Three Phase Power Review
In a three phase power system there are three voltage phasors, separated by 120 electrical degrees.
In balanced three phase systems supplies can be connected to loads in either wye (also called star) or delta configuration, as shown below. (It should be noted that it is not possible to determine if a load is wye or delta connected from the power system)
The currents in each phase leg of a balanced linear load are also phasors:
Instantaneous power flow in each leg of the load is the product of instantaneous voltage and current:
and the total power flow can be found from the sum of the power in each leg. In balanced systems, the total power is constant as the oscillating components cancel
Apparent and reactive power are given by
Up to this point the three phase system has been considered in terms of the individual phase legs of the load. The power system line quantities can be found from the phase quantities, depending on the load configuration. In a wye load the line current equals the phase current. The relationship between phase voltage and line-line voltage can either be determined mathematically or graphically.
In the phasor diagram, it can be seen that the magnitude of the line-line voltage is given by
In a delta connected load the phase voltage equals the line-line voltage, the relationship between line currents and phase currents can be seen in the phasor diagram:
and is given by
Three phase power equations
Power flow in a three phase system can be defined in terms of either phase or line quantities:
