Down-line automation is hot! Few down-line opportunities match the ROI potential of effective capacitor control. Down-line capacitor control can achieve resounding results by saving energy, increasing line and substation capacities, and improving power quality, system stability, and voltage profile.

Distribution circuits accumulate inductive load from customer appliances, mainly electric motors. Inductive load is inescapable. Inductive load imposes a requirement to supply reactive power VArs (Volt-Amperes reactive). The reactive power component requires generation and occupies valuable capacities of lines, feeders and transformers. It burdens distribution resources and often does not contribute to revenue. As VAr flow increases, the efficiency of energy delivery decreases.

Reactive power is non-productive. VArs do not register on customer's Wh meters. Generation of VArs expends fuel and other generation resources. The generation burden is commonly passed from energy supplier to distributor in the form of power factor requirements and associated penalties. Excessive reactive load is unprofitable, unnecessary, and highly visible in today's marketplace. Effective capacitor control is a genuine business asset.

In addition to efficiency and capacity considerations, effective capacitor control will improve voltage management and power quality. Capacitors have historically been used to manage distribution voltage. Voltage control is the main purpose for installing capacitors on some circuits. Down-line capacitors boost voltage and flatten voltage profile along feeders.

Effective capacitor control sometimes means managing two objectives, VAr reduction and voltage regulation. Consider the situation of light load and high voltage. Light load leading VArs can cause instability, high down-line voltage, and energy losses. In this situation, banks can be switched off to reduce down-line voltage and to improve stability. Changing from VAr control to voltage control is an important capability of the RCCS centralized capacitor control system at Florida Power & Light. FP&L power supply operations can adjust down-line target power factor setting in response to changes in system load. This method allows them to gradually shift objective between voltage (power quality and stability) and VArs (distribution efficiency).

Figure 1 illustrates a shift between voltage control and VAr control. The load graph is for FP&L transformer T-1616 on May 2, 1999. The x-axis is hour of the day. The lower white line is VArs, the upper two black lines are loads, VA above Watts. During light load, roughly midnight to 9AM, banks were tripped to restrict voltage (voltage control). During normal load, banks were switched to minimize VArs (VAr control). Capacitor switching activity is listed at the lower right.

The above illustration captures an intentional shift of objective from voltage control during light load to VAr control during normal load.

Inductive VArs, travel from customer motors up-line toward a source, typically a substation transformer. Fortunately, the inductive VAr flow can be neutralized by inserting comparably sized capacitance into the circuit. The capacitor's leading VArs add algebraically to the circuit's lagging VArs. Only the difference (VArs lagging or leading) continues up-line from the capacitor to the source. By eliminating VArs from the circuit load, the power requirement (VA's) is reduced and the circuit can carry more useable power (Watts). As Tom Marx, a noted capacitor control expert, advises in his work "The Why and How of Power Capacitor Switching" (See reference #1) the best effect is achieved when VArs are eliminated early by locating capacitors down-line, close to inductive loads. Once VArs are introduced, they continue to load the circuit's conductors until they are neutralized. The effective way to reduce circuit VArs is to use down-line capacitors. Substation capacitors do not reduce the reactive load burden in the distribution system.

Distribution capacitor banks are usually located at substations, on overhead poles or underground pads. Overhead banks typically range in size from 3-phase 300KVAr to 1800KVAr. The size of a particular bank is not variable. Distribution planners estimate a circuit's reactive load and then install banks strategically along the circuit. It is common to find from 1 to six banks along a feeder circuit. A capacitor bank is most effective when its rated KVAr matches the circuit location's maximum inductive load. Too much or too little capacitance reduces efficiency.

Enter… switched capacitor banks! Capacitor switches are essential for effective capacitor control. With automatic switches, capacitance can be added or removed in reaction to real time reactive load. Local VAr controls, with capability to monitor real-time VArs, require at least one current sensor, some circuit engineering and some site preparation. In an attempt to avoid the need for down-line VAr measurements, local controls that employ VAr predictors are commonly used. The switches rely on time, temperature, current or voltage, to substitute for real time VAr measurements. A study at FP&L "Effective VAr Control" by Fred Walker & Michael Keightly (See reference #2) solidly confirms that even diligently maintained time clock schedules compare poorly to the VAr control efficiency that can be achieved with real time VAr control.

