Novel Isolated Power Conditioning Unit for Micro Wind Turbine Applications

 

Abstract

This paper presents a novel power conditioning unit (PCU) for variable speed micro wind turbine applications. It contains a simple generator side rectifier, galvanic isolation with a simple dc-dc converter, and a single-phase full-bridge inverter at the grid side. Variable speed micro wind turbines based on a permanent magnet synchronous generator (PMSG) are increasingly used in residential and small commercial buildings, despite their relatively low output voltage. Therefore, they can be used easily for battery charging, while their grid integration requires a PCU with galvanic isolation. Most of available PCUs provide no galvanic isolation, or use relatively complicated topologies or four stage energy conversion for that purpose. The dc-dc converter proposed allows reducing the complexity of the PCU. Steady state analysis shows that the converter is capable of regulating voltage in a wide range suitable for micro wind turbines, which is supported by experimental results within the input voltage range of 40 V to 400 V. The prototype built for integration of a 1.3 kW PMSG based micro wind turbine shows good performance over the entire 1:5 range of the given wind turbine output voltage. A study of efficiency and power losses was conducted according to the wind turbine power profile.

EXISTING  SYSTEM:

Despite their relatively high price, the multi-pole low-speed Permanent Magnet Synchronous Generators (PMSGs) are a  dominant technology used in variable speed micro WTs due to small volume, high reliability and efficiency, self-excitation, and brushless design .  Usually, a small PMSG is directly driven by a three- or multi-blade horizontal wind rotor with high torque to avoid mechanical parts, i.e. gearboxes, that suffer from wearing. Residential variable speed PMSG based micro WTs of subkW level can generate output voltage as low as several tens of  volts, which varies in a wide range due to variable speed  operation. This makes them suitable for battery charging,  but an isolated converter is required for grid interfacing.  There are numerous concepts of Power Conditioning Units (PCUs) for small WTs that are mostly oriented towards applications with rated power well above that of micro WTs. The latter is usually limited by the standard household electric network: 11 kW for three-phase connection to a 400 V distribution grid, and more commonly 5.75 kW for a singlephase connection to a 230 V distribution grid. Generally, a PCU for small PMSG based variable speed WTs is nonisolated and based on: back-to-back converters a combination of a diode bridge rectifier and a dc-dc converter at the generator side and a traditional voltage source inverter at the grid side a combination of a diode bridge rectifier and an impedance-source inverter with a possibility of voltage regulation that avoids a dc-dc converter

PROPOSED SYSTEM:

There are some galvanically isolated PCUs proposed for micro WTs:   a diode bridge rectifier is followed by the fullbridge voltage source dc-dc converter that feeds two-level  full-bridge grid side inverter;   in active rectifiers supply phase-modulated high-frequency isolated dual LCL dc-ac converter that shapes current, which is injected into the grid through unfolder;  an active rectifier operates in tandem with a quasi-Z-source full-bridge dc-dc converter to supply the grid side inverter with stable voltage; a microinverter converts energy in the following four-stage sequence: diode rectifier, non-isolated boost converter, isolated flyback converter that shapes sinusoidal current, an unfolder that injects current into the grid.  Evidently, existing solutions are either based on isolated full-bridge dc-dc converters that may not be an optimal solution for sub-kW power levels, or based on a single-ended converter that requires four stages of energy conversion, which, in turn, could result in limitations of efficiency. Clearly, there is a need for a simpler approach that is better tailored for micro WTs. This work is dedicated to a novel galvanically isolated modular single-switch quasi-Z-Source  (qZS) dc-dc converter intended for micro WT applications. Its utilization in PCU for micro WTs enables low cost of realization, three stages of energy conversion despite its single-switch nature, simple control, and modular realization

CONCLUSION  

This paper proposes a novel galvanically isolated power conditioning unit intended for variable speed micro wind turbines based on the permanent magnet synchronous generator. This unit enables grid integration of sub-kW residential wind turbines into the distribution grid. Usually, reports on small scale wind energy conversion systems are dedicated to the enhanced control methods. However, performance of a power electronics converter used is often overlooked, in particular, proper analysis of the efficiency and losses according to the operating profile of a corresponding wind turbine. The proposed approach does not require any significant changes of the conventional control systems. Therefore, this paper pays special attention to energy conversion performance by means of the efficiency and power losses.  The galvanically isolated power conditioning unit proposed is based on a novel modular quasi-Z-source dc-dc converter  composed of typical single-switch power electronics modules. The modular design proposed can decrease manufacturing costs for power levels up to several kW, where the number of required modules is reasonable. The 1.3 kW experimental prototype based on the dc-dc converter composed of two modules was built for the case study of a wind turbine. It shows the capability of covering the entire operating range of the corresponding wind turbine. The prototype reaches almost 91% peak efficiency despite 1:5 voltage and more than 1:50 power variations, which usually complicates the design of the PCU for high efficiency. The approach proposed allows either reduction of the number of energy conversion stages from four to three or avoiding more complicated isolated topologies like full-bridge. Experimental study of power losses was performed in order to estimate heatsink requirements. Losses in the switches and in the qZS coupled inductors are close to each other and dominate over other losses in the prototype built. The experimental results obtained justify the power conditioning unit proposed for the given application. Focus in the future research will be on the reliability and packaging issues of the proposed concept.

REFERENCES

[1] Stefan Gsänger and Jean-Daniel Pitteloud, World Wind Energy Association. Small Wind World Report. 2015. Summary, Mar. 2015.

[2] M. Malinowski, A. Milczarek, R. Kot, Z. Goryca and J. T. Szuster, “Optimized Energy-Conversion Systems for Small Wind Turbines: Renewable energy sources in modern distributed power generation systems,” IEEE Power Electron. Mag., vol. 2, no. 3, pp. 16-30, Sept. 2015.

[3] M. Malinowski, A. Milczarek, D. Vinnikov and A. Chub, “Wind energy systems,” Chap. 12 in Power Electronic Converters and Systems: Frontiers and Applications, Ed. A. M. Trzynadlowski, London, UK: IET, 2015, pp. 351-394, DOI: 10.1049/PBPO074E_ch12.

[4] American Wind Energy Association. AWEA 2010 Small Wind Turbine  Global Market Study.

[5] R. Wiser and M. Bolinger, 2014 Wind Technologies Market Report, U.S.  Department of Energy, Aug. 2015.

[6] R. Lanzafame and M. Messina, “Power curve control in micro wind turbine design,” Energy, vol. 35, no. 2, pp. 556-561, Feb. 2010.

[7] L. Mariam, M. Basu and M. F. Conlon, “Community Microgrid based  on micro-wind generation system,” Proc. UPEC’2013, pp. 1-6, 2013.

[8] A. Chub, T. Jalakas, A. Milczarek, A. Kallaste and M. Malinowski, “Grid integration issues of PMSG-based residential wind turbines,” Proc. PQ’2014, pp. 147-154, 2014.

[9] L. Bisenieks, D. Vinnikov and I. Galkin, “PMSG based residential wind turbines: Possibilities and challenges,” Agronomy Research, vol. 11, no. 2, pp. 295–306, June 2013.

[10] M. Godoy Simoes, F. Alberto Farret and F. Blaabjerg, “Small Wind Energy Systems,” Elect. Power Components and Syst., vol. 43, no. 12, pp. 1388-1405, July 2015