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A simulation based case study for control of DSTATCOM
ISA Transactions 53 (2014) 767–775
Contents lists available at ScienceDirect
ISA Transactions journal homepage: www.elsevier.com/locate/isatrans
Research Article
A simulation based case study for control of DSTATCOM Pradeep Kumar n, Niranjan Kumar, A.K. Akella Department of Electrical Engineering, National Institute of Technology Jamshedpur, Jharkhand 831014, India
art ic l e i nf o
a b s t r a c t
Article history: Received 12 November 2012 Received in revised form 30 September 2013 Accepted 9 November 2013 Available online 19 March 2014 This paper was recommended for publication by Dr. Qing-Guo Wang
This paper presents a distribution static compensator (DSTATCOM) for power quality improvements in terms of harmonics and power factor correction in a three-phase four-wire distribution system. The DSTATCOM is implemented with PWM current controlled six-leg voltage source converter (VSC) and the switching patterns are generated through a novel synchronous reference frame controller (SRFC). The insulated gate bipolar transistor (IGBT) based VSC is supported by a capacitor and is controlled for the required compensation of the load current. The DSTATCOM is connected to the power system feeding nonlinear loads. Nonlinear loads include either current-source type or voltage-source type. Harmonic spectrum of the source current is compared in between without DSTATCOM and with DSTATCOM by considering both types of nonlinear loads. The SRFC based DSTATCOM system is validated through extensive simulation for diode-rectifier and unbalanced R–L loads with a case study. & 2013 ISA. Published by Elsevier Ltd. All rights reserved.
Keywords: Voltage source converters (VSCs) Distribution static compensator (DSTATCOM) Custom power devices (CPD) Synchronous reference frame (SRF)
1. Introduction Recently a lot of research is being encouraged for power quality and custom power problems in the distribution system due to non-linear loads [1–4]. In practical applications most of the loads are non-linear, such as power converters, SMPS, arc furnaces, UPS and ASDs [5,6]. These non-linear loads are introducing harmonic distortion and reactive power problems [7]. The harmonics in the system induce several undesirable issues, such as increased heating losses in transformers, low power factor, torque pulsation in motors, poor utilization of distribution plant and also affects other loads connected at the same Point of Common Coupling (PCC) [8,9]. The harmonic resonance is one of the most common problem reported in low and medium level distribution systems. It is due to capacitors which are used for power factor correction and source impedance [10]. There are mitigation techniques for power quality problems in the distribution system and the group of devices is known by the generic name of custom power devices (CPDs) [11]. Power converter based custom power devices (CPDs) are useful for reduction of power quality problems such as power factor correction, harmonics compensation, voltage sag/swell compensation, resonance due to distortion, and voltage flicker reduction within specified international standards [12,13]. The distribution static compensator (DSTATCOM) is a shuntconnected CPD capable of compensating current related power
n
Corresponding author. Tel.: þ 91 9905205302; fax: þ91 657 2373246. E-mail address: [email protected] (P. Kumar).
quality problems. Some of the topologies of DSTATCOM for the three phase four-wire system for the mitigation of neutral current along with power quality compensation in the source current are six-leg voltage source converter (VSC), three single-phase VSCs, three-leg VSC with split capacitors [14], three-leg VSC with zig-zag transformer [15–17], and three-leg VSC with neutral terminal at the positive or negative of dc bus [18]. The voltage regulation in the distribution feeder is improved by installing a shunt compensator [19]. Performance of any custom power device depends very much upon the control algorithm used for reference current estimation and gating pulse generation scheme. Some of the classical control algorithms are Fryze power theory, Budeanu theory, p–q theory and SRF theory [20–23], Lyapunov-functionbased control [24], the nonlinear control technique [25] and feed forward training [26] etc. In this paper, synchronous reference frame theory [21] is used for the control of the DSTATCOM. A new topology of DSTATCOM is proposed for a three-phase four-wire distribution system, which is based on six-leg VSC. Six-leg VSC compensates the harmonic current, reactive power, and improve the power factor on source side. The insulated gate bipolar transistor (IGBT) based VSC is self-supported with a dc bus capacitor and is controlled for the required compensation of the load current. The DSTATCOM is designed and simulated using MATLAB software with its Simulink and power system blockset (PSB) toolboxes. Comparative assessments of the performance of DSTATCOM feeding both types (current source and voltage source) of nonlinear loads in between without DSTATCOM and with DSTATCOM are presented. In Section 2, the description of the DSTATCOM system with operating principle is presented. Section 3
0019-0578/$ - see front matter & 2013 ISA. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.isatra.2013.11.008
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describes about the control strategy for reference current extraction. Section 4 describes a case study for the distribution system employing DSTATCOM as a power quality improvement device. In Section 5 performances have been modeled and analyzed from the obtained results under non-linear load conditions. Finally, Section 6 describes the conclusions of this work.
