Transcript
Introduction
Welcome to today’s Ask a Pro session. I’m Andrew Cornfeld, one of the software engineers here at PMI, and I’m the author of this paper. Today, we’re gonna be talking about recognizing transformer energization in power quality data.
In this presentation, we’re gonna cover why transformer energization matters in a PQ investigation, the electrical behavior that happens when a transformer is energized, how to identify energization events in your recording, and how to differentiate energization from a fault. If you have any comments or questions as we go, just type them in chat and we’ll go through them at the end.
Why This Matters
Transformer energization happens all the time on the distribution system. You’ll see it during equipment commissioning, restoration after an outage, feeder switching operations, normal distribution automation activity.
The catch is that when a transformer energizes, it can produce large transient currents and significant waveform distortion. At first glance, that activity can look like a fault, which means without proper interpretation, an energization event can trigger an unnecessary investigation or get misclassified in a PQ monitoring system entirely.
The goal of this paper and this session is to make sure that you can look at a recording and confidently say, “That’s a transformer being energized,” instead of chasing down a phantom fault.
Electrical Behavior During Energization
When voltage is first applied to a transformer winding, the magnetic core has to establish its operating flux level. Depending on exactly where on the voltage waveform that switching happens, the magnetic flux in the core can temporarily push past its normal operating range and drive the core into saturation. Once the core saturates, the magnetizing current shoots up dramatically.
Here’s the important part. This current is not limited primarily by winding impedance the way normal load current is. Instead, it’s governed by the nonlinear magnetization curve of the transformer core.
There are a few factors that determine just how big and how distorted this inrush current ends up being:
- The point on the voltage waveform at which switching occurs
- Residual magnetic flux already in the transformer core
- The size and design of the transformer itself
- System impedance at the point of connection
The bottom line is that the initial current during energization can hit five to ten times the transformer’s rated current, even though the transformer isn’t actually supplying load yet. That current waveform is highly distorted, and it decays gradually as the core flux stabilizes back to normal operation.
Four Signatures to Look For in PQ Recordings
So when you’re reviewing a PQ recording, what are the tells? There are four signatures to look for.
- A large current spike with only minor voltage disturbance. Faults usually slam the voltage down at the same time the current jumps up. Energization typically does not.
- Highly distorted asymmetric current. During an energization, one half-cycle of the current waveform can be much larger than the other, and you’ll see a clear lopsided shape that you don’t see in faults.
- A gradual decay. The transient current is large for several cycles and then steadily decreases as the magnetizing flux stabilizes. It tapers off and it doesn’t snap off.
- A strong second harmonic current. The harmonic analysis on an inrush current will show significant second harmonic, which is a very characteristic fingerprint of magnetizing inrush, and it’s actually what protection engineers use to keep transformer differential relays from tripping during energization.
Differentiating Faults from Energization
This is a point that really matters for day-to-day PQ work. Faults and energization events can both produce big current magnitudes, and they can both trigger alarms or protection. But the underlying physics is completely different, and the waveforms tell on themselves if you know what to look for. Let’s walk through them side by side.
A fault happens when an unintended low impedance path forms in the system, like a phase to ground or phase to phase short. Current rises rapidly, limited by system impedance. You’ll get a significant voltage sag at the point of common coupling, or PCC. The current waveform stays mostly sinusoidal and symmetrical, especially in the first few cycles before protection operates. Harmonic content is generally limited, and the duration is short. Protection isolates it in a few cycles.
Here you can see one of the figures mentioned. This is figure one from the whitepaper. Notice in figure one that the current goes up cleanly and is still recognizably a sine wave, just much bigger, and we can look at that in PQ Canvas here. So here you can see the current goes up cleanly. It’s still recognizable as a sine wave, just a little bit bigger. Here are the harmonic magnitudes. The harmonic content is dominated by the fundamental, with very little second harmonic to speak of, so you can see the second harmonic and everything past that are very low.
Transformer Energization Example in PQ Canvas
In contrast, a transformer energization looks fundamentally different. The current is driven by magnetic saturation and not system impedance. The voltage disturbance is modest, especially on a strong system. The waveform is highly distorted and asymmetric. One polarity has much bigger peaks than the other. Harmonic content shows a strong second harmonic, and the duration is longer. The inrush decays over many cycles or seconds, rather than ending abruptly.
Let’s take a look at an example of transformer energization in PQ Canvas. Here’s the voltage and current waveforms of transformer energization. In here, which is figure three in the whitepaper, you can see the asymmetry really clearly. One side of the waveform is dramatically larger than the other. And the voltage barely moves while all of this is happening.
This is figure four in the whitepaper, but here are the harmonic magnitudes. You can look and see how much second and third harmonic content there is compared to the fault graph in figure two from the fault. And this second harmonic is kind of the dead giveaway right here.
Putting It All Together
So if you take all these together—voltage magnitude, waveform symmetry, harmonic content, and event duration—you’ve got a reliable framework for telling a fault apart from a transformer energizing.
But the reason we can do all this confidently is that modern PQ monitoring captures the level of detail you need. High resolution waveform recording lets engineers examine the exact shape of the current and voltage during the switching event. That’s how PQ data gets used to:
- Confirm that a disturbance was a normal system operation, not a fault
- Verify the behavior of newly installed transformers
- Evaluate switching practices on a feeder
- Investigate unexpected current events
Software like PQ Canvas is what brings these signatures forward quickly so you don’t have to comb through the entire recording by hand to find them.
Recap
To recap, transformer energization is a routine normal operation, but it leaves a very distinctive fingerprint in PQ data. If you remember the four signatures of high initial current, waveform asymmetry, a strong second harmonic, and gradual decay, and pair them with a relatively undisturbed voltage waveform, you can confidently identify these events and avoid misclassifying them as faults.
As PQ monitoring keeps spreading across the grid, recognizing these kinds of patterns is gonna remain a core skill for anyone doing PQ analysis.
Well, that about does it. Be sure to check out the links and resources here for additional information. And as always, thank you to everyone for tuning in, and have a great rest of your day.
