AC mains current directional measurement

Discussion in 'Homework Help' started by ECOTEC, Oct 25, 2010.

1. ECOTEC Thread Starter New Member

Oct 23, 2010
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I am attempting to design a circuit that can measure current in an AC mains circuit. I not only want to measure the current value but also in which direction the current is flowing in the conductor.
Any suggestions would be appreciated.

2. t_n_k AAC Fanatic!

Mar 6, 2009
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AC current is just that - it alternates in either direction in the conductor.

Perhaps you really want to know the direction of the actual power flow in an AC system...??

3. beenthere Retired Moderator

Apr 20, 2004
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Current is going to follow the applied voltage. Use a stepdown transformer and diode to apply an in phase voltage to a voltage comparator. The signal will indicate one polarity quite definitely. The other polarity may be assumed.

4. thatoneguy AAC Fanatic!

Feb 19, 2009
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Can you clarify what is intended by "Current Direction"?

5. ECOTEC Thread Starter New Member

Oct 23, 2010
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Thanks for the advice. The measurement of the current is not a problem. The direction that the power is flowing in the conductor is the problem I am trying to determine. I have experimented with a pair of current clamps applied to a single conductor. The scoped output waveforms from the CTs shows two AC waveforms; by turning one CT 180 degrees to the other, the waveform moves 180 degrees out of phase with the other. Can anyone suggest a typical circuit that would identify power flow in a conductor from these scope traces?

6. t_n_k AAC Fanatic!

Mar 6, 2009
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Nothing unusual about that outcome. The CT's are simply responding to the instantaneous values of the current. If they are identical in all other respects but physically transposed by 180° on the conductor then their individual electrical output phase relationships will also be 'transposed' by 180°.

You appear to be wanting to determine the direction of power flow in a circuit. Presumably you have a valid reason for wanting to know this. If there is uncertainty in a particular physical circuit about the direction of power flow then an informed assessment of the installation should clarify the matter. An understanding of the juxtaposition of supply point(s), various load locations and conductor runs would allow one to determine this. If this is part of a system in which power flow could be bi-directional - such as with a photovoltaic system with demand related power return to the supply point of common connection - then again it may be possible to assess what's happening based on the local inverter control panel indications. If such indications exist.

The post by beenthere is relevant to the discussion if you do need to take measurements. You seem to have some useful tools at hand already. With a CT of known polarity, an isolation voltage transformer (also of known polarity - prim to sec) and your scope you could readily work out the power flow direction in a single phase system.

7. thatoneguy AAC Fanatic!

Feb 19, 2009
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Are you trying to balance the loads on each side of the CT, or are you looking for an instantaneous power "snapshot" from each side at the same moment?

8. BillB3857 Senior Member

Feb 28, 2009
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It almost sounds like he may want to feed power back to the grid and know when that is happening.

9. ECOTEC Thread Starter New Member

Oct 23, 2010
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The application that I am trying to measure is as you suggest – a solar PV system. I have a 2.2Kw grid tied system. My property is vacant during most of the daytime and therefore I estimate that around 80% of the generated power is being fed back into the grid. I would like to build a control system that will monitor the power that the system is generating and use some of this power for water / space heating. Does anyone know if such a control system exists as I do not want to re-invent the wheel? If not, can anyone suggest a suitable circuit that would achieve this?

10. blueroomelectronics AAC Fanatic!

Jul 22, 2007
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Your mains meter will show that. It's not the sort of thing you should be playing with but contact a qualified electrician if you want to modify it.

11. ECOTEC Thread Starter New Member

Oct 23, 2010
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I am a fully qualified, time served electrician and have spent the past 20 years employed as a Senior Electrical Engineer. UK digital electric meters can only display total units used. They are sealed devices and belong to the Electric Authority and therefore even if they could display instantaneous units used, would be of no use in my application.

Mar 6, 2009
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