What goes around comes around

ABB’s Stephen Clabburn MIET, UK Sales Manager – Large Motors & Generators, explains how synchronous capacitors are being reborn thanks to the renewable energy revolution.

Synchronous capacitors have been around for many years. In the 1950s and 1960s, they were commonly used to provide nearly all grid stability in the UK, but fell out of favor towards the end of the 20th century with the rise of power electronics. However, in recent decades, and especially with more renewables coming online, synchronous capacitors have come back into vogue as “renewables enablers”.

A synchronous capacitor is a large rotating machine, however its shaft is not connected to any driving or driving equipment, so it is neither a motor nor a generator. Produces or absorbs reactive power for voltage control on the grid. In addition to being widely used by grid operators, synchronous capacitors can also serve a useful purpose in providing power stability and continuity for larger industrial plants.

Synchronous capacitors: a brief history

Keeping the UK mains frequency at the required 50Hz is something we might take for granted when switching on an appliance, but in reality it always requires careful balancing. In the past, electricity grids were usually centralized around large fossil fuel power plants. Thermal energy provided through the combustion of coal or gas drives rotating mechanical equipment such as generators and turbines. In addition to creating a stable and predictable supply, this rotating equipment also inherently provided grid inertia. Inertia provides the ability to respond to fluctuations in both electricity supply and demand and is vital to grid stability.

There are three main ways synchronous capacitors can help reduce risk on the network:

1. Inertial support

In the past, inertia was provided by traditional rotary generators. However, renewable energy sources such as wind, solar, tidal and battery storage are non-synchronous resources. These sources are often intermittent and lack any electromechanical connection to the network. This results in an increase in the rate of change in frequency (RoCoF), which can lead to offline systems crashing. Synchronous capacitors provide instantaneous inertia to keep grid frequency within acceptable limits and buy operators time to respond to frequency changes.

2. Contribution of the level of failure

Non-synchronous generators are unable to provide instantaneous support like a synchronous capacitor, which can provide fault current with a much larger amplitude. Fault current is important because it is what activates many of the protection systems on the grid, which monitor the difference between a normal operating current and the actual current. The difference must be large enough to trip the protections, otherwise there is a risk of damaging equipment such as transformers or switchboards.

3. Voltage regulation

Synchronous capacitors also provide reactive power to support the grid voltage in the event of an undervoltage condition, such as when a brownout occurs. Similarly, in an overvoltage condition where the voltage becomes too high, the synchronous capacitor can draw reactive power.


The risks of network instability

Power quality events can be caused by a wide variety of factors, which can include a generator trip, overhead line failure due to a thunderstorm, or even a vehicle hitting a line or a pole, switchgear failure, or loss of an HVDC link. The effects manifest themselves in voltage variations that can lead to failures, downtime and damage to equipment. If the frequency deviates from 50Hz, this can cause the electrical equipment to trip or fail and potentially damage it. It could even make clocks run fast or slow, as happened in several European countries in 2018 when Kosovo failed to generate enough electricity to meet its needs, resulting in all grid-powered clocks slowing down to at six minutes.

As recently as 2019 in the UK, an unexpected shutdown of the Hornsea offshore wind farm is believed to have contributed to a sudden loss of frequency on the grid below 49Hz, resulting in automatic disconnection of parts of the grid. It ended up causing a blackout that wiped out electricity for much of London and the West Midlands.

Enable renewables

Modern wind and solar farms typically have no direct electromechanical connection to the grid and therefore no inherent inertia. The amount of electricity generated will also depend on prevailing conditions and can fluctuate wildly and sometimes unpredictably over the course of a single day. Thus, not only is renewable energy production generally more intermittent than with traditional fossil fuel power plants, but the traditional means of dealing with grid inertia, namely the use of generators and turbines, have also been eliminated.

As more renewables come online and conventional fossil fuel plants are increasingly phased out, this means that grid instability is a growing problem, leaving grid operators with fewer levers with which to deal with it. The addition of new power electronics to the grid, which are also inertia-free but highly sensitive to voltage variations, potentially exacerbate the problem further with each passing day. Conversely, adding more synchronous capacitors to the grid helps address these issues, helping to enable more renewables to be connected to the power grid.

Synchronous capacitors and microgrids

National power grid operators are generally well aware of the function and benefits of synchronous capacitors and are increasingly rolling them out across the grid. However, they can also serve an important purpose for large industrial plants that are particularly dependent on continuous power. Downtime, for example, at a large steel mill or an oil and gas rig can cost huge amounts of money every hour. In the past, there was no need to provide additional inertia at a site as it was provided entirely by large power plants. However, a synchronous capacitor can help such plants be self-sufficient, using locally produced energy in the event of a grid problem, without risking downtime or equipment damage. It can also help isolate microgrids in remote areas and communities from wider problems on the grid. Having the ability to compartmentalize the network in this way creates resilience and flexibility in the larger system for greater security of supply and redundancy in case of problems.

Synchronous capacitors in action

In 2022, a pair of ABB-installed synchronous capacitors will deliver more than 900 MW of inertia at a Lister Drive site in Liverpool, contributing around 0.5% of the UK’s total grid inertia. The site features two 67 MVAr synchronous condensers with 40-tonne flywheels that increase the available inertia by a factor of 3.5, and is the first design in the world to feature a high-inertia configuration.

Renewable energy is clearly vital if we are to secure a sustainable energy future, but it’s not as simple as simply plugging more solar panels and wind turbines into the grid. Each new source connected to the grid has the potential to contribute to instability and act as a tipping point for voltage variations. Despite having been out of fashion for many years, synchronous capacitors are an old technology that has learned a few new tricks and will be a vital component as an enabler of renewable energy as the UK continues to take steps to decarbonise the grid.

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