MODELLING, ANALYSIS AND CONTROL OF UNIFIED POWER QUALITY CONDITIONER
Abstract: Power quality improvement of sensitive load by an unified
power quality conditioner (UPQC) is presented. The UPQC, consists of
back-to-back connected series and shunt active filters, is modeled using
state-space-averaging technique to analyse its behaviour. The
enhancement of shunt active filter performance is achieved by applying
moving time window method. The performance of UPQC under load
nonlinearities and source disturbance conditions is investigated using
simulation as well as experimental results
1. INTRODUCTION Poor power quality in a system could be due to different factors such as voltage sag, voltage swell, voltage outage and over correction of power factor and unacceptable levels of harmonics in the current and voltage [I]. Modem solution for poor power quality is to take advantage of advanced power electronics technology. Recent research efforts have been made towards utilizing a device called unified power quality conditioner (UPQC) to solve almost all power quality problems. This concept can be traced back to 1970's [4] and its main idea is to improve power quality from the point of load terminal installation. Such kind of UPQC combines series and shunt-connected active filters and offers the best possibilities to compensate voltage imbalance, sag and distortion [2][5]. Fig.1 shows the basic scheme of such a UPQC and this is the power quality conditioner considered in this paper. The UPQC is modeled with reference to a synchronously rotating d-q-0 reference axes. This transformation technique reveals the negative sequence, zero sequence, under voltage, overvoltage and other harmonic components present in the power supply. These non-ideal quantities are reflected as ac quantities having positive or negative sequence. A new moving-window method is suggested in predicting the fundamental Positive sequence components of the load current Compared to the traditional low pass filtering methods [2], the proposed method is seen to result in a more rapid dynamic response. Simulation as well as experimental results are presented to show the effectiveness of the UPQC in improving the supply's quality. MATHEMATICAL MODELING OF UPQC In this study, the power supply is assumed to be a three phase, three-wire system. The two active filters are composed of two 3-leg voltage source inverters (VSI). Functionally, the series filter is used to compensate for the voltage distortions while the shunt filter is needed to provide reactive power and counteract the harmonic current injected by the load [5]. Also, the voltage of the DC link capacitor is controlled to a desired value by the shunt active filter. There can be negative and zero sequence components in the supply when a voltage disturbance occurs. The DC link capacitor bank is divided into two groups connected in series. The neutrals of the secondary of both transformers are directly connected to the dc link midpoint. In this way, as the connection of both three-phase to compensate these quantities in the load current. In other words, if the load current of phase "a" is expressed a:iul. = I,, co s( at - 8, ) + I uh + =I I p m co s( c d) co s( b l )+ I ~ s i n w t is + dl iv c l + (I – d li)vc2 where L1and the leakage inductance and resistance of the series transformer, vC1 and vc2 are the voltages of dc link capacitors, d li is the switch duty ratio of the series active filter. Without loss of generality, the turns ratio of the transformer is assumed to be unity. I u l. k For the transformers is Y/Y o, zero sequence voltage appears in the primary winding of the series connected transformer in order to compensate for the zero sequence voltage of the supply system. No zero sequence current flows in the primary side of both transformers. It ensures the system current to be balanced when the voltage disturbance occurs .Assuming that the load is non-linear, the power system model considered in this paper can be divided into following units: the power supply system, series active filter and shunt active filter. These constituent members of the UPQC are modeled separately in this section. First consider the power supply - Ls- - Rs is – vi h d t(2) . . . 1I.S = I .I L - 1 . r subscript refers to a, b and c phases in the power system; L, and R, are the and resistance of the transmission line; e is source voltage; vi h is the output voltage of the series active filter; I is the line current; ii L is the load current and ii h is the output current of the shunt active filter respectively .For the series active filter, To perform the first two functions, the shunt active filter acts as a controlled current source and its output current should include harmonic, reactive and negative phase sequence components in order shunt active filter, It is clear that the current output of the shunt active filter should be: L2 2 = -R2iih - V ~ F + d2ivcl + (I - d2,)vC1 (4) I ,h = I l p n , sin 8, sin u t +- I Uh + I a r = I a L – I a h = I ]pm co s@, co s wt id .k (10) where L2and R t are the leakage inductance and Hence, the current from the source terminal will be resistance of the shunt-connected transformer, d Z is the switch duty ratio of the shunt active filter. The turns ratio of this transformer is also assumed to be unity. (1 1) This is a perfect, harmonic-free sinusoid and has the same phase angle as the phase "a" voltage at the load terminal. The power factor at F is unity. It means that the reactive power of load is not provided by the source. The dc bus capacitor ''l eges can be described by the equations (5) and (6): To perform the first two functions, the shunt active filter acts as a controlled current source and its output current should include harmonic, reactive and negative phase sequence components in order to compensate these quantities in the load current. In other words, if the load current of phase "a" is expressed a: I u l. = I,, co s( at - 8, ) + I u h + II p m co s c d co s B l + I ~ n w t s +I I n o L B n~ + C I a L k (9) (3) Di is d t v I h = L1- + R is + d l iv c l + (I - l i)vc2 where L1and the leakage inductance and resistance of the series transformer, vC1 and vc2 are the voltages of dc link capacitors, d i is the switch duty ratio of the series active filter. Without loss of generality, the turns ratio of the transformer is assumed to be unity. I u l .k For the shunt active filter, It is clear that the current output of the shunt active filter should be: L2 2 = -R2iih - V ~ F + d2ivcl + (I - d2,)vC1 (4) I ,h = I l p n , sin 8, sin u t +- I U h + I a r = I a L – I ah = I ]pm co s@, co s wt id .k (10) where L2and R t are the leakage inductance and Hence, the current from the source terminal will be resistance of the shunt-connected transformer, d Z is the switch duty ratio of the shunt active filter. The turns ratio of this transformer is also assumed to be unity. (1 1) This is a perfect, harmonic-free sinusoid and has the same phase angle as the phase "a" voltage at the load terminal. The power factor at F is unity. It means that the reactive power of load is not provided by the source. 111 UPQC CONTROL A. UPQC Operating Principles Distorted voltages in a 3-phase system may contain. negative phase sequence, zero phase sequence as well as harmonic components. The voltage of phase "a" at the point S shown in Fig. 1 can be expressed as, in general: In a later section, it will be shown how the series active filter can be designed to operate as a controlled voltage source whose output voltage would be automatically controlled according to equation (8). The shunt active filter performs the following functions: to provide compensation of the load harmonic currents to provide load reactive power demand to maintain the DC-link voltage to a desired level. to reduce voltage distortions . To perform the first two functions, the shunt active filter acts as a controlled current source and its output current should include harmonic, reactive and negative phase sequence components in order to compensate these quantities in the load current. In other words, if the load current of phase "a" is expressed a: I u l. = I,, co s ( at - 8, ) + I u h + =I I p m co s c d co s B l + I ~ s i n w t s +I I n o L B n~ + C I a L k (9) (3) d i is d t v I h = L1- + R l is +d l i v c l + (I –d l )vc2 where L1and the leakage inductance and resistance of the series transformer, vC1 and vc2 are the voltages of dc link capacitors, d l is the switch duty ratio of the series active filter. Without loss of generality, the turns ratio of the transformer is assumed to be unity. I u l .k For the shunt active filter, It is clear that the current output of the shunt active filter should be: L2 2 = -R2iih - V ~ F + d2ivcl + (I - d2,)vC1 (4ii. i, h = I l p n , sin 8, sin u t +- I Uh + I a r = I a L – I a h = I ]pm co s@, co s wt I d .k (10) where L2and t are the leakage inductance and hence, the current from the source terminal will be resistance of the shunt-connected transformer, d Z is the switch duty ratio of the shunt active filter. The turns ratio of this transformer is also assumed to be unity. (1 1) This is a perfect, harmonic-free sinusoid and has the same phase angle as the phase "a" voltage at the load terminal. The power factor at F is unity. It means that the reactive power of load is not provided by the source. B. UPQC Control It is clear from the above discussion that UPQC should first separate out the fundamental frequency positive sequence from the other components. Then it is necessary to control the outputs of the two active filters in the way shown in equations (8) and (10) in order to improve overall power quality at the load terminal. To solve the first problem, a synchronous d-q-0 reference frame is used. If the 3-phase voltages are unbalanced and contain harmonics, the transformation to the d-q-0 axes results in 191 For the proper functioning of a power supply system, it is desirable that the voltages at .the load terminal should be perfect sinusoids with constant amplitude. Even under a voltage disturbance, the load still requires a constant voltage. This means that when transformed to the d-q-0 axis, the load voltage become: V m is the rated or desired voltage at the load terminal. Only one value, V m, in the d-axis would be sufficient to represent the balanced, perfect sinusoidal, 3-phase voltages in the a b c frame. As a reference quantity, it is a known quantity and more suitable for use in UPQC control than that proposed in [3]. Therefore v d p should be maintained at 3/2V,,, while all the other components should be eliminated by the series active filter. Similar expression can be obtained for the currents: Unlike load voltage, load current can change according to the connected loads. Therefore, it is not possible to assign it a reference value. Instead, a new "moving time window" method is applied here to capture the active quantity of the fundamental positive sequence component which is expressed as a dc value in the d-axis. Furthermore, from equation (14), it is evident that the average of the other components, apart from I , , COS e, , in the d-axis is zero in one fundamental cycle period because all of them are harmonics of the fundamental. Therefore a time window with a width of 0.02 seconds (for 50 Hz system ) maybe selected to calculate the dc value. The calculation for the first fundamental cycle is L y id d, = ,, p nr After T n this, the window is moved forward. If the moving frequency is also 50 Hz, the delay caused by the calculation is 0.02s. However if the moving frequency is n times of 50 Hz, the delay will be O.O2/n seconds. As the window moving frequency increases, calculation delay becomes shorter but the frequency at which the data moving into and out of the window is higher. It may need longer computation time. Fortunately, in practical power systems, Fig. 2 Control Scheme for: (a) Series Active filter, (b) Shunt Active Filter load current changes slowly. As a compromise, 500 Hz is selected as the window moving frequency in this