Time-Varying and Constant Switching Frequency Based Sliding Mode Control Methods for Transformerless DVR Employing Half-Bridge VSI
Abstract
This paper presents time-varying andconstant switching frequency based sliding mode control(SMC) methods for three-phase transformerless dynamicvoltage restorers (TDVRs) which employ half-bridgevoltage source inverter (VSI). An equation is derived for thetime-varying switching frequency. However, since the timevaryingswitching frequency is not desired in practice, asmoothing operation is applied to the sliding surfacefunction within a narrow boundary layer with the aim ofeliminating the chattering effect and achieving a constantswitching frequency operation. The control signal obtainedfrom the smoothing operation is compared with atriangular carrier signal to produce the PWM signals. Thefeasibility of both SMC methods has been validated byexperimental results obtained from a TDVR operatingunder highly distorted grid voltages and voltage sags. Theresults obtained from both methods show excellentperformance in terms of dynamic response and low totalharmonic distortion (THD) in the load voltage. However, theconstant switching frequency based SMC method not onlyoffers a constant switching frequency at all times andpreserves the inherent advantages of the SMC, but alsoleads to smaller THD in the load voltage than that of timevaryingswitchingfrequencybasedSMCmethod.
EXISTING SYSTEM:
the situation is manageable with lesspenetration of RPGs i.e. 10-15%. However in light ofincreased penetrations of such RPGs (50-60%), it will be moredifficult even for conventional power plants to participateeffectively in maintaining stability of the network. Under suchconditions, it is a real challenge for policy makers to developstrategies in real time to meet load demand with such highlyfluctuating and less predictable power generation. It leads tothe operation of future grid much more deliberately controlledon the basis of RPGs rather than consumer loads such thatpower industry is sustainable and environmentally secure.A the other problems that arise due to high penetrationsof RPGs, voltage regulation, minimizing voltage distortionsdue to flickers, sag, swell and frequency control areconsidered as prime control objectives. To solve suchproblems, solutions are presented in the literature, such as,increased rating of a stand-by DG unit and employing storagesystems for supplying deficit real time load demand. A these solutions, increased rating of a stand-by unitand high rating of the storage systems are against the motionof regulatory authorities to increase reliable penetration ofRPGs in order to reduce fossil fuel consumption and carbonfoot print to preserve environment. Increase in the rating ofstand-by DG unit increases capital and operational cost ofpower system and reduces the advantages of increasing moreunits of RPGs. Many types of storage systems have beenadvised and are in operation with power system based on theirshort term, medium term and long term storage capacity. However, their round trip efficiency, capital investment andspace requirements are always debatable. Moreover, trends areto deploy RPGs near to the load centers in order to reduce linelosses and to minimize additional reinforcements oftransmission networks. In view of all above, much morepenetration of RPGs with distribution system is expected incoming years.
PROPOSED SYSTEM:
In this paper, time-varying and constant switchingfrequency based SMC methods are presented for three-phaseTDVRs employing half-bridge VSI. The existence conditionsof the sliding mode are determined. In addition, the timevaryingswitching frequency equation is derived analyticallywhich can be used to predict the switching frequency.However, since the time-varying switching frequency is notdesired in practice, a smoothing operation is applied to thesliding surface function within a narrow boundary layer withthe aim of eliminating the chattering effect and achieving aconstant switching frequency operation. The PWM signals ofthe switching devices are generated by comparing the controlsignal obtained from the smoothing operation with a triangularcarrier signal. The theoretical considerations and feasibility ofboth SMC methods are validated by the experimental resultsunder distorted grid voltage and voltage sag conditions.
CONCLUSION
In this study, time-varying and constant switching frequencybased SMC methods are presented for three-phase TDVRemploying half-bridge VSI. An analytical equation is derivedto compute the time-varying switching frequency. Since, thetime-varying switching frequency is not desired in a realapplication, a smoothing operation is applied to the slidingsurface function within a narrow boundary layer with the aimof eliminating the chattering effect and achieving a constantswitching frequency. The control signal obtained from thesmoothing operation is compared with a triangular carriersignal to produce the PWM signals. It is observed that thesmoothing operation results in a constant switching frequency thoperation at all times. The feasibility of both SMC methods hasbeen validated by experimental results obtained from theTDVR operating under highly distorted grid voltages andvoltage sags. The results obtained from both methods showexcellent performance. However, the constant switchingfrequency based SMC method not only offers a constantswitching frequency at all times and preserves the inherentadvantages of the SMC, but also leads to smaller THD in theload voltage than that of time-varying switching frequencybased SMC method.
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