MAY/JUNE 2008 370278-6648/08/$25.00 © 2008 IEEE

Over the past couple of decades, changes have takenplace in high-voltage insulation systems that haveproduced safer and more reliable operations. Themica/epoxy insulation, which has been used inrotating machines for over 100 years, is now being

replaced by a new concept using high-voltage cross-linkedpolyethylene (XLPE) cables. Three newproducts have been recently launchedincluding the Powerformer, a newgenerator that can be directlyconnected to the transmis-sion network without theneed for a step-up trans-former. The other prod-ucts include the Dry-former, an oil-freepower transformer,and the Windformer, anew wind power gen-erating system. Due tothe Powerformer’s abil-ity to generate electrici-ty at transmission voltagelevels, it offers consider-able gains with respect toreactive power production and plant efficiency. Hence, a Powerformer bothfacilitates networkstability andd e c r e a s e s

the exploitation of natural resources. The upper limit for theoutput voltage from the Powerformer is set by state-of-the-artXLPE power-cable technology. Therefore, Powerformers revolu-tionize the age of old power generation technology and signala quantum leap in electrical engineering.

High-voltage visionConventional high-voltage generators are construct-ed in a way that limits their output voltage to a

maximum of 30 kV. The power grids with volt-ages up to 1,100 kV cannot be directly sup-

plied by these generators, which is a reasonlarge power plants use power step-uptransformers in order to transform theirgenerated voltage to a higher voltagelevel suitable for the interface with thetransmission grid. Step-up transformersimpose significant drawbacks on thepower plant as a whole, starting fromreduction in efficiency, high maintenancecosts, more space, less availability, and an

increased environmental impact. Duringthe last century, a number of attempts were

made at developing a high-voltage generatorPowerformer that could be connected

directly to the power grid, with-out going via the step-up

transformer (see Fig. 1).Recently, a Powerformer

was developed with innov-ative features that enable itto connect directly to the

transmission grid. Fig. 2shows how the rated voltage

of the Powerformer increasedduring the late 1990s. It isexpected in the comingdecade that its output volt-age will reach up to 420kV. High-voltage cableswith XLPE insulation areavailable today for volt-ages of up to 500 kV.When XLPE-insulatedcables were introducedin the 1960s there weresome initial problemswith their reliability,caused by poor controlof the manufacturing

processes. These prob-lems have since been

overcome, and today’shigh-voltage XLPE-insulated

cables have an impressivetrack record. Therefore, the

development of the Powerformeris inherently linked to the reliability

and the development of the XLPE-insulated cables.With the new technology, future trans-

formerless power plants can be constructed,

Digital Object Identifier 10.1109/MPOT.2008.915315© ARTVILLE

IBRAHIM A.METWALLY, R.M.

RADWAN, AND A.M. ABOU-ELYAZIED

Powerformers:A breakthroughof high-voltage

power generators

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leading to a new concept of energy sys-tems. The new machine Powerformerhas high efficiency, lower maintenancecosts, reduced environmental impact,and better availability, reliability, androbustness.

Going against conventionThe Powerformer, although a new

machine, is a three-phase synchronousgenerator with a rotor of conventionaldesign. The basic difference, comparedto the conventional generator, lies in thestator windings. With Powerformers thestator winding consists of high-voltagecables instead of the conventional rec-tangular cross-section windings.

In order to raise the output powerof an electric machine, either the levelof the output voltage or the current inthe stator windings must be increased.Insulation technology limited the out-put generation voltage, so the solutionwas to increase the current in themachine instead of the output voltage.However, in Powerformers, the outputpower is increased by increasing theoutput voltage using XLPE cables in thestator winding.

Figure 3 shows the quantities of steeland copper in the Powerformer and thereference (conventional) systems. Theweights are stated as net weights permegawatt-hour of electricity produced.More copper is used in the convention-al system than in the Powerformer,chiefly due to the copper content in thetransformer. The Powerformer systemhas more steel, mainly because thePowerformer stator has 2.5 times moreelectrical sheeting than the conventionalmachine. There is considerable poten-

tial for technological advances leadingto lighter and smaller machines. Thereference (conventional) systemrequires more consumption of carbon,oi l , and gas than that of thePowerformer. The highest emissionsoriginate from losses during the utiliza-tion phase. In addition, the emissions ofcarbon dioxide, nitric oxide, and sulfurdioxide for the conventional system arehigher than that of the Powerformer.

