DIFFERENTIAL PROTECTION OF AUTOTRANSFORMERS USING HIGH-IMPEDANCE RELAYS BASIC INFORMATION AND TYPICAL SCHEMATIC DIAGRAM

Some utilities provide protection for large high-voltage and extra-high-voltage autotransformers by using voltage-operated bus-type high-impedance differential relays. Typical connections of this protective system for autotransformers, with the neutral point of the wye winding solidly grounded, are shown below.

Typical schematic connections for high-impedance differential protection of a Y autotransformer with unloaded tertiary

This arrangement provides protection for all types of phase faults and ground faults, but not turn-to turn

faults. In this application, three sets of three-phase CTs are required, one set on the high-voltage side, another set on the low-voltage side, and the third set in the neutral ends of the winding.

All CTs should have the same turns ratio and should be reasonably matched in accuracy class. A single high-impedance relay connected in a ground differential scheme is also applicable for autotransformer protection.

This protection is immune to the effects of magnetizing inrush current because inrush current is cancelled by the neutral CTs. Also, there is no imbalance current in the relay circuit due to the load tap changing equipment.

Thus a high-impedance differential relay can be applied without any harmonic restraint, load bias, or time delay. Autotransformers are often provided with a Δ tertiary winding. It should be noted that with this type of scheme no protection is afforded for faults occurring in the Δ tertiary winding.

Where the terminals for this winding are not brought out to supply load, one corner of the Δ can be connected between the end of one phase of the main winding and its neutral CT. This connection is shown above.

In such an arrangement, the tertiary winding is included in the differential protection zone, and the relay would sense ground faults in the tertiary winding. This scheme does not provide protection for phase faults or turn-to-turn faults in the tertiary winding.

Where the tertiary winding is used to supply load, the Δ winding corner connection cannot be used. Hence, separate protection is required.

Information can be found here.

VOLTAGE ACROSS AUTOTRANSFORMERS BASIC INFORMATION AND TUTORIALS

Below shows the voltage relations across an autotransformer and switching contacts during a tap changing cycle using an autotransformer designed for 60% circulating current and with 100% load current at 80% power factor flowing through it.

Perfect interlacing between the autotransformer halves is assumed, and the voltage drop due to resistance of the autotransformer winding is neglected.

A study of the figure will disclose the fact that increasing the magnetizing reactance of the autotransformer to reduce the circulating current will

1. Increase the voltage across the full autotransformer winding

2. Increase the voltage to be ruptured

3. Introduce undue voltage fluctuations in the line

Since B-4 and B-3 represent the voltages appearing across the arcing contacts when the bridging position is opened at A and B, the voltage rupturing duty will increase with

1. Increase in voltage between adjacent taps

2. Increase in load

3. Decrease in power factor of the load

4. Decrease in the magnetizing current for which the autotransformer is designed

Vector relations for bridging position AB—voltage across adjacent taps; A-1 and A-2— reactance volts due to load current in only half the autotransformer winding; A-3 and A-4—induced voltage across full auto transformer winding; B-4— voltage ruptured when bridging position is ruptured

at A; B-3—voltage ruptured when bridging position is ruptured at B.

YY AUTO 3Ø / NEUTRAL = PRIM YES-SEC NO AUTO TRANSFORMER CONNECTION AND DIGRAM

WHERE USED

For increasing voltage at the end of lines or to step up voltage where line extensions are being added to existing lines, such as from 6900 VAC to 7200 VAC. Cost per kva output is less than a two-winding transformer; losses are low, regulation is good, and exciting current is low. Voltage transformation greater than 3 to 1 is not recommended.

FOR POWER FROM A 3Ø, 4W SYSTEM

When the ratio of transformation from the primary to secondary voltage is small, the most economical way of stepping down the voltage is by using autotransformers as shown. For the application, it is necessary that the neutral of the auto transformer bank be connected to the system neutral. Brand circuits shall not be supplied by autotransformers.

CAUTION

Susceptive to burnouts if the system impedance is not great enough to limit the short-circuit current to 20 to 25 times the transformer-rated current. The primary neutral should be tied firmly to the system neutral; otherwise, excessive voltages may develop on the secondary side.

RATING & FUNCTION

A considerable saving in cost may often be experienced by using autotransformers instead of two-winding transformers. When it is desired to affect a small change in voltage, or where both high and low voltages are low, there is usually no reason why an autotransformer cannot be used as successfully as a two-winding transformer.

Autotransformers should not, except under special conditions, be used where the difference between the high-voltage and low-voltage ratings is great. This is because the occurrence of grounds at certain points will subject the insulation on the low-voltage circuit to the same stress as the high-voltage circuit.

Autotransformers are rated on the basis of output KVA rather than the transformer KVA. Efficiencies, regulation and other electrical characteristics are also based on output rating.

