Advantages of Multiphase Systems
The design features discussed above for two and three phase systems are also applicable for systems with more than three phases. The reasons for the popularity of multiphase power systems can be summarised as below:
- Multiphase machines can be designed to operate under constant total power conditions, thus providing for a better mechanical design. Hence, the machines will run smoothly with less noise and vibration.
- Multiphase generators and motors provide for a more efficient design. Hence, they are smaller and more economical for a given power level.
- Multiphase transmission lines can deliver more power for the given weight and cost of conductors, since the system can be operated with a single return conductor.
- Voltage regulation of multiphase transmission lines is inherently better, since little or no current flows through the return conductor for balanced loads.
Why Three Phase Systems?
In the case of A.C. generators and motors, very little economic benefit is gained by having more than three phases. As a compromise between economy and complexity, three phase machines are invariably used.
In practice, the number of phases for a power system is determined essentially by the transmission system economics, as it provides significant economies as the number of phases are increased. Let us do a simple economic analysis of multiphase transmission systems.
Let us start by considering a single phase transmission system which is designed to transfer an average power of P at unity power factor, that is P = Erms Irms. A single phase transmission system requires two conductors – a phase conductor and a return conductor. Hence, the power transfer per conductor is 0.5P.
Let us assume a single common return conductor for a multiphase system. Let us also assume that the common return conductor has the same size as the phase conductor, which is a conservative value. Recall that the power rating of a two phase system (2P) is twice that of a single phase system (P) and so on. The power transfer per conductor for other multiphase transmission systems are as shown in Table 1.
Table 1 – Power per conductor for various systems
The incremental savings per conductor are as given below:
1-Phase to 2-Phase : the saving is 0.166P ( 33.3% saving)
2-Phase to 3-Phase : the saving is 0.084P (12.6% saving)
3-Phase to 4-Phase : the saving is 0.05P (6.7% saving)
The incremental saving decreases as the number of phases are increased. In addition, there is a ‘cost of complexity’ when the number of phases are increased.
Exception
If a substantially large power transfer is involved, then it may be feasible to justify transmission systems with higher number of phases. For example, let us say additional incremental saving justifies a six phase transmission system. In such cases, three phase generators are still the preferred system for generation.
A six phase transmission is achieved by converting the three phase power at the generation end to six phase power using special transformers. It is then converted back to three phase power at the receiving end for onward distribution. Six phase transmission systems do exist in practice, however, they are not common.
Conclusions
As a compromise between economy and complexity, the three phase generators and motors are invariably used in the present day power systems.
It may be possible to economically justify transmission systems with more than three phases at very high power levels. However, for operational convenience, it is more common to use higher voltage three phase transmission systems (for example, 66kV, 132kV, 275kV etc) rather than increasing the number of phases.
8-Aug-2021 – Version 1 (Original)
