Balance Three-Phase Circuit

The voltages in the three-phase power system are produced by a synchronous generator. In a balanced system, each of the three instantaneous voltages have equal amplitudes but separated from the other voltages by a phase angle of 120 . The three voltages (or phases) are typically labeled a, b and c. The common reference point for the three phase voltages is designated as the neutral connection and is labeled as n. We may define either a positive phase sequence (abc) or a negative phase an bn cn sequence (acb) as shown below. The three sources Van , Vbn and Vcn are designated as the line-to-neutral voltages in the three-phase system.

LINE TO LINE VOLTAGES

 First, the wye - connected voltages Van, Vbn, and Vcn are respectively between lines a, b, c, and the neutral line n. These are called phase voltages. The voltages are said to be balanced phase voltages if the voltages sources have the same amplitude and frequency w and are out of phase by 120degrees. 

                                                                Van = Vp∠0

                                                                Vbn = Vp∠-120

                                                                Vcn = Vp∠-240 = Vp∠120

     where Vp is the effective or rms value of the phase voltage. this is known as abc sequence or positive sequence, Van leads Vbn, which turn leads Vcn. This rotates counterclockwise. For negative or acb sequence, Van leads Vcn, ehich turn leads Vbn. this rotates clockwise direction.