Transcript
Introduction
Good afternoon, and welcome to today’s white paper webinar. Today, I’m here with Landon, and we’re talking about how to recognize a cap bank switch operation from a waveform capture. This is a useful technique. It doesn’t work every time, but it’s a very good indicator.
So if you see an oscillatory transient in a file, it helps you answer the question of, was that a regulator tap change? Was it a cap bank operation? I’ll let Landon walk through the paper and how you use this technique, and then I’ll show you a couple of examples, one where it is and one where it’s not a regulator or cap bank adjustment.
Three Primary Factors for Identifying a Cap Bank Switch
There’s three primary factors that we look for when we’re looking at a waveform like this, and we’re looking to see whether we have a cap bank switch. The three parameters are an oscillatory transient, a step change in voltage, and an insignificant change in current.
Factor 1: Oscillatory Transient
The first factor, an oscillatory transient, comes from the interaction between the inductance on the line and the capacitance in the cap bank switch. When we have an LC circuit, when it’s excited, when there’s energy sent into the system, it sends energy back and forth between the inductance on the line and the capacitance on the switch. This yields an oscillatory transient. This is usually in the range of 500 hertz to 5 kilohertz.
This is determined by the equation that the frequency is 1 divided by 2π times the square root of the inductance times the capacitance. Inductance is just the line inductance on the transmission line, and C is just the capacitance of the cap bank plus any parasitic capacitances in the system.
For example, if we plug in a standard value of 1 millihenry for line inductance and 100 microfarads for capacitance, plug it into this formula, we get around 500 hertz. These transients are usually in this range, 500 hertz to up to about 5 kilohertz.
Factor 2: Step Change in Voltage
The second factor that we look for is a step change in voltage. The reason why capacitor banks are used is for power factor correction. They’re used to tighten the angle between the current phasor and the voltage phasor. This allows more useful energy to flow to the customer load and less energy to be dissipated as heat.
What that means is when we do this correction, when we apply extra capacitance on the line to remove inductive loads, the voltage tends to change. The source is the same, so the current is generally the same, but the impedance changes phase angle. And so, the voltage at the load changes very suddenly from one value up to another value or down.
Factor 3: Insignificant Change in Current
The third factor that we look for is that the RMS current doesn’t change that much, because if the RMS current changes significantly, then that indicates that it’s a customer load turning on or off. It’s not necessarily a capacitor bank switch. It’s usually only about 5 to 10% of its original value. If it’s any more than that, it usually has to do with customer loads turning on or off, exciting the system rather than something on the power line exciting the system.
If the customer current is the cause for the disturbance, then the current observed will be significantly more than 5 to 10%. If the customer load is generally resistive, meaning it looks pretty linear, then the current will rise with rising voltage. When the voltage goes up with a step change, then the current will go up proportionally, based on Ohm’s law.
On the other hand, if it’s like an AC to DC converter, where it’s a constant power device, then as the voltage rises, the current will generally fall inversely to the voltage. So, the current could step up a little bit or step down a little bit based on the step change in voltage.
Limitations of This Technique
This triad of clues is useful for determining whether you have a capacitor bank switch, but it’s not necessarily definitive. It’s possible that some of these systems are heavily damped to where you don’t see the oscillatory transient. It’s possible that you could have multiple converging factors at the same time. It’s possible a customer load could turn off a light load, and that could impulse the system. So, it isn’t a foolproof system, but it is a useful heuristic for being able to look at a waveform and seeing what we can see in it.
Example: RMS Voltage and Current Waveform
In this example, this is an RMS voltage and current waveform over two hours. Start here at 4 o’clock, and we go up to 6 o’clock. What we’re looking at is the voltage in channel 3, so we have min/max average right here on channel 3, and then we have min/max average here.
This is fairly low, around 119 volts or so for the average, and then in a short amount of time, we step up to 127 volts. We go from 119 to 127 in a very short amount of time. This represents a step change in voltage. This satisfies our second clue.
This is the same plot, this time on a much shorter time scale. This is over 20 milliseconds, so that’s 10 cycles. We’re looking at channel 3 in blue hovering here around 121 volts. After this event, we end at a significantly higher voltage here, at around 126 volts. We show a step change in channel 3 voltage from 121 volts to 126 volts over the course of 20 milliseconds.
