1. Power Factor Controller -- An Integrated Power Quality Device
Richard Tinggen
Yi Hu, Le Tang and Harry Mathews
Rick Tyner
ABB High Voltage Technologies Ltd.
Zurich, Switzermd
ABB Electric systems Technology Institute
Raleigh, NC, USA
ABB Power T&D
Distribution Systems Division
Florence, SC, USA
Major concerns in the application of switched shunt
capacitors are: appropriate steady state voltage profile
control, voltage and current transients associated with
capacitor switching, and the potential problem of harmonic
resonance. The advantage of the PFC is due to its
capability of accurate control, which enables the PFC to
provide advanced solutions for dynamic reactive power
compensation as needed. The control also makes it possible
to reduce the disturbances to sensitive loads due to
capacitor switching transients. The control algorithms are
also designed to provide protection against potential
harmonic resonance.
Abstract --This paper describes a new integrated power
quality device --Power Factor Controller (PFC) for power
distribution system and industrial power circuit applications.
A PFC integrates breaker-switched capacitor banks into a
compact design with low cost sensing elements and an
intelligent control unit. The device provides more accurate
voltage control and power factor correction than traditional
shunt capacitor bank installations. The integrated design of
the PFC greatly simplifies the tasks and reduces the costs in
system design, installation and operation of shunt capacitor
compensation systems. The PFC offers advanced control
functions to m n m z capacitor switching transients, and to
iiie
protect the load and capacitors from being affected by possible
harmonic resonance associated with shunt capacitor
applications in power systems.
U. PFC AND ITS APPLICATION
--
Without reactive power compensation applied, most power
distribution systems have a lagging power factor because of
the reactive power consuming elements in the circuits such
as step-down transformers and overhead feeder sections.
The lagging angle is more pronounced in industrial circuits
because of the application of induction machines and phasecontrolled rectifiers. Typical power factors and their range
of variation for some industries without any compensation
measures applied are listed in Table 1. For a given
distribution system, reactive power drawn from its power
supply source can vary dynamically.
Index Terms Power Factor Control, Power Quality,
Distribution Network, Voltage Control, Harmonic Resonance,
Capacitor Switching
I. INTRODUCTION
Shunt capacitor banks have been applied in many power
distribution systems and industrial circuits for reactive
power compensation. This maximizes the usage of the
capacity of the power transmission and distribution circuit
and maintains a proper voltage level at the end user
connection points for improved productivity of industrial
processes.
Table 1 Typical Power Factors of Industries
This paper introduces an integrated power quality device -Power Factor Controller (PFC) for power factor correction
and voltage support at distribution voltage levels. The PFC
integrates several breaker switched capacitor banks into a
compact design with low cost sensing elements and an
intelligent control unit. The PFC provides a reliable and
affordable solution to distribution system reactive power
compensation. In this paper, the design criteria, application
limitations, performance influential factors and economical
impact of the PFC are described.
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Auto Parts
Chemical
Hospital
Office Building
Steel Works
Plastic
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75-80
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Load
Power
Factor
Induction Motor
Diode Rectifier (Small ASDs)
Fluorescent Lighting. Standard Ballast
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Electronic Ballast
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Phase Controlled Rectifier
DC Drives, Large ASDs, Arc Furnaces
Arc Welding
Resistance Welding
70-80
95-98
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MAIN AUTHOR AlTILIATION INFORMATION GOES HERE.
0-7803-5515-6/99/$10.00
0 1999 IEEE
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2. The power factors of loads have different characteristics.
Some can be nearly constant, while others can vary in a
wide range depending on the loading level as shown in
Table 2. These typical power factors shown in the tables
indicate that the capacity of a circuit could be severely
limited without proper power factor compensation. The
low power factors result in increased circuit losses and poor
system voltage profile along the feeder circuit, which, in
turn, leads to a lower energy efficiency and reduced
productivity and quality in product processing.
cause undesired disturbances to some sensitive equipment
inside a customer facility because of voltage and current
transients associated with switching.
A typical
phenomenon which has been discussed in many papers over
the last decade is voltage magnification phenomenon. This
has been identified as the root cause for nuisance tripping of
adjustable speed drives. In some cases, the magnified
transient voltage is also responsible for thyristor
commutation failures in rectifier circuits and types of
insulation failures such as bushing flashover and surge
arrestor damage.
