Key Takeaways
- Voltage sag analysis is most useful when it starts at the sensitive load bus and works back through the network to the fault source.
- Remote faults, source strength, feeder impedance, and clearing time will all shape the retained voltage that equipment actually sees.
- A sag study becomes actionable only when simulated events are checked against device ride through limits and protection timing.
Accurate voltage sag analysis starts at the equipment bus, because a short fault several buses away will still depress voltage enough to trip sensitive loads.
That reach matters because the annual cost of power interruptions to U.S. customers was estimated at US$22.8 billion to US$188.0 billion, with commercial and industrial users carrying much of the loss. A plant does not need to be faulted to lose a batch, stop a conveyor, or drop a contactor. You will get the right answer only when the network fault and the equipment ride through limit are studied as one problem. You will miss that exposure if you screen events only at the service entrance. That is the practical value of voltage sag analysis.
Voltage sag starts with fault current through system impedance
A voltage sag happens when fault current flows through the source and network impedance between the supply and your bus. The larger that current path voltage drop becomes, the lower the retained voltage at the load bus will be. Sag depth is a network response, not a mystery at the process end.
A simple feeder example makes this clear. A three-phase short on an upstream bus pulls heavy current from the utility source, and the voltage at nearby buses falls until protection clears the fault. Your motor control centre can see 0.7 per unit voltage even when no device inside the plant has failed. That dip comes from system impedance, fault type, and source strength acting at the same time.
This matters because many teams still treat a voltage sag as a local load issue. It is usually a system issue first. Once you frame it that way, the model needs accurate source data, line impedance, transformer impedance, and fault parameters before you look at the process load. Protection timing will also shape how long the process sees the dip.
Remote faults can still depress voltage at sensitive buses
A fault does not need to sit on your feeder to cause a harmful voltage sag. Remote transmission or subtransmission faults will transfer through the network and depress voltage at a sensitive bus for a short period. Distance reduces the effect, but it does not remove it.
A common plant case is a fault on a neighbouring utility line that clears in 120 ms. The service bus might only drop to 0.82 per unit, which looks mild on a utility plot, yet that same event will open a weak control relay or trip a variable speed drive undervoltage function.
“The cost shows up at the process, not at the faulted line.”
You should treat remote fault exposure as part of normal system performance. Engineers who study only on site faults will miss a large share of nuisance trips. Voltage sag analysis is useful because it ties a distant network event to the exact retained voltage seen where production stops. That link is what turns a utility disturbance into a plant reliability problem.
Start voltage sag analysis at the point of use
The best place to start voltage sag analysis is the bus feeding the device that must stay on. You need the equipment voltage tolerance first, then you work outward through transformers, feeders, and source data. If you start at the fault location, you will miss the actual ride-through question.
You’re usually trying to answer a narrow engineering question, and the input set should stay focused on that bus:
- Nominal voltage at the protected load bus
- Transformer data between the source and the load
- Feeder impedance for each cable or line segment
- Fault locations and fault types worth testing
- Ride-through limits for the sensitive device
A packaging line gives a good example. The line does not fail because the service entrance dipped. It fails because a control power supply dropped out at 480 V for 90 ms. Once you anchor the study at that bus, every model choice becomes clearer and the results will match the operating question you’re trying to answer.
Source strength sets how deep the sag will be
Source strength will largely determine how far the voltage drops during a fault. A stiff source with high short circuit capacity holds voltage better, while a weak source collapses faster under the same fault. Two feeders with the same fault current path can produce very different retained voltage.
Consider a plant served from a strong utility grid and the same plant served from a smaller islanded generation source. A fault on a downstream feeder will produce a shallower sag in the first case and a deeper sag in the second, even if the line lengths and transformer sizes stay unchanged. Short circuit level is the first screening value you should ask for.
This is why generic voltage dip assumptions are risky. A fixed 20% dip estimate will mislead you on both strong and weak systems. Good modelling turns source data into a retained voltage result at the bus that matters, which is what equipment checks actually need. You’ll get better results when the model uses the utility short circuit level instead of a placeholder source.
