Electric spring for power quality improvement

Transcription

1 Title Electric spring fr pwer quality imprvement Authr(s) Yan, S; Tan, SC; Lee, CK; Hui, RSY Citatin The 29th Annual IEEE Applied Pwer Electrnics Cnference and Expsitin (APEC 2014), Frt Wrth, TX., March In IEEE Applied Pwer Electrnics Cnference and Expsitin Cnference Prceedings, 2014, p Issued Date 2014 URL Rights This wrk is licensed under a Creative Cmmns Attributin- NnCmmercial-NDerivatives 4.0 Internatinal License.; 2014 IEEE. Persnal use f this material is permitted. Hwever, permissin t reprint/republish this material fr advertising r prmtinal purpses r fr creating new cllective wrks fr resale r redistributin t servers r lists, r t reuse any cpyrighted cmpnent f this wrk in ther wrks must be btained frm the IEEE.

2 Electric Spring fr Pwer Quality Imprvement Yan Shu 1 Siew-Chng Tan 1 1 Department f Electrical & Electrnic Engineering The University f Hng Kng Hng Kng ( yanshu@hku.hk) C.K. Lee S.Y.R. Hui 1,2 2 Department f Electrical & Electrnic Engineering Imperial Cllege Lndn Lndn, United Kingdm ( r.hui@imperial.ac.uk) Abstract In this paper, we discuss the principles f perating the electric spring (ES) as a reactive pwer cmpensatr and as a pwer factr crrectr. The thery n electric springs with capacitrs fr vltage stabilizatin is reviewed t present a general idea n the behavir f ES. Further discussin fcuses n the principle f ES with batteries t cver its eight pssible perating mdes and their usefulness in prviding line current regulatin. An input current cntrl scheme is designed fr ES with batteries t validate its capability in pwer factr crrectin. A lw-vltage single-phase pwer system with different types f lads has been built fr verifying the feasibility f prpsed thery f ES with batteries. Experimental results shw that the ES is capable f perfrming the eight perating mdes when changing the pwer cnsumptin f the nn-critical lad, and that with the prpsed input current cntrl, the ES can achieve pwer factr crrectin fr bth RL and RC lads. I. INTRODUCTION The impending energy crisis and envirnmental issues require that substantial renewable energy surces shuld be included in the future as either centralized pwer mills r distributed generatrs. Due t the dynamically changing nature f renewable energy surces, this freseeable majr change in pwer grid demands sphisticated cntrl methdlgies and a new discipline f management strategies. Smart grids based n mdern pwer electrnics and telecmmunicatin technlgies have been prpsed as a prmising slutin. T cpe with the variability and uncertainty f renewable energy surces, new methds fr lad management are required. Amng varius methds fr lad management, the electric spring (ES), which is based n pwer electrnics technlgy, can instantaneusly balance the pwer cnsumptin and generatin. This technique has the advantage ver existing demand side management [1]-[6] and energy strage slutins [7], [8] in that: i) it can cntrl the lad t reduce the fluctuatin f the generatr; ii) it can flatten the vltage fluctuatin caused by unstable pwer generatin in real time [9]. The first generatin f ES is presented in [9]. Based n Hke s law, the ES can handle reactive pwer t stabilize line vltage fr critical lads. Research in [10] als shws that ES can reduce the capacity f energy strage by up t 50%. The ptential f ES is further discvered in [11]. By replacing capacitrs with batteries n the DC side, the ES pssesses mre diverse perating mdes which can prvide bth real and reactive pwer cmpensatins, and their cmbinatins. With such a favrable feature, it is expected that the ES, as a decentralized apprach, can als be used t imprve the pwer quality f the distributin (lw-vltage) pwer grids. Cnventinally, single centralized techniques such as the series and shunt VAR