Power Quality Conditioner with Series-Parallel ...

Power Quality Conditioner with Series-Parallel Compensation Applied to SinglePhase Systems

BARRIVIERA Rodrigo

Power Quality Conditioner with Series-Parallel Compensation Applied to Single-Phase Systems Rodrigo Barriviera2 Sergio Augusto Oliveira da Silva1 Rodrigo Augusto Modesto1 Alessandro Goedtel1 Maurício Kaster1 FEDERAL TECHNOLOGICAL UNIVERSITY OF PARANA – UTFPR 1 FEDERAL INSTITUTE OF PARANÁ – IFPR 2 Av. Alberto Carazzai, 1640, CEP. 86.000-300 1 Cornélio Procópio – PR, Brazil Tel.: +55 / (43) – 3520.40.00. Fax: +55 / (43) – 3520.40.40. E-Mails: [email protected], [email protected], [email protected], [email protected], [email protected] URL: http://www.cp.utfpr.edu.br

Acknowledgements The authors gratefully acknowledge the financial support received from CNPq under grant 474290/2008-5 and 471825/2009-3, and Araucária Foundation under grant 06/56093-3.

Keywords «Harmonics», «Reactive power», «Power factor correction», «Active filter», «Unified Power Quality Conditioner (UPQC)», «Power quality»

Abstract This work deals with compensation algorithm schemes used in single-phase Unified Power Quality Conditioner (UPQC), allowing harmonic suppression and sag/swell compensation of the input voltage. In addition, reactive power compensation and harmonic suppression of the input current are also carried out, resulting in an effective power factor correction. Two different operation modes are employed to control the UPQC using Synchronous Reference Frame (SRF) based controllers. In the first mode the series converter acts as a sinusoidal current source, while the parallel converter acts as a sinusoidal voltage source. In the second mode, the series converter acts as a non-sinusoidal voltage source, while the parallel converter acts as a non-sinusoidal current source. A comparative analysis of the two operation modes is made, in which the advantages and disadvantages of each are discussed. Validation results are presented to confirm the theoretical development and performance of the single-phase UPQC.

Introduction Voltage disturbances such as sags, swells and voltage transients caused by abnormal operating conditions of the power system, plus harmonic pollution of the utility voltages caused by the interaction between the high level of undesired harmonic currents and network impedances, have helped to worsen the power quality needed for critical loads. Additionally, the interaction between the reactive current and network impedances causes drops in fundamental utility voltages. Power quality conditioners such as Unified Power Quality Conditioners (UPQC) and Active Power Filters (APF) have been developed to improve the power quality in electrical systems, providing protection against power supply disturbances and clean power to critical loads, such as industrial process controllers, computers, medical equipment, data communication systems and others [1-11].

EPE 2011 - Birmingham

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Normally the series APF’s are used to compensate utility voltage disturbances, while parallel APF’s have commonly been used to suppress the harmonic currents of the non-linear loads and compensate reactive power [1-7]. On the other hand, series and parallel active filtering capability can be performed simultaneously using the UPQC [8-10]. In this paper, a single-phase UPQC is implemented, which is composed of two single-phase fullbridge PWM converters to perform the series and parallel active power line filter functions. The single-phase UPQC will be implemented adopting two different operation modes. In the first mode, the series converter works as a sinusoidal current source, while the parallel converter also works as a sinusoidal voltage source. In the second mode, the series converter works as a non-sinusoidal voltage source, eliminating utility voltage disturbances, while the parallel converter works as a non-sinusoidal current source eliminating any current harmonics injected from non-linear loads into the power supply system. In both operation modes the output voltage regulation is considered, resulting in output voltage with constant rms value and lower Total Harmonic Distortion (THD). In order to extract the single-phase current and voltage compensation references, necessary to compensate the input current and the output voltage, Synchronous Reference Frame (SRF) based controllers are used [3, 7, 11], in which were adapted to be used for single-phase loads. The coordinates of the unit vector sinθ and cosθ, used in SRF-based controllers, are obtained from the single-phase Phase-Locked Loop (PLL) system [12-14]. A comparative analysis of the two operation modes used to control the UPQC is made, in which their advantages and disadvantages are discussed. The sinusoidal and non-sinusoidal characteristics of the voltage and current references generated by the SRF-based controllers are considered. Finally, validation results are presented to confirm the theoretical development of the single-phase UPQC.

