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This lecture discusses power electronics and the role they play in the renewable energy sector. The different topics that will be discussed in this lecture are:

• The need for power conversion
• The working principle of a power electronic converter
• The functions of a power electronic converter

The need for power conversion

Because the generation of renewable energy (such as PV’s or wind turbines) depends on the weather conditions of a certain location, this cannot always meet the demand. Therefore, it is preferred to integrate these renewables into the grid, such that power can be transferred to different locations.

However, it is not possible to directly connect a renewable energy source to the grid. PV panels have a DC output, which does not match the AC power in the grid, and wind generators typically operate on a frequency that is dependent on the wind speed. Connecting this directly to the grid can lead to failures and blackouts.

The solution to this problem is power electronic converters, which can be used as an interface between the grid and the generators. Basically, there are four types of power conversion, which are:

• DC to AC, which is used for a PV power converter
• AC to AC, which is used for a wind power converter
• AC to DC, which is used to charge laptops and phones
• DC to DC, which is used to change the DC voltage

The working principle of a power electronic converter

The operation principle is explained by looking at the basic topology of a buck converter, which is shown in the figure below. A buck converter is a device to transfer a DC voltage into a lower DC voltage.

The buck converter consists of switch, a diode, followed by a lowpass filter formed by an inductor L and capacitor C. The switch can be in two states: on, which means that it will act as a short circuit, and off, which means that it will act as an open circuit. By switching the state of the switch rapidly, the voltage over the diode will alternate between zero and the voltage of the source. The lowpass filter removes the alternating behavior and a constant voltage remains. The output voltage equals the input voltage times the so called “duty cycle”, which is the ratio of the on time divided by the total switching time. This duty cycle is determined by the using a comparator as shown in the bottom of the figure.

Similarly, a bipolar voltage can be created with two switches. The circuit for this is shown below:

By changing the states of the two switches, an alternating voltage appears on the right side. By filtering the signal with the lowpass filter, a sinusoidal wave is created.

Functions of the power electronic converters

Power electronic converters have different functions depending on the configuration. On the generation side, the major function of power electronics is to make the renewable operate on its maximum operating condition. On the demand side, the power electronics allow active and reactive power control, and other ancillary services. For different renewables, the role of power electronic converters is explained.

Wind generators can be categorized into two types: DFIG and PMSG. In DFIG (Doubly Fed Induction Generator), the generator is directly coupled to the grid via a transformer. The rotor winding is connected via an AC-DC-AC converter, such that the rotor frequency is not fixed to the grid frequency. This is done, such that the frequency can be varied to achieve the maximum operating point.

In PMSG, the stator is connected to the grid via an AC-DC-AC converter, so that the generator can adjust the frequency to achieve the maximum operating point. This was not possible in the past, because the power converters were not able to process the full-scale power of the generator. Due to development and cost reduction, this is nowadays possible.

In a PV system, the voltage is typically lower than the voltage of the grid. Therefore, a DC-DC voltage is used to boost the voltage. After this, a DC-AC converter is used to convert the DC energy into AC energy. Besides only converting DC energy into AC energy, a DC-AC converter can also control the solar cell, allowing for controllability and flexibility.

Conclusion

This lecture discussed the role of power electronic converters. Power electronic converters are used to integrate renewables into the grid and to control the operating point of the renewables. A basic buck converter was discussed to discuss the working principle of power electronic converters.