Power Factor With Vsd

[Rumrunner]

Almost all drive suppliers quote a power factor in the region of 0.98 when using their VSD.

When considering energy or cost savings, is the VSD as effective as power factor correction capacitors?

Does the answer to this depend on how the consumer is metered?

 

Would be interested in opinions on this.

[kens]

Hi, the VSD will be as effective at correction as capacitors at correcting for the motor it is connected to, if you are considering an entire facility then you would probably look at the P.F. at the metering point and correct there. Having said that the use of VSDs will reduce the amount of correction required at the supply point.

Regarding the "energy saving" of PFC, it is negligible, improving power factor will reduce the kVA not the kW, so depending on the lines charge structure you may make savings on demand charges or direct power factor penalties but you will not make savings on kWh charges. You should look at the fee structure that is applied to the site, some suppliers do not charge for poor power factor or peak demand.

[marke]

VFD manufacturers quote COS(thi), not true power factor. What they are quoting is displacement power factor and this is generally better than 0.95

What they do not tell you is that the distortion power factor can be considerably less than 0.95 due to the harmonic currents on the input of the VFD.

 

Supply losses are equally as bad with distortion power factor as with displacement power factor, but the problem is worse because distortion power factor results in voltage distortion which causes harmonic currents to flow in other equipment, causing additional losses in that equipment plus interference with the correct running or that equipment.

 

Best regards,

Mark.

[kens]

That depends on the VSD of course as to the levels of distortion caused. I am also unsure of the ability of revenue metering to record distortion power factor so the impact on the costs are not so apparent

[marke]

Hi Ken

 

Modern smart meters measure true KW and true KVA and derive PF from KW/KVA, so this is true PF which is made up of both displacement and distortion power factor.

Distortion pf, if it results in a high thdv, will increase the losses on any equipment connected to the same supply. This will show up as increased KWHr as measured by a smart meter and can affect other consumers who are "innocent parties".

Best regards

Mark

[Rumrunner]

The reason for the question is that many drive suppliers are using the power factor correction function of a VSD as reason to install drives into fixed speed applications and I felt this was misleading. I suppose, unless the drive is >6 pulse or other filtering is provided, there is no real quantifiable argument for the use of drives based on the benefits of power factor correction?

[kens]

Hi, if it is a fixed speed application the only reason I can think of for installing a VSD would be in a pumping or fan situation where the flow requirements are not yet known, otherwise all you are doing is adding inefficiency into the system, in this case you would be much better off installing capacitance either as a static unit or an automatic unit depending on the situation. (My opinion only)

Going back to the metering, the reason I mentioned it is that I am still unconvinced of the ability for some meters to measure the effect of distortion. The important thing when you are working on cost savings is to start with how the charge is generated, that is start with the meter and the charging methodology. In NZ there are more charging methods than I would care to count depending on which network you are connected to, but the common theme is usually that the charge will only apply if the P.F. at the meter is below 0.95. Then there will be some methodology of calculating the charge but it is generally based on a kVAr charge of some sort. Many of the meters that are used are only 2 channel, they record Wh and VArh and from that will calculate a VA figure, what I am not sure about is how the effect of distortion is recorded in the meter, if it results in higher VArh numbers then it can result in increased costs however I am not enough of a metering expert to know how each meter will handle it.

[marke]

Hello Rumrunner

 

Yes, the VFD will improve the displacement Power Factor and if you are charged significantly for displacement power factor, there will be a cost reduction, but if you are charged for true power factor, then there may in fact be an increase in cost.

You need to determine a) what is being paid for and how it is being measured.

 

If there is a simple power factor penalty where the power factor is derived from true KW / True KVA, then the penalty may increase. Most likely it may fall a little, but not completely.

 

The addition of a VFD WILL NOT reduce the KWHr charge unless there are mechanical inefficiencies that can be improved by slowing the machine down. If the motor runs at line frequency from the VFD rather than from the supply, then you will actually use more KWHr due to inefficiency in the VFD.

 

Kens : yes, it is totally dependent on the type of metering used and how it is configured, plus how the tariff is structured.

The trend towards is resulting in a trend towards true power factor. Many power bills I have seen use KW/KVA rather than KVAR as the power factor indicator and as such, the distortion of the current waveform begins to become significant on the power bill.

 

Best regards,

Mark.