VOLTAGE FLICKER MITIGATION EMPLOYING SMART LOADS WITH HIGH PENETRATION OF RENEWABLE ENERGY IN DISTRIBUTION SYSTEMS
Unbalanced Control Strategy for a Thyristor Controlled LC-Coupling Hybrid Active Power Filter (TCLC-HAPF) in Three-phase Three-wire Systems
Abstract
This paper proposes a control strategy for a three-phase three-wire thyristor controlled LC-coupling hybridactive power filter (TCLC-HAPF), which can balance activepower and compensate reactive power and harmonic currentsunder unbalanced loading. Compared with TCLC-HAPF withconventional control strategy, active power filters (APFs) andhybrid active power filters (HAPFs) which either fail to performsatisfactory compensation or require high rating active inverterpart for unbalanced compensation, a control strategy wasproposed for TCLC-HAPF to operate with a small rating activeinverter part for a variety of loads with satisfactory performance.The control idea is to provide different firing angles for eachphase of the thyristor controlled LC-coupling part (TCLC) tobalance active power and compensate reactive power, while theactive inverter part aims to compensate harmonic currents.Firstly, the required different TCLC impedances are deduced.Then independent firing angles referenced to the phase angle ofvoltage across TCLC are calculated. After angle transformations,final firing angles referenced to phase angle of load voltages areobtained. In this paper, a novel controller for TCLC-HAPF underunbalanced loading is proposed. Simulation and experimentalresults are provided to verify the effectiveness of the proposedcontroller in comparison with a state-of-the-art controller.
EXISTING SYSTEM:
Implementation of power filters is one of thesolutions for power quality problems. In the early days,thyristor based Static Var Compensators (SVCs) are used. Itcan inject or absorb reactive power according to differentloading situations. However, SVCs have many inherentproblems including resonance problem, slow response, lack ofharmonic compensation ability and self-harmonic generation.Later on, the remarkably progressive concept of active powerfilters (APFs) was first proposed in 1976 for dynamicallycompensating reactive power and current harmonics problems. However, APFs require high dc-link voltage levels (Vdc>•2VL-L) to perform compensation, which drives uptheir initial and operational costs. Afterwards, in order to reduce the cost of APFs, an LC-coupling hybrid active powerfilter (HAPF) with low dc-link operational voltage wasproposed by Akagi et al in 2003. Unfortunately, HAPF hasa narrow compensation range, which may require a high dc-linkoperation voltage when it is operating outside its compensationrange, thus losing its low inverter rating characteristic.Many control techniques have been proposed to improve theperformance of the APFs and HAPFs and solve the unbalancedproblem. The different current quality compensatorsand their unbalanced control methods are summarized in TableI, and also compared in the following. Akagi et al. firstly proposed instantaneous p-q controlmethod in order to eliminate the reactive power, harmonicpower and unbalanced power of the loading instantaneously. Inorder to adapt instantaneous p-q control method under differentvoltage conditions (distorted, unbalanced, etc), many othercontrol techniques were further developed such as: d-q controlmethod , p-q-r control method ,Lyapunov function-based control method, etc. However,those instantaneous power control methods are dedicated to inverter/converter-based structures and theircorresponding performances are highly dependent on thecomputation speed and the switching frequency of the digitalcontrollers and the switching devices.
PROPOSED SYSTEM:
The majordrawback of this control method is that the sequencecomponents introduced by harmonics are not taken intoconsideration. To solve this problem, combine the above instantaneous control methods with the+, – and 0 sequence control method, but the computation stepsincrease a lot, thus significantly increasing the controlcomplexity. Recently, Leszek S. et al propose a power analysis control method based on the theory of currents’physical component (CPC) to compensate the reactive power inunbalanced three-phase four-wire system. However, after thispower analysis method compensation, the active powerremains unbalanced, which means the unbalance power cannotbe completely eliminated. With all the above control methods, both APFs can effectively compensate thereactive power and harmonic currents under unbalancedloading compensation. However, both APFs and HAPFsprobably require high active inverter rating (high initial costand switching loss) to perform unbalanced currentcompensation due to the inductive coupling structures of APFsand the narrow compensation range limitations of HAPFs. In2014, S. Rahmani et al. proposed the structure of athyristor controlled LC-coupling hybrid active power filter(TCLC-HAPF) which can operate with a small rating activeinverter part for reactive power and harmonic currentcompensation in comparison to the conventional solutions. To control TCLC-HAPF, a state-of-the-art controlmethod is proposed in to reduce the steady-state error ofthe TCLC part and improve the performance of current trackingand voltage regulation of the active inverter part. However, thecontrol method proposed in was designed based on theassumption of balanced loading condition. If this controlmethod is applied to TCLC-HAPF for unbalanced loadingcompensation, it either fails to perform acceptable currentquality compensation or requires a high rating active inverterpart for compensation, which results in increasing the systeminitial cost, switching loss and switching noise. Therefore, thispaper proposes a hybrid unbalanced control method for theTCLC-HAPF, which can balance source side active power andcompensate the reactive power and harmonic currents withsmall rating active inverter part. The control idea is to generatedifferent firing angles to each phase of the TCLC in order tocompensate reactive power and balance active power, and theactive inverter part aims to compensate the harmonic currents.As a result, the voltage rating of the active inverter part can besmall, and consequently the system initial cost and switchingnoise can be significantly reduces
CONCLUSION
In this paper, a novel control strategy for a three-phase three-wire TCLC-HAPF is proposed, which can maintain itoperating with a small rating active inverter part and at the sametime it can balance the active power and compensate thereactive power and harmonic currents under unbalancedloading compensation. The design idea and operation steps ofthe proposed hybrid controller for the TCLC-HAPF underunbalanced loading is presented and discussed in details.Finally, simulation and experimental results are given to verifythe proposed control method in comparison with thestate-of-the-art control method, which shows its superiorcompensating performances under the unbalanced loadingcondition.