Conventional generatorsThe stator windings of the conven-

tional generators consist of rectangularconductors that lie in the stator slot. Themain goals of rectangular conductorshape selection are to maximize both thecurrent loading and the filling factor.

According to Maxwell’s equations,the shape of these conductors results inan uneven electric field as well as a

magnetic field distribution with values,inevitably created at each of the slotfour corners, as shown in Fig. 4. Thisintensification of corner field dictatesthe use of insulation materials with veryhigh dielectric strength (e.g., sheet micaset in epoxy resin).

The practical consequence for a rec-tangular conductor in an electricmachine is that the insulation and themagnetic materials of the machine arehighly stressed and not uniformlyloaded, which leads to an uneconomi-cal use of the involved materials.Failures in the machine related to thehigh electric stresses on the insulatingmaterials are also very likely to occur.

Therefore, intricate measures have tobe taken in the end-winding region tocontrol the electric field so as to avoidpartial discharges and corona. To mini-mize the eddy current losses in the sta-tor coils, the copper laminations consti-tuting the conductors must be trans-posed along the winding according toan elaborate scheme.

PowerformersContrary to conventional generators,

the windings of this new high-voltagegenerator have cylindrical conductors.As can be inferred from Maxwell’s equa-tions, a cylindrical conductor yields aneven electric and magnetic field distribu-tion, which is a prerequisite for a high-voltage electric machine (see Fig. 4).

As mentioned earlier, the stator wind-ing of the Powerformer consists of high-voltage cables. Consequently, the outputvoltage of Powerformers is only limitedby the state-of-the-art high-voltage cabletechnology. Recently, insulation materi-als and production techniques offer reli-able cables at operating gradients in theorder of 10 kV/mm and even more.Such high electric field is not acceptedfor the conventional mica/epoxy-basedcoil insulation.

The cable circular cross sectionsolves the two basic problems arisingfrom the use of conventional rectangu-lar stator windings:

• First, within the stator slots, theuniform electric field in the insulatormaximizes insulation performance andthe voltage rating of the cable.

• Second, bending a cable of circu-lar cross section does not result in thekinks and sharp edges that arise with arectangular cable. Thus, even in the endregions where the cable is bent to makethe transition from one slot to the next,the electric field within the insulator

Fig. 1 Schematic diagram of (a) aconventional plant with step-uptransformer, and (b) the same plant witha Powerformer connected directly to thegrid. 1) Generator, 2) generator circuitbreaker, 3) surge arrester, 4) step-uptransformer, and 5) circuit breaker.

Fig. 2 Development of Powerformer rated voltage.

1

1

24

5

5

3 3

3

(a)

(b)

0

50

100

150

200

250

300

350

400

450

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

Year

Rat

ed V

olta

ge, k

V

245 kV

45 kV

136 kV155 kV

345 kV

420 kV

Exp

ecte

d in

a D

ecad

e

38 IEEE POTENTIALS

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MAY/JUNE 2008 39

remains free of singularities. At the endregions of the Powerformer, the electricfield remains confined within the cable.Consequently, the need to control anexternal electric field, as in a conven-tional machine, is eliminated.

Design timeThe concept of Powerformers fea-

tures innovations that include commer-cial high-voltage power cables, lie incircular bores, and are accommodatedin the stator slots.

Cable designAs previously indicated, the winding

of the Powerformer consists of insulat-ed high-voltage power cables similar tothe standard and commercial powercables used in power system distribu-tion. However, the cables in thePowerformer have neither a metallicscreen nor a sheath.

Figure 5 illustrates the constructionof the Powerformer winding cable. Itconsists of a stranded conductor, aninner semiconductive layer, a soliddielectric (normally an XLPE), andfinally, an outer semiconductive layer.The purpose of the inner semiconduc-tive layer is to create a uniform electricfield at the inner surface of the insula-tion layer, while the outer semiconduc-tive layer acts to confine the electricfield within the insulator.

The word “semiconductor” describes amaterial with relatively high resistivity, inthis case XLPE doped with carbon. Sucha semiconductor is, more accurately, aresistive conductor.

In general, in the stranded conductorthere is a center wire surrounded byconcentric layers of 12, 18, 24, 30, 36,and 42 wires. This is commonly knownas a “concentric-lay” conductor. Eachlayer is applied with alternate directionof lay. The conductor cross section willbe dimensioned with respect to the pre-vailing system voltage and the maxi-mum power of the generating unit. Aconductor used in an electrical machineis exposed for a higher magnetic leak-age flux than a conductor used in trans-mission or distribution systems.

