برق و الکترونیک

Single-phase loads

Single-phase loads may be connected to a three-phase system, either by connecting across two live conductors (a phase-to-phase connection), or by connecting between a phase conductor and the system neutral, which is either connected to the center of the Y (star) secondary winding of the supply transformer, or is connected to the center of one winding of a delta transformer (Highleg Delta system) (see transformer and Split phase). Single-phase loads should be distributed evenly between the phases of the three-phase system for efficient use of the supply transformer and supply conductors.

The neutral point of a three phase system exists at the mathematical center of an equilateral triangle formed by the phase points, and the line-to-line voltage of a three-phase system is correspondingly times the line to neutral voltage. Where the line-to-neutral voltage is a standard utilization voltage (for example in a 240 V/415 V system), individual single-phase utility customers or loads may each be connected to a different phase of the supply. Where the line-to-neutral voltage is not a common utilization voltage, for example in a 347/600 V system, single-phase loads must be supplied by individual step-down transformers. In multiple-unit residential buildings in North America, lighting and convenience outlets can be connected line-to-neutral to give the 120 V distribution voltage (115V utilization voltage), and high-power loads such as cooking equipment, space heating, water heaters, or air conditioning can be connected across two phases to give 208 V. This practice is common enough that 208 V single-phase equipment is readily available in North America. Attempts to use the more common 120/240 V equipment intended for three-wire single-phase distribution may result in poor performance since 240 V heating equipment will only produce 75% of its rating when operated at 208 V.

Where three phase at low voltage is otherwise in use, it may still be split out into single phase service cables through joints in the supply network or it may be delivered to a master distribution board (breaker panel) at the customer's premises. Connecting an electrical circuit from one phase to the neutral generally supplies the country's standard single phase voltage (120 VAC or 230 VAC) to the circuit.

The power transmission grid is organized so that each phase carries the same magnitude of current out of the major parts of the transmission system. The currents returning from the customers' premises to the last supply transformer all share the neutral wire, but the three-phase system ensures that the sum of the returning currents is approximately zero. The primary side of that supply transformer commonly uses a delta winding, and no neutral is needed in the high voltage side of the network. Any unbalanced phase loading on the secondary side of the transformer will use the transformer capacity inefficiently, but equal current will be drawn from the phases feeding the primary delta winding, leaving the high voltage network unaffected.

If the supply neutral of a three-phase system with line-to-neutral connected loads is broken, generally the voltage balance on the loads will no longer be maintained. The now-virtual neutral point will tend to drift toward the most heavily loaded phase, causing undervoltage conditions on that phase only. Correspondingly, the lightly-loaded phases may approach the line-to-line voltage, which exceeds the line-to-neutral voltage by a factor of √3, causing overheating and failure of many types of loads. For example, if several houses are connected to a common three-phase transformer, each house might be connected to one of the three phases. If the neutral connection is broken at the transformer or on the distribution line somewhere upstream of the transformer, all equipment in a house might be damaged due to overvoltage. This type of failure event can be difficult to troubleshoot if the drifting neutral effect is not understood. With inductive and/or capacitive loads, all phases can suffer damage as the reactive current moves across abnormal paths in the unbalanced system, especially if resonance conditions occur. For this reason, neutral connections are a critical part of a power distribution network and must be made as reliable as any of the phase connections.

Independent (or nearly so) three phase systems are sometimes interconnected using DC transmission (with the requisite transformation equipment) in order to isolate certain potential electrical transients from propagating from one system to another.

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