Power Transformer

Power transformer 

Standards for Installation, Commissioning and Operation

•          Manufacturer’s O & M manual

•          IS 10028 ( Part 1,2,3) : 1981 selection ,operation and Maintenance of Transformer.

•          ANSI/IEEE C57.11.: 1980 guide for installation of Transformer

•          ANSI/IEEE C57.12 : 1980 guide for installation of EHV Transformer (above 345 kV)

•          CBIP Manual - Section K ( Installation & Commissioning)

•          IEC 60 354 (1991), IS 6600 : Loading guide for oil immersed Transformers

•          IS 1866 ( 1983)    : Maintenance and supervision of mineral Oil

•          Assemble all accessories , keep the transformer under vacuum for 12 ~ 24 hrs. to remove surface moisture , fill with processed oil, conduct hot oil circulation using filter machine, maintain standing time for 2 ~3 days before commissioning 

MVA Rating of Transformer

•          MVA ratings are standardized on preferred number series- Geometric series R₅ (multiplier 1.6) & R₁₀ (multiplier 1.25)

10, 10n, 10n² , 10n³ … Where n= multiplier  (ISO 3 & IS:1076 - 1985)

R₅ Series (Above 10 MVA)

1

1.6          16

2.5          25

4              40

6.3          63

10           100

R₁₀ Series (up to 10 MVA)

1              10

1.25        12.5                       

1.60        16

2.00        20

2.5          25

3.15        31.5

4.0          40

5.00        50

6.30        63

8.0          80

10.0        100 Series

Typical values of short circuit impedance of transformers :

Impedance voltage at rated current at principal tapping as a % of rated voltage. (Clauses 8.1.6.3 of IS 2026)

Rated Power                      Impedance Voltage

   KVA                                             Percent

up to 630                                             4

631-1,250                                             5

1,251-3,150                                         6.25

3,151-6,300                                         7.15

6,301-12,500                                       8.35

12,501-25,000                                    10.00

25,001-200,000                                  12.50    

Parallel Operation

- Voltage Ratio, Percent impedance at principal and extremes tappings and tapping range shall be within the tolerance as per std.

                voltage ratio ±0.5%;

                percentage impedance

                                ±7.5%    if impedance value >10%

                                ±10%     if impedance value < 10%

- Phase angle relation (clock hour number) shall preferably be same. Additional combinations are possible (see IEC 60076-8).

- KVA ratings of transformers shall preferably be within 1:2 ratio.

When there is a mismatch in voltage ratio or percentage impedance, the transformer with lower terminal voltage will get overloaded.

A mismatch of relative loading within 10 % between two transformers of non-identical design should be regarded as reasonable. 

Commissioning Test

Ratio Measurement , Winding resistance & Polarity test

Single phase low voltage excitation current

Oil properties test , IR and Power factor test

Control checks for cooler , fans, pumps and tap changer

Core earthing by Megger

Benchmark tests

IR, Power factor , single phase excitation current at 230V

DGA, Furane measurement by HPLC ( high precision liquid chromotography)

Polarization signature , Frequency Response Analysis

Low Voltage Impulse Test

Oil Particle Test & Oil Properties

Routine Maintenance

Oil Leak , hotspots at terminal connection , surface corrosion , oil level

Monitoring of oil properties ( BDV, Moisture,Tan delta)Limits as per IS 1866

Silicagel condition , bearing of fans, gas in buchholz

Special Maintenance

Re conditioning / re claimation of oil

DGA , Frequency Response Analysis , Polarization Signature

On Line Monitoring

Monitoring of combustible gases in Oil ( ABB T Monitors, Hydrans etc)

Monitoring Hot spot in winding

Monitoring condition of tap changer ( motor current, torque, temp. difference)

Monitoring Partial discharges

Monitoring of Tan delta of Bushings.