cmpensatrs are used at the highvltage level t imprve the perfrmance f AC pwer systems by prviding 1) lad cmpensatin and 2) vltage supprt [12]. Specifically, series cmpensatrs actively mdify the transmissin parameters and shunt VAR cmpensatrs change the equivalent impedance f lad. A unified PQ cnditiner integrating the series- and shuntactive filters t address the issues f vltage flicker and reactive pwer is intrduced in [13]. In recent years, static VAR cmpensatrs emplying thyristr-switched capacitrs (TSCs) and thyristr-cntrlled reactrs (TCRs) are the dminant slutins fr such applicatins, due t their simple structures, cnvenient implementatin, and affrdable price [14]-[16]. The emergence f flexible AC transmissin systems (FACTS) based n these advanced pwer electrnic technlgies pened a new area fr the peratin f transmissin systems [17]-[21]. It is wrth t mentin that such mdern techniques are based n large-capacity cmpensatrs that cnduct pwer quality imprvement in a centralized manner. Hwever, in future pwer systems where renewable energy surces are cnnected t pwer grids in a distributed manner, installing decentralized pwer cmpensatrs in numerus small capacities at the lad side can be mre favrable than the centralized apprach. Here, the ES are numerus in quantity and they act simultaneusly t achieve vltage stability and pwer cmpensatin. Thus, they can be perceived equivalently as a decentralized type f series reactive pwer cmpensatrs (RPC) which has the pwer factr (PF) crrectin features. This paper demnstrates the use f ES t perfrm tasks similar t that f RPC and pwer factr crrectrs (PFC), but at the lw vltage distributin level in achieving vltage stability in /14/$ IEEE 2140

3 grids with renewable surces thrugh input vltage cntrl and pwer quality imprvement thrugh input current cntrl. II. REVIEW ON ES BEHAVING LIKE A SERIES REACTIVE POWER COMPENSATOR The ES is a special type f RPC designed fr decentralized installatin and peratin, and fr the cre purpse f line input vltage cntrl. This is in cntrary with the series RPC, which adpts an utput vltage cntrl, as shwn in Fig. 1(a). In a series RPC, the feedback signal is acquired at the lcatin where the active pwer flws ut f the reactive pwer cmpensatr and int lad. Hwever, in an ES, the feedback signal is btained at the lcatin where the active pwer flws int the cmpensatr, as shwn in Fig. 1(b). In a series RPC, the bjective is t ensure that the utput vltage has a cnstant amplitude regardless f the varying nature f the amplitude f the line input vltage. Hwever, fr the ES, the bjective is reverse, and the functin is aided with the use f a s-called nn-critical lad, which is tlerant t a fluctuating vltage supply. These differences frm a series RPC ffer many beneficial features fr the ES: i) the line vltage can be supprted t nminal value, giving the critical lad a stable vltage; ii) the nn-critical lad cnnected in series with ES can cnsume the fluctuating pwer generated by the unstable AC pwer surces. Such useful prperties f allwing instantaneus balance f pwer supply and demand while cncurrently achieving lcal line vltage stability is particularly useful and imprtant fr future smart grids with a large penetratin f renewable energy resurces. (a) (b) Figure 1. (a) Simplified cntrl diagram f series RPC. (b) Simplified cntrl diagram f ES. Mrever, with the ES cnnected in series with the nncritical lad Z as shwn in Fig. 2, cntrlled variatin in pwer cnsumptin is easily achievable. Nn-critical lads such as water heaters, air-cnditining systems, public lighting systems, and refrigeratrs can be varied r shed ff, as and when necessary. The advantages f this arrangement are twfld. Firstly, the ES can be a small-capacity cmpensatr embedded in cmmn electrical appliances that are widely present in a distributive