Magnitude of neutral current in distribution transformer

[rubron]

How much is expected to be the neutral current read at a distribution substation of lets say 400kVA 10kV/400V, for a "normal" residential area, either in Europe or the USA. Any idea?

Thanks in advance.

 

[protoslash]

it depends on the imbalance between the three phases load fed by this transformer. If all residential load are single phase, i would expect 20% imbalance to show up on the Tx neutral.

 

[waross]

At the base frequency, as little current as possible, depending on the load balance between phases.
The neutral current may vary at different times of the day depending on the demographics of the consumers.
eg: How many cook and/or heat water with electricity versus gas.
How many can afford air conditioning.
In our little island utility (about 5000 homes) our crews tried in vain to find a load balance that would minimize neutral current at all times of the day.
Then there is the magnetizing current of each transformer that will contain harmonic currents that do not cancel on the neutral.
Non linear loads connected phase to neutral will cause harmonic currents on the neutral.
The base frequency neutral current will vary as the load balance varies.
The harmonic currents will depend on the number of transformers on the circuit and the contribution of non-linear loads.
What is normal on one circuit may be excessive on another circuit.
It depends.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamburgerHelper]

It can be more than in each individual phase. Zero sequence harmonics on each will pass through the neutral. Neutrals are sometimes larger than the phase conductors for this reason.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[rubron]

I had a quick look at the measurements of a distribution transformer for a time period of a month with a resolution of 10 minutes, and there were currents of the order of 170A in each phase, and with an imbalance on the range of 1-10 A, and with the neutral current in the range of 1-4A. I really expected higher neutral current. cuz as the loads are randomly connected during the day it is difficult to balance the grid on real time. Do you see this values normal? I will update more on the data soon, but would like to hear some thoughts. Thanks!

 

[cuky2000]

There is no universal rule allowing some maximum neutral current. Most electrical systems are configured so the neutral can carry imbalance current. See below


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[prc]

Neutral current in distribution transformers can be due to 1) Unbalance loads -value depends on the extent of unbalance in loading 2) Depending on the harmonic content in the load-will go up with K-factor.Neutral current can even reach 300 % of full load current 3)Small currents depending on grounding + nature of resistivity of soil 4) Winding manufacturing variation between phases - some users specify the neutral spill current ( with supply at HV ,LV shorted and current between shorted terminal and neutral) shall not exceed 2 % of line current.