A wind turbine generator produces electricity whose voltage and frequency wander with the wind. A power grid demands electricity at a fixed frequency, within a narrow voltage band, with controllable reactive power and the ability to ride through disturbances. The power electronic converter is what reconciles those two realities, and it has quietly become the component that determines whether a wind farm is an asset to a network or a problem for it.
Why a converter is unavoidable
Variable speed operation is worth having, because it raises energy capture and reduces structural loads. But a generator turning at variable speed produces variable frequency output, and that cannot be connected directly to a fixed frequency grid.
The converter solves this by decoupling the two. It rectifies the generator output to direct current, then inverts that direct current back to alternating current at precisely grid frequency. The rotor is then free to turn at whatever speed the wind makes optimal, while the grid sees a well behaved supply.
Two topologies
Doubly fed induction generator
In this arrangement, the stator connects directly to the grid and the converter feeds only the rotor windings through slip rings. Because the converter handles only the slip power rather than the full output, it can be rated at a fraction of machine power, which made this topology attractive on cost and it dominated onshore installations for years.
Its weaknesses are slip rings and brushes, which wear and need maintenance, and its behaviour during grid faults, which is more difficult to manage because the stator remains directly coupled to the disturbance.
Full power converter
Here the entire generator output passes through the converter, which is therefore rated for full machine power and costs more. In exchange, the generator is completely isolated from the grid, which gives far better control of fault behaviour and allows the generator to be designed without regard to grid frequency at all. This is what makes direct drive and medium speed architectures practical, as discussed in our note on geared turbines compared with direct drive.
Full conversion has become the standard for large and offshore machines, because the control capability it provides is now required rather than optional.
What grid codes demand
When wind was a negligible share of supply, network operators asked only that turbines disconnect safely during disturbances. As penetration rose, that became untenable. If every wind farm tripped during a voltage dip, the loss of generation would turn a local fault into a system event. Grid codes consequently now require a set of behaviours that go well beyond producing power.
Fault ride through obliges a plant to stay connected through a voltage dip of defined depth and duration, and to support the network while it does so. Reactive power control requires the plant to supply or absorb reactive power on command, helping hold voltage at the connection point, and modern converters can do this even at low or zero active power output. Frequency response requires output to be reduced when frequency rises, and in some codes to be increased from a deliberately curtailed level when frequency falls. Active power curtailment allows the operator to cap output on instruction.
All of this is delivered by the converter and the plant controller rather than by anything mechanical. A wind farm with modern converters is a considerably more controllable network asset than its reputation suggests.
Inertia, the genuine open problem
A conventional power station has a large rotating mass synchronously coupled to the grid. When demand suddenly exceeds generation, that mass gives up kinetic energy and slows, which slows the grid frequency gradually and buys time for control systems to respond. This inherent inertia is the reason frequency does not collapse instantly after a generator trip.
A converter connected wind turbine has rotating mass, but it is decoupled from grid frequency by the power electronics, so it contributes no inertia inherently. As the share of converter connected generation rises, system inertia falls and frequency moves faster after a disturbance.
The responses are active rather than passive. Synthetic or emulated inertia uses the converter to inject power rapidly in response to measured frequency change, extracting kinetic energy from the rotor deliberately. Grid forming control goes further, having the converter behave as a voltage source that establishes frequency rather than following it, which is an area of considerable development and is increasingly specified in markets with high renewable penetration.
Reliability and the practical consequence
Converters contain power semiconductors, capacitors, cooling systems and control boards, and field data identifies electrical and electronic components among the most frequent causes of turbine faults, though with shorter downtime per event than mechanical failures. Converter modules can usually be replaced uptower in hours rather than requiring a crane.
Thermal management is the main determinant of converter life, as it is for any power electronic equipment. Capacitors and semiconductor bond wires degrade with temperature and with thermal cycling, and a converter in a nacelle experiences both continuously. Cooling systems, filters and fans therefore belong on the maintenance schedule described in our note on operations and maintenance of a wind farm. General background is summarised in the overview of wind turbine electrical systems.
Frequently asked questions
Why can a turbine not connect directly to the grid?
Because variable speed operation produces variable frequency output. The converter rectifies and re inverts it so the grid sees a fixed frequency supply while the rotor turns at whatever speed is optimal.
What is a doubly fed induction generator?
A topology where the stator connects directly to the grid and the converter feeds only the rotor, so it handles a fraction of total power. It is cheaper but uses slip rings and manages grid faults less well.
Why has full power conversion become standard?
Because it isolates the generator from the grid, which gives much better control of fault behaviour and allows generator design independent of grid frequency, enabling direct drive and medium speed machines.
Can a wind farm support grid voltage?
Yes. Modern converters supply or absorb reactive power on command, often including at low or zero active power output, which makes the plant a useful voltage support resource.
Do wind turbines provide inertia?
Not inherently, because the converter decouples the rotor from grid frequency. Synthetic inertia and grid forming control provide equivalent behaviour actively and are increasingly required.