Application-Guide-for-the-Automation-of-Distribution-Feeder-Capacitors

Results and Findings The Application Guide for the Automation of Distribution Feeder Capacitors attempts to provide the utility engineer with the background needed to sufficiently unde

Technical Report

Distribution Feeder Capacitors

Effective December 6, 2006, this report has been made publicly available in accordance

with Section 734.3(b)(3) and published in accordance with Section 734.7 of the U.S Export

Administration Regulations As a result of this publication, this report is subject to only

copyright protection and does not require any license agreement from EPRI This notice

supersedes the export control restrictions and any proprietary licensed material notices

embedded in the document prior to publication

EPRI Project Manager

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ORGANIZATION THAT PREPARED THIS DOCUMENT

EPRI Solutions, Inc

CITATIONS

This report was prepared by

EPRI Solutions, Inc

This report describes research sponsored by the Electric Power Research Institute (EPRI)

The report is a corporate document that should be cited in the literature in the following manner:

Application Guide for the Automation of Distribution Feeder Capacitors, EPRI, Palo Alto, CA:

2005 1010655

PRODUCT DESCRIPTION

This is the fourth and final report in the Electrical Power Research Institute’s (EPRI’s) capacitor reliability study, and it deals with automating distribution capacitors Prior reports dealt with nuisance fuse operations, operating and construction practices, and lighting protection and

grounding of capacitor controllers This guide is concerned with applying automated switched capacitors to distribution systems Consideration is given to applications involving locally

controlled capacitor banks and to systems utilizing centrally controlled, switched capacitor banks The guide is designed for the distribution engineer considering capacitor automation for his or her system

Results and Findings

The Application Guide for the Automation of Distribution Feeder Capacitors attempts to provide

the utility engineer with the background needed to sufficiently understand automated capacitor control and the ways it might be applied to his or her distribution system This guide discusses commonly applied capacitor control schemes, including both locally applied and centralized control schemes The reader is presented with resources for locating a variety of capacitor

control equipment currently available from several prominent manufacturers in this area This guide also discusses the issues of system integration, capacitor protection, control schemes, and capacitor-related power quality issues

Challenges and Objectives

This guide is intended to provide the necessary background for a distribution engineer to quickly acquire a working knowledge of the issues associated with capacitor automation, including:

• Types of capacitor automation schemes (local control versus centralized control)

• Ways capacitor automation is employed

• Advantages and drawbacks of different types of capacitor controls

• Supervisory control and data acquisition (SCADA) systems for capacitor control

• Communication systems used for capacitor control

• Capacitor bank sizing and protection issues

• Capacitor power quality issues

Due to the potential variability of the capacitor control system from one utility to the next, it is difficult to assign costing figures that will cover all capacitor automation systems Therefore, this Guide attempts to describe the various payback streams that come from implementing

sophisticated capacitor automation schemes This will allow readers to assign their own dollar savings to each category and determine their own potential payback

Applications, Values, and Use

Distribution automation has emerged as a tremendous resource for increasing efficiency and decreasing operating costs for the modern electric utility Advancements in communication and control technologies have made many automation programs—never-before available—a part of the daily operation of utilities around the United States Among the array of attractive

distribution automation technologies are automated capacitor controls, which lead the way as perhaps the most desirable control technology in terms of increasing operating efficiency and providing a quick return on utility investments This guide provides a detailed look at many of the aspects of distribution capacitor automation in order to help the distribution engineer quickly gain the background needed to seriously examine capacitor automation applications

EPRI Perspective

Capacitor automation technology has advanced greatly in recent years Utilities now have access

to intelligent, automated capacitor controllers from numerous manufacturers Many controllers

on the market also have advanced communication capabilities allowing them to be easily

integrated into SCADA systems These advances in capacitor automation technology, coupled with the modern utility’s need to operate ever more efficiently, have utilities taking a closer examination of how capacitor automation can benefit their distribution systems This guide is intended to aid the distribution engineer or planner in determining how capacitor automation can

be a benefit to their distribution system as well as provide the background information and automation fundamentals needed to seriously examine how to automate the capacitors on their system

Approach

The project team began by researching all available information on state-of-the-art capacitor automation systems currently in use by utilities From this research, sections have been added to discuss the various types of control schemes used for capacitor automation and local control verses centralized control topologies The project team also researched SCADA systems used for modern capacitor automation and have attempted to provide a detailed overview of SCADA systems so readers may better understand how these systems can be utilized in distribution automation

No discussion of utility SCADA is complete without examining the many communication

channels available to transfer data from the central station to field units and back Therefore, one chapter of this Guide is dedicated to examining SCADA communication media, with particular attention paid to which companies currently offer commercial communication services for each medium Finally, basic capacitor application information is presented in chapters dedicated to capacitor installation sizing, location, protection, and power quality issues

Keywords

Capacitor automation Capacitor control

Switched capacitor Distribution automation

VARs

EXECUTIVE SUMMARY

The EPRI Capacitor Reliability Study

Utilities have a substantial investment in distribution line capacitors These investments are justified, based on certain derived benefits to the power delivery system, the utilities, and the end-users When capacitors are not available due to some failure or operating error (or are

otherwise off-line), the anticipated benefits will not be achieved Experience at utilities reveals that capacitors are unavailable for operation too frequently This project series was established, therefore, to improve capacitor reliability Initial scoping helped identify and prioritize several issues affecting the overall reliability of capacitors The EPRI capacitor reliability study spans several years, from 2002 through the present Each year a report is prepared dealing with a different aspect of capacitor reliability Reports from previous years have covered:

• Utility Survey and Literature Search (2002): This study was a utility survey and literature

search to assess the issues related to the reliability of switched capacitor banks used in

distribution systems (EPRI 1001691)

• Fusing and Transmission Support (2003): This study investigated causes of nuisance fuse

operations on capacitor banks Additionally, utility practices for providing transmission level VAR support with distribution capacitors were reviewed, and additional utility needs were assessed (EPRI 1002154)

• Grounding and Lightning Protection of Capacitor Controllers (2004): Investigate the two

primary factors influencing the magnitude of surges reaching capacitor controllers and

provide controller mounting and wiring configurations for minimizing surge magnitude The first recommendation involves the physical location at which the capacitor controller should

be mounted with regard to the control power transformer (CPT) from which it draws power The second recommendation involves grounding considerations for the controller supply power (EPRI 1008573)

This year’s report, 2005, examines automating switched capacitors at the distribution level This guide attempts to provide the utility engineer with the background needed to sufficiently

understand automated capacitor control and the ways it could be applied to their distribution system This guide discusses commonly applied capacitor control schemes, including locally applied control and centralized control schemes The reader is presented with a variety of

resources for locating capacitor control equipment from several prominent manufacturers in this area This guide also discusses the issues of system integration, capacitor protection, control schemes, and capacitor-related power quality issues

