How can I lower my bills using power factor correction?


#1

My company is being charged for wattless energy and I am seeking advice on how to tackle that. The costs are significant and it is hiding the savings we have achieved elsewhere.


#2

High reactive power and low power factor values will generally occur for a number reasons:

  • Most of the AC motors that can be found in industrial environments are induction motor type with low lagging power factor. During light loads the induction motors will work at a power factor or 0.2 to 0.3 while during full load condition they will work at 0.8 to 0.9 (for which they have been designed).
    
  • Arc lamps, electric discharge lamps and industrial heating furnaces operating at low lagging power factor are used.
    
  • The load on the power grid system is varying. It will be high during morning and evening hours and lower at other times. The supply voltage is increased during low load period, which increases the magnetisation current, resulting in low power factor.
    

Example of Power Factor variations.

Because current is inversely proportional to the power factor for fixed power and voltage, the lower the power factor, the higher the load current. This results in the following disadvantages:

  • Poor power factor means more line loss dissipation and low transmission efficiency.
    
  • More capital investment for line conductors to provide the expected transmission efficiency
    
  • All electrical equipments in connection with generation, transmission, distribution of ac power such as alternators, transformers, switch gear, cables etc are rated on KVA basis. The lower the power factor the larger is the KVA rating. Large KVA rating of equipment meaning larger size and more expensive equipment.
    
  • Poor voltage regulation. Large current at low power factor causes greater voltage drops in electrical equipment. The result is reduced voltage at the supply end. In order to keep the receiving end voltage within permissible limits, additional devices like voltage regulators are required.
    

The impact of a high reactive power and resulting low power factor principally matter to grid operators. Yet, penalties on low power factor are generally imposed to end-consumers for power factor values below a certain threshold, e.g. 90%, as a way to enforce controlled power factors levels.

How to manage poor power factor now that you have identified it? At the demand-side, the following power factor correction devices are generally used for power factor improvement before the meter:

  • Static Capacitors: they provides leading current which neutralise (totally or approximately) the lagging inductive component of load current.
    
  • Synchronous Condensers: a synchronous motor over-exited at low load provides leading current and works like a capacitor drawing leading current and partially eliminates the re-active component.
    
  • Over excited synchronous motors have to be used in place of induction motors. They are smaller in frame size and economical too.
    
  • Induction motors have maximum power factor when fully loaded, so they should be used at full load condition.
    

If you are incurring wattless charges despite having power factor correction capacitors in place, it means that they may be damaged or require tuning. You should contact your electrical contractor to confirm that they are operating correctly.

More on this can be found at [1] and [2].