POWER TRANSFORMERS SPECIFICATIONS SAMPLE 33 MVA 69kV/ 13.8 kV

33 MVA POWER TRANSFORMER 

1 Unit – POWER TRANSFORMER
Type: Outdoor use, Oil Immersed, Core type, Copper Windings, Off-Load Tap Changer, 2 Windings with Rubber Diaphram Conservator,
manufactured according to ANSI C57.12.00 Std., for use as Step-down Transformer in an Electric Utility Distribution Substation. 
Complete with necessary accessories.
Rated Capacity : 20/26/33 MVA (OA/FA1/FA2)
No. of Phases : 3 – phase
Frequency : 60hz
H.V. Voltage : 67,000 Volts 
L.V. Voltage : 13,800/7967 Volts
Connection : HV – Delta
: LV – WYE
With neutral brought to a bushing & connected to a copper conductor equivalent to 4/o AWG  for grounding.
                        BIL : HV – 350 KV
 LV & Neutral – 110 KV
%IZ : 7.44 % (@ 75 deg C, 20 MVA base)


Specification of transformer

1. GENERAL DESCRIPTION
a. Scope
i. This specification covers the following transformer
ii. Phase Three
iii. Capacity 20/26/33 MVA (OA/FA1/FA2)
iv. Voltage 67000 – 13,800/7967V  
b. Applicable standards
i. The transformer covered by this specification will be designed and manufactured in accordance with ANSI C57.12.00 Std.
c. Service conditions
i. The transformer will be suitable for operations at its ratings under the following service conditions:
          Maximum ambient air temperature         40 deg C
          Maximum altitude above sea level          1000 meters
          Annual mean ambient temperature          30 deg C
          Maximum ambient relative humidity      88%
          Maximum wind velocity                          240 km/hr
          Maximum seismic factor                          0.45 g



2. TYPE
a. Oil-immersed, self-cooled (OA) / forced-air-cooled (FA) suitable 
for outdoor installation

3. RATINGS
a. Number of phase Three
b. Rated output 20/26/33 MVA (OA/FA1/FA2) 
c. Rated voltage
i. High voltage side 67000 V
ii.   Low Voltage side 13,800 V Line-Line / 7,967 V Line-Neutral
iii. H.V. Tap voltage 67000 V + - 2  X  2.5%
d. Rated frequency 60 Hz
e. Connections
i. High Voltage Delta
ii. Low Voltage Star with Neutral brought out
f. Polarity/ Vector group Dyn1(30 °C Angular displacement)

4. ELECTRICAL PERFORMANCE:

                 a.  At rated voltage, frequency, unity p.f., and 75 deg C (@ 20 MVA base)
                                    1. No-Load Loss                          24 KW
                                    2. Load Loss                                67 KW
                                    3. Efficiency                                  99.5%
                                    4. Exciting Current                        2.0%
                                    5. Voltage Regulation                    0.7%

b.  Temperature rise
1. Insulating oil 65 deg C
2. Windings 65 deg C

c.  Dielectric strength
i. Applied voltage (for 1 minute)
           1 H.V. - L.V  E.            140 kV
2 L.V. -  E.           38 kV
 
ii. BIL: Impulse 1.2/50 microseconds (kV) crest
1 H.V. windings 350 kV
2 L.V. windings 110 kV

5. CONSTRUCTIONS
a) Core:
The core of the transformer will be constructed of the highest quality, non-aging high permeability, cold-rolled gain-oriented  silicon steel sheet especially suitable for the purpose. Every care will be taken during slitting and cutting process to avoid burrs. Both sides of each sheet will be special glass film insulated on to minimize eddy current losses. The cores will be carefully assembled and rigidly clamped to ensure adequate mechanical strength to support the windings and also reduced vibration to minimum under operating conditions.
b) Windings:
The winding for the transformer shall be made of high tensile strength electrolytic copper of high conductivity ( Class A, in accordance with ANSI) and insulation ,material of high quality shall be free from burrs, scales and plinters.
 
The insulation material of windings and connections shall not shrink, soften or collapse during service. Thermally upgraded paper shall be used for conductor insulation. The design, construction and treatment of windings shall give proper consideration to all service factors, such as high dielectric and mechanical strength of insulation, coil characteristics, uniform electrostatic flux distribution, prevention of corona formation, and minimum restriction to oil flow.

