Transcript
Introduction to Voltage Imbalance and Waveform Distortion
Hello everybody, and welcome to today’s white paper webinar. Today we’re talking about voltage imbalance, and in particular, how that affects waveform distortion.
Many people are familiar with how voltage imbalance is detrimental for traditional AC induction motors where it increases motor heating, reduces motor lifetime, but not as many people are familiar with its effects on voltage distortion and how voltage imbalance can create increased current distortion from nonlinear loads, which then translates right back to voltage distortion.
So that’s a very important side effect of voltage imbalance that’s much more common these days with more and more nonlinear loads, especially as motors are more commonly being driven by a variable frequency drive rather than connected directly across the line.
Measuring Voltage Imbalance
Just to review what voltage imbalance is, this is any sort of difference in magnitude or phase angle among the three voltage phases. And there are a couple of different ways of measuring voltage imbalance.
One of the more common is from ANSI C84 and NEMA where you take the max deviation from the average and divide it by the average. This is easy to use. It’s easy to compute with a voltmeter. You can easily do this in the field.
If you’re using this method though, then you really want to make sure that you’re using it on line to line voltages or phase voltages because that is how most three-phase equipment is connected, and that also includes the effects of phase angle shifts. If you’re measuring line to neutral, you’re not including any phase angle shifts in your measurement, and that could give you a misleading picture. It may give you a lower number than the actuality. So if you’re using the more basic method, make sure you’re measuring line to line.
Now another way of doing this, a more accurate way, is to use symmetrical components where you take the ratio of the negative sequence to the positive sequence voltage values. If you’re familiar with symmetrical components, the positive sequence value is the amount of voltage that is balanced and the negative sequence component is the amount of unbalance, and then the zero sequence is the amount of basically a DC shift.
This is a pretty powerful technique. It’s more often used in fault analysis. Symmetrical components is an easy way of turning a complex single phase to phase fault into three separate circuits to analyze. So it’s mostly used in the protection role to make fault analysis and protection simpler, but it has a role in power quality when we’re talking about voltage imbalance. Since the negative sequence voltage is the absolute level of unbalanced voltage, that ratio to the positive sequence is a better measure of voltage imbalance.
How Voltage Imbalance Translates Into Waveform Distortion
Now there are some various mechanisms for how voltage imbalance translates into waveform distortion, and these generally work where the voltage imbalance causes current distortion from loads, and then that current distortion then in turn causes voltage distortion because that distorted current has to flow through the system impedance. And with voltage drop across that, if that current is full of harmonics or has harmonic components, the voltage drop will also have harmonic components.
From a high level standpoint, voltage imbalance causes an increase in triplet harmonics. These are the 3rd, the 9th, 15th, but mostly 3rd harmonic that accumulate in the neutral. It can also increase the 5th and 7th harmonics which are the most common for the three-phase rectifier load.
Transformer Saturation
One that we talk about here is increased transformer saturation. If the voltages are not balanced going into a three-phase transformer, you have possibly circulating currents in the transformer. You have higher currents in one coil than the other and that can lead to the transformer saturating earlier than it should. That saturation starts to increase any current losses and starts to lead to a voltage distortion because that turns an ideal linear transformer into a nonlinear load itself which then leads to voltage distortion.
Nonlinear Loads and VFDs
One of the more common and probably the most prevalent way unbalanced turns into voltage distortion is with nonlinear loads, especially three-phase nonlinear loads and in particular VFDs. Here we have a figure that shows this effect.
If we think about a VFD, this is the front end of a VFD. It takes the three-phase voltage, rectifies this to a DC bus voltage and then, of course, the inverter uses high voltage transistors to pulse voltages off and on very rapidly in producing a current waveform at any arbitrary frequency. The inductance of the motor turns these voltage pulses into a smooth sine wave current.
But here we’re focused on the front end. From the utility standpoint, this is an AC to DC converter. And as the phase-to-phase voltage between two phases rises above that bus voltage, different pairs of diodes will conduct and you get a waveform that looks like this. Here we have voltage on the top, current on the bottom. These pulses of current are when a different pair of diodes conducts as the phase-to-phase voltage between two phases goes above the DC bus voltage.
