Pole mounted metering units

[Hebrew]

Hi,guys.We install two types of metering units on poles;all have three bushings on the line side and some have three or two bushings on the load side of the unit.I was asked when do we use one as opposed to the other and honestly could not say as I have seen them installed as they are available.I was told thet the 3/3 bushing types are for Y-Y systems and the 3/2 are for wye-delta installations.Reason being,if the three phases are balanced,the third one goes from line to load without being metered as it is assumed that is equal to the two that are metered.Can I have an perspective on this?Much appreciated!

[Electriceel]

Get pictures of this device and/or the form numbers of the metering.
We would never assume a customers load is balanced, you never when they change loads.

[trigger]

only balanced if all single phase ( V phase) loads are balanced to the load side

[Hebrew]

I agree,Trigger.And we know that the engineers plan for one thing,the business owners do something else.So it would make sense to have a unit that meters all three phases and that brings me to what the real issue is;cost.I understand the metering units are tens of thousands of dollars but the 3/2 configuration is cheaper.I just want to know if the two configurations are interchangeable between wye-delta and wye-wye.Thanks for the response,will see if I can get a photo or two.

[Brooks]

On the secondary side of the bank...

We used only two ct's on pump banks, wye-wye and wye-delta(closed). These banks only fed irrigation pumps so balanced loads were just about guaranteed.

We also used one ct on 1phase services.

We've moved to three on all three phase installs and two on all single phase. Wye-wye or wye-delta.

Primary metering packages were always one ct, one pt on single phase and three ct, two pt on three phase. Two phase line got two and two.

Wow, going back in time there 1980's!

[trigger]

Then the customer figures this scheme out on the sec. side and adds 1 ph load onto the unmetered leg and gets free power

[Hebrew]

Then the customer figures this scheme out on the sec. side and adds 1 ph load onto the unmetered leg and gets free power

Haha,quiet now!

[Engineer]

Blondel's theorem (1893) applies to all electrical energy metering. The theorem says that any polyphase system supplying energy with N number of conductors with a common voltage reference can be properly metered with N - 1 number of metering elements. What this translates to is that a 3 phase, 3 wire system (delta) can be metered with two meter elements. A 3 phase 4 wire system (wye) can be properly metered with 3 meter elements. In each case, for high voltage metering, a PT and a CT will be required to be connected to each element. Metering in this manner will be accurate for any load balanced or unbalanced.

In order to save money for metering, 3 phase 4 wire systems, some operators use what is called 2-1/2 element metering (non-Blondel solution). This method uses two PTs and three CTs. It has reasonable accuracy when the voltages are balanced and saves the cost of one PT.

[trigger]

sorry Mr. Engineer Theory is good but experience says not all applications apply.