Capacitor Control Concepts
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Capacitor Control

is usually done to achieve as many as possible of the following goals: Reduce losses due to reactive load current, reduce kVA demand, decrease customer energy consumption, improve voltage profile, and increase revenue. Indirectly capacitor control also results in longer equipment lifetimes because of reduced equipment stresses. Experience shows that switched feeder capacitors produce some of the fastest returns on equipment investment.

Sources of Energy Loss

Energy losses in transmission lines and transformers are of two kinds: resistive and reactive. The former are caused by resistive component of the load and can not be avoided. The latter, coming from reactive component of the load, can be avoided (Fig. 1). Reactive losses come from circuit capacitance (negative), and circuit inductance (positive). When a heavy inductive load is connected to the power grid, a large positive reactive power component is added, thereby increasing observed power load (Fig. 1). This increases losses due to reactive load current, increases kVA demand, increases customer energy consumption, usually degrades voltage profiles, and reduces revenue.

Reactive Compensation

When capacitors of appropriate size are added to the grid at appropriate locations, the above mentioned losses can be minimized by reducing the reactive power component in Fig. 1, thereby reducing the observed power demand. There are many aspects to this compensation and its effects, depending on where capacitors get to be located, their sizes, and details of the distribution circuit. Some are discussed below.

Energy Loss Reduction

More than one half of system energy loss is caused by the resistance of the feeders. To minimize energy losses it is, therefore, important to locate feeder capacitors as close to the loads as possible. Substation capacitors can not do the job - the reactive load current has already heated feeder conductors downstream from the substation. Reducing reactive current at the substation can not recover energy losses in the feeders. Another way to minimize energy losses is to use capacitor banks that are not too large. This makes it possible to put the banks on-line early in the load cycle. Since energy saved is the product of power reduction and the time the banks are on-line, the overall energy reduction is usually greater than when using large banks which are turned on for shorter amounts of time (Fig. 2).

Demand Reduction

When capacitors are on-line reactive current and, therefore, total line current is reduced. During heavy load periods this has several advantages: The peak load is increased when it is most needed (essentially releasing demand), the effective line current capacity is increased, and the operating line and transformer temperatures are reduced – prolonging equipment lifetimes. The latter makes it possible to upgrade lines and transformers less frequently. All of these contribute to reduced costs and higher revenues.

Voltage Profile

Distribution feeder demand capacity is usually limited by voltage drop along the line. The customer service entrance voltage must be stable, usually ±5% to ±10%. The feeder voltage profile can be ‘flattened’ by connecting large capacity banks to the grid. Several benefits become available: The kVA demand can be increased to arrive at the original voltage drop (this is equivalent to releasing feeder demand), the substation voltage can be lowered to reduce peak demand and save energy, or the service entrance voltage can be allowed to increase thereby increasing revenue (at the expense of less than optimum kVA demand).

System Considerations

Obviously properly switched capacitors located at appropriate locations along distribution feeders provide great financial benefits to the utility.

If there is to be only one capacitor bank on a uniformly loaded feeder, the usual two-thirds, two-thirds rule gives optimum loss and demand reduction. This means that the bank kVAr size should be two-thirds of the heavy load kVAr as measured at the substation, and the bank should be located two-thirds the length of the feeder from the substation. If the objective is voltage control the bank should be farther from the substation.

With several banks on a uniformly loaded feeder, the total capacitor kVAr can more closely match the total load kVAr. Depending on the type of the switching control used, multiple banks on a feeder can lead to ‘pumping’ as the controls affect the operating points of each other. Usually no more than three or four banks are used per feeder.

In the case of concentrated industrial loads, there should be a bank, sized to almost equal the reactive load current, located as close to each load as possible (Fig. 3).

Types of Control

VAr control is the natural means to control capacitors because the latter adds a fixed amount of leading VArs to the line regardless of other conditions, and loss reduction depends only on reactive current. Since reactive current at any point along a feeder is affected by downstream capacitor banks, this kind of control is susceptible to interaction with downstream banks. Consequently, in multiple capacitor feeders, the furthest downstream banks should go on-line first, and off-line last. VAr controls require current sensors.

Current control is not as efficient as VAr control because it responds to total line current, and assumptions must be made about the load power factor. Current controls require current sensors.

Voltage control is used to regulate voltage profiles, however it may actually increase losses and cause instability from highly leading currents. Voltage control requires no current sensors.

Temperature control is based on assumptions about load characteristics. Control effectiveness depends on how well load characteristics are know. Not useful in cases where those characteristics change often. Temperature control does not require any current sensors.

Time control is based on assumptions about load characteristics. Control effectiveness depends on how well load characteristics are know. Not useful in cases where those characteristics change often. Time control does not require any current sensors.

Power factor control is not the best way to control capacitor banks because power factor by itself is not a measure of reactive current. Current sensors are needed.

Combination control using various above methods is usually the best choice. If enough current, and/or other sensors are available, a centrally managed computerized capacitor control system taking into account the variety of available input parameters can be most effective, though expensive to implement.


on how CCEX, the NTMC capacitor control module, implements above can be found here.


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