In order to minimize the additionallosses due to the magnetic leakage fluxin the Powerformer conductors, it isnecessary to subdivide the conductorinto mutually insulated strands. Themajority of the strands may be insulat-ed, but to ensure an equal electricalpotential of the strands and the innersemiconducting layer, one or more of

the strands in the outermost layer maybe non-insulated.

The induced voltage in a Power-former generator stator winding willgradually increase from the neutralpoint to the line terminal. Therefore,the cable used for the stator windingis accordingly exposed for differentelectrical stresses along the length ofthe winding. It is therefore feasible,

in a Powerformer, to use a thinnerinsulation for the first turns of thewinding and thereafter increase theinsulat ion thickness; i t is cal led“stepped insulation” (Fig. 4).

One way of obtaining this is to usea predefined number of different cabledimensions per phase (i.e., a stepwiseincrease in the insulation thickness).This type of graded insulation facilitates

Fig. 3 Net weight consumption of copper and steel in Powerformer and the referencesystem.

30

20

10

Powerformer

Copper Steel Copper Steel

Reference

Stator

Rotor

Transformer

Other

Total

0

g/M

Wh

Fig. 4 Stator-bar of the conventional generator and stator cable-winding of thePowerformer.

Conventional Generator

Stator-Bar

Ground WallInsulation

Turn Insulation

Strand Insulation

3 kV/mm

6 kV/mmor Even Much Higher

E (

kV/m

m)

Sta

tor W

indi

ng

S

Powerformer

Cable-Winding

Ground Wall/TurnInsulation

Conductor

Ele

ctric

Fie

ld

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a better optimization of the volume ofthe laminated stator core. Also, steppedinsulation has the effect of ensuringthat the tooth width is effectively con-stant along its length, irrespective ofthe radial spread, keeping the flux den-sity constant.

Stator designThe Powerformer is fitted with a con-

ventional rotor. Therefore, only the stator-related design aspects will be coveredhere. The Powerformer stator consists of alaminated core, built up from electricalsheets. Teeth in the outer section pointinward toward the rotor (at the center).The winding is located in the slots formedby the teeth. The cross-section of the slotsdecreases toward the rotor because eachwinding turn requires less cable insulationcloser to the rotor. The cross section ofthe winding cable is taken into account bythe stator slot design. Each slot has circu-lar bores at intervals, forming narrowwaists between the winding layers.

The slots should enclose the casingof the coil as closely as possible. At thesame time, the teeth should be as broadas possible at each radial level. Thisreduces the losses in the machine andalso the excitation needed. The statorteeth can also be designed such that theradial width of the slot is largely con-stant over its entire length. This equal-izes the loading on the stator tooth.

The winding can be described as amultilayer concentric winding, whichmeans that the number of coil endscrossing each other is minimized. Thisfeature allows simpler and faster thread-ing of the stator winding. Figure 6(a)and (b) shows a sectional view of thePowerformer stator and the temperaturedistribution around a stator slot.

As a result of using a high-voltagecable in the statorwinding, an increasein the output voltagecorresponds to adecrease in theloading current inthe machine for agiven power rating.Therefore, a lowercurrent densityresults in lowerresistive losses inthe machine. Theouter semiconduct-ing layer cable isconnected to earthpotential. Hence,the electric field

outside the outer semiconducting layer isclose to zero in the coil-end region.Consequently, there is no need to con-trol the electric field in the coil-endregion as in the conventional generator.

In the conventional generators, thefield has to be controlled at severallocations per turn. This eliminates fieldconcentrations in the core, the coil-endregions, and the transition betweenthem. There is no risk for either partialdischarges or corona in any region ofthe winding. Moreover, personal safetyis increased substantially as the end-winding region is at ground potential.

Due to the lower currents and cur-rent densities, the current forces inPowerformers are considerably smallerthan those in conventional generators.As a consequence, the support for theend windings can be made simpler inthe Powerformer. Another importantaspect when designing a Powerformeris the minimization of the cable vibra-tion. To achieve this goal and to ensuregood electrical contact between thecable and the laminated core, the cableis fixed in the slot. It is based on a tri-angular silicon rubber hose that isinserted between cables and slot wallas shown in Fig. 7.