Feeder impedance shapes the sag seen by downstream loads
Feeder impedance will shape the sag profile across a plant or utility circuit. Long runs, smaller conductors, transformer steps, and large motor buses all alter how much of the upstream disturbance reaches a downstream load. One fault will not look the same at every bus.
A long 4.16 kV feeder feeding remote pumps will often show a deeper dip at the motor starter bus than at the main switchgear. Another case appears on a feeder with several transformer stages, where the retained voltage at a low voltage control panel drops below a relay threshold even though the medium voltage bus stays acceptable. Feeder detail decides which buses are safe and which are exposed.
| System condition | What the load bus will usually see |
| Short feeder from a stiff source | The retained voltage stays higher, so many loads will ride through the same remote fault. |
| Long feeder with higher series impedance | The same upstream event will appear deeper at the far end and will stress contactors and drives sooner. |
| Several transformer steps between source and load | Each stage changes the fault path, so sag depth at the end use bus can differ sharply from the main bus reading. |
| Large motor group connected near the process bus | Motor current during recovery will hold voltage down longer and can extend the process upset after fault clearing. |
| Control loads fed through small auxiliary transformers | Control power often reaches dropout first, even when larger power equipment appears to have stayed online. |
That spread across buses is why site-wide averages do not help much. You need bus-specific results. Simulate voltage dips on a feeder with enough electrical detail to keep those differences visible, or the weak point will stay hidden. That extra resolution is often what exposes the vulnerable control bus.
Fault clearing time sets equipment ride-through risk
Fault clearing time decides how long the load must survive at reduced voltage. Depth matters, but duration often separates a harmless sag from a process trip. A shallow dip that lasts too long will still defeat contactors, DC power supplies, and drive controls.
The ride-through check needs an actual time pair, not a single voltage number. The ITI curve shows this clearly, with 70% retained voltage tolerated for only 0.5 s and 80% retained voltage for 10 s. A feeder breaker that clears in 80 ms will produce a very different process outcome than backup protection that clears in 600 ms.
A control system trip often starts with small devices, not the large motor everyone is watching. A relay coil, 24 V power supply, or process controller usually won’t forgive the longer event. That is why voltage sag analysis steps must keep protection timing and equipment tolerance on the same plot. You’re checking the full sag duration against device survival limits.
Feeder studies need faults placed along the full circuit
A useful feeder study will place faults at multiple points along the circuit and will test the fault types that the feeder can actually experience. Single-line to ground, line to line, and three-phase faults produce different retained voltages at each bus. One test point will not represent the full feeder.
A lateral fault near the feeder end can expose one remote process area while leaving another nearly untouched. A main feeder fault closer to the source will depress more buses at once, but the retained voltage at each load still depends on the local impedance path. That is why engineers build a fault set instead of relying on a single worst-case guess.
SPS fits this stage well because you can place faults across buses and line sections, keep the source and feeder model transparent, and compare retained voltage at the exact loads you care about. That kind of open model matters when you’re checking assumptions with protection staff, operations teams, or students in a lab.
Ride through checks turn sag results into design action
Ride through checks convert a voltage sag plot into a clear engineering judgement about what will stay online and what will trip. That judgement needs the retained voltage, the event duration, and the actual tolerance of each sensitive device. Once those three line up, you can act with confidence.
A useful check is often simple. Match the simulated sag at the process bus against the undervoltage trip setting of a drive, the dropout curve of a contactor, or the hold-up time of a control power supply. You’ll see quickly if the weak point is the network, the protection timing, or the equipment setting. That check also keeps mitigation spending tied to the actual failure point. That is where model results stop being academic and start guiding plant fixes.
Good practice is disciplined rather than flashy.
“You place faults where they can occur, model the path the sag will take, and test the load that must survive.”
SPS SOFTWARE supports that workflow well because the value sits in clear system behaviour, visible assumptions, and checks that match the events your equipment will actually face.