2. Description of DSTATCOM system A DSTATCOM is a custom power device which is utilized to eliminate harmonics from the source current and also balance them in addition to providing reactive power compensation to improve power factor or regulate the load bus voltage. The key component of the DSTATCOM is a power Voltage Source Converter (VSC) that is based on high power electronics technologies. Fig. 1. shows single-line diagram of DSTATCOM. The DSTATCOM mainly consists of DC voltage source behind self-commutated inverters using IGBT, controller and coupling transformer. The IGBT inverter with a DC voltage source can be modeled as a variable voltage source. The distribution power system can also be modeled as a voltage source. Two voltage sources are connected by a reactor representing the leakage reactance of the transformer. The principle operation modes of the DSTATCOM output current, I which varies depending upon Vo I ¼ ðV V 0 Þ=X
ð1Þ
where V, V0, and X are the system voltage, output voltage of the IGBT-based inverter, and the total circuit reactance respectively. If V0 is equal to V, then no reactive power is delivered to the system. If V0 is greater than V, the phase angle of I is leading with respect to the phase angle of V by 90 degrees. Thus, a leading reactive power flows in the capacitive mode of the DSTATCOM. If V0 is lower than V, the phase angle of I is lagging with respect to the phase angle of V by 901. Thus, a lagging reactive power flows in the inductive mode of the DSTATCOM. The quantity of the reactive power flow is proportional to the difference between V and V0. Fig. 2 shows the power circuit of proposed six-leg VSC based DSTATCOM integrated with three phase linear transformers. It contains three single phase full bridge converters connected to a common DC bus. The DC bus voltage is held by the capacitor Cdc. Since there is no energy source connected to the DC bus, the average power exchanged by the DSTATCOM is zero if the switches are ideal and losses in the reactors and capacitor are zero. On converter ac side, the interfacing inductors (AC inductors) Lf are used to filter high-frequency components of compensating currents. The selection of the ac inductance depends on the current ripple icr(p–p). Considering, switching frequency (fs) ¼ 10 kHz, modulation index (m) ¼1, dc bus voltage (Vdc) of 400 V,
Fig. 2. Configuration of DSTATCOM.
over load factor, a¼ 1.2, the Lf value is calculated as Lf ¼ ðmV dc Þ=ð4af s icrðp pÞ Þ
ð2Þ
Considering, 7% ripple in the inductor current, and the current ripple, icr(p p) ¼0.07 18.92 240/200 ¼1.589 A, the calculated value of Lf is 5.24 mH and it is selected as 5.0 mH. A DC capacitor is connected to the DC side of VSC converter, which carries input ripple current of the converter and is the main reactive energy storage element. This capacitor could be charged by a battery source or could be precharged by converter itself. The capacitor is charged when the current in the system is higher than in the DSTATCOM and is discharged when the current is lower. The principle of energy conservation is applied as ð1=2ÞC dc ½ðV 2dc Þ ðV 2dc1 Þ ¼ 3VðaIÞt
ð3Þ
where, Vdc is the dc voltage and Vdc1 is the minimum voltage level of dc bus, a is the over loading factor, V is the phase voltage, I is the phase current and t is time by which the dc bus voltage is to be recovered. Considering, 2% voltage ripple in the dc bus, the minimum voltage level of dc bus, Vdc1 ¼392 V, and Vdc ¼400 V, V ¼ 239.60 V, I¼ 18.92 A, t¼ 750 μ s, and a ¼1.2, the calculated value of Cdc is 3863.55 μ F and it is selected as 4000 μ F.
3. Control strategy The performance of DSTATCOM depends on the control algorithm used for extraction of reference current components. For this purpose, many control schemes are reported in literature, and some of these are instantaneous reactive power (IRP) theory, instantaneous symmetrical components, synchronous reference frame (SRF) theory, and current compensation using dc bus regulation, computation based on per phase basis, and scheme based on neural network techniques [27–32]. Among this control scheme, SRF theory is most widely used. 3.1. Synchronous reference frame (SRF) based control strategy
Fig. 1. Single-line diagram of DSTATCOM.
A block diagram of the control scheme is shown in Fig. 3. The load currents (iLa, iLb, and iLc), the PCC voltages (vta, vtb, and vtc), and dc bus voltage (vdc) of DSTATCOM are sensed as feedback signals. The load currents from the a–b–c frame are first converted to the α– β frame and then to the d–q frame using the following formulation 2 3 " # ( ) i 1 1=2 1==2 6 La 7 iLα pffiffiffi pffiffiffi ¼ 2=3 4 iLb 5 iLβ 0 3=2 3=2 iLc
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Fig. 3. Block diagram of Synchronous Reference Frame control scheme.
"
iLd
#
iLq "
iLd iLq
¼
cos ωt
sin ωt
sin ωt
cos ωt
#
( ¼ 2=3
" i #
Table 1 System parameters.
Lα
iLβ
cos ωt
cos ðωt ð2π=3ÞÞ
sin ωt
sin ðωt ð2π=3ÞÞ
2 3 ) iLa 6i 7 4 Lb 5 sin ðωt þ ð2π=3ÞÞ iLc
cos ðωt þ ð2π=3ÞÞ
ð4Þ The average values of iLd and iLq are obtained as output of two identical low pass filters and are defined as " # " # iLd iLd ¼ GðsÞ ð5Þ iLq iLq The SRF controller extracts dc quantities by a low-pass filter, and hence, the non-dc quantities (harmonics) are separated from the reference signal. The DC voltage controller is designed as a proportional controller. The PCC voltage controller is designed as a PI controller. The output of the proportional (P) controller at the dc bus voltage of DSTATCOM is considered as the current iCd and the
Parameter
Value
AC source voltage and frequency
Vs ¼ 415 V, f¼ 50 Hz
Line impedance
Ls ¼40 mH, Rs ¼ 1.57 Ω
Unbalanced R–L load
Ra ¼ 50 Ω, La ¼ 200 mH Rb ¼75 Ω, Lb ¼ 225 mH Rc ¼25 Ω, Lc ¼175 mH
Voltage-source type of nonlinear load (three phase diode rectifier)
Cd ¼500 mF, Rd ¼ 126 Ω
Current-source type of nonlinear load (three phase diode rectifier)
Rd ¼125 Ω, Ld ¼ 30 mH
Filter parameter
Lf ¼5.0 mH
DC-sidecapacitance, resistance and voltage
Cdc ¼4000 mF Rdc ¼ 6000 Ω Vdc ¼ 400 V
Controller parameter (proportional and PI)
Kp1 ¼0.6, Kp2 ¼ 0.2, Ki ¼ 40
Power converter
IGBTs/diodes
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output of the proportional-integral (PI) controller at the PCC bus voltage of DSTATCOM is considered as the current iCq. u is a logical variable equal to (a) zero if PF is to be regulated and (b) one if bus voltage is to be regulated. Kq ¼1 in the latter case. When PF is to be controlled, Kq is determined by the required power factor as follows: n
K q ¼ Q s =Q L
For unity power factor, cosφ ¼1, so φ¼ 0 n
Q S ¼ VI sin φ ¼ 0
ð8Þ
Kq ¼ 0
ð6Þ
The desired source currents (in d–q component) are obtained as n
where Q nS reference reactive power is supplied by source (at PCC) and Q L is average reactive power defined by Q L ¼ jV t jiLq
ð7Þ
iSd ¼ iLd þ iCd inSq ¼ K q iLq þ uiCq
Hence 8 2 n 3 cos ωt isa > < 6 in 7 4 sb 5 ¼ 2=3 cos ðωt ð2π=3ÞÞ > : cos ðωt þð2π=3ÞÞ insc
Fig. 4. System configuration.
