Capacitor Application:Automated Controls

Automated Controls

Riding the tide of lower-cost wireless communication technologies, many utilities have automated capacitor banks. Many of the cost reductions and feature improvements in communication systems have resulted from the proliferation of cellular phones, pagers, and other wireless technologies used by consumers and by industry. Controlling capacitors requires little band- width, so high-speed connections are unnecessary. This technology changes quickly. The most common communications systems for distribution line capacitors are 900-MHz radio systems, pager systems, cellular phone systems, cellular telemetric systems, and VHF radio. Some of the common features of each are

900-MHz radio — Very common. Several spread-spectrum data radios are available that cover 902–928 MHz applications. A private network requires an infrastructure of transmission towers.

Pager systems — Mostly one-way, but some two-way, communica- tions. Pagers offer inexpensive communications options, especially for infrequent usage. One-way communication coverage is wide- spread; two-way coverage is more limited (clustered around major cities). Many of the commercial paging networks are suitable for capacitor switching applications.

Cellular phone systems — These use one of the cellular networks to provide two-way communications. Many vendors offer cellular modems for use with several cellular networks. 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 — enough, though,

to perform basic capacitor automation needs. Coverage is typically very good, the same as regular cellular coverage.

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 FM signal that uses extra bandwidth available in the commercial FM band.

Standard communication protocols help ease the building of automated infrastructures. Equipment and databases are more easily interfaced with stan- dard protocols. Common communication protocols used today for SCADA applications and utility control systems include DNP3, IEC 870, and Modbus. DNP 3.0 (Distributed Network Protocol) is the most widely used standard protocol for capacitor controllers (DNP Users Group, 2000). It originated in the electric industry in America with Harris Distributed Automation Prod- ucts and was based on drafts of the IEC870-5 SCADA protocol standards (now known as IEC 60870-5). DNP supports master–slave and peer-to-peer communication architectures. The protocol allows extensions while still pro- viding interoperability. Data objects can be added to the protocol without affecting the way that devices interoperate. DNP3 was designed for trans- mitting data acquisition information and control commands from one com- puter to another. (It is not a general purpose protocol for hypertext, multimedia, or huge files.)

One-way or two-way — we can remotely control capacitors either way.

Two-way communication has several advantages:

Feedback — A local controller can confirm that a capacitor switched on or off successfully. Utilities can use the feedback from two-way communications to dispatch crews to fix capacitor banks with blown fuses, stuck switches, misoperating controllers, or other problems.

Voltage/var information — Local information on line var flows and line voltages allows the control to more optimally switch capacitor banks to reduce losses and keep voltages within limits.

Load flows — Voltage, current, and power flow information from pole-mounted capacitor banks can be used to update and verify load-flow models of a system. The information can also help when tracking down customer voltage, stray voltage, or other power qual- ity problems. Loading data helps utilities monitor load growth and plan for future upgrades. One utility even uses capacitor controllers to capture fault location information helping crews to locate faults.

When a controller only has one-way communications, a local voltage over- ride control feature is often used. The controller blocks energizing a capacitor bank if doing so would push the voltage over limits set by the user.

Several schemes and combinations of schemes are used to control capac- itors remotely:

Operator dispatch — Most schemes allow operators to dispatch dis- tribution 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 trans- mission 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 measurement — 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.

Others — More advanced (and complicated) algorithms can dispatch capacitors based on a combination of local var measurements and voltage measurements along with substation var measurements.

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