Design of a Power Flow Control Method for Hybrid Active Front-End Converters

Transcription

1 Design of a ower Flow Control Method for Hybrid Active Front-nd Converters Tzung-Lin Lee Zong-Jie Chen Shang-Hung Hu Department of lectrical ngineering National Sun Yat-sen niversity 7, Lienhai Rd., Kaohsiung 844, TAWAN mail: tllee@mail.ee.nsysu.edu.tw Abstract Active front-end converters with bidirectional power flow capability have been extensively used for utility applications of power electronics. Large passive filters are normally required at the grid side of the converter to mitigate switching M noise. This may become a critical issue, in terms of installation space and potential resonance of the passive filter. This paper proposes a hybrid active front-end converter and its power flow control method. The hybrid converter is composed of a capacitor and a voltage source converter in series connection. Bidirectional real power of the converter can be controlled by the output voltage vector perpendicular to the grid voltage, and reactive power delivery of the converter for grid voltage regulation can be determined by the output voltage vector parallel to the grid voltage. Due to series connection capacitor, the converter can be operated between a low-voltage dc side and high-voltage grid side without any low-frequency transformer, which is the significant advantage of the proposed method. A harmonic resistance is also emulated in the proposed method to assure stable operation of the converter for unintentional voltage spike coming from the power system. Operation principles are explained in detail, and computer simulations and experimental results are provided to validate the effectiveness of the proposed approach. KYWORDS Hybrid active front-end converter, power flow control. NTRODCTON Active front-end converters have been extensively used in various power conversion applications requiring bidirectional power flow capability, such as adjustable speed drivers, uninterruptible power supply systems, inverter based distributed generators, and active power filters [], []. assive power filters, such as LC or LCL filters, are usually deployed between the converter and the grid to mitigate switching M noise produced by the converter [3], [4]. However, the required passive filters is a critical issue in terms of installation space and weight, and potential harmonic resonance between the passive filters and the grid may cause instability of the converter. Various active damping approaches have been proposed to suppress the harmonic resonances of passive filters, but power rating of the converter may be limited due to high switching frequency [5], [6], [3]. Multilevel inverters with reduced switching voltage would provide acceptable M noise at the cost of high component count and complex control [7], [8]. A shunt hybrid circuit has been proposed in active filtering applications of medium-voltage range, where a series capacitor replaces the bulky low-frequency transformer to reduce kva rating of the active filter and the resulting switching ripples [9], []. Authors have presented simulation results of transformerless interface converters applied in distributed generation systems with unity power factor operation []. This paper proposes a power flow control method for a hybrid active front-end converter. The proposed converter is composed of a capacitor and a voltage source converter in series connection. Bidirectional real power of the converter can be controlled by the output voltage vector perpendicular to the grid voltage, and reactive power delivery of the converter for grid voltage regulation can be determined by the output voltage vector parallel to the grid voltage. Since the series capacitor sustains a part of the fundamental voltage, the converter can be operated with a reduced dc voltage compared with the conventional grid-connected inverter. This algorithm allows the fundamental power control between the low-voltage dc side and the high-voltage grid side without any low-frequency transformer, which is a significant advantage in terms of both installation space and switching M noise of the converter. The proposed interface converter also emulates harmonic resistance to reduce transient voltage of the converter resulting from the capacitive switching of the power system.. ORATON RNCLS A simplified one-line diagram of the proposed hybrid active front-end converter (HAFC) is shown in Fig.. The proposed HAFC is composed of a capacitor C and a voltage source converter in series connection to the grid. The operation of the proposed HAFC includes both real power control (S is at A) and reactive power control(s is at B) modes. The reference voltage generator determines the voltage command abc according to the operational mode. The voltage controller then produces the current command i abc of the converter. Subsequently, the current controller generates the output voltage command vabc for the WM of the converter. Operational principles are detailed as follows. A. Operation modes n the real power control mode, the angle command θa is set as 9 o for maximum real power conversion and the voltage command A is determined by using a controller to regulate the real power output according to the real power command. The real power is calculated by using the instantaneous power theory []. f the HAFC is operated 33