ð9Þ
The reference for the source current in the d–q frame are first converted to the α–β frame and then to the a–b–c frame using the following formulation: " n # " in # isα cos ωt sin ωt sd ¼ insq insβ sin ωt cos ωt 8 9 2 n 3 " # 1 0 isa > > < = in p ffiffiffi sα 6 in 7 4 sb 5 ¼ 2=3 1=2 pffiffiffi3=2 insβ > > n : ; 1=2 3=2 isc 9 " # sin ωt > = in sd sin ðωt ð2π=3ÞÞ insq > ; sin ðωt þ ð2π=3ÞÞ
ð10Þ
The reference for the source current vectors (iSan,iSbn,and iScn) is computed and the desired compensator currents (iCan,iCbn,and iCcn) are obtained as the difference between the load and the source (reference) currents. The determination of reference source current vector is
Fig. 5. MATLAB based model of DSTATCOM.
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based on synchronous reference frame (SRF) theory [33].
4. Simulation of DSTATCOM
inCa ¼ iLa inSa
4.1. A case study
inCb ¼ iLb inSb
The six-leg voltage source converter based DSTATCOM connected to a distribution system having an unbalanced and nonlinear load is taken up for study [33]. The nonlinear load may be current source type or voltage source type. The system data are given in the Table 1. The system diagram is shown in Fig. 4 which shows the source impedance (Rs and Ls). A STATCOM of suitable rating is connected in parallel with the load. In general, the STATCOM is connected through a step-up transformer. Three single-phase two-winding transformers of
inCc ¼ iLc inSc Note that ω is the supply frequency expressed in radians/s. The unit vectors sin ωt and cos ωt are obtained from phase-locked loop (PLL) which is locked to the PCC voltage.
Fig. 6. MATLAB based model of synchronous reference frame theory.
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ratings 5 kVA, 200 V/240 V are selected. The selection of coupling transformer parameters has a large impact on the performance of DSTATCOM. It plays an important role in the value of voltage regulation and power compensation that the DSTATCOM can provide. In the voltage source converter a dc capacitor is connected on the dc side to produce a smooth dc voltage. The switches in the converter represent controllable semiconductors, such as IGBT or power transistors. The IGBTs are connected anti parallel with diodes for commutation purposes and charging of the DC capacitor. For converter the most important part is the sequences of operation of the IGBTs. PWM generators are used to generate the pulses for the firing of the IGBTs. IGBTs are used in this work because it is easy to control the switch on and off of their gates and suitable for the DSTATCOM.
phase is considered and a nonlinear load is modeled by three phase diode rectifiers. The main purpose of the simulation is to study two different performance aspects: (1) harmonic compensation and power factor correction of a current-source type of nonlinear load and (2) harmonic compensation and power factor correction of a voltage-source type of nonlinear load. In addition, the fast Fourier transform (FFT) is used to measure the order of harmonics in the source current. The total harmonic distortion (THD) of the source current is measured without and with DSTATCOM for the both types of non-linear load conditions. The THD measurement is compared for current and voltage source type nonlinear loads that is presented in Table 2. The system parameters used in these simulations are given in Table 1.
5. Simulation results and discussion
5.1. Harmonic compensation and power factor correction of current-source type of nonlinear load
Fig. 5 shows the basic simulation model of the DSTATCOM system that correlates to the system configuration shown in Fig. 4 in terms of source, load, DSTATCOM, and control blocks. The coupling transformer in parallel to the load, the three-phase source, and the shunt-connected six-leg VSC are connected as shown in Fig. 5. This DSTATCOM model is simulated with the synchronous reference frame theory. Fig. 6 shows the simulation models for this theory that are inconsistent with the control. The model is assembled using the mathematical blocks of SIMULINK block set. Simulation is carried out in discrete mode at a maximum step size of 1 10 3 with ode45 (Domand-Prince) solver. The total simulation period is 1 s. A three phase R–L load having different values of resistances and inductances at each
The simulation results of the DSTATCOM system with the currentsource type of nonlinear load are shown in Fig. 7. The load current (iL), compensator current(iC), supply current (iS), terminal voltage (Vt) and dc voltage (Vdc) are shown there. It is observed that load current is unbalanced and non-sinusoidal because of unbalanced R–L and diode rectifier load. Compensator current is purely sinusoidal. From Fig. 7 it is clear that the waveforms of supply current and terminal voltage are in-phase however magnitudes are different which indicate power factor correction of the distribution system. The DC link voltage Vdc remains constant at 430 V. Fig. 8(a) and (b) shows the harmonic spectrum of the supply current without and with DSTATCOM. The THD of the source current has been reduced from 5.20% without DSTATCOM to 0.01% with DSTATCOM.