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Abstract
This paper deals with modeling and control of smart loads for demand-response management under increasedpenetration of Renewable Power Generations (RPGs) atdistribution level. The increased penetration of RPGs,particularly wind energy at distribution level, is associated withadverse impact on voltage quality. A permanent magnetsynchronous generator (PMSG) based variable speed windenergy conversion system is modeled with a wind speedconsidering stochastic and periodic effects. The emulated windpower into the distribution system produces stochastic andperiodic power variations. For the load demand responsemanagement, full bridge self-commutated switches basedconverters are employed to control Smart Loads (SLs). TheseSLs are controlled for participating in grid bus voltage regulationand flickers mitigation. From various case studies, it is found thatSLs are effective in improving the voltage profiles of the testfeeder.
EXISTIN SYSTEM:
Recently, the Electric Spring (ES) has been proposed as aneffective solution for demand-side management and regulatinggrid voltages under fluctuating RPGs without the aid of anycommunication network. The ES is connected in serieswith NCLs and provides reactive power compensation forregulating grid voltage. The ES simultaneously controls NCLspower for demand-side management. Yuen et al. havereported first time the concept of an ES. Chi Kwan and ShuYuen have demonstrated reduction in energy storagerequirements under operation with RPGs using an ES. SiewChongetal.haveanalyzedgeneralsteady-statebehaviorandcontrol principles of the ES. In, authors havepresented dynamic modeling of an ES. Xiao et al. [16] havepresented a performance analysis between an ES and a staticcompensator for the grid voltage control. Shuo et al. havepresented three-phase ES for reducing power imbalance in thepower system. However, most of the reported study works onapplication of ES and have considered random variations inoutput power of RPGs (wind energy) with only slow windgust and less penetration levels (20-30%) . Inremaining part of this paper, the ES connected in series withNCLs is called as smart loads (SLs)
PROPOSED SYSTEM:
The proposed research work is motivated to explorealternative methods of voltage regulation and its qualityimprovements under high penetrations of RPGs at thedistribution level. Voltage flickers are considered as one of themost serious issue under high penetrations of wind energy atdistribution level. Most of the mitigation methods reported inthe literature for reducing flicker emissions are employed atgeneration end with appropriate control on converters ofvariable speed wind energy conversion systems (VSWECS),such as reactive power compensation, active power control,and dynamic volt-var control. However, the loaddemand response has never been explored with the bestinformation available from literature for addressing voltageflicker problems. This paper presents demand response management usingSLs under high penetration of VSWECS at distribution level.The dynamic torque and power variations of wind turbine areobtained considering stochastic and periodic effects. Thecontribution of this work are summarized as follows, The voltage flicker mitigation scheme employing loaddemand response based on SLs are presented first time. The systematic design of SLs is presented. The performance of SLs is investigated and explored under high penetration of wind power.
CONCLUSION
The electric spring based smart loads have been used for load demand response management under MW size PMSG inVSWECS under distribution system. Modeling and control fora full bridge voltage source converter based ES have beenpresented for grid bus voltage regulation. To emulate realisticwind power generation and its injection in to the grid, windspeed has been modeled considering its stochastic anddeterministic components. The study presented under Case-Ihas demonstrated that fluctuating wind power injectionresulted in voltage disturbances and severe voltage flickerspropagation at each bus of distribution network. However, thestudy presented in Case-II clearly shows that with smart loads,voltage disturbances at all buses of distribution feederdecreases and voltage flicker severity indexes are decreased. Therefore, deploying SLs using ES is proven an effectivealternative a other solutions in reducing flickers in thetest feeders. Moreover, presented sensitivity analysis hasstrengthened claim of above study that deploying SLs helps inincreasing penetration level of renewable energy. Theproposed load demand response management using SLs hasbeen effectively employed for voltage control under high windenergy penetrations in the distribution feedresulted in voltage disturbances and severe voltage flickerspropagation at each bus of distribution network. However, thestudy presented in Case-II clearly shows that with smart loads,voltage disturbances at all buses of distribution feederdecreases and voltage flicker severity indexes are decreased. Therefore, deploying SLs using ES is proven an effectivealternative a other solutions in reducing flickers in thetest feeders. Moreover, presented sensitivity analysis hasstrengthened claim of above study that deploying SLs helps inincreasing penetration level of renewable energy. Theproposed load demand response management using SLs hasbeen effectively employed for voltage control under high windenergy penetrations in the distribution feeder.
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