Conditioner Topology The topology of the single-phase UPQC is shown in Fig. 1, which uses two single-phase full-bridge PWM converters coupled to a common DC-link to perform the series and parallel APF functions. The DC-link is performed through a DC-bus capacitor. Since the voltage and current compensation references are generated using SRF-based controllers, and depending on the adopted strategy, the PWM converters can behave as a sinusoidal or a non-sinusoidal voltage/current sources. Accordingly, when the PWM series converter is working as a sinusoidal current source and the PWM parallel converter is working as a sinusoidal voltage source, the control strategy adopted is called Operation Mode 1 (OPM 1). When the PWM series converter is working as a non-sinusoidal voltage source and the PWM parallel converter is working as a non-sinusoidal current source, the control strategy adopted is called Operation Mode 2 (OPM 2). OPM 1 and OPM 2 are discussed in detail in the next section.

SRF-based controllers The SRF method must be adapted for use in a single-phase system. Therefore, a fictitious two-phase system will be created allowing the method to be used in its original form. When OPM 1 is adopted the series PWM converter is controlled to make the source currents ( i s ) sinusoidal and in phase with the power supply voltage ( v s ). Accordingly, the series converter must behave as a current source with a high enough impedance to isolate the line from the load harmonic currents. Similarly, the parallel PWM converter is controlled to act as a sinusoidal voltage source, such that regulated and sinusoidal output voltage ( v L ) is provided to the load. Any voltage disturbances such as sags, swells, harmonics and transients are handled by the series PWM converter through the series transformer. In addition, the parallel converter must behave as a voltage source with a low


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enough impedance to absorb the harmonic currents of the load. As with the source current, the output voltage is also controlled to be in phase with the power supply voltage. When OPM 2 is adopted, the series PWM converter is controlled to provide the compensating voltage ( vc ) to make the output voltage ( v L ) sinusoidal and regulated. Similarly, the parallel PWM converter is controlled to provide the compensating current ( ic ) to make the input current ( i s ) sinusoidal with low THD.

Fig. 1: Single-phase UPQC topology

Current SRF-based Controller Algorithm Applied to Single-Phase Loads (OPM 1) The current SRF-based controller applied to single-phase loads is shown in Fig. 2. Measuring the load current ( i L ) provides the two-phase stationary reference frame αβ quantities ( iα , i β ). The acquired load current is treated as the ´α´ coordinate of the fictitious two-phase stationary reference frame (αβ). Subsequently, iα has a π 2 radian phase delay, producing the fictitious β coordinate ( i β ). Therefore, a new two-phase system, represented by (1), can be studied in the αβ-axes. The d value ( id ) of the SRF is obtained by (2), where the phase-angle is obtained from a PLL system, which will be identical to the utility phase-angle θ. The DC component iddc of id is found by using a Low-Pass Filter (LPF). The total direct current component id Tdc , given by (3), is reached by adding iddc to the component obtained from the DC-bus controller ( idc ). Since only the direct component is extracted from the current SRF-based controller, it represents the active current demanded by the load ( id dc ). Finally, the sinusoidal reference current i s∗ can be achieved directly from the synchronous rotating dq reference frame given by (4). The component idc (Fig. 2) is the output signal of the DC-bus controller (PI controller), which represents the compensation of the losses related to the inductances and switching devices. In other words, idc represents the total active power demanded by the conditioners to regulate the DC-link voltage.