The shape of the cross section of therubber hose is designed to allow forelastic deformation necessary to keepthe fixation forces within certain limits.The maximum force must be limited toreduce the visco-elastic deformation ofthe cable cross section. A minimumforce has to be maintained at low tem-peratures to avoid loss of contactbetween the cable and slot wall.However, to avoid local deformation onthe cable at the end-winding region dueto vibration and bracing forces, thecables are separated from each other bya rubber distance element.

Cooling systemThe cooling system of the

Powerformer stator core is also basedon a new concept. This is due to thelow current in the cables of the statorwinding and the lower ratio betweenohmic and iron losses than that for aconventional generator. Accordingly,most of the heat is generated in the sta-tor core, which is grounded. This factgreatly simplifies the cooling system.The new cooling system is an indirectsystem that cools the stator core by axi-ally inserted water pipes made of highdensity XLPE. Thus, the stator has noradial air cooling ducts, and this leadsto a homogeneous stator core. Thismakes the gross length of the statorshorter, the efficiency improves, andthe stator assembly is more convenient,especially with respect to the cableinstallation through the slots.

As the water cooling is carried out atground potential, there is no need forde-ionized water as in the conventionalwater-cooled stator windings. Ordinarytap water may be used for the coolingof the Powerformer stator core. The useof plastic tubing also eliminates the risk

of a short circuitbetween the tubesand the core andproblems with eddycurrents in jointsand pipes. On theother hand, therotor and the endwindings are aircooled.

Faultybehavior

The performanceof the Powerformerunder fault condi-tions, whether thefault is internal or

Fig. 5 Powerformer winding-cable: 1)conductor, 2) inner semiconductinglayer, 3) XLPE insulation, and 4) outersemiconducting layer.

Fig. 6 (a) Sectional view of the Powerformer stator: 1) rotor, 2) section of stator, 3)teeth, 4) slots, 5) main winding cable, and 6) auxiliary winding and(b) temperature distribution around a stator slot.

1 2 3 4

2

1

(a) (b)

3

3

4 5 6

40 IEEE POTENTIALS

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external, and the comparison betweenthe fault currents of a plant equippedwith a Powerformer with the fault cur-rents in a generating station equippedwith a conventional generator and astep-up transformer have revealed manypositive points in favor of thePowerformer. The internal fault refers toa fault that occurs at the terminals of thegenerator, and an external fault is a faultthat occurs at the high-voltage side ofthe step-up transformer.

In the case of a Powerformer, inter-nal and external faults are basically thesame as a Powerformer that is connect-ed directly to the high-voltage bus-barin the generating station. Here is a sum-mary of different faults.

• For external faults, the fault cur-rents from a Powerformer at externalthree-phase short circuits will be of thesame magnitude as the fault currentfrom the conventional unit.

• The fault current from aPowerformer at external single-phaseground faults will be lower than thatfrom the conventional unit. The reasonis that the neutral point of aPowerformer is isolated from groundwhile the neutral point of the step-uptransformer of the conventional genera-tor is solidly grounded. Therefore, theintroduction of a Powerformer decreasesthe fault current at an external single-phase ground-fault because the elimina-tion of the step-up transformer increasesthe resulting zero-sequence reactance.

• The internal three-phase short-circuit current of a Powerformer is lessthan that of the conventional generatordue to its higher output voltage.

• In the case of the internal two-phase-to-ground and the internal phase-to-phase faults, the fault current in aconventional generating unit will besubstantially higher than the fault cur-rent in the Powerformer. The currentsat internal faults, for the studied specificcase, are summarized in Fig. 8.

• For the internal single phase-to-ground internal fault, the fault currentin a conventional generating unit ismuch lower than that of thePowerformer due to the high imped-ance grounding of the neutral of theconventional generator (Fig. 8).

In service, it is indispensable to elim-inate discharges in the intersticesbetween the main insulation of the con-ductor/cable and the walls of the slot.The damage to the insulation is pro-duced when the partially conductingcoating on the bar surface becomes

electrically isolated from the slot walls.A voltage is developed on the surfacecoating, which may be sufficient tobreak down the airgap and producearcing between the surface coating andthe slot wall. This results in the fusingof areas of the insulation surface. Thisslot-discharge activity highlights theneed to earth the surface coating to thestator core at some points along itslength.

