In the transformer field, development of new design with computer aided. measurement (CAM), high quality, low power loss yoke materials, high grade

Transcription

1 CHAPTER - 5 TRANSFORMER 5.1 Introduction In the transformer field, development of new design with computer aided measurement (CAM), high quality, low power loss yoke materials, high grade insulation, quality conductor have resulted maximum efficiency. At present highest ratings of transformer has gone upto 1500 MV A in 1200 KV with life expectancy of 30 years [33], The operation is smooth with troublefree maintenance. In the manufacturing the designs and testings have been made very7 stringent for short circuit, high voltage, impulse test, at maximum ambient temperature rise. To meet the maximum stress level high grade insulation materials are used. Intelligent digital differential relay is incorporated with on load tap changing by auto synchronous regulators [33]. Amorphous magnetic core materials, resin cast dry type and foil windings have become successful for energy saving point of view. Some further steps are suggested on transformer design for reliable and troublefree at optimum efficiency. The field commissioning results have been projected to ensure the validity of modified design from energy saving point of view. 5.2 Power Transformer Traditional Materials used in construction In general transformers consists of two coils having mutual inductance over a laminated steel core in a common magnetic field. The core is sheet steel cold rolled grain oriented (CRGO) plain strip lamination grade 51 of size 0.35 mm thickness. It is fully processed and stress relief annealed with an area of 301 to 1500 sq. mm for 5 MV A rating. The continuous magnetic path has minimum air

2 CJu-5 Transformer 5. 2 gap. The steel is high silicon content, heat treated to achieve high permeability and low hysteresis loss at usual flux density. By lamination the eddy cunent loss is minimised. The laminations thickness is 0.35 mm for 50 Hz and 0.5 mm for 25 Hz. The joints in alternate layer are staggered to avoid persuance of normal air gap right through the cross sections. This staggering is called imbricated. The heart of the transformer is the laminated sheet steel core of 3 limbs. Each limb caries two concentrically wound coils. Heavy gauge secondary inside and lower gauge primary outside with the provision of tappings. For a 5 MVA capacity 10 mm x 3 mm electrolytic copper strip and 0.5 mm size double covered insulated paper are used. The demerits of this construction are: More power loss is in laminated core. There is harmonic current loss for nonlinear loading. The design is not compact for easy and simple installation. Maximum life of 20 to 25 years. Does not give trouble free service. Maximum efficiency and maximum load do not compromise at higher percentages. To improve the efficiency and to reduce the losses some attempts have been made Modifications on design, materials use and windings As the design has large number of variables, specifications shall be stipulated for the type of electrical loading. I) Design Where applicable a third set of tertiary winding need be incorporated in large power and distribution transformers for auxiliary supply purpose, which in addition will help in suppressing harmonics and thereby minimise the losses.

3 Ch.-5 Transformer 5. 3 It is further suggested to connect synchronous condenser or high voltage capacitor across the delta connected output of tertiary winding to improve power factor and to reduce harmonic loss. II) Material Use The construction comprises of magnetic core, winding, terminal tapping switches, tanks and cooling devices. For magnetic core it shall have amorphous steel, referred as metallic glass. This is a non crystalline solid created by rapid quenching of metal, metalloid alloys. Ferromagnetic metal glass alloys can magnetise and demagnetise more easily. The compositions of amorphous steel are Feg, Bi35, Sj35 C2 of thickness 0.25 to 0.5 mm and width 175 mm. Table No. 5.1 shows the comparison of transformer core materials. III) Winding Rectangular copper conductor is the common type used in power transformer winding. To increase the short circuit strength annealed copper conductor is to be replaced by control proof stressed copper-silver alloy. This is a replacement for high conductivity copper. Example 5.1 Energy saving by copper silver alloy winding wire in place of super enamelled winding wire. From Case Study' No. 5.2 the foil load loss at 75 C of 1600 KVA transformer is watts. Core loss at no load 2160 watts. FLI2R ( ) = watts Hence yearly loss {(17040 x 24 x 330) /1000} = KWh When copper silver alloy super enamelled winding wire would be used with (Ag 92.5%, Cu 7.5%), the resistance of silver alloy will be only 94.5% of copper [113]. I2R loss ( x 0.945) = KWh

