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Power Factor Correction Manual

Power Factor Correction Methods



Individual Power Factor Correction

In the simplest case, an appropriately sized capacitor is installed in parallel with each individual inductive consumer. This completely eliminates the additional load on the cabling, including the cable feeding the compensated consumer. The disadvantage of this method, however, is that the capacitor is only utilized during the time that its associated consumer is in operation.

Additionally, it is not always easy to install the capacitors directly adjacent to the machines that they compensate (space constraints, installation costs).


Typical individual power correction







Applications:


  • To compensate the no-load reactive power of transformers
  • For drives in continuous operation
  • For drives with long power supply cables or cables whose cross section allows no margin for error


Advantages:


  • Reactive power is completely eliminated from the internal power distribution system
  • Low costs per kvar



Disadvantages:


  • The PFC system is distributed throughout the entire facility
  • High installation costs
  • A larger overall capacitor power rating is required as the coincidence factor cannot be taken into account




Group Power Factor Correction

Electrical machines that are always switched on at the same time can be combined as a group and have a joint correction capacitor. An appropriately sized unit is therefore installed instead of several smaller individual capacitors



Typical group power factor correction






Applications:

For several inductive consumers provided that these are always operated together


Advantages:

Similar to those for individual power factor correction, but more cost-effective


Disadvantages:

Only for groups of consumers that are always operated at the same time



Central Power Factor Correction

The PFC capacitance is installed at a central point, for example, at the main low voltage distribution board. This system covers the total reactive power demand. The capacitance is divided into several sections which are automatically switched in and out of service by automatic reactive power control relays and contactors to suit load conditions.

This method is used today in most instances. A centrally located PFC system is easy to
monitor. Modern reactive power control relays enable the contactor status, cos φ, active and
reactive currents and the harmonics present in the power distribution system to be monitored continuously. 

Usually the overall capacitance installed is less, since the coincidence factor for the
entire industrial operation can be taken into account when designing the system. This installed capacitance is also better utilized. It does not, however, eliminate the reactive current circulating within the user’s internal power distribution system, but if adequate conductor cross sections are installed, this is no disadvantage.


Typical central PFC system



Applications:

Can always be used where the user’s internal power distribution system is not
underdimensioned


Advantages:


  • Clear-cut, easy-to-monitor concept
  • Good utilization of installed capacitance
  • Installation usually relatively simple
  • Less total installed capacitance, since the coincidence factor can be taken into account
  • Less expensive for power distribution systems troubled by harmonics, as controlled devices are simpler to choke




Disadvantages:


  • Reactive currents within the user’s internal power distribution system are not reduced
  • Additional costs for the automatic control system



Hybrid Power Factor Correction

Economic considerations often show that it is advantageous to combine the three methods
described above.


Typical hybrid PFC system




This article was extracted from the " Manual of Power Factor Correction" by Peter Riese 


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