In the Powerformer, the originaldesign of the cable fixation in the slotswas based on the principle of ground-ing the outer semiconducting layer ofthe cable in the slot by pressing thecable against the stator laminations viafixation hoses (Fig. 7).

Differential protectionThe Powerformer winding cables can

be considered as a capacitor withcharges on the electrodes that are, in thiscase, the inner and the outer semi-con-ducting layers. The electrical charge on aphase winding of a Powerformer at volt-age maximum is 30 times larger than that

on a phase winding of a conventionalgenerator with the same rated apparentpower. The internal short circuit currentshould be investigated and simulatedaccurately for the purpose of the protec-tion of Powerformers by taking intoaccount the distributed capacitance.

Large conventional generators andlong transmission lines are confrontedwith the problem of increasing capaci-tive charging current. Therefore, theimpact on the reliability of differentialprotection should no longer be negligi-ble. In a Powerformer’s case, where thecable is considered as a capacitor withcharges on its two electrodes as dis-cussed earlier, the capacitance in theprotection zone causes two problems.First, the operating value of the differ-ential protection must be increased toavoid unwanted operation caused bythe capacitive differential current.Second, the differential protection mustsuppress transients in the differentialcurrent to avoid unwanted operationcaused by the capacitive inrush current.

The differential protection for theconventional generators rarely consid-ers the influence of capacitance in theprotection zone because the value ofthe direct earth capacitance is quitelow. Similar to the analysis of thecapacitance of the transmission line,the equivalence of winding capacitanceof the generator, in which the capaci-tance distribution along with the wind-ing is represented by a lump capaci-tance with 50% at the phase terminaland 50% at the neutral point. Thisassumption is suitable for the cases ofcapacitance evenly distributed, such asthe transmission line and the windingof a conventional generator. However,it will lead to errors in analyzing thestator winding of the Powerformer inthat the winding capacitance does not

Fig. 7 Fixation of the winding cables inthe Powerformer slots: 1) laminatedstator core, 2) XLPE insulation, 3)conductor, and 4) fixation hose.

1 2 3 4

Fig. 8 Comparison of the fault currents in (a) a conventional plant, and (b) a plantequipped with a Powerformer at internal faults.

Δ

15 kV

∼ ∼

150 kV 150 kV

ExternalNetwork

ExternalNetwork

(a) (b)

Short circuit currents3-phase-to-ground: 53 kA2-phase-to-ground: 45 kA2-phase: 45 kA1-phase to ground: <0.1 kA

Short circuit currents3-phase-to-ground: 17 kA2-phase-to-ground: 16 kA2-phase: 14 kA1-phase to ground: 16 kA

MAY/JUNE 2008 41

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42 IEEE POTENTIALS

actually distribute evenly along withthe stator winding. It is known that thewinding of the Powerformer adoptsgraded insulation, which leads to vari-ous cable thicknesses in different por-tions of the winding and the unevencapacitance distribution. Improving thereliability of the differential protectionof the Powerformer can be achieved byusing a self-adaptive compensationmethodology. The feasibility and effec-tiveness of the proposed scheme hasbeen proven with test results.

The subtransient reactance of thePowerformer is usually similar to, orslightly lower than, the total faultimpedance of the conventional genera-tor and its transformer. Consequently,there is a relatively limited impact pro-duced by the replacement of a conven-tional generator by a Powerformer onthe critical fault types governing circuit-breaker ratings in a realistic power sys-tem. The system modification requiresan increase in the breaker capacity toallow the inclusion of a Powerformerinto an existing system where the origi-nal breakers have been rated very low,according to IEEE standards. Futurework should concentrate on otherproperties—rather than critical faulttypes—such as switching capability ortransient recovery voltage.

EconomyUsing the Powerformer in a certain

power system has a significant effect onthe plant’s overall cost. The cost reduc-tion is due to high efficiency, lowerlosses, and low environmental impact.A study has been carried out to showhow a Powerformer could influence theeconomy of hydropower projects. Theaim of the study was to compare theexisting hydropower plant in India witha hypothetical plant with aPowerformer. The annual energy pro-duction is equal to 3,200 gigawatt-hours. There are four turbines, fourgenerators, and 13 single-phase trans-formers. Four Powerformers replaced

the four generators and the 13 single-phase step-up transformers. The totalcost savings associated withPowerformers, resulting from lowerbuilding and maintenance costs, wereestimated at 24% in addition to theincreased annual power generation by17 GWh (by 0.56%).

