Transcript
Introduction to Voltage Phase Angle Jumps
Hello, and welcome to another Power Monitors white paper webinar. Today, we’re gonna talk about something that’s surprisingly common in electrical power systems, but often goes unnoticed: voltage phase angle jumps, or PAJs.
Most of us are used to thinking about power quality in terms of voltage sags, swells, interruptions, harmonics, and transients. Those are the events we’re typically monitoring. There are situations where the voltage stays well within acceptable limits, yet drives trip, transfer switches operate unexpectedly, or industrial processes shut down.
The natural reaction is to ask, “If the voltage never really dropped, what happened?” The answer is often that the timing of the waveform changed. Traditionally, we’ve paid a lot of attention to the voltage magnitude, but not to the phase angle. So today, we’re gonna look at what a phase angle jump is, why it happens, and why it’s often missed by conventional monitoring.
What Is a Phase Angle Jump?
We’re gonna start with the basic concept. Every AC voltage waveform has two major characteristics. The first is the magnitude, and the second is the phase angle, which tells us where the waveform is positioned. Normally, the phase angle changes smoothly as the waveform progresses. A phase angle jump happens when the waveform suddenly shifts sideways.
Figure two here. So up here, figure one, you have a normal and a phase-shifted waveform. So you get your magnitude there, you get your phase angle that separates them, along a time axis there on the x-axis. Now, down here, we have phase angle jumps. As you can see, you go from a nice, smooth sine wave to suddenly a jarring shift, as suddenly the phase angle is jumping. You also get a magnitude dip during that same time. We’ll talk about that a little bit more.
So the voltage may only dip slightly. Depending on where your trigger thresholds are set, it may not trip your typical monitoring system, but the zero crossing and the waveform peak suddenly occur earlier or later than expected. The electrical system’s still operating at sixty hertz, but the waveform suddenly isn’t where synchronization-sensitive equipment expects it to be.
Causes of Phase Angle Jumps
So where does a shift like this come from? Most common cause is a fault. When a fault occurs, fault current flows through the source and feeder impedance. Because that impedance has both resistance and reactance, the voltage doesn’t simply decrease, it also rotates in phase. As the system moves from its normal operating condition into the fault and then back again when protection clears the fault, the voltage phasor can move abruptly between operating points.
Switching operations can produce the same effects: large motor starts, transformer energization, capacitor bank switching, feeder transfers, utility reclosing, et cetera. All of these can abruptly change current flow and system impedance, producing a sudden phase shift.
Interestingly, balanced three-phase faults often produce smaller observable phase shifts than line to ground or other asymmetrical faults because the system remains more symmetrical. So while we tend to associate these events with voltage sags, they’re often producing significant phase displacement at the same time.
Why Phase Angle Jumps Matter
So why do we care about this? A lot of modern equipment cares about synchronization, not just the magnitude. A couple examples that you may be familiar with: VFDs, or variable frequency drives, use what’s called a phase lock loop to continuously track the incoming AC waveform. The drive isn’t just looking for four hundred and eighty volts, it needs to know exactly where that sine wave is so its control electronics stay synchronized.
Grid-tied solar inverters work the same way. Before they can inject power back to the grid, they must match the grid’s voltage frequency and phase angle. If the utility waveform suddenly shifts, the inverter has to rapidly reacquire synchronization.
Static transfer switches also depend on synchronization. Before transferring a critical load between two sources, they compare the phase angle of both sources. If they’re too far apart, the switch may delay the transfer or refuse to transfer at all to avoid damaging out-of-phase currents.
Even synchronous motors and generators rely on phase angle. Their torque depends on the alignment between the rotating magnetic field and the rotor. A sudden phase shift changes that alignment immediately, producing torque oscillations and mechanical stress.
So when we say equipment is synchronization-sensitive, we’re talking about equipment that is actively tracking the timing of the AC waveform, not simply the magnitude.
VFD and Equipment Response
Let’s go back to the VFD example. As we talked about, they use phase lock loops. If the phase suddenly jumps, the PLL has to reacquire the lock. Depending on the VFD and what it’s doing at the time, this can produce DC bus overvoltage, overcurrent conditions, or simply a protective shutdown.
Static transfer switches may perform unnecessary transfers. Protective relays can also respond during these events. They’re not malfunctioning, they’re responding to a legitimate change in the waveform.
When we look only at the RMS voltage, you may see a small sag, as you can see here in figure two, and they’re represented in phasor form there in figure three. You may see a small sag, depending on the severity of the phase angle jump, but not necessarily enough to trip. This one’s obviously quite terrible with a thirty percent reduction in magnitude, but that helps you see it. But it won’t always be that severe.
Down here in figure four, we talked about how asymmetrical faults can create larger and more significant voltage drop during the phase angle jump, whereas a three-phase system typically stays more balanced during the jump.
Identifying Phase Angle Jumps
To actually identify a phase angle jump, you need a high resolution waveform capture or point on a wave recording. Looking at the waveform timing instead of only the magnitude can explain events that otherwise don’t make sense.
Here’s some examples. We’ve got some rotor torque disturbance during a phase angle jump down here in figure five. And up in figure six, we’re looking at some relay oscillography showing a phase jump. You can see the trigger there and you see the phase shift immediately on phase A. There’s your phase angle degree suddenly dropping down significantly.
And there’s a RMS trend versus the point of wave capture. So up here, you’ve got your nominal and you can see there’s a slight change. There’s your nominal, here’s your 90% threshold. So you can see that RMS sag is a small sag. It’s not crossing the threshold, but if you were to look at the phase angle, you can see there’s dramatic discontinuity there. So it’s not purely the magnitude. It’s a cleaner example.
So if you’re measuring one and not the other, but your equipment is reacting to either, you get a difference between what you’re monitoring and expecting to have happening in the system versus what your protected devices are reacting to.
Mitigation of Phase Angle Jumps
Mitigation for phase angle jumps involves equipment settings, system design, and visibility — knowing what’s happening. Many drives and inverter systems allow adjustment of that phase lock loop, that PLL bandwidth, synchronization tolerance, and ride through settings, depending on what you’re setting up and what phase angle jumps you see in your system. Depends on how tightly you want to set those bands for reacting to phase angle jumps.
Double conversion UPS systems isolate the load from changes in utility phase angle. At the system level, transformer configuration, system impedance, and switching coordination all influence how severe these events become. Utilities will often use a sync check relay and control recloser logic to prevent reconnecting systems that have drifted too far apart.
The biggest operational recommendation though is improved monitoring. If you’re investigating unexplained trips, don’t stop after looking at the RMS magnitude. Capture the waveform, look at that actual phase angle jump. As we saw here in figure seven, if you’re purely looking at magnitudes, you’re not going to see what happened and what your relays are responding to. As we increasingly become dependent on these very quick, very fast, and very sophisticated devices, we’re going to have to look at synchronization, not purely magnitude.
Closing
Thanks for coming. If you do think of a question, you can always send it in to support at powermonitors.com and they’ll relay it to the right people. If you’re interested in looking at our white papers for other topics, those are available on powermonitors.com. Thanks everybody for joining and we will see you next time.
