Power converters: enabling wind turbine integration in the grid

Many forms of electricity

Electric energy can present itself in many different forms. It can be as Direct Current (DC) or Alternating Current (AC); it may oscillate at different frequencies, it may present itself in different voltage levels, etc. In a wind farm, many power conversion steps are needed, from the electricity produced at the wind turbine generator to the electricity that is finally fed into the electric grid.

A power converter is a complex piece of equipment capable of “converting” one type of electricity into another. For example, it can have a DC input and an AC output, or it can convert between two AC sides with different voltage levels, among many options. Power converters appear in many applications, such as wind farms, solar farms, electric vehicles, power banks, etc.

Why do wind turbines need a power converter?

A wind turbine has many mechanical and electrical components. The blades move thanks to the wind. As a consequence, a shaft starts to rotate, which drives an electric generator. Thanks to this rotation, the electric generator is able to produce electricity. However, this electricity cannot be fed directly to the power cables of the farm and eventually to the power grid; this electricity must go first via a power converter and a transformer before exiting the wind turbine.

The main function of this power converter is to match the generator’s output to the required values in the power cables of the farm and grid. The output of the generator is like a slow-moving wave of electricity as shown in Figure 1 below. Since the rotational speed of the blades naturally varies depending on the wind, this makes the generator to rotate at different speeds, which means that the slow wave of electricity also changes its frequency depending on the wind. In contrast, the grid (and thus, the inter-array cables) operates at a higher and constant frequency, usually 50 or 60 Hertz (Hz) or cycles per second, which means the current changes direction 50 or 60 times every second. Therefore, the power from the generator needs to be converted to the right frequency and synchronised with the grid as indicated in Figure 1 where converter processes the electric power and create waveform in sync with the power grid. Usually, the final step is done by a transformer, whose function is to increase the voltage where the magnitude of the voltage increases to match the targeted grid voltage as shown in the Figure 1.

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Figure 1 - Function of a converter and transformer in a wind turbine

Figure 2 shows how the equipment is arranged inside the wind turbine and is similar to those used in the FlexH2 project.

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Figure 2 - A typical wind turbine with a cut-out view of the nacelle showing (from left to right) the generator, power converter, and transformer.

The converter can react to faults

The power converter can support the grid by feeding active or reactive power, as required, which helps control the grid’s voltage and frequency. Active power is usable power (i.e., feeding active power to a consumer during some time means that the consumer receives energy to function), while reactive power does not do “actual” work but is necessary for the operation of certain types of electrical equipment.

The converter also provides ‘ride-through’ support for the wind turbine. A fault (e.g., a short circuit) somewhere in the grid could cause the voltage to drop, which might result in the generator disconnecting from the grid. The converter enables the generator to ride through temporary voltage dips, helping maximise the turbine’s output and operation time, thus also ensure the security of power grid function at large.

Higher voltages enable lower losses 

The power converter is connected between the generator and transformer, as shown in Figure 3. In the FlexH2 project, the generator and converter operate at medium voltage (MV), around 3,000 volts, and the transformer steps this up to 66,000 volts for transmission to the offshore substation. Higher voltages enable lower losses and, therefore, greater efficiency when power is transmitted over long distances (the distance between a wind turbine and the offshore substation depends on many factors but might vary between 3 to 15 kilometres, approximately).

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Figure 3 - Block diagram showing electrical connections and main power converter modules. The system shown here has two parallel converters.

Clean power for the grid with FlexH2

As the diagram shows, two converters can be connected in parallel to achieve higher power ratings. The parallel configuration – used in FlexH2 - increases efficiency and reduces harmonics. Harmonics are small distortions that can appear in the voltage and current waveforms at different frequencies. Among many benefits, minimising harmonics results in optimal energy production and lower operating costs for the wind farm operator and ensures that the electricity arriving at the point of use will have a smoother, cleaner waveform.

Additionally, other equipment is needed to support the converter’s operation, such as protection devices (e.g., disconnectors, breakers, switchgear), a watercooling unit, auxiliary equipment, etc.

Electronic magic

How does the converter change the output from the generator to make it suitable for the wind farm cables and grid? As Figure 3 shows, each power converter has two sides – the generator and the grid side – connected by a link. The electricity coming from the generator is like a slow electric wave and is known as alternating current (AC). This AC is fed to the generator side of the converter, which changes it into direct current (DC) – current that flows in one direction only, like the electricity from a battery. The DC travels along the DC link, which helps to make the power more stable, and then goes to the grid side of the converter. Here, the DC is changed back to AC, but now the AC is stabilised at the right voltage and frequency for feeding to the transformer and grid.

The converter does this ‘electronic magic’ using semiconductor devices, which are of a type called integrated gate-commutated thyristor (IGCTs) in the FlexH2 converter. If we look at the grid side of the converter as an example, the IGCTs are switched on and off many times per second to convert DC to AC. This produces a series of voltage pulses, and the converter’s controller varies the width and timing of the pulses in precisely the right way to produce a clean AC output wave. This is known as pulse width modulation (PWM), shown in Figure 4. IGCTs are commonly used in MV wind converters because they handle high power levels efficiently and reliably.

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Figure 4 - By switching IGCTs on and off many times a second, the converter can feed power into the grid at the correct frequency. The blue line (V) is the pulse width modulated voltage, while the red line (B) is the resultant waveform.

Grid-forming mode for FlexH2

As we saw above, in a ‘regular’ wind turbine, the power converter adjusts the generator's output to match the grid's frequency and ensure synchronisation. This is known as grid-following mode. In the case of the FlexH2 project, the power converters will have additional functionality. A central aspect of FlexH2 is that (thanks to these new kinds of converters) the turbines will be capable of forming their own grid. In other words, the wind farm can supply power directly to the hydrogen producer (the electrolyser) without the need for an existing grid. This operating mode is called grid-forming and will require dedicated software for the converter. Grid-forming is one of the key technology innovation pillars of the FlexH2 project, and it is crucial for integrating large amounts of offshore wind energy into the energy system.

Do you want to know more about grid-forming control technology, please read the Reflexion of Dongsheng Yang.