A New Voltage Sag and Swell Compensator Switched by Hysteresis Voltage Control Method

Transcription

1 Proceedings of he 8h WSEAS Inernaional Conference on ELECTRIC POWER SYSTEMS, HIGH VOLTAGES, ELECTRIC MACHINES (POWER '8) A New Volage Sag and Swell Compensaor Swiched by Hyseresis Volage Conrol Mehod AMIR AHMAD KOOLAIYAN, ABDOLREZA SHEIKHOLESLAMI, REZA AHMADI KORDKHEILI Elecrical and Compuer Deparmen Nooshirvani Insiue of Technology Shariai Avenue, Babol, Mazandaran IRAN Absrac: - This paper inroduces a new mehod for compensaing volage sag and swell occurring in disribuion sysems. Fauls occurring in power disribuion sysems generally cause he volage sag or swell. Sensiiviy o volage sags and swells varies wihin differen applicaions. These power qualiy disorions cause serious problems, especially for sensiive loads. The new mehod, which is based on an auoransformer and an IGBT swich, is conrolling he IGBT swich in faul condiions by hyseresis volage conrol mehod. The proposed scheme is able o quickly recognize he volage sag and swell condiions, and i can correc he volage eiher by increasing he inpu volage during sag condiions or decreasing he inpu volage during swell condiions. One of he advanages of his opology is using only one conrolled swich per phase o regulae he inpu volage. Differen volage sag and swell evens have been simulaed by MATLAB/Simulink package. The resuls of simulaions verify he abiliy of proposed mehod o correc volage sag and swell evens. Key-Words: - Volage sag and swell, Auoransformer, Hyseresis volage conrol, MATLAB/Simulink Inroducion Appearance of sensiive loads in indusrial, commercial and residenial applicaions has made power qualiy issues as an imporan concern in disribuion sysems [4]. Any disorion in power qualiy of disribuion sysems can cause indusrial processes inerruped and resul in huge economical losses. Volage sag and swell are he mos harmful disurbances presen in indusrial power sysems, being capable of severely damaging, breaking, or ripping sensiive loads. Volage swell is defined as an increase in he rms supply volage beween..8 p.u., and lasing from half a cycle o min (IEEE Sd. 59 swell definiion). They are usually produced by swiching off large loads, by energizing capacior banks, or by fauls produced wihin power sysems hemselves. A volage sag is defined as a momenary decrease of he volage rms value during.5-3 cycles. A sag can cause serious problem o sensiive loads ha use volage-sensiive componens such as adjusable speed drives, process conrol equipmen, and compuers [2]. In order o increase he reliabiliy of a power disribuion sysem, many mehods of solving power qualiy problems, have been suggesed. Many volage miigaion schemes are based on inverer sysems consising of energy sorage and power swiches. Exising mehods of volage swell miigaion using gae urn-off swiches for PWM need a leas wo swiches per phase. Oher mehods use a direc AC-AC converer opology. In addiion o requiring a leas wo swiches per phase, hey require energy soring reacive componens. In an effor o achieve he advanages of a fas ime response, a hyseresis volage conrol mehod is proposed here. The proposed sysem has only one power swich per phase wih no energy sorage. Any power elecronic swich for a high volage applicaion is expensive, and he peripheral circuis such as gae drivers and power supplies are even more expensive han he device iself. The overall cos of power elecronics-based equipmen is nearly linearly dependen on he overall number of swiches in he circui opology. This paper suggess a scheme wih a new conrol mehod ha uses only one power swich wih no energy sorage. Since fewer componens are required in his scheme, he sysem becomes more reliable and less expensive. In his paper, he analysis and design of he overall sysem and simulaion resuls are presened. 2 Sysem Configuraion AC converer echnologies have been proposed for AC oupu volage conrol [5]. The well-known DC o DC conversion echnology has been adaped o AC o AC conversion echnology. AC converers consis of wo solid-sae swiches per phase and require reacive elemens such as a capacior and an inducor. Since he curren in he AC converer flows in boh direcions, saic swiches and diodes are serially conneced o ISSN: ISBN:

