title of the paper · same scheme with sensor less control. space vector control is im-plemented by...

Title of the paper

Hepzibah Gnanamani1 and Susitra D2

1Department of EEE, Sathyabama University, [email protected]

[email protected]

December 28, 2017

AbstractThis work aims at developing a current control strat-

egy for doubly fed induction machine and permanent mag-net synchronous machine. A sliding mode current controlmethod is used with a simple algorithm as a control strategyfor control of both the machines. The division summationcurrent control method is used. Space vector modulationtechniques are used. Simulation of the systems with andwithout control strategy is carried out using simpower sys-tem toolbox in MATLAB. The load parameters are esti-mated even with the absence of load details. Single sidefiring method used for power stage switching is discussed.This gives solution for dead band with the increase in PWMfrequency and sampling rate. The results are presented forboth simulated model and experimentation. The merits ofthe presented scheme over the conventional schemes are dis-cussed. From the results,it is seen that the generated windpower is more with this system compared with the earliermethod.

Keywords : DFIG, PMSG,current control, voltage reg-ulation,simulink.

1 Introduction

Recent publications on alternating current electrical machines haveproved the effectiveness of using PMSG and DFIG in low and mid

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International Journal of Pure and Applied MathematicsVolume 118 No. 16 2018, 947-960ISSN: 1311-8080 (printed version); ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

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power applications especially in wind conversion system. The cus-tomary methods of wind turbine modelling are not adequate owingto the issues related with the stability and reliability. Stability ismost required criteria during voltage dips. Voltage stability is as-sisted with the reactive power support. Fault Ride-Through andreactive power support are the major factors to be considered indeveloping the wind turbine model.

A Control scheme for interior magnet type PMSM is proposedby1,2 developed electrical equivalent circuit models for SPMSM andIPMSM and compared these models through experiment. In3, clas-sification of PM machines is discussed. As an expansion of thisauthors of 4 presented the model and control for PMSM drive. In5

investigated about the production of torque and system limitationsat low and high speeds respectively. In6 developed a fuzzy speedcontroller for synchronous motor drive. Field control of PMSMdrive by tracking reference current is presented by7.

In8 implemented fuzzy controller for the above task and com-pared it with the performance of the PI controller. In9 imple-mented DTC based control for PMSM. In10, in his work used thesame scheme with sensor less control. Space vector control is im-plemented by11.

2 Control Systems of PMSM

Variable frequency drives are used for speed control of synchronousmotors. The variable amplitude and frequency are considered forthe controlled variable. The various control methods for variablespeed drives are shown in Figure.1. The modelling of motor pa-rameters and investigation of converters for wind energy conversionsystem is dealt by12 and13 respectively.

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Figure 1: Methods of control techniques

The flux in stator is maintained constant in V/F Control. Thevariation of stator flux and rotor flux in both steady and transientstates are measured in vector control method. The electromagneticfields are efficiently controlled in field oriented control. The generalvector control approach for PMSM is presented in Figure.2 and theblock diagram for the same is shown in Figure.3.

Figure 2: Vector control strategy of existing system

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Figure 3: Vector control strategy of Proposed system

3 PMSG and DFIG

The generating performance of a PMSG is mainly affected by thecurrent control algorithm because the electromagnetic torque is de-termined by the winding currents. Proper and satisfactory currentcontrol schemes will smooth the torque ripple of a PMSG. Theswitching frequency of the RC-PWM current control is constant,but its dynamic behaviour is extremely affected by the designedfeedback controller. Moreover, the abc to dq and dq to abc trans-formations employed in the PMSM/PMSG drive requires longercomputation time of the microcontroller. The hysteresis control canbe easily implemented to achieve current source output. However,the switching frequency varies according to the hysteresis band andthe machine parameters. The division-summation (D-

∑) current

control is presented in this paper. It involves the idea of two-phaseand space vector modulation techniques.

A PMSM driver is set as the prime mover of the PMSG. Thedamping coefficient of the mechanic coupling system can be esti-mated by the torque sensor. Three-phase winding currents andencoder signals are sensed and properly filtered to implement theproposed current control algorithm. DC-link voltage is also de-tected to accomplish the one-cycle voltage regulation. All the con-trol algorithms are digitally realized by a microcontroller RenesasRX62T. The System configuration of the developed PMSG drive isshown in Figure.4.

