Electrical Engineering in Japan, Vol. 168, No. 4, 2009 Translated from Denki Gakkai Ronbunshi, Vol. 127-D, No. 8, August 2007, pp. 789–795 STATCOM Using the New Concept of an Inverter System with Controlled Gradational Voltage NOBUHIKO HATANO,1 YUKIMORI KISHIDA,2 and AKIHIKO IWATA2 1 The Kansai Electric Power Co., Inc., Japan Advanced Technology R&D Center, Mitsubishi Electric Corp., Japan 2 The authors have developed an inverter system using the gradational voltage control approach [5, 6]. This method is intended to improve efficiency and to suppress harmonics by combining inverter units with different voltages and switching frequencies [7]. Another advantage of this approach is that high-voltage output is available by series connection of single-phase inverters on the AC side [8, 9], so that transformer-less configuration can be expected. In this study, we describe a prototype three-phase 200-V STATCOM using gradational voltage control and present experimental results verifying improvement of the power factor and harmonic compensation. SUMMARY This paper presents the STATCOM using the new concept of an inverter system which consists of three inverter units connected in series, and each inverter unit generates a different input voltage such as VC, 2VC, 4VC. This inver ter system can output a high-quality voltage by a low-frequency switching oper ation. Thus, it can combine low loss and high quality. In this paper, we show the technique to apply STATCOM which uses the inver ter system,and examination results.©2009WileyPer iodicals, Inc. Electr Eng J pn, 168(4): 58–65, 2009; Published online inWileyInter Science(www.inter science.wiley.com). DOI 10.1002/eej.20823 2. Outline of STATCOM Using Gradational Voltage Control Key words: STATCOM; controlled gradational voltage; inverter. 2.1 Configuration and basic operation The STATCOM using gradational voltage control is shown in Fig. 1. The system includes three series singlephase inverters connected to the power system by reactors. Essentially, capacitors are employed for DC power supply but, as will be explained later, an auxiliary DC source is required. In gradational voltage control, the series-connected inverters have reference DC voltages 4VCN, 2VCN, and VCN (voltage ratio 4:2:1). The basic unit voltage VCN is set according to the application. For example, in a transformerless three-phase 6.6-kV system, VCN is set to 800 V so that reference DC sources of 3200, 1600, and 800 V are available. Now consider the inverter operation patterns. The basic concept is to improve efficiency by reducing the switching frequencies of the high- and medium-voltage inverters while increasing the switching frequency of the 1. Introduction With deregulation of the power market and popularization of distributed generators, there is increasing demand for flexible operation and control of power systems based on the use of power electronics. In particular, the static synchronous compensator (STATCOM) offers a simple configuration but has voltage fluctuation control, power factor improvement, and many other advanced functions of system control; there are already many examples of their practical implementation [1, 2]. Expectations for the further development of STATCOM are loss reduction, addition of active filter functions, etc. [3, 4]. One of the important prerequisites for such further development is a method of inverter control providing good output voltage waveforms at low switching frequencies. © 2009 Wiley Periodicals, Inc. 58 maintained at 4:2. This is made possible by using the fact that two switching patterns are available when the sum of the two inverter’s output voltage is ±2VCN in order to charge and discharge different capacitors. Suppose, for example, that the output voltage and current are related as shown in Fig. 3. In Switching Pattern 1, the capacitor with a reference of 4VCN is charged, while that with a reference of 2VCN is discharged. In Switching Pattern 2, the capacitor with a reference of 2VCN is charged. There can be three other relations between the polarity of the input voltage and current; however, the capacitor voltage ratio can always be maintained at 4:2 by detecting the current polarity and selecting the appropriate switching pattern. On the other hand, the switching patterns are fixed when the sum of the two inverters’ output voltage is ±4VCN and ±6VCN, so that the voltage ratio cannot be adjusted. However, this is not likely to have any strong effect on the STATCOM performance, because the inverter with a DC reference of VCN is controlled by a high-frequency PWM. In addition, the above method of maintaining the capacitor voltage ratio assumes that the STATCOM outputs sufficient current for voltage ratio adjustment in the period while the output voltage is +2VCN or –2VCN. In the experiments described below in Section 3, this assumption was satisfied because the main output component was a fundamental-wave reactive current. However, when emphasis is placed on harmonic compensation, the