Measuring kW

[JeffWalsh]

Using different devices to measure kW, we are getting different results. The scenario involves measurement using different power quality analyzers and utility metering equipment, and power factor correction devices which incorporate inductors in series with capacitors (allowing the device to be placed near inductive loads). The device connects in parallel via a disconnect.

When connected, we consistently see the following per unit: a drop in amperage of 14-16 amps; an increase in voltage by .1 to .4%; and a reduction in reactive current by 22 kVAR. We also see a reduction in kW by 1.7 to 2.2 but only when measured with certain equipment. An Amprobe DM-II Pro for example will show the drop in kW. Also, sometimes the utility?s metering equipment shows kW demand in real time, and will show the drop in kW. In switching units on and off, we don?t see any significant change in kW using certain other devices including a Fluke 435 and Amprobe DM-III, as well as analyzers from Dranetz-BMI and AEMC (each of which measure harmonics).

Without sparking a debate about power factor correction, and real and apparent power, what could be the reason for the differences in measuring kW?

 

[gar]

080918-1224 EST

JeffWalsh:

You apparently make power conditioning equipment.

The scenario involves measurement using different power quality analyzers and utility metering equipment, and power factor correction devices which incorporate inductors in series with capacitors (allowing the device to be placed near inductive loads). The device connects in parallel via a disconnect.

Can you provide a block diagram of the load(s), PF correction components, your equipment, the power source, and where you are connecting the wattmeters. With what kind of loads are you working. Some vector drives produce so much high frequency junk that many types of instruments my not function properly.

At what voltage and current levels are you operating? I assume it is three phase so is it delta or Y?

I can not help you much relative different instruments. However, I have an electronic wattmeter (a Hall device type) made by Load Controls, Inc. Years ago when I tried to use this on a vector drive, directly into the motor, it was totally useless. However, it worked very well on a standard three phase induction motor. My use was to get nut torque information from a pinion preload adjust machine that did not have a nut torque transducer.

You may have problems with harmonics causing the differences between the different instruments.

.

 

[Jraef]

Measurement tolerances. To measure kW, you need to measure current and voltage, plus you need a way to determine power factor. All of those involve a certain amount of error in measurement and/or calculation rounding, so any given device is going to show slightly different results depending on where their error tolerances and algorithms take them. In fact, the people who foist scams on PF caps as energy savers will find test instruments who's tolerances end up favoring their claim as a way to show some legitimacy. But let's face it, the ONLY measurement tool that means anything real is the one used by the utility. If it doesn't show a drop in kW, they will not lower your bill. I can guarantee that.

And please people, I know that there are power factor penalties in a lot of areas, we are not talking about that. He asked specifically about kW.

 

[JeffWalsh]

Our device is the PFC (also serves as TVSS. Typically: 480 Volts, 3-phase, delta, minimum 75 amps per unit, device connects at ot near near motor control a center or individual motor disconnect, watt readings are taken upstream near the utility meter or transformer.

 

[JeffWalsh]

the ONLY measurement tool that means anything real is the one used by the utility. If it doesn't show a drop in kW, they will not lower your bill.

I agree. The DM-II and the utility's meters are the ones showing the drop, just not the higher-end pq analyzers.

 

[gar]

080918-1423 EST

JeffWalsh:

The actual power consumed over some time period is the average value of v*i. If an instrument does a good job of calculating this instantaneous value and its average, then you get a correct result. The averaging time and whether it is synchronized with the voltage zero-crossings will depend upon the noise to be filtered out.

I like the Hall device as the multiplier because it directly multiplies the two signals. However, I have not done any error analysis of this type of wattmeter.

.

 

[robbietan]

if it is any help, always use the kW meter brands used by the utility
lol

 

[jghrist]

The difference may be in the way the different meters handle harmonic voltages and currents. The PQ meters may be including harmonic power (Vh?Ih?cos?h) while the other meters are only measuring fundamental (60 Hz) power.

 

[JeffWalsh]

That makes sense. And the device could be a sinking harmonics from other places. Does anyone suppose the the PQ meter could be set up to read more like the utility's meter, not including the harmonics?

 

[steve66]

Are you placing all your power meters in the circuit at the same point?

Steve

 

[JeffWalsh]

Are you placing all your power meters in the circuit at the same point?

Different meters were used at various locations and times, but in more than one case, the Fluke 435 and the Amprobe DM-II were placed in the same spot, under the same conditions with different results.

We have several dozen installs where the DM-II shows the kW drop. In the few cases where the utility's meter had a kW readout, those results were very similar to the DM-II. We also have cases where the customer's load is unaffected by seasonal changes, and the kWh data on the power bills indicates a reduction in consumption.

 

[beanland]

Complex Effects

Complex Effects

The addition of the PFC will interact with the entire electric system. The increase in voltage may effect the losses in other devices. The reduced current may reduce losses in an upstream step down transformer. The change in harmonic current flow may have effects. And, different meters use different methods for determining power; each leading to different results. A more complete system model (data about the rest of the system) is needed to really figure out why the differences.

 

[ATSman]

Sort of off the subject.

Sort of off the subject.

I went to a local energy conservation fair last Saturday, and there was a guy there selling a power conditioning box for homeowners. It was basically a box of capacitors that maximizes power factor, compensating for the low power factor of electric motors. He claimed that this would reduce utility bills. I said he was wrong, because the home owner isn't charged by the power company for low power factor. Industry is, because the peak currents are large and the line resistance loss in the grid is significant. But the homeowner only pays for the part of the current that's in phase with the voltage. He insisted, claiming that he timed the rpm of his meter rotor and found that the meter turns slower when his bix is connected. I said maybe something in the house switched off when his box was attached. What do you guys think? Scam or not?

 

[zbang]

I'm not sure I'd call it a "scam", but I would call the box "useless" to the homeowner.