Project Objectives

The primary focus of this guide is to provide distribution engineers with the necessary

information to examine options for applying a switched capacitor automation scheme on their distribution system This guide provides a detailed discussion of the all the key aspects of

distribution capacitor automation, including:

• Control Schemes: VAR, voltage, current, time, temperature, date, and combination control

programs

• Control Intelligence Location: Local control, central coordinated control, local control with

central station override

• Supervisory Control and Data Acquisition (SCADA) Systems: Components commonly found

in SCADA-based capacitor control systems, with examples cited from prominent

manufacturers

• Voltage and Current Measurements: Information on line parameters typically measured and

the potential for modern capacitor controllers to gather and report a wide array of line data to aid distribution engineers in investigations beyond VAR management

• Capacitor Sizing and Placement: Detailed information size and placement of capacitor

banks on the distribution system

• Capacitor Installation Protection: Detailed information on proper application of fuses to

protect capacitor banks, with additional information regarding protecting capacitor

controllers from line surges and lighting strikes

Background

There is considerable industry activity in applying distribution feeder capacitors Automated line capacitors are being added by many utilities Automation and communication technologies are more advanced, more readily available, and more reasonably priced than even before These advancements in automation control and communication allow utilities to operate switched distribution capacitors in a manner that has never before been possible Utilities are using

capacitors in a variety of ways—to supplement transmission VARs, as substitutes for substation capacitors, to manage distribution voltage profiles, and to reduce line losses Communication technology allows centralized control of distribution capacitors as if they were substation banks This adds the benefit of having the capacitors located closer to the loads they service, thereby further improving their operating efficiency

A typical switched capacitor bank installation is shown in Figure ES-1 Although Figure ES-1 only shows the capacitor assembly near the pole top, the capacitor controller is mounted lower

on the pole, approximately 10 ft (3 m) above the ground There are many types of controllers on the market, with many different configurations

Figure ES-1

Example of a Switched Capacitor Bank Configuration

Distribution line capacitors provide tremendous benefits to distribution system performance by providing volt-ampere-reactives (VARs) at or near the VAR-consuming loads—and they do this

at a low cost The main benefits that capacitors provide are:

• Reduced Losses and Increased Capacity: By canceling the reactive power to motors and

other loads with low-power factor, capacitors decrease the line current Reduced current frees

up capacity Reduced current also significantly lowers I2R line losses

• Reduce Voltage Drop: Capacitors provide a voltage boost that cancels part of the drop

caused by system loads Switched capacitors serve to regulate voltage on a circuit, having an ancillary benefit of reducing the number of operations on voltage regulators, both line (and to

a lesser to degree) substation regulators and load-tap-changers(LTCs) This reduces

maintenance costs on regulators and LTCs

• Reduced Cost of Production or Cost of Purchased Power: Because line capacitors provide

VARs, generators no longer have to produce VARs, thus capacity is freed up to produce more real power (In addition, transmission and distribution lines no longer have to transport those VARs.)

If applied and controlled properly, capacitors can significantly improve the performance of distribution circuits But if not properly applied or controlled, the reactive power from capacitor banks can create losses and can also create high voltages The most danger of overvoltage is under light loads Good planning helps ensure that capacitors are sited properly Compared to

simple controllers (like a time clock), more sophisticated controllers (such as a two-way radio with monitoring) reduce the risk of improperly controlling capacitors

Capacitors work their magic by storing energy Capacitors are simple devices—two metal plates sandwiched around an insulating dielectric When charged to a given voltage, opposing charges fill the plates on either side of the dielectric The strong attraction of the charges across the very short distance that separates them creates a tank of energy Capacitors oppose changes in voltage

It takes time to fill up the plates with charge; and once charged, it takes time to discharge the voltage

On ac power systems, capacitors don’t store their energy very long—just one-half cycle Each half cycle, a capacitor charges up and then discharges its stored energy back into the system The net real power is zero Capacitors provide power just when reactive loads need it At the time a motor with low-power factor needs power from the system, the capacitor is there to provide it Then in the next half cycle, the motor releases its excess energy, and the capacitor is there to absorb it Capacitors and reactive loads continue to exchange this reactive power This benefits the system because that reactive power (and extra current) does not have to be transmitted from the generators all the way through many transformers and many miles of lines; therefore, the capacitors can provide the reactive power locally This frees up the lines to carry real power that actually performs work

Control Strategies

Local Control

Switched capacitor banks are controlled either locally or through centralized system controls As the name implies, local controls sit on or near the same pole as the capacitor bank and govern the switching operations of only one local bank There are several local control strategies available for switched capacitor banks, as shown below (Marx 2003); (Short 2004b):

• VAR Control: The capacitor is switched on and off at an optimum point in the load cycle

based on VAR measurements on the line VAR control is the most efficient control strategy for maximizing the reduction of loss and demand on feeders having only one capacitor bank installed However, VAR control is susceptible to interaction from downstream capacitor

banks (downstream banks affect the reactive current flow upstream of their location)

Therefore, when applying multiple capacitor banks using VAR control on a single feeder, the controls should be set such that the bank furthest downstream comes on-line first, followed

by the next upstream bank, and so on Furthermore, the banks should then trip in the opposite order by which they switched in (that is, the last to switch in should be the first to trip out)

• Current Control: The capacitor is switched on and off based on the line current measured

downstream of the capacitor Reactive current can be determined from line current when the power factor of the line is known Current control engages the capacitor during periods of heavy loads which generally have the greatest VAR requirements Although not as effective

as VAR control schemes, current control provides a fairly good combination of loss

reduction and voltage control

• Voltage Control: The capacitor is switched on and off based upon the voltage To prevent

excessive operations, threshold minimum and maximum voltages are programmed into the controller, as well as time delays and bandwidths Voltage control is best suited for

applications in which the capacitor mainly provides voltage profile control and regulation Voltage controls can be influenced by both upstream and downstream capacitors, since they affect the voltage along the whole line Voltage regulators can also cause capacitor control

pumping problems In general, capacitor controllers using voltage control schemes should be

configured to operate prior to the local voltage regulators In this manner, the voltage

regulators operate only when the capacitors cannot maintain the desired voltage profile

It should also be noted that voltage control schemes provide the greatest value on feeder sections further from the substation The capacitor should have a minimum effect of 2 V (on

120 V reference), and the cap on-to-off difference should be approximately 1.5 times the expected voltage rise when the bank is switched on (Marx 2003)