paper. The two voltage-source inverters (VSIs) are used as the series and shunt active filters and these are controlled using the pulse-width-modulated (PWM) scheme. The series active filter should behave as a controlled voltage source and its output voltage should follow the pattern of voltage given in equation (8). This compensating voltage signal can be obtained by comparing the actual load terminal voltage with the desired value v F* as shown in Fig .Z(a). Since the desired VF* is already defined, it is easy calculate V h (= v F* - v s )as VF is a known unity. After obtaining the voltage signal v h , the switching duty ratio of the series active filter is obtained by comparing the reference signal V h with a triangular waveform, which is the traditional PWM control method The shunt active filter acts as a controlled current source. It means that the inverter operates in the current-regulated modulation mode. There are various ways to control the inverter in such a mode, such as the hysteresis control, and predictive control [7]. Here, the sub oscillation control is used. Its basic structure is shown in Fig.2(b). Three PI controllers are used to control I h(d ,q ,O) SO as to follow the reference current I h*(d ,q ,O). The output is added to v(d ,q ,O)a n d compared with a triangular waveform to form a control signal for the switches in the shunt active filter. C. Control of the DC-link Bus Capacitor Voltage The dc-link capacitor is divided into two units connected in series, c l and c2. The voltages v C l and vc2 are such that c l = -vc2 under balanced operating conditions. Usually, the DC link voltage is maintained at a desired value under all operating conditions. Supply voltage Fundamental load current Load displacement factor It can be shown that apart from the power loss due to line and winding resistances, a certain amount It can be shown that apart from the power loss due to line and winding resistances, a certain amount of power needs to be supplied to or absorbed by the capacitor to restore a voltage during a voltage disturbance. For example, if a voltage sag occurs in phase "a", v a h is higher than the normal value, the dc-link capacitors will supply the power through the series active filter. In the proposed scheme, a PI controller is used to control the capacitor voltage. Fig.3 shows the basic control scheme. The input to the PI controller is the error between the actual capacitor voltage and its desired value. The output of the PI controller is added to the reference current component in the d axis, id:, to form the new reference current id*. It means that the power needed to charge the two capacitors comes from the active power of the power supply system. The shunt active filter acts like a regulator. Its currents are used to adjust the capacitor voltages to within a certain range. Here only one PI controller is used to control the two capacitor voltages. Although these voltages will not be symmetrical when the system is unbalanced, which is caused by the zero sequence current in the UPQC, simulation results obtained so far have shown satisfactory performance of the closed loop system IV. NUMERICAL EXAMPLE The simulation results for the UPQC feeding a nonlinear load is presented. The parameters used are shown in Table TABLE I A. Simulation Results The UPQC system is modeled using the SIMULINK software package. Fig.4 shows the simulation results. A voltage sag is introduced in the source voltage e ,for a duration of about 7 cycles and the amplitude decreases by 50 %. As can be seen from the voltage waveform v F , it is compensated almost without any delay. The voltage v h is the compensating quantity supplied by the series active filter. During the sag, v a h is larger than the pre-sag value but V b h and V c h remain unchanged. It ensures a balanced load terminal voltage. It can also be noted that there exist itching frequency harmonics and two impulses at the beginning and end of the voltage disturbance. The two impulses result from the calculation delay. However the impulses and the switching frequency harmonics can be eliminated readily using modestly-rated passive filters [S I .These filters are necessary for the PWM inverter but have not been considered in the simulation so as to simplify the system models. M O The load current, I L contains significant 5th , 7th and1 I t h harmonics so as to simulate a rectifier load. With the shunt active filter, the current in the transmission line, is seen to be sinusoidal and harmonic free as shown in Fig. 4. The amplitude of 5th ,7th and 1 l t h harmonics in I L are 4.4 A, 3.2A and 2A respectively. From the harmonic spectra inFig.5, it can be seen that with the shunt active filter, the line current harmonics have become negligible. From the FFT of v,, it is clear that there is itching frequency harmonic of 6 kHz. It also appears in the currents and can be eliminated by the small passive filters installed at the output terminal of the shunt active filter. The instantaneous reactive and active power are calculated by applying the p q theory [6]. The results are shown in Fig. 6 and 7. The quantities q, , q L , q v h and q ,h represent the reactive power supplied by the source, drawn power supply system. Thus the load reactive power is compensated by the UPQC effectively .Next, p s is the active power supplied by the source, p~ is the power absorbed by the load (negative value indicates power absorption), p v h is the power supplied by the series active filter and Pi h is supplied by the shunt active filter. Prior to the voltage sag, (from 0.2 second to 0.3 second),p v h and Pi h are very small. These are the power from the source to balance the power losses in the UPQC in order to maintain the de capacitor voltage. During the voltage sag the series active filter supplies the power to restore the voltage at the load terminal. Meanwhile, the shunt active filter absorbs the power from the source to charge the dc link capacitors. Therefore the dc bus capacitors act as a power