AUTOTRANSFORMER CONNECTIONS BASIC AND TUTORIALS

The autotransformer is both the most simple and the most fascinating of the connections involving two windings. It is used quite extensively in bulk power transmission systems because of its ability to multiply the effective KVA capacity of a transformer.

Autotransformers are also used on radial distribution feeder circuits as voltage regulators. The connection is shown below:

The boosting autotransformer connection. The output terminals operate

at a higher voltage than the input terminals.

The autotransformer shown above is connected as a boosting autotransformer because the series winding boosts the output voltage. Care must be exercised when discussing ‘‘primary’’ and ‘‘secondary’’ voltages in relationship to windings in an autotransformer.

In two-winding transformers, the primary voltage is associated with the primary winding, the secondary voltage is associated with the secondary winding, and the primary voltage is normally considered to be greater than the secondary voltage.

In the case of a boosting autotransformer, however, the primary (or high) voltage is associated with the series winding, and the secondary (or low) voltage is associated with the common winding; but the voltage across the common winding is higher than across the series winding.

The other possible connection for an autotransformer is shown below:

The bucking autotransformer connection. The output terminals operate

at a lower voltage than the input terminals.

The autotransformer shown is connected as a bucking autotransformer because the series winding bucks, or opposes, the output voltage. The key feature of an autotransformer is that the KVA throughput of the transformer, i.e., its capacity, is different than the KVA transformed by the common and series windings. The common and series windings are wound on the same core leg.


1. The volts per turn in the common winding equal the volts per turn in the series winding. The common winding voltage divided by the series winding voltage is equal to the number of turns in the common winding divided by the number of turns in the series winding.

2. The sum of the ampere-turns of the common winding plus the ampere- turns of the series winding equal the magnetizing ampereturns.

The magnetizing ampere-turn are practically zero, so the magnitude of the ampere-turns in the common winding is approximately equal to magnitude of the ampere-turns in the series winding. The series winding current divided by the common winding current is equal to the number of turns in the common winding divided by the number of turns in the series winding.

3. The KVA transformed in the series winding equals the KVA transformed in the common winding. The capacity multiplication effect stems from the fact that the metallic connection between the input and output circuits allows part of the KVA to flow though the connection and bypass the transformation.

50 MVA 138KV AUTOTRANSFORMER SPECIFICATION SAMPLE

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 accesories.

Rating:
              HV    -     30/40/50 MVA
              LV    -      30/40/50 MVA
             TV    -      12/16/20 MVA

Cooling Method:  OA/FA1/FA2

Rated Voltage:      HV -  138KV
                            LV -    69KV
                            TV -   13.2KV

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


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.
           
BIL:
      Winding             :    HV      –  650 KV
                                  LV       -  350 KV
                                  Neutral – 150 KV
                                TV       -  110 KV

     Bushing               : HV        -  650 KV
                              LV        -  350 KV
                                Neutral   - 150 KV
                                   TV          - 150 KV

Frequency : 60 Hz
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

Vector Group : Yyna0d1

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)

Temperature Rise Limits:
             Oil         -  65 deg C
             Winding – 65 deg C

% Impedances  @ 85 deg
                  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
     
           
Audible Sound Level @ 50 MVA with all fans running: 72dB

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

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 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 jointwill 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 2 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
                                              1000 Amps, Nominal Voltage: 52 KV
                                              Maximum Voltage to Gound: 52 KV, BIL 250 KV,
                                              Creepage Distance: 1080 mm
                                              Porcelain color: Brown
                                             Threaded Standard Rod


Complete with the following accessories:

31.  One (1) Buchholz Relay with 2 contacts for alarm & tripping
32.  Two (2) Dial type Oil Level Indicators for Main Tank & OLTC with contacts for alarm.
33.  One (1) Oil Temperature Indicator & Relay type AKM OTI series 34 for alarm.
34.  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.
35.  Qualitrol type self resetting mechanical Pressure Relief Device with contacts for tripping
36.  Conservator for main Tank - Sealed Diaphragm constant pressure type.
38.  Breather type conservator for OLTC.
39.  Annunciators (Marshalling Kiosk)
40.  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
       

41.     Galvanized Steel Radiators

42.     Bushing Terminals

         HV- Universal 4 hole NEMA Flat Terminals
         LV- Universal 4 hole NEMA Flat Terminals
         TV – Universal Multi -hole NEMA Flat Terminals

43.   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.

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

45.   Insulating Oil – Shell Diala B or equivalent

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

47.  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.

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

49.  Steel Ladder with caution marking

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

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

52. Welded Tank Cover.

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

54.  All wirings must be color-coded.

55.  With replacement gaskets

56.  Anchor Bolts

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

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

59. With provisions for parallel operation of existing power transformer using
      Master - Follower method of OLTC Control.
       