• Time-Clock Control: The controller switches the capacitor, based upon the time of day

Time-clock control represents the most basic approach for switching a capacitor on and off Most time-clock controllers allow for programmable on and off time settings, as well as settings for weekends and holidays While this is the least expensive control option, it is also the most susceptible to energizing the capacitor at the wrong time; because switching is based on expected line conditions rather than on measured conditions Loads can be different than those anticipated at any time, but holidays and weekends are particularly challenging Time-clock controllers, which are susceptible to mistaken time settings and inaccurate

clocks, can switch the capacitor at times other than those planned Since time control is not based on line measurements, time-clock controls are not susceptible to interaction with other banks

• Temperature Control: The capacitor is switched based upon the temperature Like

time-clock controls, temperature controllers also provide a very basic level of capacitor control Typically, temperature controls are set to turn the bank on at 85-90º F (29.4–32.2º C) and turn the bank off again at 75-80º F (23.8–26.7º C) Since temperature control is not based

on line measurements, they are not susceptible to interaction with other banks

• Power Factor Control: The capacitor is switched based upon the power factor measured on

the line This method of control is rarely used by utilities, mostly owing to the fact that power factor is not a suitable parameter for controlling capacitor switching Since power factor is not necessarily an indication of load, power factor controls may fail to switch in the capacitor during high loads, if the power factor is also high To compensate for this shortcoming, power factor controls may also incorporate voltage and current overrides, both of which make the system more complicated Due to these reasons, VAR control is typically used rather than power factor control

Many controllers offer some or all of these control strategies Many are usable in combination; for example, they will turn capacitors on for either low voltage or high temperature

Centralized Control

Advances in wireless communication technology have made remote capacitor control more achievable and more economical than ever before Cellular phones, pagers, and other wireless technologies have become ubiquitous in modern life, turning up in new applications, such as remote capacitor control There are several control schemes available for remotely controlled capacitor installations, including:

• Operator Dispatch: Most schemes allow operators to dispatch distribution capacitors This

feature is one of the key reasons utilities automate capacitor banks Operators can dispatch distribution capacitors just like large station banks If VARs are needed for transmission support, large numbers of distribution banks can be switched on This control scheme is usually used in conjunction with other controls

• Time Scheduling: Capacitors can be remotely switched, based on the time of day and

possibly the season or temperature While this may seem like an expensive time control, it still allows operators to override the schedule and dispatch VARs as needed

• Substation VAR Measurements: A common way to control feeder capacitors is to dispatch

based on VAR/power factor measurements in the substation If a feeder has three capacitor banks, they are switched on or off in some specified order, based on the power factor on the feeder measured in the substation

• Capacitor Location VAR Measurements: The continuing advancement or capacitor

controller capabilities, coupled with increasing capability for two-way data transfer, are now making it possible for capacitor controllers to measure line parameters at their location and report that data back to a central station controller The central station controller examines the data from each capacitor location (and possible the substation as well) and makes

decisions for switching each capacitor individually A major detractor of this type of

operation is that current transformers (CTs) need to be installed at each site in order to make VAR measurements; and this carries a significant equipment cost—much higher than just measuring at the substation

• Other Methods: More advanced (and complicated) algorithms can be used to dispatch

capacitors, based on a combination of local VAR measurements and voltage measurements, along with substation VAR measurements

All of the control strategies mentioned above will typically utilize a local voltage override

feature, especially if the controller has only one-way communication capabilities Local voltage override prevents the capacitor from switching if doing so will push the voltage beyond limits set

by the user Additionally, most controllers used for centralized control will have fail-safe modes

in which they will revert to a type of local control (voltage, current, VAR, time, temperature, combination, and so on.) if communication with the central station is lost

Capacitor Controllers

The capacitor controller is really the backbone of the automated switched capacitor system Both local control schemes and centralized control schemes utilize a local capacitor controller At the

most basic level, the controller provides the interface to the capacitor switch, telling it when to open and close In local control schemes, the controller provides the switching logic In central control schemes, the controller 1) houses and interprets the signals provided by the data radio, 2) provides switching override functions based on local conditions, and 3) provides switching logic

in the event that communication with the central station is lost

There are many models of capacitor controllers available from numerous manufacturers The controllers are typically packaged in weatherproof enclosures and are intended to be mounted on the same pole as the capacitor bank and switch Some examples of capacitor controllers from various manufacturers are shown in Figure ES-2

Figure ES-2

Examples of Capacitor Controllers from Various Manufacturers

Since there is wide variability in capacitor control needs from one utility to the next, there is also

a corresponding wide variety of features among currently produced capacitor controllers Most manufacturers try to cover most, if not all, of the possible features that a utility may require, including:

• Communication: None (local control only), one-way, two-way

• Communication Channel: Radio, cellular, fiber optic, paging, copper line, and so on

• Control Type: Volt, current, VAR, time, temperature, combination control

• Monitoring: Some controllers with two-way communication ability to also report data on a

variety of parameters: voltage, current, watts, power factor, temperature

• Data Storage: Some controllers can store operational data locally for retrieval by utility field

personnel via laptop computer

• Reverse Power Detection: As part of their monitoring capability, some controllers can detect

reverse power conditions on the feeder Additionally, some controllers have the functionality

to calculate proper set points and compensate for atypical line measurements during reverse power flow conditions

• Neutral Current Monitoring: Monitoring the capacitor bank neutral current can help

diagnose problems, such as blown fuses, failing capacitor units, and high harmonic currents Further information on neutral current monitoring is available in Chapter 5, “Voltage and Current Measurements.”

Most controllers have functionality for all local control types (volt, current, VAR, time,

temperature, and so on.); and they can often run a combination program incorporating two or more of these parameters in a hierarchical manner Most manufacturers also cover both local control and centralized control with one- or two-way communication capabilities, frequently by providing different models, each with distinct communication capabilities

SCADA Systems

Basic SCADA systems, also referred to as telecontrol systems, consist of a master station(s) communicating with one or more remote terminal units (RTUs) to provide data acquisition and control functionality between a central location and dispersed field units A very simplistic diagram of a SCADA system is provided in Figure ES-3 to illustrate the concept of centralized control of dispersed field units The communication channel between the master controller and remote units can be any one of a number of technologies, including radio, cellular, modem, or hard-wired networks There are numerous protocols available that define how communications between the master station and remote units should be structured over the communication

channel, although the DNP3 protocol tends to dominate new capacitor control systems The master station runs application software that provides the human-machine interface and also provides the functionality to perform the specific tasks for which the SCADA system is used (that is, capacitor control, process control, data acquisition) Alternatively, in larger multi-

function SCADA systems, the master station may provide overall coordination and data archival, while dedicated servers run individual function programs, such as the DCC system illustrated in Figure ES-3

no communication from the field back to the control center Two-way communications offer data flow both from the command center to the field units and from the field units back to the

command center The technologies used for centralized capacitor control communications

include:

• 900-MHz Radio: These systems are very common and widely applied for centralized

capacitor control There are several spread-spectrum radios available that cover 902-928 MHz applications Implementing 900-MHz radio control on a private network requires infrastructure, including towers

• Pager Systems: Pager systems offer inexpensive options, especially for systems with

infrequent switching These systems are mostly one-way, but there are some two-way pager systems available Most commercial paging systems can be utilized, however that means that while one-way coverage is rather wide-spread, two-way systems tend to be limited to clusters around major cities

• Cellular Phone Systems: These systems use commercial cellular networks to provide

two-way communications Many vendors offer modems that are compatible with several cellular networks, and coverage is typically very good

• Cellular Telemetric Systems: These use the unused data component of cellular signals that

are licensed on existing cellular networks They allow only very small messages to be sent to perform basic capacitor automation needs Coverage is typically very good, the same as regular cellular coverage

• Very High Frequency (VHF) Radio: Inexpensive, one-way communications are possible

with VHF radio communication VHF radio bands are available for telemetry uses such as this Another option is a simulcast frequency modulation (FM) signal that uses extra

bandwidth available in the commercial FM band

• Energy Savings: In this project, energy savings (also termed loss reduction) refers to

reducing line and transformer losses by using intelligent capacitor control to effectively reduce the amount of reactive current flowing in the line Since energy wasted in heating conductors cannot be delivered to a customer, it generates no revenue It also contributes to fatigue on line conductors and apparatuses through heating

• Capacity Savings: Improving the line power factor through proper application of capacitors

reduces the total line current, thus reducing kVA demand The benefits provided by released capacity are twofold First, releasing line capacity allows more billable energy to be

transferred to customers, thus increasing the revenue that the line can generate The second benefit of releasing line capacity is that it can enable the deferral of equipment upgrades Improving the power factor releases transmission and generation capacity as well as

distribution capacity

• Operation and Maintenance Savings – Required labor hours can be greatly decreased when

upgrading to intelligent centralized capacitor controls via supervisory control and data

acquisition (SCADA) systems SCADA control greatly reduces labor costs by allowing for centralized switching control and monitoring of all capacitor banks This dramatically

reduces travel time as well as time spent adjusting capacitor bank controls Additional cost savings come from the ability to remotely monitor capacitor bank status to determine when capacitors fail This also eliminates the need to have technicians travel to capacitor

installations to annually inspect bank functioning, which amounts to a considerable savings

in work-hours The ability to quickly identify and fix failed capacitors also means that fewer capacitors would need to be installed in the system, since a very high percentage would be operational all the time Over time, then, some capacitor banks could be taken out of service and used for future installations, providing a capital cost savings

Capital costs for capacitor control systems can vary greatly, depending on the level of

sophistication being employed and what, if any, existing utility infrastructure can be utilized for the system However, the level of existing hardware also plays a role in determining the design

of the capacitor control system For example, if a utility already has an extensive 900-MHz radio system in place, then they will likely utilize that system for communication in their capacitor control system If a utility does not have any communication system is place, they may opt for a commercially provided communication system (such as a cellular control channel) rather than building their own communication network Even utilities that have a communication network in place may opt for commercially provided communications, since commercial systems require no infrastructure maintenance from the utility

CONTENTS

1 PROJECT OVERVIEW 1-1

Control Strategies 1-3 SCADA and Communications 1-4 Project Objectives 1-5

2 CAPACITOR SIZING AND PLACEMENT 2-1

Introduction 2-1 Capacitor Ratings 2-5 Released Capacity 2-9 Voltage Support 2-11 Reducing Line Losses 2-13 Energy Losses 2-16 Grounded versus Ungrounded 2-17 Impact of Switching on Capacitor Sizing and Placement 2-19 Switched Capacitor Bank Equipment Mounting Considerations 2-19 Optimal Capacitor Placement Computer Programs 2-23

3 AUTOMATION STRATEGIES 3-1

Best Use of Distribution VARs 3-1 Conservation Voltage Reduction 3-1 Optimizing Power Factor at the Substation 3-3 Distribution Capacitors for Transmission VAR Support 3-3 KCPL 3-3 Idaho Power 3-4 Cinergy Corp 3-5 Georgia Power 3-6 Summary of Utility Practices 3-7 Transmission versus Distribution Optimization 3-8

Switching Control 3-8 Station versus Feeder Evaluation 3-11 Automation and Other Infrastructure Requirements 3-12

4 CONTROL STRATEGIES 4-1

Control Strategies 4-1 Local Control 4-1 Centralized Control 4-4 Coordination of Switched Capacitors and Voltage Regulators 4-6 Coordination of Switched Capacitors and Distributed Generation 4-7

5 VOLTAGE AND CURRENT MEASUREMENTS 5-1

Basic Measurements 5-1 Neutral Monitoring 5-4

6 COMMUNICATION TECHNOLOGIES 6-1

Communications Technologies 6-1 Spread Spectrum 900-MHz Radio Systems 6-4 Pager Systems 6-5 FLEX™ Paging Protocol 6-6 Cellular Systems 6-7 Cellular Data Channel Systems 6-7 Cellular Digital Packet Data 6-9 Cellular Antennas 6-9 Commercial Support for Communication Planning and Analysis 6-11

7 CAPACITOR CONTROLLERS AND SCADA SYSTEMS 7-1

Capacitor Controllers 7-1 SCADA Overview 7-8 Master Stations 7-10 Protocols 7-10 Distributed Network Protocol (DNP3) 7-11 IEC 60870 7-13 Utility Communications Architecture 7-15 MODBUS 7-16 RTUs, IEDs, and PLCs 7-16

SCADA Security 7-17

8 SOFTWARE AND DATA APPLICATIONS 8-1

Capacitor Control Software for SCADA Systems 8-1 Device and Data Management Software 8-3 Human-Machine Interface Issues 8-4

9 CAPACITOR AND CONTROLLER SURGE PROTECTION 9-1

Primary Arrester Lead Length and Coordination with Fuses 9-1 Lead Length Considerations 9-1 Arrester Installation Clearance Considerations 9-5 Capacitor Controller Surge Protection 9-6 Modeling of Lightning Surges Originating on the Primary Conductors 9-7 Preliminary Recommendations 9-10 Key Considerations 9-12 Controller Mounting Location 9-12 Ground Loops and Shielding 9-12 Arrester Lead Length 9-15 Auxiliary Surge Suppression 9-15 Pole Ground Resistance 9-19 Consult the Manufacturer 9-19 Installation Guidelines 9-19