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Figure 4: System configuration of the developed PMSG drive

The PMSG drive implemented in this paper comprises of a fly-wheel, a motor-generator and power converters. The electrical in-put converted to mechanical energy is stored in the flywheel. Thegenerator and its associated converter unit are used in convertingthis stored mechanical energy into electrical energy. Input/outputelectrical interfaces is accomplished by the motor and generator setalong with power converters. For convenient purposes, the dynamicmodel of a FESS is replaced by a PMSG coupled with a PMSM. Asimilar emulation of the FESS can be found in.

When the flywheel is under charging, the motor is driving atmotoring mode. On the other hand, the motor is driving at gener-ator mode under the flywheel discharges. In this paper, the systemconfiguration including the current control and voltage regulationschemes is first introduced. Next, the modelling and simplificationof inductances are performed. The current control algorithm is dis-cussed. The derivation of the D-

∑current control and one-cycle

voltage regulation is accomplished. The system stability and theparameter sensitivity are also analyzed. The control of current andvoltage regulation are digitally implemented by a microcontroller.

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The current control performance is first verified by the current com-mand tracking. Then the total harmonic distortion (THD) of thecurrent is measured. The reduction of the switching losses is verifiedthrough the calculation results.The wind farm of a DFIG simulinkmodel is shown in Figure.5.

Figure 5: Wind Farm DFIG Model

4 Sliding Mode Current Control

The sliding mode current controller requires a high sampling rate toachieve high performance. With a high sampling rate, the currentcontroller can generate high-frequency switching activity, whichleads to low-current ripples in the steady state and fast transientdynamics. The sampling rate is limited by the execution time ofthe control algorithm and the frequency of the PWM signal. Logiccircuits based current controller yields faster execution speeds evenhigh sampling rates. However, raising the sampling rate depends onincreasing PWM frequency. The PWM frequency is limited mainlyby the characteristics of the switching devices and the dead time.Recently, many high-speed devices, such as insulated gate bipolartransistors and MOSFETs, have been developed, however, they stillrequire switching dead time around 12 ms. The existence of deadtime is an obstruction to raise the PWM frequency. Moreover, ifnot properly compensated for, it will lead to serious problems, suchas waveform distortion and increased torque ripples. In order toraise the sampling rate, a new switching device operating principle

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called single-side firing is presented to solve the dead-time problemand raise the PWM frequency.

The control algorithm is deduced, a dc motor model is con-sidered first, and then the results are extended to a brushless dcmotor. With certain modifications, the control algorithm for a dcmotor can be applied to each of the three phases of a PM brushlessdc motor.

5 Results and Discussion

Figure 6: Simulink Diagram

Figure 7: Existing Wind power

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Figure 8: Existing Converter output

Figure 9: Existing Inverter RMS voltage

Figure 10: Existing Phase voltage

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Figure 11: Existing Output voltage

The simulation is carried out for both the systems without slid-ing mode power control and with sliding mode power control usingSimpower system toolbox in MATLAB. Figure 6-10 shows the sys-tem results without implementation of sliding mode power control.Figure.7 depicts the wind power output. Figure.8 depicts the con-verter voltages at the input and output sides. From this figure,it is seen that the converter output voltage is 1250V for the giveninput voltage of 625V. Figure.9 shows the three phase inverter out-put voltage and Figure.10 shows the winding phase voltage of themachine and Figure.11 shows the output voltage of the system.

Figure 12: Proposed Wind Power

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Figure 13: Proposed Converter voltage

Figure 14: Proposed Inverter RMS voltage

Figure 15: Proposed Line Voltage

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Figure 16: Proposed Inverter output Voltage

The results of the system with sliding mode power control areshown in Figure. 12-16. The wind power, voltage and the per unitvalues for the proposed system is shown in Figure.12 from which,it is seen that the generated wind power is more with this systemcompared with the earlier method. The input and output voltagesof the converter are shown in Figure.13 which has the significantimprovement compared with the previous case. The three phaseinverter voltage is shown in Figure14, the line voltage is shown inFigure.15 and the inverter output voltage is shown in Figure.16.From the results, it is clearly evident that the system built withsliding mode power control shows significant improvement in re-sults.

6 Conclusion

A sliding mode current control scheme for PMSG and DFIG is pro-posed in this paper. It has been shown that the computation timefor the execution of control algorithm is very less . The implemen-tation is also made simpler with logic circuits. There is no needfor input load parameters. The dead-time problem is solved usingsingle-side firing operating principle.Without the dead-time limita-tion, the PWM frequency and the samplingrate can be raised. Theexperimental results show the validityof this scheme.

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