电能质量的主要中美. 汽车装配厂与双实用饲料
| Facility | Major U.S. automobile assembly plant — 3,200 工人 |
| Supply configuration | Dedicated substation fed from two independent transmission lines |
| 监控系统 | I-Sense monitors on each transmission line — continuous waveform recording |
| 事件 | Wind-induced line-to-line fault at substation entry of Transmission Line #1 |
| Sag duration | 4.8 周期 (0.09 秒) on the faulted line before automatic transfer |
| 中断 | 9.8 seconds on Line #1 after fault clearing — Line #2 maintained supply throughout |
| Remaining voltage during sag | 68% — above the 50% threshold for standard sag correctors |
| Key finding | Dual feed prevented a multi-hour interruption but did not eliminate the voltage sag — which still caused process disruptions |
01 Background — The Dual Feed Strategy
For industrial customers whose processes cannot tolerate supply interruptions, utilities commonly offer dual-feed service: the facility is supplied from two independent transmission lines connected to the same dedicated substation. 正常情况下, the plant load is shared between the two lines. When a fault occurs on one line, the plant load is automatically transferred to the other — a strategy designed to provide near-continuous supply despite single-line faults.
This case study, based on field monitoring data from a major U.S. automobile assembly plant employing 3,200 工人, illustrates both the strength and the limitation of the dual-feed strategy: it is highly effective at preventing long interruptions, but it does not eliminate the short-duration voltage sags that can still cause significant process downtime in sensitive manufacturing environments.
The event was captured by I-Sense monitors — part of the I-Grid™ system developed at Georgia Tech and commercialised by Soft Switching Technologies. I-Sense monitors record continuous voltage and current waveforms and timestamp events with GPS accuracy, enabling precise correlation of events across multiple measurement points. This multi-point, time-synchronised monitoring approach is essential for identifying the source and propagation path of voltage sags — a capability that single-point monitoring cannot provide.
02 The Event — Wind-Induced Transmission Fault
A windstorm caused a line-to-line fault at the point of entry of Transmission Line #1 into the dedicated substation. The physical sequence of events, reconstructed from the I-Sense monitoring data on both lines, was as follows:
- Phase 1 — Fault initiation: The line-to-line fault is fed simultaneously by both transmission lines. The fault current from both lines causes a voltage sag that propagates to all downstream load buses — including the plant loads. Both I-Sense monitors record the voltage sag simultaneously, confirming that the sag originated at a point common to both lines (the substation entry point)
- Phase 2 — Fault clearing: Circuit breakers open to isolate the faulted Transmission Line #1. The sag lasts 4.8 周期 (approximately 0.09 秒) before the breakers operate
- Phase 3 — Automatic transfer: All plant loads are transferred to Transmission Line #2, which was unaffected by the fault. The Line #2 monitor records a return to normal voltage after the sag — no interruption on this line
- Phase 4 — Extended interruption on Line #1: The Line #1 monitor records a complete interruption lasting 9.8 seconds after the sag — the line remains de-energised while the fault is cleared and the line is restored. The plant is unaffected by this interruption because it is already running on Line #2
03 Analysis — What the Dual Feed Did and Did Not Prevent
What the dual feed prevented
The automatic transfer from the faulted Line #1 to the healthy Line #2 prevented what would otherwise have been a multi-hour supply interruption — the time required to physically locate and repair the wind-damaged transmission line. For a 3,200-worker assembly plant, a multi-hour interruption represents an enormous production loss: vehicle assembly lines cannot be partially restarted, partially assembled vehicles on the line must be managed, and the restart sequence after a complete plant shutdown involves significant complexity and time.
The dual-feed strategy succeeded completely in its primary objective: the plant continued to operate on Line #2 throughout the 9.8-second interruption on Line #1. From a supply continuity perspective, the infrastructure performed exactly as designed.
What the dual feed did not prevent
The 4.8-cycle (0.09-第二) voltage sag during the fault was not prevented — and it caused process disruptions. This is the fundamental limitation of the dual-feed strategy that is often not understood by facility engineers: the automatic transfer protects against interruptions, but the voltage sag that occurs during the fault interval — before the breakers open and transfer is completed — cannot be avoided by any transfer scheme. The sag is instantaneous; the transfer takes several cycles.