manner. Secndly, by keeping the lcal line vltage stable and letting the utput f the ES t fluctuate, the nn-critical lads can absrb the fluctuating pwer while ensuring a cnstant vltage supply t the critical lads. V es Z 0 I Figure 2. V DC V DC Use f ES in electrical system. V s V s _ ref A half-bridge inverter with DC link capacitrs is a pssible way f implementing the ES. This type f ES can stabilize line vltage by handling nly the reactive pwer. Therefre, there are nly tw pssible perating mdes: i) capacitive mde when ES generates ve reactive pwer t bst line vltage and ii) inductive mde when ES generates +ve reactive pwer t suppress the line vltage. With the ES in series with Z, the phase angle difference between ES vltage and the nn-critical lad current (V es is 90⁰ leading r lagging I ) decides the perating mde f ES (inductive r capacitive mde). The relatinship f the ES vltage V es, line vltage V s, and nn-critical lad vltage V is given by V = V s V es (1) Equatin (1) shws the fundamental relatinship that can be explited t allw the ES t regulate the line vltage. Specifically, when V s remains at its nminal value, the ES is bypassed and V equals t V s, as shwn in the equivalent circuit diagram given in Fig. 3(a). When V s surges abve the nminal value, the ES is instantaneusly turned n t generate a vltage V es t suppress V s t its nminal value, as shwn in Fig. 3(b). On the ther hand, when V s drps belw its nminal value, the ES instantaneusly generates a vltage V es t bst V s t its nminal value, as shwn in Fig. 3(c). S, by incrprating such peratins, the ES vltage will instantaneusly track the fluctuatins f V s, and in the prcess, will indirectly transfer the vltage fluctuatin t the nncritical lad. Z 0 s = Vnm V V = 0 es V Vs = V es V nm Z 0 V > Vnm Vs = V es V nm Z 0 V < Vnm (a) (b) (c) Figure 3. Equivalent circuit f ES wrking in (a) neutral psitin; (b) vltage suppressing mde; and (c) vltage supprt mde. 2141

4 III. PRINCIPLES OF ES WORKING AS A PFC A. Characteristcs f ES with Batteries A mdified versin f ES is achievable by replacing the DC link capacitrs with batteries (r cnnecting the batteries acrss the capacitrs). As cmpared t an ES with capacitrs, an ES with batteries can generate a vltage with phase angle frm 0 degree t 360 degrees relative t the phase angle f the nn-critical lad current, thereby allwing bth real and reactive pwers t be exchanged. Under the same electrical cnfiguratin, an ES with batteries can prvide ther perating mdes in additin t the inductive and capacitive mdes. This characteristic imprves the capability f an ES fr use in stabilizing future smart grids. As mentined previusly, the perating mde f the ES is decided by the phasr relatinship f V es and I. Fr an ES with capacitrs, V es can nly be perpendicular t I. Hwever, fr an ES with batteries, since phase angle f V es can change freely, V es can be either ppsite t r in phase with I t give tw mre primary perating mdes: 1) negative-resistive mde when an ES generates real pwer by discharging the batteries; 2) resistive mde when an ES absrbs real pwer by charging the batteries. Thus, an ES with batteries pssesses fur primary perating mdes. Building upn this, fur ther secndary perating mdes, which are cmbinatins f the fur primary perating mdes, are pssible additins. Eight pssible perating mdes f ES with batteries can be used fr multiple purpses, such as pwer and vltage cmpensatins [12]. Here, the pssibility f line current regulatin is explred with the examinatin f the eight perating mdes. T simplify the discussin, the fllwing assumptins are cnsidered. In the distributin pwer system given in Fig. 2, the line vltage V s is cnsidered t be cnstant in phase, frequency and amplitude, and the nncritical lad Z is resistive. In this way, the perating mde f the ES