Selecting a Distribution Capacitor Control

Introduction

Capacitor banks are applied on distribution feeders to control reactive power or line voltage. They’re also used to manage critical load capacity, which can help defer capital projects for new substations and/or the need for additional generation.

Standalone (non-communicating) controls are available that switch the bank in and out based on a local automatic control strategy. Communicating controls are available too, that switch the bank in response to an automatic control strategy command from SCADA or other centralized control.

The type of control selected — along with its associated communication system, as applicable — can significantly affect total installed cost and ongoing operating cost. The selector table below lets you compare the advantages and disadvantages of each type of device, to help you choose the best solution for your application.

Automatic Control Strategies

Capacitor controls can be factory-equipped or field-programmed to switch the bank using one or more of the following control strategies:

Time. The bank is switched based on calendar date, time of day, or the season. This strategy works well if the load is highly predictable.
Temperature. This approach works well if, for example, reactive loads due to air conditioning correlate well with ambient temperature.
Voltage. Locally measured voltage is used to make switching decisions. A voltage override strategy can sometimes be implemented in addition to another control strategy.
VARs. Reactive load, measured locally or at the substation, is used to make switching decisions.
Combination strategies. Combination strategies such as time-based voltage are used to make switching decisions.
Neutral current or voltage. The bank is switched off and locked out as the result of an unbalance sensed at the neutral of the bank.

In some instances, it may be desirable to use multiple control strategies on a particular feeder, to improve coordination between capacitor banks on the feeder.

Some capacitor controls are capable of operating under multiple strategies. This can be an important cost consideration because it’s less expensive to stock one type of control and program it as appropriate for the application.

Communication Technologies

Capacitor controls that operate in response to a signal sent from a centralized control point require a communication system. One-way communicating controls do not return local status information or feeder data to the control point. Two-way communicating controls do provide such status information and feeder data.

Careful consideration must be given to the selection of the communication system used for the controls, as technology is rapidly evolving . . . the communication system can be rendered useless if equipment is no longer supported, potentially requiring replacement of all the capacitor controls. This is a real concern, as new regulatory requirements may dictate security considerations for communication and equipment access. Controls should therefore be capable of interfacing with a wide variety of communication devices.

Presently available communication technologies include:

Unlicensed radios. These radios are inexpensive but may have security and interference concerns.
Licensed radios. These radios generally provide more secure communication and have fewer interference issues. But they’re more expensive and only available from a limited number of suppliers.
Landline phones. Phone-based modems are low cost but monthly service charges are significant.
Cell phones. Cell phone service is available in urban areas and does not require repeater radios. But again, monthly service charges are significant.
Fiber-optics. The high bandwidth and rapid speed of this technology isn’t required for capacitor controls. But if fiber-optics are available, the incremental cost of adding such controls is very low.
Internet-based technology. This latest technology, utilizing WiFi or WiMax servers owned and operated by the utility or leased from a supplier, may be appropriate if a fully-functioning SCADA system isn’t already in place. Authorized personnel can log in to a specific control to determine its status, send commands, etc.

Is Decentralized Capacitor Control Acceptable?

If an interface with SCADA, a distribution management system, or engineering monitoring system isn’t required, individual capacitor banks can be successfully controlled with strategies based on local conditions.

Standalone non-communicating controls are the least-expensive approach. But they have a downside: Any problems at a bank aren’t reported, so periodic inspections of the banks are mandatory.

Is Centralized Capacitor Control Essential?

Undesirable interactions can sometimes result when decentralized controls operate without regard to coordination with other devices, for example, voltage regulators. Centralized control may therefore be required, utilizing one-way or two-way communicating controls.

A centralized control system can provide coordinated VAR support by monitoring the VAR level in the substation and sending remote switching commands to the distribution capacitor controls as necessary. Unlike a decentralized system, current sensors are only required at the substation. By determining the resulting VAR change at the substation, the central computer can verify that the banks have actually been switched. A smaller-than-expected VAR change is indicative of a blown fuse or failed capacitor pack, requiring repair.

