[solved] Why are two current transformers considered less accurate than three current transformers - isolated 3 phase

[Grossel]

Hi.

This is what I have heard several times, also from those manufacuring overcurrent protection devices. The idea is that when wanting to measure energy - say from a substation - to the busbar, you can choose either 3 current transformers one per phase - or can choose two current transformers where secondary side is connected as aaron connection scheme.
Everybody I ask say measurements is more accurate when using 3 CT's instead of two.

Some time back I was pondering about this question, and I did a relatively simple vector addition given that I know the phase shift through the CT's - but I ended up with the same result for both 2 CT scheme as for a 3 CT scheme.

Therefore I ask - Say I've thaught this wrong - why are energy measurement less accurate when using aaron connection (2 CT's) than 3 CT's - given same type of CT?

What is the physics / math that explain just that ?

 

[Grossel]

Time to close this off.

I was able to discuss this topic with one salesman for energy meters, and the explanation is as following:

In an isolated 3 phase network, under normal operation an energy meter should measure the same regardless of using 2 or 3 CT's. However, in the case of lingering earth fault, there will be a return current - that won't be taken in account when the meter calculates S ; because the sum of currents are not equal to zeero.