Utility Tie Breaker Current
Utility Tie Breaker Current
(OP)
I would like to better understand the relationship between 'var current' and 'watt current'. To do this in the best manner I know, I'll present a scenario and three questions:
'The Widget Mill' operates a T-G capable of supplying most of its electrical requirements at full mill production, and can push power out onto the utility's grid during curtailed widget production. Normally, Mvar across the utility breaker are kept at zero. It was recently found that one of the breaker's contacts was hotter than the other two, and a decision was made to limit power through it to 20 Mw until it could be replaced. The current indicator shows that, at 20Mw into the mill and zero Mvar, breaker amperage is 100.
One week later, The Widget Mill's power facility shuts down one of its boilers, reducing T-G output and necessitating 30Mw of utility power to maintain full widget production. Because of the 20 Mw limit, a decision is made to shut down a widget machine until the boiler can be put back on line.
Question 1: What would be the effect on amperage through the utility breaker if the mill imports 30Mw while pushing out 10Mvar to the utility? (i.e., is the Mvar amperage additive, subtractive, or of no consequence?)
Question 2: What would be the effect on amperage through the utility breaker if the curtailed mill imports both 20Mw and 20Mvar (as opposed to 20Mw in and zero Mvar)?
Question 3: Is it proper to assume that, in terms of heat generated in a conductor, that the current in one Mvar is exactly equal to one Mw?
With apologies for lack of brevity, I look forward to some electrical edification.
'The Widget Mill' operates a T-G capable of supplying most of its electrical requirements at full mill production, and can push power out onto the utility's grid during curtailed widget production. Normally, Mvar across the utility breaker are kept at zero. It was recently found that one of the breaker's contacts was hotter than the other two, and a decision was made to limit power through it to 20 Mw until it could be replaced. The current indicator shows that, at 20Mw into the mill and zero Mvar, breaker amperage is 100.
One week later, The Widget Mill's power facility shuts down one of its boilers, reducing T-G output and necessitating 30Mw of utility power to maintain full widget production. Because of the 20 Mw limit, a decision is made to shut down a widget machine until the boiler can be put back on line.
Question 1: What would be the effect on amperage through the utility breaker if the mill imports 30Mw while pushing out 10Mvar to the utility? (i.e., is the Mvar amperage additive, subtractive, or of no consequence?)
Question 2: What would be the effect on amperage through the utility breaker if the curtailed mill imports both 20Mw and 20Mvar (as opposed to 20Mw in and zero Mvar)?
Question 3: Is it proper to assume that, in terms of heat generated in a conductor, that the current in one Mvar is exactly equal to one Mw?
With apologies for lack of brevity, I look forward to some electrical edification.






RE: Utility Tie Breaker Current
MVA² = MW² + Mvar²
I = MVA·1000/(1.732·kVØ-Ø)
Note the Mvar is squared, so it doesn't matter which way the net reactive power flows.
RE: Utility Tie Breaker Current
"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller
RE: Utility Tie Breaker Current
Q2 - you will have a lagging PF= 0.707, V leads I by 45deg
Q3 - No. Heat generated by the current corresponding to MVA is always >= the heat generated by the current corresponding to the MW. Say, you have leads with 1 ohm resistance connected to a purely capacitive or inductive load, the current flow is 10A capacitive current(ignore the resistance component of the current), PF=0. You still have 10^2*1=100W heat on the conductor.
RE: Utility Tie Breaker Current
From a steady state condition of 20 Mw / zero Mvar in, a change is made to push 20 Mvar out to the utility (while still importing 20Mw). Assuming no change in ambient temperature, will the temperature of the utility breaker contacts tend to increase, decrease, or stay the same?
Thanks again.
RE: Utility Tie Breaker Current
"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller
RE: Utility Tie Breaker Current