Real time VAr control is the effective solution. Effective VAr control saves energy, reduces losses and improves voltage profile. Circuits operate more efficiently and with greater capacity. By reducing energy requirement and managing power factor it is possible to reduce the cost of energy. Energy cost savings can be astounding! In 1996, FP&L piloted real time VAr control at one substation and reported estimated savings in the order of $200,000 annually. Justifiably, the pilot expanded and today the program manages roughly six thousand down-line VAr controlled distribution capacitor banks. Most are 1200kVAr banks.

Effective VAr control adds life to existing infrastructure. When circuit VArs are reduced, the real power carrying capacity of expensive equipment, notably transformers and conductors is increased. Released capacity presents opportunities both to increase revenue and to conserve capital. Construction of infrastructure like substations and feeders can be reassessed and capital more efficiently allocated in light of recouped capacity. The City of Lakeland determined that effective capacitor VAr control could prevent compromising reserve capacity of heavily loaded circuits during a vital two-year construction project. Construction is progressing and reserve capacity is within plan.

CP&L recently updated their capacitor control program. Among the objectives were: extending coverage, improving VAr reduction, lowering maintenance costs, and centralizing operations. The project, coordinated by Ronnie Lawrence, manages thousands of VAr controlled down-line capacitor banks across North and South Carolina. To accomplish the coverage and centralized control objectives, CP&L deployed a new technology, 900MHZ, radio controlled capacitor switch. The switch supplied by Cannon Technologies and by Fisher Pierce operates from commands sent through commercial paging companies. CP&L uses distribution line carrier capacitor switches as well. Throughout the system, capacitors are now controlled on real-time circuit VArs. An RCCS centralized capacitor controller, at Raleigh, monitors real time feeder VArs on all circuits and switches down-line capacitors accordingly. Real time circuit loading is supplied to RCCS through direct links to CP&L's Valmet SCADA and Cannon Technologies DMS computer systems. The RCCS capacitor control software automatically monitors bank performance and reports failures. This eliminated need for routine field inspection of down-line banks. See reference #3, "Down-line intelligence: Key to distribution automation".

Practically speaking, effective, real time VAr control can be accomplished with local VAr switches, or with remotely controlled capacitor switches. Remote switches may be controlled on circuit VArs by a substation controller or a central computer. The most effective solution may require one or more approach. Local VAr control is highly effective at managing VArs at specific locations. Remote VAr control is highly effective at managing feeder and substation and system VArs.

Local VAr controls operate independently to manage VArs at a particular circuit location. They monitor real time current and voltage at the site. Commonly, only one current sensor is used and three-phase values are extrapolated from a single-phase measurement. Local VAr controls may incorporate extensive functionality taking advantage of the acquired local data. Some models accommodate control by VArs, current, voltage, temperature, or time schedules and have provision for complex overrides. Advanced models, like Fisher Pierce 4400, incorporate sophisticated features like adaptive programming that greatly simplifies installation and set up.

A key advantage of using local VAr controllers is that they can be programmed to address the unique characteristics of a particular circuit location. Choice of switching algorithms, set-points, overrides, data to be collected, events to be recorded, and measurement and scaling parameters may be programmed into each local controller. The customization can involve considerable planning and testing to determine the correct settings and measurement parameters for a given site. Managing a substantial inventory of these capable but personalized controllers can be challenging.

Modern local VAr controllers have great capacity for collecting data. Months of load history, voltages, switching events, and operating exceptions can be stored in their memories. The data are very useful for confirming set up and monitoring functioning as well as for diagnosing problems at the site. The data may be less effective for planning purposes such as evaluating substation VAr performance or siting new banks. Visiting or uploading data from numerous local controllers and correlating that data with centralized SCADA reports may be tedious.

The internal workings, firmware, within local VAr controllers is complex. The operating intelligence is built in and is permanent. Modifications can be costly if an algorithm, measurement, or other intrinsic element is faulty or new functionality is required.

