Find out more about Fault Level Studies and how our team of power systems experts can help ensure your connection to the electricity network is safe, compliant, and reliable. Whether you’re adding new generation, expanding industrial loads, or modifying existing infrastructure, understanding fault levels is crucial to maintaining system stability and preventing equipment damage.
Fault Level Studies
With the transition to low-carbon electricity generation technologies, the penetration of inverter generation sources is increasing. Traditionally, electricity generation in the GB has come from coal and gas plants, which have synchronous generators with desirable characteristics in terms of power system dynamic response (providing system inertia and contributing to system short-circuit level (SCL)). Inverter-connected generation technologies, such as solar PV, wind, and batteries, cannot provide the same dynamic reliability as synchronous generators are able to, leaving the GB power system more vulnerable and less resilient to faults.
The growing penetration of asynchronous assets introduces challenges to the bulk system’s strength. Asynchronous assets do not inherently provide SCL or inertia (Operability Strategy Report, NESO, March 2025). Based on their control approach, there are two inverter technologies: grid-forming (GFM) and grid-following (GFL) inverters. GFM inverters will help us address the issues arising from asynchronous generators because they emulate the behaviour of traditional synchronous generators. This technology will contribute to increasing the resilience of the GB power system.
A reduction of SCL implies an increase in protection response times, or even that protection may not operate if a fault condition occurs. A low SCL also leads to increased risk of control interaction and sub-synchronous oscillation issues. On the other hand, high SCL indicates significant fault currents. While the high SCL reflects the robustness and security of the electrical bulk system, fault levels must remain below the specified limits of asset ratings to avert failures that could compromise grid reliability.
The factors above can be examined through fault level studies, which serve as essential tools for analysing and designing power systems. The maximum fault level results enable the validation of electrical component ratings and the assessment of fault current infeed to the grid from a new facility. Conversely, the minimum fault levels help define the protection coordination settings for the overcurrent relays.
What is a Short Circuit Fault?
A short circuit is an abnormal connection (including an arc) of relatively low impedance, whether made accidentally or intentionally, between two points of different potential (IEEE 3002.3 series).
A sudden increase in the current characterises a three-phase short-circuit fault in a power system. Gradually, the current will reach its steady state. The current waveform will depend on the fault type, the voltage value at the time the fault was introduced, and the reactance-to-resistance ratio (X/R) as seen from the fault point. The figure below shows definitions and a typical short-circuit waveform for an AC three-phase system.
Short Circuit Fault Effects
Short-circuit currents produce thermal and dynamic stresses. According to Lorenz’s law, the electromagnetic forces are proportional to the square of the current. On the other hand, thermal effects will depend on the duration of the fault. For example, the rated currents for a 60 MVA 132 kV/33 kV Uk=12% three-phase two-winding transformer will be:
The electromagnetic forces are 69 times higher due to the fault current being 8.33 times the rated current. The effect of a short circuit on equipment such as busbars, Circuit breakers, Switch disconnectors, and cables will be similar in terms of dynamics and thermal effects. According to a cable manufacturer’s catalogue (High and extra high voltage cable TFKable Edition VI), dynamic and maximum short-circuit currents for cables can be approximately estimated using the following equations:
Short-circuit faults may cause equipment failures, affecting power system reliability, according to the 2024 CIGRE Analysis of AC Transformer Reliability. After analysing 783 major faults of power transformers, 13% were found to be caused by external short circuits. A short-circuit fault can escalate into a fire, explosion, or oil leakage, posing a risk to surrounding equipment.
Images extracted from (Analysis of AC transformer reliability, CIGRE Technical Brochure, September 2024 – Reference 939
What are Fault Level Studies?
The main objectives of a short-circuit analysis (fault level studies) are detailed as follows:
- Verify the protective device’s interrupting capability.
- Verify the equipment’s ability to withstand mechanical forces and thermal stresses caused by the short-circuit current.
- Determine branch fault currents under various conditions to establish protective relay settings and determine the associated equipment ratings.
- Determine short-circuit currents for calculating arc fault incident energy.
