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P29 Study – Voltage Unbalance

So what is a P29 study? A P29 study is shorthand for the name of a standard produced by the Electrical Networks Associate (ENA) called  P28 ‘Planning Limits For Voltage Unbalance’.

This study is relativity uncommon and is occasionally asked for by DNOs, so it is worth understanding what it is all about. The aim of the study is to assess how any newly connected loads can cause an unbalance in the the three phases of the power system. Basic power system theory shows that it is desirable that all three phases of a power system are equally balanced; if they are not balanced then the system is not optimised and can lead to excessive heating and currents flowing in the neutral. This is an undesirable state for any DNO, which is why the P29 rules exist.

Interestingly, an unbalanced power system, can also cause problems for G59 protection relays, which will see a sudden change in phase unbalance as a vector shift and trip. Aurora have direct experience of this phenomena and actually investigated a G59 relay that was nuisance tripping (see case study 6) – due to nearby factory with a heavily unbalanced lighting circuits!

In practice most loads connected at 11kV and above are usually three phase loads (i.e. motors, generators, 3 phase inverters etc..) which are inherently balanced and will not cause any change in the unbalance in the system. Most DNOs understand this, and are happy with a statement, or short report showing why this is the case, and do not require anything more formal.

The more challenging aspect of a P29 study is when a system is connected at LV (very occasionally this happens at HV) and contains lots of single phase loads such as domestic housing supplies, electrical trace heating, or industrial lighting systems. These loads are single phase, and therefore if the application designer is not careful, a significant unbalance can be introduced. Demonstrating compliance with the standard in these scenarios typically needs a computer model of the connected loads and an unbalanced loadflow analysis to confirm the total unbalance.

It should of course be noted that it is virtually impossible to get a balanced LV system, but the aim is to get it as reasonably balanced as possible so that the unbalance is limited.