Transcript
Introduction to Sizing Backup Power Sources
Good afternoon, everyone, and welcome to today’s white paper webinar. Today, we’re talking about sizing backup power sources. If you are involved in that, if you have a customer that needs a backup source or you have a new customer or a new location that doesn’t already have something you can measure, we’re going to talk about how you would size that.
It’s really a useful thing to get right because you don’t want to either over or under design your backup source. And especially if you already have something in place that already exists that you can make measurements on, that gives you a great opportunity to actually measure the actual consumption by those loads. But if you don’t, you need to estimate that from the nameplate ratings of the devices.
Watts vs. Volt Amps and Power Factor
Sometimes they’re specified in watts rather than volt amps. But really, volt amps is what burdens the generator or the utility source if this is coming from a utility, because volt amps is voltage times current, and that’s not always the same as the amount of power consumed by the load. That difference is the power factor, that inefficiency.
The power factor is the ratio of watts to volt amps, and that’s a measure of the efficiency of how that load draws power. Loads may be specified in terms of how much wattage they draw, and that’s what you’ll have sometimes if you only have billing information from this location. But the transformer or backup source is usually specified in terms of amps or volt amps, regardless of the power factor.
If you only know the watts, you can estimate the volt amps from a load by using a formula that put in the power factor, and we show here. If you don’t know the power factor, most loads these days are typically around like .8, .9, less as an inductive motor, in which case they really should have the volt amps on the nameplate of the motor itself.
Non-Linear Loads and Recording Requirements
For non-linear loads, you would typically add the nameplate ratings of all the different loads. They typically add together in terms of volt amps. But again, if you have something you can measure, if this is an existing load that you’re providing a backup source for, it’s much better to make actual measurements. And here we’ll show how you can do this with PQ Canvass.
Typically, you’d want to record for at least a week so that you get weekend and week and day and night activity from this location. And if you have some other information about its load patterns, you may need a longer recording. Or if you know that it’s exactly the same on a day-to-day basis, even through the weekends, you could get by with one day. But a one-week minimum recording is a very common requirement.
Three-Phase Measurement Example
Here we have a three-phase example, and with a three-phase load, you want to, of course, measure all three phases of voltage and current. Doing this with a PQ recorder allows you to capture RMS voltage and current, but also real power and apparent power, so you can look at the actual demand. And then you can look at waveforms. You may also be interested in crest factor or the harmonic distortion.
Analyzing Data in PQ Canvass
I’m going to jump over to PQ Canvass. Rather than showing you screenshots from the paper, I’m just going to show you PQ Canvass directly. Here I have the same recording, same location here in PQ Canvass. Again, this is a cloud-based PQ system. And as the paper describes, there are a variety of different graphs and reports to help you with any sort of PQ analysis. But here we’re going to focus on backup power generation sourcing.
RMS Current and Apparent Power
For that, you might start with just RMS current and what does this load look like. You can see that here we have a two-week recording. We have five days of the week, and then we have a lower current during the weekend, and then the five days of the week again for the next week, followed by a one day. So this is almost two weeks.
But the current is one measure, but a more direct measure is volt amps or apparent power. So I’m going to jump right to apparent power. Here we have the apparent power for all three phases, and we’re roughly in the twenty KVA range peak. Now, these are one-cycle peaks in red. You’re more interested in the steady state thermal demand. That’s a lot different here. That’s closer to ten KVA, approaching twelve KVA. We can turn on this table where we see the actual values.
Total Power Report
This graph is showing min, max, and average, one-cycle minimums, one-cycle maximums. We can also use the total power report if we go to Interval, Total Power. Here, PQ Canvass is computing the three-phase total real power, reactive power, and apparent power, and also the power factor for this. This is useful if you just need three-phase total and you don’t need individual phase powers.
Now this is just the average. We’re not looking at max, min. And we get a better representation. We can see the absolute peak on an average basis. Here we have seventeen KVA. And then we can also look at reactive power and real power, and that relates directly to power factor. The power factor is actually pretty good at this location. But if we were to size generator for this, we might round up to twenty KVA on a three-phase basis since we’re hitting peaks of seventeen here in a one-week time period.
Daily Profile
We can also see what the profile is. We look at the daily profile for apparent power. This tells us what part of the day this power is being generated or being consumed. The daily profile is an interesting graph. It’s always twenty-four hours long from midnight to midnight, and it’s an average of each day the recorder is in place.
So this is basically a two-week average giving you the twenty-four-hour profile for apparent power for this location. We can see that during the night we’re at very low power, and then during the day we’re at much higher power. So it can get a feel for what fraction of the day this power is on. This same information is also present in the strip charts, but it’s a bit harder to see because the strip chart spans the entire two-week period. The daily profile gives that for you.
Histograms and Waveforms
We can look at the histograms. That can sometimes be useful. Here I’ll look at apparent power histogram. This gives us the number of cycles of the power. This isn’t quite as useful as the daily profile for volt amps or the strip chart itself.
Then if need to know more details, we want to know what the waveforms look like. We can look at waveform captures. We can also look at the total harmonic distortion to see what kind of distortion this puts on the load itself or on the generator. These waveforms would typically be triggered by voltage sags or other disturbances unless you have periodic capture set up.
Harmonic Distortion
Here we can look at the waveforms. We can look at harmonic distortion, both for the voltage and for the current. We can look at the tabular ratings. So we can see we’ve got about 10% THD for this waveform, and we can get a better feel for that if we look at the actual THD strip charts where we’re looking at voltage and current THD.
We can see current THD is in the 10 to 20, almost 25% range. So there’s a fair amount of harmonic distortion from this load, and that might cause you to upsize the backup source a bit so it can handle the harmonic currents that are present.
Crest Factor Considerations
One aspect of generator sizing that we didn’t mention here in the paper that you might want to be aware of is the crest factor, the ratio of the peak to RMS current. A high crest factor can be a problem for inverter-based generation, for example, UPS, battery-backed UPS, or other type of generation. Not usually as much of a problem for rotating generation.
The crest factor is also something that you might want to check. You can do that here in the waveform. If we look at the meter readings, crest factor is shown for current and for voltage. And you generally want to stay under three for current. Anything above three is often outside the ratings of a typical UPS.
Summary and Net Power Report
Going back to the paper, we show examples of these graphs. It’s very important to at least have a one-week recording if it’s an existing load, so that you can see the actual amount of current that this load is consuming.
The net power report can also help with this. This gives you the three-phase total power and the maximum power. So you’re in the peak power section. This gives you the time during the recording that the power is at its maximum and the actual level. So if you want to jump right to the peak level, you can do that. It also shows the total amount of energy consumed in watt hours or kilowatt hours or megawatt hours. That may not be as important for generator sizing as the graphs themselves, but that peak power can be useful also.
Contact Information
If anyone has any questions about the paper, give us a call anytime at 1-800-296-4120 or send an email to support@powermonitors.com. Thanks for attending, and everyone have a great afternoon.