50 MVA 138 kV AUTOTRANSFORMER SPECIFICATION EXAMPLE

1. Type:   

Outdoor use, Oil-immersed, OA/FA/FA, 3 Windings with rubber diaphragm conservator vented via silica gel dehydrating breather, On-load-tap changer, manufactured according to ANSI C57.12.00 Std., All Copper Windings, For use as a Step-down transformer in an electric utility transmission substation. Complete with standard accessories.

2. Rating: 

HV    -     30/40/50 MVA

LV    -      30/40/50 MVA

TV    -      12/16/20 MVA

3. Cooling Method:  OA/FA1/FA2

4. Rated Voltage:     

HV -  138KV

LV -    69KV

TV -   13.2KV

5. Tap Voltage:   HV Side OLTC: 138 KV + 8, - 12 x 1.0%,  21 Taps

6. OLTC:

ABB type UZFRT 550/300, 138,000 Volts, 3 Phase, 60 Hz, 21 positions,

With Motor Drive Mechanism type BUF 3,

Motor : 460 Volts, 3 Phase, 60 Hz

Contactors: 230 Volts AC,

Position Transmitter: 230 Volts AC

Heating Element      : 230 Volts AC

With Manual/Automatic Change over-switch,  Raise & Lower pushbuttons.

            

7. BIL:

7.1    Winding:         HV      –  650 KV

LV       -  350 KV

Neutral – 150 KV

TV       -  110 KV

7.2    Bushing:           HV        -  650 KV

LV        -  350 KV

Neutral   - 150 KV

TV          - 150 KV

7.3    Frequency :     60 Hz

7.4    Connection:

HV  -  Star with Neutral (Auto-Star) brought out to a bushing

LV  -   Star with Neutral (Auto-Star) brought out to a bushing

TV  -   Delta

7.5    Vector Group : Yyna0d1

7.6    Guaranted Losses at rated voltage, frequency, unity pf & @ 85 deg C (50 MVA):        

No -Load Loss:         18.6 KW

Load Loss @ 50 MVA:   157.6 KW

Efficiency :    99.65% @ 50 MVA( Without Auxiliary Loss)

7.7    Temperature Rise Limits:

Oil :    65 deg C

Winding:        65 deg C

7.8    % Impedances  @ 85 deg C:

                        HV – LV@ 50 MVA         HV - TV@ 20 MVA         LV - TV@ 20 MVA

         8L     -   149,040 V   -   10.28               -         10.45

         N      -   138,000 V   -   10.50               -         10.31                   -            5.6          

        12R    -   121,440 V   -   11.09               -         10.52

   

  

            

8. Audible Sound Level @ 50 MVA with all fans running: 72dB

9. Service Condition:

Maximum ambient air temperature:                                   40 deg C

Average ambient air temperature for any 24h period:       30 deg C

Maximum altitude above sea level:                                   1000 meters

Maximum ambient relative humidity:                               88%

Mean annual rainfall:                                                         2400 mm

Maximum wind velocity:                                                   220 km/hr

Maximum seismic factor:                                                  0.45g

10. 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 of the transformer shall be made of  high tensile strength electrolytic copper of a high conductivity (Class A, in accordance with ANSI)  and insulation material of high quality shall be used. The windings shall be  free from burrs, scales and splinters.

          

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 strenght 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 strength 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 arising under short-circuit conditions based on the maximum durations of fault:

Primary  Winding:                   2          seconds

Secondary  Winding:              2          seconds

Tertiary    Winding                 2          seconds

The maximum sustrained 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 the seconds 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):

138 KV: Ik"  60 KA         69 KV : Ik" = 50 kA                     13.2 KV : Ik" = 40 kA

The transformer shall be capable of withstanding the resulting successive short-circuits, without cooling to normal operating temperature between successive occurrence 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 hand hole 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: 460 VAC, 3 phase, 60 Hz.

h)   On-load tap-changer:

The following tap-changer will be equipped on H.V. side for the regulation of voltage under loading conditions.

Type                                        Type UZFRT 550/300

                                                3 phase,60 Hz, 21 positions

Number of tap positions          21 taps positions (138KV + 8 X 1380 V, -                                                    12 X 1380 V)

Manufacturer                          ABB

Motor Drive Mechanism:

                                    Type:                                        ABB type BUF 3

                                    Motor Voltage:                       460 Volts, 3 Phase, 60 Hz

                                    Contactors Voltage:                  230 VAC

                                    Position Transmitter:                 230 VAC

                                    Heating Element:                       230 VAC

                             

Motor-Drive Mechanism Accessories:

                                   1. Standard Accessories

                                   2. Phase Failure Relay

                                   3.Circuit Breakers for Control & Auxiliary circuits

4. Accessories for paralleling with 3 transformers using MASTER-   FOLLOWER method.

OLTC Accessories:

