Choosing electric turbocompressor drivers

The Dutch NAM, a joint venture of Essoand Shell, as the owner and operator ofthe giant Groningen gas field in thenorth of The Netherlands, is the largest

producer of natural gas in the Netherlands. After35 years of production, the reservoir pressure ofthis field has declined, and three undergroundgas storage (UGS) reser-voirs were installed tocope with peak demand.This article reports onthe injection compres-sor drivers.

The single casing,double barrel Sulzerturbocompressors andtheir Siemens drivers areidentical and are de-signed for an injectioncapacity of 12 millionnm3/d at discharge pres-sures of 320-340 bar anda compressor shaft speedof some 11,000 rpm (seeFig.1).

The compressor driv-ers are speed controlledelectric motors with rating data as shown in Table I.

The selection of the driver type has been basedon environmental, operational, technical, and eco-nomical aspects; and the types of drivers investi-gated were

� fixed speed electric motors ,with and withouta hydraulic speed converter,

� variable frequency controlled electric motors,� gas turbines.During the detailed engineering phase for

Norg and Grijpskerk, a follow-up project of a

much larger scale was initiated to ensure thelong-term production capacity of the Groningengas field. Up to 29 booster compressors in ratingsof 23 and 11 MW, respectively, will be requiredto this end, and high-speed electric motor drivershave also been chosen for this giant task. A proto-type supersynchronous motor rated 23 MW @

6,300 rpm has been cho-sen and is already operat-ing successfully sinceWinter of 1998. As thecentrifugal compressorfor this application canbe built to match theshaft speed of the electricmotor, thus eliminatingthe step-up gear, an-other bold step towardsnew technology could betaken by the NAM forthese compressor-driverstrings. Dry gas seals forthe compressor and ac-tive magnetic bearingsfor both compressor andmotor were chosen, al-lowing for a “dry ,”

oilless system with outstanding performance andeconomy (see Fig. 2).

The Operator’s Choice of SystemEnvironmental AspectsTo start an undertaking like the UGS project, theDutch government requires companies to preparean environmental impact report which addresses:air, soil, and water pollution; noise; lighting; hori-zon contour; ecological aspects; etc. In this report,predictions are made towards noise level to be ex-

1077-2618/01/$10.00©2001 IEEE IEEE Industry Applications Magazine � July/August 2001 45

Fritz Kleiner, Bas de Wit, and Bernd Ponick

Fritz Kleiner and Bernd Ponick are with Siemens AG in Erlangen and Berlin, Germany, respectively. Bas de Wit iswith Nederlandse Aardolie Maatschappij in Hoogezand, The Netherlands. This article first appeared in its originalform at the 1999 IEEE Petroleum and Chemical Industry Conference.

Fig. 1. This 50,000 hp (38 MW)supersynchronous motor (left) has

been chosen to drive the single barrelturbo injection compressors at NAM.

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pected, NOx emissions, etc. This does not meanthat the values of the predictions will be acceptedby the authorities, but the figures can be used anddiscussed in the decision making process.

Environmental aspects of concern for the Norgunderground storage facilities were air, soil, andhorizon pollution, as well as noise due to the closevicinity of housing in a nature preserve area.

Some examples are:� Air pollution: in the Norg case CO, CO2, and

NOx emissions must be kept very low, whichturned out to be a disadvantage for the appli-cation of gas turbine drivers.

� Noise contours for the total plant were lim-ited to a maximum sound power level L(Aeq)= 40 dB(A) at night.

Ecological aspects dealt with in the environ-mental impact report are subsoil water flows af-fected by building installations or soil pollution,stock of game, effects of the installation, etc.

Operational AspectsThe design of the injection facilities and the selec-tion of equipment are based on an unmanned re-mote operation. Only during an initial period ofabout three years will the plant be manned. After-wards, monitoring and control of the facility willbe done from a centralized control center assistedby roving crews. A lifetime of 60 years is foreseenfor the UGS installations.