2 Loads Vs V dc HAFC C L i abc LL ω e qd e qd 9 A θ A B θ B Sinusoidal Generator Sinusoidal Generator A B S abc abc i abc ω abc to d e q e e qd e qd i e qd HF ĩ e qd i qd ω d e q e to abc i abc i abc,h abc R i abc Current regulator v abc,f v abc,h v abc WM Reference voltage generator Voltage controller Current controller Fig.. The proposed HAFC and its associated control. in the reactive power control mode, the angle command θb should be set as o for maximum reactive power delivery. The voltage command B is based on the required reactive power for restoring the grid voltage to the nominal value, which can be implemented by a synchronous reference frame (SRF) controller [3]. Note that, if a dc capacitor is used in the dc side of the converter, the dc voltage can be controlled by drawing reactive current from the grid []. Therefore, the voltage command abc can be obtained for different operating modes. B. Voltage and current controls A -based voltage controller in the SRF is implemented to generate the current command i e qd of the converter according to the measured voltage abc. Based on the current command i abc, the measured current i abc, and the measured voltage abc, the current regulator in the stationary frame calculates the voltage command vabc,f of the fundamental frequency as follows [], v abc,f = abc L i ΔT (i abc i abc ). () L i is the output inductor of the inverter, and ΔT is the sampling period. Harmonic damping for suppressing the transient voltage coming from the upstream of the power system is also emulated, whose operation is defined as follows, v abc,h = R i abc,h. () R represents harmonic resistance, i abc,h is harmonic current component, and v abc,h is harmonic voltage command of the HAFC, respectively. Harmonic current component i abc,h can be extracted by a high pass filter (HF) in the SRF. Finally, the space vector WM is employed to synthesize the gating signals of the inverter. Based on this algorithm, the HAFC current can be controlled with the desired power flow requirement. = o V c = θ Fig.. X c A single phase equivalent circuit of the HAFC. C. ower flow analysis Fig. shows the single-phase equivalent circuit of the proposed HAFC at the fundamental frequency. The inverter and its output inductor are considered as an ideal voltage source, due to the proposed voltage control. is the grid voltage, θ is the angle of leading, and X c is the reactance of series capacitor, respectively. nverter output real power, inverter output reactive power, the reactive power injected into the grid can be expressed as follows, = 3X c ( sinθ) =3X c ( cosθ) (3) =3X c ( cosθ) 34

3 Obviously, maximum real power transfer is at θ= ± 9 o and maximum reactive power delivery is at θ= o or 8 o, respectively. Therefore, the real power or the reactive power can be separately and adequately controlled by regulating voltage command A or B with the corresponding angle condition. n this paper, θ A = 9 and θ B = are used for power controlling. The converter is at the discharging mode > in Fig. 4(a) and at the charging mode < in Fig. 4(b), respectively. n addition, the converter is operated as an inductor > in Fig. 4(c) or a capacitor < in Fig. 4(d), respectively ower. (a) >. r Degree (a),, and for =. pu and =.3 pu, respectively, with fixed X c=. pu. (c) >. (d) <. Fig. 4. hasor diagrams of the proposed HAFC..4.3 L l X =. X =. X c X c X c X c = Degree (b),, and for X c=. pu and X c=.3 pu, respectively, with fixed =.3 pu. Fig. 3. xamples of power flow analysis. Fig. 3(a) and Fig. 3(b) show power-flowing examples between and for various conditions. Fig. 3(a) illustrates maximum real power and maximum reactive power are increased with when X c is fixed. The controllable range of both real power and reactive power is also extended with increasing X c as shown in Fig. 3(b) if is fixed. Fig. 4 shows phasor diagrams of power flow in the proposed HAFC. Vs 35 Fig. 5. Ls Simulation circuit. C L i HAFC V dc. SMLATON RSLTS Fig. 5 shows the simulation circuit and circuit parameters are given as follows: ower system: V(line-to-line), kva, 6 Hz. C= μf, L s =.mh, L i =.mh, L l =3 mh. V dc = V, rms =7 V. The converter is a conventional three-phase voltage source inverter. The WM frequency and the sampling frequency are khz and khz, respectively. Fig. 6 shows kw power conversion for both discharging and charging modes. At t=s, the HAFC starts in operation with = kw. The converter voltage is increased and maintained 9 o lagging the grid voltage for supplying real power to the grid as shown in Fig. 6(a). At the steady