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Fig. 2: Current SRF-based controller (OPM 1) ⎡iα ⎤ ⎡ iL ( ωt ) ⎤ ⎢i ⎥ = ⎢ ⎥ ⎣ β ⎦ ⎣iL ( ωt − π 2 )⎦

(1)

id = iα cos θ + iβ sin θ

(2)

idTdc = id dc + idc

(3)

is∗ = idTdc cos θ

(4)

Voltage Reference Generation (OPM 1) The output voltage reference ( v ∗L ) is controlled to be in phase with the utility voltage ( v s ). The PLL phase-angle reference θ*, which must be identical to θ, will be used to generate v ∗L , as represented by (5), where V Lp is the required amplitude of the output voltage. In this work the single-phase PLL scheme presented in [14] (Fig. 3), was adopted for all the implementations of the SRF-based controllers, in which, using software and knowing the measurement of the single-phase of the utility voltage, all coordinates of the unit vector ( sin θ and cos θ ) can be obtained. v ∗L = V Lp sin θ *

(5)

Current SRF-Based Controller Algorithm Applied to Single-Phase Loads (OPM 2) In OPM 2, the current SRF-based controller generates the reference current of the parallel PWM converter to perform the following tasks: suppress the harmonic currents of the load, perform the compensation of the reactive power, and control the DC-link voltage. Similar to Fig. 2, the current SRF-based controller used to achieve the parallel reference current is shown in Fig. 4, where the direct and quadrature currents are obtained from (6).


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Fig. 3: PLL scheme of the single-phase PLL control system To achieve the harmonic part of i L in d-axis ( id h ), the DC component of the direct current id ( id dc ), found by using an LPF, is subtracted from id . Thus, the total reference current ic∗ for the OPM 2, that represents the total harmonic parcels (dq-axes) and the reactive parcel (q-axis) of i L , is obtained by (7), in which the current idc , which represents the total active power demanded by the conditioners to regulate the DC-link voltage, is considered. ⎡id ⎤ ⎡ cos θ ⎢i ⎥ = ⎢ ⎣ q ⎦ ⎣ − sin θ

[

sin θ ⎤ ⎡iα ⎤ ⎢ ⎥ cos θ ⎥⎦ ⎣iβ ⎦

(6)

]

(7)

ic∗ = ( id − id dc ) + idc cos θ − iq sin θ

Fig. 4: Block diagram of the current SRF-based controller for single-phase load (OPM 2)


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Voltage SRF-based Controller Algorithm Applied to Single-Phase Loads (OPM 2) In OPM 2, the voltage SRF-based controller, shown in Fig. 5, generates the reference voltage to the series PWM voltage to suppress the harmonic voltages of the utility. In addition, it must maintain the regulated output voltage. The adopted strategy to obtain the series compensation voltage is similar to that used to generate the reference current to the parallel PWM converter in OPM 2. Measuring the input voltage ( v s ) provides the two-phase stationary reference frame αβ quantities ( vα , v β ), as shown in Fig. 5. The acquired input voltage is treated as the ´α´ coordinate of the fictitious two-phase stationary reference frame (αβ). Subsequently, vα has a π 2 radian phase delay, producing the fictitious β coordinate ( v β ). Therefore, a new fictitious two-phase system, represented by (8), can be studied in the αβ-axes.

Fig. 5: Voltage SRF-based controller for single-phase load (OPM 2) The two-phase voltages ( vα , v β ) are transformed from a two-phase stationary reference frame into a two-phase synchronous rotating (dq) reference frame (SRF), based on (9). In the dq-axes, the quantities vd and vq are filtered to achieve the quantities v d h and v q , that have all the harmonic h

components of the input AC voltage. Finally, the reference voltage of the series converter is obtained by (10). The output voltage ( v L ) must be controlled, and the two ways to perform this both use PI controllers, as shown in the AC output voltage controllers of Fig. 6. In Fig. 6 (a) the output voltage ( v L ) is measured and compared to the output voltage reference ( v ∗ ). The output signal of the PI controller L

( v pi ) is added to the harmonic reference voltage ( vc ) to generate the definitive reference voltage ( vc∗ ). In Fig. 6 (b), the output voltage is measured, phase-delayed and transformed into the two-phase synchronous rotating dq reference frame v Ld and v Lq . Subsequently, v Ld and v Lq are filtered and compared to the reference output voltages in the dq-axes ( v ∗Ld and v ∗Lq ). Output voltage control is obtained by adding the PI output signals v d pi and v q to the respective signals in the dq-axes, pi generating v d hT and v qhT , as found by (11) and (12). Thus, (10) must be replaced by (13), which represents the final reference voltage of the series PWM converter, now considering the control of v L and the harmonic suppression of v s .


