A Series Compensation Technique for Enhancement of Power Quality Isolated Power System Venkateshwara Rao R K.Satish Babu

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1 A Series Compesatio Techique for Ehacemet of Power Quality Isolated Power System ekateshwara Rao R K.Satish Babu PG Studet [P.E], Dept of EEE, DR & DR. H S MIC College of Tech, A.P, Idia Assistat Professor, Dept of EEE, DR & DR. H S MIC College of Tech, A.P, Idia Abstract I Isolated power systems the power quality problem is compouded as the drive coverter loads are likely to fluctuate i cojuctio with miig or exploratio areas. Amog power system disturbaces, voltage sags, swells ad harmoics are some of the severe problems to the sesitive loads. Adequate reactive power cotrol solves power quality problems like flat voltage profile maiteace at all power trasmissio levels, ad improvemet of power factor, trasmissio efficiecy ad system stability. The series compesatio method is best suited to protect such loads agaist those disturbaces. The use of a series compesator (SC) to improve power quality is a isolated power system is ivestigated. The role of the compesator is ot oly to mitigate the effects of voltage sag, but also to reduce the harmoic distortio due to the presece of o liear loads i the etwork. I this paper, a series compesator is proposed ad a method of harmoic compesatio is described ad a method to mitigate voltage sag is ivestigated. The proposed series compesator cosists of Eergy Storage System (ESS) ad oltage Source Iverter (SI), Ijectio Trasformer. The ESS ca be a capacitor of suitable capacity. ESS would act as a buffer ad geerally provides the eergy eeded for load ride-through durig voltage sag. A cotrol strategy for the SC is developed to regulate power flow. This is achieved through phase adjustmet of load termial voltage. alidity of the techique is illustrated through simulatio. Keywords: Isolated Power Systems, Series Compesator, Harmoics compesatio, Eergy Storage System, SI, Harmoics; oltage Sag; MATLAB. Itroductio Isolated power systems are commoly foud i rural ad remote areas of the world. Isolated power systems are characterized by limitig geeratig capacity. The sesitive loads which are preset i the isolated power systems are much more affected by the power quality problems. Power Electroics ad Advaced Cotrol techologies have made it possible to mitigate power quality problems ad maitai the operatio of sesitive loads. Power quality problems ecompass a wide rage of disturbaces such as voltage sags/swells, flickers, harmoics distortio, impulse trasiet, ad iterruptios. Amog power system disturbaces, voltage sags, swells ad harmoics are some of the severe problems to the sesitive loads, because (i) the occurrece of voltage sag i the system ca cause devices/process dow time, effect o product quality, failure/malfuctio of equipmets etc., (ii) the occurrece of harmoics i the system ca cause excessive losses ad heatig i motors, capacitors ad trasformers coected to the system.

2 This paper aalyses the key issues i the power quality problems, I the proposed system oltage sag occurs due to the three phase fault i the trasmissio lie ad harmoics occurs due to the coectio of cotrolled six pulse coverter (rectifier) to the mai drive load(o liear load). All these factors affect the sesitive load which is coected i parallel to the mai drive load. So the proposed system protects the sesitive load by mitigatig the voltage sags ad harmoics usig series compesatio techique. A alterative method to improvig the power quality of the isolated systems is through the use of series compesators (SC). A SC is viable because it is of smaller capacity compared to a shut compesator to achieve the same level of voltage quality cotrol. It is based o the well-established voltage-source iverter (SI) techology. Also, as reported i [6, 7], the SC ca fuctio to mitigate the effects of voltage sag/swell although i these previous works; harmoic voltages/curret distortios i the etworks have bee igored. Mai Sources, Causes ad Effects of Electrical Power Quality Problems: Power Quality is Ay power problem maifested i voltage, curret, or frequecy deviatios that results i failure or misoperatio of customer equipmets []. Power systems, ideally, should provide their customers with a uiterrupted flow of eergy at smooth siusoidal voltage at the cotracted magitude level ad frequecy. However, i practice, power systems, especially the isolated systems, some of the primary source of distortio [2] ca be idetified as No Liear Loads, Power Electroic Devices, IT ad Office Equipmets, Arcig Devices, Load Switchig, Large Motor Startig, Larger capacitor bak eergies, Embedded Geeratio, Electromagetic radiatios ad Cables, Storm ad Eviromet Related Causes etc. System Model The simple isolated power system model show i Fig. is used to explai the priciple of the proposed harmoics compesatio method of the SC. The upstream geerators are represeted as a ideal voltage source ad ZS represets the equivalet source impedace. The mai drives or machiery loads are modeled as a lumped resistace-iductace load coected to the source through a power coverter which is assumed to be a ucotrolled six-pulse rectifier i this study. Fig : A typical isolated power system with Series Compesator The mai drives or machiery loads are modeled as a lumped resistiveiductive load coected to the source through a power coverter, assumed to be a six-pulse rectifier. The much smaller capacity sesitive loads are assumed to be 2

