Grid compliance is one of the earliest technical considerations when planning an offshore wind connection.
A new development needs to connect without creating unacceptable voltage fluctuations or disturbing other users of the network. That principle applies across international markets, but the methods to assess compliance vary.
For offshore wind developers working across different countries, experience remains invaluable. However, the assumptions used for one network cannot automatically be applied to another.
Voltage fluctuations under UK Engineering Recommendation P28
In the UK, Engineering Recommendation P28 governs voltage fluctuations and flicker caused by equipment connecting to the network.
It considers the size of a voltage change and how often the event occurs. An occasional transformer energisation may be assessed differently from a voltage change that happens repeatedly during normal operation.
P28 permits a voltage dip of around 3% for frequent energisation events; repeated fluctuations are subject to lower limits.
Flicker is assessed using measures including Pst, the short-term flicker severity index. This helps determine whether repeated changes in voltage could cause light flicker or affect connected equipment.
Where the proposed connection exceeds the relevant limits, the network operator may require mitigation before granting approval.
Japanese grid compliance requirements
Japan does not use a direct equivalent of P28. Projects need to comply with JEAC 9701-2019, utility-specific requirements and the practices applied by the relevant network operator.
Japanese standards place particular emphasis on delta V10. This is a voltage fluctuation measure based on the equivalent effect of 10 Hz voltage modulation.
Our paper, Offshore Wind Grid Integration at Weak Grid Connection Points, Challenges and Solutions in Japan, identifies a tolerable level of 0.45 V on a 100 V base, equivalent to approximately 0.45% voltage change. This creates a demanding requirement for offshore wind projects. Transformer energisation, reactive power steps and switching operations may need to produce only a limited effect on the network voltage.
Different measures, shared purpose
P28 and the Japanese approach use different metrics. P28 focuses on percentage voltage change and flicker severity; Japanese practice places greater emphasis not only on delta V10, but also on requirements set by individual utilities.
A direct conversion between the two is therefore difficult. Both approaches seek to protect other network users. They also create a need to understand how the proposed offshore wind connection will behave before construction and commissioning.
This is particularly important where a large wind farm connects at a weak point of the network. Lower system strength can make switching events and voltage changes more visible at the connection point.
What this means for project design
A switching strategy developed for a UK connection may need further assessment before being used in Japan. The equipment itself may be similar; the acceptable network response may differ.
Transformer energisation studies may identify the need for Point-on-Wave Switching or pre-insertion resistors. Reactive power changes may also need to be divided into smaller steps.
The commissioning sequence should reflect the strength of the network and the limits applied by the local operator. These decisions are more effective when they are made early. Once equipment has been selected, the number of available mitigation options may narrow.
Learning across international markets
Experience from established offshore wind projects can still provide valuable insight. For instance, the Viking Wind Farm in Shetland connected within a weak electrical environment (read our case study here). Although the project was assessed against UK requirements, the methods used to control transformer energisation and voltage disturbances provide useful technical lessons.
Those lessons can inform work elsewhere, provided they are reconsidered within the local compliance framework.
Successful international knowledge transfer depends on understanding both parts of the problem: the electrical behaviour of the system and the rules applied at the connection point.
Download the white paper Grid Integration of Offshore Wind at Weak Grid Points, Technical Challenges and Solutions in Japan, to read the full UK and Japan comparison.
