What goes around, comes around

Synchronous condensers have been around for many years. In the 50s and 60s, they were commonly used to provide almost all grid stability in the UK, but fell out of favour towards the end of the 20th century with the rise in power electronics. However, in recent decades, and particularly as more renewables come online, synchronous condensers have roared back into fashion as an “enabler of renewables”.

A synchronous condenser is a large rotating machine, however its shaft is not attached to any driven or driving equipment, and so it is neither a motor nor generator. It produces or absorbs reactive power for voltage control on the grid. As well as being widely used by grid operators, synchronous condensers can also serve a useful purpose in providing stability and continuity of power for larger industrial facilities.

Synchronous condensers: a brief history

Keeping the frequency of the UK electrical grid at the required 50Hz is something that we may take for granted when we switch on an appliance, but in fact it requires a careful balancing act at all times. In the past, power grids were typically centralised around large fossil fuel power stations. Thermal energy provided through the combustion of coal or gas would drive rotating mechanical equipment such as generators and turbines. As well as 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 demand and supply, and is vital for grid stability.

There are three main ways that synchronous condensers can help to reduce risks on the grid:

1. Inertia support

In the past, inertia was provided by traditional rotating 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 electro-mechanical connection to the grid. This results in an increased rate of change of frequency (RoCoF), which can result in systems tripping offline. Synchronous condensers provide instantaneous inertia to keep grid frequency within acceptable limits, and buy time for operators to respond to frequency changes.

2. Fault level contribution

Non-synchronous generators are unable to provide instantaneous support like a synchronous condenser, which can provide fault current in a far higher amplitude. Fault current is important because this is what triggers many of the protection systems on the network, which monitor the difference between a normal operating current and actual current. The difference must be big enough to trigger the protections, or else it risks damaging equipment such as transformers or switchgear.

3. Voltage regulation

Synchronous condensers also deliver reactive power to support grid voltage in the event of an undervoltage condition, such as when there is a voltage dip. Equally, in an overvoltage condition where voltage is becoming too high, the synchronous condenser can absorb reactive power.

The risks of grid instability

Power quality events can be caused by a wide range of factors, which can include a generator set tripping, failure of an overhead line due to a storm or even a vehicle striking a line or pole, switchgear failure, or loss of an HVDC link. The effects manifest themselves in voltage variances which can lead to trips, downtime and equipment damage. If the frequency deviates from 50 Hz, this can cause electrical equipment to trip or fail, and potentially cause damage to it. It may even cause clocks to run fast or slow, as was the case across several European countries in 2018 when Kosovo failed to generate enough electricity to meet its needs, resulting in all mains powered clocks on the network slowing down by up to six minutes.

As recently as 2019 in the UK, an unexpected shutdown at Hornsea offshore wind farm is thought to have contributed to a sudden loss of frequency on the grid to below 49Hz, causing parts of the network to disconnect automatically. It ended up causing a blackout that wiped out power for large parts of London and the West Midlands.

Enabling renewables

Modern wind and solar farms typically have no direct electro-mechanical connection to the grid, and therefore no inherent inertia. The amount of electricity generated will also depend on the prevailing conditions, and can vary wildly and sometimes unpredictably throughout the course of a single day. So not only is renewable power generation generally more intermittent compared to conventional fossil fuel power stations, but also the traditional means of dealing with grid inertia, i.e., using generators and turbines, have also been removed.

As more renewables come online, and conventional fossil fuel plants are increasingly retired, this means that grid instability is a growing problem, while leaving grid operators with fewer levers with which to address it. The addition of new power electronics to the grid, which also have no inertia yet are extremely sensitive to voltage variances, potentially compounds the problem further with each passing day. Conversely, adding more synchronous condensers to the grid helps to address these issues, helping to enable the connection of more renewables to the power network.

Synchronous condensers and microgrids

National power grid operators are generally well aware of the function and benefits of synchronous condensers, and are increasingly rolling them out across the grid. However, they can also serve an important purpose for large industrial facilities that are especially reliant upon continuous power. Downtime at, for instance, a large steelworks or oil and gas platform can cost huge amounts of money every hour. In the past, no additional inertia would need to be provided at a site as it was all supplied by the large power plants. However, a synchronous condenser can help such facilities to be self-sufficient, falling back on locally produced energy in the event of grid issues, without risking downtime or damage to equipment. It can also help to insulate microgrids in remote areas and communities from wider problems on the grid. Having the ability to compartmentalise the grid like this builds resilience and flexibility into the wider system for greater security of supply, and redundancy in the event of any issues.

Synchronous condensers in action

In 2022, a pair of synchronous condensers installed by ABB is providing more than 900 MWs of inertia at a site at Lister Drive in Liverpool, contributing roughly 0.5 percent of the UK’s total grid inertia. The site features two 67 MVAr synchronous condensers with 40 tonne flywheels that boost the available inertia by a factor of 3.5, and is the first project anywhere in the world to feature a high-inertia configuration.

Renewables are clearly vital if we are to ensure a sustainable energy future, but it is not as straightforward as simply plugging more solar panels and wind turbines into the grid. Every new source that is connected to the grid can potentially contribute to instability, and act as a pinch point for voltage variances. Despite falling out of fashion for many years, synchronous condensers are an old technology that’s learnt some new tricks, and will be a vital component as a renewables enabler as the UK continues to take steps to decarbonise the grid.

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