ET201: THREE PHASE SYSTEMS (PART 1)

1.0    Introduction

Three Phase Systems

Most domestic supplies are single-phase alternating current (AC). As you are aware, to transmit power with single phase AC, we need two wires (live wire, L and neutral wire, N). However, you would have seen that distribution lines usually have 4 wires. This is because distribution is done using three phase and the 4^th wire is the neutral. (4-wire supplies are normally used to distribute domestic supplies since they can provide an earthed neutral. 3-wire systems are commonly used for transmission of high voltage supplies between substations because money is saved by not providing a neutral wire).

Therefore, most of the electrical power generated and transmitted in the world today is three phase. Industrial supplies are three phase, whilst domestic supplies receive only one of the phases. 

Three phase system contains three AC voltages. The three phases usually designated R, Y, B corresponding to Red (R), yellow (Y) and Blue (B), which are phase displaced from one another. We can obtained single phase from three phase system by connecting any phase (R or Y or B) and neural (N) to form R-N, Y-N or B-N single-phase system. The single phase load of domestic supplies is shared out across the three phases by supplying different groups of users with different phase voltages.

Three phase supply voltages and load systems have two basic configurations, the star (wye) connection and the delta connection.

Figure 1.0 shows the three phase RYB system with star and delta connections and also single phase systems obtained from the three phase system as discussed earlier.