2 Proceedings of he 8h WSEAS Inernaional Conference on ELECTRIC POWER SYSTEMS, HIGH VOLTAGES, ELECTRIC MACHINES (POWER '8) allow boh direcions curren. This paper suggess a new compensaion scheme wih a new conrol mehod. In his scheme, only one power swich is used [6]. The scheme configuraion is shown in Fig.. In his configuraion an auoransformer is used as a boosing ransformer insead of a wo-winding ransformer. The auoransformer in Fig.2 does no offer elecrical isolaion beween primary side and secondary side bu has advanages of high efficiency wih small volume. The relaionships of he auoransformer volage and curren are expressed in equaion (), where a is he urns raio. In his paper, a ransformer wih raio N : N2 = : is used o boos up o a 5% volage sag in sag condiions and o decrease he inpu volage during swell condiions. In his configuraion, he swich curren is wo imes he load curren. primary curren I L (low volage side). From equaion (), i is clear ha V 2V and I 2I. V a I, a N V I N N The basic configuraion of he proposed Mehod is presened in Fig.3. The model consiss of a single IGBT swich in a bridge configuraion, a hyrisor bypass swich, oupu filers, an auoransformer, and he sysem conroller. In normal condiions, he IGBT swich is off and he power flows hrough ani-parallel hyrisor as a bypass swich. This bypass swich connecs he inpu power o he load unless he sag condiion is presen. When volage sag or swell occurs, he bypass swich urns off and he swiching command urns on he IGBT swich. Uilizing auoransformer, he IGBT improves he oupu volage in such a way ha he volage across he load remains consan. Since he IGBT swich operaes only under sag or swell condiions, he efficiency of he sysem is high. However, swiching he power devices always produces noise in he sysem, which can cause he waveform having harmonic conens. To preven he swiching noise effec of IGBT and hyrisor on he load curren and volage waveforms, wo filers (a capacior filer and a noch filer) uilized. Fig. The main configuraion of sysem. 2 3 Kv Fig.2 General srucure of auoransformer. This volage compensaor works for only a few seconds and remains in he off-sae mos of is operaion ime. Remaining in he off-sae for mos of he ime, he swich mus wihsand he volage across i. Therefore, he volage across he swich becomes an imporan facor. The volage across he swich in he off-sae is equal o half of he inpu volage. As he urns raio equals :2, he magniude of he load curren I H (high volage side) is he same as ha of he Fig.3 Overall srucure of suggesed compensaor. When he swiches urn off, a high di/d occurs, which can damage he swiches. To keep he power swiches safe, an RC snubber circui is designed. The snubber circui of IGBT swich consiss of a resisor and a capacior. The snubber suppresses he peak volage across he IGBT swich when he IGBT urns off. When he IGBT swich urns off, he curren flowing in he IGBT in he on-sae insanly divers o ISSN: ISBN:

