Transformers, Power Quality, and Standby Systems
What this domain tests
Transformers, Power Quality, and Standby Systems tests transformer ratings and connections, voltage regulation, harmonics, UPS and generator systems, transfer switches, and separately derived systems. Expect questions on kVA, turns ratio, primary and secondary current, inrush, neutral switching, and grounding.
Core rules
- Transformer apparent power is rated in kVA: for three-phase, kVA = √3 × V_L × I ÷ 1000. Primary and secondary currents are inversely related by voltage ratio.
- A delta-wye transformer creates a separately derived system; bond the secondary neutral to ground at the transformer or first disconnect, and keep neutral and bonding separated downstream.
- Transformer impedance limits fault current and affects voltage regulation; high inrush can trip a primary breaker if it is not selected for the transformer.
- Harmonics from VFDs, LED drivers, and electronic power supplies heat neutral conductors and transformers; use appropriate K-rated or derated equipment and avoid shared neutrals where harmonic current is high.
- Standby generators need a transfer switch that switches the neutral where required, correct bonding, and load sequencing to avoid overload.
- UPS systems need bypass provisions, ventilation, and battery maintenance.
Worked example
A 75 kVA, three-phase transformer has a 600 V delta primary and a 120/208 V wye secondary. Primary current is 75,000 ÷ (1.732 × 600) = 72.2 A. Secondary current is 75,000 ÷ (1.732 × 208) = 208 A. The reasoning: kVA is constant, so the lower secondary voltage carries higher current. The primary protection and conductors are selected from the primary current and code rules, while the secondary panel and bonding are designed from the secondary current and the separately derived system requirements.
Traps
Using single-phase kVA formula on a three-phase transformer; ignoring transformer impedance when coordinating protection; bonding the secondary neutral in a downstream panel instead of at the source; failing to switch the neutral on a generator that requires it; undersizing a neutral for harmonic loads; forgetting UPS battery ventilation.
Key points
- Three-phase transformer current uses kVA ÷ (√3 × V_L).
- Delta-wye secondaries are separately derived systems.
- Bond the secondary neutral at the source, not downstream.
- Harmonics increase neutral and transformer heating.
- Transfer switches and generator neutrals must match the system grounding design.