Power factor correction
Once you’ve decided that your facility can benefit from power factor correction, you’ll need to choose the optimum type, size, and number of capacitors for your plant.
There are two basic types of capacitor installations: individual capacitors on linear or sinusoidal loads, and banks of fixed or automatically switched capacitors at the feeder or substation.
Individual vs. banked installations
7 advantages of individual capacitors at the load:
- Complete control // Capacitors cannot cause problems on the line during light load conditions
- No need for separate switching // Motor always operates with capacitor
- Improved motor performance due to more efficient power use and reduced voltage drops
- Motors and capacitors can be easily relocated together
- Easier to select the right capacitor for the load
- Reduced line losses
- Increased system capacity
3 advantages of bank installations at the feeder or substation:
- Lower cost per kVAR
- Total plant power factor improved – reduces or eliminates all forms of kVAR charges
- Automatic switching ensures exact amount of power factor correction, eliminates over-capacitance and resulting overvoltages
Table 1 // Summary of Advantages/Disadvantages of Individual, Fixed Banks, Automatic Banks, Combination
|Individual capacitors||Most technically efficient, most flexible||Higher installation and maintenance cost|
|Fixed bank||Most economical, fewer installations||Less flexible, requires switches and/or circuit breakers|
|Automatic bank||Best for variable loads, prevents overvoltages, low installation cost||Higher equipment cost|
|Combination||Most practical for larger numbers of motors||Least flexible|
Consider the 5 particular needs of your plant
When deciding which type of capacitor installation best meets your needs, you’ll have to weigh the advantages and disadvantages of each and consider several plant variables, including load type, load size, load constancy, load capacity, motor starting methods, and manner of utility billing.
1. Load type //
If your plant has many large motors, 50 hp and above, it is usually economical to install one capacitor per motor and switch the capacitor and motor together. If your plant consists of many small motors, 1/2 to 25 hp, you can group the motors and install one capacitor at a central point in the distribution system.
Often, the best solution for plants with large and small motors is to use both types of capacitor installations.
2. Load size //
Facilities with large loads benefit from a combination of individual load, group load, and banks of fixed and automatically-switched capacitor units. A small facility, on the other hand, may require only one capacitor at the control board.
Sometimes, only an isolated trouble spot requires power factor correction. This may be the case if your plant has welding machines, induction heaters, or DC drives. If a particular feeder serving a low power factor load is corrected, it may raise overall plant power factor enough that additional capacitors are unnecessary.
3. Load constancy //
If your facility operates around the clock and has a constant load demand, fixed capacitors offer the greatest economy. If load is determined by eight-hour shifts five days a week, you’ll want more switched units to decrease capacitance during times of reduced load.
4. Load capacity //
If your feeders or transformers are overloaded, or if you wish to add additional load to already loaded lines, correction must be applied at the load. If your facility has surplus amperage, you can install capacitor banks at main feeders.
5. Utility billing //
The severity of the local electric utility tariff for power factor will affect your payback and ROI (return of investment). In many areas, an optimally designed power factor correction system will pay for itself in less than two years.
Reference: Power factor correction – A guide for the plant engineer – EATON (Download paper)