Another study was carried out adopt-ing a new methodology for analyzing thegeneration capacity of power systems.The method is validated on the 24-busIEEE-RTS system, augmented with realis-tic market and plant data, by using it tocompare the financial viability of severalgenerator investments applying eitherconventional or Powerformer technolo-gies. The significance of the results isassessed using several financial risk mea-sures. Comparative results of differentrisk indices have confirmed that thePowerformer is a better option within theset of assumptions used for this study.The results suggest that the Powerformerwill be superior compared with its con-ventional generator counterpart. The dif-ferences in viability, however, are notlarge, and more work is required toconfirm the observed trends.

Reliability and robustness The Powerformer has higher avail-

ability, more reactive power margin,and extra short-term overloading capac-ity. Several studies on its impact on thesystem dynamic behavior have beencarried out. It has been confirmed thatit can delay the system voltage collapseby several seconds. Similarly, its designis capable of producing perceptiblechanges on the system fault behavior.

Table 1 shows that there are sixPowerformers in operation: four are inSweden, one in Japan, and another onein Canada, and all are running withoutany difficulties. The impact of aPowerformer on the composite systemreliability has been examined and com-pared with the existing conventionalgenerators using the 24-bus IEEE-RTStest system. A number of sensitivity

analyses are performed, and it has beenobserved that the improvement in thesteady state adequacy is largely attrib-uted to its higher availabilities. Its impactdepends on its location, load level, andthe system topologies. Operating historyof a Powerformer is limited, thus long-term viability assessments are made sole-ly on the available cable data and someof the extensive laboratory testing by themanufacturers.

In an attempt to evaluate the failurerate and reliability of stator windings,an electrical ageing test is conducted ina realistic Powerformer environmentwith an applied voltage of 220 kV cor-responding to 25 kV/mm of field stressto insulation. The calculated failure rateof the high-voltage stator windings isequal to 0.53 faults/100 generators-yearand with this the mean time to failurebecomes 1/0.0053 ≈ 190 years. If amajor fault occurs inside the stator core,complete stator laminations need to bereplaced. The mean repair time is esti-mated to be 13 days. The unavailabilityof the Powerformer stator winding is aslow as 0.019%.

Another independent finding sup-ports the estimated values which revealthat the failure rate of a Powerformer issignificantly lower than the recordedfailure rates of the conventional genera-tor in the hydropower plants in Nordicelectricity generation and transmissionsystems. It is close to or lower than therecorded failure rates of the generatorsin the nuclear power plants in Sweden.

Station-originated failures due to sta-tion components such as breakers,transformers and busbar sections canhave a significant impact on overallpower system reliability, where severalpotential points of failures can be elimi-nated. Powerformer arrangementshould be more reliable for the longrun. If failure occurs in a transformer,its replacement time can be very long,causing a high loss of power supply,especially if one transformer is servingfor more than one generator. It is fairlylogical to assume equal failure rates forthe conventional rotors andPowerformer rotors, because withminor modifications a conventionalgenerator rotor could be converted tofit the Powerformer.

The risk of voltage collapse requiresthat the system operator use all availablereactive resources in the receiving areato maintain the voltage level. There is aneed to boost the reactive generationfrom synchronous machines in the

Table 1. Existing Powerformer generators.Rated voltage Rated power

Location Commissioning Type (kV) (MVA)

Porjus 1998 Hydro 45 11Eskilstuna 2000 Thermal 136 42Porsi 2001 Hydro 155 75Holjebro 2001 Hydro 78 25Miller Creek 2002 Hydro 25 32.8Katzurazawa 2003 Hydro 66 9

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MAY/JUNE 2008 43

receiving area. Powerformers have abetter overload capability than conven-tional generators, so the Powerformersproved their robustness under voltagecollapse. In 2001, statistical recordsrevealed that the first Powerformer,located in Porjus, Sweden, accumulatedmore than 13,700 hours of successfuloperation without any disturbances. Theunit has been synchronized to the gridmore than 150 times and has been sub-jected to thermal and electrical cycling.Thorburn and Leijon also presented acase study of upgrading Powerformerrated voltages. Powerformers with arated voltage of 245, 345, and 420 kVare being considered.

Extensive tests on the firstPowerformer, including heat run test,load rejection, efficiency measurements,and short-circuit tests on the terminal at100% magnetization, have been con-ducted to evaluate its performance andbehavior. The response on the auxiliarywinding during a short-circuit test onthe main winding revealed that thePowerformer is robust.

