1
A Novel Automatic Power Factor Regulator
Jinn-Chang Wu
Abstract—A novel automatic power factor regulator (APFR)
comprising a conventional APFR and a power converter based
protector is proposed in this paper. The APFR can compensate
for the reactive power step by step, while the power converter
based protector is used to protect the APFR from the harmonic
damage. Additionally, a hybrid power switch is used to switch the
AC power capacitor to avoid the inrush current of the power
capacitor in the process of switching on. Hence, the proposed
APFR can compensate the reactive power and avoid the problems
of harmonic damage and inrush current of AC capacitor.
Computer simulation is made to verify the performance of the
proposed novel APFR. The simulation results show that the
proposed novel APFR has the expected performance.
Index Terms-- power factor, harmonic, power converter
I.
INTRODUCTION
M
ost loads in the industry power systems are inductive,
which absorb reactive power, and where the phase of its
current lags from that of voltage. Hence, the power factor is
low, which results in poor power efficiency of the power
distribution system; poor voltage regulation in the load side; as
well as de-rating of the transmission, substation and
distribution facilities. Generally, AC power capacitor is
installed to supply a leading reactive power and improve the
power factor.
Recently, harmonic pollution of the industrial power
system has become a serious problem due to the wide use of
nonlinear loads [1,2]. The AC power capacitor for power
factor correction provides a low impedance path for harmonic
current. Hence, AC power capacitors are frequently damaged
by harmonics. In addition, this results in the harmonic
resonance between the AC power capacitor and the
distribution power system, so the AC power capacitor may be
damaged due to over-voltage or over-current [3-7].
Furthermore, the over-voltage of AC power capacitor caused
by the harmonic resonance may damage the neighboring
electric power facilities and even result in public accidents.
In order to properly adjust the reactive power provided
by the AC power capacitor, the APFR shown in Fig.1 is
developed. The APFR consists of several sets of AC power
capacitors connected to the utility via switches S1 through SN.
In general, every set of AC power capacitors contain an
inductor to suppress the inrush current in the process of
switching on. The reactive power supplied by the APFR can be
J. C. Wu is with the Department of Electrical Engineering, Kun Shan
University of Technology, Tainan 710, Taiwan, R.O.C. (e-mail:
jinnwu@mail.ksut.edu.tw).
adjusted by switching on/off the set number of AC power
capacitors. Although, the APFR can supply an adjustable
reactive power to respond to variations of loads, the AC power
capacitor is still directly connected to the utility. Therefore, the
problem of harmonic damage cannot be solved by the APFR.
Figure 2 illustrates a facility based on the power electronic
technology to be applied in a distribution power system to
compensate for the reactive power [8-10]. This facility is
known as the active type reactive power compensator. This
active type reactive power compensator uses a power
converter via an inductor shunt connected to the utility, and it
may provide leading or lagging reactive power. The supplied
reactive power can be linearly adjusted to respond to the
variation of loads. Hence, the input power factor can be
maintained to be close to unity. Meanwhile, the active type
reactive power compensator will not result in harmonic
resonance, so it can avoid the damage to the harmonic
resonance generated by the AC power capacitor. However, the
active type reactive power compensator must compensate the
full reactive power demanded by the loads, and this requires a
large capacity of power converter in the active type reactive
power compensator. As a result, it is very expensive and its
wide application is limited.
Fig. 1 circuit diagram of APFR.
Fig. 2 circuit diagram of active type reactive power compensator.
2
In this paper, a novel APFR is proposed, consisting of a
conventional APFR and a power converter based protector
shunt connected to the APFR bus, and then connected to the
utility via a link inductor. The proposed APFR can supply an
adjustable reactive power in response the load variation. The
reactive power is primarily compensated by the APFR, and the
power converter is used to protect the AC power capacitors of
APFR from harmonic damage. To solve the problem of inrush
current in the process of switching on the AC power capacitor,
a hybrid power switch constructed by an electromagnetic
switch shunt connected to a thyristor series connected to a
resistor is used to switch the AC power capacitor. The
proposed APFR can compensate for the reactive power and
avoid the problems of harmonic damage and inrush current of
AC capacitors. Hence, its reliability can be improved. To
verify the performance of the novel APFR, a computer
simulation is made.
II.
