Q: I see a big metal box with bushings on top - a test transformer? When should I use this instead of the reactor stack?
A: Yes - a conventional test transformer is a simple step-up transformer (220/380V in, 10–300 kV out). Best tool for small, low-capacitance loads like CTs, PTs, bushings, and short cables (< 25 m).
When to use it:
|
You are testing... |
Capacitance |
Test TX? |
Why? |
|
CT (current transformer) |
50–200 pF |
✅ Best |
Tiny load - TX drives it easily |
|
PT (potential transformer) |
100–500 pF |
✅ Best |
Same reason |
|
Small bushing |
100–500 pF |
✅ Best |
Simple resistive load |
|
Short cable < 100 m |
< 20 nF |
✅ Good |
Low enough charging current |
|
GIS bay (circuit breaker) |
5–50 nF |
✅ Acceptable |
Moderate C - OK with margin |
|
5 km power cable |
1 μF+ |
❌ Bad |
Huge current - see FAQ 5 |
|
Generator stator |
1 μF+ |
❌ Not good |
Same problem |
Approximate cost (new, 2025):
50 kV / 5 kVA test TX: $8,000–15,000
100 kV / 10 kVA test TX: $20,000–40,000
200 kV / 50 kVA: $50,000+ - but rare; people switch to SR at this level
Where used: Factory test labs (CT/PT/bushing mfr), switchgear factories, small repair shops.
Pro tip: "100 kV / 10 kVA" rating means it can supply 10 kVA into a resistive load at 100 kV. But into a capacitive cable, the charging current may exceed 10 kVA - even at correct voltage. That is the #1 way beginners destroy test transformers.