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⎡ vα ⎤ ⎡ v s ( ωt ) ⎤ ⎢v ⎥ = ⎢ ⎥ ⎣ β ⎦ ⎣v s ( ωt − π 2 )⎦

(8)

⎡v d ⎤ ⎡ cos θ ⎢v ⎥ = ⎢ ⎣ q ⎦ ⎣− sin θ

(9)

sin θ ⎤ ⎡vα ⎤ ⎢ ⎥ cos θ ⎥⎦ ⎣v β ⎦

v c∗ = v d h cos θ − v qh sin θ

(10)

vd hT = ( vd − vd dc ) + vd pi

(11)

vqhT = ( vq − vq

(12)

dc

) + vq

pi

v c∗ = v d hT cos θ − v qhT sin θ

(13)

(a)

(b) Fig. 6: Voltage SRF-based controller for single-phase load (OPM 2)


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Simulation results To verify the application of the SRF-based controllers the single-phase UPQC was simulated. The PWM converters operate at 20 kHz switching frequency, while the load comprises a diode full-bridge rectifier, followed by R-L loads (Fig. 1). The utility voltage is assumed to have harmonic contents and has nominal rms phase-voltage equal to 127 V, while the values of the R-L loads are: R = 7.5 Ω, L = 20 mH. Table I shows the simulation parameters of the series and parallel PWM converters.

Table I: Simulation parameters PWM Converters

DC-bus Voltage Vdc (V)

UPQC DC-bus Capacitor Cdc (F)

300

2.2 m

Series Parallel

Filter Inductors (H) L fs = 500 μ

Filter Capacitors (F) C fs = 280 μ

L fp = 500 μ

C fp = 180 μ

The UPQC working in the OPM 1 and 2 are shown in Figs. 7 and 8, respectively. Figs. 7 and 8 (a), (b) and (c) show, respectively, the DC-bus voltage, output voltage and input current, when the load transient occurs at 150ms for the UPQC. The controlled output voltage and the compensated input current are sinusoidal with low THD. The DC-bus voltage is controlled at 300 V. The UPQC performing the compensation of the harmonic voltages and sag/swell disturbances, are shown in Figs. 9 and 10 for OPM 1 and 2, respectively. In Fig. 9 from 0s up to 104.16 ms the utility operates at nominal voltage. After that, from 104.16 ms up to 187.5 ms sag disturbance occurs. The swell disturbance occurs between 295.83 and 404.16 ms. Fig. 9 (b) shows the controlled and sinusoidal output voltage. Fig. 9 (c) shows the amplitude of the sinusoidal input current varying to control the balance of power flow through the UPQC, due to the sag and swell disturbances. It is controlled by the DC-bus controller shown in Figs. 2 and 4. Fig. 10 shows the same conditions for OPM 2, in which the output voltage is regulated using the AC output voltage controller shown in Fig. 6 (b). The dynamic performance of the UPQC working in OPM 1, is better than the UPQC working in OPM 2, because in OPM 2, besides the DC-bus controller, the UPQC requires the AC-output voltage controller, while in OPM 1 only the DC-bus controller is required.

Fig. 7: UPQC (OPM 1): (a) DC-bus voltage; (b) Output voltage; (c) Input current


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Fig. 8: UPQC (OPM 2): (a) DC-bus voltage; (b) Output voltage; (c) Input current

Fig. 9: UPQC (OPM 1): (a) DC-bus voltage and input voltage; (b) Output voltage; (c) Input current

Fig. 10: UPQC (OPM 2): (a) DC-bus voltage and input voltage; (b) Output voltage; (c) Input current


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Conclusion This paper presented SRF-based controllers used in single-phase UPQC, allowing harmonic current suppression and reactive power compensation, resulting in an effective power factor correction. The compensation of the input voltage disturbances was also taken into account. Two operation modes (OPM 1 and OPM2), were implemented to analyze the UPQC performance, when series-parallel active filtering was carried out. In OPM 1 the series PWM converter operates as a sinusoidal current source while the parallel PWM converter operates as a sinusoidal voltage source. It represents an advantage when compared with OPM 2, due to the voltage and current references are sinusoidal, making the PWM control easier to accurately track the references. Besides, in OPM 2, the series converter must to regulate the output voltage, simultaneously with the output compensation voltage task.

