A Variable Structure Unified Power Flow Controller for Advanced Industrial Applications

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1 A Variable Structure Unied ower Flow Controller for Adanced ndustrial Applications T. T. Ma, Member, EEE AbstractThis paper presents a new application example of flexible ac transmission systems (FACTS) deices in the adanced industrial power control schemes. An integrated multifunctional power quality controller and energy saing system based on the unied power flow controller (UFC) is proposed. The proposed integrated system can compensate the reactie power, harmonic current and the unbalanced power of the controlled distribution system with a capacitylimitation manner by its shunt branch. t also can regulate multiple load oltages simultaneously based on the prescribed V characteristics of distinct loads for the purpose of energy saing ia its series branch. The detailed design procedure of the controllers proiding flexible hardware conguration and multiple control functions is presented. A 5KVA testsystem with the ariable structure UFC conguration performing actie power lter functions and simultaneously regulating an 7H induction motor for energy saing is numerically inestigated with EMT programs. The feasibility and effectieness of the proposed system and the related control techniques are conrmed through some simulation results. ndex Terms flexible ac transmission systems, power quality controller, unied power flow controller, energy saing, actie power lter, oltage regulation. NTRODUCTON n recent years FACTS technology has been moing forward rapidly since the introduction of highcapacity power electronic switches, highleel digital controllers and fast computer systems to the maret. Of those adanced FACTS deices the UFC is considered one of the newest and the most powerful FACT deices []. The basic hardware conguration of a UFC system can be seen as the combination of a STATCOM and a SSSC operating from a common dc capacitor. This particular arrangement is actually a practical realization of an ac to ac power conerter with independent reactie power control capability on its input and output terminals of the two inerter modules/branches. As what it was named, it has been well accepted that a UFC can perform almost all power flow control functions proided by any other FACTS deices with much better flexibility and control performance. Many application examples using FACTS deices to improe the performance of power system operation and control can be found in the literature []. Howeer, research results concerning practical applications of these adanced (or conerter based) FACTS deices in industrial control aspects hae not yet been found. TsaoTsung Ma is with the Department of Electrical Engineering, National United Uniersity, Miaoli City 36, Taiwan, R.O.C. (telephone: , tonyma@nuu.edu.tw). This paper aims to presents a new application example of the most ersatile flexible ac transmission system deice, UFC, in the eld of power quality control and energy saing. Based on a general report on the types of major loads and the electronics related industrial equipments used in our industrial sectors, there has been a continuous proliferation of nonlinear type of loads due to the intensie use of power electronic controllers in arious applications as well as those general consumptions of electric energy. As a result, the utility normally has to proide large reactie power to the load center so as to support a satised system oltage prole and maintain a secure system operating status. n fact, the punitie tariffs leied by utilities against excessie ars and the threat of stricter harmonic standards hae led to extensie research in the eld of load compensation. n this paper, a new control scheme based on a ariable structure UFC woring as a multifunctional power quality controller particularly designed for distribution leel operations (relatiely low oltage) is inestigated. Considering both demands on power quality control and energy saing requirements, we propose an integrated multifunctional control scheme in which a realtime power quality conditioner and arious energy saing functions can be performed by using a new UFC hardware conguration without increase the oerall system complexity too much. Although research results concerning the design of power conditioners employing the rectierinerter topology nown as the actie power lters (AF) for realtime ltering unwanted components of power system parameters hae been reported [37], in our new control scheme the UFC with multiple identical inerter modules and with the ariable structure (ratings) flexibility is utilized as a new controller topology. Fig. shows the conceptual UFC structure and routes of controlled power flow. As can be seen in Fig., the seriesbranch inerter of the UFC is connected in series with the power wires connecting arious loads for eliminating oltage flicer, unbalance and fluctuation, while its shuntbranch inerter can be operated as a shunt actie power lter to compensate the load reactie and harmonic powers as well as the unbalanced load currents. n this paper, the proposed control system can further adjust the load oltage from the iewpoint of energy saing. The optimal output oltage for achieing highest operating efciency is regulated by the seriesbranch of the UFC. t is obious that the oltage command for power saing is loaddependent and normally based on a set of prescribed load V characteristic cures. n practice the oltage command must be updated on a realtime basis to ensure that the desired optimal