Moreover, under any load condition, none of the material used shall disintegrate, carbonizer or become brittle under the action of hot oil.

The coils must be capable of withstanding movement and distortion caused by abnormal operating conditions. Adequate barriers shall be provided between windings and core as well as between high voltage and low voltage windings. All leads or bars from the windings to the terminal boxes and bushings shall be rigidly supported. Stresses on coils and connections must be avoided.

Due to very unfavorable short-circuit conditions and numerous short-circuits in the network, special measures have to be taken to increase the capability of the winding to withstand short-circuit currents. Winding and arrangement of coils shall be designed so as to unify the initial potential distribution caused by impulsive traveling waves, as much as possible, to avoid potential oscillation and in order to withstand abnormal high voltage due to switching.

To increase the capability of the transformer windings to witstand electromagnetic forces under short circuit conditions, modern technology in design and construction shall be applied. (e.g. low current density, provision of pressure limiting devices and spring elements, use of perfectly dried pre-compressed pressboard, maintaining a balance of ampere-turns between windings, ets.)

Measures against coil displacement as generated by the radial and longitudinal forces shall be considered. Computation of strenght against these forces including the description of the method being applied shall be submitted in detail.

The tank, conservation, coolers and bushings shall be adequately braced to withstand ocean shipment, and earthquake with seismic coeffecient of 0.45 g (horizontal)

               c)         Short Circuit Withstand Capability
The transformer shall withstand the combined effects of thermal, mechanical and electromagnetic stresses arsing under short-circuit conditions based on the maximum durations of fault:

High Voltage Winding: 2 seconds

Low Voltage Winding: 2 seconds

The maximum sustained short-circuit current in each windings shall be stated by the manufacturers. The maximum temperatures of the windings shall not exceed 250 deg C within second duration of fault. All transformer accessories, parts, components (CT's, bushings, tap-changer, etc.) shall be capable of withstanding the cumulative effects of repeated mechanical and thermal over-stressing as produced by short-circuits and loads above the nameplate rating.

For design purposes, the following network data shall be take into consideration. The available system fault currents as as follows (in rms):

67 KV : Ik" = 40 kA 13.8 kV : Ik" = 25 kA

The transformer shall be capable of withstanding the resulting successive short-circuits, without cooling to normal operating temperature between successive occurence of the short-circuit, provided the accumulated duration of short-circuit does not exceed the maximum duration permitted for single short-circuit defined above.

The upper limits of the symmetrical overcurrent due to such short-circuits as a multiple of rated current shall also be specified by the manufacturer.

      d)       Overload Capability
The short-time overload rating and operation of the transformer shall be in accordance with ANSI C57.92 or IEC 354. All other auxiliary equipment (bushings, CT's, etc) affected shall be rated to match the transformer overload rating.
e) Transformer Tanks:
The tank should have sufficient strength to withstand full vacuum and internal pressure of 1.0 kg/cm2, with cooling equipment & conservator connected. The tank cover will be clamped with bolts and nuts, and will be provided with handhole or manholes of suitable size. All seams and joint will be oil tight. Guides within the tank will be furnished to facilitate tanking and untanking, and to prevent movement of the core and coil assembly, in transit. The casing will be provided with suitable lugs for lifting the completely assembled transformer filled with oil. All gaskets will be synthetic rubber bonded cork.
f) Radiators:
The transformer will be provided with a number of sufficient radiators for self-cooled (OA) operation. The radiator will be installed on the tank via radiator valves, so that each radiator can be detached from the tank independently of the oil in the main tank. The radiator valves will have the open and close positions clearly marked. Radiators will be equipped with provisions for draining. Radiators shall be made of galvanized steel.
g) Forced-air-cooling system:
For forced-air-cooled (FA) operation, the transformer will be provided with automatically controlled three phase motor-fans actuated from winding temperature. Fan motor, weather proofed, three phase, Hz, and will be thermal protected. The cooling-fans will be mounted on the radiators and the control box will be mounted on the wall of the tank. Motor Voltage: 230 VAC, 3 phase, 60 Hz
h) On-load tap-changer:
The following tap-changer will be equipped on H.V. side having the following taps:

