INTRODUCTION

This electrical Current Transformer(CT) training course introduces basic safe operational and field diagnostics of CTs with a focus on safe operation, testing, and maintenance of the CTs normally installed in substations.

How are current transformers Classified?

CTs are classified according to structure, service, insulation, and tank design.

Different Parts of Current Transformer

1.   Top cover

2.   Oil level indicator

3.   Oil expansion device

4.    Metal box LV screen and         cores

5.    Bar-type or wound-type primary

6.    Paper-oil insulation

7.    Porcelain insulator

8.    LV screens

9.    Secondary terminal box

10.   Base

Parts of CT	Material Used for Construction
PRIMARY WINDING	ring type or hairpin type(high purity, annealed, high conductivity electrolytic copper)
SECONDARY WINDINGS:	Suitably insulated copper wire of electrolytic grade
PRIMARY TERMINALS	heavily tinned electrolytic copper or Aluminium alloy of 99.9% conductivity
SECONDARY TERMINALS	The studs, nuts, and washers shall be of brass, duly nickel plated.
Core	Core laminations shall be of CRGO silicon steel or other equivalent alloys
TANK	high-quality steel
PORCELAIN HOUSING	homogeneous, vitreous porcelain of high mechanical and dielectric strength
INSULATING MEDIUM ( OIL TYPE)	purified oil
Fittings and Accessories	Oil level gauge.   Oil filling hole and cap.   Pressure relieving device. Phase terminal connectors. Lifting lugs for core

Tests (Type Test/Special Test)Conducted for CT

• Lightning Impulse Voltage Test.
• High Voltage power frequency wet withstands voltage Test.
• Short-time current test.
• Temperature rise test.
• Instrument Security Factor Test.
• IP-55 Test on Secondary Terminal Box.

•  Radio Interference voltage test.
• Corona Extinction test.
• Thermal stability test.
• Thermal Co-efficient test.
• Fast transient test.
• Seismic withstand test.
• Mechanical terminal load on the bushing.
• Magnetization and internal burden tests..
• Effectiveness of sealing tests.
• Capacitance and dielectric loss angle test.

Routine tests conducted for CT

• Verification of Terminal Marking and polarity.
• Appearance and Dimensional Check.
• Measurement of Insulation Resistance.
• Power Frequency Dry withstands Tests on Primary and Secondary winding including primary intersections.
• Over – Voltage Inter turn test.
• Partial discharge Test for 400 KV,220 KV, and 132KV C.TS
• Knee point voltage and Excitation current measurement for ‘PS’ class cores.
• Secondary winding resistance measurement.
• Determination of errors.
• ISF Test.
• Leakage Test.
• Magnetization Characteristics of the Current Transformers.
• Turn ratio error on ‘PS’ class cores.
• Measurement of capacitance for 400 KV,220KV and 132KV C.TS.
• Measurement of the tan delta at 0.3, 0.7, 1.0, and 1. lUм/√3 for 400 KV,220KV &           132KV C.Ts.

CT Console box

It acts as the necessary interface between the equipment and the control room panel. The three-phase control wiring takes place for the purpose of the specific use of the CT.

Reasons for CT Failures

moisture entry into solid insulation

opening of the secondary winding

opening of tan delta point

dielectric failure due to premature aging

lOther dielectric failures due to the improper wrapping of paper, improper flux distribution, etc.

Conclusion

Current Transformers are the sensors for the electrical system and play the most vital role in the protection and measurement Circuit.