can be equivalently bserved by learning the vectr psitins f V es and V. In all the eight perating mdes, the intrductin f the V es will lead the pwer system int a new steady-state cnditin. As cmpared with the state when the ES is absent, the new steady-state cnditin reshapes the pwer cnsumptin f the nn-critical lad and, as a result, changes the behavir f the line current. Meanwhile, either real r reactive pwer r bth, are exchanged between the ES and the pwer surce. Frm Figs. 4(a) and 4(b), pure capacitive and inductive mdes can be realized by ensuring V es t be perpendicular t V (V es 90 leading V fr inductive mde, V es 90 lagging V fr capacitive mde). Fr these perating mdes, the ES nly exchanges reactive pwer with the pwer surce. The riginal nn-critical lad vltage (withut an ES) V (blue-dtted line), which is equal t the line vltage V s, is relcated t a new psitin V (blue-slid line) after the intrductin f V es. As a result, V s is decmpsed int V es and V, which reduces the pwer cnsumptin f the nn-critical lad. Thus, an ES in the inductive mde can perfrm lad reductin and changes the equivalent lad t be mre inductive. An ES in the capacitive mde can perfrm lad reductin, but changes the equivalent lad t be mre capacitive. (a) (c) (e) (g) Figure 4. Vltage vectrs f the eight perating mdes f ES with batteries. (a) inductive mde. (b) capacitive mde. (c) resistive mde. (d) negative-resistive mde. (e) inductive plus resistive mde. (f) capacitive plus resistive mde. (g) inductive plus negative-resistive mde. (h) capacitive plus negative-resistive mde. Figs. 4(c) and 4(d) shw the vltage vectrs f the ES wrking in resistive and negative-resistive mdes. In bth cases, nly real pwer is exchanged between the ES and the pwer surce. An ES in resistive mde intrduces vltage V es, which suppresses V t V and thus reduces the pwer cnsumptin f the nn-critical lad. In cntrast, V es f negative-resistive mde increases V t V and thus bsts the pwer cnsumptin f the nn-critical lad. In summary, an ES in the resistive mde makes the equivalent lad less resistive while an ES in the negative-resistive mde makes the equivalent lad mre resistive. Based n these fur primary perating mdes, fur hybrid secndary perating mdes with vltage vectrs as shwn in Figs. 4(f) t Fig. 4(h), in which real and reactive pwers are simultaneusly exchanged between the ES and the pwer surce, wuld be pssible. Specifically, the fur secndary mdes are namely, the resistive plus inductive mde, resistive plus capacitive mde, negative-resistive plus inductive mde, and negative-resistive plus capacitive mde. Since the resistive, inductive, and capacitive mdes have lad reductin effect, the resistive plus inductive mde, and resistive plus capacitive mde reduce the pwer cnsumptin f the nncritical lad. Hwever, the negative-resistive mde, negative- (b) (d) (f) (h) 2142

5 resistive plus capacitive, and negative-resistive plus inductive can be used t bst pwer cnsumptin f the nn-critical lad. B. Principles f the ES with Batteries fr PF Crrectin One particular applicatin f the ES with batteries (with eight perating mdes) is fr PF crrectin, which can minimize reactive pwer exchange by cntrlling the input current t be in phase with the input vltage. This technique is cmmn in high-vltage transmissins with centralized cmpensatin. In future smart grids, the ES can be installed in lw-vltage distributin grid t perfrm the same task. Using as a PFC, an ES adpts the same electrical cnfiguratin as that f an ES acting as a series RPC, but pssesses distinct perating principle. In line vltage regulatin, V s is fed back and cntrlled instantaneusly. An input vltage cntrl is necessary fr an ES with capacitr t achieve this functin. Hwever, fr an ES acting as a PFC, the input vltage cntrl fails t be an ptin, since the input