Power Factors and Harmonics Levels

where is the third harmonic? A square wave is the sum of all odd order harmonis.

when I see a series like that 100...??...1.2.....12

I suspect HO3 is 12?

you can actually chart this and see what your wave is, or use an oscilliscope.

http://www.allaboutcircuits.com/worksheets/mixed.html

I would be careful with the application of your Power Factor correcting capacitors.

The 5th and 7th harmonics suggest you have variable speed drives at your facility. If you add power factor correcting capacitors at the wrong point you may discover you have unexpected drive failures. Most OEMs specify no PF caps on the ac supplies.

Another phenomenon you may find is uncontrolled ringing of the system. The caps form a tuned circuit. Unless there is sufficient series resistance the system will ring at each commutation notch.

Another "watch out" is the fact that the capacitor bank will actually enhance (increase) the harmnonics and "broadcast" them upstream to the utility substation, where they will be carried out to all other users on the power feed from that substation.

This happened to me when I was plant engineer at a foundry. Our HV induction melt ovens had a problem in their circuitry. The harmonics actually caused equipment failures at a "neighboring" plant that was over a mile away. The power company checked their site and found the harmonics, then traced it back to the substation and ultimately back to us. Fortunately, we were able to fix our problem without having to pay for the damages at the victimized plant.

Have a certified power professional determine what is causing your harmonics and fix that first, or find out a way to isolate it, before you add the Pf capacitors.

thank u all

i have seen all the harmonics except the 5th and the 7th harmonics they are below 1% and beside this the THD% OF THE VOLTAGE IS AROUND 1.3 and THD of the current is 7.4 .i want to know how much the capacitors can magnifies the existing harmonic level and what is the acceptable levels .if u can give me something which can i use for the future too would be better.i know the IEEE HAS A STANDARD "IEEE519 RECOMMENDED PRACTICES AND REQUIRMENTS FOR HARMONIC CONTROL IN ELECTRICAL POWER SYSTEMS" but i couldnt find it on the internet.

with best regards

If you go to the IEEE site you can purchase the standards.

Nebiyou,

As you can see from the responses, this is not a simple thing to do. In general, adding some capacitance will help but too much will cause resonance problems. Nobody can tell you how much is acceptable because we need to know the details of your plant impedance such as the load distribution and the transformer impedance.

This is typically done by an engineer properly trained for this type of work.

One thing that you must be aware of is that adding a small capacitor bank will keep the resonance frequencies high enough to avoid problems but it might not really help improve your PF. Another problem is that the harmonics currents can damage your bank by exceeding its capacity. It all depends on where the bank is placed. As you see, it is not simple. Detuning the bank to the 4,5th harmonics helps but is no guaranty if you don't know the rest of the story.

The harmonics voltage levels that you provide are well within the IEEE recommendations. I would not worry too much about it unless somebody is complaining. I regularly see plants operating with 2,5% VTHD without any problem. Adding the capacitor is much more likely to cause problems.

I see that the PF is low. This is probably why you wanted the capacitors to begin with. I would look at the source of the low PF. It is probably from a motor. You might be able to add a small capacitor bank in parallel with it to correct the PF locally at the motor (not connected to a drive). Avoid going above a PF of 0.9 as you risk resonances on light loading. Look at the ABB capacitors site for application notes.

You might also consider installing an active harmonics filter or a synchronous capacitor if your plant can afford it or use the motor.

Finally, hiring a professional might not be a bad investment as you will get a solution that works. Make sure that he has real experience and call his customers. You would be surprised to see how many experienced experts have caused more problems than they solved...

Buying the IEEE standard is also a good investment as it contains good examples.

That .73 pf suggests he may have a lot of lightly load machines, or a DC drive running at 75% rated speed.

It would be interesting what his plant produces.

"plastic factory"

Chemical Manufacturer?, extrusion?

Types of machine being run?

Your typical 20 year old screw extruder is a DC drive, so PF is approx same as running speed / max speed. (I know, thats really loose math.)

thank u all for ur concernes and fast response

let me give u further detail

the machines are all topfine plastic mold machines (around seven machines)and i am planning to put the capacitor banks at the main distribution board (i.e which supply power to all machines) and this distribution board gets power from 315kva transformer.

the main supply is from 315kva transformer with a short circuit impedance 0f 4.17%

THAT MEANS THE RESONANCE HARMNICS LIES AT n=8.69

and since the harmonic level around the n=8.69 is almost 0 we will not worry about the resonance.

what i am woring is not about the the magnification of current and voltage due to resonance but when we add a capacitor bank to the circuit. harmonic levels of almost all orders increase and i dont know how much it could increase and whether it is acceptable or not unless i instoled the capacitor bank and analyse it with a harmonic analyzer.which is not possible

if we are not sure about these we are planning to use a 7% detuned power factor corrector.WHICH IS MORE EXPENSIVE.

I CAN GET CONFIRMATION ABOUT THIS IDEA FROM OUR AGENT .THEY YES OR NO BUT I COULDNT DISSCUSS IT WITH THEM IN DETAIL THAT IS WHY I BROUGHT THIS IDEA TO THIS FORUM

During transient switching of equipment, the current pulse contains "all" the frequencies. Same thing happens during a power failure and lightening strikes. This is where the resonance frequency can be the most damaging, especially after a power loss when most of the loads (dampening) are still off. Make sure that the capacitor banks are automatically disconnected.

A resonance at 8.69 is closed to the 9th that might be small but is never zero. Make sure it won't be a problem. Don't forget that some equipment do produce inter-harmonics. It might come from your neighbor's plant.

De-tuned capacitor banks usually give a more stable operation as the resonance frequency is less dependant on what else is operating in the plant and the neighborhood.

The resonance problems are usually not visible during steady state operation. They show up during transients. Make sure that you survey these conditions.

Good luck.