4 Ch.-5 Transformer 5. 4 The other alternative is to use wide strips of aluminium conductor in the winding. a) 1.2 mm dia annealed aluminium wire with medium covering may be used. b) In another case 1 mm x 3 mm aluminium strips with double insulating paper of 0.5 mm thickness are to be used for optimum energy saving. Above all whereever it is possible the conventional transformers may be replaced by cast resin transformers from EMC point of view. The power consumption pattern at no-load of power transformer with amorphous magnetic core and with CRGO core are shown in Table No. 5.2, for 10 nos. different KVA rating transformers. Case Study No. 5.1, (Taking modified design, use of new material and new winding) hi M/s J.K. Paper Mills, Rayagada, India, 2000 KVA Power Transformer with CRGO core The test results of power block transformer: KVA capacity: 2000 KVA, SI. No. V8793/1 Make: M/s Crompton Greaves Ltd., India Date of test: HV: 6.6 KV, 175 Amps. LV: 433 V, Amps. Vector group:delta/ Star No load loss: 2544 watte Full load loss: I2R at 75 C = KW Total no load loss yearly (2.544 x 24 x 330) = KWh In case of AMT core The no load loss is 0.5 KW for 2000 KVA transformer Total no load loss yearly (0.5 x 24 x 330) = 3960 KWh Energy saving ( ) = KWh per year

5 Ch.-5 Transformer Distribution Transformers Its construction comprises of magnetic circuit, winding, tanks, and cooling devices similar to power transformer. In earlier construction before 1970 inferior grade of laminated steel were used for which hysteresis and eddy current losses were higher and also had quick ageing effects. Gradually there was improvement in material use with small quantity of silicon alloyed, low carbon content steel. This had improved losses and ageing period. There was further development in steel manufacture. The core material was cold rolled grain oriented (CRGO) silicon steel. It has minimum losses and universally used in all transformers. Japanese started use of carlite (a commercial name) [95] HIB. It is of colloidal silica of small thermal expansion and amorphous structure. This is applied at last stage of production process and this has narrowed main domain width in high permeability steel. Further cutting a fine groove in the surface had reduced losses upto 40% [117]. The hot rolled steel alloyed with silicon lamination has specific loss of 2 watt/kg at 1.5 Tesla. In the case of grain oriented steel it is 1 watt/kg at 1.5 Tesla. British Steel Electrical Ltd. further produced a steel material of low loss of 0.62 W/kg at 1.7 Tesla. This loss will be less at lower field strength of 1.5 Tesla [22], The lamination joints influence the core loss and step lap stacking results in smaller losses, lower magnetising current and noise. Amorphous magnetic material is a substitute of grain oriented electrical steel to be used as core material [16, 117]. This application reduces the losses upto 60 to 70% Improvements of Distribution Transformers Like power transformers, this distribution transformers has many demerits as mentioned in As the losses in CRGO transformers are more, and the efficiency can be improved with the modifications in magnetic path, winding material and construction features.

6 Ch. -5 Transformer 5. 6 I) Magnetic path in Core Use of laser scribed core material grade ZDKH and high B grade ZH100 material. Boltless bottom yoke. Use of nonmagnetic steel for clamp plates to reduce stray losses and better overfluxing capacity. Use of yoke shunts / wall shunts / flux trap for control of stray losses. II) Winding Materials Use of special type of copper conductor such as bunched, glued and workhardened etc. Bimetalic HT termination suitable for XLPE cable connection. Interleaved disc winding for high voltage application and compactness to axial and radial forces. Use of ceramic bottom space for oil ducts in the core. III) Constructional feature during assembly Use of guided oil flow and force directed oil flow in winding for control of hot spot gradient Better oil sealing gasket arrangement to arrest the leakage. Welded tank covers with oval shaped. Use of vertical stiffeners in tank. Reduction in solid press board insulation by suddivided barrier arrangement. Optimisation of radial and axial clearances based on permissible stress distribution. Case Study No. 5.2 (Energy saving in Distribution Transformer incorporating modifications in Core, Winding and their Construction) In J.K. Paper Mills, Rayagada, India, Power Block 1600 KVA Distribution Transformer with CRGO core

