Switched Banks capacitors
Switched banks provide benefits under the following
situations:
• More loss reduction
- As the reactive
loading on the circuit changes, we reduce losses by switching banks on and off to
track these changes.
• Voltage limits
— If optimally applied banks under the average loading
scenario cause excessive voltage under light load,
then use
switched banks. In addition, automated capacitors — those with
communications — have the flexibility to also use distribution vars for
transmission support.
Fixed banks are relatively easy to site and size
optimally.
Switched banks are more difficult. Optimally sizing
capacitors, placing them, and deciding when to switch them are difficult tasks.
Several software packages are available
that can optimize this solution. This is an intensely
studied area, and technical literature documents several approaches (among
these Carlisle and El-Keib, 2000; Grainger and Civanlar, 1985; Shyh, 2000). To place switched capacitors using the 1/2-kvar
method, again place the
banks at the location where the line kvar equals half
the bank rating. But instead of using the average reactive load profile (the
rule for fixed banks),
use the average reactive flow during the time the
capacitor is on. With timeswitched banks and information on load profiles (or
typical load profiles),
we can pick the on time and the off time and determine
the proper sizing based on the average reactive flow between the on and off
times. Or, we can place a bank and pick the on and off times such that the
average reactive line flow while the bank is switched on equals half of the
bank rating. In
these cases, we have specified the size and either the
placement or switching time. To more generally optimize — including sizing,
placement, number of banks, and switching time — we must use a computer, which
iterates to find
a solution [see Lee and Grainger (1981) for one
example].
Combinations of fixed and switched banks are more
difficult. The following approach is not optimal but gives reasonable results. Apply
fixed banks to the circuit with the 1/2-kvar rule based on the light-load case. Check voltages. If there are undervoltages, increase
the size of capacitors, use more capacitor banks, or add regulators. Now, look
for locations suitable for switched banks. Again, use the average reactive line
flows for the time when the capacitor is on (with the already-placed fixed
capacitors in the circuit model). When applying switched capacitors, check the
lightload case for possible overvoltages, and check the peak-load case for
undervoltages.