A centralized control system must either interface with SCADA, or — when sharing a common communication system — a distribution management or engineering monitoring system.

Centralized control systems can have a major disadvantage: The high-level control software may be quite expensive to develop or purchase. Training charges and annual software maintenance fees can be substantial too.

One-way communicating controls are low in cost. But they’re totally dependent on availability of the associated communication system. If communication to the control point becomes unavailable, the utility’s ability to control their distribution capacitor banks will be totally lost.

One-way communicating controls with voltage override have the ability to mitigate this situation. They can still normalize voltage if communication to the control point is unavailable or a remote command adversely changes feeder voltage.

Two-way communicating controls are preferable if feeder status data is desired. Such data provides the voltage profile along the feeder, which can be used to correct power quality issues. Two-way communicating controls can also report any problems at the bank.

With “intelligent” two-way communicating controls, automatic switching decisions are normally made locally at each capacitor bank . . . but with SCADA monitoring and override. Under severe load conditions, a SCADA command can be issued to turn all banks on — even though voltage and VARs may not be optimal for a particular bank.

When these controls are applied in a centralized control system, they can provide back-up automatic switching decisions if a communication issue arises.

Determining the Best Approach

The advantages and disadvantages of presently available capacitor bank control technologies are summarized in the table below.

To determine the best approach for a particular application, the desired benefits and performance goals must first be defined. Then the total cost of each alternative must be identified and weighed against the advantages and disadvantages. Intangibles such as the long-term stability and reliability of the supplier should be considered.

DISTRIBUTION CAPACITOR CONTROL SELECTION TABLE
Type of Control	Characteristics	Advantages	Disadvantages
Standalone Non-Communicating Control	Operates based on locally measured values.	Lowest cost. No communication system needed.	Bank can’t be operated remotely. Historical data is only available locally, by connecting to control. Frequent inspections are needed to verify bank is actually in service.
Standalone Control With Communicated Alarms	Operates based on locally measured values.	Low-cost control. Lowest-cost communication system. Problems are communicated immediately.	Bank can’t be operated remotely. Historical data is only available locally, by connecting to control.
One-Way Communicating Control	Receives switching command from SCADA or other centralized control, based on measurements made at substation.	Low-cost control. Lower-cost communication system. No sensors needed on the feeder. VAR change due to switching can be determined at the substation.	High-cost central control system. Communication problem can disable VAR support at many banks. Multiple-contingency support is difficult to implement. System changes and expansion require extensive reprogramming. Alarms and data are not communicated back.
Two-Way Communicating Control	Receives switching command from SCADA or other centralized control, based on measurements made at substation. Transmits status information and feeder data.	Local feeder voltage and control information available remotely. No sensors needed on the feeder. Capacitor bank problems are reported. Remote configuration and data reporting may be available.	Higher-cost control and communication system. High-cost central control system. Communication problem can disable VAR support at many banks. Multiple-contingency support is difficult to implement. System changes and expansion requires extensive reprogramming.
“Intelligent” Two-Way Communicating Control	Operates based on locally measured values, with SCADA monitoring and override. If necessary, receives switching command from SCADA. Can be overridden or controlled by centralized control system. Transmits status information and feeder data.	Local feeder voltage and control information available remotely. Capacitor bank problems are reported. Remote configuration and data reporting may be available. Communication problem won’t affect VAR support. Control problem won’t affect other installations. Automatically responds to multiple contingencies. System changes and expansion are easily accommodated.	Higher-cost control and communication system. Current sensor needed at each capacitor bank.
Web-Based Communicating Control	See “Two-Way Communicating Control” and “‘Intelligent’ Two-Way Communicating Control.”	Low initial cost. Can be accessed from at any Internet-equipped computer.	Significant on-going monthly cost. Long-term stability and reliability of suppliers not known. Potential for security issues.