7 Ch.-5 Transformer 5. 7 The test results of the transformer: KVA capacity: 1600 KVA, SI. No. Ml707/1 Make: Crompton Greaves, India HV: 6.6 KV, 140 Amps. LV: 433 V, 2134 Amps. Vector group: DYn 11, Delta/Star No load loss: 2160 watts Load loss at 75 C = watts Date of test: 25/ Total losses supplied during test watts Yearly no load loss {( ) x 24 x 330 / 1000) = KWh per year In case of AMT core The no load AMT core loss 0.49 KW Yearly energy loss (0.49 x 24 x 330) = 3881 KWh per year Energy saving ( ) = KWh per year Table Nos. 5.3 and 5.4 show the energy saving of distribution transformer with amorphous core and CRGO core Energy saving on the use of Dry Transformers in place of Traditional Distribution Transformers For low loading pattern, Dry type transformers may be used for better eficiency. The added advantage of dry type transformer is that it is very compact in construction, requires less space and can be installed in any awkward place. Normally the distribution transformers are mounted on poles at a minimum distance of 50 to 100 metres from the consumer premises. In the LT supply side power is availed through tropodour under ground cables to consumer premises. But in case of dry type transformer the length of LT cables are very less.

8 Ch.-5 Transformer 5. S Example 5.2 Consider a 50 KVA 11/0.4 KV transformer Average load in Amps.: 60 A Cable resistance : 0.12 Q/Km, Length: 100 m The extra length of 3 x 120 sq. mm tropodour cable is 50 mtrs. for the distribution transformer. Energy loss in traditional distribution transformer cable (I2R loss) [3 x (100 / 1000) x {(0.12 x 60 x 60 x 24 x 330) / 1000}] = 1026 KWh per year Energy loss in dry transformer cable for 50mtrs. long [(3 x 50 x0.12 x 60 x 60 x 24 x 330) / (1000 x 1000)] = 513 KWh per year Hence energy saving on the use of dry transformer ( ) = 513 KWh, say 500 KWh per year Application of Amorphous Transformer (AMT) in Distribution System in place of CRGO Transformer The amorphous core transformer is having upto 80% reduction in no load losses when it is compared with a same KVA rating traditional distribution transformer [22]. Core Construction The core is amorphous like metallic glass. The metallic molecules occur in random pattern and having a non-crystalline alloy steel like glass. There are no ( grains. The flux experiences least resistance in any direction. The rigid grain structure of silicon steel opposes the flux with a resistive path. The unique structure Gf non-crystalline alloy steel has easy magnetization and demagnetization property. The material s unique structure results from rapid solidfication manufacturing process. The cooling rate of molten alloy is one million degree celcius per second. The core is cast in extremely thin ribbons of size mm to achieve required magnetization properties.

9 Ch.-5 Transformer 5. 9 The core is annealed in an oven in a strong magnetic field. High Voltage Winding (HVW) This is made of circular or rectangular copper wire, insulated with enamel coating. The inter layer insulation is high temperature curing epoxy dotted high strength paper. Oil ducts are provided between the layers for efficient cooling. Low Voltage Winding (LVW) The low voltage winding is a foil winding made of copper with the same type of insulation as provided in the HVW. The connection between foil and terminal are Tungsten Inert gas welded to windings. The assembled coil HV over LV is compact under pressure and baken in an oven. The epoxy dots on inter layer insulating paper undergoes a transformation and cures compatibility, making the entire mass homogeneous. The special feature in construction is that there is no axial force developed to pull HV and LV coils apart and hence no axial clamping of coils is necessary. There is uniform impulse voltage distribution in all turns of winding. This is due to large surface area of layers of winding of both HV and LV. The winding to earth capacitance is low as compared to winding to winding capacitance. For hermitically sealed construction, the oxidation and moisture contamination is eliminated. 5.4 Rectifier Transformer The design of rectifier transformer differs widely from power and distribution transformer. In a rectifier transformer the voltage is stepped up or down in conjunction with rectifier unit for conversion of AC power to DC power. The salient features are [61]: Secondary windings carry sinusoidal current only over a portion of cycle. The harmonic components of these waves contribute for heating of transformer. So it has to be designed for higher rating.