OPERATION PRINCIPLE OF THE PROPOSED APFR
Fig. 3 shows the circuit diagram of the proposed APFR,
which consists of a conventional APFR and a power converter
based protector connected in parallel and then connected to the
utility via a link inductor. The proposed APFR can supply an
adjustable reactive power to respond to the load variation. The
conventional APFR consists of several sets of AC power
capacitor, and it compensates the reactive power step by step.
Every set of AC power capacitors comprises an inductor and a
power capacitor connected in series, and then shunt connects
to the APFR bus by a hybrid power switch. The capacity of
AC power capacitors switched into the APFR bus depends on
the instantaneous reactive power demanded by the loads. The
power converter based protector is used to protect the AC
power capacitor of the conventional APFR. In contrast, the
power converter based protector comprises a power converter
serially connected to a filter inductor and an AC power
capacitor, and then shunt connects to the APFR bus by a
hybrid power switch. A DC power capacitor is placed on the
dc side of the power converter to act as an energy buffer. A
link inductor is inserted between the utility and APFR bus.
This link inductor is used to block the power interference from
utility and reduce the power capacity of power converter.
The APFR supplies the reactive power to respond to load
variation by switching on/off a set number of AC power
capacitors. The power converter acts as a virtual harmonic
resistor to suppress the harmonic effect from nonlinear loads
and the distorted utility voltage. From Fig. 3, it shows that an
AC power capacitor is connected in series with the power
converter in the power converter based protector. The AC
capacitor is used to withstand the fundamental voltage of the
APFR bus. Since the voltage of the APFR bus is almost the
fundamental component in the normal condition, the power
rating of the power converter in this power converter based
protector can be reduced significantly due to the connected AC
power capacitor [11]. Hence, the dc side voltage of the power
converter can be kept to below the peak value of APFR bus
voltage, and the current rating of the power converter is only
the connected AC power capacitor’s current. Hence, the power
capacity of the power converter is very small as compared with
the APFR capacity, and the capacity is dependent on the set
number of AC power capacitors and the distortion of load
current and the utility voltage.
The power converter based protector is expected to
suppress the harmonic current injected into the APFR. The
conventional control algorithm of the hybrid power filter
[12,13] can be used for this purpose. The difference between
the proposed power converter based protector and the
conventional hybrid power filter is that the current flowing
through the link inductor is expected to be sinusoidal in the
proposed power converter based protector, whereas the current
of the utility side is expected to be sinusoidal in the
conventional hybrid power filter. The output voltage vc1(t) of
power converter for harmonic suppression is represented as:
v c1 ( t ) = ki ah ( t )
(1)
where iah(t) is the harmonic component of the APFR current in
the link inductor, and k is a constant value. The power
converter used in the proposed APFR is used to avoid the
power resonance of conventional APFR. If the power
converter generates a voltage proportional to the harmonic
component of the injecting current shown in (1), it can act as a
virtual harmonic resistor k serially connected to the link
inductor at the harmonic frequency [12,13]. Hence, the power
converter can avoid the harmonic current injecting into the
proposed APFR due to the inserted virtual harmonic resistor.
Therefore, the power converter can improve the APFR
performance and protect the APFR under the conditions of the
distorted utility and the neighboring nonlinear load.
III.
SYSTEM ANALYSIS
The harmonic equivalent circuit of the proposed APFR is
shown in Fig. 4(a). The APFR impedance can be simplified as
Zah, and the impedance of link inductor and utility impedance
are denoted as ZLih and Zsh. Since the voltage-source power
converter generates the voltage shown in (1), it can be
regarded as a dependent voltage source, Vch(t). The nonlinear
load in the load side is simplified as a current source iLh(t). The
Fig. 3 the circuit diagram of the proposed APFR.
3
Fig. 4 the equivalent circuit of the hybrid compensation system.
equivalent circuit of Fig. 4(a) can be simplified to Fig. 4(b) by
the current/voltage source transformation, and the following
equations can be derived
veqh (t ) = i Lh (t ) Z sh
(2)
Z eq = Z sh + Z Lish
(3)
From the viewpoint of the harmonic voltage source, the
equivalent impedance before applying the power converter (
k=0) can be derived as:
Z eqh Z ah + Z eqh Z ch + Z ah Z ch
Zh =
(4)
Z ah + Z ch
The harmonic current injecting into the APFR can be derived
as:
Veqh
I Lih =
(5)
Zh
From (4), it can be found that the equivalent impedance Zh is
nearly zero at the frequency of power resonance. As seen in
(5), the power resonance will result in a very large harmonic
current injecting into the APFR, which will damage the APFR.
