--- title: "Parallel operation conditions and dark lamp synchronizing" subject: "Electric Machines II" chapter: 1 key: "semester_5/electric_machines_ii:parallel-conditions-dark-lamp" source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/parallel-conditions-dark-lamp site: BCT Engineering --- # Parallel operation conditions and dark lamp synchronizing Electric Machines II · Chapter 1: Three Phase Synchronous Generator - Asked in: 2076 Chaitra, 8 marks; 2070 Chaitra, 8 marks; 2069 Chaitra, 8 marks (asked 3 times) - Video: [SM33 Synchronization Methods](https://www.youtube.com/watch?v=wnXLevAYNo4) — Lectures in Electrical Engineering - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/parallel-conditions-dark-lamp - Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-1 ## Question What are the different conditions to be satisfied for parallel operation of alternators? Describe in detail the full process of synchronization using dark lamp method. ## Short answer Conditions: equal voltage, frequency, phase sequence, and phase agreement. In the dark lamp method, three lamps connected across like phases flicker together. The synchronizing switch is closed when all lamps are dark, indicating zero phase difference; prime mover speed is adjusted to make flickering very slow. ## Answer **Parallel operation** means connecting two or more alternators to the same bus-bars so that they share the load. Connecting an incoming alternator to live bus-bars is called **synchronising**. #### Conditions for parallel operation 1. **Equal voltage**: the rms terminal voltage of the incoming machine must equal the bus-bar voltage. 2. **Equal frequency**: the frequency of the incoming machine must equal the bus-bar frequency. 3. **Same phase sequence**: the phase sequence (R-Y-B) of the incoming machine must be the same as that of the bus-bars. 4. **In phase**: the incoming voltage must be in phase with the bus voltage at the instant of closing the switch (zero phase difference). 5. The waveforms should be of similar shape (both nearly sinusoidal). If these are not met, large circulating currents and heavy mechanical shocks occur at the instant of switching. #### Dark lamp method Three lamps are connected across the three poles of the open synchronising switch, i.e. each lamp is connected between a bus-bar phase and the **same** phase of the incoming alternator (R–R', Y–Y', B–B'). ```text Bus-bars R Y B | | | (L1) (L2) (L3) lamps across | | | open switch Incoming R' Y' B' alternator |_________|_________| G2 + prime mover ``` The voltage across each lamp is the difference of the bus voltage and the incoming voltage of the same phase. #### Procedure 1. Start the incoming alternator with its prime mover and bring it near synchronous speed. 2. Switch on its field and adjust the excitation until its voltmeter reads the bus-bar voltage. 3. Watch the lamps: - If all three lamps **become bright and dark together**, the phase sequence is correct. If they flicker one after another (rotating effect), the phase sequence is wrong; interchange any two leads of the incoming machine. - The rate of flickering equals the frequency difference $(f_1 - f_2)$. Adjust the prime-mover speed until the flickering is very slow. 4. When all three lamps are **dark** (at the middle of the dark period), the voltages are equal and in phase. Close the synchronising switch at this instant. 5. After closing, the incoming machine is "floating" on the bus; it takes load when its prime-mover input (steam/water) is increased. The reactive load is shared by adjusting the excitation. #### Merits and limitations - Simple and cheap; it checks phase sequence as well. - Lamps go dark at about one-third of rated voltage, so the exact instant of synchronism is uncertain; a voltmeter across a lamp or a **synchroscope** is used for accuracy in large stations. - Filament failure may be mistaken for darkness. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which set lists the conditions for paralleling an incoming alternator with bus-bars? A. Equal voltage, equal frequency, same phase sequence, in phase B. Equal power, equal speed, same number of poles, same kVA C. Equal current, equal torque, same phase sequence, same rating D. Equal voltage, equal kVA, same frequency, same load Answer: A. Voltage, frequency, phase sequence and phase agreement must match at the instant of closing. (Key point: Conditions) 2. How are the lamps connected in the dark lamp method? A. Each between different phases (cross-connected) B. Each across the open switch between a bus phase and the same phase of the incoming machine C. In series with the bus-bar of each phase D. Across the field winding of the incoming machine Answer: B. Lamps connect R-R', Y-Y', B-B' across the open synchronising switch. (Key point: Lamp connection) 3. What indicates correct phase sequence in the dark lamp method? A. The lamps glow one after another in rotation B. All lamps remain steadily bright C. All three lamps become bright and dark together D. All lamps remain steadily dark Answer: C. Correct sequence gives simultaneous flicker; rotating glow means two leads must be interchanged. (Key point: Phase sequence check) 4. When should the synchronising switch be closed, and why is the method limited? A. When lamps are brightest; bright means voltages equal B. As soon as the speed reaches synchronous; lamps are unnecessary C. Middle of the bright period; filaments burn out otherwise D. Middle of the dark period; lamps are dark over a range of voltage so the instant is uncertain Answer: D. Voltages are equal and in phase when lamps are dark, but darkness spans up to about 1/3 of rated voltage, so a synchroscope is used for accuracy. (Key point: Closing instant) ## Related in Electric Machines II - [Need for parallel operation and dark lamp method](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aparallel-operation-need-dark-lamp) — Electric Machines II, chapter 1 - [Parallel operation conditions and three lamp method](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aparallel-conditions-three-lamp) — Electric Machines II, chapter 1, asked 3× ## Related in other subjects - [Synchronizing alternators by dark lamp method](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-alternators-dark-lamp) — Electrical Machines, chapter 6 - [Criteria for synchronizing two alternators](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Asynchronizing-two-alternators-criteria) — Electrical Machine, chapter 6 - [Criteria for synchronizing two alternators](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Async-alternators-synchronizing-criteria) — Electrical Machines, chapter 6 - [Need and conditions for parallel operation of transformers](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Atransformer-parallel-operation-conditions) — Electrical Machines I, chapter 2