Pts for Metering

[keyrose]

I have seen 2Pts with open delta used for metering as well 3 pts with star or delta connection used for metering.

What is difference using 2 pts for metering and 3 pts for metering.

When 2 pts are able to measure circuit power why we are using 3 pts for metering?


Kiroj

 

[pwrtran]

Because Blondel's Theorem states - in a system of N electrical conductors, N-1 electrical meter elements, when properly connected, will measure the electrical power or energy taken. The connection must be such that all potential coils have a common tie to the conductor in which there is no current coil.
In other words:
1) a 3-wire delta system can be metered by using 2 PTs connected between Red-White and Blue-White phase and no CT on white phases.
2) a 4-wire grounded wye system can be metered by using 3 PTs star connected with the neutral as the common and no CT on the neutral.

 

[scottf]

Keyrose-

Really depends on the voltage class, the circuit configuration, and if the PTs will also be used to feed any protection applications.

Some of the scenarios are covered above.

At 46 kV and 69 kv (in the US), 3 x single-bushing PTs became cheaper to purchase than 2 x 2-bushing PTs, especially when dry-type single-bushing PTs are used at that voltage, which is becoming the norm.

Also, some protection applications require a line-to-ground voltage measurement.

 

[pwrtran]

Keyrose,

If your are talking about metering IT's (wholesale and retail), they have to be Blondel compliance. 2 elements metering is for ungrounded 3phase system and 3 elements metering is for grounded 3phase system. Some legacy installations are 2-1/2 elements metering, they can also be used to measure a grounded system with correction factor. However, when any substantial changes need to be done, you need go with full Blondel compliance.


If they are for protection or indication/SCADA, then - it is a different story.

 

[waross]

Two PTs may be used when you are confident that the voltages are equal. The accuracy will be the same as a three PT system.
If the voltages are unbalanced, the two PT system may meter high or low, depending which phase is high. I had some metering working well with two PTs,
But. due to two factors;
1> The loss of an undersea phase conductor in a triplexed cable that was replaced by a single conductor, and
2> The island ran on two phases for quite awhile before a cable was located and installed and the load was never able to be properly balanced between phases.
The primary voltages could not be balance acceptably and I had to change to new meters with three PTs to properly meter the industrial plant.

Bill
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"Why not the best?"
Jimmy Carter