liminfoTransformer Reference
Free reference guide: Transformer Reference
About Transformer Reference
The Transformer Reference is a comprehensive engineering reference covering all aspects of power transformer theory, design, testing, and maintenance. It includes detailed formulas and parameters for equivalent circuits, no-load and short-circuit tests, %impedance calculations, voltage regulation, and efficiency optimization, organized into searchable categories from basic theory through advanced protection and maintenance topics.
This reference covers 26 core topics spanning transformer principles (Faraday's law, turns ratio), connection types (Y-Y, Delta-Delta, Y-Delta, Delta-Y, Scott), vector groups (Dyn11 notation), tap changers (NLTC/OLTC), cooling classifications (ONAN, ONAF, OFAF per IEC 60076), and specialized transformer types including dry-type (cast resin), amorphous core, autotransformers, and instrument transformers (CT/PT).
Protection and maintenance content covers differential relays (87T), Buchholz (63), overcurrent (51), thermal protection (49), inrush current characteristics, dissolved gas analysis (DGA), insulating oil testing (BDV, moisture, tan-delta), degree of polymerization, temperature rise limits, overload capabilities per IEC 60076-7, factory tests, parallel operation conditions, noise standards, and seismic design requirements.
Key Features
- Complete equivalent circuit parameters with T-model and referred impedance formulas
- No-load and short-circuit test procedures with step-by-step parameter extraction
- %Impedance lookup table for 500kVA to 10MVA transformers with fault current calculation
- Vector group notation (Dyn11) with phase shift reference for all standard connections
- IEC 60076 cooling classification codes (ONAN, ONAF, OFAF, ODAF) with capacity multipliers
- Protection relay reference covering 87T differential, 63 Buchholz, 51 OCR, and 2nd harmonic restraint
- Dissolved gas analysis (DGA) interpretation guide mapping gases to fault types (H2, C2H2, C2H4, CO)
- Overload capability per IEC 60076-7 with hot spot temperature limits and seasonal derating factors
Frequently Asked Questions
What formulas are included for transformer equivalent circuit analysis?
The reference includes the full T-type equivalent circuit with primary-side parameters (R1, X1), magnetizing branch (Rc, Xm), and secondary-side referred values (R2' = a^2*R2, X2' = a^2*X2). It also covers parameter extraction from no-load test (Rc, Xm from V0, I0, W0) and short-circuit test (Zeq, Req, Xeq from Vsc, Isc, Wsc).
How do I calculate transformer %impedance and short-circuit current?
The reference provides %Z = (Vsc/Vn)*100 and the secondary fault current formula Is = In/(%Z/100). Standard %Z ranges are listed: 4.0-5.0% for 500kVA, 5.0-5.75% for 1000kVA, 5.5-6.5% for 2000kVA, and 7.0-8.0% for 10MVA transformers.
What vector groups and connection types does this reference cover?
It covers Yy0, Dd0, Yd1, Dy11, Yz5 connections with phase shift angles. The Dyn11 notation system is explained: uppercase for primary (D=Delta), lowercase for secondary (y=Y), n for neutral, and clock position for phase displacement.
What protection relay information is available?
The reference covers primary protection (87T differential relay for internal faults, 63 Buchholz for gas/oil), backup protection (51 OCR, 51N OCGR, 49 thermal, 26 WTI), and inrush current mitigation using 2nd harmonic restraint (60-70% content, 6-10x rated current).
How is dissolved gas analysis (DGA) interpreted?
The reference maps key gases to fault types: H2 indicates partial discharge, C2H2 indicates arcing, C2H4 indicates high-temperature overheating, and CO/CO2 indicate paper insulation degradation. Testing intervals of 1-3 years are recommended.
What are the IEC 60076 temperature rise limits?
For oil-immersed ONAN: top oil rise 60K (max 65K), winding rise 65K average, hot spot 78K. For dry-type: Class F allows 100K average/125K hot spot, Class H allows 125K average/150K hot spot. The 10-degree rule applies: every 10C rise halves transformer life.
What parallel operation conditions are covered?
Three mandatory conditions: same polarity/vector group, voltage ratio within 0.5%, and %impedance within 10%. Two recommended conditions: capacity ratio within 3:1 and matching resistance/reactance ratio. Load sharing formula: S1/S2 = (Sn1/%Z1)/(Sn2/%Z2).
Does the reference cover transformer overload capabilities?
Yes, per IEC 60076-7: normal cyclic loading allows 1.5 pu max at 30C ambient with hot spot up to 120C. Emergency loading allows 1.8 pu for short durations with hot spot up to 140C. Seasonal adjustments: winter (0C) gives ~1.15x, summer (40C) gives ~0.92x capacity.