Current Remains Relatively Constant
The top plot is voltage, bottom plot is current. If we look at the corresponding channel 3 current, we’re seeing it hover around 50 amps at the start. Then after the event, it jumps up to 70 and then jumps back down, and then stays consistent around 50 amps. This satisfies our condition 3, that the current stays relatively constant throughout the event.
Waveform Capture Shows Oscillatory Transient
When we look at the waveform data, this is the waveform capture of this same event. We see here in channel 3, we have a sharp spike, and we have a couple of oscillatory rings. We see a spike, and then a spike, and then a positive spike, and then it kind of dies out after about a couple of milliseconds here.
This is a zoomed-in capture of this. This is just showing two cycles; the previous view was showing around ten cycles. You can see negative spike, positive spike, negative spike, positive spike, and then it kind of dies out. The oscillatory transient is over by about this point here in the waveform.
This satisfies our first criterion, the oscillatory transient. The waveform capture over nine cycles shows an oscillatory transient occurring 35 milliseconds in and lasting for 3 milliseconds. It starts at millisecond 35 in this capture, and then it lasts for about three milliseconds here.
Conclusion: Highly Indicative of a Capacitor Bank Switch
Given these clues, we have a step change in voltage, low to high. We have current relatively constant throughout, around 50 amps before the event and still 50 amps after the event. The current doesn’t change very much, voltage changes significantly, and we see an oscillatory transient in the voltage waveform. Given these clues, this is highly indicative of a capacitor bank switch on this one.
Additional Examples in PQ Canvass
I got a couple more examples here in PQ Canvashttps://powermonitors.com/videos/visualizing-loose-neutrals-w-pmi-rms-capture/s. Here we have two different recordings with two different waveform captures, but both of these are oscillatory transients. Here on the left, we have an oscillatory transient and we can see that it rings at its resonant frequency, so something in both these waveforms has excited the system resonance.
That can be an impulsive transient, it can be something like a lightning strike or a switching event. Could be a customer load energization that presented a step change in current. Something’s excited the system, it’s ringing, like Landon said, at its resonant frequency.
The question was, was this a cap bank energization or a regulator tap change that would not only produce an oscillatory transient, but also result in a step change in voltage? When a capacitor switches in or out, that changes the var flow, and even though the capacitor ostensibly is for power factor correction, they’re often used for voltage control. In any case, they’ll typically change the RMS voltage on a steady state basis. And so of course, so will a regulator switch. That’s its purpose. But if it’s something like a customer load coming on, like Landon said, you’ll have a big change in current.
So the triad is, you’ve got an oscillatory transient, and you’re looking for a step change in voltage, but with no significant change in current. That’s the indication that it was some sort of cap bank or regulator.
Left Example: Likely Not a Cap Bank Energization
If we zoom in for this one, for example, we can look at the RMS graph. I click on RMS here, and we can see that the voltage before and after is roughly the same. We have 121.8 here on phase A. We’ve got 121.7. The voltage is basically the same before this transient and after. So the steady state voltage did not change, so it was likely not a cap bank energization or a regulator tap change, because there would have been a change in voltage.
Right Example: Likely a Cap Bank Energization
On the other hand, if we look at this one on the right, we have a nice strong ring that’s a very high Q resonance for a distribution system. The current doesn’t really change. We have load current before and after. Let’s look at the RMS value. The current doesn’t change much, but the voltage changes.
Before, we’re at 489.0. I’ll circle where this is so you can see this when I go back. So 489.0, 4.1 on phase A, and after, we’re at 492.8. So we’ve got roughly a three volt change in voltage, three volt increase in voltage after this event.
This is likely a cap bank energization, given that very high Q. Could also be a tap changing event, but it’s probably a cap bank switch. We know that it’s not caused by the customer load, because the current is roughly the same before and after, 49 amps, 30 amps. The current’s only rising slightly because the voltage rose slightly. On the other hand, if there’s a giant change in current, a big load switching all the way on, that would be the root cause of that transient in the first place.
So here, we have that triad event. We have an oscillatory transient, we have a small step change in voltage, in this case an increase, and we don’t have a large change in current, indicating that this is likely a cap bank energization.