10 Ch.-5 Transformer 5.10 The secondary windings carry high current and connections are made by heavy busbars. The size and layouts are decided to reduce stray losses and reactive drops. The secondary may have more numbers of windings and may be used to 6 pulse or 12 pulse operation with increased numbers of pulses the output ripple content is reduced. In addition, associated equipments, like autotransformer, interphase transformer, transductors and even sometimes rectifier units are housed in the same tank Modification of Rectifier Transformer with the use of Microprocessor CRGO M4 or M5 steel sheets laminations are presently used with vacuum controlled impregnation of quality varnish. These transformers are of oil cooled type. The rectifier transformers are subjected to mechanical vibrations due to electrical stresses. The failure of transformers occurs at weak parts, when operating stresses develops. Paper and press board insulations when heated in oil loose mechanical strength and become brittle. These materials will fail after loosing mechanical strength. The transformer consists of mainly cellulose insulating materials which have 8 to 10% moisture at ambient temperature. The impurities such as dust, dirt and fibres are the main enemy to deteriorate the life of insulation. Moreover the rectifier transformers are subjected to electromagnetic forces under short circuit condition while in operation of furnace core materials. This phenomenon is transient and dynamic in nature. Therefore, at design stage proper attention shall be given to avoid hot spot in winding laminated core and at mechanical clamping parts. These rectifier transformers are used for furnace and electrostatic precipitator (ESP) applications. They are of voltage and current control types. For ESP application, voltage control rectifier transformer, the manual tap changing in

11 Ch.-5 Transformer 5.11 primary side is replaced with microprocessor control unit. The steep voltage rise was controlled by 6 pulse gate triggering circuit, resulting power saving. This modification helps in improvement of working capability of ESP. Case Study No. 5.3 M/s J.K. Paper Mills, Rayagada, India There are 4 nos. of Electrostatic Precipitator (ESP) in liquor Fired Boiler (LFB) No. IV. The voltage control was manual. ESP operates at 80 KV peak and 400 ma, DC Average volts: 65 KV at 300mA Load on each ESP = 65 KV x 300 ma AC side load voltage = 300 V at 50 A ESP DC load [65 x 1000 x 300 x 10'3 /1000] = 19.5 KW AC load [2.2 x 300 x 40 /1000] = 26.4 KW (Taking losses 10%) = KW Considering DC load as constant, i.e., 19.5 KW with microprocessor efficiency 90%. Input power [19.5 / 0.9] = KW Power saving ( ) = 7.37 KW Total energy saving for 4 nos. ESP per year [7.37 x 4 x 24 x 330] = KWh per annum 5.5 Foil winding Transformer The foil winding power transformers are capable of withstanding severe mechanical stress under short circuit condition, but offers lower efficiency Constructional Feature Use of thin sheet conductor in place of round or rectangular conductor in winding is the main feature. In normal spiral winding there are number of turns in axial direction and number of layers in radial directions. In foil widing there is only one turn occupying the entire height of the secondary winding and there are number of layers equal to the total number of turns in the primary winding. The

