Leading or Lagging PF? 1

[dalas]

Hi,

in order to run / operate electrical network,

which type of power factor should we keep having?

if a system runs on leading power factor, what is the main effect ? does it matter to have leading or lagging?

since it just depends on the type of loads

thank you

 

[itsmoked]

Normally all networks have lagging pf.

If you achieve leading you may cause network stability issues.

You want pf=one or a little lagging.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Synchronous generators have limited ability to absorb VARs, so generating companies usually prefer a neutral or slightly lagging power factor as this keeps the generator operating at a stable condition where it can withstand system disturbances without tripping or losing synchronism. Transmission system operators would ideally have a unity PF because it maximises line capacity. Distribution companies are generally grateful for any leading loads which help to compensate the majority lagging load caused by induction motors.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Truemanator]

Correct me if I'm wrong, but having a unity pf on a transmission would be unwise for stability purposes, wouldn't it?

My experience in transmission system control tells me you need VAr's to keep the voltage up in case of a fault. We'd all like zero cost for transmission, but without VAr's my understanding is you are much more subceptible to instability.

 

[davidbeach]

Unity pf on a transmission system would mean that a sufficient quantity of VArs are being supplied to compensate for all reactive "loads". Wouldn't be a bad condition, having VArs supplied near where they are "consumed".

 

[cranky108]

Sence Vars typically cost money to produce (I've never had a large capacitor handed to me). It seems prudent that you would want to operate as lagging as the utility will allow you to.

As far as stability, capacitor created Vars are less stable than motor/generator created Vars, from a system perspective. Under a disturbance condition the capacitor created Vars will decrease with the square of the voltage, where the motor/generator created Vars won't decrease as fast.

 

[bacon4life]

Truemanator/DavidBeach

You are both right, just looking at different time periods. For transient stability after a fault, overexcited machines (thus producing VARs) have a longer (better) critical clearing time than underexcited machines. For longer term voltage stability it it is better to produce the VARs near the loads to prevent voltage collapse.

 

[davidbeach]

bacon4life, no disagreement about critical clearing time for overexcited machines, but nobody pays for delivery of VArs, only for Watts. The system would be far better off is generators, in general, ran far more lagging than they do but the economics aren't there.

 

[Rodmcm]

Not quite correct Davidbeach, often the ability to support voltage by the ability to supply and absorb Vars is a mandatory requirment for connection to the grid. Also, in my country there are generators that run as synchronous condensors and they do get paid for that use.