--- title: "Need and conditions for parallel operation of transformers" subject: "Electrical Machines I" chapter: 2 key: "semester_4/electrical_machines_i:transformer-parallel-operation-conditions" source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/transformer-parallel-operation-conditions site: BCT Engineering --- # Need and conditions for parallel operation of transformers Electrical Machines I · Chapter 2: Transformer - Asked in: 2082 Kartik (new course), 2+4 marks - Video: [Transformer Parallel Operation | Conditions for Transformer Parallel Operation | Hindi](https://www.youtube.com/watch?v=GY3nqk2Rpcs) — Electrical With Abhishek Joshi - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/transformer-parallel-operation-conditions - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-2 ## Question Why do we need to connect transformers in parallel? Also explain the conditions that need to be fulfilled for parallel operation of transformers with detailed explanation for operation. ## Short answer Parallel operation meets load growth, improves reliability and efficiency. Essential conditions: equal voltage ratios (to avoid circulating current), same polarity, equal per-unit impedances and X/R ratios for proportional load sharing; for three-phase, same phase sequence and vector group. ## Answer #### Need for parallel operation Two or more transformers are connected in parallel (primaries to a common supply bus, secondaries to a common load bus) because: - **Load growth:** a second unit can be added when the load exceeds the rating of the existing one. - **Reliability:** if one transformer fails or is taken out for maintenance, the others still supply (part of) the load. - **Efficiency:** at light load some units can be switched off so the rest run near maximum efficiency. - **Transport and spares:** one very large unit is hard to transport, and a small spare unit is cheaper to keep. ```text HV bus ==========+==============+===== | | [T1] [T2] | | LV bus ==========+==============+===== --> load ``` #### Conditions for parallel operation 1. **Same voltage ratio (same rated primary and secondary voltages).** If the ratios differ, the no-load secondary emfs are unequal and a **circulating current** $I_c = (E_A - E_B)/(Z_A + Z_B)$ flows round the loop even with no load. Since the impedances are small, even 1% difference gives a large current, which causes extra copper loss and heating and reduces the useful capacity. 2. **Same polarity** (essential). The terminals must be connected with correct polarity. With wrong polarity the two secondary emfs add in the closed loop, which acts like a dead short circuit, and very large currents flow. 3. **Same per-unit (percentage) impedance.** Load is shared in the inverse ratio of impedances: $S_A/S_B = Z_B/Z_A$. If the per-unit impedances are equal, each transformer takes load in proportion to its kVA rating, so none is overloaded while the others are under-loaded. 4. **Same X/R ratio.** If X/R differ, the currents in the two units are not in phase; the total current is less than the arithmetic sum, and the units work at different power factors, so the combined capacity is not fully used. 5. **For three-phase transformers, additionally:** - **Same phase sequence** (essential) – otherwise pairs of phases are short-circuited. - **Same vector group / phase displacement** (essential) – e.g. a Dyn11 unit cannot be paralleled with a Dyn1 unit, as the 60° phase difference between secondary voltages would cause huge circulating currents. Units of the same group (0°, 180°, −30°, +30°) can be paralleled. Conditions 2 and 5 (polarity, phase sequence, phase displacement) must be met exactly; conditions 1, 3 and 4 should be met as closely as possible. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is a reason for operating transformers in parallel? A. To raise the frequency of supply B. Reliability and load growth: other units keep supplying if one fails, and units can be added C. To avoid needing any impedance D. To make the voltage ratio unimportant Answer: B. Parallel units give continuity, flexibility and better efficiency at light load. (Key point: Reasons) 2. What happens if the voltage ratios differ? A. Nothing, as the load is shared by ratio B. A circulating current (EA − EB)/(ZA + ZB) flows even at no load, causing loss and heating C. The primaries short-circuit at once D. The frequency changes Answer: B. Impedances are small, so even 1% difference gives a large circulating current. (Key point: Unequal ratio) 3. How does load divide between two parallel units? A. Directly as their impedances B. Equally regardless of rating C. Inversely as their impedances; equal per-unit impedance gives sharing in proportion to kVA D. In proportion to turn counts Answer: C. SA/SB = ZB/ZA, so matched per-unit impedance avoids overloading one unit. (Key point: Load sharing) 4. Which conditions must be met exactly? A. Equal kVA rating and equal weight B. Equal X/R ratio and impedance C. Equal losses and efficiency D. Polarity, phase sequence and phase displacement (vector group) Answer: D. Wrong polarity, sequence or group causes a near short circuit; the others should be matched closely. 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