POWER QUALITY AND RELIABILITY SUPPLY IMPROVEMENT USING A POWER CONDITIONING SYSTEM WITH ENERGY STORAGE CAPABILITY

Transcription

1 POWER QUALITY AND RELIABILITY UPPLY IMPROVEMENT UING A POWER CONDITIONING YTEM WITH ENERGY TORAGAPABILITY Domenico Caadei, Gabriele Grandi, Claudio Roi Department of Electrical Engineering Univerity of Bologna Viale Riorgimento, I406 Bologna ITALY ABTRACT A Power Conditioning ytem (PC) with energy torage capability i propoed a a viable olution for improving the quality and the reliability of the electric energy upply. everal tak can be performed at the ame time, uch a reactive power compenation, current harmonic reduction, and moothing of pulating load. Moreover, the PC can operate a Uninterruptible Power upply (UP) during hort time interruption of the grid upply. In order to how the PC performance, experimental tet have been carried out uing a flywheel a torage device. The effectivene of the PC ha been uccefully verified in everal operating condition. KEY WORD Uninterrupted Power upply, Power Factor Compenation and Conditioning, Phae and Voltage Imbalance, Energy torage ytem. Introduction In practical application a high reliability power upply i required for critical load. In general, thi requirement i fulfilled by a tandard UP configuration baed on the erie connection of a Voltage ource Inverter (VI) between the grid and the load. The energy i uually tored in a lead acid battery bank. It can be noted that, about 90% of grid fault are very hort (le of ), and in the remaining cae the UP upplie the load for the time interval required for tarting up a dieel or turbine generator et. In thee cae the UP upplie the load for no more than 0 econd. A a conequence, for thee high power and low energy application the lead acid battery bank i often overized, and an extra cot i paid for an energy reerve that will be never ued. Uing new type of energy torage device, uch a uperconducting magnet, upercapacitor or flywheel, which are more uitable for high power and low energy application, it i poible to better exploit the energy reerve. Thee new torage device allow additional tak to be performed uing the ame hardware tructure required for the UP operation. The additional tak conit of reactive power compenation, current harmonic reduction, load unbalance compenation, and moothing of pulating load. In thi way, the UP behave a a power conditioning ytem when the grid upply i preent, improving ignificantly the power quality in the grid ection next to the Point of Common Coupling (PCC). The cheme of the propoed PC i hown in Fig.. The energy torage Device (D) i connected to a tatic converter, which allow the bidirectional energy flow with the dclink bu. The topology of thi converter depend on the type of the energy torage device. The threephae Voltage ource Inverter (VI) i hunt connected to the PCC through three aclink inductor. The PCC i upplied by the main through a tatic witch that mut be turned off during the operation of the power conditioning ytem a UP. In thi cae, the PC utilize the tored energy to upply the load. When the grid upply i preent, the tored energy can be ued to compenate flicker phenomena due to witchingon and off of the load [4]. During thee P D L ac P F PCC P C c P L torage Device D converter dclink capacitor inverter Power Conditioning ytem (PC) Fig. chematic drawing of the PC tructure aclink inductor load tatic witch """" """"! ource