current matters mre than the input vltage in achieving a satisfactry PF. Thus, different frm an ES acting as a series RPC, an ES acting as a PFC shuld dynamically shape the input current t be in phase with the input vltage by cmpensating bth the real and reactive pwers. T realize this bjective, the input current rather than the input vltage is fed back and an input current cntrl is devised, accrdingly. Cnsider a standard setup f an ES in an electric pwer system as shwn in Fig. 5, in which the input vltage V s is assumed t change nly slightly in phase and amplitude due t the limited inner impedance f the pwer surce. Figure 5. Practical setup f ES as a PFC. The line current I can be expressed as: Vs Ves Vs I = + Z Z where I is the line current, V s is the line vltage, V es is the utput vltage f ES, Z is the impedance f the nn-critical lad, and Z s is the impedance f the critical lad. T relate the cntrl f the line current given in (2) t the pwer surce vltage V g, the effect f the inner resistance f pwer surce Z in must be taken int cnsideratin, that is, s (2) Zin Vg + Ves Z Vs = Zline Z 1+ + Z Z s By substituting (3) int (2), the relatinship between I and V s is derived as line (3) Zin + ( ) Zs Z + Zs Z Z 1 I = ivg + i Ves (4) Zin Zin Zin Zin Z Zs Z + + Zs Z Equatin (4) shws that in a pwer system with a given Z and Z s, fixed Z in, and a cnstant V g, the phase angle f the input current I can be cntrlled by changing V es. C. Input Current Cntrl f the ES fr PF Crrectin Equatin (4) is a straightfrward descriptin f hw V es can cntrl I. Hwever, it is als a highly cupled relatinship between the phase angle f I and the amplitude and phase angle f V es. Thus, re-rganizing (4) is necessary fr the design f a feasible cntrl scheme. In a pwer system with fixed perating frequency (f s = 50 Hz), all parameters can be regarded as a rtating vectr within a framewrk, which cnsists tw perpendicular axis (d and q) and rtates at a frequency f s = 50 Hz. When a reference vectr t which the phases f ther vectrs are cmpared is chsen, all parameters are represented by vectrs with certain amplitudes and phase angles. The reference vectr is set alng the d axis, which gives it an angle f 0⁰. Befre applying the abve transfrmatin fr (4), a few simplificatins are required. Parameters are redefined as given in (5) and (6) Z Z s Zline Z 1+ + Z Z s line = b + b j Zline ( + ) Zs Z Z 1 = b3 + b4j (6) Zline Zline Z 1+ + Zs Z By substituting (5) and (6) int (4) and cnverting I, V es, and V g int the d +jq frm with V g chsen as the reference n the d axis ( V = V ), 0 we have g g I = bv + bv bv I = bv + bv + bv d 1 g 3 esd 4 esq q 2 g 3 esq 4 esd Equatin (7) is fundamental fr the design f the input current cntrller. Fr the d cmpnent f line current, V esd will be used as a feedback fr the design f the d lp. V esq is fed back fr the design f the q lp. (5) (7) 2143

6 Figure 6. Cntrl diagram f ES fr PF crrectin. Fig. 6 shws the structure f the prpsed input current cntrller. The phase angle f the V s and the amplitude reference f I is instantaneusly sent t a Plar t d-q blck, which perfrms respective plar t d-q transfrmatin. Current references alng bth f the d and q axes are generated and then cmpared with the input current alng the d and q axes. The derived errrs are fed the respective PI cntrllers t generate the references fr V es. The V es references in the d-q frm are integrated int a single sinusidal frm V es_ref. The mathematical expressin f PI cntrller is given as: V = ( K + K / s)( V V ) es _ mag _ ref p _ Vs i _ Vs s _ mag _ ref s _ mag IV. EXPERIMENTAL SETUP OF THE ELECTRIC SPRING (11) The system cnsists f a cnstant AC pwer surce, a resistive nn-critical lad, and a critical lad. Three types f critical lad, namely a resistive, a resistive-inductive, and a resistive-capacitive