Compression units need to be available for opera-tion during a nine-month uninterrupted periodeach year. Each compression unit, therefore, shallhave a target reliability of 100% during this period.

Unpredictable fluctuations in the gas marketresult in a relatively high estimated number of 250start/stop cycles of the compressors per year. Also, astart up time of 20 min from cold state to maxi-mum gas injection flow rate was required toquickly react to demand changes.

Economical AspectsEconomics for the injection plant is split into capi-tal expenses (CAPEX) and operating expenses(OPEX). Life cycle costs for the injection plant areof paramount importance to the NAM, because theinjected gas has already been produced at a certaincost in a different place.

CAPEX are primarily affected by the drivertype selection: gas turbine, electric motor, etc.Early budgetary quotations for the possible com-pressor drivers showed a price difference of approx-imately 15% between a 38 MW gas turbine and a38 MW speed controlled electric motor in favor ofthe latter.

The overall price comparison made in 1992 cov-ered the main costs like design and engineering,equipment, materials, installation costs, etc., and stillfavored the electric solution over the gas turbines.

Application of a fixed speed electric motor, in-cluding the option with hydrodynamic fluid cou-pling, was no longer considered after investigationof the plant flexibility requirements.

TechnologyTechnology in this context represents the degree ofproven designs of manufacturers at the time of theinitial studies and driver selection. As the availableoptions are all tailor made, to a degree, it is a difficultsubject to judge on. In the final analysis, however, theNAM was convinced that required upratings of exist-ing equipment were well within the vendor’s experi-ence and did not involve new technology or unprovenelectrical or mechanical stress levels.

Selection of Compressor DriversThe comparison of the compressor drivers was basedon the aspects discussed above. For the selection,four driver options have been evaluated: gas tur-bines, fixed speed electric motors, speed controlledelectric motors, and fixed speed electric motors withhydraulic torque converters for speed control.

Each drive system was ranked for operatingrange covered; suitability for frequent starting andstopping; starting reliability; turndown ratio;operability; availability; reliability and maintain-ability; environmental aspects; etc.

The speed controlled electric motors of 38 MWrating were selected for the following reasons:

IEEE Industry Applications Magazine � July/August 200146

Fig. 2. The NAM GLT “dry” motor-compressor withmagnetic bearings operates outdoors at grade levelwithout a protective shelter.

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Table I. Rating Data Of LCI Drive System

Rated power and speed 38,000 kW @ 3 898 rpm

Starting current and torque 120% Irat for 1 min

Speed control range 2,800...4,200 rpm

Constant power speed range 3,898...4,200 rpm

Idle/stabilizing speed 680 rpm

Run-up time from idle to nmin 7 s

Constant output of converter 38,000 kW

Rated power of conv.transformer 50 MVA @ 40°C and 55K

� No contribution to the NOx, CO, or CO2emissions at the plant location. (Emissions ofthese exhaust gases at the power station can-not be avoided but can be better controlled,particularly at part load, and reduced.)

� High flexibility and reliability for remoteplant operation, that is, no problems are an-ticipated with the required number and reli-ability of starts and stops.

� Low maintenance costs for the driver com-pared to a gas turbine.

� No proven manufacturer experience, at thattime, in this power range for the hydraulictorque converters for drives of the size required,and the efficiency at part load is too low.

� High reliability and availability ranking forelectrical drives (the same can be said for themaintainability ranking).

� Less contribution to overall plant noiseemissions.

Environmental and Power Quality ConstraintsEnvironmental Constraints

NoiseThe Norg UGS facilities are installed in a naturereserve close to a small village. Noise limitationswere based on an agreement with the governmentof a 40 dB(A) sound power level at 500 m distanceduring gas injection and on the noise contour cal-culations supplied by manufacturers. An unex-pected source of noise turned out to be thecapacitor banks, including their vertical steelstructures, of the harmonic filters. They, and thealready silenced converter transformers, were sub-sequently enclosed by tuned sound barriers to stopthe neighboring resident’s complaints.