3 Proceedings of he 8h WSEAS Inernaional Conference on ELECTRIC POWER SYSTEMS, HIGH VOLTAGES, ELECTRIC MACHINES (POWER '8) he snubber circui. The energy sored in he curren pah is ransferred ino he snubber capacior and resisor. 3 Sysem Characerisics The sysem can be divided in six pars: he source, he load, he IGBT swich, he hyrisor bypass swich, he RC snubber circui, and he oupu filers. In his sysem, he magniude of he inpu hree-phase volage is 2 kv rms. So, he rms value of line o neural single-phase volage is.5 kv (6.3 kv peak). The load of sysem is a passive RL load ( MVA). The power facor of load is.8 (lag). 3. Oupu Filer To reduce harmonic componens of he oupu volage, wo filers are used. The firs one is a noch filer and he oher is a capacior filer. Usually less han 5% THD (Toal Harmonic Disorion) of he volage is accepable in power sysem. To selec he filer values, firsly he equivalen circui is derived as shown in Fig.4. In Fig.4, he oal effecive inducance L consising of he source and he leakage inducances of he ransformer represens 4L source +2L leakage. Fig.4 The equivalen circui of compensaor for filer design. From Fig.4, i can be observed ha he combinaion of he effecive inducance L and he oupu capacior filer named C filer form a low-pass filer. As he source and leakage inducance work as a low-pass filer, i seems large source impedance is preferred o reduce he harmonics. The leakage inducance helps o reduce he harmonics, bu he source inducance does no. The impedance of he filer is given by equaion (2). 2 To selec he capacior value of he noch filer, he common design rule is o choose he capacior kva abou 25%_3% of he oal raing kva. Since he oupu filer is always energized regardless of he operaion mode, i is desirable ha he oupu capacior has a lower capaciance. The capacior reacive power (VAR) is given by equaion (3). 3 Using equaion (3), he oal capaciance is obained as C=2µF. 4 Volage Deecion Mehod Volage deecion is imporan because i deermines he dynamic performance of volage compensaor. The magniude of oupu volage deermines he behavior of hyseresis volage conrol mehod and he saring momen of compensaion. Therefore, precise and fas volage deecion is an essenial par of he volage sag and swell supporer. Several volage deecion mehods have been documened for use in various volage compensaion schemes. Many approaches use he DQ ransformaion of he volage in he synchronous reference frame. If hree-phase parameers such as currens and volages are balanced, he value of he DQ ransformaion resuls in consan DC values. In addiion, he resuling DC values make he volage conroller design easier. The DQ ransformaion uses insananeous values. Therefore, he deecion ime is much faser han oher mehods such as average, rms, and peak deecion. However, if he faul is unbalanced, a ripple will appear in he DC componen. To solve his problem, i is necessary o use a filer. However, using he filer will cause delay in he volage deecion. So, his mehod is no appropriae for unbalance volage sag or swell evens. Moreover, his mehod requires he values of all phases. To conrol and deec he volage sag, he volage conrol scheme used in his paper requires only he peak values of oupu volage. Therefore, a simple mehod called he "peak deecion mehod" is used for deecing volage [2]. While he DQ ransformaion needs hree-phase informaion, he peak deecion mehod needs only a single phase value. The peak deecion mehod is implemened as shown in Fig.5, and equaion (4) forms he peak deecion value as follows. sin cos 4 The process of measuring he peak value can be explained as follows. The single-phase line-o-neural volage is measured, and he cosine value of his volage is deermined using a 9⁰ phase shifer. Assuming a fixed value (5Hz) for he line frequency, he 9⁰-shifed value can be found. The signal waveforms of The single-phase line-o-neural volage and he ISSN: ISBN:

4 Proceedings of he 8h WSEAS Inernaional Conference on ELECTRIC POWER SYSTEMS, HIGH VOLTAGES, ELECTRIC MACHINES (POWER '8) 9⁰-shifed value of his volage are shown in Fig.6. Boh componens of volage are squared and summed o yield. Obaining he square roo of resuls in he peak value of he deeced volage. 5 Hiseresis Volage Conrol Mehod A hyseresis volage conrol echnique is implemened wih a closed loop sysem where an error signal, e(), is used o deermine he swiching saes and o conrol he load volage [],[3]. e() is he difference beween he reference volage, V ref, and he acual volage, V acual. As shown in Fig.8, here are bands above and under he reference volage. When he error reaches o he upper limi, he volage ges forced o decrease and when he error reaches o he lower limi, he volage ges forced o increase. Fig.5 The block diagram of peak deecion mehod. 2 x 4 9-shifed Vinpu V - Vinpu Fig.6 The waveforms of V inpu and he 9⁰-shifed V inpu. The oupu value of peak deecion block is compared wih he inpu volage (V inpu ) in Fig.7. The comparison verifies he abiliy of mehod in deecing he peak value of he inpu signal in he leas possible ime. The deecion ime in his mehod used o be less han a quarer of a cycle. 2 x 4 Peak Deecion of Vinpu Fig.8 Hyseresis swiching mehod. The inpu signals of he conrol block are V inpu, V load and V ref. When a volage sag (swell) occurs, he magniude of he inpu volage decreases (increases). As menioned, he hyseresis mehod is based on he difference beween he acual volage and he reference volage. On he oher hand, his scheme is based on uilizing an auoransformer wih :2 urn raio. So, o compensae volage sag and keep he load volage consan, i is necessary o implemen half of nominal source volage in sag condiions o he primary of auoransformer. This will be done by IGBT swiching. So, he signal ½ V will play he role of acual volage in conrol block. Therefore, in his mehod he error signal ( e() ) is defined in equaion (5). Which is presened in equaion (6). 5 V Vinpu Fig.7 Peak deecion signal and V inpu versus ime. The simulink block of hyseresis volage conroller is shown in Fig.9. As shown in Fig.9, he acual signal (V acual ) acs as he inpu of decision block. If V acual is posiive, he upper pah of decision block produces IGBT swiching pulses, and if V acual is negaive, he lower pah of decision block produces IGBT swiching pulses. ISSN: ISBN:

