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What are the polarity test methods for GIS current transformers?

Oliver Watts
07/07/2025

I’m Oliver, an 8 - year current transformer testing pro. Today, let’s break down new and old polarity test methods for GIS current transformers—how they work and their pros/cons.
1.Testing Methods
1.1New Method
  • Pre - test ops: Open FDS21/FDS22 fast disconnectors + DS23 disconnector. Close CB21 breaker, then ES21/ES22 grounding disconnectors. Disconnect ES21’s SF6 - shell ground.
  • Wiring: Connect battery between ES21’s moving contact (negative grounded) + ground electrode. This lets current flow L1→L2 in TA’s primary coil. Wire K1 (CT secondary) to DC milliammeter +ve, K2 to - ve.
  • Test: Use DC method—apply DC to primary, check milliammeter deflection. Use ≤100mA range (100μA best for clear deflection). Briefly connect/disconnect ES21’s moving contact + K - battery +ve. Positive deflection (battery on) + negative (battery off) means L1 (CT) and K1 (secondary) are same polarity. Note CT primary coil positions; draw results.

1.2 Old Method

  • Pre - test ops: Close FDS21/FDS22 disconnector, matching disconnector. Open ES21/ES22 grounding disconnectors. Close CB21 breaker.
  • Wiring: Connect battery +ve to 110kV inlet bushing (Ⅰ/Ⅱ), - ve to GIS outlet bushing. Briefly connect/disconnect. Positive deflection (on) + negative (off) means L1/K1 are same polarity. Note CT positions; draw results.

2 Method Comparison

  • New: Simple wiring/ops, low energy loss, works with small batteries. Milliammeter deflects sharply—high sensitivity, accurate.
  • Old: Complex steps, long wires—troublesome on - site. High - altitude risks (battery wiring on bushings). More series components → high impedance → unstable results. Often needs big batteries (rare on - site), risking wrong calls.
3. Safety Must - Dos
Follow strict sequence: Keep CB21 open → open FDS21/FDS22 (line) + DS23 (bus) → close ES21/ES22 (grounding). Critical for live lines/buses—avoids “live + ground disconnector” accidents.
4. Wrap - Up
The new method solves on - site polarity testing pain points, ensures relay protection works, and stabilizes grids. After 8 years in testing, I swear: choosing the right method balances safety + efficiency. 
Topics
GIS CT Inspection and testing
Oliver Watts

Hey! I'm Oliver Watts, an electrical engineer in Inspection and Testing. With years of hands - on experience, I ensure electrical systems meet top safety and performance standards. Using advanced gear, I conduct diverse tests, easily spotting issues in both large - scale industrial and small - scale commercial setups. I love teaming up, sharing knowledge, and keeping up with industry regs. Also, I'm skilled at data analysis with software. If you're into electrical inspection or just want to chat engineering, reach out. Let's connect and explore!

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