Harmonic Disturbance on PF correction cap

[Cowboyjoe]

We have a company doing a load eval for a big property that keeps getting charged from the POCO for low power factor (< .85). I want to have a cap sized for their lines, looks like its probably the best option at this point.

They need to just monitor the lines for a week or so in order to evaluate the usage and calculate any NON-linear loads. That part I can keep up with. But they sent me an email today saying that they may have to increase the time (and $ amt.) of the evaluation if they find any harmonic disturbances playing into the equation.

I am trying to think of some examples of "harmonic disturbance" to explain to the customer, but I'm having trouble figuring it out. Would it be disturbance caused by equipment that might cause feedback? Or maybe a transformer issue? Not sure.:dunce: Anyone want to take a stab at shining some light on their terminology? I know its something I should be familiar with, I just can't seem to place what exactly it could be...

 

[__dan]

"Harmonic disturbance" and "nonlinear load" are probably interchangeable in the discussions, basically anything that does not draw or reflect a perfect 60 cycle sinewave on the line. The nonlinear loads are the rectifier front ends on UPS's and VFD's, voltage converting power supplies, anything with SCR waveform voltage choppers, induction heaters. Also defective or worn equipmemt could be an issue, say something supposed to have a good or corrected PF but now bad because of some type of component in the process of failure, dried out or damaged filter caps at the front end of power supplies .. The point being that bad pf could be caused by something localized to specific equipment that is calling for service and out of factory spec rather than something that is natively bad pf and needing pf correction at the supply.

Look at the character of the loads, what should be good pf (factory pf corrected power supplies and rectifier front ends) and what is natively lower pf that can be corrected (straight motor loads with no vfd's). I would want the pf caps closer to the target loads, either right at the motor or at the MCC, and as far away from the UPS's and VFD's as possible.

Pf correcting caps will probably want to work with a 60 cycle sinewave load (straight on the line motors) and not like the higher frequency components of nonlinear load waveform distortion. There could also be unpredictable issues, resonance, with loads that read as having bad pf but do not like excess capacitance.

 

[T.M.Haja Sahib]

. But they sent me an email today saying that they may have to increase the time (and $ amt.) of the evaluation if they find any harmonic disturbances playing into the equation. ]
Ask the following question.
Since a harmonic disturbance persists as long as its source is operating and one week time is enough to detect any harmonic disturbance,is the more time required based on any standard practice?
As the over voltage caused by power company switching is a different issue,this may be dealt with them separately.

 

[ron]

They need to just monitor the lines for a week or so in order to evaluate the usage and calculate any NON-linear loads. That part I can keep up with. But they sent me an email today saying that they may have to increase the time (and $ amt.) of the evaluation if they find any harmonic disturbances playing into the equation.

The NEC considers a month as being long enough for load calculations. That is how long I monitor before deciding what is going on regarding loads and harmonic characteristics. That way have have a standard to go by for reference.

 

[Besoeker]

We have a company doing a load eval for a big property that keeps getting charged from the POCO for low power factor (< .85). I want to have a cap sized for their lines, looks like its probably the best option at this point.

They need to just monitor the lines for a week or so in order to evaluate the usage and calculate any NON-linear loads. That part I can keep up with. But they sent me an email today saying that they may have to increase the time (and $ amt.) of the evaluation if they find any harmonic disturbances playing into the equation.

I am trying to think of some examples of "harmonic disturbance" to explain to the customer, but I'm having trouble figuring it out. Would it be disturbance caused by equipment that might cause feedback? Or maybe a transformer issue? Not sure.:dunce: Anyone want to take a stab at shining some light on their terminology? I know its something I should be familiar with, I just can't seem to place what exactly it could be...

A lot of different things are non-linear loads these days. Almost anything with electronics. Computers, televisions, printers, dimmers, anything with a switch-mode power supply. Each on its own might not be much but the cumulative effects can lead to significant supply voltage distortion.

A word of caution. The harmonics have frequency components that are higher than the fundamental (60Hz) and this can, and does result in PFC capacitors taking more than their rated current. De-tuning reactors (chokes) are one way of dealing with the problem. It's a bit of a specialist field and not for the faint hearted.

 

[robbietan]

We have a company doing a load eval for a big property that keeps getting charged from the POCO for low power factor (< .85). I want to have a cap sized for their lines, looks like its probably the best option at this point.

They need to just monitor the lines for a week or so in order to evaluate the usage and calculate any NON-linear loads. That part I can keep up with. But they sent me an email today saying that they may have to increase the time (and $ amt.) of the evaluation if they find any harmonic disturbances playing into the equation.

I am trying to think of some examples of "harmonic disturbance" to explain to the customer, but I'm having trouble figuring it out. Would it be disturbance caused by equipment that might cause feedback? Or maybe a transformer issue? Not sure.:dunce: Anyone want to take a stab at shining some light on their terminology? I know its something I should be familiar with, I just can't seem to place what exactly it could be...

as long as the facility being monitored is on "normal load", one week of harmonic monitoring is more than enough. Harmonics is a steady-state phenomena, there is no need for long term monitoring, unless the facility loads differ greatly every day

there are IEEE standards for voltage harmonics, as long as the limits for harmonics are not met then there is no problem. providing capacitors would need more time to "calculate" (not monitor) since the monitoring would already identify what harmonic number is prevalent. it would be a poor service to provide capacitors that would lead to a resonance condition (and more facility problems) if the harmonic numbers were ignored

if the monitoring is done right, one week (if the facility is operating "normally) is more than enough to monitor the amount of harmonics and the total load of the facility. then more time to design a capacitor system that would correct p.f. and not cause resonance

 

[weressl]

We have a company doing a load eval for a big property that keeps getting charged from the POCO for low power factor (< .85). I want to have a cap sized for their lines, looks like its probably the best option at this point.

They need to just monitor the lines for a week or so in order to evaluate the usage and calculate any NON-linear loads. That part I can keep up with. But they sent me an email today saying that they may have to increase the time (and $ amt.) of the evaluation if they find any harmonic disturbances playing into the equation.

I am trying to think of some examples of "harmonic disturbance" to explain to the customer, but I'm having trouble figuring it out. Would it be disturbance caused by equipment that might cause feedback? Or maybe a transformer issue? Not sure.:dunce: Anyone want to take a stab at shining some light on their terminology? I know its something I should be familiar with, I just can't seem to place what exactly it could be...

Sounds like bait-and-switch to me. Any reputable company that does business in the power quality field knows that harmonic content is an issue in sizing and selecting pf corrective capacitors and should be able to furinsh a firm quote accordingly. Not knowing the load profile or the type of business is usually the problem, so it's a longstanding power company practice to have a load reading for a month. It is somewhat arbitrary as it can't account for seasonal load changes, but they had to get a number within a reasonable time, so they settled on a 30-day period.