In addition, the second Powerformer,located in Eskilstuna, Sweden, hasundergone workshop tests and has suc-cessfully operated at 177 kV during anover-excitation test. In 2000, thePowerformer stood the sudden short-cir-cuit test under 100% of rated voltage. Asexpected, the currents at the test wereclose to 1,000% of normal load current.This is the highest possible current,which may flow from Powerformer incase of faults in the transmission net-work. The unit also performed wellwhen subjected to a sudden singlephase-to-ground fault. In this case, thevoltage on the two healthy phases tem-porarily exceeded 175% of normal oper-ating voltage. This is the highest possiblevoltage, which may stress the insulationof the stator winding in case of faults inthe cable connecting the Powerformer tothe transmission network.

Voltage collapse, loadability, and sta-bility studies were made on the Swedishtest systems by examining the effects ofremoval of step-up transformers at select-ed generator busses (i.e., direct connec-tion Powerformers to the high voltagebus). The 100% overload for up to 30minutes was observed with reinforcedrotor cooling. Three power systems wereused in the studies of the 32-bus Swedishtest system. The results indicate that thelocation of the Powerformer has a notice-able impact on the loadability and loca-tion of voltage collapse in the system.

As discussed earlier, the electricfield is fully confined to the cable. Soin the Powerformer, the bracing of thecoil ends is reduced to simply support-ing the cables constituting the over-hang (whose surface is held at groundpotential) such that the vibrations ofthe coil ends are minimized. The forcesexerted on the end windings are highlyreduced during operation of thePowerformer compared to the conven-tional generator. This is due to its lowerrated current for a given output power.

Objection on Powerformer technology

Objections on Powerformer technol-ogy by designers of conventional gener-ators generally fall under one of threeheadings: efficiency, cable technology,and possible applications.

Using a circular conductor will meanthat the cable occupies a larger volumewithin the stator. This will result in alarger, more expensive machine. Also,iron losses will be higher, offsetting thesavings in transformer losses. However,the generator will be larger, but thestep-up transformer has been eliminat-ed. Using Maxwell’s equations it is pos-sible to show that, in terms of energyproduced per unit volume, thePowerformer must be more efficient.Again, iron losses in the conventionalgenerator are indeed higher, but thelow-current nature of the Powerformerdesign means that these extra losses aremore than offset by lower stator copperlosses and lower ventilation losses (theresult of reduced demands on the air-

based cooling system). Generally, iti s estimated that an optimizedPowerformer design will be between0.5% and 1.5% more efficient than aconventional generator and its step-uptransformer solution.

XLPE cable, especially at 400 kV, is arelatively new product. There are seriousdoubts over its long term suitability asstator windings. Also, the cable is verystiff, especially at higher voltages, mak-ing it difficult, if not impossible, to bendthe cable at the end regions. However,the cable in the machine “XLPE” is ratedfor continuous operation at 90 ◦C, withthe capacity to operate at up to 130 ◦Cfor several hours. In normal operation,the Powerformer windings are kept at 70◦C. The machine capacity to operate at ahigher temperature (higher current),without harm for short periods meansthat it can play a valuable role in systemmanagement, as a controllable source ofreal and reactive powers.

It is believed that when the adop-tion of Powerformers becomes morecommon, reliable data can be obtainedregarding its impact on power systems.Fair and reliable judgment on this newtechnology can be convincing topower utilities.

ConclusionsPowerformers, wound by XLPE power

cables without sheath, represent a break-through of high-voltage power genera-tors, where they provide direct connec-tion to the high-voltage power grid with-out going via a step-up transformer atvoltage rating up to 155 kV.Powerformers enable very clean andcompact power plants that are not onlyeconomical, reliable, and environmentfriendly but also more efficient than con-ventional ones.

Powerformers are fitted with conven-tional rotors, while the stator consists ofa laminated core. The cross- section ofthe slots decreases toward the rotorsince each winding turn requires lesscable insulation closer to the rotor.

The cooling system of the stator corein a Powerformer is also based on a newconcept, where an indirect system coolsthe stator core by axially inserted waterpipes made of high-density XLPE.

