Three-phase four-wire systems

Three-phase, four-wire, Wye-connected systems in which the neutral is used as a circuit conductor... 

A star-connected system is also called a three-phase four-wire system and allows us to connect single-phase loads to a three-phase system. 

In any star-connected system currents flow along the lines (/l), through the load and return by the neutral conductor connected to the star point. In a balanced three-phase system all currents have the same value and when they are added up by phasor addition, we find the resultant current is zero. Therefore, no current flows in the neutral and the star point is at zero volts. The star point of the distribution transformer is earthed because earth is also at zero potential. A star-connected system is also called a three-phase four-wire system and allows us to connect single-phase loads to a three-phase system. 

When a three-phase four-wire system feeds non-linear or single-phase loads this balance is upset and the unbalanced current flows through the neutral. The same relationship can be expressed as... 

A three-phase four-wire, ungrounded system. 

A three-phase four-wire system may be grounded in the following ways ... 

This is not a method of providing an artificial neutral, as in the previous case, but to detect an unbalance or residual voltage (zero sequence voltage) in a three-phase three-wire or a three-phase four-wire ungrounded system. The residual or zero sequence voltage that may appear across the open delta will be the reflection of an unbalance or a ground fault in the system (Figure 20.10). Also refer to. Section 15.4.1 for more details. 

Figure 21.5(b) Unrestricted G/F protection schemes for three-phase four-wire systems... 

A scheme for a ground fault protection will depend upon the type of system and its grounding conditions, i.e. whether the system is three-phase three-wire or three-phase four-wire. A three-wire system will require an artificial grounding while for a four-wire system the type of grounding must be known, i.e. whether it is effectively (solidly) grounded or non-effeclively (impedance) grounded. 

Figure 21.15 Scheme lor restricted G/F protection lor a three-phase four-wire system. Healthy condition... 

The CT provided in the ground circuit is now removed and the same scheme becomes suitable for an unrestricted G/F or a combined G/F and phase fault protections. It is true for a three-phase three-wire or a three-phase four-wire system. This scheme may also be arranged for a combined 0/C and G/F protections as illustrated in Figures 21.5(a) and 21.5(b). 

Power measurements can be made in three-phase, four-wire circuits by a three-element wattmeter (Figure 3.55). Such a design is seldom utilized because the two-element wattmeter movement can be modified to permit measurement on three-phase, four-wire systems by reconnecting one of the fixed coils for each element. Meters of this type are known as 21/2-element wattmeters. They will correctly indicate power for three-phase, four-wire loads as long as the line-to-neutral voltages are balanced for all three phases. 

Three-phase, Four-wire,— The fourth wire a neutral lamps connected between any outside wire and a neutral wire Barely used six-wire system also in infrequent use. 

A load connected to the three phases of a star-connected three-phase four-wire supply system from the local substation would have a voltage of ... 

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