Advanced local controls can be configured with remote communications capability. Remote communication is vital if you need to know what's happening at the capacitor. RCCS centralized control programs can monitor and manage local controls with communicating capability.

Centralized capacitor control typically makes use of existing SCADA telemetry. This approach has a number of advantages. It avoids the cost associated with down-line measurements. VAr control performance can be evaluated through consistent and verifiable reporting. Substation instrumentation is likely to be more accurate and capable than down-line equipment. Measurement facilities are more likely to be diligently maintained when part of a SCADA infrastructure.

The load data that SCADA presents to centralized capacitor control generally consists of Watts, VArs, and voltage. The data may relate either to feeders or transformers. It may represent individual phases or three phase values.

Centralized control may also use VAr predictors (time, amps, volts, system load) substituting for VAr data. Down-line measurements can also be used when available.

Down-line capacitor switches are typically controlled over wireless facilities such as licensed radio, paging services, cellular services. Various combinations of communication methods can be employed by a centralized system. CP&L uses 900MHZ commercial paging and distribution line carrier. FP&L uses VHF private paging together with dedicated VHF radio.

Radio switches are uncomplicated. Acceptance testing and preparation for installation is straight forward and fast. At Public Service of New Mexico their radio department built a neat radio switch tester for this purpose. Once an inventory number and radio address is assigned, a radio switch is ready for service. Installation on a pole or in a pad enclosure is fast and uncomplicated. There are no high voltage connections and no current sensors to deal with. Installation goes something like this: disconnect three capacitor fuses, connect AC power (typically 120V) and two low voltage leads, or, simply snap into a four or six jaw meter plug. Request a trip/close/trip sequence from the central station and observe that the line switches operate accordingly. Record the inventory information and reconnect the fuses. Installation is complete.

Centralized control requires an up to date capacitor inventory. Recently, I called on a newly assigned planner who had no idea where his hundred or so down-line capacitors were installed. With centralized control, inventory and control settings are immediately available. A centralized database can include static banks, locally controlled banks as well as centrally controlled banks. Figure 3, below, generated automatically by FP&L's RCCS capacitor controller, shows a static bank hashed (see "WHITFIED P" next line from the bottom) in the substation capacitor diagram.

Centralized control provides an up to the minute view of capacitor activity and availability. The number of VArs on line and VArs available is always known. The number of VArs is presented for a system, regions, substations, or feeders.

Figure 3 below, is a central control substation display. It presents the switch position (trip/close) and the respective order of closing for all banks on the substation. The diagram is colored so that red indicates closed and green, tripped.

Any bank or group of banks may be isolated and operated independently of programmed control. Blue background identifies banks that may be operated independently.

RCCS centralized software confirms the operation of every capacitor bank. It automatically monitors circuit VArs as commands are dispatched. If VAr change is successful, the operation passes. If not, the bank is reported suspect. Troubled banks can be re-tested individually from the central station. Common service problems such as one or more blown fuse are distinguishable from the reports. Self-checking eliminates the tedious and costly routine of physically inspecting cap banks. In the diagram below (figure 3), suspected mis-operations, are reported with white border.

The central station database may be organized by regions so operators may interrogate a particular region or alternatively the entire system.

Central station software tracks real time circuit load and capacitor switching activity so that system performance can be observed daily or over longer periods of time. Daily circuit performance and annualized cost of VAr losses are calculated and may be confirmed by comparison with SCADA reports. Such information is invaluable to planning.

As the number of down-line capacitors increases, their impact on system stability becomes apparent. It becomes increasingly important for power supply management to have access to down-line VArs for system stability interests. RCCS software, with appropriate authorization, allows override of planned VAr control strategy so that power supply can use available distribution VArs when stability requires. For a distribution system the size of FP&L, with giga-VArs down-line, this is a serious consideration.

In conclusion

Effective distribution capacitor control is a highly visible asset. An effective capacitor control program is premised on an ability to manage VArs and/or voltage in real time appropriate to actual load. Local control and centralized control have individual merits and they can profitably co-exist.