lad, are used. The parameters f the system setup are given in Table 1. The pwer cnverter implementing the ES is a half-bridge inverter with the dc vltage rail fed by batteries. A bypass relay is cnnected acrss the utput capacitr f the inverter t bypass the ES when peratin f ES is nt required. The specificatins f the ES are given in Table 1. The derivatin f the system frequency is achieved by a phase-lcked lp (PLL) blck fr multiple uses: 1) the framewrk t which all variables are transfrmed needs a rtating frequency; 2) the d and q cmpnents f the V es reference signal have t be synchrnized int the sinusidal frm using the system frequency. Instead f V g, the line vltage is used as the reference whse instantaneus value is sent t the PLL blck. V s is tracked n the d axis with a zer degree phase angle. This arrangement facilitates the setting f the phase reference fr the line current. Since V s is used as the reference, the phase reference f line current can be set t zer at all times. The data acquisitin and prcessing blck is included t prcess the instantaneus values f the line current and vltage int the frmat suitable fr use by the input current cntrller. Its majr functin is t transfrm the sinusidal line current int the decupled d+jq frm. V. EXPERIMENTAL RESULTS AND DISSUSSIONS T demnstrate the functins f the ES, tw sets f experiments are cnducted based n the setup (Fig. 7). The first set f experiments demnstrates that the ES with batteries can change the lad cnsumptin f the nn-critical lad by perfrming eight pssible perating mdes. In the secnd set f experiments, the ES is prgrammed t perfrm PF crrectin with input current cntrl. A resistivecapacitive and a resistive-inductive critical lad are used respectively t examine the PF crrectin capability f the ES fr varius types f lads. Figure 7. Overview f the experimental setup. Pwer Surce Nn-critical Lad Critical Lad 1 Critical Lad 2 Critical Lad 3 TABLE I. SPECIFICATIONS OF EXPERIMENTAL SETUP Pwer System V g = 120 V RMS 200 Ω 500 Ω Switching Frequency Filter Inductr Filter Capacitr Electric Spring 20 khz 500 uh 13.2 uf j115 Ω DC link Capacitr 4500 uf 115 j115 Ω Battery Vltage 2*120 V Fig. 7 shws the blck diagram f the setup f a singlephase pwer system with the ES fr experimental verificatin. A. Operating Mdes f the ES with Batteries This experiment is dne under the cnditins: V s = 120 V (50 Hz), Z = j0 Ω, and Z s = j0 Ω. Withut the ES, the steady-state perating cnditin f the system is I = 0.6 A and I s = 0.24 A. The steady-state cnditin f the system with the ES activated will be cmpared t this riginal cnditin in rder t shw the effect f the ES n the current flw and pwer cnsumptin f the nn-critical lad. 1) Resistive Mde: In resistive mde, V es and I are in phase with nly real pwer being exchanged between the pwer surce and the ES. Meanwhile, V es reduces the amplitude f V (nn-critical lad vltage) and thus perfrms a lad pwer reductin. As shwn in Fig. 8, after the intrductin f V es = 43 V, the nn-critical lad current I 2144

7 drps frm 0.61 A t 0.38 A with n change in phase angle, signifying a reductin in real pwer cnsumptin. 5) Inductive Plus Resistive Mde: When the ES wrks in the inductive-plus-resistive mde, V es leads I by Here, V is reduced with the intrductin f V es, causing a reductin in pwer cnsumptin f the nn-critical lad. (Fig. 12) Figure 8. Captured wavefrms f ES wrking in the resistive mde. Measured RMS values: V es = 43 V; I = 0.38 A. 2) Negative-Resistive Mde: In Fig. 9, ES generates a vltage f V es = 40 V in ppsing phase t V s. As a result, I is increased frm 0.61 A t 0.78 A, leading t an increase in pwer cnsumptin f the nn-critical lad. This pwer is supplied by bth the pwer surce and the ES. Figure 12. Captured wavefrms f ES wrking in inductive plus resistive mde. Measured