This month, Statkraft and ABB announced the construction of two synchronous condensers on Lister Driver in Liverpool. The Lister Drive project will provide a range of grid stabilisation services to National Grid ESO, including inertia, short circuit current and voltage control. Grid stability in the area was previously supported by the Fiddler’s Ferry coal-fired power station which closed in March 2020. The contract with ABB is over ten-years and the £25 million project is due to begin operations later this year.
Last year a similar grid stabilisation project was announced for Rassau in Wales. The scheme which comprises a synchronous condenser and flywheel is a collaboration between Welsh Power, Quinbrook, National Grid ESO, Siemens and Western Power Distribution, is due to come online by autumn 2021.
“Within 15 minutes of an instruction, our facility can provide approximately 1% of the inertia needed to operate the grid safely – with zero emissions,” – Chris Wickins, director of grid services at Welsh Power
Why are these schemes suddenly gaining interest?
The electricity system is changing. The growth in renewables and the decline in traditional generation isn’t just decreasing the carbon content of electricity, it is also reducing the amount of inertia on the grid – the property of large, heavy turbines which resist changes to grid frequency. This isn’t just a quirky bit of physics, the frequency of the electrical oscillations of the electricity system must stay within a narrow band (50 Hz ±1%) otherwise electrical equipment can trip.
Renewable generators also cannot supply reactive power to the grid. Reactive power occurs when current becomes out of phase with voltage in ac systems, and oscillates inside the circuit in a similar way to a pendulum, with energy being absorbed in and released from the magnetic fields created by electrical equipment. However, reactive power does not travel well so unless it is minimised within electrical systems, components can become over-heated leading to voltage instability.
Thermal power stations traditionally provided reactive power as well as inertia, and this also needs to be replaced as the energy transition progresses. Grid operators can use static or dynamic reactive power compensating devices. Static devices such as capacitor banks are relatively low cost but are slow to respond, and their output drops when voltage drops. They can also take up lot of space.
Other static devices include static var compensators – switches made of shunt capacitors and reactors connected by thyristors (solid-state semiconductor-based switches). These offer more voltage control than capacitors and can absorb and supply reactive power, but their reactive power output varies according to the square of the voltage, so they struggle to operate in conditions of voltage instability. Static Compensators (“STATCOMs”) are shunt devices which use force-commutated power electronics to control power flow with fast response times in microseconds. They need much less space than capacitors, but are about 20% more expensive.
Dynamic devices include synchronous condensers which can continually adjust their output level in tiny increments to smoothly balance the system. National Grid is beginning to use synchronous condensers to meet these inertia and reactive power needs. These can be either existing generators that turn without generating electricity, or purpose built machines which can never generate power. These purpose-built machines are sometimes fitted with flywheels to increase their mass and therefore their inertia.
The idea of synchronous condensers is not new – in the 1960s and 70s some substations deployed stand-alone synchronous condensers, and in the early days of electricity, they were used quite widely, but once synchronous grids were established, they were considered to be obsolete and largely fell out of use.
Synchronous generators can be adapted to also operate as synchronous condensers by installing a synchronous self-shifting clutch between the turbine and the generator. In this scenario, the turbine brings the generator up to speed, enabling it to synchronise with the grid. At this point, the turbine disconnects from the generator with the generator using grid power to keep spinning. The clutch disengages the prime mover and the generator when reactive power is needed, and re-engages for power generation when real power is needed.
As the electricity transition progresses and traditional generation is increasingly replaced by asynchronous renewable generation, technologies such as synchronous condensers will be more widely used.