2 Q T sh V sh = V dc Q dc T se V se Q Fig. The conceptual UFC structure and routes of controlled power flow operating efciency can be held when the load operating point is changing from time to time. This paper also presents a new method to nd the compromised optimal operating oltage for a compound load that may inole arious load types with different ratings, V characteristics and operating constraints. To ealuate the performance of the proposed control scheme, a threephase 3V power supply system equipped with a 5KVA UFC power conditioner is inestigated, in which the proposed UFC woring as an actie power lter and simultaneously regulating the terminal oltage of a 7 H threephase induction motor to achiee its optimal efciency operation for energy saing. A number of assumed system operating conditions are inestigated with comprehensie EMT simulations. Typical results are presented and discussed in full.. CONFGURATON OF THE ROOSED UFC CONTROL SCHEME The detailed UFC based multifunctional integrated power quality control scheme employing the bactobac inerter topology is shown in Fig.. The DClin with a common DC capacitor is designed to proide a path for bidirectionally flowing real power between the two inerter modules/branches of the UFC system. The seriesbranch inerter is connected in series with the load through a coupling transformer normally equipped with multiple secondary windings for regulating multiple loads with different V characteristics. The seriesbranch inerter is also responsible for alleiating the flicer and imbalance of the utility oltages such that the load oltages are low distorted, balanced and insensitie to external or internal load disturbances. The oltage regulator regulates the inerter oltages based on the utility oltages and the load oltage commands generated by the energy saing controller. Since distinct loads may hae distinct V characteristics the energy saing controller calculates the instantaneous consumed or output (mechanical power) power of each load and determines a compromised operating oltage for the combined load based on the minimum total input power criteria or the optimal operating efciency criteria. The turn ratio of the coupling transformer is selected such that all load oltages are within their allowable range and the compound load has the best oerall efciency with the UFC based power conditioner in operation. The shuntbranch inerter behaes as a shunt actie power lter. t regulates the DClin oltage by absorbing real power from the connected source terminal and recycling the real power from the seriesbranch inerter to feed the internal power losses of two inerter systems. t also supplies the load reactie power, harmonic current, unbalanced power such that the input currents on the utility side are balanced, sinusoidal and inphase with the oltages (unit power factor). To ensure a regulated DClin oltage and a secure operation of the controller, the output current components supplied by the shuntbranch inerter including the reactie power, harmonic current and unbalanced power must be limited. A capacitylimitation control technique proposed in [8] is adopted here for the design of the actie power lter controller. t should be noted that in practical implementation of the proposed controller the starting and fault protection circuits must be used to protect transient and fault conditions. The fault conditions may include oeroltage of the utility and the load, oercurrent of the inerter and the load and oertemperature. As a fault condition is detected, the protection circuits should be able to shutdown the trigger signals of power switches and bypass the seriesbranch inerter to ensure normal operation of the system. V uc i SC V ub i SB V ua i SA L V sh = V d dc phase b ob L L L phase c phase a V Voa V oc Fig. UFC based multifunctional integrated power quality controller oj i LC i LB i LA V ui