Modern industrial process equipment — particularly programmable logic controllers, 可变频率驱动器, and robotics — typically has voltage sag immunity of 8–20 cycles depending on the manufacturer and configuration. A 4.8-cycle sag at 68% remaining voltage may or may not trip sensitive equipment depending on the specific immunity characteristics of each device in the plant. In an automobile assembly plant, even a single equipment trip on the line can halt the entire assembly process — which is why the 4.8-cycle sag still caused “some process interruptions” despite the successful automatic transfer.
The mitigation gap — sag correctors
The 4.8-cycle sag with 68% remaining voltage is within the operating range of commercially available voltage sag correctors — dynamic voltage restorers (DVR) or ferroresonant constant-voltage transformers (CVT) — which can typically compensate sags down to 50% remaining voltage for durations up to 10–30 cycles. Had such devices been installed on the critical process equipment feeders, the 4.8-cycle sag would have been invisible to the sensitive loads and no process disruptions would have occurred.
Dual utility feeds provide excellent protection against supply interruptions but provide no protection against voltage sags. A comprehensive voltage reliability strategy for a sensitive industrial facility requires both: dual feeds to address interruption risk, and sag mitigation equipment (DVR, UPS, or ride-through controls on VFDs) to address the sags that occur during the transfer interval and from other network events that do not cause a transfer at all.
04 电能质量视角
This case study is a clear example of the difference between supply reliability and power quality — two concepts that are often conflated but address different failure modes. The dual feed addresses reliability: the risk of a sustained interruption due to a fault on one supply path. Voltage sags address power quality: the short-duration voltage depressions that occur during faults anywhere on the connected network, regardless of the supply configuration.
From a utility engineering perspective, the dual-feed case study also illustrates the value of multi-point, time-synchronised monitoring. Without monitors on both lines, it would be impossible to confirm from the data alone that the sag originated from a fault on Line #1 rather than from a load-switching event within the plant. The simultaneous sag recorded on both lines, and the subsequent different behaviour (Line #1 interrupts, Line #2 recovers), is the definitive signature of a transmission fault at a point common to both lines — in this case, the substation entry point.
The I-Grid monitoring approach demonstrated here — time-synchronised monitors at multiple points in the network — is exactly the monitoring architecture that separates utility-side PQ assessment from facility-side PQ assessment. A single monitor at the plant service entrance would have recorded the sag but could not distinguish a utility transmission fault from an internal plant fault. Two synchronised monitors, one on each feed, provide unambiguous source attribution. This principle scales: a well-designed utility PQ monitoring network with GPS-synchronised recorders at multiple substations can locate the source of any disturbance to within a specific feeder segment. That is the utility power quality engineering perspective — and it is what this case study demonstrates at a small scale.
参考文献
- Divan D, Brumsickle W, Eto J. A New Approach to Power Quality and Electricity Reliability Monitoring — Case Study Illustrations of the Capabilities of the I-Grid™ 系统. 欧内斯特·奥兰多·劳伦斯·伯克利国家实验室, LBNL-52048, 四月 2003.
- IEEE StD里 1159-2019. IEEE Recommended Practice for Monitoring Electric Power Quality. IEEE, 纽约, 纽约, 2019.
- SEMI F47-0706. Specification for Semiconductor Processing Equipment Voltage Sag Immunity. SEMI, Milpitas, 加利福尼亚, 2006.
Divan D, Brumsickle W, Eto J. A New Approach to Power Quality and Electricity Reliability Monitoring — Case Study Illustrations of the Capabilities of the I-Grid™ 系统. Lawrence Berkeley National Laboratory, LBNL-52048, 四月 2003.
This case study is presented in summary and commentary form for educational purposes. Original material is attributed to the authors and Lawrence Berkeley National Laboratory. The PQ Perspective section (部分 4) and SVG diagram are original IPQDF editorial content by Denis Ruest, M.Sc. (Applied), P.Eng. (ret.). IPQDF does not claim authorship of the original research.