Power Factor 1

[govender1]

What are some of the causes of a low power factor. Usually the power factor at most of our traction substations is .96 lagging however one particular substation is 0.5.

Substation layout:

Primary of transformer : 88 kV 3340kVA
Secondary : 2x2364 V note there two secondary windings 2x2362 kVA
Load : 6 pulse diode rectifier : Output voltage 3kV dc

3kV feeds onto overhead power lines where trains acquire their power via pantographs.

 

[mpparent]

Possibly you have some high induction type loads, such as induction furnaces, etc.

 

[jwerthman]

My guess is that the harmonic-rich currents to the 6-pulse rectifier are causing the low power factor. Some instruments that measure only displacement power factor would not respond to this, but newer digital meters that actually calculate power factor from kW and kVA or kVAR measurements will respond to the true power factor. Harmonic-rich load currents will generally have lower power factors since the harmonic currents contribute little to the real power delivered to the load.

 

[electricuwe]

Where did you measure the power factor ?

On primary or on secondary of substation transformer?

Your transfomer seems to be an old unit used for mercury arc rectifiers. For these applications usually midpoint connection was used. When the mercury arc rectifiers were replaced by diodes also midpoint connection had to be used whereas on new diode recifiers for that voltage bridge connection is the standard.

On the secondary side of such a midpoint connection power factor is quite low, but on the primary it's similar to a bridge rectifier.

 

[mpparent]

LOL....didn't see your load profile. It's more than likely attributed to the six pulse rectifier.

Mike

 

[jghrist]

Rectifiers can reduce the power factor in two ways. First, the current distortion adds current without adding to real power. Total power factor being watts divided by volt-amperes, will decrease because the volt-amperes increases without an increase in watts. This is the distortion component of the power factor and is independent of the phase angle, or displacement.

Second, if the rectifiers are thyristor-controlled, conduction of each phase can be delayed to control the dc output. Delaying the conduction will result in a lagging current phase angle. This is the displacement component of the power factor.

 

[Alex68]

What are the main differences between this particular substation and all the others?
All the previous answers are correct, but they don't explain the difference.

Electricuwe asked correctly:<<Where did you measure the power factor?>>
Maybe the difference is the measuring point!

 

[busbar]

Good comments so far.

First off—it is ridiculously, agonizingly, and disgustingly easy to convert electric power into heat.

For similar loads, {traction?} one would not expect to see such a wide dissimilarity in readings.

With power-frequency measurements, there are likely two conflicts—measurement methods {and probable misregistration} versus ‘actual’ load. With otherwise-similar loads on different transformers, it may be a fundamental problem of something like AC-side phase voltage/current imbalance, transformer /reactor/capacitor configurations that foster this, or the means used to arrive at and makes sense of the end measurement and calculation.

I suppose you are aware that with nonsinusoidal voltage and current waveforms, the traditional Pythagorean power relationship no longer applies. The methods and algorithms of various power quantities have to be approached differently. Distillation of other power quantities surely gets interesting.

Step further is to parse positive- negative- and zero-sequence characteristics on both arithmetic and mechanical levels. With our penchant for defining the real world, what best models the actual system and {often primarily/ultimately thermal} effects on it?

 

[RPGu]

Would this particular substation which has a 0.50 power factor does not only supply DC traction power but also AC power and is located on the Maintenance Depot.

I am also working in traction substation and our pf. is the same with yours (above .90). The power factor were measured in the metering of the Utility company (34.5kV).

There is a period when our pf in the Maintenance Depot dropped between .70 - .80, however there is on-going project which utilized lots of motors.After this project,our pf is back to normal and we are longer being penalized by the utility company.

But your pf of 0.50 is to low.

 

[reactive]

This has to be a measuring problem.

 

[coyney]

Low power factor = high inductive content of the load.