Table 2 Measured THD under various non-linear load condition. Non-linear load
Without DSTATCOM (%)
With DSTATCOM(%)
Current source type Voltage source type
5.20 5.43
0.01 0.01
Fig. 7. Steady state response of the DSTATCOM with a three-phase current source type of non-linear load.
P. Kumar et al. / ISA Transactions 53 (2014) 767–775
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Fig. 9 shows the simulation results for the compensation of harmonics by the DSTATCOM system with a voltage-source type of nonlinear load. The waveforms of the load current (iL), compensator current(iC), supply current (iS), terminal voltage (Vt) and dc voltage (Vdc) are presented to demonstrate the filtering performance of the DSTATCOM for a voltage-source type of nonlinear load. From Fig. 9 it is observed that the waveforms of supply
current and terminal voltage are in the same phase hence power factor correction of the distribution system is achieved. Fig. 10 (a) and (b) shows the supply current spectrum without and with DSTATCOM. The corresponding THD of the supply current is reduced from 5.43% without DSTATCOM to 0.01% with DSTATCOM. The FFT analysis of the DSTATCOM confirms that the THD (total harmonic distortion) of the supply current is less than 5% for the both cases of non-linear loads (current-source type and voltagesource type) that are in compliance with IEEE-519 and IEC 61000-3 harmonic standards.
Fig. 8. Spectrum of the source current in phase 1. (a) Without DSTATCOM (b) and with DSTATCOM.
Fig. 10. Harmonic spectrum of the source current in phase 1. (a) Without DSTATCOM and (b) with DSTATCOM.
5.2. Harmonic compensation and power factor correction of voltage-source type of nonlinear load
Fig. 9. Steady state response of the DSTATCOM for voltage source type of non-linear load.
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Fig. 11. Switching patterns. (a) Current source type of non-linear load and (b) voltage source type of non-linear load.
Fig. 11 illustrates the switching patterns of DSTATCOM for the case of current-source and voltage-source types of nonlinear loads.
6. Conclusion The modeling and simulation of a new topology of DSTATCOM consisting of six-leg VSC integrated with three phase linear transformer have been carried out and the performance has been demonstrated for power factor correction and harmonic
elimination. The DSTATCOM is implemented with PWM current controlled voltage source converter. This converter switching patterns are generated from a synchronous reference frame controller. It has been shown that the system has a fast dynamic response and is able to keep the THD of the supply current below the limits specified by the IEEE 519 and IEC 61000-3 harmonic standards. The DSTATCOM employing the proposed synchronous reference frame control has been found suitable for voltage and current-source types of nonlinear loads. The obtained results have demonstrated the effectiveness of the proposed
P. Kumar et al. / ISA Transactions 53 (2014) 767–775
synchronous reference frame control strategy and the high performance level of the DSTATCOM. Hence, the DSTATCOM system can be an effective and economic solution for harmonic problems caused by large-capacity nonlinear loads.
Acknowledgments The authors are thankful to Government of India. One of the authors Mr. Pradeep Kumar is thankful to All India council of Technical Education (AICTE), Ministry of Human Resources Development (MHRD), and Govt. of India for providing financial assistants to do the research work under Technical Quality Improvement Programme (TEQIP). References [1] YugyiLG, Trycula ECS. Active AC power filters. In: Proceedings of the IEEE/IAS annual meeting. vol. 19-C; 1976. p. 529–35. [2] Hosseini Mehdi, Ali Shayanfar Heidar, Fotuhi-Firuzabad Mahmoud. Reliability improvement of distribution systems using SSVR. ISA Trans 2009;48:98–106. [3] Karuppanan P, Mahapatra Kamala Kanta. PI and fuzzy logic controllers for shunt active power filter —a report. ISA Trans 2012;51(1):163–9. [4] Hooshmand Rahmat Allah, Esfahani Mahdi Torabian. Adaptive filter design based on the LMS algorithm for delay elimination in TCR/FC compensators. ISA Trans 2011;50:142–9. [5] Duffey Christopher K, Stratford Ray P. Update of harmonic standard IEEE-519: IEEE recommended practices and requirements for harmonic control in electric power systems. IEEE Trans Ind Appl 1989;25(6):1025–34. [6] Hooshmand Rahmat Allah, Esfahani Mahdi Torabian. A new combined method in active filter design for power quality improvement in power systems. ISA Trans 2011;50:150–8. [7] Subjak Jr. S, Joseph S, Mcquilkin John. Harmonics—causes, effects, measurements, and analysis: an update. IEEE Trans Ind Appl 1990;26(6):1034–42. [8] Emanuel Alexander E, John AOrr, David Cyganski M, Gulachenski Edward. A survey of harmonic voltages and currents at the customer's bus. IEEE Trans Power Deliv 1993;8(1):411–21. [9] Grady WM, Samotyj MJ, Noyola AH. Survey of active power line conditioning methodologies. IEEE Trans Power Deliv 1990;5(3):1536–42. [10] Lee Tzung Lin, Hu Shang Hung. Discrete frequency-tuning active filter to suppress harmonic resonances of closed-loop distribution power systems. IEEE Trans Power Electron 2011;26(1):137–48. [11] Ghosh A, Ledwich. G. Power Quality Enhancement Using Custom Power Devices. London, U.K.: Kluwer; 2002. [12] IEEE Std.519. IEEE recommended practices and requirement for harmonic control on electric power System; 1992. [13] Lee Tzung-Lin, Hu Shang-Hung, Chan Yu-Hung. DSTATCOM with positivesequence admittance and negative-sequence conductance to mitigate voltage fluctuations in high-level penetration of distributed generation systems. IEEE Trans Ind Electron 2013;60(4):1417–28.