assumption is not necessarily valid, and measures must be taken in order to assure sufficient current in the period while the output voltage is +2VCN or –2VCN. For example, a fundamentalwave reactive current proportional to the error of the capacitor voltage ratio can be added to the target output. On the other hand, for the low-voltage inverter with DC reference of VCN [Fig. 2(d)], the switching pattern is fixed, and the capacitor charging and discharging cannot be adjusted, in contrast to the voltages of 4VCN and 2VCN. Fig. 1. STATCOM using inverter system with controlled gradational voltage. low-voltage inverter in order to improve the output voltage/current waveforms. Specifically, the high- and medium-voltage inverters with DC references of 4VCN and 2VCN are operated, respectively, in one-pulse and three-pulse modes, as shown in Figs. 2(a) and 2(b). The pulse width and timing are set so that the output voltage VSH + VSM of the two inverters follows as faithfully as possible the target Vref, as shown in Fig. 2(c). The greatest difference between the output voltage and target voltage is VCN. The low-voltage inverter with DC reference of VCN is operated by a high-frequency PWM so as to compensate this difference. The STATCOM output voltage VSH + VSM + VSL is a 15-level waveform, as shown in Fig. 2(e). In the proposed system, the capacitor voltage ratio of the inverters with DC references of 4VCN and 2VCN is always Fig. 3. Adjustment of DC capacitor voltage ratio. Fig. 2. Operational timing chart. 59 Hence, here the auxiliary DC source is used instead of the capacitor. 2.2 Control of output current We next describe the method of output current control aiming at improving the power factor and compensating for harmonics. The main variables are listed in Table 1, and the model circuit is shown in Fig. 4. The block diagram of STATCOM output current control is illustrated in Fig. 5. The difference between the reactive power Qp output by the power source and its target Qref is integrated and multiplied by cos(ωt + φk) [k = a, b, c] to obtain the reactive current component, and the active power Pp output by the power source is multiplied by sin(ωt + φk)/Vpeak to obtain the active current component. Then the target values of the source current Irefa − Irefc are calculated by summing the reactive and active components. In addition, the voltage components ∆VSa − ∆VSc for current control are obtained from the proportional and differential elements of the source current IPa − IPc minus the target values Irefa − Irefc and the components ICa − ICc used to adjust the capacitor voltage (explained later). The voltage components ∆VSa − ∆VSc found in this way are added to the voltage defined by the peak value Vpeak and phases φa – φc, to obtain the target Vrefa − Vrefc of the output voltage. The active power Pp, reactive power Qp, system voltage reference Vpeak, and φa – φc (Fig. 5) are calculated as shown in Fig. 6. The active power Pp and reactive power Qp output by the source are obtained from the instantaneous values of the source current IPa − IPc and load voltage Vta − Vtc, based on pq-theory [10]. As regards the system voltage reference, the peak voltage Vpeak is calculated as the root-sum-square of Vtα, Vtβ obtained by αβ transformation of the load voltage Vta − Vtc, and φa – φc are obtained by synchronization with Vta. For convenience, φa is taken as the reference below (φa = 0). The calculation of the current components ICa − ICc used to adjust capacitor energy is illustrated in Fig. 7. Here Fig. 4. Model system. Fig. 5. Calculation of output voltage reference. Fig. 6. Calculation of power and grid voltage. Table 1. Main variables Fig. 7. Adjustment of DC capacitor energy. 60 IC1a − IC1c are components used to adjust the capacitor energy ECave of the whole STATCOM; they are active current components proportional to the difference ∆EC with respect to the target Eref. The set of current elements IC2a − IC2c consists of negative-sequence components used to handle unbalance of the system voltage. In the STATCOM using gradational voltage control, there is no energy exchange between the capacitors on the DC side (see Unit a, Unit b, Unit c in Fig. 4). Therefore, when a negative-sequence current is output, the capacitor energies ECa − ECc of Unit a, Unit b, Unit c diverge, even though the total energy ECave remains unchanged, which makes continuous operation impossible. In this study, we attempt to solve this problem by obtaining the negative-sequence current output from the STATCOM so as to cancel the divergence of ECa − ECc. For example, Fig. 8 shows the relationship between the current components used to compensate for the deviation of ECa (Unit a) from the target Eref, and the system voltage reference determined by the peak voltage Vpeak and phase φa. The active powers Pa, Pb, and Pc output, respectively, by Unit a, Unit b, and Unit c are given by Eq. (1): namely, IC2a − IC2c in Fig. 8 are negative-sequence current components defined so that Unit a outputs