Transformers and Power Distribution Flashcards
6 cards from real 309A practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Transformers and Power Distribution flashcards as text
What is the purpose of paralleling transformers in a distribution system?
Answer: To increase capacity and provide redundancy
Paralleling transformers increases the total available capacity of the system and provides redundancy — if one transformer fails, the other(s) can continue to supply the load at reduced capacity.
What conditions must be met to parallel two transformers successfully?
Answer: Same voltage ratio, same impedance percentage, same polarity, and same phase relationship
Successful paralleling requires matching voltage ratios, impedance percentages (within 7.5%), polarity (for single-phase), and phase relationship (for three-phase) to prevent circulating currents.
What is the total kVA of a three-phase transformer bank using three single-phase 50 kVA transformers?
Answer: 150 kVA
The total three-phase kVA of a transformer bank equals the sum of the individual transformer ratings: 3 × 50 = 150 kVA.
What test is performed on a transformer to determine its core losses (iron losses)?
Answer: Open-circuit test
The open-circuit (no-load) test is performed with the secondary open and rated voltage applied to the primary. The measured power represents core losses (hysteresis and eddy current losses).
What is the percentage impedance of a transformer and why is it important?
Answer: The percentage of rated voltage needed to circulate rated current through a shorted secondary; determines fault current levels
Percentage impedance (%Z) is the percentage of rated primary voltage required to circulate rated full-load current through a short-circuited secondary. It determines maximum available fault current: Ifault = IFLA / (%Z/100).
What is the purpose of a grounding transformer in a delta system?
Answer: To create an artificial neutral point for grounding and ground fault detection
A grounding transformer (zigzag or wye-delta) creates an artificial neutral point on an ungrounded delta system, enabling ground fault detection and providing a path for ground fault current.