Power Transformer Faults and Protections Schemes

 Power Transformer:

An electric power transformer is a device that transfers electrical energy between two or more circuits through electromagnetic induction. It typically consists of two or more coils of wire wound around a core, and it can step up or step down voltage levels to facilitate efficient transmission and distribution of electrical power. Transformers play a crucial role in power distribution networks by adjusting voltage levels to minimize losses during transmission.

Type of fails that lead to faulty condition of transformer:

Transformer winding related failures:

Insulation deterioration because of;

• Moisture
• Overheating
• Voltage Surges
• Mechanical Stresses from through faults
• Vibration and impact forces due to through fault current

Transformer Bushing related failures:

• General aging
• Contamination
• Cracking
• Internal moisture

Transformer core related problems:

• Core insulation failure
• Core overheating
• Shorted lamination

On Load Tap Changer (OLTC) related failures:

• Malfunction of mechanical switching mechanism
• High resistance contacts
• Over Heating
• Contamination of insulating oil

Others causes:

• Current transformer (CT) related problems: (i) Oil leakage (ii) Oil contamination
• Metal particles
• Moisture

Types of Transformer faults:

• Internal faults
• External faults

Internal faults are two types:

• Slow acting faults
• Quick acting faults

Internal faults:

Faults develop inside Transformer tank, busing, conservators or radiators may be defined as internal faults. Transformer internal faults can be classified into two groups.

Quick acting faults:

• Electrical faults which causes serious damage immediate after the inception of faults.
• Phase to phase or phase to earth fault in winding, inter-turn fault in winding, core fault, busing failure etc.

Slow acting faults:

• Faults those develop slowly over time. These type of faults may suddenly convert into quick acting faults.
• Poor electrical connection of conductors that causes limited arching (partial discharge) in oil, leakage in Tank, clogged or contaminated oil formation of sludge in oil etc.
• Oil quality degradation causes rise of temperature even below full load operation.

External faults:

• continuous overloading
• power system faults
• over voltage
• under frequency
• environmental impact etc.

Overloading of Transformer reduces lifetime of transformer. Power system faults cause mechanical stress to transformer. Over voltage, transient over voltage or under frequency cause over fluxing which increases iron loss and damages insulation.

Protection measures applied in transformer:

• Transformer differential protection (87T)
• restricted Earth fault protection (87N)
• Time delayed over current and earth fault protection (51/51N)
• Instantaneous over current and earth fault protection (50/50N)
• Directional over current and earth fault protection (67/67N)
• Over fluxing protection (24)

Mechanical protection provided within transformer:

• Main tank buchholz protection (alarm/trip)
• OLTC buchholz protection (trip)
• Pressure relief device (PRD) protection(trip)
• Winding temperature protection(alarm/trip)
• Oil temperature protection(alarm/trip)
• Oil level (alarm)