RMS values: V es = 42 V; I = 0.41 A. 6) Capactive plue Resistive Mde: When the ES wrks in the capacitive-plus-resisitive mde, V es lags I by Here, V is reduced with the intrductin f V es, causing a reductin in pwer cnsumptin f nn-critical lad. (Fig. 13) Figure 9. Captured wavefrms f ES wrking in the negative-resistive mde. Measured RMS values: V es = 40 V; I = 0.78 A. 3) Inductive Mde: In the inductive mde, V es is 90 leading I as shwn in Fig. 10. The nn-critical lad current is decreased t 0.36A, leading t a reductin in the pwer cnsumptin f the nn-critical lad. Figure 13. Captured wavefrms f ES wrking in capacitive plus resistive mde. Measured RMS values: V es = 42 V, I = 0.42 A. 7) Inductive Plus Negative-Resistive Mde: In the inductive-plus-negative-resistive mde, V es leads I by I is increased after the intrductin f V es, leading t a bst in the pwer cnsumptin f the nn-critical lad. Figure 10. Captured wavefrm f ES wrking in the inductive mde. Measured RMS values: V es = 62 V; I = 0.45 A. 4) Capacitive Mde: The capacitive mde ccurs when the phase f the ES is 90 lagging I. In Fig. 11, it is shwn under such a cnditin that I is reduced t 0.39A, resulting in a reductin in pwer cnsumptin f nn-critical lad. Figure 14. Captured wavefrms f ES wrking in inductive plus negative-resistive mde. Measured RMS values: V es = 52 V; I = 0.63 A. 8) Capactive plus Negative-Resistive Mde: In the capacitive-plus-negative-resistive mde, V es lags I by I is increased after the intrductin f V es, leading t a bst in the pwer cnsumptin f the nn-critical lad. Figure 11. Captured wavefrms f ES wrking in the capacitive mde. Measured RMS values: V es = 62 V; I = 0.42 A. Figure 15. Captured wavefrms f ES wrking in inductive plus negative-resistive mde. Measured RMS values: V es = 51 V; I = 0.65 A. 2145

8 B. PF Crrectin 2) Using the ES as a PFC fr an RL lad: The system has an equivalent resistiv-plus-inductive lad f Zequal = 88 + j41 = 97 26⁰Ω, which leads t a PF f (lagging). After the ES in inductive-plus-negative-resistive mde is intrduced, the phase angle f the line current I is reduced t 6, changing the PF t Figs. 17(a) and 17(b) give the captured wavefrms f the system with and withut perating the ES. Fig. 17(c) shws the crrespnding phase angles f VS and I. (a) (a) (b) (b) (c) Figure 16. (a) Withut ES fr capacitive plus resistive lad. Measured RMS values: VS = 120 V; I = 1.2 A; Ves = 0 V; I = 0.61 A. (b) With ES wrking as PFC fr capacitive plus resistive lad. Measured RMS values: VS = 120 V; I = 0.98 A; Ves = 70.5 V; I = 0.88 A. (c) Phase angles f line vltage and line current befre and after ES is turned n. 1) Using the ES as a PFC fr an RC lad: In this experiment, the system has an equivalent RC lad Zequal = 88 j41 = Ω, which gives a PF f (leading). After the ES is activated, the phase angle f I is reduced t 8, which crrects the PF t Figs. 16(a) and 16(b) give the captured wavefrms f the system with and withut perating the ES. Fig. 16(c) shws the crrespnding phase angles f Vs and I. (c) Figure 17. (a) Withut ES fr inductive plus resistive lad. Measured RMS values: VS = 120 V; I = 1.2 A; Ves = 0 V; I = 0.61A; (b) With ES wrking as PFC fr inductive plus resistive lad. Measured RMS values: VS = 120 V; I = 0.98 A; Ves = 76 V; I = 0.67 A. (c) Phase angles f line vltage and line current befre and after ES is turned n. 2146