the power P in Eq. (1), which is returned to the system at Unit b and Unit c, at P/2 each: 3. Experimental Results 3.1 Experimental setup The circuit configuration, switching patterns, and control method described in the previous section were implemented in a prototype three-phase 200-V STATCOM, and experiments were performed to evaluate the power factor improvement, harmonic compensation, and handling of system voltage unbalance. A single-line diagram of the experimental system and a three-line load diagram of load are shown in Figs. 9 and 10, respectively; the circuit and control parameters are given in Table 2. In the experimental setup, the STATCOM is connected to the power system in parallel to the load. The secondary side of the source transformer has two taps with ratios of 100 and 80%, and the unbalance of the system voltage can be generated by tap switching on each phase. Two loads were provided: an RL load for the experiments of the power factor improvement, and a rectifier load for the experiments of harmonics compensation. 3.2 Capacitor setting The capacitors CH and CM were selected so that the fluctuation of the capacitor voltages was about 1 V when the STATCOM output the rated fundamental-wave reactive current. This constraint can be formulated as follows, with ∆VCH and ∆VCM denoting the voltage fluctuations of capacitors CH and CM, and with the peak value of rated current being 30 Apeak: (1) The same applies to the current components IC2b, IC2c obtained from ∆ECb and ∆ECc; the three phase components IC2a − IC2c equalize the respective capacitor energies ECa − ECc. Fig. 8. Example of ICa − ICc and grid voltage. Fig. 9. Experimental setup. 61 In addition, capacitor CL was set at 9.9 F, which is very large compared to CH and CM because it operates as auxiliary DC source. 3.3 Setting of control parameters The control parameters given in Table 2 were determined experimentally using the circuit presented above. The gain K1 for controlling overall capacitor energy was selected by confirming the stability of the capacitor voltages with the functions of power factor improvement and harmonic compensation disabled, that is, with Pp and Qp equal to 0 in Fig. 5. As indicated by IC1k [k = a, b, c] in Fig. 7, the gain K1 is intended to output an active current from STATCOM proportional to the deviation of the capacitor energy from its reference, while the relationship between the active PSK power output from the STATCOM’s k-phase [k = a, b, c] and the average capacitor energy ECave of the capacitor energy can be expressed by Eq. (5) in the Laplace transform domain. Then the capacitor energy ECave is found by solving Eq. (6): Fig. 10. Load circuit. (2) (3) (4) (5) Considering the above expressions and the constants Vn, VCHN, and VCMN in Table 2, the capacitors were chosen as CH = 66 mF and CM = 136 mF. (6) Thus, the gain K1 determines the time constant to recover the capacitor energy ECave to its reference value after a transient fluctuation. Under our experimental conditions, the time constant is about 9.5 ms, as estimated from K1 and Vn in Table 2. The gain K2 for controlling capacitor energy at each phase was selected by confirming the stability of the capacitor voltages under an unbalance condition of the system voltage, while confirming the stability of the capacitor voltage. When the system voltage includes a negative-sequence component with a peak value Vneg, the negative-sequence current flowing into the load is Vneg/Vn times the rated current. The STATCOM control shown in Fig. 5 provides compensation of such a negative-sequence current; for example, when the a-phase outputs an active power Vneg/Vn times as the rated phase power, half of this power returns to other phases. By this, the a-phase capacitor voltage drops and capacitor voltages of other phases increase by half as much. As a result, control by IC2k [k = a, b, c] in Fig. 7 works. The relation between the a-phase active power PSa and the capacitor energy ECa can be estimated by Eq. (7). Solving Eq. (7) gives the capacitor energy ECa at steady state in the time domain, as shown in Eq. (8): Table 2. Circuit and control parameters 62 (7) (8) Thus, the gain K2 reduces the deviation of the capacitor energy ECa from the average value. In our experiments, Vneg was 10% of the rated voltage Vn; assuming that the voltage ratio VCHa:VCMa is maintained at 4:2, and converting the error of ECa into the error of VCHa + VCMa by means of the constants K2, Vn, VCHN, VCMN, RRL, and LRL (Table 2), the deviation from three phases average voltage is about 4 V. The time constant T is associated with the low-pass filter that removes oscillations caused by the voltage Vta − Vtc and the current Ipa − Ipc from the Pp, Qp, and Vpeak of the control block in Fig. 6. This time constant was set so as to remove oscillations above a few tens of hertz from Pp, Qp, and Vpeak. The gains KP, KQ, and K1 for controlling the STATCOM output current were selected while confirming the current waveforms during operation with power factor improvement. Fig. 11. Experimental results of power factor improvement. 