Transcript
Introduction to Impact Loading
Good afternoon, everyone, and welcome to today’s webinar. Today we’re going to be talking about impact loading. This is an interesting power quality issue that can be really puzzling for a utility, because it can cause a very unexpected transformer failure, and many utilities aren’t on the lookout for this type of a PQ issue, because it’s more commonly seen inside a customer facility than at the utility service transformer.
What Is an Impact Load?
An impact load is a load on a transformer that exceeds its nameplate current rating for a brief amount of time. We’re not talking about steady state loads. We’re talking about abrupt changes in load. This would be like motor starting currents. For example, if you have inrush current from a hard start motor, that’s going to cause a surge in current that’s many times the running current of that motor.
In most cases, the transformer that serves an entire building is much larger than even the inrush current of any one load in a building. Where this is traditionally seen is in what’s known as a unit transformer, a transformer that is feeding just one motor. This is something you normally see inside a facility where the transformer is changing the voltage to suit a single motor.
There the transformer is sized generally for the steady state running current of the motor. It’s sized for thermal reasons. But if it’s a hard start motor, the inrush current will exceed the nameplate rating of the transformer. And if that happens repeatedly, that can damage or cause the transformer to fail.
That looks like a fault current to the transformer. That puts a lot of mechanical stress, due to induced magnetic fields, on the core of the transformer and the winding. So that repeated mechanical stress will cause the transformer to beat itself up, and it’ll eventually fail.
The Impact Load Reference Curve
As the paper talks about here, towards the end, there is a standard reference for assessing an impact load on a transformer. This curve is taken from a paper from the 1970s that’s still actually the main reference and guideline for assessing impact loads.
The way to use this graph is to determine the per unit pulse swing on that transformer. That’s defined as the ratio of the kVA of the transformer to the kVA of the inrush pulse. That ratio is the pulse swing. If that’s greater than one, that means that the inrush current that the transformer’s seeing is over its nameplate rating. And this graph gives guidelines for how often a transformer can see that before failure may happen.
We have pulse swing on the X-axis and how many times per hour this happens to the transformer on the Y-axis. So for example, if we have a pulse swing of two, where we have twice the nameplate rating of the current happening, say, 50 times per hour, roughly once a minute, that would put us right here on the graph. That’s outside the safe operating area.
If the pulse swing is in this region, that’s okay. The transformer should be able to take that. If you end up in this region, the transformer may fail early. It needs to be upsized or somehow reduce that pulse swing. Anything at one or below is under the nameplate rating of the transformer, so it can take that indefinitely. And anything above four is four times the rating of the transformer. You should never do that. That’s a fault on the transformer winding.
When Utilities See Impact Loads
In a power quality recording, an unexpected transformer failure from an impact load is often a surprise. Utilities don’t normally see this, again, because most utility transformers are feeding more than one load. They’re feeding an entire building or even a collection of buildings, and no one load in the building is usually sized comparable to the transformer itself.
A couple exceptions to that are where the utility transformer really is serving just one or two loads, for example, a water pump or a sewage system where the load really is just one pump, or a situation where the loads inside the building are synchronized together. And so you have what looks to the transformer like one giant load.
Poultry Plant Case Study
That’s the situation we have in these graphs. This was a poultry plant, and it was fed by a one MVA transformer, and that transformer failed in about six months. The utility replaced the transformer, made a power quality recording to try to assess why did that transformer fail.
One of the common reasons for transformer failure is harmonic currents causing excessive heating. But if we look at the waveforms here, when these motors start, it’s all sine waves. All sinusoids. No VFDs, no harmonic distortion. So harmonics are not an issue for this location. But we do have very high peak to average currents, as you can see in figure one. This is the peak current, and this is the average current.
Viewing the Recording in PQ Canvass
I’m going to jump to PQ Canvass to show you this in more detail. Here we have the recording. We’re using our cloud-based PQ Canvass system to demonstrate this. We can look at the interval graph, and I’m going to look at voltage and current.
We can immediately see very high peak currents. This purple is the one cycle maximum current, and the dark blue is the average current. We have very high peak to average currents, and they’re repeated very often. If we take a look and zoom in to get a sense for how often this is happening, this is on a two-minute interval, so every two minutes we’re seeing a spike in current. And this current spike is over 1,500 amps. That’s a lot of inrush current.
Often in these sort of situations, you’ll see high flicker levels, because these high current spikes cause lots of voltage sags. You can see a deep voltage sag every time this happens. That will cause high PST values. If we look at the PST flicker graph, we’re above one in many cases. So another symptom of impact load is high flicker.
Computing the Impact Load
I’m going to jump to a separate presentation that goes in a little bit more detail on this type of issue with this recording in particular. Here, we compute the impact load. If you look at the peak current, we’re getting about 840 kVA per phase or about two and a half MVA total on a three-phase basis. And remember, this is in a one MVA transformer.
So that means the pulse swing is 2.5 over one or 2.5, and it’s happening every two minutes, so that’s roughly 30 times per hour. On this graph, that puts us right about here. We’re at a two and a half pulse swing, and it’s happening 30 times per hour. That’s outside the safe operating area of a standard transformer. So it’s not a surprise that that transformer failed early, very early.
The utility upsized the transformer to one and a half MVA, and that put it inside the safe operating area.
Thermal Sizing vs. Impact Load Sizing
From a thermal standpoint, one MVA was a reasonable choice. If you just go by the steady state current here in blue, this is the current level that’s going to relate to transformer heating. These current spikes are very brief, and so they’re not going to heat the transformer up. They’re going to be an impact load, but not really cause thermal issues. So normally, you would size the transformer based on thermal load, and that’s this much lower current here. So this transformer was sized reasonably well from a thermal standpoint. But given the impact load, it needed to be upsized.
In this poultry plant, it had a long conveyor system that had many, many induction motors, and they were synchronized together. The conveyor system would stop and start repeatedly. And when it did that, we had many, many motors stopping and starting simultaneously. So from the transformer’s standpoint, it looked like one large load, one huge motor, not a lot of small motors. There was no load diversity in this building like you would normally have in most situations.
Key Takeaways
An impact load for a utility is kind of an unusual situation. But when it happens, it can be very puzzling and very surprising. You have a transformer that fails for no apparent reason. And harmonics are good. There’s no current distortion. So it can be really puzzling as to what happened to kill that transformer.
Be on the lookout for synchronized loads or the clues of an impact load: very high peak currents compared to the average current, lots of flicker complaints, and low current distortion. This is easy to see in a PQ recording where you have min, max, and average, either through PQ Canvass or through using standard PC-based software.
If you have questions about the paper, feel free to give us a call anytime at 1-800-296-4120, or send an email to support@powermonitors.com. We’d be happy to chat. Everyone, have a great afternoon, and thanks for attending.