Leading PF

[charlie b]

What is the real hazard in overcorrecting PF with excessive capacitance, resulting in a leading PF? I probably should be able to answer this myself, but school was so, soo, sooo long ago.

 

[adamscb]

What is the real hazard in overcorrecting PF with excessive capacitance, resulting in a leading PF? I probably should be able to answer this myself, but school was so, soo, sooo long ago.

I don't think there is any harm in doing so. Each load is getting the correct amount of power that it needs, and the excessive reactive power is being pushed back onto the grid. The utility doesn't like it, but I don't think there's any harm in it.

Correct me if I'm wrong though, anyone

 

[Besoeker]

What is the real hazard in overcorrecting PF with excessive capacitance, resulting in a leading PF? I probably should be able to answer this myself, but school was so, soo, sooo long ago.

Probably even longer ago for me, young fellow.......
Increased voltage is the usual technical issue cited. You need to draw out the vectors. I did post them on this site But that was a few years ago. And several computers ago.
We, and many others, would aim for around 0.95 lag.
Commercially, you don't want to pay for more than you need.

 

[ptonsparky]

From limited personal experience it raises the voltage. We are blessed with good voltage in this area and one project ended up making it too high. The customer was the only one on the transformer bank and the POCO was able to change taps for us. IIRC the resulting increase in voltage was textbook for the kvar we added.

 

[Ingenieur]

depends

if you go from 0.9 to 1.1 no real benefit since i remains the same

if local at the motor and left online when motor is turned off severe resonant at a f > 60 hz
likely motor and swgr damage

a bit over for bulk correction on a system loaded close to capacity
no issue

if the system is lightly loaded same as the motor scenario, not good

 

[Besoeker]

depends


if local at the motor and left online when motor is turned off severe resonant at a f > 60 hz
likely motor and swgr damage

Possible rather than probable.
Off is off.

 

[Ingenieur]

Possible rather than probable.
Off is off.

likely, almost certain

 

[kwired]

I don't think there is any harm in doing so. Each load is getting the correct amount of power that it needs, and the excessive reactive power is being pushed back onto the grid. The utility doesn't like it, but I don't think there's any harm in it.

Correct me if I'm wrong though, anyone

Unless you go too extreme, utility probably doesn't care - less correction they need to apply on their own. Yes they still put their own correction capacitors in their distribution where they feel is needed. Larger industrial type customers are mostly the ones paying penalties for poor power factor, residential and other smaller services typically are not forced to correct poor power factor - so there still is poor PF within the system and they do place capacitors in the system to lessen the effects.

 

[Ingenieur]

nominal system v elevation is minimal

v rise ~ Qc/Qsc pu
Qc = cap var
Qsc = system sc kva at cap location

say you have 1 mva xfmr at 5% puz
Qsc ~ 20 mva inf bus
Qc ~ 200 kva or 0.2 mva, 20% of system S, quite high
v rise 0.2/20 ~ 1%
this is load independent

 

[Besoeker]

likely, almost certain

A little tale for you that I have posted here before.
A good many years ago we had the bright idea that we could run a submersible pump and induction motor in reverse. The pump running as a turbine and the induction motor as an induction generator. The idea was cheap power where there was no utility grid.

I set it it up in our test bay with a variable speed DC drive as the prime mover in place of the pump/turbine.
As I'm sure you know, just spinning an induction motor won't make it a generator. It needs excitation. Enough residual and a PFC bank worked a treat. I got it up to 30kW which was the rating of the machine. All good. And no severe resonances.

Perhaps a more relevant point here is that if you leave PFC connected to the motor when the supply is disconnected and the motor load can drive it you can get regeneration, desirable or otherwise. That's why we, and other responsible manufacturers, always include a contactor for the PFC bank.

 

[kwired]

A little tale for you that I have posted here before.
A good many years ago we had the bright idea that we could run a submersible pump and induction motor in reverse. The pump running as a turbine and the induction motor as an induction generator. The idea was cheap power where there was no utility grid.

I set it it up in our test bay with a variable speed DC drive as the prime mover in place of the pump/turbine.
As I'm sure you know, just spinning an induction motor won't make it a generator. It needs excitation. Enough residual and a PFC bank worked a treat. I got it up to 30kW which was the rating of the machine. All good. And no severe resonances.

Perhaps a more relevant point here is that if you leave PFC connected to the motor when the supply is disconnected and the motor load can drive it you can get regeneration, desirable or otherwise. That's why we, and other responsible manufacturers, always include a contactor for the PFC bank.

To be NEC compliant though you must have discharge resistors installed on the capacitor.

 

[Besoeker]

To be NEC compliant though you must have discharge resistors installed on the capacitor.

PFC units normally come with pre-fitted with discharge resistors.