10 CAPACITOR FUSING 10-1

Fusing Guidelines 10-1 Reasons for Relaxing Fusing 10-4 Maximum Fuse Sizes 10-6 Nuisance Fuse Operation 10-8 Outrush and Inrush 10-9 Fuse Installation Issues 10-16 Proposed Fusing Guidelines 10-18

11 CAPACITOR BANK POWER QUALITY AND RELIABILITY ISSUES 11-1

Harmonics 11-2 Solutions to Harmonics 11-5 Switching Surges 11-6

Adjustable Speed Drive (ADS) Tripping 11-9 Solutions to Switching Transients 11-10 Telephone Interference 11-12 Voltage Flicker 11-13

12 ECONOMICS 12-1

Energy Savings 12-2 Capacity Savings 12-2 Operation and Maintenance Savings 12-4 Estimated Cost Breakdown 12-4

13 REFERENCES 13-1

LIST OF FIGURES

Figure 1-1 Example of Switched Capacitor Bank Configuration 1-3 Figure 2-1 Capacitor Components 2-2 Figure 2-2 Overhead Line Capacitor Installation 2-3 Figure 2-3 Released Capacity with Improved Power Factor 2-10 Figure 2-4 Extra Capacity as a Function of Capacitor Size 2-10 Figure 2-5 Voltage Profiles After Addition of a Capacitor Bank 2-12 Figure 2-6 Optimal Capacitor Placement Using the “2/3’s” Rule 2-14 Figure 2-7 Placement of 1200-kVAR Banks Using the ½-kVAR Method 2-15 Figure 2-8 Sensitivity to Losses of Placing One Capacitor on a Circuit with a Uniform

Load 2-16 Figure 2-9 Example of Real and Reactive Power Profiles on a Residential Feeder on a

Peak Summer Day with 95% Air Conditioning (Data from East Central Oklahoma

Electric Cooperative, Inc.) 2-17 Figure 2-10 Comparison of Grounded-wye and Ungrounded-wye Banks During a Failure

Courtesy of Donald M Parker at Alabama Power 2-22

Figure 2-14 Example Location of Fuses and Lightning Arresters in a Switched Capacitor

Installation Courtesy of Donald M Parker at Alabama Power 2-22

Figure 3-1 Three Steps for Applying Capacitors for Peak Shaving 3-7 Figure 3-2 Optimal Capacitor Location for Loss Reduction as the VAR Profile Changes 3-10 Figure 4-1 Example Feeder with a Switched Capacitor Located Just Upstream of a

Distributed Energy Resource 4-8 Figure 5-1 Typical Capacitor Controller Mounting Configuration with a Meter Socket

Courtesy of S&C Electric Company 5-1

Figure 5-2 Example of Connections in a 6-Jaw Meter Socket Used for Capacitor

Controller Installations 5-2 Figure 5-3 Generic Example of Pole-Top Connections for Input Signals to a Capacitor

Controller Note: Protection devices and other apparatuses have purposely been

omitted from this drawing for clarity Actual installations would also utilize hardware,

such as surge arresters, cutouts, and fuses .5-3

Figure 5-4 Series 1301 PowerFlex® Current Sensors From Joslyn Hi-Voltage Courtesy

of Joslyn Hi-Voltage 5-4

Figure 5-5 S&C Electric Company’s CSV Line Post Current and Voltage Sensor

Courtesy of S&C Electric Company 5-4

Figure 5-6 Neutral Monitoring of a Capacitor 5-5 Figure 5-7 Neutral Current Drawn by Failing, Grounded-Wye Bank, Depending on the

Portion of Bank Failed 5-5 Figure 6-1 Reflection of Radio Signals 6-3

Figure 6-2 SkyTel Telemetry Services Advanced Messaging Network Courtesy of SkyTel 6-7

Figure 6-3 Example of an Omni-Directional Antenna and Resulting Coverage Pattern 6-10 Figure 6-4 Example of a Yagi Directional Antenna and Resulting Coverage Pattern 6-10 Figure 7-1 Examples of Capacitor Controllers from Several Manufacturers 7-1 Figure 7-2 Beckwith Electric’s M-2501B Autodaptive® Capacitor Control (left), M-2937

CAMP™ Remote Communication Module (middle) and M-2980 CAMP™ Utilinet®

Remote Communication Module (right) Courtesy of Beckwith Electric 7-3

Figure 7-3 Cannon Technologies’ CBC-5000 (left) and CBC-7000 (right) Remote Power

Factor Control Courtesy of Cannon Technologies 7-4 Figure 7-4 S&C Electric’s Intellicap® Automatic Capacitor Control Courtesy of S & C

Electric 7-4 Figure 7-5 S&C Electric’s Intellicap PLUS® Automatic Capacitor Control Courtesy of S &

C Electric 7-5 Figure 7-6 A Fisher Pierce AutoCap™ Series 4400 Capacitor Control Courtesy of Fisher

Pierce / Joslyn Hi-Voltage 7-6 Figure 7-7 A Fisher Pierce AutoCap™ Series 4500 Capacitor Control Courtesy of Fisher

Pierce / Joslyn Hi-Voltage 7-6 Figure 7-8 ProCap™ 150T Capacitor Controller by Maysteel LLC Courtesy of Maysteel

LLC 7-7

Figure 7-9 MicroCap (left) and MiniCap (right) Capacitor Switching Controllers from QEI,

Inc Courtesy of QEI Inc .7-8 Figure 7-10 Capacitor Switching Controller eCAP-9040, QEI Inc Courtesy of QEI Inc .7-8

Figure 7-11 Components of a Basic SCADA System 7-9 Figure 7-12 Example of Basic SCADA Based Centralized Capacitor Control Using a

Master Station and a Dedicated Capacitor Control Server 7-9 Figure 7-13 The ISO Seven-Layer, Open Systems, Interconnection Model 7-12 Figure 7-14 DNP3 Implementation Using the Enhanced Performance Architecture (EPA)

Model 7-13 Figure 8-1 Example of Multiple Interfaces to Single Capacitor Control System 8-1

Figure 8-2 Example Screen from WinMon® Graphical User Interface Courtesy of S&C

Electric Company 8-4

Figure 9-1 Arrester Lead Length 9-2 Figure 9-2 Example of Considerable Lead Length on a Riser Pole 9-3 Figure 9-3 Example of Almost Zero Lead Length on a Riser Pole 9-4