12 Ch.-5 Transformer 5.12 LV side has less number of turns of bigger cross section, so foil winding is for LV side, whereas HV will have more number of turns of round or rectangular cross section Advantages of foil winding It has greater strength to withstand short circuit. As foil winding occupies the full height, so nullifies the axial unbalance between windings by distribution of current in L.V. corresponding to H.V. So it has built in capacity to have no axial unbalance at anytime during life. It has special thermosetting epoxy coated interlayer paper insulation to withstand high strength. It has improved heat transfer characteristic for its special construction. The unit is compact with better space factor, so less floor area is required. It is robust against any short circuit and thermally superior. It has better overloading capacity. The no load power losses are very less. The eddy current loss is the square of conductor thickness. As the conductor are parallel and 2 nos. in LV side, so circulating current is also less. It has reduced noise level and has longer life with less maintenance. This unit is suitable for frequent switching surges. This type windings are suitable for furnace transformer, rectifier. transformer and traction transfoimers, which are subjected to frequent short circuits, overloads and switching surges in the network system. 5.6 Energy saving on welding transformer Welding transformers generally remain on throughout the working hours [55, 61]. There is wastage of no load power, due to the magnetising current of the machine. The no load current in primary side for a transformer varies from 6 to 10 Amps. Welding work is not a continuous job. In a shift there may be only 2 to 3 hours of welding work. An electronic auto control switch (operates on load) is attatched to welding transformer in the primary side. By this device die power supply will be cut automatically just after 15 to 20 seconds when welding work is

13 Ch.-5 Transformer 5.13 stopped. The power supply will be on to the machine again when the job is connected. Case Study No. 5.4 J.K Paper Mills, Rayagada, India There are 6 nos. of 14 KVA, 415 V welding transformers are in operation in workshop. The no load current is 6 Amps. No load power factor is 0.3. No load working hours is 6 a) Energy consumption (^3 x 415 x 6 x 0.3 x 6 hours x 330 days / 1000) = 2561 KWh per year Energy saving with auto control switch only (For 6 M/cs) Total energy saving is KWh b) Energy saving with auto control switch and capacitor connected at primary side No load power saving for a 14 KVA transformer = 2561 KWh (c.f.a) With the connection of capacitor p.f. is changed to 0.8 from 0.5. Welding work is done for 2 hours. Energy consumption on job without capacitor (V3 x 415 x 28 x 0.5 x 2 x 330 days /1000) = 6641 KWh Energy consumption with capacitor (V3 x 415 x 15.8 x 0.8 x 2 x 330 /1000) = 5996 KWh Energy saving ( ) = 645 KWh Total energy saving ( 2561 (a) (b)) = 3206 KWh For 6 machine total energy saving KWh per year Table No. 5.5 shows recommended capacitor at welding transformer.

14 Ch.-5 Transformer Reasons for failure of Transformers The failure arises from the occurence of voids or cavities in the insulating material during operation. Existance of mechanical and electrical stresses breeds chemical changes in due course resulting in acid formation or products of oxidisation (CO, C02, H20). The voids then become pocket of gas and get ionised in the area of electrical stress concentration. This phenomenon of partial' discharge in AC voltage causes energy dissipation known as dielectric loss. There is another reason of surface corona discharge due to ionisation of gas on HV side which forms a potential gradient. Internal corona may exist in a void deep within a coil insulation. Humidity has also effect on this failure. The cellulose insulating materials have 8 to 10% moisture at ambient temperature. This will injure system insulation and failure in course of time Suggested method for long life on EMC point The conventional thermoplastic material like Shellac or Bitumen bonded micaflake does not give good service. This materials swell on heating and conductors become weak to develop earth fault after some service period. The insulation of thermosetting synthetic resin using glass cloth as baking materials either mica paper insulation or mica flake to be more resistant for these conditions is suggested. The benefit is that at increased temperature it will be a solid mass. The coefficient of synthetic resin micaflake is 8-12 x 10"6 and of copper is 16.8 x 10"6, where the figure for this composite mass is x 10^. It is very near to copper. In resin rich system, the insulation base is a glass baked mica paper bonded with epoxide novolac resin. Mica will be in fine splilting with resin in baking stage. The next step is vacuum pressure impregnation process system, which will be taken up to improve heat transfer characteristic. This will improve voltage stability. Entire assembly with core pocket is to be immersed in a thermosetting blend of resin and hardners impregnated using vaccum and pressure with heat