4 Time rms Time (a) kw discharging operation from dc side to grid side. Fig. 7. Simulations of reactive power delivery for grid voltage regulation Fig Time (b) kw charging operation from grid side to dc side. Simulations of real power conversion. state, rms =35 V, rms = A, = kw, and =-8 var. Subsequently, is changed to - kw at t=3s. Fig. 6(b) shows the HAFC enters the charging mode and draw kw from the grid when reaching the steady state. At this time, is kept 9 o leading the grid voltage. Fig. 7 shows the reactive power control for grid voltage regulation. Due to inductive load L l, the grid voltage drops to 6.4 V, which is slightly lower than nominal value. After the HAFC is started at t=s, the converter voltage is regulated to 8 o out of phase with the grid voltage. Finally, the grid voltage can restore to the nominal value, and the HAFC absorbs.7 kvar at rms = V and rms = A. V. XRMNTAL RSLTS The experimental setup is similar to the simulation circuit in Fig. 5 except the inductive loading L l =3 mh is absent. The converter control is implemented by using T TMS3F8335 chip to perform signal processing, such as power calculation, frame transformation, controllers, filters, and WM algorithm. Fig. 8 shows both steady-state and transient results of real power conversion operation from the dc side to the grid side for A =3V. The converter output current lags the grid voltage by 9 o and maintains in phase with the converter output voltage as shown in Fig. 8(a). Fig. 8(b) shows the transient of the real power and reactive power after the HAFC is in operation. TABL summarizes test results of,, when A =5V, 3V, 35V, 4V, respectively. With increasing A, more real power is delivered to the grid. TABL RAL OWR CONVRSON. A 5V 3V 35V 4V.7A A.4A.8A 6W 775W 934W 95W -88var -var -6var -5var Fig. 9 shows the reactive power delivery for grid voltage regulation. The reference grid voltage is set as 8V(peak). Before the HAFC is started, the grid voltage is 85 V(peak) due to no loading. After the HAFC is engaged, the grid voltage is restored to its reference value, 8 V(peak), with =8 var and =8. A as illustrated in Fig. 9(b). n contrast, the converter output current lags the grid voltage and the converter output voltage by 9 o simultaneously as shown in Fig. 9(a). TABL gives and when B =5V, 3V, 35V, 4V, respectively. More reactive power delivery to the grid with increasing B is verified. 36

5 (a) nverter output voltage, inverter output current, and grid voltage of phase a. (a) nverter output voltage, inverter output current, and grid voltage of phase a. peak (b) nverter output real power and inverter output reactive power. Fig. 8. Voltage, current, real power output and reactive power output when the HAFC is in real power operation mode. ( :5 W/div,:5 var/div,, : V/div, : A/div) (b) nverter output reactive power and grid voltage peak value peak (with 8V offset). Fig. 9. Voltage, current and reactive power output when the HAFC is in reactive power operation mode. ( :5 W/div,:5 var/div,, : V/div, : A/div, peak : V/div) TABL RACTV OWR DLVRY. B 5V 3V 35V 4V 8.8A 8.7A 8.5A 8.3A 47var 559var 637var 78var V. SMMARY A power flow control method for a hybrid active frontend converter is presented in this paper. The maximum real power flow can be converted between the dc side and ac side with bidirectional capability, and the maximum reactive power can be controlled for grid voltage regulation by adequately adjusting the converter voltage vector. Thanks to a series capacitor between the converter and the grid, the converter can be operated with a reduced dc voltage without any lowfrequency transformer, compared with the conventional gridconnected inverter. This is a significant advantage, in terms of both installation space and switching M noise of the converter. Fig. shows switching ripples between the proposed hybrid active front-end converter and the conventional gridconnected converter, where both converters deliver the same real power (kw) to the grid. Obviously, the conventional gridconnected converter produces about 5 times current switching ripple compared with the proposed method. As shown in (3), the power delivery of the proposed HAFC is dependent on both and X c, where is related to the required dc voltage of the converter and X c is the reactance of the series capacitor. Fig. shows the relationship of power conversion for to X c. is roughly inverse to X c for fixed power delivery, and required or X c is increased with power output. Based on Fig. (a) and Fig. (b), the maximum dc voltage and the series capacitor can be determined for the required power output. 37

