A couple of 240D 3P questions ............

[VernB]

Greetings all! In order to throughly understand these PITA 240D 3P entrances I'm blessed with dealing with, I've got a couple of questions on behavior of these.

I understand that if the load on these is severely unbalanced, that these entrances can become quite unstable in terms of voltage, much more so than 208Y120 3P. Any rules of thumb for how much load unbalance to trigger this?

Second, I understand that this unbalance can cause a watthour meter to read inaccurately. Would this tend to make the meter read low (less power indicated than used) or high (more power indicated than used)?

Thanks all! Vern

 

[robbietan]

I dont think that unbalanced loads will affect your kWHr meter.

 

[VernB]

I dont think that unbalanced loads will affect your kWHr meter.

I've seen a number of articles that suggest severely unbalanced load on 3P delta will negatively affect meter accuracy but I'm not finding details on what conditions it occurs under. I'm thinking it has something to do with the metering method (these entrances don't use 3P meters, they appear to use only 3 wire meters and only directly measure 2 of the 3 phases).

Vern

 

[winnie]

Take the following with a large grain of salt, since I am remembering something that someone else said and which I possibly misinterpreted....

Many meters are designed with the assumption of balanced supply voltage, and become less accurate as imbalance increases. The issue is the number of 'potential coils' measuring the supply voltage. The imbalanced _load_ isn't a problem, but instead the imbalanced supply voltage, and depending upon transformer impedance, the load current will cause the supply voltage imbalance.

See:

http://www.uomschool.org/Meter_Book/Table%20of%20Contents/Blondel/Blondel's%20Theorem.htm

Then for center tapped delta, see:
http://www.uomschool.org/Meter_Book/Table of Contents/Self-Contained Diagrams/15S - 240V - 4W.htm

It appears that a standard meter used for this sort of service only has 2 stators, when 3 would be required to meter correctly in imbalanced conditions.

Apparently there is a form 17S meter that has the necessary 3 stators and would read correctly with imbalanced supply voltages.

-Jon

 

[ptonsparky]

I understand that if the load on these is severely unbalanced, that these entrances can become quite unstable in terms of voltage, much more so than 208Y120 3P. Any rules of thumb for how much load unbalance to trigger this?
Thanks all! Vern

I believe this will depend on each system. Smaller transformer KVA will not be able to handle the unbalance as well as larger. Type of loads, how they are started, etc. Just my thoughts.

 

[VernB]

The delta meters are all form 5A (with CTs) or form 12S (without CTs). As a comparison, the wye meters are all 9A (with CTs) or 14S (without CT).

Vern

Take the following with a large grain of salt, since I am remembering something that someone else said and which I possibly misinterpreted....

Many meters are designed with the assumption of balanced supply voltage, and become less accurate as imbalance increases. The issue is the number of 'potential coils' measuring the supply voltage. The imbalanced _load_ isn't a problem, but instead the imbalanced supply voltage, and depending upon transformer impedance, the load current will cause the supply voltage imbalance.

See:

http://www.uomschool.org/Meter_Book/Table%20of%20Contents/Blondel/Blondel's%20Theorem.htm

Then for center tapped delta, see:
http://www.uomschool.org/Meter_Book/Table of Contents/Self-Contained Diagrams/15S - 240V - 4W.htm

It appears that a standard meter used for this sort of service only has 2 stators, when 3 would be required to meter correctly in imbalanced conditions.

Apparently there is a form 17S meter that has the necessary 3 stators and would read correctly with imbalanced supply voltages.

-Jon

 

[robbietan]

without going into math (which I personally dont like anyway) the two-wattmeter method that the form 5A and 12S use is very accurate despite having loads (or voltages) unbalanced.