Tap 1               70,350 V
       2               68,675 V
       3               67,000 V
       4               65,325 V
       5               63,650 V
             
i) Oil preservation system:
Conservator system with sealed diaphragm will be used. Conservator with low-profile design having a moisture-proof barrier made with an oil-resisting diaphragm will be applied and placed at the level slightly higher than the transformer tank.
j) Bushing Assembly:
HV – ABB model: GOB 325/800LF123027-K 72.5 KV, 800 Amps, 350 KV BIL, Creepage Distance: 1980MM, Condenser type

LV & Neutral – Model # FB - 229Y - 33B
                        15 KV, 1500 Amps, 110 KV BIL 
 Creepage Distance: 400 mm
k) Painting:
The standard finish will consist of one rust resisting primary coat and two finish coats of paint. The paint will be applied after the steel surface had been thoroughly cleaned by shot blasting and treated with rust inhibitor. The color of the final coat for transformer will be ANSI 70 light gray.

                       l)         Audible Sound Level:
                                               The average sound level shall be 70dB @ 33 MVA 
6. ACCESSORIES
a. Off-load tap changer
b. Dial type Oil Level Indicator with contacts for alarm
c. Oil Temperature Indicator & Relay type AKM OTI series 34, with alarm & trip contacts
d. Drain valve with sampling cock
e. Upper filter press valve
f. Pressure Relief Device with Trip contact (Qualitrol, self resealing type)
h. Hand-hole
i. Buchholz relay with alarm and trip contacts
j. Lifting lug for the completely assembled unit filled with oil 
k. Lifting eye for the tank cover
l. Cooling fans, must be  3 phase, 230 VAC, 60 Hz, Winding temperature controlled for automatic operation, with automatic/manual change over switch. With Circuit Breakers for motor overload & short circuit protection.
m. Radiator and radiator valves
n. Stainless steel nameplate
o. Two (2) Winding Temperature Indicators & Relays  for HV & LV windings, with three (3) contacts each for alarm, tripping & fan control, AKM type WTI series 35
p. Terminal box (with fan control)
q. Diaphragm type conservator and conservator connection valve
r. Grounding pads for Grounding Cables for HV Arresters, LV Arresters & Neutral Conductor.
s. Dehydrating breather
t. Steel Ladder
u. Jacking pad
v. Anchor Bolts
w. Skid base with pulling eye
x.   Mounting Brackets for Surge Arresters nearest to the HV & LV bushings
y. Insulating Oil
z. Bushing Current transformers
i. HV : 400/300/250/200/100:5A, Class 0.3B-2.0
ii. LV & Neutral: 
                   2500/2000/1500/1000/500:5A, Relaying Class C-400
7. TESTS
The following tests will be carried out at our works with the presence of
user representative and the records of testing will be submitted.
a. Resistance measurements of the winding
b. Ratio test
c. Polarity and phase relation test on rated voltage
d.  Measurement of No-Load Loss and Excitation Current at 90%, 100% & 110% of rated voltage & frequency.  
e. Load loss, efficiency, regulation and impedance test at rated voltage
f. Insulation oil dielectric test
g. Applied potential test
h. Induced potential test
i. Impulse voltage test
j. Temperature rise test
k. Insulation power factor test on winding
l. Sound level test
m. Leak Test
n. Vacuum Tests on transformer tank, conservator & radiators; pressure tests on tank & oil filled compartments
o. Tests on auxiliary equipment & accessories ( functional tests only including cooling fans)
p. Voltage regulation

8.   Protection:
                 All power & control circuits must be protected by circuit breakers.

9. Wirings & Terminations:
 All wiring connections & terminations must be in accordance with  ANSI standard, using crimp type terminal lugs with insulator caps.

10.   Other Accessories, Tools

                 1. Terminal Lugs
                                          i.  HV Terminals                4 Holes NEMA (Flat)
                                          ii.  LV Terminals                Multi-Hole NEMA (Flat)

2.  Three-dimensional Impact Recorder with time period recording chart of at least 3-month period for use during transport of the transformer.

                 3.   Touch up paint (reserve for retouching works)

                 4.   Spare Gaskets for Assembly
                 
                 5.   Silica-gel breather 

6.    Pressure gauge with Nitrogen tube and automatic filling device which fill the transformer through the tube in case of any leakage shall be supplied.


      







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