Q&A: Detrimental Effects of Oscillatory Transients
We have a question: “Oscillatory transient. We talked about this being a transient or ringing, meaning it should be analyzed. So how’s the application of the multi-cap banks impact the oscillatory transient that may be observed?”
So if you’ve correctly identified that it’s probably a cap bank switching, should you analyze this in more detail? The detrimental effects of this transient depend on usually two factors. There’s the peak voltage that it achieves. Here, if we look at the instantaneous voltage where it was a negative 703 volts, we go down to minus 825. That’s about 120, 130 volt increase in the peak voltage.
These transients really cause problems in two ways. The maximum peak voltage that is achieved can be high enough to puncture insulation, cause equipment damage. Surge suppression is supposed to help clamp that, but that can also have its energy limits exceeded.
The other way that these transients cause trouble is by distorting the voltage through zero crossing. We can see in phase A, the zero crossing looks terrible. In fact, there’s multiple zero crossings here, as the waveform crosses through zero more than once in the same half cycle. Phase B looks pretty good through zero crossing, and then phase C also has some zero crossing distortion.
That zero crossing distortion can confuse equipment that’s using that waveform for timing purposes. It can also confuse LED lights into thinking there’s a dimmer present. It can cause UPSs to misoperate or just beep at the customer. If the signature happens to match the profile of an arc-fault breaker, you may have a customer with an arc-fault breaker that trips or opens up falsely based on that sort of waveform. That’s very hard to predict, what that shape’s gonna be and what sort of equipment on that circuit is going to be sensitive.
In general, the higher the Q or the more ringing there is, the more likely this is to cause problems for someone on that circuit. But these are very common. In most cases they don’t cause trouble, but if the peak voltage rises too high, they can.
Q&A: Multiple Cap Banks on a Circuit
Another question: “If you have multiple cap banks on a circuit, how does that impact the transient you would observe?”
That’s a good question. If you have multiple cap banks, then you have multiple resonances. For example, if you have two cap banks, you’re gonna have three resonances. Each cap bank individually has a resonant frequency with the substation transformer, and then the parallel combination gives you a third resonance, and you’ll have multiple frequencies ringing at the same time.
In that situation, in the time domain, you’ll see a very confused picture where you don’t have a clear one-frequency ring. It’s not really possible to tell in the time domain what that frequency is, but with the frequency analysis, you’ll see multiple peaks. You’ll see one peak here and another peak at the other resonant frequency. In the frequency domain, you can separate those. But having multiple resonances means that you’ll have a more muddled waveform.
Typically, the peak voltage isn’t necessarily higher, but the wave shape just looks uglier, and I can show an example of what that looks like here.
Example: Two Cap Banks on a Circuit
Here, we have an example of two cap banks on a circuit. We got 1200 kVAR and 600 kVAR a bit down the line, and that gives you three different resonant frequencies. Here, I’m showing the handy shortcut formula for estimating the resonant frequency of a circuit if you know the var radians and the short circuit current. We go through the math here, and that gives us 722 hertz, 1022 hertz, and 590 hertz.
Here is the waveform capture from that example, and you can see that you don’t have one dominant frequency. It sort of rings and then stops and rings again. That’s what happens when you have multiple frequencies that are not too far apart from each other, ringing, and they’re all superimposed on top of each other. You have a very confused, muddled look there.
So if you see a waveform capture like that where it looks like it’s ringing for a little bit but it’s not a clear one-frequency sine wave, that means you have multiple resonances. Here in the frequency domain, we see these three broad peaks, and if we look at those, they’re roughly at the frequencies that we get with this formula.
In this situation, this isn’t necessarily any worse than a single resonance. The peak voltage isn’t that much higher and zero crossing really isn’t that much worse than you might have been with just one. In fact, if this is a lower Q, lower quality factor, then this may be a better situation. But the challenge with multiple cap banks is it gives you more chances for these resonant frequencies to be on a common harmonic that customers are drawing current on. And of course, if these capacitors are switching, then your resonant frequency changes every time they switch.
Closing
That’s all the questions we have now. If you have a question later, give us a call anytime at 1-800-296-4120 or send us an email to support@powermonitors.com, and we’d be happy to chat. Everyone have a great day and thanks for attending.
