Grid-connected Switched Reluctance Generator based Wind Farm Control using Modified Hybrid Multicell Converter

This study deals with analysis of grid integrated Switched Reluctance Generator (SRG) based wind farm control using Modified Hybrid Multicell Converter (MHMC). A Finite Element Analysis (FEA) model is developed for the existing machine geometry using MagNet 7.1.1 and solved to obtain the parameters like flux linkage, inductance and torque. The data obtained from FEA Model is utilized to develop model of SRG in the MATLAB/SIMULINK environment. Asymmetric Half Bridge Converter (AHBC) with dedicative buses for source and load is used for excitation. Developed wind turbine with Maximum Power Point Tracking (MPPT) algorithm to extract maximum power from the wind turbine. Output of the SRG is integrated with grid through Modified Hybrid Multicell Converter. The MHMC uses a flying capacitor module on each cell to generate a positive variable dc-link voltage. The variable dc link is inverted at fundamental switching frequency by an H-bridge to generate the cell output voltage. Phase-Disposition level-shifted Pulse Width Modulation (PD-PWM) Technique is used to obtain the high performance waveforms. In this study wind farm having four SRG’S controlled through MHMC is proposed.


INTRODUCTION
A Switched Reluctance Generator (SRG) is very simple in construction associated with the absence of permanent magnets or windings in the rotor, which results in lower manufacturing costs and makes it suitable for variable speed applications, Boldea (2006).Structure of the machine and unipolar power converter topology used are very robust.Switched Reluctance Generator is a very good alternative to conventional variable speed drives in many applications, especially in the extraction of maximum energy in wind energy generation system with the variable wind speeds, Sawata (2001).Cornea et al. (2008) proposed a Switched Reluctance Motor drive model in the MATLAB/SIMULINK environment using standard library components.This work is studied to develop Switched Reluctance Generator drive model using library components and data obtained from FEA model.
Operation of SRG and its control is addressed by Torrey (2002).Operating variables are identified from this paper and it is used to control the grid-connected SRG based wind energy conversion system.Chang and Liaw (2008) reported the design of power circuit and control scheme for SRG.Authors proposed power circuit with common bus for source and load.This work is studied and extended to develop asymmetric half bridge converter with dedicative buses for source and load to avoid absorbing large current from the source.
Speed control of grid-connected switched reluctance generator driven by variable speed wind turbine using adaptive neural network controller is proposed by Hasanien and Muyeen (2012).This study is referred to integrate SRG with grid.Chang and Liaw (2011) reported switched reluctance generator based common DC microgrid system.In this study wind form having four SRG's controlled through MHMC is proposed.Lezana and Aceiton (2011) introduced new Hybrid Multicell Converter (HMC) (Lezana et al., 2013).This topology is based on the series connection of macrocells which are composed by a flying capacitor converter that generates a variable dc-link voltage and an H-bridge that applies this voltage to the load with positive or negative polarity.The HMC topology is modulated with a simple Phase-Shifted Pulse Width Modulation (PS-PWM) scheme.
Phase-Disposition Pulse Width Modulation (PD-PWM) scheme is implemented for a HMC to obtain the high-performance by Lezana et al. (2013).
In this study analysis of Grid-Connected Switched Reluctance Generator Based Wind Farm Control using Modified Hybrid Multicell Converter is discus results are reported.

METHODOLOGY
Switched reluctance generator: Switched Reluctance Generator is an electromechanical device which converts mechanical energy into electrical energy by properly synchronizing phase current with rotor position.To achieve generating mode operation phase current pulses are applied during the period where the rotor leaves the aligned position and phase winding is excited by means of DC source bus.During generation, SRG produces negative torque that tends to oppose rotation, thereby extracts energy from the win (Torrey, 2002).The cross section of the 1 8/6 SRG is shown in Fig. 1.Specifications of the SRG are given in Table 1.
The supporting diagram which explains the generating mode operation of the machine is shown in Fig. 2.
One arm of the power converter used to excite the phase winding of SRG is shown in Fig. 3. equation of the phase voltage (v) is given as:   The flux linkage (λ) is a function of current (i) and rotor position (θ), therefore the volta rewritten as Chang and Liaw (2008): Considering one phase of the converter, during magnetization period both switches S1 and S2 connect the DC bus to the winding, thus the phase current increases.Equation corresponding to excitation is described as: (2) Considering one phase of the converter, during magnetization period both switches S1 and S2 connect the DC bus to the winding, thus the phase current increases.Equation corresponding to excitation is  The variation of flux linkage with respect to current is shown in Fig. 4.
It is evident from Fig. 5 that inductance is maximum at the aligned position.When the rotor leaves the aligned position inductance decreases and it is minimum at the unaligned position.The inductance profile of the machine is periodic function of rotor position as seen in Fig. 5.
Torque obtained from FEA model for various values of current and position is shown in Fig. 6.It is negative from 0-30° and positive from 30-60°.
Wind turbine modeling: Wind Turbine (WT) is a machine for converting the kinetic energy in the wind into mechanical energy.The kinetic energy in a flow of air through a unit area perpendicular to the wind direction, per mass flow rate is computed as (Chen et al., 2013;Khanna et al., 2014): where, v = Wind speed (m/sec) For an air stream flowing through an area 'A' the mass flow rate is ρAv, therefore the power in the wind is given as: The rotor torque T w is given as: The maximum power extractable from the wind turbine depends not only on the strength of the source but also on the operating point of the energy conversion system.Therefore the Maximum Power Point Tracking (MPPT) is of the principal significance in wind energy conversion systems.Wind farm with MPPT algorithm is developed to optimize the generator speed relative to the wind velocity intercepted by the wind turbine such that the power is maximized.MPPT techniques mainly used are: • The lookup table based • The state space linearization and non-linear state space based • The neural network and fuzzy logic based • The hill climbing based • Hybrid technique Among these methods hill climb search control is feasible method and it does not require prior knowledge of the system.The principle of hill climb search control is, keep perturbing the control variable in the same direction until the power is decreased as shown in Fig. 7.
The flow chart for hill climb search law is shown in Fig. 8.
Coding is developed in MATLAB to obtain the response of Wind Turbine (WT) with MPPT capability.The Wind Turbine characteristic for variable speed operation is shown in Fig. 9.

