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°.
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.
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.
- 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).
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.
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