Question
I need to provide resistive isolation between the 4 wires of a 3-phase supply(incl. neutral) and the burden resistor circuit ground used by currenttransformers. The traditional way to do this is to use high value resistors onall phase inputs including the neutral followed by a 1k to 2k attenuatingresistor. If I introduce a 2M resistor into the neutral line will this causea problem. It will be feeding into a 1k resistor so the ADE7880 VN pin willsee a 1k source impedance.Is this approach OK?
Answer
Adding a Series Resistance to the Neutral line for resistive isolation cancertainly be done and we have an application note to explain the impact:http://www.analog.com/media/en/technical-documentation/application-notes/AN-1334.pdf.
There are a few takeaways,· The Series Resistance on the neutral should be matched to that of the phases,for example if the phases use 2MOhm + 2kOhm for the divider then so should theneutral.· For a balanced system where all the voltages are the same there is no issue.· If the System becomes unbalanced (i.e. one voltage line drops lower than therest), it could cause some gain and phase error. This is because with theseries resistance, the ground of the ADE7880 no longer has a strong tie to aparticular potential but rather has a weak tie to all the phases and neutral.This means that the ground point can move when the system is unbalanced.