--- title: "Unit generator-transformer schemes" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:unit-generator-transformer-schemes" source: https://www.bctengineer.com/q/semester_8/power_plant_design/unit-generator-transformer-schemes site: BCT Engineering --- # Unit generator-transformer schemes Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2074 Magh, 5+3 marks; 2071 Magh, 5+3 marks; 2070 Bhadra, 5+3 marks (asked 3 times) - Video: [How Power Plant Works II Generator Transformer Unit II Single Line Diagram](https://www.youtube.com/watch?v=OuiGXfYk5wo) — Smart Educare - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/unit-generator-transformer-schemes - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question Explain any two scheme of unit generator transformer combination used in power plant on the basis of fault level, maintenance outage and efficiency. Also draw the single line diagram for both of the scheme. ## Short answer Unit scheme: one generator per GSU; LV fault level lower; transformer fault affects only that unit; each transformer runs at full load, idle transformer can be disconnected. Grouped scheme: two generators share one GSU via LV bus; LV fault level higher; transformer fault stops both units; single large transformer gives lower total losses at full load but no-load loss remains when one unit runs, and switchyard cost is lower. ## Answer In a power plant, the generator is connected to the grid through a step-up transformer. Two common schemes are (1) the **unit scheme**, one generator to one transformer, and (2) the **grouped (combined) scheme**, two generators to one transformer. #### Scheme 1: Unit connection (one generator – one transformer) ```text G1 G2 | | GCB GCB |---UAT |---UAT GSU1 GSU2 | | HV CB HV CB | | ===================== HV bus ``` - **Fault level:** a fault on the generator bus is fed by one generator plus the grid through one GSU impedance; LV fault level is lower, so GCB and bus duct ratings are smaller. - **Maintenance outage:** a GSU fault or maintenance takes out only its own unit; other units keep running. Each unit is independent. - **Efficiency:** each transformer runs close to full load when its unit runs; when a unit is off, its transformer can be disconnected, so no-load losses are saved. But total no-load and load loss of several small transformers is higher than one large transformer, and cost is higher. - **Reliability:** highest; widely used for large units. #### Scheme 2: Grouped connection (two generators – one transformer) ```text G1 G2 | | GCB1 GCB2 | | ================ LV (11 kV) bus |---- Station transformer GSU (2 x unit rating) | HV CB | ================ HV bus ``` - **Fault level:** a fault on the LV bus is fed by both generators plus the grid, so the LV fault level is much higher; GCBs and bus need higher breaking capacity. - **Maintenance outage:** a GSU fault or maintenance takes out **both** units, so energy loss is larger. A generator can be taken out by its own GCB while the other runs. - **Efficiency:** one large transformer has lower cost per MVA and lower total losses at full load; but when only one unit runs (dry season), the transformer runs at half load and its full no-load loss remains. - **Cost:** fewer transformers, HV breakers and bays, so cheaper and smaller switchyard; common for small and medium hydro in Nepal. #### Comparison | Basis | Unit scheme | Grouped scheme | |---|---|---| | LV fault level | Lower | Higher | | GCB rating | Smaller | Larger | | Outage on transformer fault | One unit | Both units | | Transformer losses | Higher total, but idle transformer can be switched off | Lower at full load; no-load loss always present | | Switchyard cost | Higher (more bays) | Lower | | Flexibility | High | Lower | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which scheme has the higher LV fault level? A. Unit scheme B. Both equal C. Grouped scheme, since both generators feed the LV bus D. Neither Answer: C. In the grouped scheme two generators feed a fault on the common LV bus, so GCBs need higher breaking capacity. (Key point: Fault level) 2. A transformer fault in the grouped scheme: A. Takes out only one unit B. Takes out both units C. Has no effect D. Takes out the whole grid Answer: B. Both generators share one GSU, so both are lost; in the unit scheme only one unit is lost. (Key point: Maintenance outage) 3. Which is correct about the grouped scheme? A. Higher total losses and more bays B. Idle transformer is switched off C. One large transformer gives lower loss and cost, but at half load (one unit) its no-load loss remains D. It is the most reliable scheme Answer: C. Fewer, larger transformers are cheaper and more efficient at full load, but no-load loss always exists. 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