References [1] M.I.M. Monteiro, E.R. Cadaval, and F.B. González, “Comparison of Control Strategies for Shunt Active Power Filters in Three-Phase Four-Wire System,” IEEE Trans. on Power Electronics, vol. 22, no. 1, pp. 229-236, January 2007. [2] A. Pigazo, V.M. Moreno, E.J. Estébanez, M. Liserre, and A. Dell´Aquila, “Harmonic Compensation in Shunt Active Power Filters by Applying Kalman Filtering for Estimation of the Averaged Load Conductance,” in Proc. of IEEE International Symposium on Industrial Electronics, 2010, pp. 1874-1880. [3] S. Bhattacharya, T.M. Frank, D.M. Divan, and B. Banerjee, “Parallel Active Filter System Implementation and Design Issues for Utility Interface of Adjustable Speed Drive Systems,” in Proc. 31st Industry Applications Society Annual Meeting, 1996, pp. 1032-1039. [4] R. Griñó, R. Cardoner, R. Costa, and E. Fossas, “Digital Repetitive Control of a Three-Phase Four-Wire Shunt Active Filter,” IEEE Trans. on Industrial Electronics, vol. 54, no. 3, pp. 1495-1503, June 2007. [5] M. Aredes and E.H. Watanabe, “New Control Algorithms for Series and Shunt Three-Phase Four Wire Active Power Filters,” IEEE Trans. on Power Delivery, vol. 10, no. 3, pp. 1649-1656, 1995. [6] S.A.O. Silva, A. Feracin, S.G. Cervantes, A. Goedtel, and C.F. Nascimento, “Synchronous Reference Frame Based Controllers Applied to Shunt Active Power Filters in Three-Phase Four-Wire Systems,” in Proc. of IEEE International Conference on Industrial Technology, 2010. [7] M.C. Benhabib, and S. Saadate, “New Control Approach for Four-Wire Active Power Filter based on the use of Synchronous Reference Frame,” Electric Power Systems Research, ELSEVIER Science, Switzerland, No. 73, pp. 353 - 362, March 2005. [8] H. Fujita and H. Akagi, “The Unified Power Quality Conditioner: The Integration of Series and Shunt Active Filters,” IEEE Trans. on Power Electronics, vol. 13, no. 2, pp. 315-322, March 1998. [9] S.A.O. Silva, R.A. Modesto, A. Goedtel, C.F. Nascimento “Compensation Algorithms Applied to Power Quality Conditioners in Three-Phase Four-Wire Systems,” in Proc. of IEEE International Symposium on Industrial Electronics, 2010. [10] M. Aredes, R.M. Fernandes, “A Dual Topology of Unified Power Quality Conditioner: the iUPQC,” in Proc. 13th International European Power Electronics Conference, 2009, paper 0731. [11] S.A.O. Silva, P.F. Donoso-Garcia, P.C. Cortizo and P.F. Seixas, “A Three-Phase Line-Interactive UPS System Implementation with Series-Parallel Active Power-Line Conditioning Capabilities,” IEEE Trans. on Industry Applications, vol. 38, no. 6, pp. 1581-1590, 2002. [12] L.G.B. Rolim, D.R. Costa and M. Aredes, “Analysis and Software Implementation of a Robust Synchronizing PLL Circuit Based on the pq Theory,” IEEE Trans. on Industrial Electronics, vol. 53, no. 6, pp. 1919-1926, 2006. [13] R.M. Santos Filho, P.F. Seixas, P.C. Cortizo, L.A.B. Torres, and A. F. Souza, “Comparison of Three SinglePhase PLL Algorithms for UPS Applications,” IEEE Trans. on Industrial Electronics, vol. 55, no. 8, pp. 2923-2932, 2008. [14] S.A.O. Silva, A. Goedtel, C.F. Nascimento, L.B.G. Campanhol, and D. Paião, “A Comparative Analysis of p-PLL Algorithms for Single-Phase Utility Connected Systems,” in Proc. 13th International European Power Electronics Conference, 2009, paper 0390.


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