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Large Converter-Fed Synchronous Motorsfor High Speeds and Adjustable Speed Operation: Design Features and ExperienceGerhard J. NeidhoferABB Power Generation Ltd

DepartmentKWB7CH - 5401 Baden, SwitzerlandAbstract - The use of higher than the line frequency enab lesynchronous motors to be operated at high adjustable speedsespecially in high-power electrical drive systems. The paperoutlines the special design measures for such m otors with solid -iron cylindrical rotors and describes the various novel applica-tion fields including operating experience.

I. INTRODUCTIONDue to their particular potential and features, synchro-nous motors are best suited for high-power and high-speeddrive duties. They emerged from a fascinating course ofdevelopment which the synchronous machine has undergone

from the very beginnings up to the present day [l]. Thenewest advances came from the progress in power elec-tronics, enabling static frequency conversion and even fre-quency variation, thus offering individual and optimumspeeds in drive systems. The challenge for high-speed motoroperation required the development of a machine based onthe cylindrical solid-iron rotor principle and adapted to theparticular converter supply conditions.11. HIGH-SPEED AND ADJUSTABLE SPEED DRIVE SYSTEMS:

MAIN FEATURES AND APPLICATIONSFig.1 shows the simplified diagram of a converter-motordrive system [2]. The electrical energy is transmitted fromthe medium-voltage mains via an isolating transformer to

the rectifier side of the converter. Me r rectification, thecurrent flows through the smoothing reactor in the dc link tothe inverter which feeds the synchronous motor with variablevoltage and frequency.Static frequency converter (LCI)

1~

Mains

i Line- Load-Isolating ,commutated commutated ! Synchronoustransformer ! converter Reactor converter I motorI

I

I IStatic

Fig. 1 Diagramof converter-motordrive systemWith respect to turn-off times and high current blockingcapability of power thyristors, frequencies as high as 120Hzare possible. Choosing the lowest number of poles, namelytwo, for the synchronous motor, the present maximumattainable speed is 7200 rpm. Such high rotational speedsnecessitate the motor to be constructed with a cylindrical,solid-iron rotor. This helps master the significant centrifugalforces as has already been well proven in practice on turbo-generators. With the inclusion of direct air cooling in the

Anders G. TroedsonABB ndustrial Systems Inc.16250W. Glendale Drive

New Berlin, WI 53 151 , USA

field winding of the rotor, very high single-unit ratings arepossible. The combination with variable-frequency supplythus leads o drive systems which offer:highest unit ratings (in the range of 10 - 40 MW)high-speed operation (up to 7200 rpm)variable-speed operation direct drives without gearsprecise speed control loss-free process controlhigh overall efficiency low maintenance.

Although the initial investment is slightly higher than forconventional drives, this is soon recovered due to the betterefficiency and simpler maintenance.

Fig. 2Themainapplications can be outlinedasfollows:wherever unit ratings andor speeds are required beyondthe limits for conventional dc and ac drivesreplacement of steam and gas turbine drives,especially for:boiler feed pumps in thermal power stationsturbocompressors in petrochemical and other process

industries.One example of these applications can be seen in Fig. 2,showing drive systems for feedwater pumps in a thermalpower station [2], [3]. Another application is, for example,turbocompressors in petrochemical plants [4], [6]. Mean-while, more than 50 units of similar rating and speed havebeen put in service by the same manufacturer. The newestdrive systems were supplied for gas transport in North Seagas pipelines [7].

Feedwaterpump and motorrated 12MW at6000rpmforvariable-sped operation

111 SYSTEM DESIGN CONSIDERATIONSThere are several important system design aspects thathave to be taken into account when designing a converter-fed drive system. These factors tend to become increasinglyimportant and the effects tend to be more pronounced as thesize of the drive becomes larger. These include:

0-7803-39464/97/$10.00 1997 EEE. MA26.1

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A. Network HarmonicsThe recommendations outlined in IEEE 519 provideguidelines for maximum acceptable harmonic distortion seenin the supplying network. Suitable remedies, such as the useof 12- or 24-pulse network connection, tuned filters, etc. arecommonly applied to bring the distortion to acceptablelevels. These countermeasures are well documented else-where and we will not elaborate further on this subject here.

