Wind Power Generation System

There are many topologies available for the wind power generation system. They can be sorted into three groups, the systems without power electronics, the systems with partially rated power electronics, and the systems with fully rated power electronics. Fig. 7.1(a) shows the system without power electronics, which normally uses induction generator to work at a fixed rotation speed since its output frequency cannot be modified and need to be the same as the grid frequency. The input power of this system is limited by the wind turbine pitch control [95], therefore this type of system cannot optimise the utility of wind energy. Furthermore, any fluctuation in wind speed naturally causes the mechanical power of the turbine to vary. This could lead to the torque fluctuation of the turbine rotor and stress the mechanical component such as gear box.

Fig. 7.1(b) shows a widely used system topology, doubly-fed induction generator (DFIG), in the second category. Partially rated power converters are employed hence greatly improved the system control performance. By using the DFIG, the system is able to operate at variable rotor speed while the amplitude and frequency of the generated voltage remain constant. This is achieved by adjusting the amplitude and frequency of the AC currents that fed into the generator rotor windings. With the ability of variable speed operation, the torque fluctuation at the mechanical component is reduced and more power can be generated from wind energy. Furthermore, the power rating of the converter is about 30% of the nominal generator output power, which saves the cost for power electronics and reduces the power

Gear box Reactive compensator Grid Induction generator (a) Gear box Grid Induction generator AC DC AC DC (b) Grid PM generator AC DC AC DC (c)

Figure 7.1: Different topologies of wind power generation system. (a) topology without power electronics, (b) topology with partially rated power electronics, (c) topology with

Wi_{nd turbine Generator Rectifier}

Inverter Grid

DC bus Inverter input

Power generation Power conversion

Load

Figure 7.2: Basic structure of wind power generation system.

loss during power conversion. However, the DFIG requires a complex power conversion and control circuit and the slip rings used for the rotor excitation requires periodic maintenance, which is not desirable for the off-shore operation.

Permanent magnet (PM) generator can be used to avoid the excitation circuit as well as the slip rings. The topology of this system is shown in Fig. 7.1(c), which is sorted into the third category. This system is able to operate at variable speed as well, but the amplitude and frequency of the generator output voltage vary as the wind speed fluctuates. Hence its power conversion unit needs to be the same rating as the generator. Furthermore, the increasing cost of PM elements also limits the application of the PM generator. However, this type of system eliminates the gear box and slip rings which have high failure rates, hence improve its reliability.

This work focuses on the reliability prediction of power converters thus the PM gener- ator topology is selected for the simulation due to its simplicity in control and electrical circuit. The basic structure of such wind power generation system consists of the power generation unit, the power conversion unit, and the load, as shown in Fig. 7.2. As the DC bus voltage is kept almost constant, the output of the rectifier can be considered as a constant voltage source, whose output current is determined by the output power of the PM

0 5 10 15 20 25 30 0 200 400 600 800 1000 1200 Wind speed (m/s) Pow er ( W)

Cut-in wind speed (around 3m/s)

Cut-out wind speed (around 25m/s) Power rating

Figure 7.3: Inverter input power curve.

generator. Therefore it is essential to obtain the output power variation due to the wind speed fluctuation.

The output power of the PM generator depends on the power generated by the wind turbine and the efficiency of the generator. The wind turbine output power is determined by the wind speed and can be obtained from its power curve. The power curve of an actual wind turbine (RES 1 MW) is scaled down to the power rating of the test modules (SKM50GB123D), as shown in Fig. 7.3. The modified power curve is used as a LUT to generate the wind turbine output power according to wind speed profile. The efficiency of the power generation and conversion unit is assumed to be 90%. Since the DC bus voltage is constant, the wind turbine output power can be used to derive the inverter output current. Therefore the detailed modelling of the wind turbine, the generator, and the rectifier can be neglected.

The main electrical circuit of the grid connected voltage source inverter model is designed and shown in Fig. 7.4. This inverter model is similar to the general purpose inverter model

1 3 5 2 4 6 Gate signal A Am1 A Am2 A Am3 1 2 3 V 4 f ( u) f ( u) f ( u) f ( u) f ( u) f ( u) demux

Gate Fcn Gate Fcn Gate Fcn

Gate Fcn Gate Fcn

Gate Fcn

CG

Figure 7.4: PLECS model of the grid connected voltage source inverter.

introduced in chapter 4, with some small modifications on the circuit. The resistive load is changed to a constant three phase AC voltage source which represents the ideal grid. The DC bus voltage is set to be 800 V and the three phase grid voltage is set to be 220 V. The capacitors are removed since they tend to short circuit the AC voltage source. Inductors (0.01 H) are used to ensure the current conducts continuously. They also work as filters to reduce the harmonics generated by the inverter. When applying this model to a real application, the ambient temperature may change, this can be modified by change the constant temperature sourcesTCGin Fig. 7.4to an variable temperature source which value is the recorded ambient

temperature.