Synchronous condensers (SCs) produce reactive power and are a useful solution for stabilizing the power produced by renewable resources.
SCs are rotating electrical machines that resemble synchronous generators in their design. They are neither motors - as they do not drive a load - or generators because they are not driven by an engine or turbine.
SCs were previously used as sources of reactive power to compensate for induction motors and other highly inductive loads. Advances in power electronics led to the development of static compensators and these replaced SCs as they offered better controllability, flexibility and speed. Demand for SCs slumped and for many years they looked like an obsolete technology.
Energy resources like solar and wind are intermittent and variable. They lack the capability to tolerate network faults or play a grid support role. As a result, the increasing penetration of these sources of power in the energy mix is reducing the resilience and stability of electricity networks.
Many solar farms, for example, use an inverter to synthesize alternating current (AC) for the network from the direct current (DC) produced by photovoltaic (PV) panels. If a fault occurs somewhere in the network causing the network parameters to diverge too far from their normal levels, the inverter will be switched off by its protection unit. The solar farm has no capability to ride through network faults.
In many areas the rise of solar and other renewables is happening at the same time that conventional power plants are being decommissioned. As a result, the power plants’ large, high inertia synchronous generators, which have supplied the grid with short-circuit power and rotating mass, are being taken out of service. In this type of situation, synchronous condensers can provide additional short-circuit power to strengthen the grid, help to maintain grid quality, and provide fault ride-through capability.
SCs can also be used to strengthen the short-circuit capacity of networks that are subject to seasonal variations in power demand or major fluctuations due to local heavy industry. In addition, they can be used to produce reactive power locally in order to minimize losses in grids where power production and consumption are separated by long distances.
Because system performance requirements and site conditions vary so much, SCs should be tailored for their specific application. ABB supplies SCs in the power range 1 – 75 MVA at 3 – 15 kV system voltage. The network voltage is generally much higher, so a step-up transformer is used. The SCs are modules and several can be combined for higher outputs – a solution that also provides better redundancy and availability than a single large unit.
They can be started by a frequency converter, direct online, or using a pony motor. A small (200-300 kW) pony motor supplied by a frequency converter is the ideal starting method, because it enables the SC to be designed without having to allow for special start-up requirements. Like synchronous generators, SCs feature a salient pole rotor, brushless excitation and epoxy-resin insulated stator windings. They are generally water-cooled, but other cooling methods are also available. For example, ABB’s air-water-air (CAWA) cooler with a closed cooling circuit that enables SCs to be installed outdoors at sites where no water is available.
With continued growth in the use of renewables, SCs are attracting a lot of interest from electricity utilities and grid operators. Synchronous condenser technology is well-established and proven, and it can play an important role in strengthening the quality and resilience of today’s electricity grids.
|Weak network (lk)
||Short circuit capacity is reduced due to reduced capacity / number of large fossil fuel power plants and increased number of new industries
||The number of new SC's will improve the total lk and provide redundancy for service shut downs
|Unstable network (Hz)
|Frequency fluctuating due to increased new load types and faults
||With high inertia it can stabilise the frequence
|Long distribution lines (V)
||Voltage sags due to many consumers over long distances
||Stabilise the voltage with boost output for several 100 ms
|Temporary / periodically peak loads
||Process industries can represent large uneven rapid changing loads
|SC's have high* overload capability for 15-30 min
|Can be from e.g. lightning strokes or equipment failure
|With high inertia it has good capability to ride through the faults
* SC's will be optimised based on specific project requirements at each locations