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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: anuryan.com)

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

Where:

  • 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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About Author //

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Edvard Csanyi

Edvard - 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 fascilities. Professional in AutoCAD programming. Present on

4 Comments


  1. Daniel
    Apr 03, 2017

    Hi
    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


  2. Deepak
    Apr 01, 2017

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


  3. Deepak
    Apr 01, 2017

    what company relay was that


  4. Manuel Bolotinha
    Mar 31, 2017

    Very básica.

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