regulator. It can be seen from Figure 8 that the capacitor voltage can be kept to within a certain range even if the voltage imbalance lasts for a considerable length of time. It is also noted that the average values in p s and P I h change in the opposing directions against each other during and after the sag. This is caused by the control of dc capacitor voltage by ,the PI controller. Figure 8 also shows that the capacitor voltages behave similar to p s and ph during transient. In other words, the PI controller acts to cause the effect on the average values of p s and P I h . V. HARDWARE IMPLEMENTATION Since voltage distortions are of prime concern in practical use, only the series active filter is implemented to illustrate the capability of the UPQC. The experimental circuit is shown in figure 9. The controller is implemented on a TI320C31 DSP based hardware system. The inverter uses IGBTs as switching elements whose voltage rating is 600 V, current rating is 100 A. A three-phase rectifier is connected parallel to supply to maintain DC link voltage instead of the shunt active filter. Table I1 shows the parameters of the experimental circuit. A. Experimental Results During the experiment, the voltage amplitude in source terminal of phase a is deliberately reduced by 50% for 200 ms to simulate a single-line fault in the supply as shown in Fig. 10. This figure also shows the load terminal voltage waveforms in phases a and b. From this result, it is clear that the load voltage in phase a is not affected by the voltage sag on the source side. The voltage is restored very quickly by the proposed UPQC. At the begining and end of the sag, as shown by Fig.11, it can be seen that there are some slight distortions in the load voltage. They are mainly caused by the system delay, which is about 0.2 ms in this experimental system. Fig.12 shows the current from the AC supply during the voltage sag. DC bus voltage can be kept constant because Fig. 11 The Source and Load voltage The Source and Load voltages in Phase A at the Beginning of Sag Phase A at the End of Sag the DC bus capacitors are of relatively large capacitance and the duration of voltage sag is short. The source current increases over this period because additional power is needed to restore the load voltage and it comes from the AC supply. The experimental results seem to agree quite well with the simulation results. VI. CONCLUSIONS The following conclusions can be drawn from this investigation: 0 The UPQC considered in this project is a multifunction power conditioner which can be used to compensate for various voltage disturbance of the power supply, to correct any voltage fluctuation and to prevent the harmonic load current from entering the power system. 0 The proposed direct compensation control method used in the series active filter and the moving window current calculation method used in the shunt active filter voltage change make the UPQC response very quickly to any sudden Fig. 12 Source Voltage and Current 0 The simulation results show that during the voltage sag period, the series active filter will supply an active power which is drawn from the shunt active filter through the dc bus capacitors from the source The control scheme used in the shunt active filter not only keeps the voltage of the dc bus capacitor to within a certain range but also causes the capacitor charging current to be obtained from the active component of the supply current. Under normal operating conditions, this capacitor charging current will be minimal. 0 The experimental results have shown that the proposed scheme is feasible although further work is needed to optimize the parameters of the UPQC. The simulation program developed can be used toward achieving this. 0 The simulation results show that during the voltage sag period, the series active filter will supply an active power which is drawn from the shunt active filter through the dc bus capacitors from the source. The control scheme used in the shunt active filter not only keeps the voltage of the dc bus capacitor to within a certain range but also causes the capacitor charging current to be obtained from the active component of the supply current. Under normal operating conditions, this capacitor charging current will be minimal. 0 The experimental results have shown that the proposed scheme is feasible although further work is needed to optimize the parameters of the UPQC. The simulation program developed can be used toward achieving this. VII. REFERENCES [l] Narain G., Hingorani, "Introducing Custom Power", I€E€ SPECTRUM, June 1995, pp.41-48. [2] F.Z.Peng, H.Akagi, A.Nabae, "Compensation Characteristics of the Combined System of Shunt Passive and Series Active Filters". I€€€ U S Annual Meeting 1989, pp.959-966. [3] G. Blajszaak, "Direct Method For Voltage Distortion Compensation In power Networks By Series Converter Filter", IEE froc.-Elecfr. Powr Appl., 1995, pp.308-312. [4] Himfumi Akagi, Hideaki Fujita, "A New Power Line Conditioner for Harmonic Compensation in Power Systems". IEEE Transacrions Power Delivery, Vol. 10, No. 3, July 1995 [SI Mauricio Aredes, etc. "A Combined Series and Shunt Active Power Filter", IEEWKTH Stockholm Power Tech Conference, Stockholm. Sweden, June 18-22, 1995. [6] Edson H. Watanabe, Richard M. Stephan, etc. "New Concepts of Instantaneous Active and Reactive Power in Electrical System With Generic Loads", IEEE Transactions on Power Delivery, Vol. 8, No. 2 April 1993. [7] Joachim Holtz, Bemd Beyer, "Fast Current Trajectory Tracking Control Based on Synchronous Optimal Pusewidth Modulation", IEEE IAS Annual Meeting 1994, pp. 734-741. [8] Ned Mohan, Tore M. Undeland, William P. Robbins, foiwr Electronics: Converters. Applications and Design, John Wiley \& Sons, Inc., 1989. (91 Subhashish Bhattacharya, Deepak Divan, "Synchronous Frame Based Controller Implementation For a Hybrid Series Active Filter System". IEEE IAS Annual Meeting I99S, pp.253 1-2540. BIOGRAPHS B.Naga Lakshmi Prasanna (08L41A0258) studying b. tech final year (e e e) in mekapati raja mohan reddy institute of science and technology, udayagiri Nellore(dist) P.M.pavithra(08L41A0259) studying b. tech final year (e e e) in mekapati raja mohan reddy institute of science and technology, udayagiri Nellore(dist)