Now ideally, all of the voltages are the same amplitude, and if that’s the case, the voltage across every set of diodes will be the same and the currents will be the same height. Here we see a difference. Some of the pulses are smaller than other pulses and that’s due to the voltage imbalance. The diodes that see slightly higher voltage will have slightly higher current. That will also be detrimental to the VFD itself. Some diodes are heating faster than others. Those diodes will run hot. They’re gonna fail early. You could even have thermal runaways where the VFD fails. So voltage imbalance is bad on the VFD for that reason.
Triplen Harmonics From Uneven Current Pulses
That’s not where we get the voltage distortion. What happens is this uneven pulse height gives rise to triplet harmonics, in particular the 3rd harmonic. In theory, if the pulses are all the same height, you have the characteristic harmonics of a six-pulse VFD, the 5th and 7th or 11th, 13th. These are all multiples of six plus or minus one.
However, if the voltage is unbalanced, the current pulses are at different heights, and that gives rise to third harmonic and other triplen harmonics. So these harmonics shouldn’t be there in an ideal perfect VFD driven by perfect voltages. But if the voltage is unbalanced, that will generate third harmonic currents, which then generates third harmonic voltage distortion. And it could be a pretty significant effect.
Here is a bigger version of the chart that’s in the white paper, and here we’re looking at harmonic number on the X-axis and the amount of current distortion on the Y-axis. With no voltage imbalance, we have very little third harmonic current. As you’d expect with a six-pulse VFD, there should be no triplen harmonics.
Now, if we have just 1% voltage imbalance, we’re already over 20% distortion on the third harmonic. And this is a harmonic that no one’s expecting to be there. If the customer’s filtering harmonics, they’re probably not filtering the third harmonic because their VFD shouldn’t be producing that in the first place.
And you notice the other harmonics are still high. That doesn’t reduce the already present harmonics, the fifth and seventh. That just adds extra harmonics in addition to what was already there. And as the voltage imbalance gets higher, 3%, 5%, it gets even worse, but even 1% voltage imbalance is a significant amount of voltage distortion resulting from this current distortion.
So if you have a feeder that has a lot of third harmonic voltage distortion, and it’s mostly three-phase loads which don’t generally produce third harmonic, check your voltage imbalance because as that imbalance gets worse and worse, you’re gonna see more and more voltage distortion on that third harmonic.
Impact on Advanced Drive Architectures
Now, another way in which voltage imbalance gives rise to current distortion is through mechanisms where the customer is using a more advanced architecture. Here is an 18-pulse drive, and in this situation we have three AC voltages presented to three sets of diode blocks. A phase shifting transformer produces a negative 20 degree phase shifted voltage set and a positive 20 degree set, and each of these is rectified to its own DC value, and these are diodored together to produce one overall DC bus.
Now, this arrangement inherently has much less current distortion than the traditional VFD, but this phase shifting technique starts to fall apart as the voltage becomes more unbalanced. Here we have a graph of the amount of VFD load and the amount of current distortion versus voltage imbalance. With no voltage imbalance, current distortion is very low, this technique works well. But as the voltage becomes more and more unbalanced, this technique becomes less and less useful.
So the customer has spent money to put in a more advanced drive to reduce current distortion, and some of that work is undone as the voltage imbalance creeps up. And of course, this current distortion not only is going to cause the transformer to run hotter, but it’s going to lead to voltage distortion.
Summary
Going back to the paper, the bottom line is voltage imbalance, either directly or indirectly, will cause non-linear loads, particularly three-phase non-linear loads, to draw harmonic orders that aren’t expected, in particular the third harmonic and other triplens. And that increased current distortion will of course make your transformers run hot and also increase voltage distortion.
So as your voltage becomes more unbalanced, your voltage distortion will also become worse, to the point where it’s possible that even if customers are meeting their current limits, you may have trouble meeting the voltage distortion limits because the voltage unbalance is contributing.
If you have questions about the paper, give us a call anytime, 1-802-964-120, or send an email to support@powermonitors.com.