Managing High Fault Levels: Causes, Assessment and Solutions

As power systems grow and more generation is connected to the network, fault levels at substations can rise to the point where they exceed the ratings of existing equipment. This page explains why circuit breakers have fault current limits, how fault levels are assessed, and what solutions are available when those limits are exceeded.

Why Do Circuit Breakers Have a Maximum Fault Current Rating?

Circuit breakers (CBs) have a maximum fault current rating because there is a limit to the amount of fault energy they can safely withstand and interrupt. Under normal operating conditions, a circuit breaker carries load current without issue. During a fault, however, current can increase to tens of kiloamps within a fraction of a second. When the breaker opens to clear the fault, an electrical arc forms between the separating contacts – the breaker must not only interrupt this current but also extinguish the arc while withstanding significant thermal and mechanical stresses.

The maximum fault current of a circuit breaker is primarily limited by three factors:

Thermal Stress

Fault currents generate heat in proportion to I²R. Extremely high fault currents can rapidly raise temperatures within the breaker, potentially melting contacts, damaging insulation, or deforming internal components before the fault is cleared.

Mechanical Stress

Large fault currents create substantial electromagnetic forces between conductors. These forces increase approximately with the square of the current. If the fault current exceeds the breaker’s design capability, contacts, busbars, and other current-carrying components may become damaged or distorted.

Interrupting Capability

When the breaker contacts separate, an electrical arc forms. The breaker must be capable of extinguishing this arc and preventing it from re-establishing. If fault current levels are too high, the arc may become too powerful to interrupt successfully, potentially resulting in restrikes, equipment damage, or catastrophic failure.

For these reasons, power system engineers calculate the prospective short-circuit current at each busbar and select circuit breakers with a short-circuit breaking capacity greater than the maximum expected fault level, typically with an appropriate safety margin.

System Fault Levels and the SQSS

In the UK, the Security and Quality of Supply Standard (SQSS) requires transmission licensees to plan and operate the network such that faults do not result in unacceptable overloading of primary transmission equipment.

To demonstrate compliance, companies carry out fault level studies to determine the maximum fault current that may occur at each substation. This ensures that all primary equipment, including circuit breakers, busbars, disconnectors, current transformers, and transformers, is capable of withstanding and interrupting the calculated fault currents.

Why Are Fault Currents Lower on Lower-Voltage Networks?

Fault current is governed by Ohm’s Law:

For a given network impedance, reducing the system voltage results in a lower fault current. Consequently, fault currents are generally lower on lower-voltage circuits than on higher-voltage circuits when all other factors remain constant. However, in practice, fault levels depend on both the network voltage and the total impedance between the source and the point of fault, so detailed fault level studies are always required.

Determining Maximum Fault Levels Within a Substation

To determine the maximum fault level at a substation, the network must be modelled in its most onerous operating condition. In the UK this is typically during periods of maximum demand – generally in winter, when the network is most heavily loaded and the greatest amount of generation and system infrastructure is connected.

A maximum fault level study will typically assume:

• All available generation is in service.
• HVDC links and mechanically switched capacitors (MSCs) are in service where appropriate.
• All normal network circuits are switched in.
• The network is configured in the arrangement that produces the highest fault current.
• Busbar voltages are set at their maximum operational limits.

By assessing the network under these worst-case conditions, engineers can confirm that all equipment ratings remain adequate throughout the life of the substation and that the system continues to operate safely and in accordance with industry standards.

Solutions to High Fault Levels

When a fault level study reveals that equipment ratings are being approached or exceeded, several solutions are available. These range from targeted equipment replacements to more significant substation reconfigurations.

Replace Overstressed Circuit Breakers

The most targeted solution is to replace only those circuit breakers whose ratings are exceeded. This avoids the cost and disruption of wider substation works, but may not address the root cause if fault levels continue to rise over time.

Replanting All Substation Equipment In-Situ

Where multiple items of equipment are overstressed, it may be more cost-effective to replace all primary equipment at the substation with higher-rated alternatives on the existing site. This avoids the need for land acquisition but requires careful outage planning to manage the risk of disruption to supply.

Replace the Substation with a New Substation

In some cases, particularly where the existing site is constrained or the substation is reaching the end of its operational life, it may be more practical to build an entirely new substation to a higher specification. This is the most capital-intensive option but offers the opportunity to design for future fault level growth.

Splitting the Substation Busbar

Splitting a substation is an effective way of reducing fault levels because it limits how many sources can contribute current to a fault. When a busbar is fully connected, all incoming circuits, transformers, and generators are electrically tied together. A fault at the bus is then supplied by multiple sources in parallel, each feeding current through relatively low impedances, making the overall equivalent impedance at the fault point very low, and therefore the fault current very high.

When the substation is split, typically by opening a bus section or coupler breaker, the single bus is divided into two independent sections. Each section then contains only a subset of the connected equipment. If a fault occurs on one section, only the sources connected directly to that section can contribute; sources on the other section are electrically separated and cannot feed the fault.

From an electrical perspective, this removes parallel current paths and increases the equivalent impedance seen at the fault location. Since fault current is inversely proportional to impedance, this increase in impedance leads directly to a reduction in fault level.

Figure 1: A fully coupled busbar (left) allows all sources to contribute to the fault. Splitting the busbar (right) isolates sources and increases the equivalent impedance, reducing the fault level.