circuit Breakers

The configuration of a substation has a significant impact on power flow patterns across the network. By altering switching arrangements, engineers can modify the electrical impedance seen by the system and thereby influence both the magnitude and direction of power flows. Additional circuit breakers enhance operational flexibility by creating alternative switching options, helping to minimise overloads during normal operation and post-fault conditions.

2. Quadrature Booster (QB) Tapping

Quadrature Boosters are specialised transformer arrangements used to control power flow on transmission circuits. A QB consists of a shunt transformer and a series transformer working together — by injecting a voltage component in quadrature with the system voltage, the device changes the phase angle across the circuit. Since active power flow is strongly dependent on phase angle difference, a QB can increase or decrease flow on a specific circuit, or redistribute flow between parallel circuits. This makes QB tapping a valuable operational tool for relieving thermal overloads under both normal and contingency conditions.

3. Short-Term and Cyclic Ratings

Many items of power system equipment possess thermal inertia, their temperature does not rise instantly when loading increases. This means equipment can often operate above its continuous rating for a limited period without exceeding safe temperature limits.

Short-term ratings typically apply for durations of up to approximately 20 minutes and are used to manage temporary overloads immediately following faults or switching operations.

Cyclic ratings are commonly applied to transformers and are based on predictable daily demand patterns. A transformer may be permitted to operate above its continuous rating for up to six hours, provided that sufficient off-peak operation follows to allow cooling. This maximises asset utilisation while maintaining acceptable levels of equipment ageing.

4. Operational Intertripping Schemes

Operational intertripping schemes automatically disconnect selected generation following predefined network contingencies. Through commercial agreements with generators, these schemes can reduce power flows that would otherwise exceed thermal limits after an outage. Intertripping is particularly useful during planned outages and maintenance periods where network security margins are reduced, and can often provide a cost-effective alternative to major infrastructure reinforcement.

5. Hotwiring

For overhead line circuits, conductor temperature is often limited by conductor sag. As current increases, the conductor heats up, expands, and sags closer to the ground and safety clearances between the conductor and the ground must always be maintained. Hotwiring involves re-tensioning the conductors to increase the available clearance at higher operating temperatures, allowing the line to carry more current safely. Where supporting engineering assessments confirm the conductor and grease condition are satisfactory, hotwiring provides a relatively low-cost method of increasing circuit capacity without requiring full asset replacement.

6. Reconductoring

Reconductoring involves replacing an existing overhead line conductor with a new conductor that has a higher current-carrying capability. Modern conductors can often operate at higher temperatures while maintaining acceptable sag and mechanical performance, enabling a significant increase in transmission capacity without establishing an entirely new route. However, reconductoring studies must also assess tower structural capability, mechanical loading, insulator suitability, and electrical clearances. If tower replacement is required, overall project costs can increase substantially.

7. Construction of a New Circuit

Building a new transmission circuit is generally the final solution when operational measures and asset upgrades are insufficient. New circuits provide the largest increase in network transfer capability and often bring additional benefits including improved network resilience, better voltage performance, and enhanced system stability. However, transmission line projects involve significant capital expenditure, lengthy design and construction programmes, environmental assessments, and planning consent requirements — making this the longest-term and most expensive solution.