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.
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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.
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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.
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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.
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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
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(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.
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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.
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