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Home / Technical Articles / Wiring of power factor relay on LV and MV side (circuit diagrams)

Circuit diagram of PF relay

In order to measure electrical power the relay needs to receive the line voltage and current from a current transformer (CT) installed close to the metering point, if possible. To measure reactive power at the power factor relay especially, it must be ensured that the vectors of voltage and current are shifted by 90°.

Wiring of power factor relay on LV and MV side
Wiring of power factor relay on LV and MV side (photo credit:

In three-phase systems this is simple to realize by measuring the current in phase L1 and the voltage path is taken from the other two phases L2 and L3. The so-called current path of the power factor relays is standardized to either 5 A (mainly) or 1 A.

One has to take into consideration that the ratio of the current transformer is adapted to the load expected and the primary current will be transformed to the secondary side proportionally.

An oversized ratio, for example using 1000 A/5 A, but expecting no more than 200 A, leads to an inaccuracy in the control of reactive power. The current transformer must seize both the load of the consumers and the compensation bank.

Figure 1 illustrates a simplified circuit diagram of how to wire the current path to the terminals k and l of the power factor relay.

Simplified connecting diagram of the current path to the reactive power relay
Figure 1 – Simplified connecting diagram of the current path to the reactive power relay


  • V – loads (consumers)
  • T – current transformer (CT)
  • C – compensation (capacitor) bank
  • Q – circuit breaker
  • N – power factor relay
  • K – terminals

In general the reactive power of a three-phase system is measured by means of the current transformer in one phase only chosen as desired, mainly L1 (or A).

The correct installation of the current transformer is very important, showing on its side K the feed-in point of the electricity utility and its side L to the consumers including the compensation bank(s).

According to Figure 2, there is the possibility of measuring the load either on the LV or on the MV side.

Alternatively, there is the method of mixed LV/MV measurement which is rarely used. In this method, voltage path for the power factor relay is taken from the LV side. However, the current path is taken from a current transformer installed at the MV side.

Circuit diagram of compensation taking the current and voltage path either from the LV or from the MV side
Figure 2 – Circuit diagram of compensation taking the current and voltage path either from the LV or from the MV side

Although the mixed measurement is rarely in use, it is very important to discuss. It refers mainly to large industrial plants like car factories or steelworks. This method is considered if a voltage transformer on the MV side is not available. Incidentally, voltage transformers for metering purposes are not allowed to be used.

The method has the advantage that all consumers downside (L-side) from the current transformer are to be compensated, including the power transformer(s), by the compensation bank at the LV side.

Most applications use the LV method, because a separate current transformer on the MV side is not available all of the time, not to mention the costs.

It is a matter of course that the power transformers are not compensated by the automatic compensation bank on the LV side. Recall that the desired power factor cosφ, preset at the relay, is to be achieved at the location of the current transformer exclusively and consumers will be compensated only just downside (the L side of the current transformer), referring to the flow of energy.

Power Factor Correction Capacitors

Reference // Reactive power compensation by W. Hofmann

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Pathik Megha
    Nov 09, 2021

    You can series connection in apfc controller of CT connection.

  2. Tom jose
    Feb 17, 2021

    If your MSB and energy meter is far away. Then there will be a drop in powerfactor at energy meter. To overcome the issue you have to provide a capacitor at your msb panel -on direct.

    Nov 30, 2020

    Dear sir
    How much % of capacitor (kvar) will connected against our transformer rating and our plant load indivisually.

  4. Puneet Chouhan
    Sep 18, 2020

    This Electrical engineering portal is very nice for better understanding of the Electrical concepts So here my friends having number of problems arises in the power factor concepts & I am deeply concerned with this p.f concepts .Just we know that there are two types of loads inductive & capacitive. Meanwhile capacitive gives the leading p.f & inductive gives the lagging p.f. First of all check what kind of load are you consuming?? What kind of relay are you using?? Is it single or double or three phases??Is it 1CT or 3CT relay??? Out of these which Relay as well as the Capacitor are you using are of better quality??? The CT should be placed in the correct phase as well?? What kind of programming is done for the inputs of the capacitor bank?? Manual or auto sensing relays are also used for the p.f.Correct programming should be done as well as location of the CT with correct ratio is most important for the power factor correction. Most of most important is the voltage for the achievement of the unity power factor.
    Puneet Chouhan
    Electrical Engineer & Consultant

  5. Rajitha
    Aug 02, 2020

    Please make sure system harmonics level (THDi) is low.Utility energy meter read true power factor (=displacement pf x distrotion pf).But capasitor bank pf controller measure only displacement pf.Thats why two reading.

  6. Chirag
    Nov 28, 2019

    How can I connect two apfc panel with one current transformer..

  7. Kandasami
    May 27, 2019

    I installed a APFC Panel. There is one issue is there that power factor reading in the controller and EB energy meter both are not matching. Please guide what could be the reason. I fixed CT in R phase in consumer load side and not in APFC panel input cable.

  8. [email protected]
    Feb 27, 2018

    Dear all lecturer have.

  9. [email protected]
    Feb 27, 2018

    methods and diagram very good if you attach a lecturer is very very good and easy.

  10. Daniel
    Apr 03, 2017

    how can i correct automatic closing up and down

    -the sampling current between 0.9 — 2 amp
    -the load current is between 200 amp –600 amp
    -CT 800/5 A
    – the capacitor banks rated 2*10 + 5*30 + 4*50 KVR units installed in parallel
    -set up is don as usual
    The problem is always the units are fully closed no increment or decrements seen
    what should be the possible cusses

    thank you

  11. Deepak
    Apr 01, 2017

    what is te genral percentage increase in watt consumption on adding capacitors so that pf rises from .90 to 1

  12. Deepak
    Apr 01, 2017

    what company relay was that

  13. Manuel Bolotinha
    Mar 31, 2017

    Very básica.

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