Reactive compensation for onshore wind farms

Grid code revisions in UK lack clear guidance on solutions for reactive compensation for onshore wind farms.
Published: Wed 21 Sep 2016

The growth of onshore wind farms across the UK has led to a growing need for Reactive Compensation Equipment (RCE). At the same time reactive compensation technology has advanced to enable a greater range of solutions.

However, there have been doubts about the compliance of incorporating Switched Shunt Device (SSD) to a Static Compensator (STATCOM) with the UK grid code, which led to its revision with modifications. The revised grid code comes into effect in 2017 and clarifies the future use of SSDs. Nevertheless, while an important advance, there remains little guidance for wind farm developers and owners on the various solutions that are available.

To fill this gap, Engerati sat down with Aazzum Yassir, Application and Tendering Engineer at S&C Electric Company, to discuss the background to the revised grid code and what this means for RCE solutions such as STATCOMs + SSDs (also known as a hybrid solution) in particular.

“Whilst customers are aware of new UK grid code revisions, how these changes could impact the solutions in favour of the customer is not always clear,” Aazzum told Engerati.

UK grid code modifications

Aazzum explains that the issue dates back about a decade with the emergence of what is now known as a hybrid solution. Traditionally STATCOMs are used for compensating reactive power in wind farms. “A STATCOM is able to quickly react to changes from the wind farm, essentially absorbing or generating reactive power dependent on the output of the wind farm. Without a STATCOM in place the wind farm would not be grid code compliant and the systems efficiency would likely be affected. In real terms this means, without grid code compliance the system would not be allowed to connect to the network."

Developments in turbine technology have meant that in some cases, the wind turbines themselves can provide adequate reactive compensation to enable compliance with the grid code. However, this is very site specific and additional RCE may be required depending on the overall windfarm impedance and turbine performance at full output. Whilst technology is advancing, the pressure to reduce total windfarm costs is increasing. To help address this, RCE solutions are available from S&C which allow for a reduction in the STATCOM size. As the STATCOM size is reduced the additional compensation can be covered by either solution below.

· Bias solution – This solution comprises a STATCOM with the addition of Bias Shunt Devices (BSD), i.e. a reactor/capacitor bank or combination of both. In this solution the shunt devices are fixed and always on.

· Hybrid solution - This solutions comprises a STATCOM with the addition of the SSDs, i.e. reactor/capacitor banks or combination of both. In this solution the shunt devices are switched so can be turned on and off dependant on the required compensation needed. The latest SSD technology uses point on wave switching to provide a much faster response time and reduces the past concern related to capacitive currents and repeated switching. This technology is what allows the hybrid system to meet the revised grid code requirements.

“We have been delivering hybrid solutions in the UK since 2008 but there has always been a question of compliance with the grid code due to its ambiguity,” says Aazzum. Following a query raised by a wind farm developer on the issue, an investigation was instigated and S&C along with other parties worked with National Grid on the revisions which “essentially clarify” the compliance of the hybrid solutions.

The requirement was set at response to a maximum of 5 events in a 5-minute period, and 25 events in a 24-hour period. This allows response to 96% of events that arise during a winter storm, and is applicable only to new farms connected from 1 December 2017 (because of the higher costs that would accrue to farms existing or under construction). The response itself is that 90% of the change in reactive power output following a voltage step change is to be provided within 1 second, or within 2 seconds for the installations after 1 December 2017.

STATCOM use in wind farms

Aazzum says that as a very general rule of thumb from S&C’s experience, a STATCOM only solution is likely be the most cost effective for smaller compensation requirements and a bias or hybrid solution for larger compensation. However, there are several factors that influence the choice of solution, such as the local geography, the nature of the connection and the amount of compensation that is required. He stresses that a reactive compensation study is the key and should be always be conducted.

“As standard practice we always recommend a study to take all the factors into account, even with the new generation turbines which generally have some reactive compensation capability built in by the manufacturer,” he says. He adds that S&C also recommends the study be done by the company that will provide the solution, rather than by an external consultant, so that it’s correct size is ensured.

Further, a full lifecycle approach is required taking into account not only the initial capital costs but also the lifetime operating costs and losses. STATCOM only solutions generally have higher CAPEX, OPEX and losses than hybrid solutions, while bias solutions are somewhere in between.

“Even though the hybrid is the latest solution and in most cases is the most cost effective, it shouldn’t be the default. Each of the solutions has its place and one needs to find the balance, which is why the sizing of the system is so important.”

An established solution provider

In conclusion, Aazzum says that in addition to its UK deployments, S&C has provided hybrid solutions elsewhere in the world, mostly in the US, Canada and Mexico.

“We want to guide customers and to give them the information they need about the various solutions that are available so that they can then decide how to meet their reactive compensation needs. This is the guidance we feel is needed.”

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