After applying the power converter to the APFR, the
independent voltage source, generated the voltage shown as
(1), is operated. Then, the equivalent impedance from the
viewpoint of the harmonic voltage source can be rewritten as
Z eqh Z ah + Z eqh Z ch + Z ah Z ch + kZ ah
Zh =
(6)
Z ah + Z ch
Comparing (6) with (4), it can be found that the term kZah is
adding in the numerator of (6) after applying the power
converter. Hence the equivalent impedance Zh will be
amplified under the resonant frequency. This means that the
harmonic current caused by the power resonance can be
suppressed. The larger k is, and the better suppression effect
will be. The frequency response diagram of the injecting
current and the power converter current are shown in Fig. 5.
The parameters of the proposed APFR system are shown in
Tab. 1. In Fig.5, two sets of AC power capacitors and the
power converter based protector are applied to the APFR bus.
As seen in Fig. 5(a), the injecting harmonic current of APFR is
reduced when the gain k is increased. However, Fig. 5(b)
shows that the current of the power converter is amplified
significantly when the gain k is increased. This is due to that a
power resonance occurs in the inner loop Zah and Zch of APFR
when the power converter is applied. To solve this problem,
the tuned frequency of Zah must be smaller than that of Zch [14].
Hence, the filter inductor of power converter based protector is
reduced to 150uH, and the frequency response diagram of the
injecting current and the power converter current are redrawn
and shown in Fig. 6. This figure shows that the power
converter current is clearly reduced after reducing the filter
inductor of power converter based protector, and the injecting
harmonic current is not affected.
Although the harmonic component of the power converter
current is reduced after using a small filter inductor in the
power converter based protector, the power resonance still
occurs in the inner loop Zah and Zch of APFR. To further
suppress the power resonance of inner loop Zah and Zch, a
damping resistor should be inserted into the inner loop Zah and
Zch. However, the practical resistor cannot be inserted into the
inner loop Zah and Zch due to the power loss. Hence, the power
converter must function not only as a virtual harmonic resistor
serially connected to the link inductor but also as a virtual
harmonic resistor serially connected to Zch. To obtain the
function of a virtual harmonic resistor serially connected to Zch,
the power converter must generate a voltage proportional to
the harmonic current of Zch branch and it is represented as:
v c 2 ( t ) = k r i ch ( t )
(7)
where ich(t) is the harmonic current of the Zch branch. If the
power converter generates a voltage shown as (7), it can act as
a virtual harmonic resistor kr serially connected to Zch at the
harmonic frequency. The frequency response diagram of the
injecting current and the power converter current after adding
the function of virtual harmonic resistor serially connected to
Zch are shown in Fig. 7. As seen in Fig. 7(b), the power
converter current can be further reduced when the power
converter act as a virtual harmonic resistor serially connected
to Zch at the harmonic frequency. Furthermore, it also can be
found that the injecting current is slightly increased when the
power converter acts as a virtual harmonic resistor serially
connected to Zch at the harmonic frequency. Hence, a trade-off
must be made between the power rating of power converter
and the injecting harmonic current of APFR.
Tab. I parameters of the proposed APFR
Utility
380V, 60Hz
Utility inductance
50uH
Link inductor
50uH
Power capacitor segment
L:200uH, C:300uF
Utility inductance
50uH
DC bus voltage of Power converter
150V
Switching frequency of power converter
20kHz
4
Fig. 5 the frequency response diagram, (a) injected current, (b) power
converter current.
Fig. 6 the frequency response diagram, (a) injected current, (b) power
converter current.
The power converter acting as a virtual harmonic resistor
will result in real power flowing through the power converter
and the variation of dc side voltage in the power converter. To
balance the real power flow, the power converter must absorb
or regenerate the real power from or into the utility under the
fundamental frequency. And then, the dc side voltage of power
converter can be maintained at a desired value to operate the
power converter normally. Hence, the power converter must
generate a voltage proportional to the fundamental component
of the Zch branch current, which can be represented as:
v c3 ( t ) = k f i c1 ( t )
(8)
where ic1(t) is fundamental component of Zch branch current.
The power converter is operated as a virtual fundamental
resistor kf when it generates the voltage shown in (8).