2 tranient the PC exchange a given quantity of the tored energy in order to deliver the difference between the intantaneou load power and it average value, which i upplied by the ource. Fig. how that the dc/ac ection of the PC connected to the main ha the ame topology of a hunt active power filter [][]. Then, by mean of a uitable control of the PC it i alo poible to compenate the load reactive power, to reduce the current harmonic of non linear load, and to compenate unbalanced load. Thee feature are achieved by the direct control of the current through the aclink inductor in order to force the ource current to be balanced and inuoidal for any operating condition. The ource current are ynchronized with the fundamental poitive equence component of the ource voltage. A a conequence, balanced and inuoidal ource current with unity power factor can be obtained, even in preence of voltage perturbation coming from the main [5], [6]. The flicker phenomena compenation and the active filter operation are achieved operating the PC in "current ource mode". When a perturbation of the main i detected, the control ytem commutate the operating tatu from "current ource mode" to "voltage ource mode" and the PC behave a UP. Thi commutation hould be carried out without dicontinuity in the voltage applied to the load. A power conditioning ytem having all the decribed feature ha been realized in laboratory, according to the cheme repreented in Fig.. The performance of the PC ha been verified by experimental tet and good reult have been achieved both in active power filter operation and UP operation.. Analyi of the Energy Control ytem The implemented control ytem i contituted by two ection that allow the operation of the PC repectively in current ource mode when the grid upply i preent and in voltage ource mode when a grid fault occur. A commutation trategy baed on the monitoring of the ource voltage vector e allow the commutation () R E D P L P P F inverter P F = P F R () D converter torage Device E D D P R ( ) D P D capacitor Fig. Block diagram of the Energy Control ytem E D between the two control ection, without dicontinuity in the voltage applied to the load.. Current ource mode During the operation of the PC a current ource the tranfer of energy among the torage device, the dclink capacitor and the ac network i performed by the Energy Control ytem (EC). The analyi of the EC can be uefully carried out in term of power flow and energy balance [9], [0]. In order to explain the principle of operation of the EC, reference i made to a load power perturbation. Any load power change determine a variation of the power upplied by the PC and then a variation of the energy tored in the dc link capacitor. Thi energy hould be quickly retored to it reference value uing energy coming from the torage device. A a conequence, a power flow i etablihed between the PC and the point of common coupling. In thi way at a witchingon of the load, the ource i not required to upply intantaneouly the full load power, but only an increaing percentage, being the remaining amount of power upplied by the PC. During thi tranient the energy upplied by the PC can be conidered a coming directly from the D unit. The ue of the D unit for moothing load variation i neceary becaue the dc link capacitor ha very low tored energy, and large dclink voltage variation mut be avoided in order to enure a correct operation of the inverter. The behavior of the EC can be analyzed in term of Laplace tranform by the control cheme hown in fig.. Here, the input control variable are the energy in the dclink capacitor E C, and the energy in the torage device E D. The regulator R generate the reference ource power P on the bai of the dclink capacitor energy error. The inverter power P F that hould be exchanged from the main to the inverter i obtained by ubtracting the load power to the ource power, P F =P P L. The regulator R varie the reference value of the D energy on the bai of the error in the dclink capacitor energy. R act to keep the D energy cloe to it reference value by exchanging power with the dclink. In order to analyze the compenation of flicker phenomena, it i ueful to conider the block diagram hown in Fig., which ha been derived rearranging the block diagram of Fig.. By uing the cheme of Fig. it i poible to determine the tranfer function linking the ource power to the load power. Auming for the regulator the following tranfer function R () =K, R () =K, R ()=K, () the ource power i given by the following equation C P () = G C ()E G D ()E G L () PL. () The expreion of G C (), G D (), G L () are given in Appendix. Being the energy reference and E D D

3 P F P L P PF P D R () P R() D converter P D = P D capacitor E D () R E ~ D K Fig. E D Block diagram howing the relationhip between P L and P contant value, G C () and G D () do not introduce teady tate error in P (). A a conequence the open loop tranfer function F() between the ource power P and the load power P L can be expreed by K( K ) F() =. () ( KK ) It can be verified that thi tranfer function enure a table operation of the PC for any load perturbation [] A mooth variation of the ource power in repone to a load power variation can be obtained by a proper tuning of the parameter K, K and K. The parameter K of the regulator R i related to the capability of the control ytem in following quick change of E ~ D. The higher i K, the fater i the D energy regulator. The parameter K affect the behavior of the regulator R in keeping contant the energy tored in the dclink capacitor. The error in the capacitor energy determine the reference value of the ource power, and through the parameter K, determine alo the variation of the D energy level. In thi term, the parameter K define the capability of the PC to utilize the tored energy. With reference to a load witchingon tranient, the energy level of the D will be decreaed by an amount related to the value of K. A high value of K determine a deep dicharge of the torage device, with a large amount of energy flowing from the PC toward the PCC. Thi mean a high moothing effect on the ource power variation, but require torage device with high rated energy. A a concluion, the tuning of K define the capability of the PC in compenating flicker phenomena introduced by pulating load. With reference to the poibility to compenate reactive power and current harmonic uing the EC decribed in thi ection, a detailed analyi ha been preented in [] and [6]. Here only ome comment will be given. The energy variation on the dclink capacitor determined either by compenation of load unbalance or by operation with unbalanced ource voltage are very mall compared to thoe of pulating load, and they do not produce appreciable change on the reference value of the D energy. Then, with reference to the EC of Fig. it can be aumed that the upper control loop i alway active, wherea the lower control loop operation i involved only during large variation of the load power. Fig. 4 Block diagram of the Energy Control ytem ued in the voltage ource mode. Voltage ource mode During grid fault, the operation of the PC a voltage ource i obtained by a imple control ytem which allow the upplying of the load uing the energy tored in the D. The D converter dicharge the torage device determining a power flow from the D to the dclink in order to keep it voltage cloe to the reference value. The block diagram of the EC employed in the voltage ource mode i hown in Fig. 4. The inverter i controlled by a pace Vector Modulation (VM) technique that upplie the load with threephae balanced ytem of inuoidal voltage, correponding to the grid rated voltage.. Implementation of the Control ytem The energy control ytem required for operating the PC in the current ource mode ha been implemented introducing ome change with repect to the baic cheme of Fig.. The energy level in the dclink capacitor i kept under control uing the capacitor voltage V C a control variable. The ource power i regulated uing the ource current i a control variable [6]. Fig. 5 and 6 how the control ytem modified according to thee conideration. The reference ource current i i obtained by multiplying the unity vector vˆ, by the reference ource current magnitude I, where: the unity vector vˆ i in phae with the poitive equence fundamental component of the line to neutral voltage vˆ, and it i generated by a Three Phae Locked Loop algorithm [7]. Thi algorithm operate correctly even in the cae of unbalanced and non inuoidal voltage [8]. the reference ource current magnitude I i generated by the regulator R (), which operate on the intantaneou error between the reference value V C and the actual value V C of the dclink voltage.

