Transformers, Distribution, and Power Quality
What this domain tests
Transformer ratings and protection, distribution arrangements, fault levels, and the power-quality issues that affect real installations: harmonics, power factor, neutral loading and voltage unbalance.
Core rules
- Transformer size is quoted in kVA; phase current I = kVA × 1000/(√3 × V).
- Short-circuit current is set by impedance: I_sc = I_rated/(%Z/100). Distribution transformers are typically 4–6% Z.
- Vector groups state winding connection and phase displacement, e.g. Dyn11 is delta HV, star LV with neutral, LV leading HV by 30°. Parallel operation needs the same vector group, ratio, polarity and similar impedance.
- Tap changers give typically ±2.5% steps, off-circuit or on-load, to correct supply voltage.
- Triplen harmonics (3rd, 9th, 15th) add in the neutral, so neutrals may need to be full size or larger; 5th and 7th harmonics add heating and can resonate with power-factor capacitors.
- Capacitor banks are switched in steps, never connected on the load side of a VFD, and are detuned where harmonic levels are high.
Worked example
A 1000 kVA, 400 V transformer with 5% impedance. I_rated = 1 000 000/(1.732 × 400) = 1443 A. I_sc = 1443/0.05 = 28.9 kA. Every downstream device needs a breaking capacity above 28.9 kA, and the busbar short-time withstand must be checked against the same value.
Traps
- Treating %Z as an ohm value rather than a per-unit figure.
- Adding parallel transformer fault currents without combining impedances correctly.
- Assuming harmonic current cancels in a star neutral: triplens add.
- Ignoring transformer inrush when selecting HV protection.