Here is the rest of my post about this topic.
Part 1 was
Anyway, on any 3 phase supply you will show small harmonic currents on the neutrals.
Many modern items of office equipment contain switched mode PSU's which can generate a range of harmonic currents. Most of these currents can be disregarded when considering installations, however the magnitude of triple harmonics has been a cause for concern, and raised awareness of the problem to ensure they are allowed for in installation design.
Triple Harmonics effect:
Cable current carrying capacity
Voltage Drop
Overcurrent device operation
UPS, Transformer and Stand-by systems ratings.
For single phase systems the neutral current will normally always equal the phase current so can be ignored.
In 3 phase 50Hz supplies that are balanced, the loads cancel out in the neutral due to the natural consequence of the 120 degree displacement of each phase, however the third, and other triple, harmonics combine to give a neutral current that has a magnitude equal to the sum of the third harmonic of each phase. The heating effect of this neutral current can have a noticeable effect on the temperature at which the cable operates in 4 wire systems.
The current rating of cables found in BS7671 and related standards assumes that at full load the neutral current will be practically zero. Whilst in practise the current is very rarely zero on standard 50Hz supplies, this is simply an indication of an out of balance loading of the installation. Assuming the phase conductors are not overloaded, then neither will the neutral conductor.
In most current rating tables the rating of 4 core cables is the same as that of 3 core cables, but theoretically the 4 core cable should be given a higher current rating. This additional capacity of a 4 core cable can offset, to some extent, the heating effects of the harmonic currents induced into the neutral conductor.
For this overload protection is only provided if;
Iz = It Ca Cg Ci Ct
>In
> Ib
Where
Iz is the current carrying capacity of the cable for this particular installation
It is the tabulated current carrying capacity of the cable
Ca,Cg,Ci,Ct are enviromental and installation correction factors
In is the nominal current of the overload protective device on the phase conductors
Ib is the load design current
If third harmonics are present then the formula holds true for 4 core cables where the triple harmonic content of each phase does not exceed 15% of the fundermental, for greater harmonic content, allowances must be taken into account.
Triple Harmonics
Triple harmonics are multiples of the 3rd harmonic, ie 3rd, 9th, 12th, 15th etc. The third harmonic will oscillate at 3 x 50 Hz or 150 Hz. These harmonics are drawn by power supplies to computers, printers, other office electronics and certain types of lighting equipment.
Due to their nature, triple harmonics and multiples of the triple harmonic are not cancelled out in the neutral, they actually add together. For a load that is pure thrid harmonic, the neutral current (Ibn) is actually 3 times the phase current (Ibph). This could and does lead to overheating of cables and equipment, increases the voltage drop of the cable and ultimately degrades the current carrying capacity of the cable.
When designing installations, the designer needs to make allowances for the third harmonic content, both in cable sizes and in switchgear, along with overcurrent protection to match.
The Neutral Current(Ibn) can be easily calculated if the third harmonic current is known.
Ibn = 3h / 100 x Ibph
Where:
Ibn = neutral current
Ibph = phase current
h = triple harmonic as a percentage of the phase current.
OK, so I could bang on about this for hours, and I have better things to do, but you can see that harmonic current s are a problem that requires addressing, usually at the design stage of an installation, however it is also something that needs to be seriously looked at when addressing unbalanced loading issues in existing installations.
As electricians we are taught that Neutral = Phase current = zero, that is not true, single phase supplies attatched to unbalanced 3 phase installations can have triple harmonic currents imposed upon them, having a negative effect on the equipment within the installation and th infra-structure of that installation.
Have any of you actually wondered why the neutral is normally the one that suffers heat stress in badly loaded or installed systems? Well now you know what the reason is...outside of loose terminals anyway.