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Contents lists available at ScienceDirect
ISA Transactions journal homepage: www.elsevier.com/locate/isatrans
Research Article
A simulation based case study for control of DSTATCOM Pradeep Kumar n, Niranjan Kumar, A.K. Akella Department of Electrical Engineering, National Institute of Technology Jamshedpur, Jharkhand 831014, India
art ic l e i nf o
a b s t r a c t
Article history: Received 12 November 2012 Received in revised form 30 September 2013 Accepted 9 November 2013 Available online 19 March 2014 This paper was recommended for publication by Dr. Qing-Guo Wang
This paper presents a distribution static compensator (DSTATCOM) for power quality improvements in terms of harmonics and power factor correction in a three-phase four-wire distribution system. The DSTATCOM is implemented with PWM current controlled six-leg voltage source converter (VSC) and the switching patterns are generated through a novel synchronous reference frame controller (SRFC). The insulated gate bipolar transistor (IGBT) based VSC is supported by a capacitor and is controlled for the required compensation of the load current. The DSTATCOM is connected to the power system feeding nonlinear loads. Nonlinear loads include either current-source type or voltage-source type. Harmonic spectrum of the source current is compared in between without DSTATCOM and with DSTATCOM by considering both types of nonlinear loads. The SRFC based DSTATCOM system is validated through extensive simulation for diode-rectifier and unbalanced R–L loads with a case study. & 2013 ISA. Published by Elsevier Ltd. All rights reserved.
Keywords: Voltage source converters (VSCs) Distribution static compensator (DSTATCOM) Custom power devices (CPD) Synchronous reference frame (SRF)
1. Introduction Recently a lot of research is being encouraged for power quality and custom power problems in the distribution system due to non-linear loads [1–4]. In practical applications most of the loads are non-linear, such as power converters, SMPS, arc furnaces, UPS and ASDs [5,6]. These non-linear loads are introducing harmonic distortion and reactive power problems [7]. The harmonics in the system induce several undesirable issues, such as increased heating losses in transformers, low power factor, torque pulsation in motors, poor utilization of distribution plant and also affects other loads connected at the same Point of Common Coupling (PCC) [8,9]. The harmonic resonance is one of the most common problem reported in low and medium level distribution systems. It is due to capacitors which are used for power factor correction and source impedance [10]. There are mitigation techniques for power quality problems in the distribution system and the group of devices is known by the generic name of custom power devices (CPDs) [11]. Power converter based custom power devices (CPDs) are useful for reduction of power quality problems such as power factor correction, harmonics compensation, voltage sag/swell compensation, resonance due to distortion, and voltage flicker reduction within specified international standards [12,13]. The distribution static compensator (DSTATCOM) is a shuntconnected CPD capable of compensating current related power
n
Corresponding author. Tel.: þ 91 9905205302; fax: þ91 657 2373246. E-mail address: [email protected] (P. Kumar).
quality problems. Some of the topologies of DSTATCOM for the three phase four-wire system for the mitigation of neutral current along with power quality compensation in the source current are six-leg voltage source converter (VSC), three single-phase VSCs, three-leg VSC with split capacitors [14], three-leg VSC with zig-zag transformer [15–17], and three-leg VSC with neutral terminal at the positive or negative of dc bus [18]. The voltage regulation in the distribution feeder is improved by installing a shunt compensator [19]. Performance of any custom power device depends very much upon the control algorithm used for reference current estimation and gating pulse generation scheme. Some of the classical control algorithms are Fryze power theory, Budeanu theory, p–q theory and SRF theory [20–23], Lyapunov-functionbased control [24], the nonlinear control technique [25] and feed forward training [26] etc. In this paper, synchronous reference frame theory [21] is used for the control of the DSTATCOM. A new topology of DSTATCOM is proposed for a three-phase four-wire distribution system, which is based on six-leg VSC. Six-leg VSC compensates the harmonic current, reactive power, and improve the power factor on source side. The insulated gate bipolar transistor (IGBT) based VSC is self-supported with a dc bus capacitor and is controlled for the required compensation of the load current. The DSTATCOM is designed and simulated using MATLAB software with its Simulink and power system blockset (PSB) toolboxes. Comparative assessments of the performance of DSTATCOM feeding both types (current source and voltage source) of nonlinear loads in between without DSTATCOM and with DSTATCOM are presented. In Section 2, the description of the DSTATCOM system with operating principle is presented. Section 3
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describes about the control strategy for reference current extraction. Section 4 describes a case study for the distribution system employing DSTATCOM as a power quality improvement device. In Section 5 performances have been modeled and analyzed from the obtained results under non-linear load conditions. Finally, Section 6 describes the conclusions of this work.