Considering the function of power resonance suppression
and DC bus voltage regulation, the output voltage of the power
converter is
v c ( t ) = ki ah ( t ) + k r i ch ( t ) + k f i c1 ( t )
(9)
IV. HYBRID POWER SWIRCH
Conventionally, an electromagnetic switch is used to turn
on or off the AC power capacitor to the power feeder at low
level voltages such as 220V, 380V or 480V. However, it
requires about ten milliseconds or more to actuate the
electromagnetic switch, and then a significant transient current
may be generated. This is known as the inrush current, which
flows through the switch and affects the reliability and life of
the switch, as well as shortening the life of the AC power
capacitor. In general, the capacity of the electromagnetic
switch must be significantly increased to withstand the inrush
current of the AC power capacitor at the instant of switching
on [15]. However, the electromagnetic switch used in the
APFR is switched on/off frequently, and the problem of the
Fig. 7 the frequency response diagram, (a) injected current, (b) power
converter current.
inrush current is very serious, so the resolution to this problem
is very important.
A conventional high voltage/current endurable thyristor is
also suitable to act as a switch to turn on/off the AC power
capacitor from a power feeder [15]. This thyristor can be
precisely controlled and connected the AC power capacitor to
the power feeder. Hence, it can reduce the inrush current of the
AC power capacitor during the process of switching on.
However, a significant voltage will drop on the thyristor
during the conduction period. This results in a significant
power loss on the thyristor switch of the AC power capacitor
and the thyristor’s temperature increases. In order to avoid the
problem of overheating in the thyristor, an additional huge
heat-sink and cooling fan are required. Therefore, the overall
efficiency of the thyristor used as the AC power capacitor’s
5
switch is lower than that of a conventional electromagnetic
switch.
Figure 8 illustrates the circuit configuration of the hybrid
power switch. This hybrid power switch includes two
switching loops. One is an electromagnetic switch, and the
other is a solid-state switch constructed by a thyristor and a
resistor connected in series. The resistor in the solid-state
switch is very small, and it is used to damping the oscillation
caused by the AC power capacitor and the inductor when the
set of AC power capacitor is applied to the distribution power
system. The solid-state switch of the hybrid switch acts as an
auxiliary switch during the transient states of both switching
on and switching off processes, and the electromagnetic switch
is the main switch in the steady state. In the process of turning
on, the solid-state switch is firstly turned on at the zerocrossing point of bus voltage. At the same time, the
electromagnetic switch is also actuated, and then it is closed
after several milliseconds. The on state dropped voltage of the
electromagnetic switch is much smaller than that of the solidstate switch. Hence, the power capacitor current will flow
through the electromagnetic switch after the electromagnetic
switch is closed. The conduction time of the solid-state switch
in the process of switching on is only one to two cycles of the
utility. Hence, the heating problem of thyristor and resistor can
be neglected. Therefore, the hybrid switch can effectively
solve the problem of the inrush current. Since the inrush
current of AC power capacitor at the instant of switching on is
suppressed by the proposed hybrid switch module, the capacity
rating of the electromagnetic switch can be significantly
reduced compared with that using a singular electromagnetic
switch to turn AC power capacitor on/off. Consequently, this
may increase the life and reliability of the electromagnetic
switch.
V. SIMULATION RESULTS
In order to verify the performance of the proposed APFR, a
computer simulation is made. The main parameters used in the
computer simulation system are also shown in Tab. I. The
filter inductor used in power converter based protector is
150uH.
Figure 9 shows the current waveforms of the power
capacitor at the instant of switching on. In Fig 9(a), the power
capacitor is switched on via an electromagnetic switch, and a
significant inrush current and current oscillation occurs. Since
an inductor is connected in series with the AC power capacitor
in every set of AC power capacitor, it can suppress the inrush
Fig. 8 the circuit configuration of the hybrid type power switch for APFR.
Fig. 9
the current waveforms of the power capacitor at the instant of
switching on, (a) by the electromagnetic switch , (b) by the thyristor
switch, (c) by hybrid type power switch.
current but generates a current oscillation of long duration.
