Transformator 2

8/9/2019 Transformator 2

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5.2 POWER TRANSFORMERS5.2.1 GeneralThe primary function of a power transformer is to transform system voltage from one nominal level toanother. The transformer has to be capable of carrying (within the guidelines of ANSI/IEEE Std.

!".#$% the power flow for its particular location in the system under various operating conditions and

contingencies& such as line or transformer outages.This section deals primarily with oil'filled power transformers with nominal primary voltage ratings of $ ) *+ and below and utili,ing one of the following methods of cooling-

. Self'cooled ( A%$. Self'cooled and assisted by forced'air ( A/0A for one stage1 A/0A/0A for two stages%

. Self'cooled and assisted by forced'air and forced'oil ( A/0A/0 A or A/0 A/0 A for twostages%

ther methods of cooling such as with water are available and are described in IEEE Std. !". $.)).The type of cooling used is based on the re2uirements of the specifications& the si,e of the transformer&and the manufacturer3s standard design.4eeting these specific re2uirements usually results in the power transformer3s being the largest&heaviest& and most costly piece of e2uipment used in a substation.

5ecause of their great importance and comple6ity& power transformers re2uire special care in their application& specification& and procurement. This is best accomplished by ta*ing full advantage of applicable industry standards and guides of national organi,ations such as the American NationalStandards Institute (ANSI%& Institute of Electrical and Electronic Engineers (IEEE%& National Electrical4anufacturers Association (NE4A%& etc.& and 78S I9 $)$' & :;ist of 4aterials Acceptable for 8se onSystems of 78S Electrification 5orrowers.<The following discussion highlights various aspects of power transformers and provides guidance andrecommendations to assist cooperatives in obtaining the proper e2uipment for their systems. 4ost of this discussion will apply to step'down transformers& especially those not included in the ;ist of 4aterials. 7efer to 78S 5ulletin !)'# & :78S Specification for Step'=own =istribution SubstationTransformers&< for step'down transformers.5.2.2 Types9ower transformers may be either autotransformers or multiwinding conventional transformers. Athree'phase installation may consist of a three'phase unit or three single'phase units. The decision astowhat type of transformer to purchase depends on such factors as initial installed cost& maintenancecosts& operating cost (efficiency%& reliability& etc. Three'phase units have lower construction andmaintenance costs and can be built to the same efficiency ratings as single'phase units. The initial costof a three'phase transformer is usually appro6imately one'third less than four single'phase units.Additionally& the e6posure of three'phase units to long outages can be minimi,ed system'wide when amobile substation or transformer is available for bac*up in case of failure. 78S 5ulletin > ' >&:?uidefor Economic Evaluation of =istribution Transformers&< provides information for evaluatingsubstationtransformer costs.The *+A ratings for various si,es of transformers are covered by the standards. IEEE Std. !". $.))&Table & defines the preferred continuous ratings for both single ' and three'phase units. ANSIStd. !". $. )& Tables $ and $& further define the ratings of transformers with and without cooling.Transformers )&))) *+A and below can accommodate one stage of cooling only& while transformerslarger than )&))) *+A can include up to two stages of cooling. Each stage of cooling increases thecapacity of the transformer by a fi6ed percentage of the base ( A% rating. 0or three'phasetransformersrated between "!) and $))) *+A& increasing the base level of cooling to forced air cooling willincrease the continuous *+A capacity by ! percent. 0or transformers rated $!)) *+A to )&))) *+A&the increase is $! percent. 0or transformers above )&))) *+A& additional stages of cooling may be

used to increase the continuous *+A rating of the transformer by percent per stage. Transformerslarger than those listed in these tables can be purchased and would normally be triple rated or wouldhave provision for the future addition of two stages of cooling e2uipment to produce a triple rating.



The choice between conventional two' or three'winding transformers and autotransformers involvestheir basic differences as they may affect the application and cost factors. In general& autotransformersare considered primarily because of cost advantages where the voltage transformation ratio isfavorable&up to possibly / . 5eyond this ratio& the cost advantage of autotransformers diminishes. Also&autotransformers are wye connected and thus provide only an in'phase angular relationship between

primary and secondary voltages.ther advantages of autotransformers are smaller physical si,e& lighter weight& lower regulation

(voltage drop in transformer%& smaller e6citing currents (easier no'load switching%& and lower losses.The main disadvantages of autotransformers are lower reactance (impedance%& more comple6 design

problems& and adverse affect on ground relaying. These problems can usually be resolved.