Ideally, the power factors at each load bus should be
compensated close to 100%. In the past, the power factor
compensation was traditionally done by installing shunt
capacitors at different load busses in a circuit. The design
and installation of traditional shunt capacitors can be an
involved and costly process. Traditional shunt capacitors
are typically single bank installations. They may be
permanently connected to a load bus or via breakers to
permit switching. The capacitor, as well as the switching
breaker and the control unit (if it is a switched shunt
capacitor application) for each installation, are typically
selected by the utilities and purchased directly from
manufacturers. All components are then assembled and
tested in the field, and put into operation. The selection of a
shunt capacitor installation in a distribution system should
first start with an estimate of the total, amount of reactive
power compensation needed in the system.
Then,
according to system voltage requirements and the principle
of minimized total system losses, the number of
installations needed, the steps required for each installation
and the locations of the shunt capacitor banks are
determined. During this decision making process, system
studies may often be required to ensure that the shunt
capacitor will be able to deliver satisfactory performance
and will not cause potential problems to the system, such as
harmonic resonance, and to devise proper countermeasures
if problems do exist (e.g. de-tune the circuit if resonance
occurs). The capacitors and the breaker need to be properly
sized for each individual application. The installation of a
traditional shunt capacitor requires all components to be
assembled and tested in the field along with a scheduled
outage for each test.
The PFC was developed to provide a low cost solution to
the discussed problems in distribution systems and
industrial circuits. A PFC typically consists of one or more
breaker switched capacitor banks along with an intelligent
control unit. A PFC could be a single-phase design or
three-phase design. All capacitor banks, their switching
breakers, required sensors, protective components and the
control unit are packaged into the PFC enclosure, which is
manufactured and fully tested in the factory.
By integrating breaker switched capacitor banks into a
compact design with intelligent control unit, the application
of the PFC offers a reliable and affordable reactive power
compensation solution for distribution systems. Since it is
manufactured and tested in the factory, the reliability of a
PFC is ensured. The integrated design of PFC greatly
simplifies the field installation (no more field assembling
and testing), and allows for a fast deployment with reduced
labor cost and system down time for installation. With its
integrated intelligent control unit, a PFC provides greatly
improved performance and compensation than traditional
shunt capacitors, as well as other advanced functions.
I
PFC
Control
.....
-.-.-.-.-.-.-.-.-.-.-.-.-.Load
For traditional switched shunt capacitors, usually only very
simple controls (voltage relay, timer, etc.) are employed.
Incorporating complex control for traditional switched
shunt capacitors is possible, but may incur high cost due to
the additional cost for measurement components, control
unit, additional wiring, control unit programming, testing,
etc. The development or customization of a control system
for each individual application, either done by utilities
themselves or by outsourcing, could be a major effort,
which may increase the cost of a switched shunt capacitor
application substantially and make it economically not
justifiable. However, the simple control has difficulty
maintaining var compensation and voltage profile at the
desired level, especially when the actual system condition
deviates from that used in the original design. Also,
frequent switching operation of these capacitor banks may
Figure 1 Typical PFC configuration
The power factor control of the PFC is performed by
controlling the opening and closing of the capacitor
switches based on the measured power factors. The control
unit of the PFC measures three phase voltage and current on
the feeder side. The active power, reactive power and
power factor are computed from the measured voltage and
current by the PFC control unit, and the results of the
computed power factor is compared to the pre-determined
(input by user) target power factor setting. Using measured
and target power factors and the capacitor information, the
control unit determines if one or more capacitor banks need
to be switched ON or OFF to bring the actual power factor
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3. as close as possible to the targeted power factor setting.
The PFC thus offers ii more precise power factor
compensation compared to traditional shunt capacitors.
Capacitor SMtchlng at Voltage Peak (BUSSA)
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The PFC also provides a more economical alternative to the
Distribution Static VAR Compensators (DSVC). A DSVC
typically consists of a reactor and several capacitor banks
switched by power electronic switches. The reactive power
that the DSVC supplies can be regulated continuously by
controlling the firing angle. The main differences between
the PFC and the DSVC are: the DSVC can vary reactive
power in the designed rmge continuously and can supply
inductive reactive power if needed, while the PFC changes
reactive power in steps by switching ON or OFF one or
more capacitor banks, and provides only capacitive reactive
power. However, the power electronic valves used by
DSVC are more costly than the mechanical switches used
in a PFC. For the same reactive power capacity, a DSVC is
more expensive than a PFC due to high cost associated with
the power electronic devices, protection circuit for power
electronics devices, and the reactor used in a DSVC. In
addition, the configuration and control of an DSVC is more
complicated than that of a PFC due to its use of power
electronics devices, which requires complicated firing
control and protection functions to ensure it will operate
correctly. For many industrial distribution system power
factor compensation applications where continuous reactive
power regulation and inductive reactive power supply are
not required, the PFC provides a more economical solution
to the customer than a DSVC.