2. Description of DSTATCOM system A DSTATCOM is a custom power device which is utilized to eliminate harmonics from the source current and also balance them in addition to providing reactive power compensation to improve power factor or regulate the load bus voltage. The key component of the DSTATCOM is a power Voltage Source Converter (VSC) that is based on high power electronics technologies. Fig. 1. shows single-line diagram of DSTATCOM. The DSTATCOM mainly consists of DC voltage source behind self-commutated inverters using IGBT, controller and coupling transformer. The IGBT inverter with a DC voltage source can be modeled as a variable voltage source. The distribution power system can also be modeled as a voltage source. Two voltage sources are connected by a reactor representing the leakage reactance of the transformer. The principle operation modes of the DSTATCOM output current, I which varies depending upon Vo I ¼ ðV V 0 Þ=X
ð1Þ
where V, V0, and X are the system voltage, output voltage of the IGBT-based inverter, and the total circuit reactance respectively. If V0 is equal to V, then no reactive power is delivered to the system. If V0 is greater than V, the phase angle of I is leading with respect to the phase angle of V by 90 degrees. Thus, a leading reactive power flows in the capacitive mode of the DSTATCOM. If V0 is lower than V, the phase angle of I is lagging with respect to the phase angle of V by 901. Thus, a lagging reactive power flows in the inductive mode of the DSTATCOM. The quantity of the reactive power flow is proportional to the difference between V and V0. Fig. 2 shows the power circuit of proposed six-leg VSC based DSTATCOM integrated with three phase linear transformers. It contains three single phase full bridge converters connected to a common DC bus. The DC bus voltage is held by the capacitor Cdc. Since there is no energy source connected to the DC bus, the average power exchanged by the DSTATCOM is zero if the switches are ideal and losses in the reactors and capacitor are zero. On converter ac side, the interfacing inductors (AC inductors) Lf are used to filter high-frequency components of compensating currents. The selection of the ac inductance depends on the current ripple icr(p–p). Considering, switching frequency (fs) ¼ 10 kHz, modulation index (m) ¼1, dc bus voltage (Vdc) of 400 V,
Fig. 2. Configuration of DSTATCOM.
over load factor, a¼ 1.2, the Lf value is calculated as Lf ¼ ðmV dc Þ=ð4af s icrðp pÞ Þ
ð2Þ
Considering, 7% ripple in the inductor current, and the current ripple, icr(p p) ¼0.07 18.92 240/200 ¼1.589 A, the calculated value of Lf is 5.24 mH and it is selected as 5.0 mH. A DC capacitor is connected to the DC side of VSC converter, which carries input ripple current of the converter and is the main reactive energy storage element. This capacitor could be charged by a battery source or could be precharged by converter itself. The capacitor is charged when the current in the system is higher than in the DSTATCOM and is discharged when the current is lower. The principle of energy conservation is applied as ð1=2ÞC dc ½ðV 2dc Þ ðV 2dc1 Þ ¼ 3VðaIÞt
ð3Þ
where, Vdc is the dc voltage and Vdc1 is the minimum voltage level of dc bus, a is the over loading factor, V is the phase voltage, I is the phase current and t is time by which the dc bus voltage is to be recovered. Considering, 2% voltage ripple in the dc bus, the minimum voltage level of dc bus, Vdc1 ¼392 V, and Vdc ¼400 V, V ¼ 239.60 V, I¼ 18.92 A, t¼ 750 μ s, and a ¼1.2, the calculated value of Cdc is 3863.55 μ F and it is selected as 4000 μ F.
3. Control strategy The performance of DSTATCOM depends on the control algorithm used for extraction of reference current components. For this purpose, many control schemes are reported in literature, and some of these are instantaneous reactive power (IRP) theory, instantaneous symmetrical components, synchronous reference frame (SRF) theory, and current compensation using dc bus regulation, computation based on per phase basis, and scheme based on neural network techniques [27–32]. Among this control scheme, SRF theory is most widely used. 3.1. Synchronous reference frame (SRF) based control strategy
Fig. 1. Single-line diagram of DSTATCOM.
A block diagram of the control scheme is shown in Fig. 3. The load currents (iLa, iLb, and iLc), the PCC voltages (vta, vtb, and vtc), and dc bus voltage (vdc) of DSTATCOM are sensed as feedback signals. The load currents from the a–b–c frame are first converted to the α– β frame and then to the d–q frame using the following formulation 2 3 " # ( ) i 1 1=2 1==2 6 La 7 iLα pffiffiffi pffiffiffi ¼ 2=3 4 iLb 5 iLβ 0 3=2 3=2 iLc
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Fig. 3. Block diagram of Synchronous Reference Frame control scheme.
"
iLd
#
iLq "
iLd iLq
¼
cos ωt
sin ωt
sin ωt
cos ωt
#
( ¼ 2=3
" i #
Table 1 System parameters.
Lα
iLβ
cos ωt
cos ðωt ð2π=3ÞÞ
sin ωt
sin ðωt ð2π=3ÞÞ
2 3 ) iLa 6i 7 4 Lb 5 sin ðωt þ ð2π=3ÞÞ iLc
cos ðωt þ ð2π=3ÞÞ
ð4Þ The average values of iLd and iLq are obtained as output of two identical low pass filters and are defined as " # " # iLd iLd ¼ GðsÞ ð5Þ iLq iLq The SRF controller extracts dc quantities by a low-pass filter, and hence, the non-dc quantities (harmonics) are separated from the reference signal. The DC voltage controller is designed as a proportional controller. The PCC voltage controller is designed as a PI controller. The output of the proportional (P) controller at the dc bus voltage of DSTATCOM is considered as the current iCd and the
Parameter
Value
AC source voltage and frequency
Vs ¼ 415 V, f¼ 50 Hz
Line impedance
Ls ¼40 mH, Rs ¼ 1.57 Ω
Unbalanced R–L load
Ra ¼ 50 Ω, La ¼ 200 mH Rb ¼75 Ω, Lb ¼ 225 mH Rc ¼25 Ω, Lc ¼175 mH
Voltage-source type of nonlinear load (three phase diode rectifier)
Cd ¼500 mF, Rd ¼ 126 Ω
Current-source type of nonlinear load (three phase diode rectifier)
Rd ¼125 Ω, Ld ¼ 30 mH
Filter parameter
Lf ¼5.0 mH
DC-sidecapacitance, resistance and voltage
Cdc ¼4000 mF Rdc ¼ 6000 Ω Vdc ¼ 400 V
Controller parameter (proportional and PI)
Kp1 ¼0.6, Kp2 ¼ 0.2, Ki ¼ 40
Power converter
IGBTs/diodes
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output of the proportional-integral (PI) controller at the PCC bus voltage of DSTATCOM is considered as the current iCq. u is a logical variable equal to (a) zero if PF is to be regulated and (b) one if bus voltage is to be regulated. Kq ¼1 in the latter case. When PF is to be controlled, Kq is determined by the required power factor as follows: n
K q ¼ Q s =Q L
For unity power factor, cosφ ¼1, so φ¼ 0 n
Q S ¼ VI sin φ ¼ 0
ð8Þ
Kq ¼ 0
ð6Þ
The desired source currents (in d–q component) are obtained as n
where Q nS reference reactive power is supplied by source (at PCC) and Q L is average reactive power defined by Q L ¼ jV t jiLq
ð7Þ
iSd ¼ iLd þ iCd inSq ¼ K q iLq þ uiCq
Hence 8 2 n 3 cos ωt isa > < 6 in 7 4 sb 5 ¼ 2=3 cos ðωt ð2π=3ÞÞ > : cos ðωt þð2π=3ÞÞ insc
Fig. 4. System configuration.