1. INTRODUCTION Poor power quality in a system could be due to different factors such as voltage sag, voltage swell, voltage outage and over correction of power factor and unacceptable levels of harmonics in the current and voltage [I]. Modem solution for poor power quality is to take advantage of advanced power electronics technology. Recent research efforts have been made towards utilizing a device called unified power quality conditioner (UPQC) to solve almost all power quality problems. This concept can be traced back to 1970's [4] and its main idea is to improve power quality from the point of load terminal installation. Such kind of UPQC combines series and shunt-connected active filters and offers the best possibilities to compensate voltage imbalance, sag and distortion [2][5]. Fig.1 shows the basic scheme of such a UPQC and this is the power quality conditioner considered in this paper. The UPQC is modeled with reference to a synchronously rotating d-q-0 reference axes. This transformation technique reveals the negative sequence, zero sequence, under voltage, overvoltage and other harmonic components present in the power supply. These non-ideal quantities are reflected as ac quantities having positive or negative sequence. A new moving-window method is suggested in predicting the fundamental Positive sequence components of the load current Compared to the traditional low pass filtering methods [2], the proposed method is seen to result in a more rapid dynamic response. Simulation as well as experimental results are presented to show the effectiveness of the UPQC in improving the supply's quality. MATHEMATICAL MODELING OF UPQC In this study, the power supply is assumed to be a three phase, three-wire system. The two active filters are composed of two 3-leg voltage source inverters (VSI). Functionally, the series filter is used to compensate for the voltage distortions while the shunt filter is needed to provide reactive power and counteract the harmonic current injected by the load [5]. Also, the voltage of the DC link capacitor is controlled to a desired value by the shunt active filter. There can be negative and zero sequence components in the supply when a voltage disturbance occurs. The DC link capacitor bank is divided into two groups connected in series. The neutrals of the secondary of both transformers are directly connected to the dc link midpoint. In this way, as the connection of both three-phase to compensate these quantities in the load current. In other words, if the load current of phase "a" is expressed a:iul. = I,, co s( at - 8, ) + I uh + =I I p m co s( c d) co s( b l )+ I ~ s i n w t is + dl iv c l + (I – d li)vc2 where L1and the leakage inductance and resistance of the series transformer, vC1 and vc2 are the voltages of dc link capacitors, d li is the switch duty ratio of the series active filter. Without loss of generality, the turns ratio of the transformer is assumed to be unity. I u l. k For the transformers is Y/Y o, zero sequence voltage appears in the primary winding of the series connected transformer in order to compensate for the zero sequence voltage of the supply system. No zero sequence current flows in the primary side of both transformers. It ensures the system current to be balanced when the voltage disturbance occurs .Assuming that the load is non-linear, the power system model considered in this paper can be divided into following units: the power supply system, series active filter and shunt active filter. These constituent members of the UPQC are modeled separately in this section. First consider the power supply - Ls- - Rs is – vi h d t(2) . . . 1I.S = I .I L - 1 . r subscript refers to a, b and c phases in the power system; L, and R, are the and resistance of the transmission line; e is source voltage; vi h is the output voltage of the series active filter; I is the line current; ii L is the load current and ii h is the output current of the shunt active filter respectively .For the series active filter, To perform the first two functions, the shunt active filter acts as a controlled current source and its output current should include harmonic, reactive and negative phase sequence components in order shunt active filter, It is clear that the current output of the shunt active filter should be: L2 2 = -R2iih - V ~ F + d2ivcl + (I - d2,)vC1 (4) I ,h = I l p n , sin 8, sin u t +- I Uh + I a r = I a L – I a h = I ]pm co s@, co s wt id .k (10) where L2and R t are the leakage inductance and Hence, the current from the source terminal will be resistance of the shunt-connected transformer, d Z is the switch duty ratio of the shunt active filter. The turns ratio of this transformer is also assumed to be unity. (1 1) This is a perfect, harmonic-free sinusoid and has the same phase angle as the phase "a" voltage at the load terminal. The power factor at F is unity. It means that the reactive power of load is not provided by the source. The dc bus capacitor ''l eges can be described by the equations (5) and (6): To perform the first two functions, the shunt active filter acts as a controlled current source and its output current should include harmonic, reactive and negative phase sequence components in order to compensate these quantities in the load current. In other words, if the load current of phase "a" is expressed a: I u l. = I,, co s( at - 8, ) + I u h + II p m co s c d co s B l + I ~ n w t s +I I n o L B n~ + C I a L k (9) (3) Di is d t v I h = L1- + R is + d l iv c l + (I - l i)vc2 where L1and the leakage inductance and resistance of the series transformer, vC1 and vc2 are the voltages of dc link capacitors, d i is the switch duty ratio of the series active filter. Without loss of generality, the turns ratio of the transformer is assumed to be unity. I u l .k For the shunt active filter, It is clear that the current output of the shunt active filter should be: L2 2 = -R2iih - V ~ F + d2ivcl + (I - d2,)vC1 (4) I ,h = I l p n , sin 8, sin u t +- I U h + I a r = I a L – I ah = I ]pm co s@, co s wt id .k (10) where L2and R t are the leakage inductance and Hence, the current from the source terminal will be resistance of the shunt-connected transformer, d Z is the switch duty ratio of the shunt active filter. The turns ratio of this transformer is also assumed to be unity. (1 1) This is a perfect, harmonic-free sinusoid and has the same phase angle as the phase "a" voltage at the load terminal. The power factor at F is unity. It means that the reactive power of load is not provided by the source. 