B. Machine Side Harmonics and Special Machine DesignConsiderationsCurrent-source converters cause line currents which, inthe case of six-pulse equipment, have an approximatelyrectangular or trapezoidal waveform. Consequently, thereare current harmonics of the order

i.e. m =5 and 7, 11and 13, 17 and 19 etc. With 12-pulseconverters, harmonic pairs associated with k =1,3, 5 etc.can be avoided. The amplitudes I, depend on the order itself,but also on current overlapping during commutation,determined by the commutation reactance and firing angle.Potential harmonic effects are countered as follows:The armature winding in the stator has to be designedwith thinner strands for limiting the more pronounced skineffect at the higher frequencies. In addition, a suitableinsulation system masters the slightly increased dielectricstress due to the voltage spikes from current commutation.A very effective damper winding is needed in the rotor,provided by conductive slot wedges and suitable end-con-nections. It acts as a squirrel-cage for compensating theharmonic current sheets. These are travelling waves movingeither in the same direction as the rotor (m=7, 13 etc.), orin the opposite direction (m =5, 11 etc.). Their velocity,referred to the rotor, is thus (m - 1) or (m +1) times thesynchronous velocity (i.e. 6 times, 12 times etc.).A proper term for estimating the rotor surface electricload is the so-called equivalent negative-sequence current,leading to the same amount of unbalanced load losses causedby a negative-sequence current i2with twice the synchronousvelocity or frequency, referred to the rotor [SI:

(2)Considering the stator current-harmonics, each pair m

m = 6 k 21 k = 1,2,3 etc. (1)

p2 z constant i i J;i

p,,, = constant (i$ -I-i 2 )J;;produces losses of approximately

where n is the relative frequency, referred to the rotor (n=6,12, 18,24 etc.) and with ml =n - 1, m2=n +1.The sum of these losses would be caused by an equivalentnegative-sequence current of the magnitude

(31

(4)In a typicalexample for 6-pulse operation withml.2 5 7 11 13 17 19 23 25n 6 6 12 12 18 18 24 24i, 0.20 0.12 0.074 0.053 0.033 0.025 0.013 0.01the result is i2equ0.35 p.u. (12 pulses: i2equ0.15 P.u.). Ofcourse, for real rotor structures detailed considerations oflocal current and loss conditions are necessary.

M A 2 6 . 2

A handy way for qua nw ng the rotor surface losses isoffered by introducing the rotor wave resistance Rw, acharacteristic quantity of typical rotor and damper designs[SI. The current-harmonic losses, per unit area at the rotorsurface, are thus(5)

where AZequs the equivalent linear current density, basedon Lqu.Example: For a synchronous motor rated 12MW at6000 rpm (f=100Hz),having a cylindrical rotor with acomplete damper cage, the rotor wave resistance, referred to2 x 100= 200 Hz, mounts to z 25 @, and the equivalentlinear current density, based on i2equ s 27 W m . Thus, thecurrent-harmonic losses come to p =9110 (1670) W/m2 andP =26 ( 5 )kW according to 6 (12)-pulse operation.A suitable comparison may be helpful: In 6-pulse opera-tion the rotor surface specific losses amount to =20% of thespecific heat production of a domestic iron, and the losses intotal correspond to =0.2% of the rated motor power. Suchmodest values result from the very effective rotor dampercage reflected by a low rotor wave resistance.

For reducing the current-harmonic effects and thecommutation reactance of the machine converter to aminimum, the damper bars are also arranged in the rotorpole regions [SI. The ends of all wedges are joined togetherby the retaining rings. SuEcient electrical contact is ensuredeven at low speeds and at standstill by special pretensioningmeasures 121. The damper bars serve a second purpose, thisbeing to act as electrical bridges across the flexibility slotsneeded for mechanically isotroping the rotor in both axes inorder to counter the double-frequency bending vibrations.C. Drive Train Considerations

The entire mechanical drive train needs to be analyzed asto its mechanical natural frequencies and their interactionwi th the total electrical system. The current harmonics, seenby the motor, can excite torsional oscillations in the shaftline. Interaction of the air-gap main flux with current-harmonic travelling waves results in forward- or backward-directed harmonic air-gap torques. The individual fre-quencies are 6, 12 etc. times the operating frequency. Foreach pair of current harmonics, with the common frequencyn referred to the rotor, the air-gap torque tn can analyticallybe approachedas follows:t, = (6)) i)t+l sin(n wt+p,,+I -with Q1 amplitude of the fundamental voltage

( P ~-~ , the respective current phase anglesThe resulting current-harmonic air-gap torques in thegiven example for 6-pulse (12-pulse) operation are:

n 6 12 18 24tn [P.u.] 0.20 (-) 0.08 (0,08) 0.036 (-) 0.015 (0,015)In order to avoid mechanical resonances, the shaft linehas to be designed in such a way that the natural torsionalfrequencies do not coincide with the electrical torquefrequencies within the dominant speed range. It may also benecessary to provide the shaft coupling with special damp-

