Shunt capacitors offer a convenient and practical means of relieving lines and source equipment of wattless current. They can be installed in relatively small banks and placed near the load points.
They usually are arranged in three phase banks of 45 kvar or more and are distributed over the system at distribution voltage, usually 2400 volts and up, in accordance with local requirements. At present it is not economical to apply capacitors on the secondary side of distribution transformers because of the much greater cost.
Where the transformers are expensive, such as network units, secondary capacitors may be justified.
The capacity of a distribution feeder can be limited by current or by voltage drop. Where current is the limiting factor, the effect of capacitors in reducing the current is dependent upon load power factor.
If the power factor is low, a large reduction in feeder current or kva can be obtained. If the load
power factor is high, shunt capacitors cannot materially change feeder loading. Where voltage is the limiting factor, the capacitor kvar to decrease voltage drop is dependent not only on load power factor but also on the ratio of resistance to reactance of the distribution feeder.
In applying shunt capacitors to distribution circuits, certain system data are required.
1. Determine variation, preferably by graphic instruments, of kw and kva on each feeder for a typical 24-hour period at both minimum and maximum daily loads. Usually the minimum reactive kva determines the amount of fixed capacitors to apply without automatic control.
This gives about unity power factor at minimum load. In certain cases more fixed capacitor kvar can be applied where voltage conditions at light load permit and where leading power factor is not objectionable.
2. Obtain actual voltage measurements on the feeder during full load and light load at a sufficient number of points to determine the optimum location for capacitors. Fixed shunt capacitors raise the voltage level at the point where they are applied on a given circuit by practically a constant value as given by XI.
To calculate the voltage at various points on the feeder the circuit characteristics and the load distribution must be known. Where the individual loads are not known, it is reasonable to assume they are proportional to the installed transformer capacity for minimum and maximum feeder load.
To simplify calculations single-phase loads can be grouped together to form balanced three-phase loads and adjacent three-phase loads can be grouped to simplify the calculations.
3. It is desirable to supply the kvar required by the load as close to the load as possible to reduce feeder
losses. Therefore, capacitor units should be located at load centers or near the ends of feeders. Ideally each load point would have the exact amount of capacitor kvar to supply the necessary load kvar.
This, however, is not possible because standard size units must be used. Also it is more economical to use the large size units, namely, 15 or 25 kvar. Overcompensation of feeder branch circuits with capacitors to obtain a higher voltage results in increased copper losses because at lower and lower leading power factors, the current increases.
4. Calculate the released feeder capacity in kw and kva for the capacitor kvar installed. This may involve capacitors installed at several locations on a given feeder. Released substation, transmission, and generator capacity is also immediately available.
5. Calculate the reduction in kw losses and the reduction in kvar losses in the feeder. The effect on all
equipment back to and including the source generator should also be evaluated when the total capacitor
kvar become appreciable relative to the total source circuit or system reactive kva.
6. Summarize the tangible effects namely, the released feeder capacity, the released capacity back to and including the source generator, the reduction in losses, the effect on voltage, etc. and evaluate the economics to determine whether or not capacitors are justified.
Also compare the cost of capacitors with other ways of doing an acceptable job, such as construction
of a new feeder, installation of voltage regulators, raising the distribution voltage, etc.
From the above brief summary on applying shunt capacitors to distribution systems, it is evident that no fixed rules can be stated regarding the location of capacitors nor can the degree of importance of each of their effects be stated.