Installing the current transformers (CT's) correctly on a three phase system is critical if the power zappi measurements are going to be correct.
Understanding the theory behind three phase power is beyond the scope of this article and we'll aim to produce a separate article to cover the theory in more detail (Three phase power - the theory). This article provide some practical tips to get the three phase CT's set up correctly.
- If you have a single phase zappi or an eddi then you will need a harvi to measure the current on all three phases - you cannot connect the three CT's from different phases directly to the single phase zappi.
- If you have a three phase zappi then the three CT's can be wired directly to the zappi.
In theory it's simple:
- The CT must be clipped around the right cable (CT1 on phase 1, CT2 on phase 2 and CT3 on phase 3)
- The arrow on the CT must point towards the consumer unit / fuse board
- The "phase rotation" setting on the zappi must agree with the way the three phase wiring is connected.
- Simplest is to have zappi wired L1 - terminal 1 / L2 - terminal 2 / L3 - terminal 3 + Neutral and Earth
- This is the default setting for zappi. More information on the zappi phase rotation setting will be covered in a separate article (The zappi phase rotation setting)
Unfortunately we find that the wiring in the house often swaps / rotates the phases - and this is where the problems occur.
Start by checking the Grid CT's
Make sure that any local generation (PV / wind) is turned off and that there is enough load in the house to draw at least 1A per phase.
Now look at the CT readings.
For directly connected CT's these readings are on the menu:
Other Settings -> Advanced -> CT Config -> Readings...
For CT's that are connected to a harvi, the readings are on the menu:
Settings -> Advanced -> Linked Devices - Devices -> Harvi
- Each CT should read a +ve power and have a power factor close to 1.0 (at least 0.9)
- A power factor reading of 0.5 means that the CT is clipped round the wrong phase.
If it's not easy to see which CT is on the wrong phase, try the following:
- Clip all the CT's around the same wire.
- Check the CT readings.
- One CT will show a power factor close to 1.0. The others will show power factors close to 0.5
The CT with the power factor of 1.0 is on the correct phase
- Now move the other two CT's to a different wire.
- Check the CT readings. One CT will show a power factor close to 1.0, the other will show a power factor close to 0.5
The CT with the power factor or 1.0 is on the correct phase
- Now move the last CT to the remaining wire and check the CT readings again.
The Grid CT's should now all be on the correct phase and giving the correct power reading.
If there is a PV panel or wind turbine and you have CT's on the inverter output then you can follow the same approach to get these CT's right as well.