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ening devices to change the dynamic behaviour of the entiredrive train.A comparison of the current harmonic losses and theproduced torque for a 12-pulse with a 6-pulse load sideconnection, clearly demonstrates the advantages of the 12-pulse system. This preferably is configured as a single-channel, 12-pulse converter feeding two three-phase wind-ings, displaced by 30 electrical degrees, in the stator [2].Adjustable speed motors also require special attention tothe bearing design. A three-bearing system is used: Twobearings support the main rotor and the third is arranged atthe outboard end of the exciter shaft. The bearings are ofpedestal type, forced lubricated, and of four-lobe sleeve type,derived from and previously used with good experience forindustrial turbines. Inherent damping properties ensure lowsensitivity to system unbalances, which enables the motor tosafely operate in close proximity to or even at the firstcritical speed (at the bending resonance state).D. xcitation System Considerations

Since the brushless excited motor requires a source ofexcitation at standstill to provide torque for starting andacceleration, a three-phase induction type exciter is used.The exciter rotor field is oriented to rotate in the oppositedirection to the stator field, providing a slip of 1.0 atstandstill which increases with speed. The exciter statorwinding is energized from a three-phase thyristor controlledac supply which induces voltage at the slip frequency in thethree-phase rotor winding. Thus, the motor is alwayssynchronized, even during start-up [6] .IV. PERFORMANCE TESTS

The first large converter-motor drives, built with this newtechnology, were thoroughly tested in a full scale back-to-back type test in the manufacturers workshop [ 3 ] .Two com-plete sets were installed on a test bed and coupled together(Fig. 3). Both sets could operate at full load, one of them in amotoring mode and the other in generator mode, while theDower losses were covered bv the factorys network.

Fig. 3 Load-testarrangement oftw o 12 MW/6000 rpmdrive systemsinthe testbayThe development and acceptance tests confirmed theproper functioning over the entire speed/power range, therestrained impacts of harmonics and all other specificationsincl. temperature rises, overall efficiency and sound level.The excellent results also provided a safe platform for sub-sequent drive equipment in other application fields. Further-

more, those tests have proven that individual componenttesting is sufficient for subsequent systems. The completesystem can be comfortably tested on site after installation [6].V. OPERATING EXPERIENCE

For more than ten years the modem drive systems havefound increasing access to various fields of application.Accordingly, the operating experience, which was positivefrom the very beginning, has grown continuously. Approxi-mately 50 drive sets are in operation to date, which repre-sents a total of two million service hours; individual figuresof approximately 100 000 service hours per drive unit dem-onstrate the high degree of reliability.A few problems of rather secondary nature arose in someisolated cases during commissioning or test runs, inparticular control and protection settings, and excitationrectifier failure [6] .They were quickly eliminated or resolvedduring the next planned shutdown, for example by theinstallation of a complete monitoring and fault diagnosticsystem. Another corrective action, related to the floatingspacer shaft between a motor and ethylene compressor, wasto fit an auxiliary bearing [5]. But these incidents did not atall influence the principle and capability of the new driveconcepts which continue to provide successful and reliableservicein a variety of power stations and industrial plants.

VI. CONCLUSION ANDOUTLOOKBased on the complete confirmation of all expectations

and the excellent operating experience it can be concludedthat the present state-of-the-art for high-power electricaldrive systems with high and adjustable speeds has consoli-dated itself. It can be expected that this advanced technologywill continue to take over various drive duties in new powerplants or in existing power stations with modernisationprograms, as well as in oil or petrochemical and otherprocess industries.

REFERENCESG . Neidh6feq The Evolution of the Synchronous Machine,ABB Review,K.M. Weber and W. Heil, Converter-Fed Synchronou s Motor as aGearless, High-speed Drive for Boiler Feed Pumps, Brown BovenReview,Vol. 73,No. 6, pp. 284.291, June 1986.H.E. Schweicka rdt, H. Kobi and AS. Mitchel, 12 MW / 6000 rpmvariable speed load commutated inverter drives for Matimba powerstationCIGRESession Aug. 2 - Sept. 4, 1986, Paper 1 -1 5.S. Comas, K.-H. Metzger, K. Schweizer and P. Steimer, Adjustablespeed, converter-fed synchronous motor used to drive a 13-MW turbo-compressor,ABB Review, No. 7, pp. 19-26, 1990.A Grgic, W. Heil and H. renner, Large converter-fed adjustable spee dAC drives for turbomachines, Proceedings of the 21st TurbomachinerySymposium and Short Courses, Texas University, S e p t . 28 -Oct. 1,1992,B.M . Wood, W .T. Oberle, J.H. Dula s and F. Steun, Application of alSOOOhp, 6000r/min Adjustable Speed Drive in a PetrochemicalFacility, 41st Annual Petroleum and Chemical Indushy Conference,Vancouver BC, Sept. 12-14,1994. IEEE Paper No. PC IC-94-31.K.M. Weber, Adjustable spe ed AC drive systems for North Sea gaspipelines,ABB Review, No . 9, pp. 4-9, 1 994 .G.J. eidh6fer and B.N . &se,