5 Proceedings of he 8h WSEAS Inernaional Conference on ELECTRIC POWER SYSTEMS, HIGH VOLTAGES, ELECTRIC MACHINES (POWER '8) x Fig.9 The simulink block of hyseresis volage conroller. The swichig pulses produced by Relay and Relay2 are shown in Fig. and Fig.. Moreover, he IGBT swiching pulses, which is he sum of swiching pulses produced by Relay and Relay2, is shown in Fig.2. The oupu signals of he upper and lower pahs of decision block is shown in Fig.3 and Fig.4. In his figures, solid lines are he oupus of decision block, and dashed line is he reference volage ( V ref ) Fig.3 The oupu signals of he upper pah of decision block. Dashed line is V ref and solid line is V acual. x Fig.4 The oupu signals of he lower pah of decision block. Dashed line is V ref and solid line is V acual Fig. The swiching pulses produced by Relay Fig. The swiching pulses produced by Relay Fig.2 The IGBT swiching pulses. 6 Simulaion Resuls To verify he abiliy of he proposed mehod in compensaing volage sag and swell, differen simulaions have been carried ou by MATLAB sofware. In hese simulaions, he load of sysem is an R-L load. Volage sag evens wih deph of % o 5% have been simulaed. Moreover, %-3% volage swell was simulaed. In normal condiions, he IGBT swich is off and he conrol sysem ransfers he power o he load hrough hyrisor (bypass swich). When he faul (sag or swell) occurs, he conrol sysem of compensaor makes he hyrisor urn offf and commands he IGBT o urn on, and power flows hrough IGBT and auoransformer. The inpu signals of conrol block are V in pu, V load and V ref. Considering he load volage fed back o he conrol block, he hyseresis volage conroller swiches he IGBT. Swiching he IGBT will apply half of he desirable volage o he primary of auoransformer. Considering ha he urn raio of auoransformerr is :2, he oupu of auoransformer will be equal o he nominal volage ( kv rms). Fig.5 and Fig.6 show he resuls of simulaions for 4% volage sag and 2% volage swell. ISSN: ISBN:

6 Proceedings of he 8h WSEAS Inernaional Conference on ELECTRIC POWER SYSTEMS, HIGH VOLTAGES, ELECTRIC MACHINES (POWER '8) 2 x 4 The resuls of simulaions obviously confirm he abiliy of he proposed mehod o miigae volage sag and swell evens wih differen magniudes and differen duraions. Vinpu Vou x Fig.5 Inpu and oupu volage having 4% volage sag. Vinpu 2 x 4 7 Conclusion A new volage compensaor based on an auoransformer and an IGBT swiched by hyseresis volage conrol mehod has been presened in his paper. Hyseresis volage conrol mehod suggesed in his paper is a mehod in which he real volage of sysem compares wih a reference volage hrough a hyseresis band, and hyseresis band produces swiching signals of IGBT. Differen volage sag and swell evens have been simulaed by MATLAB/Simulink sofware. The resuls of simulaions verify he abiliy of proposed mehod o miigae volage sag and swell evens. References: [] I. Faouh A powerful and Efficien hyseresis PWM conrolled inverer, EPE Journal, vol. 4, no.4, December 994. [2] H. Y. Chu, H. L. Jou, and C. L. Huang, Transien response of a peak volage deecor for sinusoidal signals, IEEE Trans. Ind. Elecron., vol. 39, no., pp , Feb [3] K. M. Rahman, M. R. Khan, M. A. Choudhury, Variable band hyseresis curren conrollers for PWM volage source inverers, IEEE Trans. on Power Elecronics, vol. 2, no. 6, pp , November997. [4] M. H. J. Bollen, Undersanding power qualiy problems: volage sags and inerrupions, leee Press, New York, 2. [5] Bimal K.Bose, Modern Power Elecronics and AC Drives, Prenice-Hall Inc., 22. [6] D. M. Lee, T. G. Habeler, R. G. Harley, J. R. Rosron, and T. L. Keiser, A Volage Sag Supporer Uilizing a PWM-Swiched Auoransformer, in Proc. IEEE 24, Aachen, Germany,pp , Nov. 2 6, x 4 Vou Fig.6 Inpu and oupu volage having 2% volage swell. ISSN: ISBN: