The successful operation of a power factor correction depends largely on the correct positioning of the capacitors in the network. The relevant factors are: tariff in force; metering point; details of light, average and full load KVA, KW and power factor; position of motors, welding equipment, transformers or other equipment causing bad power factor; and supply system problems such as harmonics.
The siting of the capacitors, does to some extent, depend on whether each piece of equipment (e.g. a motor or a transformer) is being individually corrected or the installation as a whole or part is being corrected as a block (generally known as bulk or group connection). In the first case the capacitor and the load (motor, transformer etc.) are as close together as possible. In the second case the capacitor is located at some convenient point in the system, such as a substation.
Before power capacitors can be placed, the physical location of the utility meter should be determined since all power capacitors must be installed “downstream” of the meter.
There are three basic locations for Power Capacitors.
Individual capacitor installation at the level of each machine
Load side of the AC motor, commonly referred to as “at the load” or “motor switched”
When the capacitors are installed directly at the induction motor terminals (on the secondary of the overload relay), the capacitors are turned on and off with the motors, eliminating the need for separate switching devices or over current protection. The capacitors are only energized when then motor is running.
Capacitors installed near the loads in a plant provide spot delivery of magnetizing current (KVAR) just at the load, which eliminates unnecessary reactive current in the feeder lines thereby reducing the line losses, minimizing voltage drops and maximizing system capacity. This is one of the most economical and efficient way of supplying these kilovars, which relieves both you and your utility of the cost of carrying this extra kilovar load.
The drawback is that large number of capacitors may be needed for individual motor correction, increasing the installation costs ($ per KVAR compensation). Also overload relay settings need to be changed to account for lower motor current draw. If the capacitors are installed between the contactor and the overload relay, the overload relay can be set for nameplate full load current of motor.
Group or bank installation
Small motors operating from a common starter
Load side of the utility transformer on the distribution bus
Bank installation at Feeders, Sub-stations, or Transformers
Installing capacitors between the upstream circuit breaker & the contactor or at the main distribution bus ensures lower installation cost since you install fewer banks in large KVAR blocks. The drawback is that since reactive current must be carried a greater distance, there are higher line losses and larger voltage drops. Another drawback is that overcorrection may occur under lightly loaded conditions. A separate disconnect switch and over current protection is required. It is recommended to consider automatic switched banks with such an installation
Mixed installation, at both the individual and group level
Installations not operating continuously and which may be supplied at high voltage but with low voltage loads should employ low voltage capacitors for power factor improvement. Low voltage switch gear is much cheaper than high voltage switch gear and obviously is available with much lower ratings which enable relatively small capacitor steps (100 KVAR and below) to be employed for automatically controlled capacitors. This ensures flexibility of operation without excessive switchgear costs.
Benefit of Capacitor Installation
In providing the power factor correction it should be remembered that distribution boards and circuits can carry a greater useful load if the capacitors are installed as near as possible to the source of low power factor. For this reason either bulk or individual correction, rather than correction at the intake point, can almost invariably be justified.
In an installation where the low voltage load is supplied from several distribution substations, local automatic control at each substation is generally much cheaper as well as operationally superior to an elaborate method of overall control operated from the point of incoming supply.
After careful consideration of the advantages and disadvantages of the various installation options, care must be taken in sizing and placing power factor correction capacitors. Leading power factor, greater than 100%, must be avoided. The capacitors should only be on line when the load requires KVAR and disconnected when the load is reduced.
Conclusion
The greatest power factor correction benefits are derived when you place capacitors at the source of reactive currents. It is thus common to distribute capacitors on motors throughout an industrial plant. This is a good strategy when capacitors must be switched to follow a changing load. If your plant has many large motors (25 hp and above), it is usually economical to install one capacitor per motor and switch the capacitor and motor together. Switched capacitors don’t require separate switch control equipment when they are located on the load side of motor contactors.
Thus, capacitors installed on the larger motors are nearly as economical as fixed banks installed at motor control centres. When some switching is required, the most economical method is to install a base amount of fixed capacitors that are always energized, with the remainder on the larger motors and switched when the motors are energized. Observe load patterns in order to determine good candidate motors to receive capacitors.
If your plant contains many small motors (in the 1/2 to 10 hp size range), it may be more economical to group the motors and place single capacitors or banks of capacitors at, or near, the motor control centres. If capacitors are distributed for loss reduction and also need to be switched, you can install an automatic power factor controller in a motor control centre; this provides automatic compensation and may be more economical than capacitors on each of the small motors fed from that control center.
Often the best solution for plants with large and small motors is to specify both types of capacitor installations. Sometimes, only an isolated trouble spot requires power factor correction. This may be the case if your plant operates welding machines, induction heaters, or DC drives. Facilities with very large loads typically benefit from a combination of individual load, group load, and banks of fixed and automatically-switched capacitor units.