9 VI. CONCLUSIONS The principles and peratins f the electric springs (ES) with DC-link capacitrs and with batteries are investigated. The riginal prpsal f the ES with capacitrs prvides reactive pwer cmpensatin fr mains vltage stabilizatin and autmatic nn-critical lad pwer variatin fr balancing pwer supply and lad demand. By replacing the DC-link capacitrs with batteries (r cnnecting the battery acrss the dc link capacitr f the inverter), the ES can perate in eight perating mdes, which enable the ES t prvide pwer factr crrectin. A detailed discussin int this aspect is prvided in this paper. It is shwn theretically that the ES with batteries is capable f perfrming line current regulatin as well as pwer factr crrectin. A design f an input current cntrller allwing the ES t perate like a pwer factr crrectr is presented and practically verified. REFERENCES [1] M. Parvania and M. Ftuhi-Firuzabad, Demand respnse scheduling by stchastic SCUC, IEEE Transactins n Smart Grid, vl. 1, n. 1, pp , Jun [2] M. Pedrasa, T. D. Spner, and I. F. MacGill, Scheduling f demand side resurces using binary particle swarm ptimizatin, IEEE Transactins n Pwer Systems, vl. 24, n. 3, pp , Aug [3] A. J. Cnej, J. M. Mrales, and L. Baring, Real-time demand respnse mdel, IEEE Transactins n Smart Grid, vl. 1, n. 3, pp , Dec [4] A. J. Rsce and G. Ault, Supprting high penetratins f renewable generatin via implementatin f real-time electricity pricing and demand respnse, IET Renewable Pwer Generatin, vl. 4, n. 4, pp , Jul [5] P. Palensky and D. Dietrich, Demand side management: demand respnse, intelligent energy systems, and smart lads, IEEE Transactins n Industrial Infrmatics, vl. 7, n. 3, pp , Aug [6] A. Mhsenian-Rad, V. W. S. Wng, J. Jatskevich, R. Schber, and A. Len-Garcia, Autnmus demand-side management based n gametheretic energy cnsumptin scheduling fr the future smart grid, IEEE Transactins n Smart Grid, vl. 1, n. 3, pp , Dec [7] A. Mhd, E. Ortjhann, and A. Schmelter, Challenges in integrating distributed energy strage systems int future smart grid, IEEE Sympsium n Industrial Electrnics, pp , 2008 [8] J. A. McDwall, Status and utlk f the energy strage market, PES 2007, Tampa, July [9] S. Y. R. Hui, C. K. Lee, and F. F. Wu, Electric springs - a new smart grid technlgy, IEEE Transactins n Smart Grid, vl. 3, n. 3, Sep [10] C. K. Lee, Hui, and S. Y. R. Hui, Reductin f energy strage requirements in future smart grid using electric springs, IEEE Transactin n Smart Grid, vl., n. 99, pp. 1-7, Apr [11] S. C. Tan, C. K. Lee, and S. Y. R. Hui, General steady-state analysis and cntrl principle f electric springs with active and reactive pwer cmpensatins, IEEE Trans. Pwer Electrn., vl. 28, n. 8, pp , Aug [12] J. Dixn, L. Mran, J. Rdriguez, and R. Dmke, Reactive pwer cmpensatin technlgies: state-f-the-art review, Prc. IEEE, vl.93, n. 12, pp , [13] H. Fujita and H.Akagi, The unified pwer quality cnditiner: The integratin f series active filters and shunt active filters, IEEE Pwer Electrnics Specialists Cnference, vl.1, pp , [14] L. Gyugyi, Reactive pwer generatin and cntrl by thyristr circuits, IEEE Trans. Ind. Appl., vl. IA-15, n. 5, pp , Sep./Oct [15] L. Gyugyi, R. Ott, and T. Putman, Principles and applicatins f static, thyristr-cntrlled shunt cmpensatrs, IEEE Trans. Pwer App. Syst., vl. PAS-97, n. 5, pp , Oct [16] Y. Sumi, Y. Harumt, T. Hasegawa, M. Yan, K. Ikeda, and T. Mansura, New static VAR cntrl using frce-cmmutated inverters, IEEE Trans. Pwer App. Syst., vl. PAS-100, n. 9, pp , Sep [17] N. Hingrani and L. Gyugyi, Understanding FACTS, Cncepts and Technlgy f Flexible AC Transmissin Systems. New Yrk: IEEE Press, [18] R. Grünbaum, M. Nrzian, and B. Thrvaldssn, FACTS pwerful systems fr flexible pwer transmissin, ABB Rev., pp. 4 17, May [19] H. Okayama, T. Fujii, S. Tamai, S. Jchi, M. Takeda, R. Hellested, and G. Reed, Applicatin and develpment cncepts fr a new transfrmer-less FACTS device: the Multimde Static Series Cmpensatr (MSSC), IEEE PES Cnf. Exp, [20] A. Edris, FACTS technlgy develpment: an update, IEEE Pwer Eng. Rev., vl. 20, n. 3, pp. 4 9, Mar [21] R. Grünbaum, Å. Peterssn, and B. Thrvaldssn, FACTS imprving the perfrmance f electrical grids, ABB Rev. (Special Reprt n Pwer Technlgies), pp ,