Transcript
Introduction to the Voltage Unbalance Compliance Report
Good afternoon, everyone, and welcome to today’s white paper webinar. Today we’re talking about the Compliance Report in PQ Canvass. This is the Voltage Unbalance Compliance Report, and this gives you a pass/fail and some more information about your three-phase voltage imbalance.
Voltage imbalance is a measure of how different your three voltages are from each other in a three-phase system. In theory, there are multiple ways of measuring voltage imbalance. The most common is the one that is recommended in ANSI C84.1, which is what most utilities use, and that’s using the formula shown here, where we take the max deviation from the average divided by the average voltage.
How the Voltage Unbalance Formula Works
The way this works is that at any instant in time, you take the three voltage readings from the three phases and average them together to get an average voltage, and then you find which phase is most different from that average. Here in this example, it’s AC 112 volts. So you subtract that from the average, which gives you 6.3 volts on the top of the fraction, and the bottom of the fraction is the average.
You divide times 100% and that gives you a percentage. So in this case, we’re at 5.3% imbalance because the average is 118.3 volts, and the max deviation from that average is 6.3 volts.
Now this is meant to be applied to steady state RMS values. Voltage imbalance is a steady state power quality phenomenon, so typically this would be done on one-minute averages or even five or ten-minute averages because this isn’t something that you are concerned with when it comes to sags or swells or disturbances. This is a steady state type of issue.
Symmetrical Components Method
Now, this formula works well when you’re reading line-to-line voltages, but if you’re looking at line-neutral voltages, a better way of measuring this, and is available in newer PMR quarters, is the one that’s based on symmetrical components. If you’re familiar with positive, negative and zero sequence components, these are factors that are ways of breaking down a three-phase system into its balanced, unbalanced and basically DC components. We’re using the nomenclature that’s common with symmetrical components.
This is something that is more often used in fault analysis and system protection rather than power quality, but from a technical standpoint, the ratio of negative sequence voltage to positive sequence voltage is a better way of measuring voltage imbalance. It’s not commonly done this way because these components aren’t always available on a power quality recorder or used very commonly in power quality, but this is available in the Volt and the Seeker.
With the symmetrical components, the negative sequence voltage represents the counterrotation voltage, balance portion of the three-phase set, and the positive sequence is the positive value. So if you think about motor torque, for example, an induction motor spinning, the negative sequence voltage causes counter-torque on that shaft. We have a white paper to learn more about that topic in particular.
Voltage Unbalance in PQ Canvass
Here we’re using the ANSI C84 max deviation from the average method, and this report works on the voltage averages that have been recorded by the recorder in the voltage strip chart. In PQ Canvass, for example, if we look at a file, you have the interval report. It records the recording voltage, RMS, min/max and average. If we take a look at this example, here is the min/max and average voltage for all three phases. We’re running 48490 volts, roughly, but the averages are what is used for the voltage imbalance.
You can actually graph that directly in PQ Canvass. We can look at the voltage imbalance graph, and here we can see that 3% is the limit, as recommended by ANSI C84. We can see that we’re over that sometime, but we can get a more formal analysis of that using the analytics report.
Running the Compliance Report
Here the paper talks about how to do that in PQ Canvass, so it uses an example of running this report. We can show this to you live, and here we have David, author of the paper, that can guide you through how to do that.
First, in the header report, there’s an analytics section to run the compliance reports. You select that. You will see that there are selections for the voltage unbalance. You choose that one by clicking it, hit Configure for what you can change, your averaging interval if we have it, defaults to five minutes on this one. And it’s computing that from the interval data you recorded.
So if you’re at one-minute intervals in your recording, it’ll compute five-minute intervals for you. Then when you run it, it takes a moment to run, but then you can list the report here, you can click on, and it gives you a nice PDF format that you can save off, store, share with those who need to see it.
Report Sections and Details
It gives you sections. First, it’s a summary. It describes the recording, what kind of instrument was used, what time period it covered. Then it gives you the quick overview. What was your max unbalance, your average unbalance, minimum balance for voltage and current, ratios, so you can quickly see at one glance, was I in compliance, yes or no, and for how long if I was out of compliance? What percent of the time did we exceed the environment?
Then we start into more detail. We give you graphs. This is the same one Chris showed in PQ Canvass, but we also have the unbalanced one. Again, he showed you that in PQ Canvass, but here it has a line showing where the compliance is. Here we have the three percent. Where were you over it in the recording?
Then also we show other graphs of interest like the ratios, current to voltage unbalance ratio. The current to voltage unbalance ratio can be useful if you’re trying to determine the load’s effect on voltage unbalance, and this can be useful if it’s a very high ratio, that may indicate that you have a real unbalanced load.
Daily Profile Graphs
Next is the daily profile of graphs, again, showing the threshold where you exceeded so you can see, well, what times of day do I exceed this? Is it common at a particular time? Again, more information to help you figure out when and where your problem is.
This can be very helpful in determining daily patterns. Like here, it seems to be roughly the same time of day each day that the unbalance creeps above the limit. So that can help determine a root cause or determine if it’s related to load patterns or your switching operations or what’s going on.
Voltage Exceptions and Observations
Lastly in the report, we give you all the voltage exceptions that the recorder sent in with the recording, where this recorded the times and durations of your voltage unbalance events. This is the most detailed, “Okay, it happened this time for this long.” And last, we just give you some observations about what we saw in the report.
Again, these can be saved and these can be retrieved later. If you run a number of these reports, you don’t have to rerun them each time. You can go and see the saved recordings.
Other Compliance Reports in PQ Canvass
So this is a useful compliance report. This gives you a more formal output based on voltage unbalance. We have other similar reports here in PQ Canvass for not just voltage unbalance, but also for voltage sags per 1668, and voltage regulation per NCC-84, and coming up soon, our flicker, or 1453 and the IEEE 519 report for harmonics.
This section here, it’s labeled Analytics, it’ll soon be Compliance, will be where you see the more formal reports that give you a pass/fail and some information about that pass or failure, how close you were to the edge, any patterns it sees, to help you stay within IEEE guidelines. And again, these are available to anyone here in PQ Canvass. Thanks for attending everyone. Everyone have a great afternoon.