4 V C _ V C v R () vˆ I i i i L i F i F i F K v v F PWM VI v F v L F i F Fig. 5 dclink voltage controller The ac current regulator then yntheize the reference ource current. Thi regulator operate in order to keep the ource current i cloe to it reference value i []. The regulator R () of the energy control ytem keep under control the energy level in the torage device. In the implemented control ytem thi ha been achieved controlling the voltage acro the upercapacitor, or the rotating peed of the flywheel. Thi control i performed uing the converter between the dclink and the torage device. The topology of the converter i a dc/dc chopper in the cae of upercapacitor, and a threephae VI in the cae of flywheel driven by an induction machine. The commutation trategy between current ource mode and voltage ource mode i implemented in a upervior algorithm. When the grid i preent, the upervior algorithm calculate the intantaneou value of the ource voltage vector v by monitoring the ource voltage. Auming balanced and inuoidal upply voltage, the voltage vector v rotate at contant peed decribing a circular locu. In cae of unbalanced or non inuoidal upply voltage, the trajectory decribed by the voltage vector v change from circular cauing variation of the magnitude and angular velocity. Perturbation of the upply voltage, uch a voltage ag, large voltage drop, zero voltage condition, caue large deviation of the voltage vector trajectory. A critical circular region ha been defined in order to detect thee perturbation and to tart the commutation equence. If the voltage vector lay in the critical region for a time interval greater than a given value, the upervior algorithm diable the current ource mode, drive the opening of the tatic witch, and when the ource current are almot zero, enable the operation of the PC in voltage ource mode. During the commutation equence, the phae angle of the ource voltage i etimated allowing the PC to upply the load without appreciable voltage dicontinuity. During the operation of the PC in voltage ource mode, the grid voltage i continuouly monitored and when the ource voltage vector leave the critical region permanently, the control ytem commutate from the voltage ource mode to the current ource mode. During thi commutation a PLL algorithm allow to ynchronize the voltage vector applied to the load with the retored ource voltage. In thi way, the commutation i carried out without dicontinuitie on the voltage applied to the load. 4. Experimental reult Fig. 6 ac current controller A PC having the characteritic hown in Tab. I ha been realized in laboratory. A flywheel driven by an induction machine ha been ued a energy torage device. The ytem configuration i the ame a in Fig., therefore the PC i able to compenate flicker phenomena, to behave a active power filter, and to enure the continuity of the load upply during hort time interruption of the grid. The control ytem ha been implemented on a PPC MHz DP baed board, including A/D converter and an encoder interface. The board include alo a econd DP (TM0F40) to generate the PWM ignal for both the converter. A a firt tep, the capability of the PC to operate a active filter ha been verified. Fig. 7 how the compenation of the reactive power of a RL linear load. Fig. 8 how the compenation of the current harmonic of a non linear load repreented by a three phae diode rectifier. A inglephae load ha been connected to the PCC to repreent an unbalanced load. The reult obtained, howing the effectivene of the compenation are illutrated in Fig.9. Pulating load have been conidered to verify the capability of the PC to compenate flicker phenomena. Table I Main data of the PC prototype utility voltage V 0 [V] dclink capacitance C C 0 [mf] utility frequency f 50 [Hz] flywheel inertia J D 0. [kg.m ] phae aclink inductor L ac [mh] reference value of dclink voltage V C 450 tatic witch TRIAC phae reference value of flywheel peed n D 000 [rpm] [V]