6 .8 =.5pu =.pu =.5pu =pu.6.4. (a) Current spectrum of the proposed HAFC X c (a) The relationship of to X c for real power conversion..8 =.5pu =.pu =.5pu =pu.6.4 (b) Current spectrum of the conventional grid-connected converter. Fig.. Comparison of switching ripples between the proposed HAFC and the conventional grid-connected converter when they produce kw real power output concurrently. ACKNOWLDGMNT This research is funded by the National Science Council of TAWAN under grant NSC Fig X c (b) The relationship of to X c for reactive power conversion. The relationship of to X c for various power output. RFRNCS [] T. G. Habetler, A space vector-based rectifier regulator for AC/DC/AC converters, Trans. ower lectron., vol. 8, no., pp. 3 36, Jan [] H. Akagi, Active harmonic filters, roc., vol. 93, no., pp. 8 4, Dec. 5. [3]. C. Loh and D. G. Holmes, Analysis of multiloop control strategies for LC/CL/LCL-filtered voltage-source and current-source inverters, Trans. nd. Appl., vol. 4, no., pp , Mar./Apr. 5. [4]. J. Gabe, V. F. Montagner, and H. inheiro, Design and implementation of a robust current controller for vsi connected to the grid through an lcl filter, Trans. ower lectron., vol. 4, no. 6, pp , June 9. [5] V. Blasko and V. Kaura, A novel control to actively damp resonance in input LC filter of a three-phase voltage source converter, Trans. nd. Appl., vol. 33, no., pp , Mar./Apl [6]. Twining and D. G. Holmes, Grid current regulation of a threephase voltage source inverter with an lcl input filter, Trans. ower lectron., vol. 8, no. 3, pp , May 3. [7] J. Rodriguez, J. S. Lai, and F. Z. eng, Multilevel inverters: a survey of topologies, controls and applications, Trans. nd. lectron., vol. 49, no. 4, pp ,. [8] B. Wu, High-ower Converters and AC Drives. iscataway, NJ: ress, 6. [9] H. Akagi, S. Srianthumrong, and Y. Tamai, Comparison in circuit configuration and filtering performance between hybrid and pure shunt active filters, in ndustry Applications Conference 38th AS Annual Meeting, 3, pp. 95. [] S. Srianthumrong and H. Akagi, A medium-voltage transformerless ac/dc power conversion system consisting of a diode rectifier and a shunt hybrid filter, Trans. nd. Appl., vol. 39, no. 3, pp , May/Jun. 3. [] T.-L. Lee and Z.-J. Chen, A transformerless interface converter for a distributed generation system, in 3th ower lectronics and Motion Control Conference (-MC), 8. [] H. Akagi, Y. Kanagawa, and A. Nabase, nstantaneous reactive power compensator comprising switching devices without energy storage components, Trans. nd. Appl., vol. A-, pp , May/Jun [3] S. Bhattacharya, D. Divan, and B. Banerjee, Synchronous frame harmonic isolator using active series filter, in the 4th uropean Conference on ower lectronics and Applications, 99, pp