5.2.3 Ratings5.2.3.1 Capacity: The selection of substation transformer *+A capacity should be based on anacceptable up'to'date engineering study. The selection should consider the effects of load cycle& loadfactor& and ambient temperature as described in ANSI/IEEE Std. !".#$& :?uide for ;oading 4ineral

il'Immersed 9ower Transformers up to and Including )) 4+A @ith !! or >! Average

@inding 7ise.<Since cooling efficiency decreases with increase in altitude& the transformer manufacturer should beinformed when the transformer will be operated at an elevation above ))) meters ( )) feet% so thatthe proper cooling system can be provided. See Table s !' and !'$ for guidance on the effect of altitude on temperature rise. Also& multiwinding transformers with loads on various windings atdifferent power factors have higher load losses and may re2uire additional cooling capacity.

In addition to selecting a transformer capable of satisfying the basic capacity re2uirements& it is alsodesirable to give due consideration to inventory and standardi,ation with the obBective of simplifyingspare parts& testing& maintenance& and unit sparing problems.

5.2.3.2 Temperatures - Normal transformer design is based on ambient temperatures of C)ma6imum& ) average over $C hours& and D$) minimum. Abnormal ambient temperatures should

be made *nown to the manufacturer at the time of purchase since they usually re2uire modifications inthe design of the transformer.5.2.3.3 Voltage: Nominal voltage ratings of a transformer are selected to conform to systemvoltage conditions. Tables !' & !'C& !'!& and !'> list standard voltages through $ ) *+. According toANSI/ IEEE Std. !". $. )& transformers should not be subBected to operating voltages or volts per hert, above )! percent of any rated secondary tap when operating loaded to nameplate *+A ratingwhen the load power factor is ) percent or higher and the fre2uency is at least #! percent of rated

value. In addition& transformers should not be operated continuously above ) percent of ratedsecondary tap when operating at no'load. 4ultiwinding transformers and autotransformers may be



restricted further depending on the specific design criteria specified when the transformer was purchased. 7efer to the original specifications or the manufacturer for guidance. Also refer to ANSIStd. C. for further guidance on ma6imum permissible operating voltages.



5.2.3.4 Basic Insulation Levels (BIL): Tables !'"& !' & and !'# list basic insulation levelscommonly used for various system voltages. Neutral terminal 5I; may be specified at a differentlevelthan the line terminals depending on the type of system grounding being used. Table !' ) lists theminimum insulation levels for neutral terminals. ontinuous improvements over the years in the

protective margins provided by surge arresters have enabled users to select reduced insulation levelsfor transformers& at appreciable cost reductions& without sacrificing reliability.

5.2.3.4 Basic Insulation Levels (BIL): Tables !'"& !' & and !'# list basic insulation levelscommonly used for various system voltages. Neutral terminal 5I; may be specified at a differentlevelthan the line terminals depending on the type of system grounding being used. Table !' ) lists theminimum insulation levels for neutral terminals. ontinuous improvements over the years in the

protective margins provided by surge arresters have enabled users to select reduced insulation levelsfor transformers& at appreciable cost reductions& without sacrificing reliability.



Any selection of a transformer with reduced 5I; is a user responsibility and re2uires *nowledge of certain system characteristics.

n effectively grounded systems& a reduced 5I; of one step below full basic insulation level may beappropriate for transformers with nominal ratings of ! *+ and above. An insulation coordinationstudy may be re2uired to ensure that ade2uate margin is maintained between transformer insulationstrength and the protective level of protective e2uipment.5.2.3.5 Loading - A transformer can supply a load beyond its nameplate rating for various periodsof time& which may or may not affect its normal life& depending on several factors related totemperature conditions in the transformer. See ANSI/IEEE Std. !".#$ for additional guidance on

transformer loading.



