Current Transformer Question

Discussion in 'General Electronics Chat' started by steve5x5, Sep 12, 2010.

  1. steve5x5

    Thread Starter New Member

    Sep 11, 2010
    My project concerns a 50KW 3 phase 120/208 volt photovoltaic inverter system that I’m about to install. The Electric Utility uses 400A. polarized 1 turn CT’s for their metering. The drawings included show only 1 phase for simplicity.

    Previous installations have worked as shown in figures A and C. Figures A and C show power back fed to the Utility Grid when pv production exceeds the building consumption and thus turns the Utility Meter backwards. When building consumption exceeds pv production, the Utility Grid needs only to supply any additional power for the load not supplied by the pv system. This reduces power supplied from the Utility Grid.

    My question is will I achieve the same metering results by making the connections as shown in figures B and D ? Does the stronger magnetic field of the conductor carrying more current cancel or retard or modify the lesser current carrying conductor, or vice versa ? Do the different directions the two currents travel through the CT nullify anything ? a-b2.BMP c-d2.bmp
  2. timrobbins

    Senior Member

    Aug 29, 2009
    Steve - in general you will get the same meter measurement with either configuration, as the currents in the two cable configuration do effectively acts as just being from a single wire (ie. the magnetic fields cancel). However, reality may bite, and there may be a small residual offset whenever PV and building currents are both flowing, such that the CT does not see them as perfectly being the sum of currents. The measured offset should be very small - but in a kWh sense, that will accumulate over a year and could possibly be noticable, but may be within the class accuracy of the meter. Try and keep the cables going through the CT as 'centred' as possible in the hole, and at right angles to the CT face. It is unlikely that you would have additionally accurate enough metering to check the outcome.
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