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