3 3. CONTROLLER DESGN FOR THE ROOSED UFC SYSTEM A. Modelling of the shuntbranch inerter f the sinusoidal WM switching technique is employed for the control of the shunt and seriesbranch inerters shown in Fig., neglecting the highfrequency switching terms, the power system parameters, control ariables and the two inerter circuits can be expressed by the following differential equations based on KVL. d L = pwmconi Vui (i = a, b or c) () dt doj L = pwmconj Voj (j = a, b, or c) () dt capj dvoj ' = C = oj Lj (j = a, b, or c) (3) dt where coni and conj (i = a, b or c; j = a, b or c) are the control oltages of the i and jleg and pwm is the gain of the threephase inerter, where V d pwm = (4) tm tm is the amplitude of the triangular waeform of the WM control. B. Controllers design of the shuntbranch inerter ) Current controller Fig. 3 shows the control bloc diagram of the currentloop designed with the control model of (). Where s is the sensing factor of the halleffect sensor. The current controller of each phase contains two parts, i.e. the feedforward controller and the feedbac controller. The feedforward controller use the synchronous signals to produce the feedforward control signal f (i = a, b, c) for eliminating the disturbance directly caused by the input oltages. Once the input oltage disturbance is eliminated almost by the feedforward control signal, the current tracing response can be determined with the feedbac control loop as: i i u = s u, u = (5) pwm s L where u is equialent to the bandwidth of the currentloop, which can be adjusted by the feedbac control gain. i i sini coni pwm sl fba ff s ffa V ui Fig. 3 Current controller of the shuntbranch inerter (i = a, b or c) ) Actie power lter controller For simplifying the control circuits, the controller design is better performed on the dq domain. As the current commands for the inner controllers are calculated by the actie power lter controllers, they must be transformed bac to the abc domain before entering the current control loop. The outerloop controllers in the daxis and the qaxis both include the reactie power, harmonic current and unbalanced power compensators. The control system also requires a oltage regulator designed for both axes. The limiter behind each compensator is then used to limit the respectie regulating capacity. t follows that the nal current commands in each axis are the sum of the current commands from all compensators and the oltage regulator. The detailed operation of the aboe addressed actie power lter can be obtained in [8]. C. Controllers design of the seriesbranch inerter ) Voltage regulator The oltage controller as shown in Fig. 4 can be deised with the aboe equations () and (3). The dualloops control method [8] with feed forward plus feedbac control technique can be employed to mae the output oltage and current of the seriesbranch inerter trac their commands closely. With both and 3 as constant gains the closedloop response of the current loop in the seriesbranch inerter can be approximated to be: µ / s s ' capj = icapj Lj, s s µ µ 3 pwm s = L µ (6) where µ can be seen as the bandwidth of the current loop. The oltage regulating response can be deried as: s us d ' oj =, oj C Lj s us d s us d ' = H ( s) oj Z o ( s) Lj oj oj o 9 phase shifter sc s / i capj pw m i capj Fig. 4 The oltage controller fj 3 pwm conj ower circuit s u d = C s sl oj capj ' Lj sc (7) ) Energy saing controller Assuming that there are two critical loads to be controlled for energy saing, the oltage controller must be able to real V oj

4 4 time regulate a load oltage based on the prescribed optimal V characteristics of the loads. As addressed preiously the optimal oltages for the two loads, V and V, are to be L L determined separately, the compromised optimal load oltage V L is then obtained with the following relationship: V L w V L w VL = (8) Where the weighting factor w i (i =, ) is set in proportional to its load power rating. The aim of the linear combination of (8) is to regulate two load oltages simultaneously with a single seriesbranch inerter. This compound oltage relies on the capacity as well as the operating power of each load to approximate the optimal operating oltage for energy saing. Finally, the inerter oltage command is obtained with the difference between the input oltage and the load oltage command. f the inerter output oltage can trac its command closely through the oltage regulator, then each load oltage will be: L i ui Ns / N poi L i ui Ns / N poi = (9) = () The transformer turn ratio gies another freedom for further optimizing the system efciency. V. EMT SMULATONS AND RESULTS A. AF control (UFC shuntbranch inerter) A 5KVA UFC power quality control system is designed and numerically studied in this paper. The system parameters are listed as below: nput oltages 3V/6Hz, V d = V, C d = 9 µf, =.8, =.34, L = L = 8mH, C = 4 µ F, ˆ = 8V/KHz, =, = 8, 3 =.5 To demonstrate the distinct functions and performances of the proposed UFC power quality controller, the load is assumed unbalanced, reactie and distorted. t composes a linear part (Z La = Ω 5mH, ZLb=5 Ω 4mH, ZLc=3 Ω ) plus a nonlinear part (a sixpulse threephase diode bridge with DCside connected to a LC lter (3.5mH µ F ) and a 45 Ω resistie load). All components of the load are designed within the limited capacity. The load is regulated by the seriesbranch inerter of the UFC through a 5: transformer. Fig. 5 (a) to (f) show a set of typical EMT simulation results. These gures indicate that with the AF functions of the UFC in operation the input currents are inphase with the oltage and sinusoidal. t conrms that the reactie power and harmonic current