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Figure 2 Switching-ontransients at voltage peak
The overvoltage associated with capacitor switching causes
two major concerns: (1) the impact of overvoltage on
equipment insulation level, and (2) the impact of the
transients on power quality. The following discusses each
of these concerns.
!
In addition to being a reliable and affordable solution for
power factor compensation, the PFC also supplies a
solution to other problems associated with switched shunt
capacitor applications. The following two sections discuss
the two main concerns in switched shunt capacitor
application, i.e., capacitor switching transients and potential
problems of a harmonic resonance, as well as the solutions
provided by PFC.
4
4
1
Industrialload
Large industrial drive load
SCI: 900kVAR Switched
Sc2: 600 LVAR, Fixed
S C 3 1,200 LVAR. Switched
Figure 3 One-line diagram of the simulated system
Although the highest magnitude of a capacitor switching
transient overvoltage at the bus where capacitor switching
occurs may not exceed 2 pu, the transient overvoltage at
other buses in the circuit could be much higher due to so
called "voltage magnification". Figure 4 illustrates such a
problem. The magnitude of the transient overvoltage at the
terminal of the switched capacitor is 1.57 pu, but results in
2.26 pu on the low voltage side of the downstream
transformer. Excessive overvoltage exposes the equipment
in the system to the danger of possible damage. It could
become a problem for some applications from the
equipment insulation level point of view. For these
applications, reducinflimiting the magnitude of capacitor
switching transient overvoltage is preferred.
Iu. CAPACITOR SWITCHING TRANSIENTS
Capacitor switching is known to generate voltage transients
in the system. Switching-on a fully discharged capacitor at
voltage peak may result in a 2 pu transient overvoltage at
the switched capacitor terminal. The capacitor switching
transient usually dies out quickly (typically in about one
cycle). Figure 2 shows a waveform of this type of capacitor
switching transient.
All waveforms in this paper are obtained fiom a simulated
industrial distribution network unless otherwise indicated.
The one-line diagram of the simulated system is shown in
Figure 3. There are three shunt capacitors in the system
(SC1 at bus 5, SC2 at bus 6 and SC3 at bus 9). The load at
bus 8 is predominantly drive load which acts as the
harmonic current source. The waveform in Figure 2 is
obtained at the bus 9 when the capacitor SC3 is switched.
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4. the operation of capacitor breaker with the voltage
waveforms.
When several capacitor banks are connected to the same
bus, switching ON a fully discharged capacitor bank when
some capacitor banks are already in operation ("back-toback" switching situation) may result in a high in-rush
current flowing from energized banks to the one being
switched. Depending upon the magnitude of the in-rush
current and the ratings of the capacitors and their breakers,
a current limiting reactor may be needed. Similar to the
normal capacitor switching situation, the in-rush current
magnitude of "back-to-back" capacitor switching is also
depends on the point on the voltage wave where switching
occurs. The highest magnitude occurs when a capacitor is
switched on at a voltage peak. "Back-to-back" capacitor
switching at the voltage zero results in the lowest in-rush
current magnitude. With synchronized capacitor switching
control, the in-rush current magnitude of "back-to-back"
capacitor switching can also be effectively reduced/limited.
Figure 4 Overvoltage magnification case
In addition, for effective protection of power electronic
switching components of PWM drives, most drive
manufacturers set the protective tripping level of the PWM
drive at a dc bus voltage level of 1.2 pu. The capacitor
transient can easily generate a temporary dc bus
overvoltage and cause nuisance tripping of the drive.
Synchronized capacitor switching allows the PFC to offer
improved power factor compensation function with reduced
switching transients. This reduces the possibility of
potential equipment damage and improves the power
quality in shunt capacitor applications.
Iv. SYNCHRONIZED CAPACITOR SWITCHING
The magnitude of the transient overvoltage of a capacitor
switching transient is related to the point on the voltage
waveform that the capacitor is switched, i.e. it is point-ofwaveform dependent. The magnitude of the switching
transient overvoltage could be very small if a capacitor is
switched on at a voltage zero instead of a voltage peak, as
can be seen from Figure 5. The capacitor is switched on at
the same bus (bus 9) as before but at the voltage zero
instead of at voltage peak (slightly before 0.2 second). The
figure shows there is very small transient after the capacitor
switching, and the voltage magnitude remains close to 1.0
per unit.