3.4 Experiments of power factor improvement The improvement effect of power factor was examined using the RL load as shown in Fig. 10(a). Since the control constant Qref in Table 2 is 0, the STATCOM (Fig. 9) acts to match the phase of the source current Ipa − Ipc to the voltage Vpa − Vpc. The results thus obtained for the a-phase are presented in Fig. 11. The load current Ita lags Vpa by about 3 ms; however, the delay of the source current Ipa is only 0.2 ms due to compensation by the STATCOM output current ISa, and the power factor from the voltage Vpa to the load side is improved to about 1. The capacitor voltages VCHa and VCMa are maintained near the respective reference values of 88 and 44 V. 3.5 Experiments of harmonic compensation Compensation for harmonics current was verified by using the rectifier load as shown in Fig. 10(b). Since the control constant Qref in Table 2 is 0, the STATCOM (Fig. 9) acts to compensate harmonics in the source current Ipa − Ipc, while matching the current phase to the voltage Vpa − Vpc. The results thus obtained for the a-phase are presented in Fig. 12. The THD of the load current Ita is about 20%, while the THD of the source current Ipa is only about Fig. 12. Experimental results of harmonics compensation. 63 5%. In addition, the delay of the source current Ipa with respect to Vpa is only about 0.2 ms, and the power factor seen from the voltage Vpa to the load side is improved to about 1. The capacitor voltages VCHa and VCMa are maintained near the respective reference values of 88 and 44 V. 3.6 Experiments of operation under unbalanced system voltage Unbalance of the system voltage was produced by switching the secondary tap to 80% for phase in Fig. 9. In the experiments, the RL load was used as shown in Fig. 10(a). As explained in Section 2.2 and in Fig. 7, the STATCOM acts to compensate the negative-sequence current caused by the unbalanced system voltage, and maintains the voltage of the three-phase capacitor. Since the control constant Qref in Table 2 is 0, the phase of the source current Ipa − Ipc is matched to the voltage Vpa − Vpc. The results thus obtained for the a-, b-, and c-phases are presented in Fig. 13. The peak value of Vpa decreases to 80% relative to Vpb and Vpc, so that an unbalance occurs. However, the capacitor voltages VCHa − VCHc and VCMa − VCMc are equal for three phases, and the reference values of 88 and 44 V are maintained. In addition, the power Fig. 14. Comparison results (without adjustment of unbalanced voltage). factor from the voltage Vpa − Vpc to the load side is improved to about 1. For comparison, Fig. 14 presents experimental results obtained with the control constant K2 (Fig. 7) set to 0, that is, without any compensation for the negative-sequence current. Here the capacitor voltages VCHa − VCHc and VCMa − VCMc show the deviation between the three phases. For example, the voltage VCHb reaches about 120 V, while VCHa and VCHc drop below 80 V, which degrades STATCOM operation. These results confirm the effectiveness of the proposed method in handling unbalanced voltage. 4. Conclusions We built a three-phase 200-V STATCOM using gradational voltage control, and performed experimental verification of power factor improvement and harmonics compensation. The system with gradational voltage control achieves high system efficiency and low output distortion by using a combination of inverter units with different voltage and switching frequencies. However, system control becomes complicated because multiple inverters must be operated in coordination according to specific applications, while the energy of the individual DC sources must be adjusted. In this paper, we applied high-frequency switching to the inverter with the lowest DC voltage. And we used adjustment of the negative-sequence current output by STATCOM so as to handle unbalance of the system voltage. The experiments confirmed the effectiveness of the proposed method in terms of both power factor improvement and harmonic suppression. Fig. 13. Experimental results of operation under unbalanced voltage source. 64 6. Hatano N, Kishida Y, Iwata A. 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(in Japanese) AUTHORS (from left to right) Nobuhiko Hatano (member) graduated from Osaka University in 1987, completed the M.E. program in 1989, and joined the Kansai Electric Power, Co., Inc. His research interests are power electronics technologies for power systems. Yukimori Kishida (member) completed the M.E. program at Tsukuba University in 1993 and joined Mitsubishi Electric Corp., Advanced Technology R&D Center. His research interests are power circuit breakers; he is now involved in power converters. Akihiko Iwata (member) graduated from Yatsuhiro National College of Technology in 1981 and joined Mitsubishi Electric Corp. His research interests are development of power supply units for arc welders, laser devices, and other machines. 65