Q: My 50 kV / 5 kVA test transformer - boss says "just connect it to the 3 km cable." Something feels wrong. Am I right?
A: YOU ARE RIGHT. DO NOT DO THIS. This is the #1 beginner mistake that destroys transformers.
Problem #1: Massive overload
• 3 km 35 kV cable: C ≈ 0.5 μF. I_charge = 50,000 × 314 × 0.5×10⁻⁶ = 7.85 A.
• Required power: 50 kV × 7.85 A = 393 kVA.
• Your transformer is rated 5 kVA. You are asking for 78× rated power. The winding will smoke in seconds.
Problem #2: Waveform distortion
• Capacitive load → leading power factor → transformer core saturates.
• Output waveform becomes peaked - crest factor rises from 1.414 to 1.6–1.8.
• A 50 kV RMS test may produce 90 kV peak - uncontrolled overstressing.
Problem #3: Voltage regulation fails
• At no load, you set 50 kV.
• When the cable connects, the voltage drops to 10–20 kV (the transformer can't supply current).
• Turn up input → TX saturates more → voltage collapses → fuse blows.
When is a test transformer workable for cables?
|
Cable at 35 kV |
C |
Current @ 50 kV |
OK for 5 kVA TX? |
|
< 100 m |
< 15 nF |
< 0.24 A |
❌ Still 2.4× overload |
|
< 50 m |
< 8 nF |
< 0.12 A |
❌ 1.2× overload |
|
< 20 m |
< 3 nF |
< 0.05 A |
✅ Within rating |
Hard rule: Only use a test transformer for cables shorter than ~25 meters. For anything longer, use a series resonant system.
Quick Selection Guide
|
If you test... |
Choose... |
Reason |
|
CT, PT, bushing, short bus bar |
Test transformer |
Simple, cheap, enough power |
|
Switchgear / 1–2 GIS bays |
Test transformer or small SR |
Both work |
|
Distribution cable < 500 m |
Small SR (30–60 kV) |
Need Q-factor power saving |
|
HV cable > 1 km |
SR (100–300 kV) |
Only viable option |
|
Generator stator |
SR (20–30 kV high I) |
Clean waveform, high current |
|
Transformer induced test |
SR with > 100 Hz |
Frequency > rated needed |
|
Factory routine (same load) |
Test TX or fixed SR |
Standardized choice |
|
Field test (different loads) |
Modular SR |
Flexibility to reconfigure |