The synchronized switching in the PFC is implemented by
tracking the voltage waveforms, and controlling the
capacitor breaker's opening or closing operation based on
the breaker operating time and voltage waveform. For a
three phase PFC, capacitors on each phase must be
switched on at a different moment due to the phase
difference between phases. This will require breakers of a
three-phase PFC to be operated on an individual phase
basis (a three phase breaker could not be used).
CaDacitor Switchinq at Vokaae Zero fBUSQA)
RESONANCE
V. HARMONIC
Another major concern in shunt capacitor application is the
potential problem of a harmonic resonance. Switching ON
or OFF shunt capacitors in a distribution network changes
the topology of the system. This may tune one of the system
resonance frequencies to one of the frequencies of the
harmonic sources in the circuit. A harmonic resonance
could also occur as a result of the change of other system
parameters which causes the circuit to be tuned to a
harmonic resonance condition. Application of a PFC faces
the same problem.
0.15
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02
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t
The harmonic resonance caused by switching-on a
capacitor can be seen in Figure 6 and Figure 7 obtained
from the simulated industrial distribution network.
Switching-off a capacitor could also cause a harmonic
resonance to occur as can be seen from Figure 8. The
capacitor SC3 is switched on (or off) at bus 9 with
synchronized switching control at about 0.2 seconds.
03
.
(sec)
Figure 5 Switching-on transients at voltage zero
The magnitude of capacitor switching transient overvoltage
thus could be effectively controlled by synchronizing the
capacitor breaker's operation so that it closes at, or close to,
the zero point on the voltage waveform, i.e., to synchronize
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5. wide range variation of system impedance, it will
significantly increase the cost for each individual PFC
application in additional to the capital investment on
equipment. The solution only helps an individual
application to avoid possible harmonic resonance for the
studied system conditions, and may not completely prevent
a possible resonance if the system condition deviates from
that used in the system study after a PFC is installed on the
field.
Switching-on capacitor cause resonance (BUS9A)
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s
>
o
Switchingotl capacitor cause resonance (BUS9A)
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Figure 6 Voltage waveform of a capacitor switching-ON
harmonic resonance case
Harmonic resonance results in the system voltage and
current being distorted with high magnitude harmonic
contents. Prolonged harmonic resonance in the distribution
circuit could overheat and cause damage to equipment. The
application of shunt capacitors must ensure that either the
capacitor will not cause any potential harmonic resonance
or must devise proper countermeasures to overcome the
problem.
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0.2
0.15
0.25
03
.
(sec)
Figure 8 Switching-offcapacitor cause resonance
The harmonic resonance which results from capacitor
switching could be resolved by taking a reverse action to
de-tune the system (e.g., switch OFF a capacitor if the
resonance is caused by the switch ON of the same
capacitor). A capacitor switch-ON resulting in a harmonic
resonance case is shown in Figure 6 and Figure 7, switchOFF the same capacitor effectively eliminated the
resonance in the system as can be seen from the voltage and
current waveforms shown in Figure 9.
The application of a PFC encounters the same problem as
the traditional shunt capacitor. For the PFC, one possible
solution is to conduct detailed system study, as has been
done for traditional shunt capacitor applications, for each
individual application, to size capacitor and determine
operating procedure accordingly, and de-tune the circuit if
there is a resonance possibility.
Swltchingoff capacitor remove resonance (BUS9A)
1 . 5 1 !
01
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0.05
g o
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1
0.15
02
.
0.25
03
.
t (sec)
Figure 7 Current waveform of a capacitor switchingON harmonic resonance case
However, the traditional solution has its own drawbacks.
Because the study must be conducted for each application
and various system operating conditions that may result in a
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6. Figure 10 Harmonic spectrum before capacitor
switching
Voltage signal spectrum after capacitor switching
10,
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0.24
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t (Sec)
(b) Current waveform
Figure 9 Switch-off capacitor remove resonance
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20
25
30
t (sec.)
Thus a better solution for a PFC would be to implement a
resonance protection function in its intelligent control unit.
This will allow the PFC to take a reverse action if a
capacitor switching harmonic resonance did occur. System
studies for each individual application will not be needed,
and this solution could reduce the cost for each PFC
application. This solution is adopted by the PFC.