Several internationally accepted methodologies exist for calculating and analysing fault currents in AC electrical systems, including IEC 60909, ENA EREC G74, ANSI, and IEEE standards. Regardless of the standard used to obtain the fault current results, the procedure to conduct the fault-level study includes modelling the electrical system, including transformers, generators, motors, DC/AC converters, cables and transmission lines, reactors, capacitors and switchgear equipment based on their technical specifications, the Single line diagram of the project and the electrical design. A grid equivalent will model the connection to the distribution or transmission grid; this data should be provided by the DNO or TSO company where the project will be connected.
The following table summarises the importance of the fault currents and types of faults.
What are Equipment Ratings and Why are they Important?
Equipment is designed to operate within specific technical parameters, and operating outside of these limits can cause safety and operational issues, as mentioned above.
Switchgear
For medium-voltage switchgear, the rated breaking capacity, peak making capacity, and short-circuit duration are crucial parameters to consider in a fault level study. DNO and TSO companies provide guidelines for the minimum ratings of switchgear (such as the making, breaking, and peak current), which are mandatory for the elements to be connected to their grid.
Cables
Manufacturers provide short-circuit ratings for both the conductor and the screen of medium-voltage and high-voltage insulated cables; the maximum temperature of the insulation material limits the short-circuit current of the conductors. In contrast, the maximum temperature of the cable sheath material constrains the maximum short-circuit current of the cable’s metallic screens. Short-circuit ratings for insulated cables are typically specified for a fault duration of 1 second. These parameters are then compared with the results of the fault level study to validate the ratings of the conductor during a fault condition.
Power Transformers
Transformers are designed and tested to withstand the thermal and dynamic effects of external short-circuit conditions (IEC 60076-5 Power transformers Part 5: Ability to Withstand Short Circuit). The symmetrical and peak short-circuit current used for transformer design and testing should be calculated based on the apparent short-circuit power available in the system where the transformer will be installed. When this data is not specified, the transformer short circuit design can be defined according to Table 2 of the IEC 60076-5 standard.
Fault short-circuit current values are used to establish protection settings that prevent damage to transformers and cables, as well as to validate the ratings of switchgear. The manufacturer provides short-circuit limits for assets.
The following figure presents the results of a three-phase maximum fault conducted using the IEC 60909 method on a 33 kV switchgear with a 25 kA breaking capacity and a 63 kA making capacity. The loading of the breaker (considering the peak current) is 54.43% and 75.13% for the breaking ratings. It is essential to keep both values below the rating limits to ensure that the circuit breaker operates correctly during worst-case fault conditions.
The results for the cables are presented in terms of thermal loading for each cable feeder. The incoming feeder (H04 feeder, 300 mm2 Cu XLPE) experiences less thermal loading during the fault compared to the outgoing feeders (H01, H02, and H03 feeders, 150 mm2 Cu XLPE) due to the conductor size.
Fault Level Study – When is it necessary to conduct & what is required to do so?
Fault level studies are necessary when any changes to the electricity network are planned, such as when a new generator or industry will be connected to the grid, or when it is suspected that the fault level available at the point of connection has changed. When a detailed electrical design is not available due to the early state of the project, a short-circuit study can be conducted by assuming the technical specifications of the equipment to model the system. However, the accuracy of the results will be impacted by the quality of the data.
Model and data validation are among the most critical tasks to be carried out before any system calculations can be conducted. The accuracy of simulation results from any software cannot exceed the accuracy of its input data and the models used to represent system equipment. From this perspective, the significance of model and data validation cannot be overstated (IEEE 3002.3 series).
At Blake Clough, we assess fault level studies to plan the deployment of generation facilities, advising on equipment ratings and proposing solutions to preserve the reliability of the client’s system. If you are developing a project connected to a distribution or transmission system or planning an expansion for an existing facility, and you need to validate the impact in terms of fault level, protection coordination, and arc flash assessment, we would be happy to help. Contact us to explore how we can assist you with these challenges with confidence. We also have experience in carrying out the full range of studies for both grid forming and grid following technologies.