The amplitude of inrush current is dependent on the voltage
angle that the electromagnetic switch is turned on. Fig. 9(b)
shows the power capacitor is switching on via the thyristor
switch. It can be found that the inrush current of the AC power
capacitor is suppressed by switching at the precise zero
crossing point of the APFR bus voltage. However, it still
generates the current oscillation. Fig. 9(c) shows the AC power
capacitor is switched on via the hybrid power switch, and it
shows that almost no transient current appears at the instant of
switching on the AC power capacitor. Hence, the hybrid power
switch can suppress both the inrush current and the current
oscillation, and it is very suitable to be the switch in the APFR
application.
Figs. 10 and 11 show the simulation results for the
proposed APFR before applying the power converter. Three
sets of AC power capacitors are switched on, and the load is a
phase-controlled current-fed rectifier. As seen in Fig. 10(d), it
can be found that the current of APFR contains rich harmonic.
Figure 11 shows the spectrum of the utility voltage and APFR
current. As seen in Fig. 11(b), the power resonance between
the utility impedance and APFR occurs near 420Hz. The
power quality is degraded due to the power resonance, and the
total harmonic distortion (THD) of utility voltage is 8.6%.
Figures 12 and 13 show the simulation results for the proposed
APFR after applying the power converter based protector with
k=kr=1and two sets of AC power capacitors. As seen in Fig.
12(d), the power converter based protector can effectively
suppress the injecting harmonic current of APFR. Figure 13
shows the spectrum of the utility voltage and the APFR current.
Figure 13(b) shows that the power resonance at 420Hz is
suppressed effectively. Hence the power quality is improved
when the power converter based protector is applied, and the
THD of the utility voltage is improved to 4.5%.
Figure 14 shows the simulation results after applying the
power converter based protector with k=1 and kr=0. This
indicates that a large harmonic current flows through the
power converter because the power resonance occurs in the
inner loop Zah and Zch of APFR. Comparing Fig. 12(e) with
Fig. 14(b), it can be found that the virtual harmonic resistor kr
can suppress the power resonance of inner loop Zah and Zch of
APFR, and it is very consistent with the above analysis. Fig.
6
Fig. 14
the simulation result of the proposed APFR under the power
converter but kr=0, (a) APFR current, (b) power converter current.
Fig. 10 the simulation result before applying the power converter, (a) utility
voltage, (b) utility current, (c) load current, (d) APFR current.
Fig. 15 the simulation result of the proposed APFR under the load variation,
(a) utility voltage, (b) utility current, (c) load current, (d) APFR
current, (e) reactive power of utility.
VI. CONCLUSION
Fig. 11 the spectrum, (a) utility voltage, (b) APFR current.
Fig. 12 the simulation result of the proposed APFR after applying the power
converter, (a) utility voltage, (b) utility current, (c) load current, (d)
APFR current, (e) power converter current.
Since nonlinear loads are widely used in modern power
systems, the problems of harmonic pollution are very serious.
The APFR is widely used to improve the power factor in the
distribution power system. However, harmonic amplification
may occur due to the power resonance between the power
capacitor of APFR and the system impedance under the
harmonic-polluted power system. The power resonance will
induce a high voltage and high current on the AC power
capacitor, and it may damage the AC power capacitor and
inductor of the APFR.
In this paper, a novel APFR is proposed to supply a
reactive power tracing the load variation and avoiding
harmonic damage. The proposed APFR consists of a
conventional APFR and a power converter based protector.
Because the power capacity of the power converter is very
small compared with the capacity of APFR, it can be used in
practical applications from the viewpoint of cost. The
simulation results show that the performance of the proposed
APFR is as expected.
VII. ACKNOWLEDGEMENT
The authors would like to express their acknowledgement to
the financial support under NSC 92-2213-E-168-010.
VIII. REFERENCES
Fig. 13 the spectrum, (a) utility voltage, (b) APFR current.
15 shows the simulation results of the proposed APFR under
the load variation. The load is linear and inductive. It
indicates that the reactive power supplied by the proposed
APFR can effectively trace the load variation. It also shows
that the transient performance of the proposed APFR is
excellent.
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BIOGRAPHIES
Jinn-Chang Wu was born in Tainan, Taiwan on 1968. He graduated from
National Kaohsiung Institute of Technology, Kaohsiung, Taiwan in 1988, and
received his M.S. and Ph.D. degree from National Cheng Kung University,
Tainan, Taiwan in 1992 and 2000, all in electrical engineering. Since 2002,
he has been an associate professor at the Department of Electrical
Engineering, Kun Shan University of Technology. His research interests are
in power quality and power electronic applications.