Figure 9-4 Simulation of Protection Provided by Arresters at Adjacent Poles Only 9-5 Figure 9-5 Blown Arrester with a Dangling Ground Lead 9-6 Figure 9-6 Example of Switched Capacitor Bank Configuration 9-7 Figure 9-7 CPT Secondary Voltage for Scenarios with and Without Secondary Arrester

and Ground Loop 9-9 Figure 9-8 Voltage at the Controller Terminals for Scenarios with and Without Secondary

Arrester and Ground Loop 9-10 Figure 9-9 Example Configuration Using Shielded Control Cable 9-13 Figure 9-10 Ground Loop Created by Grounding the CPT Output and the Capacitor

Controller Neutral Terminal 9-14 Figure 9-11 Cooper Power Systems Storm Trapper H.E Secondary Surge Arrester

Courtesy of Cooper Power Systems 9-16

Figure 9-12 Axiomatic 120Vac Surge Protector Courtesy of Advanced Surge Suppressor 9-16 Figure 9-13 Example Configuration for Surge Protection Covering Incoming Lines for All

Surge Modes 9-17 Figure 9-14 Approximate Size Relationship of Meter Socket and Typical Auxiliary Low-

Side Surge Protection (Note: Actual sizes will vary depending on what equipment is

used) 9-18 Figure 10-1 Capacitor Bank with a Blown Fuse (EPRI 1001691 2002) 10-1 Figure 10-2 Capacitor Unit with a Failed Element 10-5 Figure 10-3 Fuse Curves with Capacitor Rupture Curves 10-7 Figure 10-4 Comparison of Grounded-wye and Ungrounded-wye Banks During a Failure

of One Unit 10-8 Figure 10-5 Outrush from a Capacitor to a Nearby Fault 10-10 Figure 10-6 Outrush as a Function of the Resistance to the Fault for Various Size

Capacitor Banks (The sizes given are 3-phase kVAR; the resistance is the

resistance around the loop, out and back; the distances are to the fault) 10-13

Figure 10-7 Damaged Fuse Tubes from Loose Connections Courtesy of C W (Charlie)

Williams at Progress Florida 10-17

Figure 10-8 Infrared Thermovision Scan of Cutouts Tested with 83 Amps of Current

Courtesy of C W (Charlie) Williams at Progress Florida 10-17

Figure 11-1 Waveform and Harmonic Spectrum of Typical 6-Pulse ac Motor Drives 11-3 Figure 11-2 Harmonic Resonance 11-4 Figure 11-3 Tuned Harmonic Filter 11-6 Figure 11-4 Example Capacitor Switching Transient 11-7 Figure 11-5 Scenario for Magnified Transients 11-8 Figure 11-6 Example of a Transient Magnified to Individual Customers 11-8 Figure 11-7 Effect of Capacitor-Switching Transient on the Direct Current Bus of an

Adjustable Speed Drive 11-10 Figure 11-8 Transient Caused by Synchronous Switching of a Capacitor 11-12 Figure 11-9 Telephone Influence Factor (TIF) Curve 11-13 Figure 11-10 GE Flicker Curve 11-14

LIST OF TABLES

Table 2-1 Substation versus Feeder Capacitors 2-4 Table 2-2 Common Capacitor Unit Ratings 2-5 Table 2-3 Maximum Permissible Power-Frequency Voltages 2-7 Table 2-4 Expected Transient Overcurrent and Overvoltage Capability 2-7 Table 2-5 Maximum Ambient Air Temperatures for Capacitor Application 2-9 Table 2-6 Percent Voltage Rise for Various Conductors and Voltage Levels (Impedance

is for all-Aluminum Conductors with GMD=4.8 feet) 2-12 Table 3-1 Substation Versus Feeder Capacitors 3-12 Table 6-1 Frequency Bands for Typical Applications (Young 1999) 6-2 Table 6-2 Pro’s and Con’s of Radio Network Ownership 6-4 Table 10-1 Fusing Recommendations for ANSI Tin Links From One Manufacturer

[Cooper Power Systems, 1990] 10-3 Table 10-2 I2t Comparisons on a 3-phase, 1,200-kVAR Bank at 12.47 kV (I load=55.6 A) 10-11 Table 10-3 I2t Comparisons on a 3-phase, 600-kVAR Bank at 12.47 kV (Iload=27.8 A) 10-12 Table 10-4 Example Fuse Application Guidelines for a 12.47/7.2-kV System 10-19 Table 12-1 Estimated Benefits from Instituting Automated Capacitor Control on the

Kansas City Power & Light Distribution Systems 12-5 Table 12-2 Estimated Cost of Instituting Automated Capacitor Control on the Kansas

City Power & Light Distribution Systems 12-5

1

PROJECT OVERVIEW

The EPRI Capacitor Reliability Study

Utilities have a substantial investment in distribution line capacitors These investments are justified based on certain derived benefits to the power delivery system, the utilities, and the end-users When capacitors are not available due to some failure or operating error (or are

otherwise off-line), the anticipated benefits will not be achieved Experience at utilities reveals that capacitors are unavailable for operation too frequently This project series was established, therefore, to improve capacitor reliability Initial scoping helped researchers identify and

prioritize several issues affecting the overall reliability of capacitors EPRI’s capacitor reliability study spans several years, from 2002 through the present Each year a report is prepared dealing with a different aspect of capacitor reliability Reports from previous years have covered:

• Utility Survey and Literature Search (2002): A utility survey and literature search to assess

the issues related to the reliability of switched capacitor banks used in distribution systems (EPRI 1001691)

• Fusing and Transmission Support (2003): An investigation of the causes of nuisance fuse

operations on capacitor banks, utility practices for providing transmission-level VAR support with distribution capacitors, and assessments of additional utility needs (EPRI 1002154)

• Grounding and Lightning Protection of Capacitor Controllers (2004): Investigate the two

primary factors influencing the magnitude of surges reaching capacitor controllers and

provided controller mounting and wiring configurations for minimizing surge magnitude The first recommendation involved the physical location at which the capacitor controller should be mounted with regard to the control power transformer (CPT) from which it draws power The second recommendation involved grounding considerations for the controller supply power (EPRI 1008573)

This year’s report, 2005, examines automating switched capacitors at the distribution level This guide attempts to provide utility engineers with the background needed to sufficiently understand automated capacitor controls and ways they could be applied to his or her distribution system This guide discusses commonly applied capacitor control schemes, including locally applied control and centralized control schemes The reader is presented with a variety of capacitor control equipment from several prominent manufacturers in this area This guide also discusses the issues of system integration, capacitor protection, control schemes, and capacitor-related power quality issues