The use of a cylindrical conductoryields an even distribution of the elec-tric and the magnetic fields, which is aprerequisite for a high-voltage electricmachine. On the contrary, the conven-tional generators use rectangular con-ductors that lead to overstressing the

“SEMICONDUCTOR”DESCRIBES A

MATERIAL WITH RELATIVELY HIGH

RESISTIVITY, IN THISCASE XLPE DOPED

WITH CARBON. SUCH A SEMICONDUCTOR IS,MORE ACCURATELY, A

RESISTIVE CONDUCTOR.

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44 IEEE POTENTIALS

insulation and the magnetic materials,and a high rate of electrical failures.

To mitigate partial discharges inPowerformers, the XLPE cables (statorwindings) are fitted in the stator slotswith a certain amount of clearance. Agood contact is kept between the outersemiconducting layer of the XLPE cablesand the slot walls via fixation hoses.

Additional magnetic losses in thePowerformer conductors can be mini-mized by subdividing the conductorinto mutually insulated strands. Themajority of the strands may be insulat-ed. To ensure equal electrical potentialin the strands and the inner semicon-ducting layer, one or more of thestrands in the outermost layer may benoninsulated.

The limitation of the Powerformeroutput voltage is set solely by state-of-the-art high-voltage XLPE cable

technology (stator windings). Thecable insulation of the Powerformercannot withstand the similar tempera-ture as the insulation materials in con-ventional machines. The criterion oftemperature in the available cables fordesign practice is 70 ◦C. It is expectedthat Powerformers with voltage rat-ings up to 420 kV can be realized in adecade.

Read more about it• M. Leijon, L. Gertmar, H. Frank, J.

Martinsson, T. Karlsson, B. Johansson, K.Isaksson, and U. Wollstrom, “Breakingconventions in electrical power plants,”1998, presented at Paper 1 1/37-3,CIGRE, Paris.

• M. Leijon, “Powerformer—A radi-cally new rotating machine,” ABB Rev.,pp. 21–26, 1998.

• M. Leijon, F. Owman, T. Sorqvist, C.

Parkegren, S. Lindahl, and T. Karlsson,“Powerformer: A giant step in powerplant engineering,” in Proc. Int. Conf.Electric Machines and Drives “IEMD,”May 1999, pp. 830–832.

• S. Lindah, “Improved control offield current heating for voltage stabilitymachine design-Powerformer,” in Proc.IEEE Power Engineering Society WinterMeeting, 2001, vol. 1, pp. 209–214.

• T.R. Limbu and T.K. Saha,“Investigations of the impact ofPowerformer on composite power systemreliability,” in Proc. IEEE PowerEngineering Society General Meeting 1,2005, pp. 406–413.

• Q. Tian, X. Lin, and W. Lu, “ANovel Current Differential ProtectionScheme for Powerformer,” inInternational Conference on PowerSystem Technology, PowerCon 2006, Oct.2006, pp. 1–7.

• W. Shishan, L. Zeyuan, L. Yanming,G. Yinna, and G. Hong, “Calculation ofshort-circuit mechanical strength forPowerformer,” in Proc. Int Conf PowerSystem Technology, PowerCon 2006, Oct.2006, pp. 1–6.

• Q. Tian, X. Lin, and P. Liu, “A novelself-adaptive compensated differentialprotection design suitable for the genera-tor with considerable winding distributedcapacitance,” IEEE Trans. Power Delivery,vol. 22, no. 2, pp. 836–842, 2007.

• X. Lin, Q. Tian, Y. Gao, and L. Liu,“Studies on the internal fault simulations ofa high-voltage cable-wound generator,”IEEE Trans. Energy Conversion, vol. 22, no.2, pp. 240–249, 2007.

• IEEE Trial-Use Guide to theMeasurement of Partial Discharges inRotating Machinery, IEEE Standard1434–2000, 2000.

About the authorsIbrahim A. Metwally (metwally@

squ.edu.om) earned an M.Eng. and Ph.D.in high-voltage engineering. He is a pro-fessor with the Department of ElectricalEngineering, Mansoura University.

R.M. Radwan (roshdyradwan@yahoo.com) earned a B.Sc. degree inelectrical engineering and a Ph.D. inelectrical engineering. He is a full-timeprofessor in the Faculty of Engineering,Cairo University.

A.M. Abou-Elyazied (abouelyazied@gmail.com) earned B.Sc. and M.Scdegrees in electrical engineering. He isa Ph.D. student in the Department ofElectrical Energy, Systems, andAutomation at Ghent University,Belgium.

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