Transcript
Introduction to IEEE 1668
Hello, everyone. Welcome to today’s white paper webinar. Today we’re gonna talk about the IEEE 1668 standard. This is a not well-known standard, especially among utilities, but it’s a really important standard to be familiar with, and I will talk about why that is here in this white paper.
What Is IEEE 1668?
So let’s start with what is 1668? This is a standard that has some couple different parts to it. One is that it defines what voltage sags are, how you quantify sags. It gives methods for measuring the duration and magnitude of a sag in terms of the retained voltage and the duration of that low voltage.
It also defines different types of sags: a Type I, Type II, Type III. These are single-phase, line-to-ground sags or line-to-line sags or three-phase sags. It gives definitions for all of those, and some of that isn’t really present in any other standard.
It also importantly gives backgrounds on why voltage sags are kind of a fact of life, that they’re impossibly rid of. It’s not practical for a utility to eliminate all voltage sags. These are notable due to faults, insulation breakdown, equipment failures, insulator failures. And a good portion of the standard is devoted to a primer on why sags happen, why utilities can’t just eliminate all sags, and what the typical sag characteristics are. This is really meant to be educational for the readers of 1668.
Sag Immunity Testing
The rest of the standard is devoted towards how to test sag immunity, both for equipment and for an entire facility. It goes into a significant amount of detail on the type of sag testing that should be done, both single-phase and three-phase sags, and importantly gives immunity levels to these sags.
The Effect of Sags on Equipment
It talks in detail about the effect of sags on equipment. It talks about even where the sag starts in terms of the point on the waveform. For example, here in figure three, we have a sag that starts at the peak of the waveform. That’s where a lot of them start. Where insulation failure happens, that’s where the peak voltage is highest.
But as the standard points out, that’s not where they always start. Surprisingly, they can start pretty much anywhere in the waveform, and it does make a difference. Even if the duration and magnitude’s the same, a sag that starts near zero crossing will result in different equipment behavior in some cases, because the equipment’s ride through capacitors have less energy or less charge stored in them at that point. So the equipment is starting off with less stored energy to ride through a sag than it is if the sag happens at the peak of the waveform.
It gives some information about different characteristics of equipment based on where on the point on the waveform these sags happen. And again, it defines a sag in terms of voltage falling below 90% retained voltage for a certain amount of time, and that gives you a duration and magnitude, and these sags are quantified by that duration and magnitude.
Measuring Sags in ProVision
You can measure this in, for example, ProVision by using the RMS capture report. As we show here in figure four, a sag will trigger a waveform capture in most cases. Normally in the PC software or the cloud-based software, you’d start with a sine wave graph like this, where you see no shape, no change in the waveform shape. This is not a oscillatory transient or other kind of disturbance because it’s a six-hertz sine wave all the way through.
Once you see that, the next step is to go to the RMS capture graph in ProVision or in PQ CanvaSs. Here the software is computing the RMS value with a one-cycle window and sliding that window one sample at a time, so you have a very fine time scale resolution RMS graph. This is the best way to measure the duration of the voltage sag. It’s also much easier to see the amplitude. You can use the ProVision point vertical line to mark the start and end of the sag and also look at the retained voltage to determine what this sag level is.
ProVision Demonstration
I can show this in ProVision. We jump to ProVision. Here is a waveform. This is a little bit uglier looking sag. This is the RMS capture graph. This is the waveform capture. When you see a waveform capture like this, it’s hard to see what’s happening in terms of sags because when you’re looking at sine waves, you’re looking at the positive peak and the negative peak.
Here we have an 800-volt span for each of these plots, and it’s hard to see a 10 or 20-volt RMS change when you’re looking at 800 volts on the scale. But if you look closely, you can see that the peak of the waveform is a little bit lower. But again, the best way to use this type of graph is to first rule out there’s no oscillatory transients, there’s no waveform shape changes. These are pure sine waves, so this is an RMS event.
And we move to the RMS capture graph of the same event. Now it’s a lot clearer what’s happening because when looking at RMS values, this auto-scaling works in our favor. That’s going from the lowest RMS value to the maximum RMS value. You see there’s a quick change in voltage as the current ramps up, and then the voltage recovers. So this is a much better way to analyze RMS events like sags.
We can turn on the point table, mark the beginning of the sag. As the white paper points out, the start of the sag is technically where it goes to 90% of the nominal voltage. That’d be roughly in here. And you can note the time here at 41 milliseconds. And then also put the marker on the endpoint where the voltage is back up to 90%. And that gives you the duration.
Sag Withstand Standards and the SEMI F47 Curve
The standard gives certain voltage and magnitude sag characteristics that equipment should be able to withstand. These are extensions of the SEMI F47 curve. They’re not exactly the same, but very similar. These are defined in terms of duration and magnitude, and equipment that meets these standards are considered well-designed.
How Utilities Can Use IEEE 1668
Really the way utilities would use the standard is to ask customers if they have equipment that’s malfunctioning due to sags, is that equipment designed to meet the standards of IEEE 1668 and has it been tested at that standard? This is a really useful way for utilities to be able to push back a little bit on sag complaints to make sure that the customer’s equipment isn’t overly sensitive.
Customer equipment in the entire facility should be able to withstand sags that were within the limits of IEEE 1668. That’s the whole purpose of 1668, to really provide that compatibility point where utilities can mitigate sags to where it’s within the boundaries of 1668. And customer equipment also has immunity to those level of sags so that we have compatibility when they’re connected together.
So if utilities are doing their part to meet the sag limits in 1668, then customers should also ensure their equipment is also capable of operating in that environment. This gives end users of power, utility customers, ways to push this back onto the equipment manufacturer, asking, hey, have you been tested to 1668? Are you compatible with those sort of sags? Is there any extra conditioning equipment that needs to be included to ensure 1668 compatibility?
Benefits for Both Sides of the Meter
This is a really useful standard for really both sides of the meter. For utilities, it gives some guidelines to themselves for what sags they should be able to mitigate or not. That mitigation usually involves maybe stiffening the system, a lower impedance so that the fault currents don’t produce as much sag, or perhaps some sectioning, more reclosers to section off different loads if there are faults on a certain path.
But also for customers, if they find their equipment is frequently misoperating due to what would normally be acceptable sags, then their equipment may be overly sensitive and they should ensure that it’s been tested to 1668 or at least meets the requirements of 1668 so that they know that the equipment they purchase and install is going to be compatible in a normal utility environment.
So even though this is not a utility standard per se, it’s a very useful standard for utilities to be aware of because this gives them some sort of meat to teach their customers why sags are kind of a fact of life. It’s impossible to completely get rid of them, but also some expectations for how sensitive equipment should be. And to learn more about 1668, of course, you can purchase the standard from IEEE.
Contact Information
If anyone has any questions about IEEE 1668, give us a call anytime at 1-800-296-4120 or send an email to support at powermonitors.com. Everyone, thanks for attending. Have a great day.