15 C/i.-J Transformer 5.15 application for correct impregnation. All these procedures will make the winding durable and impervious product [38]. 5.8 Modifications / Neutral Conductor to be adopted for Energy Conservation In a 3 phase 4 wire system neutral conductors will be severely affected by non-linear loads connected to 4 wire distribution system. Now-a-days computer loads are very commonly used which generates harmonic current. In a normal 3 phase balanced linear load no harmonic current will flow in linear conductor. In case of non-linear loads some numbered harmonics do not cancel but flow together in the neutral conductor. The 3rd, 9th, 15th and 21st etc. called Triplers come in this category. These harmonic currents flow to the transformer neutral. In such situations it may be possible for neutral current to exceed the individual phase current. This will overload the neutral conductor and create more loss. Voltage drop between neutral conductor and the ground will be more. Recommendations and standards are formulated in developed countries [81] to monitor, filter and eliminate the harmonics generation from consumers end. It is not the case in developing countries. In all transformers manufactured in developing countries, the size of neutral conductor is half the power conductor size. Hence the power loss is more due to generation of harmonics. To reduce this loss, size of conductor shall be increased and filters may be provided with neutral conductor. 5.9 Suggested Procedure for testing Improvement of efficiency, p.f. and long life of transformers can be ascertained from the perfect test results. Internal sparking gas formation and other short comings on workmanship, winding and materials can be prevented when proper tests are conducted, a) Preliminary Test: After rewinding before tanking, core insulation, core loss ratio, polarity, vector group and winding resistances shall be taken.

16 CH.-5 Transformer 5.16 b) Intermediate test: The test of winding resistances, voltage ratio, vector group, impedance voltage, load loss, no load loss, current dielectric test, insulation. c) Final Test- Temperature rise test, impulse voltage test, short circuit withstand test, vibration test. Any moisture and dirt content in oil severely deteriorates the life of insulation and causes failure. Regular oil testing is essential to maintain the standard of oil as per specifications. Table Nos. 5.6 and 5.7 show the standard of specifications of transformer oil Conclusion The transformers used for voltage regulation in the network shall have minimum impedance which can be achieved by compact winding designs and minimum HV and LV clearances between windings, using an oil press board barriers system. The design must meet the stringent short circuit and high voltage impulse test and other tests in view of technological advancements. There shall be computer aided measurement (CAM) system to reduce the tolerance limit and on material specifications designed optimally. The general essential specifications of a transformer is shown in Table No In transformer the basic engineering data are in continuous process of advance modification with innovation. The present components are not well guarded against all type of faults in all cases. The rate of failure and system disturbances result heavy power loss. The present switchgears, controls, relay settings, synchronisations, plant monitoring communication with alarm management are based on Computer applications for easy approachable nature. In near future Power Electronics Transformers [92] will be introduced to eradicate all complex nature of fault and for best EMC applications.

17 CH.-5 Transformer 5.17 SI No Table No. 5.1 Compar'son of Transformer Core Materials Properties unit Amorphous alloy CR.GO Silicon steel 1 Saturation flux density T at 25 C at 100 C Core loss at 1.4T watt/kg Exciting power at 1,4T VA/kg Specific Resistance Hardness DPH 10.3 RB 76 6 Thickness in micron Space factor % Annealing Temperature C Special Annealing requirement magnetic annealing - 10 Annealing atmosphere Inert gas. Inert gas

18 Ch.-5 Transformer 5.18 SI No Table No. 5.2 Comparison of no load loss of Amorphous core and CRGO core Power Transformer 11/0.433 KV Transformers KVA No load loss in KW rating CRGO Amorphous Transformer Transformer Yearly saving in KWh t , Table No. 5.3 Comparison of No load Loss Yearly power KWh SI KVA rating No load oss in watts consumption Savings in No of Transformer CRGO core Amorphous core CRGO core Amorphous core Kwh per year

19 Ch.-5 Transformer 5.19 Table No. 5.4 Single phase Transformer Amorp rous core ^ow loss Silicon Stee KVA Core Winding % Exc. % Core Winding % Exc. % rating loss loss Amps. impeda loss loss Amps. impeda watts watts nee watts watts nee Table No. 5.5 SI. Welding Transformer Capacitor rating No. KVA KVAR

