Power Factor Controller -- An Integrated Power Quality Device
Richard Tinggen

Yi Hu, Le Tang and Harry Mathews

Rick Tyne...
The power factors of loads have different characteristics.
Some can be nearly constant, while others can vary in a
wide ra...
as close as possible to the targeted power factor setting.
The PFC thus offers ii more precise power factor
compensation c...
the operation of capacitor breaker with the voltage
waveforms.
When several capacitor banks are connected to the same
bus,...
wide range variation of system impedance, it will
significantly increase the cost for each individual PFC
application in a...
Figure 10 Harmonic spectrum before capacitor
switching
Voltage signal spectrum after capacitor switching
10,

0.2

0.P

0....
supply are not required. The advantage of the PFC is further
enhanced by its solutions to two major shunt capacitor
applic...
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power factor controlling using intregarted device controller

  1. 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. I Auto Parts Chemical Hospital Office Building Steel Works Plastic I 75-80 65-75 75-80 80-90 65-80 I 75-80 Load Power Factor Induction Motor Diode Rectifier (Small ASDs) Fluorescent Lighting. Standard Ballast I Electronic Ballast I Phase Controlled Rectifier DC Drives, Large ASDs, Arc Furnaces Arc Welding Resistance Welding 70-80 95-98 70 MAIN AUTHOR AlTILIATION INFORMATION GOES HERE. 0-7803-5515-6/99/$10.00 0 1999 IEEE 572 (%I 90 40-90 75-90 35-60 40-60 I
  2. 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 573
  3. 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) 1.5i I I 1 1 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. 0.5 -. 05 1 I -,.U 0.15 I I 0.2 0.25 t (sec) 0.3 03 .5 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. 574
  4. 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 0.25 02 . 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 575
  5. 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) I I I 1 05 . 3 s > o Switchingotl capacitor cause resonance (BUS9A) -0.5 0.81 I I I 1 I -1.51 0.15 I 02 . 0.25 I 03 . t (sec) 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. I -0.8' 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 . 0.05 g o -0.05 -0.1 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 576 ! ! ! ! ! ! ! 1 I
  6. 6. Figure 10 Harmonic spectrum before capacitor switching Voltage signal spectrum after capacitor switching 10, 0.2 0.P 0.24 0.26 0.28 0.3 0 3 0 3 0.36 .2 .4 1 0.38 0.4 t (Sec) (b) Current waveform Figure 9 Switch-off capacitor remove resonance 0 5 10 15 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. 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 57 8

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