Background

There is considerable industry activity in applying distribution feeder capacitors Automated line capacitors are being added and operated by many utilities Automation and communication technologies are more advanced, more readily available, and more reasonably priced than ever before These advancements in automation control and communication allow utilities to operate switched distribution capacitors in a manner that has never before been possible Utilities are using capacitors in a variety of ways—to supplement transmission VARs, as substitutes for substation capacitors, to manage distribution voltage profiles, and to reduce line losses

Communication technology allows centralized control of distribution capacitors as if they were substation banks This provides the added benefit of having the capacitors located closer to the loads they service, thereby further improving their operating efficiency

Distribution line capacitors provide tremendous benefits to distribution system performance by providing VARs at or near the VAR-consuming loads, and they do this at a low cost The main benefits that capacitors provide are:

• Reduced Losses and Increased Capacity: By canceling the reactive power to motors and

other loads with low-power factor, capacitors decrease the line current Reduced current frees

up capacity Reduced current also significantly lowers I2R line losses

• Reduce Voltage Drop: Capacitors provide a voltage boost that cancels part of the drop

caused by system loads Switched capacitors serve to regulate voltage on a circuit, having an ancillary benefit of reducing the number of operations on voltage regulators, both line (and to

a lesser to degree) substation regulators and load-tap-changers(LTCs) This reduces

maintenance costs on regulators and LTCs

• Reduced Cost of Production or Cost of Purchased Power: Because line capacitors provide

VARs, generators no longer have to produce VARs, thus capacity is freed up to produce more real power (In addition, transmission and distribution lines no longer have to transport those VARs.)

A typical switched capacitor bank installation is shown in Figure 1-1 Although Figure 1-1 only shows the capacitor assembly near the pole top, the capacitor controller is mounted lower on the pole, approximately 10 ft (3 m) above the ground There are many types of controllers on the market, with many different configurations

Figure 1-1

Example of Switched Capacitor Bank Configuration

The capacitor controller is really the backbone of the automated switched capacitor system Both local control schemes and centralized control schemes utilize a local capacitor controller At the most basic level, the controller provides the interface to the capacitor switch, telling it when to open and close In local control schemes, the controller provides the switching logic In central control schemes, the controller 1) houses and interprets the signals provided by the data radio, 2) provides switching override functions based on local conditions, and 3) provides switching logic

in the event that communication with the central station is lost

Properly applied and controlled capacitors offer many benefits to the distribution system

Capacitors provide both energy savings and capacity savings, which can increase revenue and defer system upgrades By serving distribution-level VAR needs on the feeder close to the load, capacitors help reduce VAR flow at all levels of the utility system—generation, transmission, and distribution And by reducing VAR flow, properly applied capacitors will reduce wear and tear on equipment at each level of the utility system

Control Strategies

Automated control of switched capacitor banks takes two primary forms—locally controlled automation and centrally controlled automation In local control schemes, the control logic resides at the capacitor location via an intelligent capacitor controller The controller measures line parameters, such as voltage or VARs, and makes switching decisions based on these

parameters There are also less-sophisticated control schemes based on time, date, or

temperature More information on the various control schemes is available in Chapter 4, Control Strategies Centrally controlled automation systems also use local capacitor controllers but in a somewhat different role In centrally controlled systems, switching decisions are made by a master controller and then sent to the local capacitor controller, which tells the local switch to either open or close The capacitor controller may measure line parameters and transmit that information back to the central station master (although not all central control schemes

incorporate local measurements) Some centralized control schemes make use of feeder data gathered at the substation rather than at the capacitor location In centralized control schemes, the local capacitor controller provides local overrides to prevent the capacitor from switching, if doing so would cause the line voltage to move out of ANSI C84.1 specifications The local controller also provides switching control intelligence in the event that communication is lost between the central station and the local controller

SCADA and Communications

Capacitor automation via centralized control utilizes supervisory control and data acquisition (SCADA) technology to provide both overall data collection and control of the switched

capacitor banks Most utilities have at least some type of SCADA system as part of their

operational infrastructure, although the extent to which utilities make use of SCADA varies greatly When trying to integrate new capacitor automation functionality into existing SCADA systems, the challenge is to provide a method for the new system to interface with the existing system Depending on the system in place, newly added capacitor control functionality may reside on a separate server in parallel with the existing SCADA master station Other

installations may add capacitor control software directly into the master station For some

utilities, this will be their first foray into SCADA; so they are unencumbered by compatibility issues

There are several technologies currently in use for communicating with the capacitor controllers Some offer one-way communication while others offer two-way communication With one-way communication, commands can be dispatched to the capacitor controllers in the field, but there is

no communication from the field back to the control center Two-way communications offer data flow, both from the command center to the field units and from the field units back to the

command center The technologies used for centralized capacitor control communications

include:

• 900-MHz Radio

• Pager systems

• Cellular phone systems

• Cellular telemetric systems

• VHF radio

Project Objectives

The primary focus of this guide is to provide the distribution engineer with the necessary

information to evaluate applying a switched capacitor automation scheme to his or her

distribution system This guide provides a detailed discussion of the all the key aspects of

distribution capacitor automation, including:

• Control Schemes: VAR, voltage, current, time, temperature, date, and combination control

programs

• Control Intelligence Location: Local control, central coordinated control, local control with

central station override

• Supervisory Control and Data Acquisition (SCADA) Systems: Components commonly found

in SCADA-based capacitor control systems, with examples cited from prominent

manufacturers

• Voltage and Current Measurements: Information on line parameters typically measured and

the potential for modern capacitor controllers to gather and report a wide array of line data to aid distribution engineers in investigations beyond VAR management

• Capacitor Sizing and Placement: Detailed information size and placement of capacitor

banks on the distribution system

• Capacitor Installation Protection: Detailed information on proper application of fuses to

protect capacitor banks, with additional information regarding protecting capacitor

controllers from line surges and lighting strikes

• Reduce Avoidable Losses and Free-up Capacity: By canceling the reactive power to motors

and other loads with low-power factor, capacitors decrease the line current Reduced current frees up capacity, thus the same circuit can serve more load Reduced current also

significantly lowers the I2R line losses

• Improved Voltage Profile: Capacitors provide a voltage boost that cancels part of the drop

caused by system loads Switched capacitors can regulate voltage on a circuit

If applied properly and controlled, capacitors can significantly improve the performance of distribution circuits But if not properly applied or controlled, the reactive power from capacitor banks can create losses and can also create high voltages The most danger of overvoltage is under light load Good planning helps ensure that capacitors are sited properly Compared to simple controllers (like a time clock), more sophisticated controllers (such as two-way radio with monitoring) reduce the risk of improperly controlling capacitors

Capacitors work their magic by storing energy Capacitors are simple devices—two metal plates sandwiched around an insulating dielectric When charged to a given voltage, opposing charges fill the plates on either side of the dielectric The strong attraction of the charges across the very short distance that separates them creates a tank of energy Capacitors oppose changes in voltage

