Power Transformer – Protection & Testing

Author: Engr. Jahangir Amir

• PROTECTION OF POWER TRANSFORMER

1. DIFFERENTIAL RELAY:
   • Differential Zone (High Voltage – Current Transformer to Low Voltage -Current Transformer, Clear all fault Between them.
2. OVER CURRENT & EARTH FAULT RELAY:
   • None Directional Relay , Their Zone is the Circuit / Equipment On Which Installed
3. MAIN BUCHHULZ RELAY & TAP BUCHHULZ RELAY:
   • Detect the failure of the winding insulation will result in some form of arcing, which can decompose the oil into hydrogen, acetylene, methane, etc. localized heating can also precipitate a breakdown of oil into gas.
   • Detects the Incipient (Early Stage) Fault in the Transformer
   • It has Ability to Retain the Gas Developed within the Transformer After A Fault.
   • It has Two Contacts, One For Alarm And Other For Trip.
4. WINDING TEMPERATURE GAUGE & OIL TEMPERATURE GAUGE:
   • Watches the Winding and Oil Temperature of the Transformer
   • It has Three Contacts, which Operates at Three Temperature Settings Respectively;
   • 65*C Fans Start,        55*C Fans Off
   • 90*C Alarm On          80*C Alarm Reset
   • 100*C Trip Signal      90*C Trip Signal Reset
   • Note: Oil Temp: Also has Three Settings, But each 10*C Less.
5. PRESSURE RELIEF DEVICE:
   • On Severe Faults, Certain Pressure Develops in the Transformer Tank, Such as Explosion Vent Or Pressure Relief Valve.
6. RESTRICTED EARTH FAULT RELAY:
   • To Protect the Transformer from Earth Faults within the Windings,
   • It is Especially Required when Transformer is Resistance Or Impedance Earthed 
7. OVER FLUXING OR OVER EXCITATION:
   • Voltage Rise of the Transformer Supply (Load Drop)
8. NEUTRAL / BODY GROUNDING  :
   • Protection from the Heavy Fault Current
9. LIGHTING ARRESTOR (132 & 11 KV) :
   • Preventing from Lightening  

• TESTING OF POWER TRANSFORMER 

1. WINDING RESISTANCE TEST (MEGGER):
   • 15 Seconds = R15 (Form Factor =R60/R15)
   • 60 Seconds = R60
2. C&DF OF TRANSFORMER  & BUSHING:
   • Measurement Of Capacitance And Dissipation Factor Of The Insulation System Of Windings And Bushing ( For Minor Fault In Major Insulation)
3. TTR, TRANSFORMER TURN RATIO:           
   • Measurement Of Voltage Ratio
   • Check The Phase Displacement    
4. OPEN CIRCUIT OR NO LOAD TEST:
   • To Determine  No-Load Loss Or Core Loss And No-Load ‘I’ Which Is Helpful In Finding X And R
5. SHORT CIRCUIT OR IMPEDANCE TEST:
   • It Is Economical Method For Determining The Following;
     • Copper Loss At Full Load ( And At Any Desired Load), This Loss Is Used In Calculating The Efficiency Of The Transformer
     • Equivalent Impedance Z, Leakage Reactance X And Total Resistance R Of The Transformer As Referred To The Winding In Which The Measuring Instruments Are Placed
     • Total Voltage Drop In The Transformer As Referred To Primary Or Secondary  Can Be Calculated And Hence Regulation Of The Transformer Determined
6. WINDING  INSULATION RESISTANCE TEST:
   • Primary Winding To Secondary Winding
   • Primary Winding To Earth
   • Secondary Winding To Earth        
7. OIL INSULATION TEST:
   • Dielectric Strength Of Oil
   • Efficient Heat Transfer And Cooling
   • To Preserve The Core And Assembly
8. SEQUENCE IMPEDANCE (Z_1, Z_2, Z₀ ):
   • Positive And Negative Sequence Impedances (Z₁ And Z₂) Of A Transformer Are Identical
   • Zero Sequence Impedance (Z₀) Is Dependent Upon The Path Available For The Flow Of Zero Sequence Current And The Balancing Ampere Turns Available Within The Transformer
9. PERCENTAGE IMPEDANCE, Z%:
   • The %Z Of A Transformer Is The Volt Drop On Full Load Due To The Winding Resistance And Leakage Reactance Expressed As A Percentage Of The Rated Voltage,
   • It Is Also The Percentage Of The Normal Terminal Voltage Required To Circulate Full-Load Current Under Short Circuit Conditions