20 Ch.-5 Transformer 520 Table No. 5.6 General Standard of Transformer Oil SI Permissi ale Value No Type of Test Minimum Maximum 1 Electric Strength BDB in kv (Minimum of 6 Test value) Dielectric dissipation factor Tan S at 90 C Specific Resistance in ohm/cm at 90 C xlo12 4 Water content in Ppm Neutralisation value total acidity in KOH/gram Sludge content percentage in weight nil - 7 Flash point by 0 C Interfacial Tension in N/M Density in gm/cm2 ISC-335/ Kinetic viscocity in CST at 27 C ' - 27

21 Ch.-5 Transformer 521 Table No. 5.7 Suggested Standard of Oil for EHV / HV Transformers SI No Characteristics As per IS 335 of 1963 Permissible Value Minimum Maximum 1 Appearance : (A representative sample of the oil Oil shall be clear and shall be examined in 100 mm thick layer at 27 C) transparent and free from suspended mater or sediment 2 Density at 29.5 C g/cm Viscocity kinematics Cst at 27 C Interfacial Tension N/m at 27 C Flash point Pensky-Martin closed 0 C Pour point0 C Neutralization value a) Total acidity mg KOH/gm 0.03 b) Inorganic Acidity / Alkalinity nil nil 8 Corrosive sulpher copper strip 19 hrs. at the rate 140 C non-corrosive 9 Dielectric dissipation factor Tan S at 90 C Surface resistance ohm/cm a) at 90 C 35 x b) at 27 C 150 x Oxidation Stability a) Neutralisation value after oxidation for 164 hrs at 104 C mg KOH/gm b) Total sludge at 104 C mg KOH/gm 0.10 after 164 hrs. at 100 C by wt % 12 Accelerated ageing test Coper banker method with copper catalyst 94 hrs. at 115 C a) Specific resistance ohm-cm at 27 C 2.5 x 1012 b) Specific resistance ohm-cm at 90 C 2 x 1012 c) Dielectric dissipation factor Tan 6 at 90 C d) Total acidity mg KOH/gm e) Total Sludge value Pressure oxidation inhibitor (The oil should not Th have any anti oxidation additives) Water content in PPM - 15

22 Ck-5 Transformer 522 Table No. 5.8 General Technical Perticulars for an EHV Transformer unit Parameter 1 Name of manufacturer 2 Service 3 Rating NIVA 4 Rated Voltage (HV and LV) KV RMS 5 Rated Current (HV / LV) K Amps 6 Rated frequency Hz 7 Nos. of phases and Vector group Winding connections HV and LV Winding conductor material OFF/ON load tap changer with range and steps Type of cooling Temperature rise over an ambient 50 C top oil by thermometer and winding by resistance method 13 Losses 1) Total loss at rated voltage in Principal tapping at rated Hz H) No load loss at rated voltage in Principal - - tapping at rated Hz Impedance voltage at rated current for the principal tapping at rated Hz 15 No load current at rated voltage and rated Hz 16 Efficiency at 70 C a) atu.p.f. at 100%, 75%, 50%, 25% of rated load % b) at 0.8 p.f. lagging, 100%, 75%, 50%, 25% of rated load % - 17 Audible sound at 1 metre distance Insulation level a) Separate source power frequency withstand voltage KVRMS - HV/LV winding b) Induced over voltage withstand level HV/LV KVRMS winding 19 Full wave lightning impulse withstand voltage KV - HV/LV winding Peak 20 Weight, core and winding, tank filling and accessories, kg - oil and total mass 21 Transformer oil, quantity for l5* filling, type of Lts. manufacturer, dielectric strength and specific gravity (It is given in Table No. 5.5) 22 Overall dimension 23 For EHV Bushings(continued in next page)

23 Ch.-5 Transformer 523 i) Type and make ii) Momentary power frequency dry withstand voltage KV - iii) Visible power frequency discharge voltage KV - iv) 1 minute dry withstand power frequency voltage KV - v) Under oil flash over or puncture withstand power frequency voltage KV - vi) Full wave impulse withstand voltage vii) Creep distance in air viii) Recommended gap setting KV