ð9Þ
The reference for the source current in the d–q frame are first converted to the α–β frame and then to the a–b–c frame using the following formulation: " n # " in # isα cos ωt sin ωt sd ¼ insq insβ sin ωt cos ωt 8 9 2 n 3 " # 1 0 isa > > < = in p ffiffiffi sα 6 in 7 4 sb 5 ¼ 2=3 1=2 pffiffiffi3=2 insβ > > n : ; 1=2 3=2 isc 9 " # sin ωt > = in sd sin ðωt ð2π=3ÞÞ insq > ; sin ðωt þ ð2π=3ÞÞ
ð10Þ
The reference for the source current vectors (iSan,iSbn,and iScn) is computed and the desired compensator currents (iCan,iCbn,and iCcn) are obtained as the difference between the load and the source (reference) currents. The determination of reference source current vector is
Fig. 5. MATLAB based model of DSTATCOM.
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based on synchronous reference frame (SRF) theory [33].
4. Simulation of DSTATCOM
inCa ¼ iLa inSa
4.1. A case study
inCb ¼ iLb inSb
The six-leg voltage source converter based DSTATCOM connected to a distribution system having an unbalanced and nonlinear load is taken up for study [33]. The nonlinear load may be current source type or voltage source type. The system data are given in the Table 1. The system diagram is shown in Fig. 4 which shows the source impedance (Rs and Ls). A STATCOM of suitable rating is connected in parallel with the load. In general, the STATCOM is connected through a step-up transformer. Three single-phase two-winding transformers of
inCc ¼ iLc inSc Note that ω is the supply frequency expressed in radians/s. The unit vectors sin ωt and cos ωt are obtained from phase-locked loop (PLL) which is locked to the PCC voltage.
Fig. 6. MATLAB based model of synchronous reference frame theory.
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ratings 5 kVA, 200 V/240 V are selected. The selection of coupling transformer parameters has a large impact on the performance of DSTATCOM. It plays an important role in the value of voltage regulation and power compensation that the DSTATCOM can provide. In the voltage source converter a dc capacitor is connected on the dc side to produce a smooth dc voltage. The switches in the converter represent controllable semiconductors, such as IGBT or power transistors. The IGBTs are connected anti parallel with diodes for commutation purposes and charging of the DC capacitor. For converter the most important part is the sequences of operation of the IGBTs. PWM generators are used to generate the pulses for the firing of the IGBTs. IGBTs are used in this work because it is easy to control the switch on and off of their gates and suitable for the DSTATCOM.
phase is considered and a nonlinear load is modeled by three phase diode rectifiers. The main purpose of the simulation is to study two different performance aspects: (1) harmonic compensation and power factor correction of a current-source type of nonlinear load and (2) harmonic compensation and power factor correction of a voltage-source type of nonlinear load. In addition, the fast Fourier transform (FFT) is used to measure the order of harmonics in the source current. The total harmonic distortion (THD) of the source current is measured without and with DSTATCOM for the both types of non-linear load conditions. The THD measurement is compared for current and voltage source type nonlinear loads that is presented in Table 2. The system parameters used in these simulations are given in Table 1.
5. Simulation results and discussion
5.1. Harmonic compensation and power factor correction of current-source type of nonlinear load
Fig. 5 shows the basic simulation model of the DSTATCOM system that correlates to the system configuration shown in Fig. 4 in terms of source, load, DSTATCOM, and control blocks. The coupling transformer in parallel to the load, the three-phase source, and the shunt-connected six-leg VSC are connected as shown in Fig. 5. This DSTATCOM model is simulated with the synchronous reference frame theory. Fig. 6 shows the simulation models for this theory that are inconsistent with the control. The model is assembled using the mathematical blocks of SIMULINK block set. Simulation is carried out in discrete mode at a maximum step size of 1 10 3 with ode45 (Domand-Prince) solver. The total simulation period is 1 s. A three phase R–L load having different values of resistances and inductances at each
The simulation results of the DSTATCOM system with the currentsource type of nonlinear load are shown in Fig. 7. The load current (iL), compensator current(iC), supply current (iS), terminal voltage (Vt) and dc voltage (Vdc) are shown there. It is observed that load current is unbalanced and non-sinusoidal because of unbalanced R–L and diode rectifier load. Compensator current is purely sinusoidal. From Fig. 7 it is clear that the waveforms of supply current and terminal voltage are in-phase however magnitudes are different which indicate power factor correction of the distribution system. The DC link voltage Vdc remains constant at 430 V. Fig. 8(a) and (b) shows the harmonic spectrum of the supply current without and with DSTATCOM. The THD of the source current has been reduced from 5.20% without DSTATCOM to 0.01% with DSTATCOM.