                                   1.  Oil Conservator

                                   2.  Oil Level Indicator with contacts for Alarm

                                   3.  Dehydrating Breather

                                   4.  Pressure Relief Valve/Device with contacts for tripping

                                   5.  Pressure Relay with contacts for tripping

                                   6. Oil Flow Controlled Relay with contacts for alarm

                                   6. Thermoswitch Housing

                                   7. Valve for oil filtration mounted on the top

                                   8. Valve for oil filling, draining & filtration

                                   9. Earthing terminal

                                  10. Prepared for on - line oil filter unit

                                   

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)    Bushings:

Primary:           ABB type GOB 650-1250-0.3 Brown, Cat # 123 193-K

1250 Amps, Nominal Voltage: 170 KV rms,

Phase to Earth Voltage:145 KV rms, BIL: 650KV,

 Creepage Distance: 4080 mm

 Porcelain Color: Brown

 Short end shield

                        Secondary:       ABB type GOB 380-800-0.3 Brown, Cat # 123-185-K 

800 Amps, Nominal Voltage: 100 KV rms,

Phase to Earth Voltage:72.5 KV rms, BIL: 380 KV

Creepage Distance:2210 mm

Porcelain Color: Brown

Short end shield

                       Tertiary:             CEDASPE s.p.a. Italy type Dt 30 Nf 1000

                                                 1000 Amps, Nominal Voltage: 36 KV,

                                                 Maximum Voltage to Ground: 30 KV, BIL:170 KV,

                                                 Creepage Distance: 640 mm

                                                 Porcelain Color: Brown

                                                 Threaded Extended Rod

                      Neutral:              CEDASPE s.p.a. Italy type Dt 52 Nf 1000

                                                 1,000 Amps, Nominal Voltage: 52 KV

                                                 Maximum Voltage to Gound: 52 KV, BIL 250 KV,

                                                 Creepage Distance: 1080 mm

                                                 Porcelain color: Brown

                                                 Threaded Standard Rod

11. Complete with the following accessories:

11.1           One (1) Buchholz Relay with 2 contacts for alarm & tripping

11.2           Two (2) Dial type Oil Level Indicators for Main Tank & OLTC with contacts for alarm.

11.3           One (1) Oil Temperature Indicator & Relay type AKM OTI series 34 for alarm.

11.4           Three ( 3) Winding Temperature Indicators & Relays for HV, LV & TV windings with 3 contacts each for alarm, tripping & fan control, AKM type WTI series 35.

11.5           Qualitrol type self resetting mechanical Pressure Relief Device with contacts for tripping

11.6           Conservator for main Tank - Sealed Diaphragm constant pressure type.

11.7           Breather type conservator for OLTC.

11.8           Annunciators (Marshalling Kiosk)

11.9           Bushing Current Transformers

HV:     300/200/100:5A;    0.6 - B 0.5

LV & Neutral:  600/500/400/300/200/100:5A;   C-400

TV :    1200/1000/900/800/600/500/400/300/200/100:5A;  C-400

11.10         Galvanized Steel Radiators

11.11         Bushing Terminals

HV- Universal 4 hole NEMA Flat Terminals

LV- Universal 4 hole NEMA Flat Terminals

TV – Universal Multi -hole NEMA Flat Terminals

11.12      Sets of Surge Arresters mounted nearest to the HV, LV & TV transformer bushings, with Surge Counters & 4/0 AWG THW Copper conductors connected to Grounding terminals.

HV     : 120 KV Voltage Rating, 98 KV MCOV, Station Class, Polymer housing, Metal Oxide, Line Discharge Class 4 per IEC, 12 KJ/KV Energy capability 65 KA Pressure Relief Capability, Grey Silicone Insulator ABB type PEXLIM-P Complete with top clamps to hold a 336.4 MCM Aluminum Conductor and 4/0 AWG THW Copper green wire ground conductor connected to ground terminal.

LV     :  60 KV voltage Rating, 48 KV MCOV, Station Class, Polymer housing, Metal Oxide, Line Discharge Class4 per IEC, 12 KJ/KV Energy Capability 65 KA Pressure relief Capability, Grey Silicone Insulator, ABB type PEXLIM-P Complete with top clamps to hold a 795 MCM Aluminum Conductors and  a 4/0 AWG THW Copper green wire ground conductor connected to ground terminal.

TV     :18 KV Voltage Rating, 15 KV MCOV Station Class, Polymer housing, Metal Oxide, Line Discharge Class 3 per IEC, 9.0 KJ/KV energy capability 65 KA pressure relief capability, Grey Silicone Insulator, ABB type POLIM-S 15N Complete with top clamps to hold a 795 MCM Aluminum Conductors and a 4/0 AWG THW Copper green wire ground conductor connected to Ground terminal.