It takes time to fill up the plates with charge; and once charged, it takes time to discharge the voltage

On ac power systems, capacitors do not store their energy for very long—just for one half-cycle Each half-cycle, a capacitor charges up and then discharges its stored energy back into the

system The net real power is zero Capacitors provide power just when reactive loads need it When a motor with low-power factor needs power from the system, the capacitor is there to provide it Then in the next half-cycle, the motor releases its excess energy, and the capacitor is there to absorb it Capacitors and reactive loads repeatedly exchange this reactive power This benefits the system because that reactive power (and extra current) does not have to be

transmitted from the generators all the way through many transformers and many miles of lines,

as the capacitors can provide the reactive power locally This frees up the lines to carry real power—power that actually performs work

Capacitor units are made of series and parallel combinations of capacitor packs or elements put together as shown in Figure 2-1 Capacitor elements have sheets of polypropylene film, less than 1-mil thick, sandwiched between aluminum-foil sheets Capacitor dielectrics must withstand voltage on the order of 2000 V/mil (78 kV/mm) No other medium-voltage equipment has such high-voltage stress An underground cable for a 12.47-kV system has insulation that is at least 0.175 inches (4.4 mm) thick A capacitor on the same system has an insulation separation of only 0.004 inches (0.1 mm)

Courtesy of General Electric

Figure 2-1

Capacitor Components

Utilities often install substation capacitors, as well as capacitors at points on the distribution feeders Most feeder capacitor banks are pole-mounted, which is the least expensive way to install distribution capacitors Pole-mounted capacitors normally provide 300-3600 kVAR at each installation Many capacitors are switched, based either on a local controller or from a centralized controller through a communication medium A line capacitor installation has the capacitor units in addition to other components, possibly including arresters, fuses, a CPT,

switches, and a controller (see Figure 2-2 for an example)

Figure 2-2

Overhead Line Capacitor Installation

While most capacitors are pole-mounted, some manufacturers provide pad-mounted capacitors

As more circuits are put underground, the need for pad-mounted capacitors will grow

Pad-mounted capacitors contain capacitor cans, switches, and fusing in a dead-front package that follows standard, pad-mounted enclosure integrity requirements (ANSI C57.12.28-1998) These units are much larger than pad-mounted transformers, so they must be sited more carefully to avoid complaints The biggest obstacles are cost and aesthetics The main aesthetic-related complaint is that pad-mounted capacitors are large Customers complain about the intrusion and the appearance of such a large structure

Substation capacitors are typically offered as open-air racks Normally elevated to reduce the hazard, individual capacitor units are stacked in rows to provide large quantities of reactive power All equipment is exposed Stack racks require a large substation footprint and are

routinely engineered for the given substation Manufacturers also offer metal-enclosed

capacitors, where capacitors, switches, and fuses (normally current-limiting) are all enclosed in a metal housing

Substation capacitors and feeder capacitors both have their uses Feeder capacitors are closer to the loads—capacitors closer to loads more effectively release capacity, improve voltage profiles, and reduce line losses This is especially true on long feeders that have considerable line losses and voltage drop Table 2-1 highlights some of the differences between feeder and station

capacitors Substation capacitors are better when more precise control is needed System

operators can easily control substation capacitors wired into a SCADA system to dispatch VARs

as needed Modern communication and control technologies applied to feeder capacitors have reduced this advantage Operators can control feeder banks with communications just like station banks, although some utilities have found the reliability of switched feeder banks to be less than desired Further, the best times for switching in VARs, as needed by the system, may not

correspond to the best time to switch in the capacitor for the circuit on which it is located

Table 2-1

Substation versus Feeder Capacitors

Feeder Capacitors Advantages Disadvantages

• Reduces line losses

• Reduces voltage drop along the

feeder

• Frees up feeder capacity

• Lower cost

• More difficult to control reliably

• Size and placement important

Substation Capacitors Advantages Disadvantages

• Better control

• Best placement if leading VARs

are needed for system voltage

support

• No reduction in line losses

• No reduction in feeder voltage drop

• Higher cost

Substation capacitors may also be desirable if a leading power factor is needed for voltage

support With a leading power factor, moving this capacitor out on the feeder increases losses Substation capacitors cost more than feeder capacitors This may seem surprising, but station capacitors must be individually engineered; and the space they take up in a station is often

valuable real estate Pole-mounted capacitors installations are more standardized

Utilities normally apply capacitors on 3-phase sections Capacitors are applied on single-phase lines as well, but this is less common Application of 3-phase banks downstream of single-phase protectors is also uncommon because of ferroresonance concerns Most 3-phase banks are

connected, grounded wye on 4-wire, multi-grounded circuits Some are connected in floating wye On three-wire circuits, banks are normally connected as a floating wye

Most utilities also include arresters and fuses on capacitor installations Arresters protect

capacitor banks from lightning overvoltages Fuses isolate failed capacitor units from the system and clear the fault before the capacitor fails violently In high-fault-current areas, utilities may use current-limiting fuses Switched capacitor units normally have oil or vacuum switches in addition to a controller Depending on the type of control, the installation may include a CPT for power and voltage sensing and, possibly, a current sensor Because a capacitor bank has a

number of components, capacitors normally are not applied on poles with other equipment

Properly applied capacitors provide a return on investment very quickly Capacitors save

significant amounts of money in reduced losses In some cases, reduced loadings and extra capacity can also delay building more distribution infrastructure

Capacitor Ratings

Capacitor units rated from 50-500 kVAR are available Table 2-2 shows common capacitor unit ratings A capacitor’s rated kVAR is the kVAR at rated voltage 3-phase capacitor banks are normally referred to by the total kVAR on all three phases Distribution feeder banks normally have one, two, or (more rarely) three units per phase Many common-size banks have only one capacitor unit per phase

2400 50, 100, 150, 200, 300, and 400 1 and 3 75, 95, 125, 150, and 200

2770 50, 100, 150, 200, 300, 400, and 500 1 and 3 75, 95, 125, 150, and 200

• 135% of nominal rms current, based on rated kVAR and rated voltage

Capacitor dielectrics must withstand high-voltage stresses during normal operation—on the order

of 2000 V/mil Capacitors are designed to withstand overvoltage for short periods of time

IEEE Std 18-1992 allows up to 300 power-frequency overvoltages within the time durations in Table 2-3 (without transients or harmonic content) New capacitors are tested with at least a 10-second overvoltage, either a dc-test voltage of 4.3 times the rated rms or an ac voltage of twice the rated rms voltage (IEEE Std 18-2002)