You've reached the review station. These questions summarize the entire transformer journey, from the basic principle to protection, and each question is accompanied by a link to the page that explains its answer in detail. Test yourself before checking the answers.
Basic Principles Questions
- Why doesn't a conventional transformer work on direct current? And what happens if it's connected to one? — Answer here
- What is the difference between energy transfer by conduction and by induction? — Working principle
- Explain the relationship between the number of turns, voltage, and current. — Turns ratio
- What is the difference between a step-up transformer and a step-down transformer? Where does each sit in the grid? — Step-up and step-down
- Why are transformers rated in kVA rather than kW? — Answer
Construction and Components Questions
- Why is the transformer core made of insulated laminations? — Answer
- What is the function of silica gel? And what does a color change mean? — Breather unit
- How does a Buchholz relay respond to a small amount of gas? When does it trip? — Buchholz relay
- Explain the symbol ONAF. What happens if the fans fail? — Cooling systems
- Compare a dry-type transformer with an oil-filled transformer in terms of cooling and application. — Components
Connections and Operation Questions
- What is the relationship between line voltage and phase voltage in star and delta connections? — Star vs. delta
- Why do distribution transformers need a neutral on the secondary side? — Dyn11
- What are the most important conditions for connecting two transformers in parallel? (turns ratio, vector group, phase sequence, impedance) — Vector group
- What is the difference between OLTC and OCTC? — Tap changers
Testing and Protection Questions
- Why use a megger instead of an ohmmeter for insulation testing? — Insulation resistance test
- What are the three sources of losses in a transformer? — Losses
- What does the open-circuit test measure, and what does the short-circuit test measure? — Open-circuit test and short-circuit test
- How does differential protection distinguish inrush current from a fault? — Inrush current
- Why is it prohibited to open a CT secondary under load? — CT and VT
Your calculated load is 990 kVA, and the available standard ratings are 900 and 1000 kVA — which do you choose? The answer: 1000 kVA (the next size up — never select a transformer smaller than the load), and with the usual 25% growth margin, it may be worth studying a 1250 kVA unit depending on the growth plan.
Sample answer: You should never select a transformer smaller than the calculated load, so 900 kVA is ruled out, and the next standard rating up, 1000 kVA, is chosen. In practice, a safety and growth margin is added (a common factor is 1.25), bringing the requirement to about 1240 kVA, at which point the appropriate next standard rating (such as 1250 kVA) is studied, balancing the purchase cost and no-load losses of an oversized transformer.
Excessively oversizing the transformer "to be safe." A transformer much larger than the load always operates at low partial load: no-load losses are paid for nothing, operating efficiency is lower, and the purchase cost is higher. Engineering sizing is a balance, not an inflation.
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