5 Fig. 0 how the behaviour of the PC during the tranient caued by a load witchingon. The effect of the PC i to mooth the correponding tep of the ource power by upplying the load at the beginning of the tranient. Fig. how the tranient repone caued by a load pulating at a frequency of Hz, with a duty cycle of 0%. The effect of the PC i to keep the ource current amplitude almot contant and equal to the mean value of the load current. Thi i achieved exchanging a fraction of the energy tored in the flywheel. Fig. 4 how the behaviour of the ytem during a hort interruption of the ource upply. It can be noted that the ytem i able to upply the load during the grid fault by employing the mechanical energy tored in the flywheel (Fig. ). The detail of the commutation between the two operating mode, i.e. current ource and voltage ource, are repreented in Fig. and 4. Both the commutation are carried out without ignificant dicontinuitie in the load current that mean continuity of ervice of the critical load. 5. Concluion The performance a PC with the capability to meet everal power quality requirement ha been analyed and verified by experimental tet. An analytical approach ha been ued to determine the parameter of the three regulator. A PC prototype with a rated power of 0 kva ha been realized in laboratory. everal tet have been carried out in different operating condition. It ha been verified that thi ytem i able to compenate reactive power and current harmonic due to nonlinear load. Furthermore, the reult obtained have hown that the preence of an energy torage device make it poible to reduce the ource current variation due to pulating load, and to enure the continuity of the upply during hort time utility fault. The experimental reult are quite atifactory, howing the effectivene of the propoed PC in improving the power quality and the reliability of the power upply VI. Reference [] H.Fujita, H.Akagi, The Unified Power Quality Conditioner: The Integration of erie Active Filter and hunt Active Filter, Proc. IEEEPEC Conf., Baveno, ITALY, 996, [] L.A. Moran, J.W. Dixon, R.R. Wallace, A threephae active power filter operating with fixed witching frequency for reactive power and current harmonic compenation, IEEE Tran. on IE, Vol. 4, No.4, 996, [] D. Caadei, G. Grandi, U. Reggiani, C. Roi, Control method for active power filter with minimum meaurement requirement, Proc. IEEE APEC Conf., DallaTX UA, 999, Vol., [4] D. Caadei, G. Grandi, U. Reggiani, G. erra, Analyi of a Power Conditioning ytem for uperconducting Magnetic Energy torage, Proc. IEEEIIonf., Pretoria outh Africa, 998, [5] D. Caadei, G. Grandi, U. Reggiani, G. erra, A. Tani, Behavior of a Power Conditioner for µme ytem under Unbalanced upply Voltage and Unbalanced Load, Proc. IEEE IIonf., Bled lovenia, 999, Vol [6] D. Caadei, G. Grandi, C. Roi, Effect of upply Voltage nonidealitie on the Behavior of an Active Power Conditioner for Cogeneration ytem, Proc. IEEEPEC Conf., Galway, Irland, 000, Vol. 7. [7] F. M. Gardner, Phaelock Technique. (New York: Wiley, 979). [8]. K. Chung, A Phae Tracking ytem for Three Phae Utility Interface Inverter, IEEE Tranaction on Power Electronic, Vol. 5 no., May 000, 4 48 [9] D. Caadei, G. Grandi, U. Reggiani, C. Roi, Active AC Line Conditioner for a Cogeneration ytem, Proc. European Confer. on Power Electronic and Application EPE99, Lauanne, witzerland, 999. [0] F. Negrini, U. Reggiani, C. Roi, G. Grandi, D. Caadei, et. al, Recent Development on micro ME ytem Project at the Univerity of Bologna, Proc. of EEAT, OrlandoFL, UA, 000. [] G.Grandi, D.Caadei, C.Roi, A Parallel Power Conditioning ytem with Energy torage Capability for Power Quality Improvement in Indutrial Plant, Proc. European Confer. on Power Electronic and Application EPE'0, Graz, Autria, Appendix With reference to the control cheme of Fig., the expreion of G C (), G D (), G L ()are G G G C () = D L () = K K K K K ( K K KK ) KK KK K K K ( K K KK ) KK K K KK K () =. ( K K K ) KK K

6 i A i LA V A Pload Pource CH 0Kw/div P L, P 0 kw/div CH 0Kw/div CH I A 50A/div A/div Iource (0 A/div) 5ec flywheel CH4 000rpm/ peed div 000 peed rpm/div Fig. 7 Compenation of load reactive power i LA Fig. 0 Repone of the PC to a load witchingon CH V C 0 0V/div Vdc CH I LA 50 0A/div Ia ource 0 (A/div) i A Fig. 8 Compenation of current harmonic 0.5 Fig. Repone of the PC to a pulating load I CH A 50 0A/div A/div Ia load flywheel CH4 peed 00rpm/div peed 000 rpm/div i LA i LC i LB i la ir A i B i C T 0 (A/div) Fig. 9 Compenation of load unbalance CH 00V/div VEa A 00 V/div CH 5A/div I A 5 Iource A/div CH 500V/div V A 500 V/div Ea CH 50A/div I LA 50 Iload A/div CH 50A/div I A 50 Iource A/div flywheel CH4 000rpm/ peed 000 rpm/div peed Fig. Operation of the PC during a.5 grid fault CH 00V/div Ea V A 00 V/div CH 0A/div I A Iource 5 A/div CH I LA 5A/div 5 A/div Iload CH 0A/div I LA 0 Iload A/div 0mec CH4 tatic 5V/div witch I. Command 0mec CH4 tatic 5V/div witch I. Command Fig. UP operation: detailed view of the commutation: from current ource mode to voltage ource mode Fig. 4 UP operation: detailed view of the commutation: from voltage ource mode to current ource mode