111 UPQC CONTROL A. UPQC Operating Principles Distorted voltages in a 3-phase system may contain. negative phase sequence, zero phase sequence as well as harmonic components. The voltage of phase "a" at the point S shown in Fig. 1 can be expressed as, in general: In a later section, it will be shown how the series active filter can be designed to operate as a controlled voltage source whose output voltage would be automatically controlled according to equation (8). The shunt active filter performs the following functions: to provide compensation of the load harmonic currents to provide load reactive power demand to maintain the DC-link voltage to a desired level. to reduce voltage distortions . To perform the first two functions, the shunt active filter acts as a controlled current source and its output current should include harmonic, reactive and negative phase sequence components in order to compensate these quantities in the load current. In other words, if the load current of phase "a" is expressed a: I u l. = I,, co s ( at - 8, ) + I u h + =I I p m co s c d co s B l + I ~ s i n w t s +I I n o L B n~ + C I a L k (9) (3) d i is d t v I h = L1- + R l is +d l i v c l + (I –d l )vc2 where L1and the leakage inductance and resistance of the series transformer, vC1 and vc2 are the voltages of dc link capacitors, d l is the switch duty ratio of the series active filter. Without loss of generality, the turns ratio of the transformer is assumed to be unity. I u l .k For the shunt active filter, It is clear that the current output of the shunt active filter should be: L2 2 = -R2iih - V ~ F + d2ivcl + (I - d2,)vC1 (4ii. i, h = I l p n , sin 8, sin u t +- I Uh + I a r = I a L – I a h = I ]pm co s@, co s wt I d .k (10) where L2and t are the leakage inductance and hence, the current from the source terminal will be resistance of the shunt-connected transformer, d Z is the switch duty ratio of the shunt active filter. The turns ratio of this transformer is also assumed to be unity. (1 1) This is a perfect, harmonic-free sinusoid and has the same phase angle as the phase "a" voltage at the load terminal. The power factor at F is unity. It means that the reactive power of load is not provided by the source. B. UPQC Control It is clear from the above discussion that UPQC should first separate out the fundamental frequency positive sequence from the other components. Then it is necessary to control the outputs of the two active filters in the way shown in equations (8) and (10) in order to improve overall power quality at the load terminal. To solve the first problem, a synchronous d-q-0 reference frame is used. If the 3-phase voltages are unbalanced and contain harmonics, the transformation to the d-q-0 axes results in 191 For the proper functioning of a power supply system, it is desirable that the voltages at .the load terminal should be perfect sinusoids with constant amplitude. Even under a voltage disturbance, the load still requires a constant voltage. This means that when transformed to the d-q-0 axis, the load voltage become: V m is the rated or desired voltage at the load terminal. Only one value, V m, in the d-axis would be sufficient to represent the balanced, perfect sinusoidal, 3-phase voltages in the a b c frame. As a reference quantity, it is a known quantity and more suitable for use in UPQC control than that proposed in [3]. Therefore v d p should be maintained at 3/2V,,, while all the other components should be eliminated by the series active filter. Similar expression can be obtained for the currents: Unlike load voltage, load current can change according to the connected loads. Therefore, it is not possible to assign it a reference value. Instead, a new "moving time window" method is applied here to capture the active quantity of the fundamental positive sequence component which is expressed as a dc value in the d-axis. Furthermore, from equation (14), it is evident that the average of the other components, apart from I , , COS e, , in the d-axis is zero in one fundamental cycle period because all of them are harmonics of the fundamental. Therefore a time window with a width of 0.02 seconds (for 50 Hz system ) maybe selected to calculate the dc value. The calculation for the first fundamental cycle is L y id d, = ,, p nr After T n this, the window is moved forward. If the moving frequency is also 50 Hz, the delay caused by the calculation is 0.02s. However if the moving frequency is n times of 50 Hz, the delay will be O.O2/n seconds. As the window moving frequency increases, calculation delay becomes shorter but the frequency at which the data moving into and out of the window is higher. It may need longer computation time. Fortunately, in practical power systems, Fig. 2 Control Scheme for: (a) Series Active filter, (b) Shunt Active Filter load current changes slowly. As a compromise, 500 Hz is selected as the window moving frequency in this paper. The two voltage-source inverters (VSIs) are used as the series and shunt active filters and these are controlled using the pulse-width-modulated (PWM) scheme. The series active filter should behave as a controlled voltage source and its output voltage should follow the pattern of voltage given in equation (8). This compensating voltage signal can be obtained by comparing the actual load terminal voltage with the desired value v F* as shown in Fig .Z(a). Since the desired VF* is already defined, it is easy calculate V h (= v F* - v s )as VF is a known unity. After obtaining the voltage signal v h , the switching duty ratio of the series active filter is obtained by comparing the reference signal V h with a triangular waveform, which is the traditional PWM control method The shunt active filter acts as a controlled current source. It means that the inverter operates in the current-regulated modulation mode. There are various ways to control the inverter in such a mode, such as the hysteresis control, and predictive control [7]. Here, the sub oscillation control is used. Its basic structure is shown in Fig.2(b). Three PI controllers are used to control I h(d ,q ,O) SO as to follow the reference current I h*(d ,q ,O). The output is added to v(d ,q ,O)a n d compared with a triangular waveform to form a control signal for the switches in the shunt active filter. C. Control of the DC-link Bus Capacitor Voltage The dc-link capacitor is divided into two units connected in series, c l and c2. The voltages v C l and vc2 are such that c l = -vc2 under balanced operating conditions. Usually, the DC link voltage is maintained at a desired value under all operating conditions. Supply voltage Fundamental load current Load displacement factor It can be shown that apart from the power loss due to line and winding resistances, a certain amount It can be shown that apart from the power loss due to line and winding resistances, a certain amount of power needs to be supplied to or absorbed by the capacitor to restore a voltage during a voltage disturbance. For example, if a voltage sag occurs in phase "a", v a h is higher than the normal value, the dc-link capacitors will supply the power through the series active filter. In the proposed scheme, a PI controller is used to control the capacitor voltage. Fig.3 shows the basic control scheme. The input to the PI controller is the error between the actual capacitor voltage and its desired value. The output of the PI controller is added to the reference current component in the d axis, id:, to form the new reference current id*. It means that the power needed to charge the two capacitors comes from the active power of the power supply system. The shunt active filter acts like a regulator. Its currents are used to adjust the capacitor voltages to within a certain range. Here only one PI controller is used to control the two capacitor voltages. Although these voltages will not be symmetrical when the system is unbalanced, which is caused by the zero sequence current in the UPQC, simulation results obtained so far have shown satisfactory performance of the closed loop system IV. NUMERICAL EXAMPLE The simulation results for the UPQC feeding a nonlinear load is presented. The parameters used are shown in Table TABLE I A. Simulation Results The UPQC system is modeled using the SIMULINK software package. Fig.4 shows the simulation results. A voltage sag is introduced in the source voltage e ,for a duration of about 7 cycles and the amplitude decreases by 50 %. As can be seen from the voltage waveform v F , it is compensated almost without any delay. The voltage v h is the compensating quantity supplied by the series active filter. During the sag, v a h is larger than the pre-sag value but V b h and V c h remain unchanged. It ensures a balanced load terminal voltage. It can also be noted that there exist itching frequency harmonics and two impulses at the beginning and end of the voltage disturbance. The two impulses result from the calculation delay. However the impulses and the switching frequency harmonics can be eliminated readily using modestly-rated passive filters [S I .These filters are necessary for the PWM inverter but have not been considered in the simulation so as to simplify the system models. M O The load current, I L contains significant 5th , 7th and1 I t h harmonics so as to simulate a rectifier load. With the shunt active filter, the current in the transmission line, is seen to be sinusoidal and harmonic free as shown in Fig. 4. The amplitude of 5th ,7th and 1 l t h harmonics in I L are 4.4 A, 3.2A and 2A respectively. From the harmonic spectra inFig.5, it can be seen that with the shunt active filter, the line current harmonics have become negligible. From the FFT of v,, it is clear that there is itching frequency harmonic of 6 kHz. It also appears in the currents and can be eliminated by the small passive filters installed at the output terminal of the shunt active filter. The instantaneous reactive and active power are calculated by applying the p q theory [6]. The results are shown in Fig. 6 and 7. The quantities q, , q L , q v h and q ,h represent the reactive power supplied by the source, drawn power supply system. Thus the load reactive power is compensated by the UPQC effectively .Next, p s is the active power supplied by the source, p~ is the power absorbed by the load (negative value indicates power absorption), p v h is the power supplied by the series active filter and Pi h is supplied by the shunt active filter. Prior to the voltage sag, (from 0.2 second to 0.3 second),p v h and Pi h are very small. These are the power from the source to balance the power losses in the UPQC in order to maintain the de capacitor voltage. During the voltage sag the series active filter supplies the power to restore the voltage at the load terminal. Meanwhile, the shunt active filter absorbs the power from the source to charge the dc link capacitors. Therefore the dc bus capacitors act as a power regulator. It can be seen from Figure 8 that the capacitor voltage can be kept to within a certain range even if the voltage imbalance lasts for a considerable length of time. It is also noted that the average values in p s and P I h change in the opposing directions against each other during and after the sag. This