11.13      Neutral Conductor: 4/0 AWG THW Copper wire colored green connected to ground pad.

11.14      Insulating Oil – Shell Diala B or equivalent

11.15      With provision for Built-in OLTC Insulating Oil Filter Machine, such as mounting brackets, connecting flange, connecting valves, etc.

11.16      Cooling Fans must be 3 Phase, 460 VAC, 60 Hz, Winding Temperature Controlled for Automatic Operation, with automatic/manual change over switch.            With Circuit breakers for motor overload & short circuit protection.

11.17      Grounding Pads for HV Arresters, LV Arresters, TV Arresters & Neutral Cables.

11.18      Steel Ladder with caution marking

11.19      All External Power & Control cables must be flexible, multicore,  PVC insulated & enclosed in conduit pipes & flexible hoses.

11.20      All power & control circuits must be protected by circuit breakers.

11.21      Welded Tank Cover.

11.22      All wiring connections & terminations must be ANSI standard using crimp type terminal lugs with insulator caps.

11.23      All wirings must be color-coded.

11.24      With replacement gaskets

11.25      Anchor Bolts

11.26      One (1) Spare bushing each for HV, LV & TV

11.27      One (1) Spare OLTC Tap Position Indicator for remote use

11.28      One Set - Paralleling Equipment for 3 OLTC Power Transfomers using Master - Follower method of OLTC Control.

        

12.       TESTS:

The following tests shall be carried out at the factory with the presence of user representative and records of testing will be submitted.

12.1     Winding Resistance Tests

12.2     Turns Ratio Tests

12.3     Polarity & Phase Relation Tests on rated voltage

12.4     Measurement of no-load losses and excitation current @ 90%, 100% & 110% of rated secondary voltage @ rated frequency on rated voltage connection.

12.5     Measurement of impedance voltage and load loss tests at rated current and rated frequency.

12.6     Low frequency tests (Applied Voltage & Induced Voltage) including partial discharge measurement in terms of RIV.

12.7     Leak Test.

12.8     Routine test certificate for the bushings, current transformers and surge arresters shall be submitted.

12.9     Temperature Rise Test at OA, FA1 & FA2 ratings on the tapping with maximum losses.

12.10   Lightning Impulse Test on HV, LV , Neutral & Tertiary terminals. (Full wave, Chopped wave & reduced full wave)

12.11   Audible Sound level Test at no-load and rated frequency and with all fans operating.

12.12   Measurement of zero-sequence impedance.

12.13   Insulation Power factor

12.14   Insulation Resistance Tests at ambient temperature.

12.15   Vacuum test on transformer tank, conservator & radiators; and pressure test on tank and oil-filled compartments.

12.16   Determination of Capacitances (windings to ground & between windings)

12.17   Tests on auxiliary equipment & accessories ( functional tests including cooling control)

12.18   Voltage regulation

12.19   Measurement of the power taken by the fan and oil pumps motors.

12.20   Functional tests on Tap Changer

12.21   Test on Current Transformers ( Check on polarity, ratio & wiring)

12.22   Mechanical inspection, ( check of layout, dimensions, nameplate data, clearances, etc)

12.23   Oil tests

12.24   Efficiency at principal tap and full load for unity & 0.8 power factor.

       

13.       SPECIAL TEST

Certificate of Short Circuit Test on power transformers of similar rating shall be submitted. 

14.       Other Accessories, Tools

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

b.      Three-dimensional impact recorder with time period recording chart of at least 3 months for use during transport of the transformers.

c.      Silica -gel breathers for main and OLTC conservators.

  

15.       Painting

Special attention should be given to the protection of all iron-work. The methods proposed and the means adopted should be fully described in the offer.

All surfaces shall be thoroughly cleaned of rust, scale, grease and dirt and other foreign matters and all imperfections shall be removed by means of approved methods.

The following treatments shall be applied:

a.  External surfaces

All steel surfaces shall be sand-blasted in accordance with SIS 055900, and shall then be painted in the following sequence:

        

1.   two (2) primer coats:   2 x 35 um (micrometer)

      Binder  :          epoxy resin hardened with polyamide              

            Pigment:          titanium dioxide, zinc oxide, zinc phosphate, tinting additives

2.   one (1) intermediate coat:                     35 um

            Binder:  epoxy resin hardened with polyamide

            Pigment: titanium dioxide, micaceous iron oxide, tinting additives

3.   one (1) top coat (polyurethane base):    35 um

Binder:  epoxy resin hardened with isocyanat

Pigment: titanium dioxide, micaceous iron oxide, tinting additives

Coating thickness:                   Total        140um

The color code shall be Munsell Gray No. N7.0

        b. Internal surfaces

Inside the transformers vessel, sand-blasting shall be performed in accordance with SIS 055900. After that solvent-free, oil-resistant coating shall be applied. The minimum dry film thickness shall be 40 um

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