of the load hae been well compensated. The unbalanced load current is also compensated such that the input currents are balanced. These all demonstrate that the controllers designed s tri for the UFC based actie power lter are ery effectie. EMT simulation results of power system parameters on the utility side [s].3 (le UFC3V.pl4; xar t) t: VSA t: VSB t: VSC t: SA t: SB t: SC Fig. 5 (a) The currents and oltages (3phase) on utility side (f ile UFC3V.pl4; x ar t) t: VSC 4.4 EMT simulation results of power sy stem parameters on the load side t: LA t: AFA t: LB t: LC t: VSA 4.4 Fig. 5 (b) The currents and oltages (3phase) on load side (le UFC3V.pl4; x ar t) t: AFB t: AFC [s] t: VSB 4.4 EM T simulation results of the output currents from the UFC based AF branch Fig. 5 (c) The output currents of the UFC_AF branch [s] t: VSA 4.4

5 EMT simulation of current commands and practical output currents from the UFC_AF B. ower saing control (UFC seriesbranch inerter) To demonstrate the power saing control function and performance of the proposed controller, a 7H squirrelcage induction motor with the prescribed efciency characteristics (in terms of operating oltage and output power) as shown in Fig. 6 is used. Based on the design principle addressed preiously, the energy saing controller is then used to realtime optimize the motor operating efciency. Fig. 7 shows the results of a comparison study on the cases without and with the energy saing controller (f ile UFC3V.pl4; xar t) t: OAC t: AFB t: AFC t: OBC t: OCC [ms] t: AFA efcien cy V(p.u.) Fig. 5 (d) The current commands for the UFC_AF branch and the practical output currents.5. Output power (p.u.) Fig. 6 The prescribed efciency characteristics of the studied induction motor Fig. 5 (e) The current commands for the UFC_AF branch and the practical output currents Fig. 7 The input and load power of the cases without and with the energy saing control Fig. 5 (f) The current commands for the UFC_AF branch and the practical output currents By obsering the results shown in Fig. 7 and referring to the prescribed efciency characteristics of the studied induction motor shown in Fig. 6, one can erify the argument that the power saing control scheme is feasible and the designed oltage controller is ery effectie. n fact the result is also consistent with conclusion gien in [4] that the reduced oltage driing has higher efciency at the lightload conditions. t should be noted that in our design case integrating AF with power saing controller the efciency improement seems better than that achieed in [4]. This is mainly because a lower input VA required by the induction motor at a certain operating point can be achieed when the unity power factor and the sinusoidal and low distorted oltage waeform are offered by the compensated power source.

6 6 V. CONCLUSONS This paper has demonstrated the feasibility and effectieness of a new multifunctional power quality control scheme, in which control functions of actie power lter, oltage regulator and energy saer are integrated into a single power quality controller based on a ariable structure UFC. Based on the numerical results presented in this paper it seems that the proposed reduced oltage driing method has higher efciency only at the lightload conditions; howeer, in practice the induction motors abundantly used as the main driing system in many industrial and commercial applications are usually with high power rating. Therefore een a few percentage of efciency improement can produce a great impact on the longterm energy consumption. Based on the comprehensie simulation studies carried out in this paper, the possibility and potential of applying FACTS deices in the adanced power control schemes on the distribution leel is conrmed. REFERENCES [] Narain G. Hingorani, Flexible AC transmission, EEE Spectrum, April 993, pp [] A. Edries, FACTS Technology Deelopment: An Update, EEE ower Engineering Reiew, March,, pp [3] S. Bhattacharya and D. Dian, Actie lter solution for utility interface of industrial loads, in EDES 96, 996, pp [4] H. Tomita and T. Haneyoshi, An optimal efciency control for energy saing of ac motor by thyristor oltage controller, in EEE ECON, 988, pp [5] K. Chatterjee, B. G. Fernandes, and G. K. Dubey, An instantaneous reactie oltampere compensator and harmonic suppressor system, EEE Trans. on ower Electronics, Vol. 4, No., pp. 3839, 999. [6] H. Fujita and H. Aagi, The unied power quality conditioner: the integration of series and shunt actie lters, EEE Trans. On ower Electronics, Vol. 3, No., pp. 353, 998. [7] F. Kamran and T. G. Habetler, Combined deadbeat control of a seriesparallel conerter combination used as a uniersal power lter, EEE Trans. on ower Electronics, Vol. 3, No., pp. 668, 998. [8]. J. Chiang and W. J. Ai, arallel operation of threephase fourwire actie power lters without control interconnection, presented in EEE ESC. ACKNOWLEDGMENT The authors would lie to acnowledge the nancial support of the National Science Council in Taiwan, R.O.C., through its grant NSC 93ET739ET.