is caused by the control of dc capacitor voltage by ,the PI controller. Figure 8 also shows that the capacitor voltages behave similar to p s and ph during transient. In other words, the PI controller acts to cause the effect on the average values of p s and P I h . V. HARDWARE IMPLEMENTATION Since voltage distortions are of prime concern in practical use, only the series active filter is implemented to illustrate the capability of the UPQC. The experimental circuit is shown in figure 9. The controller is implemented on a TI320C31 DSP based hardware system. The inverter uses IGBTs as switching elements whose voltage rating is 600 V, current rating is 100 A. A three-phase rectifier is connected parallel to supply to maintain DC link voltage instead of the shunt active filter. Table I1 shows the parameters of the experimental circuit. A. Experimental Results During the experiment, the voltage amplitude in source terminal of phase a is deliberately reduced by 50% for 200 ms to simulate a single-line fault in the supply as shown in Fig. 10. This figure also shows the load terminal voltage waveforms in phases a and b. From this result, it is clear that the load voltage in phase a is not affected by the voltage sag on the source side. The voltage is restored very quickly by the proposed UPQC. At the begining and end of the sag, as shown by Fig.11, it can be seen that there are some slight distortions in the load voltage. They are mainly caused by the system delay, which is about 0.2 ms in this experimental system. Fig.12 shows the current from the AC supply during the voltage sag. DC bus voltage can be kept constant because Fig. 11 The Source and Load voltage The Source and Load voltages in Phase A at the Beginning of Sag Phase A at the End of Sag the DC bus capacitors are of relatively large capacitance and the duration of voltage sag is short. The source current increases over this period because additional power is needed to restore the load voltage and it comes from the AC supply. The experimental results seem to agree quite well with the simulation results. VI. CONCLUSIONS The following conclusions can be drawn from this investigation: 0 The UPQC considered in this project is a multifunction power conditioner which can be used to compensate for various voltage disturbance of the power supply, to correct any voltage fluctuation and to prevent the harmonic load current from entering the power system. 0 The proposed direct compensation control method used in the series active filter and the moving window current calculation method used in the shunt active filter voltage change make the UPQC response very quickly to any sudden Fig. 12 Source Voltage and Current 0 The simulation results show that during the voltage sag period, the series active filter will supply an active power which is drawn from the shunt active filter through the dc bus capacitors from the source The control scheme used in the shunt active filter not only keeps the voltage of the dc bus capacitor to within a certain range but also causes the capacitor charging current to be obtained from the active component of the supply current. Under normal operating conditions, this capacitor charging current will be minimal. 0 The experimental results have shown that the proposed scheme is feasible although further work is needed to optimize the parameters of the UPQC. The simulation program developed can be used toward achieving this. 0 The simulation results show that during the voltage sag period, the series active filter will supply an active power which is drawn from the shunt active filter through the dc bus capacitors from the source. The control scheme used in the shunt active filter not only keeps the voltage of the dc bus capacitor to within a certain range but also causes the capacitor charging current to be obtained from the active component of the supply current. Under normal operating conditions, this capacitor charging current will be minimal. 0 The experimental results have shown that the proposed scheme is feasible although further work is needed to optimize the parameters of the UPQC. The simulation program developed can be used toward achieving this. VII. REFERENCES [l] Narain G., Hingorani, "Introducing Custom Power", I€E€ SPECTRUM, June 1995, pp.41-48. [2] F.Z.Peng, H.Akagi, A.Nabae, "Compensation Characteristics of the Combined System of Shunt Passive and Series Active Filters". I€€€ U S Annual Meeting 1989, pp.959-966. [3] G. Blajszaak, "Direct Method For Voltage Distortion Compensation In power Networks By Series Converter Filter", IEE froc.-Elecfr. Powr Appl., 1995, pp.308-312. [4] Himfumi Akagi, Hideaki Fujita, "A New Power Line Conditioner for Harmonic Compensation in Power Systems". IEEE Transacrions Power Delivery, Vol. 10, No. 3, July 1995 [SI Mauricio Aredes, etc. "A Combined Series and Shunt Active Power Filter", IEEWKTH Stockholm Power Tech Conference, Stockholm. Sweden, June 18-22, 1995. [6] Edson H. Watanabe, Richard M. Stephan, etc. "New Concepts of Instantaneous Active and Reactive Power in Electrical System With Generic Loads", IEEE Transactions on Power Delivery, Vol. 8, No. 2 April 1993. [7] Joachim Holtz, Bemd Beyer, "Fast Current Trajectory Tracking Control Based on Synchronous Optimal Pusewidth Modulation", IEEE IAS Annual Meeting 1994, pp. 734-741. [8] Ned Mohan, Tore M. Undeland, William P. Robbins, foiwr Electronics: Converters. Applications and Design, John Wiley \& Sons, Inc., 1989. (91 Subhashish Bhattacharya, Deepak Divan, "Synchronous Frame Based Controller Implementation For a Hybrid Series Active Filter System". IEEE IAS Annual Meeting I99S, pp.253 1-2540. BIOGRAPHS B.Naga Lakshmi Prasanna (08L41A0258) studying b. tech final year (e e e) in mekapati raja mohan reddy institute of science and technology, udayagiri Nellore(dist) P.M.pavithra(08L41A0259) studying b. tech final year (e e e) in mekapati raja mohan reddy institute of science and technology, udayagiri Nellore(dist)
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