EmissionsEmissions into the air and water are restricted andmust be minimal. Hydrocarbon emissions to the airare prevented by the installation of a high integritypressure protection system (HIPPS) instead of safetyrelief valves. Emissions of exhaust gases to the aircannot be totally avoided (for safety reasons, there isa flare system, and for process reasons, two furnacesare installed) but, by selecting electric motor driversfor the compressors, these are drastically reduced.

Power QualityLoad commutated inverter (LCI) drives, as in-stalled in Norg and Grijpskerk, are main contribu-tors to the harmonic pollution of the electrical gridand, as such, are a source of voltage/current distor-tion and “low” power factor. Requirements andregulations to be met for this project are power fac-tor and noise distortion.

� The power factor of the plant load at thepoint of common coupling (PCC) should bemaintained between 0.85 (lagging) and 1.0during steady-state operation under allload/speed conditions.

� The limits of the voltage distortion to be main-tained at the various voltage levels can be seenin the examples of Table II. The limit to be metin the 110 kV grid is at the PCC and is the limitfor the plant contribution to the overall distor-tion (without possible predistortion).

Voltage distortion at the ripple control frequencyof 228 Hz is limited to 0.2% of the rated voltage atthe 110kV bus. Transient voltage dips, commutationnotches, and commutation oscillations were as perVerband Deutscher Elektrotechniker (VDE) 0160and restricted to 3% at the 110 kV levelectrical Staticvoltage changes at the plant distribution systemsshould be limited to ±10% for the 30 kV bus, and+10/−7% for the 0.4 kV bus bars. Limits in flickervoltage were set as given in IEC 1000-2-2

Supply VoltageThe Norg plant is located in a rural area where thenearby 10 kV electrical grid is weak (maximumpower available for NAM is 1-2 MW). Initial dis-cussions with the electricity board resulted in twooptions to supply the compressor plant: a) supplyfrom a 220 kV transmission line about 8 km awayand b) supply from a 110 kV substation at approxi-mately 14 km distance. Initially, the first option wasselected, but this had to be changed later to the 110kV solution. At the time the specification for thecompressors and drivers were ready and contractswere in place, the revised supply from the weakergrid with an underground cable had a significantimpact on the size of the harmonic filter plant.

In the requisition, the manufacturer was re-quested to perform an extensive network study tak-ing all above requirements into account beforestarting detailed design work for the driver units andfilter systems. Results of the study were discussedwith the electricity supply board for their acceptance.

Power System StudyThe purpose of the power system and harmonicanalysis for large variable speed drive systems(VSDS) loads is to assure compliance with the lim-its spelled out in the previous paragraph and com-prises of these steps:

IEEE Industry Applications Magazine � July/August 2001 47

Table II. Permissible Voltage Distortion

OrderNumber

Voltage Distortion Limit at Voltage Level (p.u.)

110 kV 30 kV 0.4 kV

5 0.65 5.0 4.0

7 0.6 4.0 4.0

11 0.4 3.0 4.0

13 0.3 2.2 4.0

17 0.25 1.2 4.0

19 0.25 0.95 4.0

THD 3.0 — 5.0

� Determine number and level of harmoniccurrents and voltages as referred to the con-tractual PCC

� Assure proper functioning of all connectedequipment

� Prevent network resonances� Identify possible harmonic/interharmonic

interferences with ripple control and tele-phone systems

� Check power factor (PF) and reactive powerbalance over entire speed range and with allnormal operating (load) conditions.

To maintain a specific maximum harmonicvoltage level at the PCC with a given networkshort-circuit capacity and frequency-dependentimpedance, the only variable for the LCI drive sys-tem designer is the number and amplitude of har-monic currents injected into the network orproduced by the drive. Besides changing the con-verter circuitry (six- or 12-pulse, or even higher)the generated harmonic currents can only be di-verted from the PCC into filter circuits which ab-sorb them.

Factors considered in the sizing of harmonic fil-ters to meet the prescribed limits are:

� Various short-circuit levels of the network atdifferent switching states

� Predistortionof thenetworkbyother consumers

� Operating speed and load envelopes of thecompressor and motor in transient and insteady-state modes

� Inrush currents of motors and transformerson the same distribution bus.

