Effective performance and power transfer operation of a current controlled WRIG based WES in a hybrid grid
Introduction
Utilization of renewable energy (RE) sources effectively reduces dependence on fossil fuels and increases reliability in supply. Among the green RE sources, wind is an imperative energy source to overcome the energy demand in areas with high wind velocity. Wind power can be supplied to loads when connected to isolated systems or can supplement utility when connected to grid. Owing to many renewable sources, power generation moves from large conventional power plants to small renewable power plants increasing the micro grids. Hybrid renewable micro grids are eco friendly, expandable and delivers uninterruptible power [1], [2].
Focusing on power extraction from wind at all conditions, Self-Excited Induction Generator (SEIG) based systems were developed [3], [4], [5]. SEIGs gain fame owing to its rugged construction, simple maintenance and absence of brushes and slip rings. SEIGs supplying frequency sensitive loads suffer from torque oscillations, temperature effects and machine vibration. Owing to vibration less operation with frequency sensitive loads, enhanced performance, usage of reduced converters and bidirectional slip power extraction, WRIGs are attractive and are widely employed nowadays [6]. Grid connected WRIG dealt in Ref. [7] focuses on voltage regulation using mathematical model rather than power transfer. Grid connected DFIG in Refs. [8], [9] details the operation at low wind with fractional converters. The method of power generation from variable speed DFIG in Ref. [10] focuses on harmonic mitigation rather than effective utilization of wind. All the above mentioned literature involves two converters connected back-to-back leading to increased cost and switching losses.
Micro grids in islanded operation offer the ability to effectively use renewable energy during grid outage. In any isolated WES, regulation of voltage and frequency is a confronting task. Moreover, isolated system needs to be reliable and should withstand dynamic wind speed/load changes. The system should be able to deliver power during all conditions so that wind power is better utilized rather than wasting it on some dump loads during adverse conditions. In order to extract the available wind power, the WES should be able to operate over a wide speed range. To achieve the above objectives, many control techniques are proposed in literature such as, slip control, vector control, direct torque/direct current/direct power control, Integral sliding-mode control etc., [11], [12], [13], [14], [15], [16], [17], [18]. The control plan for the hybrid operation of micro grid discussed in Refs. [19], [20] involve mathematical manipulation. A control strategy for the stable operation of micro grid with battery dealt in Refs. [21], [22] has a limitation on charging. These control techniques for WRIG involve mathematical modeling, parameter estimation, sensing, and sophisticated instrumentation. Effective power supply operation even during adverse conditions is not possible.
A sensor less voltage control along with harmonic compensation discussed in Ref. [23] involves a four leg converter leading to additional cost. Sensor less speed control proposed in Ref. [24] involves frequency loops and machine oriented analysis. The autonomous power generation system with UPS function dealt in Ref. [25] is devoid of sensors but involve two converters. A current control approach discussed in Ref. [26] is applied to provide both voltage and frequency regulation. It involves an additional pitch control technique for improving the dynamic behavior of micro grid employing DFIG. The autonomous cascaded doubly fed induction generator (CDFIG) dealt in Ref. [27] operates in a variable speed constant frequency (VSCF) mode requires dynamic modeling of the system. The stand alone DFIG discussed in Ref. [28] focuses on autonomous frequency control rather than power supply at various conditions.
A WES feeding AC loads discussed in Refs. [29], [30] aims mainly at harmonic elimination with the usage of a battery. Although effectiveness of storage and optimum energy storage techniques are dealt in literature [31], [32] battery usage has a limitation on charging. Extraction of maximum wind energy in any autonomous system is essential to provide reliable power to load [33]. An autonomous wind farm employed with a quantitative analysis based control proposed in Ref. [34] focuses on frequency regulation, making the generator suffer from stress and losses. A DFIG system feeding unbalanced loads with interest on harmonic compensation by current control as in Ref. [35] focuses only on load side converter. The current control proposed in Refs. [36], [37] focuses on frequency disturbances with the usage of resistance. The above mentioned isolated systems involve two converters to deliver power to load and fail to address the problems of power transfer during various wind/load conditions. Power extraction during machine stall, short circuit faults, over load, no load and low wind by an isolated WRIG WES is not addressed in the literature. The comparison of present work with some of the other findings in literature is diagrammatically represented in Fig. 1 to put forth the merits of the WRIG based WES dealt in this paper.
In this study, performance and possible operational modes of WRIG based WES supplying hybrid grid is discussed. It can operate both in islanded and utility mode delivering power irrespective of wind speed/load conditions. Occasional modes such as overload, no load, low wind speed, short circuit faults on stator and rotor are also analyzed indicating the system flexibility to operate at any wind velocity. During short circuit fault, WRIG operates as cage machine, either as Shorted Stator Induction Generator (SSIG) or Shorted Rotor Induction Generator (SRIG) depending on short in the windings. If there is any reduction in DCM voltage, SSIG mode can be provoked. The primary objective of the system is to effectively transfer the maximum generated power during all possible modes with a single current controlled RSC. In islanded mode, PI based decoupled voltage vector control is adopted for RSC with an outer stator voltage loop to maintain stator voltage and an inner rotor current loop to improve the stator voltage dynamics with less overshoot. The requisites of a standalone generator such as, wide speed range, suitable excitation etc., are also achieved. In utility mode, RSC is current controlled by reference current generation technique using IPT to effect bidirectional slip power flow by rectification/inversion. Hysteresis controller is used to obtain pulses for RSC. Experimental examination is done and results indicate the smooth power transfer operation, regulation of stator voltage and frequency and bidirectional slip power transfer.
This paper is organized as follows. In Section 2, the system under study is described. In section 3, the control scheme employed for RSC and possible modes of power transfer are detailed. Section 4 involves experimental performance evaluation of the system. Section 5 deals with conclusion.
Section snippets
System description
The laboratory work bench schematic of WRIG based WES in a hybrid grid, delivering power during both islanded and utility tied modes is shown in Fig. 2. To impart reliable and quality power, DCM is framed with renewable such as Solar PV, fuel cells, hybrid electric vehicle etc., integrated with suitable power conditioners. A battery is connected to support any failure in renewable/to store excess power thereby increasing power reliability. AC micro grid is framed with WRIG based WES, AC loads
Control strategy
The control scheme shown in Fig. 3 is detailed in three subsections. Section-3.1 deals with the calculation of maximum power (Pgmax) generated by WRIG at various rotor speeds through a power calculator. Section-3.2 details the current control of RSC during islanded and utility modes. The power transfer operations during various modes are detailed in Section-3.3.
Experimental setup
To investigate the performance, a prototype of the system is developed and tested with a 500 W WRIG coupled to a DC machine. The photograph of the hardware setup is given in Appendix-D. The laboratory available Semikron IGBTs (SKM75GB063D) along with gate driver (SKHI22BR) and associated control circuits are used. The maximum operating terminal voltage and maximum rotational speed of WRIG are 130 V and 1650 rpm respectively. A dS1104-DSP is used to implement the control algorithm and to
Conclusion
Effective power transfer by WRIG based WES in a hybrid grid operating in both islanded and utility mode is reported. Depending on wind speed/load conditions, all possible operational modes are formulated with a concern over conditions like short circuit fault, overload, no load, stall and operation at low wind. To analyze the performance, simple control schemes, devoid of mathematical manipulations are proposed for the single RSC to perform bidirectional slip power flow and regulation of stator
Acknowledgement
This work is supported by Anna Centenary Research Fellowship, Anna University, Chennai, India.
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