# Problem with unbalance detection in three phase system

1. ## Problem with unbalance detection in three phase system

Hello! I want to check voltage unbalance in a three phase system. I have used the below circuit for checking unbalance. But in normal operation that all three phase are approximately equal there is'nt zero voltage difference between points N and N'. I have near 5 Vrms in normal operation, while in some weeks ago there was near 0 voltage difference. Do anyone know what is the problem??

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2. ## Re: Problem with unbalance detection in three phase system

No problem. Just asymmetrically loaded power grid.

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3. ## Re: Problem with unbalance detection in three phase system

Do you mean the problem is with power grid that supplies 3 phase?! When I measure phase voltages there is little difference between them! and 3 resistors also are equal.

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4. ## Re: Problem with unbalance detection in three phase system

Phase voltage isn't the full information. You should also measure phase-to-phase voltage.

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5. ## Re: Problem with unbalance detection in three phase system

There are two distinct unbalance conditions: The three phases have unequal peak voltages AND/OR the phase relations between the phases are not equal.

In general, we expect the load too be symmetric on the three phases. This condition is often not fulfilled. The effect depends on the source impedance.

Your load is not the only load on the system; other large loads on the same circuit can affect the phase and amplitude. If the line voltages are 220/440V, then 5V on the neutral due to unbalanced current is rather small and nothing to worry.

If the voltage gets over 10% of the line voltage, you need to redistribute the other loads so that some symmetry is attained.

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6. ## Re: Problem with unbalance detection in three phase system

Harmonic distortion can easily be several percent.
So even if the rms voltages are identical, the zero point can be moving around quite a lot at harmonic frequencies.

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