Table 2 Measured THD under various non-linear load condition. Non-linear load
Without DSTATCOM (%)
With DSTATCOM(%)
Current source type Voltage source type
5.20 5.43
0.01 0.01
Fig. 7. Steady state response of the DSTATCOM with a three-phase current source type of non-linear load.
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Fig. 9 shows the simulation results for the compensation of harmonics by the DSTATCOM system with a voltage-source type of nonlinear load. The waveforms of the load current (iL), compensator current(iC), supply current (iS), terminal voltage (Vt) and dc voltage (Vdc) are presented to demonstrate the filtering performance of the DSTATCOM for a voltage-source type of nonlinear load. From Fig. 9 it is observed that the waveforms of supply
current and terminal voltage are in the same phase hence power factor correction of the distribution system is achieved. Fig. 10 (a) and (b) shows the supply current spectrum without and with DSTATCOM. The corresponding THD of the supply current is reduced from 5.43% without DSTATCOM to 0.01% with DSTATCOM. The FFT analysis of the DSTATCOM confirms that the THD (total harmonic distortion) of the supply current is less than 5% for the both cases of non-linear loads (current-source type and voltagesource type) that are in compliance with IEEE-519 and IEC 61000-3 harmonic standards.
Fig. 8. Spectrum of the source current in phase 1. (a) Without DSTATCOM (b) and with DSTATCOM.
Fig. 10. Harmonic spectrum of the source current in phase 1. (a) Without DSTATCOM and (b) with DSTATCOM.
5.2. Harmonic compensation and power factor correction of voltage-source type of nonlinear load
Fig. 9. Steady state response of the DSTATCOM for voltage source type of non-linear load.
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Fig. 11. Switching patterns. (a) Current source type of non-linear load and (b) voltage source type of non-linear load.
Fig. 11 illustrates the switching patterns of DSTATCOM for the case of current-source and voltage-source types of nonlinear loads.
6. Conclusion The modeling and simulation of a new topology of DSTATCOM consisting of six-leg VSC integrated with three phase linear transformer have been carried out and the performance has been demonstrated for power factor correction and harmonic
elimination. The DSTATCOM is implemented with PWM current controlled voltage source converter. This converter switching patterns are generated from a synchronous reference frame controller. It has been shown that the system has a fast dynamic response and is able to keep the THD of the supply current below the limits specified by the IEEE 519 and IEC 61000-3 harmonic standards. The DSTATCOM employing the proposed synchronous reference frame control has been found suitable for voltage and current-source types of nonlinear loads. The obtained results have demonstrated the effectiveness of the proposed
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synchronous reference frame control strategy and the high performance level of the DSTATCOM. Hence, the DSTATCOM system can be an effective and economic solution for harmonic problems caused by large-capacity nonlinear loads.
Acknowledgments The authors are thankful to Government of India. One of the authors Mr. Pradeep Kumar is thankful to All India council of Technical Education (AICTE), Ministry of Human Resources Development (MHRD), and Govt. of India for providing financial assistants to do the research work under Technical Quality Improvement Programme (TEQIP). References [1] YugyiLG, Trycula ECS. Active AC power filters. In: Proceedings of the IEEE/IAS annual meeting. vol. 19-C; 1976. p. 529–35. [2] Hosseini Mehdi, Ali Shayanfar Heidar, Fotuhi-Firuzabad Mahmoud. Reliability improvement of distribution systems using SSVR. ISA Trans 2009;48:98–106. [3] Karuppanan P, Mahapatra Kamala Kanta. PI and fuzzy logic controllers for shunt active power filter —a report. ISA Trans 2012;51(1):163–9. [4] Hooshmand Rahmat Allah, Esfahani Mahdi Torabian. Adaptive filter design based on the LMS algorithm for delay elimination in TCR/FC compensators. ISA Trans 2011;50:142–9. [5] Duffey Christopher K, Stratford Ray P. Update of harmonic standard IEEE-519: IEEE recommended practices and requirements for harmonic control in electric power systems. IEEE Trans Ind Appl 1989;25(6):1025–34. [6] Hooshmand Rahmat Allah, Esfahani Mahdi Torabian. A new combined method in active filter design for power quality improvement in power systems. ISA Trans 2011;50:150–8. [7] Subjak Jr. S, Joseph S, Mcquilkin John. Harmonics—causes, effects, measurements, and analysis: an update. IEEE Trans Ind Appl 1990;26(6):1034–42. [8] Emanuel Alexander E, John AOrr, David Cyganski M, Gulachenski Edward. A survey of harmonic voltages and currents at the customer's bus. IEEE Trans Power Deliv 1993;8(1):411–21. [9] Grady WM, Samotyj MJ, Noyola AH. Survey of active power line conditioning methodologies. IEEE Trans Power Deliv 1990;5(3):1536–42. [10] Lee Tzung Lin, Hu Shang Hung. Discrete frequency-tuning active filter to suppress harmonic resonances of closed-loop distribution power systems. IEEE Trans Power Electron 2011;26(1):137–48. [11] Ghosh A, Ledwich. G. Power Quality Enhancement Using Custom Power Devices. London, U.K.: Kluwer; 2002. [12] IEEE Std.519. IEEE recommended practices and requirement for harmonic control on electric power System; 1992. [13] Lee Tzung-Lin, Hu Shang-Hung, Chan Yu-Hung. DSTATCOM with positivesequence admittance and negative-sequence conductance to mitigate voltage fluctuations in high-level penetration of distributed generation systems. IEEE Trans Ind Electron 2013;60(4):1417–28.
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