5.2.4 Taps No'load tap changers (N;T % and/or load tap changers (;T % can be obtained on power transformers.The addition of no'load taps in the primary of a substation transformer ma*es it possible to adapt thetransformer to a range of supply voltages (usually a ) percent overall range of which ! percent isabove nominal and ! percent below nominal& usually in $.! percent steps%. Since no'load taps are notcapable of interrupting any current including transformer charging current& the transformers have to bede'energi,ed when the manual no'load tap position is changed. All taps should have full capacityratings. The individual tap voltages should be as specified in ANSI/IEEE Std. !". $. )& paragraphC.!.Any decision to use load tap changing transformers should be based on a careful analysis of the

particular voltage re2uirements of the loads served and consideration of the advantages anddisadvantages& including costs& of alternatives such as separate voltage regulators. @hen load tapchanging transformers are specified& they have to comply with the re2uirements of ANSI/IEEEStd. !". $.))& and ANSI/IEEE Std. !". $. ).5.2.5 I pe!an"eTransformer impedance affects transformer voltage regulation& efficiency& and magnitude ofthroughshort'circuit

currents. 5oth regulation and efficiency are generally improved with lower impedance.Fowever&these desirable results should be viewed along with higher through'fault currents permissiblewithlower impedance.Figher load'side fault currents can be potentially damaging to the transformer and may also re2uirehigher fault current ratings of load'side e2uipment at increased cost. 9rudent compromises are thusoften re2uired in specifying transformer impedance.@here through'fault currents are not a significant factor& it is generally desirable to specify as lowimpedance as possible that will not result in increased transformer cost. Standard impedance rangesfor various transformer 5I; ratings are listed by manufacturers& and cost penalties may apply when theimpedance falls above or below these ranges. The standards permit manufacturing tolerances of –".! percent of the specified impedance for two'winding transformers and – ).) percent for multiwinding transformers and autotransformers. These are important to remember if transformer

paralleling is being considered and if the margin between transformer through'fault current ande2uipment ratings is very close. Substation transformer impedance needs to comply with there2uirements of ANSI/IEEE Std. !". $. )& Section >. Standard impedances for various voltageratings are given in Table !' . =istribution substation transformers (!)) *+A or smaller% should bespecified with standard impedance where possible. These impedances are sufficient to ma*e thetransformer self protecting under any secondary faults.5.2.# P$ase Relati%n9roper phase relationships between the various winding voltages are e6tremely important intransformer application. These have to be selected to fit e6isting or planned conditions in the particular system.Standard single'phase substation transformers are built with subtractive polarity. The polarity of athree'phase transformer is fi6ed by its connections between phases and by relative location of leads. Astandard delta'wye or wye'delta& three'phase& step'down transformer will result in the high'sidevoltages leading their respective low'side voltages by ) degrees. An installation of three single'phasetransformers can be connected to accomplish this same relationship.Autotransformers are connected wye'wye& and no phase angle e6ists between the high' and low'sidevoltages. This may preclude the use of autotransformers& in some cases& even when they are otherwise

preferred. See Section !.$." concerning the phase relationship re2uired for parallel operation of transformers.



5.2.& Parallel Operati%n %' Trans'%r ersIn most cases& the purchase of two smaller si,e transformers& to be operated in parallel in one circuit&inlieu of one full'si,e transformer& is not recommended. Two transformers will cost more than a singletransformer of e2uivalent capacity& their combined losses are higher& and they re2uire a more elaborateand e6pensive substation structure to accommodate them. Fowever& where a situation e6ists for

possible parallel operation& such as where continuity of at least partial service in event of failure of oneunit is of great importance& the transformers should be individually protected and the followingguidelines considered.Any two or more transformers can be operated in parallel& provided their impedances are in the sameorder of magnitude when considered on their own *+A base& their voltage taps and voltage ratios areessentially the same& and their polarity and phase voltage displacement are or can be made ali*e.E2ual impedances will permit proportionate sharing of the load between transformers. If not e2ual&the

load will be divided in inverse proportion to the magnitude of the impedance. This condition issatisfactory within reasonable limits& as determined by re2uirements& and may be of little conse2uencewhere the larger of two transformers has the lower impedance and will carry more than its

proportionateshare of the load. Fowever& if the smaller unit has the lower impedance& it will carry more than itsshare of the load and may even become severely overloaded& while the larger unit still has availablecapacity. This is demonstrated in the following e6ample.9rotecting transformers operating in parallel as a single unit is not recommended. The sensitivity ofthehigh'side protection is significantly reduced& and the occurrence of nuisance tripping duringenergi,ation is increased due to incorrect differential relay harmonic restraint unit operation.

Transformator 2

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