Re: Harmonic filter for better power quality

Home Electrical Engineering Forum General Discussion Harmonic filter for better power quality Re: Harmonic filter for better power quality


Hi, Owen,

It is pretty amazing to watch ‘technologies’ emerge when the cost of electricity increases or something is done in the name of reduced green house gasses or….

The three companies you have referenced deal with passive components (inductors and/or PF capacitors). They assembly them in unique ways and are touted as ‘new’, ‘energy saving’, ‘harmonic filter’, and ‘transient protector’, to name a few points.

As near as I can tell the device that employs a series inductor and star-delta transformers provides load balancing and transient protection primarily. Since it is an inductor, it will cause lagging PF to get worse – that is draw more phase shifted current for worse PF. The amount of transient protection depends upon the actual inductive impedance it contains. [This will cause the I2R losses to increase as well. See below discussion.]

Benefits would be balanced voltage and current draw per phase for cooler AC motor operation. Is there a ‘big’ problem here? Might this offset the phase shift caused by the addition of the inductor? Might this cover the added I2R losses the inductors/transformers cause? Maybe – maybe not.

The inductor does act as a harmonic filter. It helps to decrease the harmonic current drawn from the utility, but not at the nonlinear device. The amount of the benfit depends upon the impedance rating of the inductor. Low impedance has little effect while larger impedances (3-7.5%) have good effects. No idea what is designed into this product.

The other two products are PF capacitors in a box. They either correct for poor displacement PF (lagging current) of add leading current to cause leading PF (when no lagging is required). There is no KW savings for the user. The KVA is reduced and the KVAR is normally reduced (if lagging loads are present). Neither action provides any benefit to the users utility bill. If a device could generate KW and replace the utility KW supply, then there would be a user benefit – think solar panels or wind turbines.

PF capacitors (alone) do not filter harmonic current. They will cause resonance if high levels of harmonic current are present. PF caps won’t help but could harm installations when harmonic producing loads are present (say above 15% of total loading). [Here’s an interesting thought: More and more of the loads in residential homes are energy saving devices = electronic loads = harmonic producing loads. What happens as more and more electronics are installed in homes and these capacitors are in place?]

The discussion regarding energy savings typically hinges on I2R (I squared R) losses. This is a real loss within the electrical network and does waste energy. BUT at the user level it is usually very small (under 2% potetnial). However to the utility the sum of all the savings can be large. This is true because the energy to provide the I2R losses at the customer delivery point must be generated at the generation facility and ‘pushed’ through the whole electrical network (HV, MV and LV + transformers, etc). This can be 3-4 times the actual I2R losses for the sum of the users savings at a PCC with the utility.

Today there is no economic benefit for users to limit I2R losses. The utility charges for the KW delivered and that makes up more than 98% of the KW needed for operating the facility. If the utilities got serious and offered an incentive to save the 2% I2R losses, then there would be cash back for users, albeit small.

I made a point above that more of the loads in homes are based upon electronics because they save real energy – KW. This savings far exceeds any benefits PF caps will provide for I2R losses. Utilities sometimes offer rebates for installing energy savings devices because they can reduce the quantity of future generations facilities. Rebates for heat pumps, high efficiency furnaces, the new washing machines (VFD driven), to name a few exist. Why? Because the really save energy.

Just to make another point, when utility bill savings are proclaimed, it is usually based upon the utility bill. How much of the utility bill is affected by the weather – not what the user does or doesn’t do? Is the weather for any period of this year equal to same period as last year (the usual basis of comparison)? Not very often. The variables also are was the heat or air conditioning turned on or off – for the same periods of time as last year? My point is that such compariosns are hard to do and typically are subject to many variables – which by the way the equipment suppliers from above use to their advantage when charges increase.