3 supplied through poit of commo couplig ad are modeled by the resistor R i parallel with the capacitor C. The SC is coected upstream from the sesitive load through a ijectio trasformer. It is series coected with the sesitive load. The fuctio of the SC is to esure that the voltage across the sesitive load termials is of high quality. The cetral part of the SC is a eergy storage system (ESS) ad a SI where a PWM switchig scheme is ofte used[5]. The ESS ca be a capacitor of suitable capacity. Because of the switchig, harmoics are geerated, ad filterig is required. Priciple of harmoics compesatio: The uderlyig priciple whe the SC is used to compesate for upstream voltage sag/swell has bee discussed i [8]. I Fig. 2, distorted voltage L will appear o the upstream source-side of the sesitive load ad the phase voltages ca be expressed as La Lb Lb [ 0 si( wt l) 2 si( wt 2)] () [ 0 si( wt l 2 /3) 2 si( wt 2 2 /3)] (2) [ 0 si( wt l 2 /3) 2 si( wt 2 2 /3)] (3) where is the harmoic order; 0 is the zero phase sequece voltage compoet; ad φ are the magitude ad phase of the positive phase sequece voltage compoets; 2 ad φ 2 are the magitude ad phase of the egative phase sequece voltage compoets. Clearly, the distorted voltage is udesirable at the sesitive load termials. The proposed voltage ijectio method is to compesate for the differece betwee L ad the desired voltage described by ()-(3). This is achieved by ijectig a ac voltage compoet i series with the icomig three-phase etwork. Hece from ()-(3), the desired ijectio voltages are, (4) ija la La l 2 si( wt 2 ) 2 o i si( wt ) ijb ijc lb lc Lb Lc l 2 si( wt 2 2 / 3) 2 o i si( wt 2 / 3) o i si( wt 2 / 3) (5) l 2 si( wt 2 2 / 3) 2 (6) The above equatios ca be writte i a compact form. From (4)-(6), defie Lf =[ la(t), lb (t), lc (t)] T ad from ()-(3), deote as, Lf =[ la(t), lb (t), lc (t)] T, Let Lh be the vector cotaiig all the harmoic compoets i ()-(3). Hece, from (7)-(9), the ijectio voltage of the SC would be * = Lf- L= Lh (i) 3

4 L ca be measured olie ad its fudametal voltage Lf ca be obtaied usig, for example, a Phase Locked Loop (PLL) scheme. Hece the ijectio voltage ca be geerated olie ad used to mitigate the harmoic distortios i the maer described below. Series Compesator Cotroller The harmoics is geerated i the load termials usig six pulse coverters with fixed firig agle are coected to the mai drive o liear load which is parallel to the sesitive load. oltage sag is created at load termials via a three phase fault. The above voltage problems are sesed separately ad passed through the sequece aalyzer. The magitude compoet is compared with referece voltage (ref). Pulse width Modulatio (PWM) cotrol techique [6] is applied for iverter switchig so as to produce a three phase 50 Hz siusoidal voltage at the load termials. Choppig frequecy is i the rage of few KHz. The IGBT iverter is cotrolled with PI cotroller i order to maitai per uit voltage at the load termials. PI cotroller (Proportioal Itegral Cotroller) is a closed loop cotroller which drives the plat to be cotrolled with a weighted sum of the error (differece betwee the output ad the desired set poit) ad the itegral of that value. Oe advatage of a proportioal plus itegral cotroller is that the itegral term i a PI cotroller causes the steady-state error to be zero for a step iput. For the coveiece of aalysis ad to avoid complicated mathematical expressios, a sigle-phase equivalet system is used to describe the three-phase system show i Fig. If the fuctio of the SC is solely for the purpose of harmoics compesatio, the istataeous power at the SC output will be of the form (7) out L si( wt ) s ( t) sh p ( Ik si( kt k ) k2 k2 t t I Load t) k si( k t k ) The average power is k2 (8) P cos k 2 I k k (9) Note that oly power compoets associated with the harmoics are cotaied i P. P is either imported ito or exported from the SC to the exteral system. The losses i the SI would be low ad ca be igored. Hece the eergy exchage betwee the SC ad the exteral power system is T E (0) cos 2 I k k k k 2 4

