INSIGHTS
TECHNOLOGY UPDATE: POWER QUALITY Mark Stephens and Alden Wright, EPRI
Power conditioning to match the power quality environment Voltage sags depend on the facility. An uninterruptible power supply (UPS) may be needed, but for large-scale issues, a UPS might not be the best option. See five UPS alternatives.
V
Figure 1: Sensitivity curves illustrate the vulnerabilities of a few control components still found in the field. All figures courtesy: EPRI
oltage sags have negative impacts on sensitive control circuit components and reliability. Understand the facility’s source of power to decide if the answer requires an uninterruptible power supply (UPS) or power quality mitigation technologies. Part 1 of this series “Why is my industrial process so sensitive to power blips?” described the power quality (PQ) event defined as a voltage sag, but known to many informally as a surge, blip or outage. Part 1 also presented a typical ac control circuit and components used in many industrial processes: 120-volt control power transformer (CPT) with ac “ice cube” relay(s), a programmable logic controller (PLC), adjustable speed drive (ASD), etc. The sources of process sensitivity were identified as control components vulnerable to voltage sags, or momentary reductions in supply voltage below 90% of nominal voltage. This article, part 2 identifies and examines possible solutions to the impact of voltage sags on sensitive control circuit components. These impacts may depend largely on the facility’s source of power. Is it supplied from a transmission circuit or a distribution circuit? Transmission systems tend to be more interconnected, and at much higher voltage levels, than distribution circuits.
What’s a voltage problem?
Figure 2: Mitigation curves for battery-less mitigation solutions (events above lines are mitigated). The coil hold-in device, for relays and contactors, can mitigate sags down to 25% of nominal. The constant voltage transformer can mitigate all sags above ~45% of nominal. The fast tap changer can mitigate all voltage sags above 50% of nominal as can the static series compensator. The latter also can mitigate very brief, deep sags and interruptions. DC modules can support dc controls through an interruption for 0.2 second up to 38 seconds.
8
•
May 2020
control engineering
The PQ data (5 years) for the system under Figures 1 and 2 reveals multiple interruptions at 0 volts, and many more, much deeper, and much longer voltage sags. This pattern is characteristic of distribution-fed electrical systems. Note that voltage sags above 90% of nominal, while recorded at right, should not be a problem as most equipment is designed to function normally at that level. Figure 1 illustrates the sensitivities of various control system components. The green, brown, red, blue, and dark green lines represent respectively a sensitive PLC, a sensitive dc power supply loaded at 100%, a sensitive ac “ice cube” relay, sensitive PLC input/output (I/O) connections, and the same sensitive dc power supply loaded at 50% of full load – a remarkable improvement merely by under loading the power supply. The process control circuit is most susceptible to voltage sag events largely because sophisticated power www.controleng.com