Figure 11Harmonic spectrum after capacitor switching
The control unit of the PFC determines if the capacitor
switching has caused a harmonic resonance and takes the
reverse action according to the following steps:
Step 1: before sendingthe command to thebreaker, store
pre-switching spectrum analysis results for
comparison;
Step 2: send breaker switch command to initiate breaker
operation;
Step 3: perform spectrum analysis on post-switching
voltage waveforms and compare them to preswitching results;
Step 4: if sudden change in harmonic contents is detected
to be exceeded a preset threshold, send reverse
action command to breaker
Step 5: continue to perform spectrum analysis to
determine if resonance has been removed, take
additional action if necessary
The resonance protection function is a built-in function for
the PFC. The function allows the user to benefit from the
simplified and economical application of the PFC, and at
the same time ensures that the possible harmonic resonance
problem in shunt capacitor applications has been properly
handled by the advanced control of the PFC.
VI. HARMONIC
RESONANCE PROTECTION
The indication of a harmonic resonance caused by capacitor
switching is the sudden increase of the harmonic distortion
in the voltage waveform after the switching operation. This
can be seen more clearly from the harmonic spectrums of
the sampled voltage waveforms.
The following two graphs ( Figure 10 and Figure 11 ) show
the harmonic spectra of the voltage waveform before and
after the capacitor is switched ON for the capacitor
switching-on case shown in Figure 6. As can be seen from
these two figures, the 5* harmonic in the voltage waveform
jumped from about 1% before the capacitor switching to
above 8% after the capacitor is switched on.
Voltage signal spectrum before capacitor switching
VII. CONCLUSIONS
The new integrated power quality device -- Power Factor
Controller -- provides a reliable and affordable solution to
distribution system reactive power compensation. Since it is
manufactured and tested in the factory, the reliability of the
PFC is increased. With its integrated design, a PFC is easy
to install, commission and operate in the field. It provides
more accurate power factor compensation compared to
traditional shunt capacitors. A PFC also requires much less
capital layout in comparison to a Distribution Static Var
Compensator with a comparable size. It provides an
economical solution for applications where continuous
reactive power regulation and inductive reactive power
t (Sec.)
577

7. supply are not required. The advantage of the PFC is further
enhanced by its solutions to two major shunt capacitor
application concerns: i.e. capacitor switching transients and
potential harmonic resonance.
Vm. REFERENCES
Allan Greenwood, "Electrical Transients in Power Systems", Wiley
Interscience, 1991
A. J. Schultz, I. B. Johnson, and N. R. Schultz, "Magnification of
Switching surges", IEEE Transactions on Power Apparatus and
Systems, Vol. PAS-77, pp 1418-1425, February 1959
S.S. Mikhail and M. F. McGranaghan, "Evaluation of Switching
Concerns Associated with 345 kV Shunt Capacitor Applications",
IEEE Transactions on Power Apparatus and Systems, Vol. PAS-105.
NO. 4, pp 221-230, April 1986
M. F. McGranaghan, W. E. Reid, S. W. Law, and D. W. Gresham,
"Overvoltage Protection of Shunt Capacitor Banks Using MOV
Arresters", IEEE Transactions on Power Apparatus and Systems,
Vol. PAS-103, NO. 8, pp 2326-2336, August 1984
G. Hensley, T. Singh, M. Samotyj, M. F. McGranaghan, and T.
Grebe, "Impact of Utility Switched Capacitors on Customer Systems,
Part I1 -- Adjustable Speed Drive Concerns", IEEE-PES Winter
Power Meeting, 1991
Le Tang, et al. "Analysis of Harmonic and Transient Concerns for
PWM Adjustable-Speed Drives Using the Electromagnetic
Transients Program", ICHPS V Conference, September, 1992,
Atlanta
Le Tang, et al, "Analysis of Harmonic and Transient Concerns for
PWM Adjustable-Speed Drives -- Case Studies", FQA Conference,
September, 1992, Atlanta
R.W. Alexander, "Synchronous Closing Control for Shunt
Capacitors", IEEE Transactions on Power Apparatus and Systems,
Vol. PAS-104, No. 9, pp 2619-2626, September, 1985
E.P. Dick, D. Fischer, R. Mamila, and C. Mulkins, "Point-of-Wave
Capacitor Switching and Adjustable Speed Drives", IEEE
Transactions on Power Delivery, Vol. 11, pp 1367-1368, July 1996
[lo] T.A. Bellei, R. P. O'Leary, andE. H. Camm, "Evaluating CapacitorSwitching Devices for Preventing Nuisance Tripping of AdjustableSpeed Drives Due to Voltage Magnification", IEEE Transactions on
Power Delivery, Vol. 11, pp 1369-1378, July 1996
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