5 over the time iterval T. The SC supplies eergy to the exteral system whe E>0. As the oly sigificat source of eergy storage i the SC is the ESS, the export of the eergy to the exteral system will result i a decrease i the voltage DC. Hece DC has to be cotrolled withi certai rage. I order to achieve this, there must be cotrol o the eergy flow. This ca be achieved by adjustig the phase of the fudametal compoet of the referece voltage of the SC. If a phase shift α is itroduced to the referece voltage for, say phase a of (), oe obtais L t) si( wt ) () ( Sice oly the fudametal voltage compoet is ivolved i the phase shift, the secod subscript i (4) has bee omitted i ( ). Furthermore, otice that the itetio is ot to chage the magitude of the fudametal compoet of the loadside voltage L. Hece L has the same magitude as without the phase shift. With a assumed costat impedace load model, I will also remai costat followig the phase shift. It the follows that the ew ijectio voltage is ij si( t ) si( t ) Lh (2) Refer to above equatios, the eergy flow betwee the ESS ad the exteral power system ow becomes T E ( cos( ) cos ) 2 I I k k k 2 (3) From (3), it ca be see that E could be forced to be zero if α is selected to be α 0 where ar (4) 0 cos( k I k cos k ) I k2 Simulatio Results ad Discussios The example show o Fig. may ow be used to verify the effectiveess of the SC i ehacig the voltage quality of the power system. The upstream geerator is represeted as a 220- voltage source, with its AR actio igored. The source impedace is assumed to be 0.05 p.u. ad q=20. The mai load coverter is assumed to be a six-pulse cotrolled rectifier. 5

6 Fig 2: Isolated Power System without Series Compesator Fig 3: Isolated Power System with Series Compesator with 25% ad 40% load chage. 6

7 Fig 4: Termial voltage () of the sesitive load before ad after a 25% mai load chage: without SC Fig 5: Termial voltage () of the sesitive load before ad after a 25% mai load icrease: with SC Fig.6. Harmoic power flows i the isolated p.s prior to ad after the 25% mai load icrease. Fig: 7. Termial voltage of ESS for a 40% load icrease 7

8 Fig: 8. Termial voltage of ESS(v) followig load chage:70% voltage sag Fig 9.Load curret of the sesitive load with sc for 40% load icrease Fig 0.Source curret of the sesitive load with sc for 40% load icrease Coclusio oltage quality improvemet i a isolated power system through series compesatio has bee ivestigated. The power system cotais sigificat proportio of fluctuatig oliear load ad a high level of harmoic distortios is observed. The SC is also desiged to maitai the fudametal frequecy compoet of the termial voltage of protected sesitive load. I this paper, a complete simulated series compesator system has bee developed by usig Matlab Simulik software. It is show that the simulated SC developed works successfully to improve power quality. PWM techique is used to cotrol the ijectio voltage of the SC so that it ca mitigate the effects of the harmoics ad voltage sag has bee proposed. The proposed system performs better tha the traditioal methods i mitigatig harmoics ad voltage sags. The proposed SC ca hadle both balaced ad ubalaced situatios without ay difficulties ad would iject the appropriate voltage compoet to correct rapidly ad aomaly i the supply voltage to keep the load voltage balaced ad costat at the omial value 8

9 Refereces [] Roger C. Duga, Mark f. Mcgraagha, Dr. Surya Satoso ad H. Waye Beaty., Electrical power systems quality, Tata McGraw Hills publicatios, [2] Devaraju.T, Dr.eera Reedy..C ad Dr.ijayakumar.M, Modelig ad simulatio of customer power devices to mitigate power quality problems Iteratioal Joural of Sciece ad Techology, ol. 2(6), 200, pp [3] Wag.T.X, Choi S.S. ad E.K.K. Sg Series compesatio method to mitigate harmoics ad voltage sags ad swells IET Geer. Trasm. Distrib., ol., No., Jauary 2007,pp [4] Choi, S.S., Wag, T.X., ad Sg, E.K.: Power quality ehacemet i a isolated power system through series compesatio. Proceedigs of 5th Power System Computatio Coferece, Liege, Belgium, August 2005, Sectio 22, pp.-7 [5] Wag.T.X. ad Choi S. S., Ehacemet of oltage Quality i Isolated Power Systems IEEE Trasactios o power delivery, ol. 22, No. 2, April [6] Leela.S, Dash. S.S., Cotrol of Three Level Iverter Based Dyamic oltage Restorer Joural of Theoretical ad Applied Iformatio Techology, ol.8, No., Pp-3-7 [7] Rosli Omar, Nasrudi Abd Rahim, Mariza Sulaima Modelig ad Simulatio for oltage Sags/ Swells Mitigatio usig Dyamic oltage Restorer (DR) Joural of Theoretical ad Applied Iformatio Techology, ol.5, No. 4, 2009, pp [8] Eg Kia Keeth Sg, Choi, S.S., ad D.Mahida vilahgamuwa: Aalysis of series compesatio ad DC-lik voltage cotrol of a trasformerless self-chargig Dyamic oltage Restorer. IEEE trasactios o power delivery, ol. 9, No. 3, July 2004, pp