Simulation model of converter and SRG:
The converter used for the SRG is shown in Fig. 10.Asymmetric Half Bridge Converter with dedicative buses for source and load is used to avoid absorbing more current from the source.To get generator mode operation, winding is energized during the drooping inductance region of the inductance profile, where the variation of inductance with respect to the rotor position is negative.The sign of the generated torque is negative and it extracts energy from the wind turbine.
The equivalent model of SRG is a set of differential equations obtained using dynamic electric machine theory.The flux linkage Vs current, inductance Vs rotor position and torque characteristics of the machine are obtained using FEA.The electrical model of the SRG is created and linked to the mechanical model.
The SRG is modeled as an inductor and a resistor in series as shown in Fig. 3.The phase of SRG is modeled according to Eq. ( 1) in SIMULINK.The Simulation results of the SRG are reported below.For 4-phases 8/6 SRG, minimum dwell angle is 15° and maximum dwell angle is 30°.The results are obtained for θ on -5° and θ off -30°. Figure 12 gives generated voltage of the SRG.
Phase current of the generator mode operation is shown in Fig. 13.The sign of the current is always positive due to the unidirectional elements present in the converter.
SRG generates negative torque and it is shown in Fig. 14.

Modified hybrid multicell converter:
Conventional two-level inverters are mostly used to generate an AC voltage from a DC voltage.The two-level inverter can only create two different output voltages for the load.To build up an AC output voltage these two voltages are usually switched with PWM.Though this method is effective it creates harmonic distortions in the output voltage, EMI and high dv/dt.
The concepts of Multilevel Inverters (MLI) do not depend on just two levels of voltage to create an AC signal.Instead several voltage levels are added to each other to create a smoother stepped waveform, with lower dv/dt and harmonic distortions.With more voltage levels in the inverter the waveform it creates becomes smoother, but with many levels the design becomes more complicated, with more components and a more complicated controller for the inverter is needed.The variable dc link is inverted at fundamental switching frequency by an H-bridge to generate the cell output voltage.Phase-Disposition level-shifted Pulse Width Modulation (PD-PWM) Technique is used to obtain the high performance waveforms, (Lezana et al., 2013).The macrocell multilevel dc-link voltage V m is implemented by the FC converter, which is handled by the output H-bridge to generate the cell output signal V oc .The output voltage response of the MHMC is show in Fig. 20.There is a small distortion in the output voltage due to the non-linearity exists in the SRG.

HMC topology shown in
The output of the MHMC is integrated to the grid through transformer to match the voltage and frequency of SRG with grid.Voltage and current waveforms of the load which is integrated with the grid is shown in Fig. 21 and 22.
From the results it is clear that the wind farm having four SRG's is properly integrated with grid.SRG gives satisfactory performance in the generating mode and supplies power to the load.From the above discussion it is very apparent that, the SRG is a very good wind generator for wind energy conversion system and modified hybrid multicell converter is suitable for wind farm control.

CONCLUSION
This paper has presented a grid connected switched reluctance generator based wind form control using modified hybrid multicell converter.Developed SRG machine model in the MATLAB/SIMULINK environment using the data obtained from FEA model and the response of the SRG is found satisfactory.Wind turbine model with Maximum Power Point Tracking (MPPT) algorithm is developed to extract maximum power from the wind turbine.Switching signal for triggering the power device in the MHMC is obtained accurately using Phase-Disposition level-shifted Pulse Width Modulation (PD-PWM) Technique and operation of MHMC is found reasonable.The output of the MHMC is integrated to the grid through transformer to match the voltage and frequency of SRG with grid.Results are reported and this wind farm is suitable for rural electrification.

Fig. 1 :
Fig. 1: Cross section of SRG exc = Current during excitation L = Inductance, function of current (i) and rotor position (θ) e = Back emf During generation both switches S1 and S2 are OFF, then the DC bus is connected 1 flux linkage (λ) is a function of current (i) and rotor position (θ), therefore the voltage equation can be 2008): , function of current (i) and rotor During generation both switches S1 and S2 are connected in reverse polarity

Fig. 14 :
Fig. 14: Torque response of SRG Fig. 15 is customized to obtain Modified Hybrid Multicell Converter (MHMC).MHMC allows additional SRG's to be connected in series, increasing the overall converter voltage and power ratings while improving the quality of the output signals.In this study wind farm having four SRG'S controlled through MHMC is proposed.MATLAB/SIMULINK model of the grid-connected SRG based wind farm control using MHMC is shown in Fig. 16.

Table 1 :
Specifications of SRM Electrical specifications of SRM PWM) Technique.It is a technique used to obtain high performance output.Wind farm proposed in this study is having four SRG, therefore four triangular carriers are used and which are compared with sinusoidal.Modulation signals are shown in Fig. 17.Switching Signals for the MHMC and H-bridge inverter are shown in Fig. 18 and 19, respectively.

Table 2 :
Switching state and output voltage