Present trend of ever increasing power demand and increase in power failures are creating deficiency of power. Research and development on alternative and eco-friendly energy sources have been showing excellent potential as a form of contribution to conventional power systems. Renewable energy technologies offer clean, eco-friendly and abundant energy gathered from self-renewing resources – such as sun, wind waves, tides and so on. Advancements in power electronics provides a wide area control on flow and conversion of power. This article presents review on recent advancements in power electronic converters and their control to form hybrid power generation systems.
Recent developments and trends in the entire world indicate an increase in electric power consumption in a large scale. This makes the world face the challenge to overcome the deficiency of energy. The ever increasing demand for conventional energy sources and need for safe globe and better life of all living beings, are driving the society towards research and development of alternatve eco-friendly energy sources. Renewable energy sources have attracted wide attention because of their abundant nature and availability. Renewable energy sources are generally defined as an energy extracted from resources that are naturally replenished on a human time scale. For example: sunlight, wind, tides, waves and geothermal heat.
Virtually, all regions of the world have renewable sources of one type or another. Solar and wind energy systems are the two renewable energy sources most commonly in use. Photovoltaic (PV) power or solar power is global, as it is directly captured from sunlight, and is a clean energy source since it is extracted without any rotational generators. Wind Energy Conversion Systems (WECS) offers a feasible solution for distributed power generation. Wind power can be captured easily by generators through wind turbines with high capacity and also they are free from pollution, this makes them more attractive. Wind can also be said as a promising and clean energy source.
PV and WECS are complementary in nature. During sunny days, winds will be weak and strong winds will flow during nights and cloudy days. Hence, to get an uninterrupted supply PV-WECS hybrid power generation systems can be designed. Not only WECS-PV, but a hybrid power generation system can be design with any combination of renewable sources. This depends on the availability of the natural resources for generation. For the loads near sea shores, the energy sources available are wave and tidal, and hence these generation systems can be combined with the WECS or PV or with both to form hybrid power generation systems. Use of biomass instead of fossil fuels can also stay advantageous – because of its properties of low green house gases emission, energy cost savings, improved security of supply, waste reduction opportunities. Bio energy can be converted into power through thermal-chemical processes i.e., combustion, gasification and pyrolysis or by bio-chemical processes like anaerobic digestion. This electric power can be obtained mostly in villages.
Hence in villages, biomass energy can also be combined to form hybrid power generation systems. Likewise many other sources like fuel cells and hydrogen fuel cells can also be integrated.
Converter topologies for hybrid power generation systems: A hybrid power generation system basically can be formed by using individual converters with each renewable energy source. An example of integration of PV-WECS with individual DC-DC converters is shown in Fig 1.
It can be observed in the figure that each subsystem such as PV & WECS are connected to individual DC-DC converter. The control is provided by MPPT technique. The output of the hybrid system is connected to a common DC bus. The DC power available at DC bus is supplied to the grid through inverter. Thus the entire system will provide continuous energy supply.
For the purpose of individual control the DC-DC converter topologies used commonly are listed below:
- Cuk converter
- Zeta converter
- SEPIC converter
- Four-Switch converter
The basic buck boost converter used for this purpose is a Cuk converter. This converter circuit is shown in Fig. 2.
This type of converter is a inverting type power converter, where the output voltage polarity is reversed. So as to avoid this complexity non-inverting converters are developed, which are Zeta and SEPIC converters. The Zeta converter is shown in Fig. 3.
Similar to the Cuk converter, this converter also consists of a MOSFET switch and a diode and two inductors and two capacitors. So, as to avoid inverting of voltage polarity the switch position is interchanged with inductor. The voltage is controlled by adjusting the duty ratio of the power switch. Duty ratio is the ratio of active time of the switch to the total time period.
Vo = [D/ (1-D)].Vs
Where D is duty ratio of the switch. Another combination of the same elements to avoid inverting of voltage polarity is SEPIC converter. The circuit of the converter is shown in Fig. 4.
It can be observed from the figure that instead of changing the position of power switch, the position of diode is interchanged to obtain non-inverting characteristics. Similarly voltage is controlled by adjusting duty ratio of the power switch. Beyond non-inverting so as to obtain a synchronous output from the converter Four-switch converter can be used. The circuit of the converter is shown in Fig. 5.
So as to understand the operation of the circuit the switching of the four switches must be studied. The switches Q1 and Q3 work as one group and Q2 and Q4 work as another group, such that when Q1 and Q3 are turned on, the switches Q2 and Q4 will be turned off, and vice versa. Thus we can regulate the voltage to higher or lower than the source voltage through proper control of duty ratio for the power switches.
Multi Input Converters (MIC): Further integration of renewable energy sources with individual converters requires individual controllers. This increases the complexity of circuits. Hence MICs are introduced, which take several inputs and give a single output. Due to this, the controller required for several sources is a single one. An MIC topology is introduced with the combination of Cuk converter and SEPIC converter either in series or in parallel. This is shown in Fig. 6.
This can be observed from the figure that PV-WECS are integrated together with a single converter topology. Hence, a single controller will be sufficient for simultaneous power management in this type of hybrid power generation system. A new MIC topology is introduced, which also adds energy storage device like battery. This converter topology is shown in Fig. 7.
From the figure it can be observed that the two sources are connected parallely and a battery is also connected to the system. This circuit operates in three modes of operation. Here in this circuit for two sources four switches are used. Further by reducing the number switches for each source a new MIC topology is developed. This MIC uses only one switch for each input port connected to a source. This topology is developed such that ‘m’ number of sources can be controlled simultaneously. The converter topology is shown in Fig. 8.
This MIC consists of a Low-Voltage-Side (LVS) circuit and a High-Voltage-Side (HVS) circuit connected by a high frequency transformer TX. The LVS circuit consists of m ports in parallel, one energy storage capacitor cs, and the primary winding of the transformer. Each port contains a controllable power switch, a power diode, and an inductor. The HVS circuit consists of secondary winding of the transformer connected to a full-bridge diode rectifier, and a low frequency LC filter. This converter has three operating modes:
- All switches are ON.
- Switch S1 is OFF while at least one of the other switches is ON.
- All Switches are OFF.
Thus an m sources can be integrated and a single controller can control the power flow from m sources to load.
Conclusion
This article has presented a complete picture on various converter topologies for hybrid power generation systems. An overview of each converter has been explained. These converter topologies allow for integration of different electrical sources, which are designed based on the requirements. Converter topology should be selected based on the requirements. Converter topology should be selected based on the power generation systems designed. An attempt is made to provide quick reference for the researcher, practising engineers and academicians those are working in the area of hybrid power generation systems.
