London Array's substations transform electricity from the wind turbines from 33,000 volts to 150,000 volts before exporting it to land.

By Richard Horan

The offshore substation is essential for getting offshore wind power back to shore. Without it, wind turbines would sit idle. Once the installed capacity exceeds about 100MW, power from the turbines needs to be stepped up in voltage to get it back to shore. This is the job of the offshore substation.

A typical offshore substation houses two power transformers rated at around 150MW. A 150MW transformer carries approximately £125,000 of wind farm revenue every day. Given the typical offshore transformer repair time of 8 to 12 months, the importance of maintenance is clear.

However, offshore maintenance is very costly and experienced high voltage offshore technicians are difficult to find. As a result, many operators only carry out maintenance every 3 months.

Limited track-record

Substations contain many components that are critical for the transmission of power to shore, and the wind industry is breaking new ground by putting all this high voltage equipment on offshore substations.

The equipment is subjected to extreme weather conditions, vibrations and operating parameters that are quite different to traditional onshore substations. As such, early life failures in this new operating environment should come as no surprise.

Offshore substations can be designed with some level of spare capacity, but it is a difficult balance to strike. Expensive substations may save money in the long-term due to increased availability, but at the outset it is often difficult to justify the additional expenditure - particularly given the limited experience in predicting the lifetime of high voltage equipment in the offshore environment.

High voltage equipment has been developed over many years for use in our traditional onshore substations, where the footprint of the substation is driven by the size of the equipment the electrical engineers say they need. By contrast, offshore substations must be small.

The structural engineers require that equipment be laid out according to structural constraints, while keeping the overall cost down. While the electrical engineers do their best to fit the equipment into the available space, the resulting substation isn’t always easy to maintain.

Regular maintenance at the early stage, however, would enable detection of early signs of failure. If suppliers recommend monthly maintenance then operators should heed this advice – at least until hard evidence supports an alternative maintenance regime.

For example, regular maintenance of the ‘life support’ systems for the electrical components, such as air filtration systems, emergency diesel generators, fire detection and protective coatings would minimise the risk of early failures. The oil and gas industry knows only too well that the cost of repairing protective coatings can escalate rapidly if effective regular maintenance is not carried out.

Frequent maintenance in the early years of operation provides a robust database that can support a reliability-centred maintenance regime. Detailed maintenance and failure records help to identify weak points, as well as opportunities to develop products designed to withstand the offshore environment.

In addition to inspecting the substation equipment and carrying out manufacturers’ recommended maintenance, predictive maintenance techniques such as partial discharge monitoring and thermal imaging, should be used.

These techniques identify early stages of insulation breakdown and electrical connection failure, allowing equipment to be replaced and remedial works to be carried out before a failure occurs. Downtime and health and safety risks are minimised, as are revenue losses, given that maintenance can be carried out when wind is low and switching out electrical plant will not result in loss of revenue from the wind turbines.

Weak points

The limited experience to date has already highlighted power transformers as a weak point. The large offshore power transformers step up the voltage from the turbines for transmission back to shore.

These transformers use oil as electrical insulation and as a way of transferring heat to large radiators, which are usually cooled with fans.
The radiators have to be made from thin steel in order to radiate the heat from the oil, and must be located outside to expel heat. This makes them highly susceptible to corrosion.

Some offshore substation designers are considering installing transformers indoors and using an oil/water separator to transfer the heat into a seawater cooling system. Others are considering gas-insulated transformers that will eliminate the health and safety risk of oil fires.

However, the industry is wary of moving too quickly into these new technologies, which could potentially introduce a new set of problems. The old technology may not be ideal, but it has been tried and tested in onshore substations.

At this early stage, nobody can claim to be an expert in the maintenance of offshore substations. Until clearer track records have been established, more caution should be exercised and more regular maintenance should be carried out. Only when the causes of early life failures have been identified and engineered out, would moving to longer maintenance intervals be advisable.

To respond to this article, please write to:

Richard Horan: richard@horanengineering.com

Or write to the Editor: Rikki Stancich

Image credit: London Array