60. TESTS:
      The following tests shall be carried out at the factory with the presence
      of user representative and records of testing will be submitted.
         a. Winding  Resistance Tests
         b. Turns Ratio Tests
         c. Polarity & Phase Relation Tests on rated voltage
        d. Measurement of no-load losses and excitation current @ 90%, 100%
           & 110% of rated secondary voltage @ rated frequency
           rated voltage connection.
        e. Measurement of impedance voltage and load loss tests at rated current
           and rated frequency.
        f. Low frequency tests (Applied Voltage & Induced Voltage) including
           partial discharge measurement in terms of RIV.
        g. Leak Test.
        h. Routine test certificate for the bushings, current transformers and surge
            arresters shall be submitted.
        i. Temperature Rise Test at OA, FA1 & FA2 ratings on the tapping
          with maximum losses.
       j. Lightning Impulse Test on HV, LV , Neutral & Tertiary terminals.
        (Full wave, Chopped wave & reduced full wave)
      k. Audible Sound level Test at no-load and rated frequency and
         with all fans operating.
            l.  Measurement of zero-sequence impedance.
           m. Insulation Power factor
           n.  Insulation Resistance Tests at ambient temperature.
           o. Vacuum test on transformer tank, conservator & radiators; and pressure
               test on tank and oil-filled compartments.
           p. Determination of Capacitances (windings to ground & between windings)
          q.  Tests on auxiliary equipment & accessories ( functional tests
               including cooling control)
           r.  Voltage regulation
          s. Measurement of the power taken by the fan and oil pumps motors.
          t. Functional tests on Tap Changer
          u. Test on Current Transformers ( Check on polarity, ratio & wiring)
          v. Mechanical inspection, ( check of layout, dimensions, nameplate
             data, clearances, etc)
         w. Oil tests
         x. Efficiency at principal tap and full load for unity & 0.8 power factor.
       
61.  SPECIAL TEST
       Certificate of Short Circuit Test on power transformers of similar rating
       shall be submitted.
   
62.  Other Accessories, Tools

    a. Pressure gauge with nitrogen tube and automatic filling device which
         fill the transformert 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.
 
63.  Painting
      Special attention should be given to the protection of all iron-work.
      The methods propised 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.

ADVANTAGES AND DISADVANTAGES OF THE AUTOTRANSFORMER CONNECTION TUTORIALS

ADVANTAGES AND DISADVANTAGES OF THE AUTOTRANSFORMER CONNECTION
What Are The Advantages & Disadvantages Of Auto-transformer Connection? 


Summarizing the advantages of the autotransformer connection:

• There are considerable savings in size and weight.
• There are decreased losses for a given KVA capacity.
• Using an autotransformer connection provides an opportunity for achieving lower series impedances and better regulation.

Summarizing the disadvantages of the autotransformer connection:

• The autotransformer connection is not available with certain threephase connections.
• Higher (and possibly more damaging) short-circuit currents can result from a lower series impedance.
• Short circuits can impress voltages significantly higher than operating voltages across the windings of an autotransformer.
• For the same voltage surge at the line terminals, the impressed and induced voltages are greater for an autotransformer than for a twowinding transformer.

In many instances, the advantages of the autotransformer connection outweigh its disadvantages.

For example, when very large MVA capability is required and where a Grd.Y-Grd.Y connection is suitable, an autotransformer is usually the design of choice.

Because an autotransformer cannot provide a Δ-Y connection, autotransformers are not suitable for use as generator step-up transformers.

ADVANTAGES AND DISADVANTAGES OF THE AUTOTRANSFORMER CONNECTION

AUTOTRANSFORMERS CONNECTION ADVANTAGES AND DISADVANTAGES
What Are The Advantages And Disadvantages Of Autotransformer Connection?


Summarizing the advantages of the autotransformer connection:

 • There are considerable savings in size and weight.

• There are decreased losses for a given KVA capacity.


• Using an autotransformer connection provides an opportunity for achieving lower series impedances and better regulation.

Summarizing the disadvantages of the autotransformer connection:

• The autotransformer connection is not available with certain threephase connections.

• Higher (and possibly more damaging) short-circuit currents can result from a lower series impedance.

• Short circuits can impress voltages significantly higher than operating voltages across the windings of an autotransformer.

• For the same voltage surge at the line terminals, the impressed and induced voltages are greater for an autotransformer than for a two winding transformer.

In many instances, the advantages of the autotransformer connection outweigh its disadvantages.

For example, when very large MVA capability is required and where a Grd.Y-Grd.Y connection is suitable, an autotransformer is usually the design of choice.

Because an autotransformer cannot provide a Δ-Y connection, autotransformers are not suitable for use as generator step-up transformers.