--- title: "Need for UPS and on-line UPS" subject: "Industrial Power Distribution and Illumination" chapter: 9 key: "semester_6/industrial_power_distribution_and_illumination:ups-online-industrial" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/ups-online-industrial site: BCT Engineering --- # Need for UPS and on-line UPS Industrial Power Distribution and Illumination · Chapter 9: Emergency and Backup Supply Systems - Asked in: 2072 Magh, 4 marks - Video: [On Line UPS (Basics, Block Diagram, Working, Applications & Specifications) Explained](https://www.youtube.com/watch?v=lKRW4fEB6bE) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/ups-online-industrial - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-9 ## Question Why UPS is required for industrial electrification? Explain about On-line UPS. ## Short answer UPS provides no-break clean power to critical loads during supply disturbances and bridges the gap until the DG starts. On-line UPS always supplies the load from the inverter via rectifier and battery, giving zero transfer time and full isolation; it has lower efficiency and higher cost. ## Answer #### Why UPS is required in industrial electrification A **UPS (Uninterruptible Power Supply)** gives continuous, clean AC power to critical loads during supply failure and disturbances. - PLCs, DCS/SCADA, computers and instruments lose data or trip on even a few milliseconds' interruption. - It protects equipment from voltage sags, spikes, surges, harmonics and frequency variation. - It bridges the 10–15 s gap until the DG set starts and takes load. - It enables safe, orderly shutdown of processes and avoids loss of production. - It supplies emergency lighting, communication, alarm and security systems. #### On-line UPS In an **on-line (double-conversion) UPS**, the load is **always supplied by the inverter**. 1. **Rectifier/charger** converts mains AC to DC, feeds the inverter and keeps the battery charged (float). 2. **Battery** is connected to the DC bus permanently. 3. **Inverter** converts DC to regulated sine-wave AC for the load. 4. **Static bypass switch** transfers the load to mains if the inverter fails or is overloaded. When mains fails, the battery continues feeding the inverter, so there is **zero transfer time**. ```text ┌────── static bypass ──────┐ AC ─►[Rectifier]─►DC─►[Inverter]───┴─► Load │ [Battery] ``` **Advantages:** no break, complete isolation from mains disturbances, stable voltage and frequency. **Disadvantages:** higher cost and losses (efficiency ≈ 90–94%). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does an industry need a UPS for PLCs, SCADA and computers? A. They lose data or trip even on a few milliseconds' interruption B. They need DC supply only C. They operate at a higher frequency than the mains D. They consume too much power from the mains Answer: A. Sensitive control and IT loads cannot tolerate even brief interruptions, so a no-break supply is required. (Key point: Need for UPS) 2. What gap does a UPS bridge in an industry with a DG set? A. The 10-15 s taken for the DG set to start and take load B. The several hours of a utility shutdown C. The 1 ms between supply cycles D. The time to refuel the DG set Answer: A. The battery-backed UPS carries critical loads during the DG start-up delay of about 10-15 s. (Key point: DG bridging) 3. In an on-line (double-conversion) UPS, what supplies the load during normal operation? A. The mains directly through the static bypass B. The battery directly C. The inverter, always D. A separate generator Answer: C. The load is always fed by the inverter, which is supplied by the rectifier and battery on the DC bus. (Key point: On-line UPS operation) 4. Why is the transfer time of an on-line UPS zero when the mains fails? A. The static bypass switch closes instantly B. The battery is permanently on the DC bus, so the inverter keeps running C. The DG set starts immediately D. The rectifier output is stored in a capacitor Answer: B. The battery is already connected to the inverter input, so nothing needs to switch when mains fails. 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