• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

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


Well-Known Member

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 ?


Well-Known Member
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.

Latest threads

EE World Online Articles