With the commonly used 12-pulse LCI cir-cuits, the predistortion of the network must becarefully measured over an extended period of timebecause it, rather than the drive’s contribution, de-termines the size of the harmonic filters.

A specific problem of this installation in TheNetherlands is the possibility of series resonancesbetween the network impedance and the drive sys-tem transformers in the plant due to an under-ground 110 kV three-phase cable of about 15 kmlength with significant capacitance. The excitingfrequencies of this resonance are the most powerful11th and 13th harmonics generated by the vsds,and these harmonics must be prevented from en-tering the feeding network. This is accomplishedby using additional blocking filters, thereby, arti-ficially increasing the impedance of the system atthe exciting frequencies (see Fig. 3).

Major Drive System ComponentsSupersynchronous MotorThe only electric motor suitable for this applicationis the brushless synchronous type-induction motor


110 kV NetworkFault Level

1.5...2.3 GVA

15 km CablePoint of CommonCoupling (P.C.C.)

Main Transformers

Blocking Filters

60 MVA15%

60 MVA15%

60 MVA2%

60 MVA2%

60 MVA2%

60 MVA2%

Tuning Frequency550 Hzq.f.:20

Tuning Frequency550 Hzq.f.:20

Tuning Frequency228/650 Hzq.f.:20

Tuning Frequency228/650 Hzq.f.:2030 kV

n.c.

Aux. Load

Drive 238 MW

Drive 138 MW

Order Number ofTuning Frequency:

Reactive Power/MVAr.

Quality Factor:

5. 5.7. 7.11. 11.13. 13.17. 17.23. 23.

2.47 2.471.21 1.213.0 3.01.79 1.791.19 1.191.19 1.19

5.7 5.76.5 6.5

Hormonic Filters, Each Side 10 MVAr at Rated Voltage

Fig. 3. Harmonic and blocking filters are required at NAM Norg to comply with local requirements and to avoid systemresonances.

driving a 38 MW compressor at variablespeeds up to 11,000 rpm. This type of motorhas never been built, and suitable frequencyconverters are not on the market. Output rat-ings of two-pole synchronous motors are un-limited in terms of power, but their shaftspeeds are limited for these reasons:

� The centrifugal forces acting on theembedded field windings and the di-ode wheel, or the mechanical strengthof the rotor materials in general

� The length of the rotors, bearingspan, is limited to cope with the lat-eral vibrations

� The recovery time of the thyristors islimited.

Resulting output limitations for the com-monly used frequency converters and syn-chronous as well as asynchronous, induction,motors are shown in Fig. 4.

Consequently, the motor to be used for this pro-ject had to be a synchronous one with a solid steelturborotor (see Fig. 5) and a step-up gear to matchthe compressor speed of 10,500 rpm. To staywithin proven equipment designs and to keep thegear-ratio as low as possible, a maximum operatingspeed of 4,200 rpm was chosen. This includes the105% overspeed margin according to AmericanPetroleum Institute (API) 617. The rotor isoverspeed tested for 2 min at 4,950 rpm. The statoris equipped with two three-phase windings, 30°electrical offset to each other, to reduce torque os-cillations and harmonic currents.

The brushless exciter, which must supply fullfield power from standstill to rated speed, is solidlyflanged to the nondrive end of the motor and has itsown bearing, resulting in a well-defined, in termsof lateral vibration behavior, rotor system withthree bearings. All three bearings are of the pedes-tal-type with proven forced-oil fed radial cylindri-cal sleeves; there is no need for the additionaldamping a tilting-pad bearing would offer.

Operating in a Zone 2 hazardous area, the motorenclosure is totally enclosed, purged, and pressur-ized to meet the EEx(p) certification requirementsof the German Physikalisch TechnischeBundesanstalt (PTB). For reliability improvements,all sensors and fans for the pressure monitoring andpressurization system are executed redundantly.

A synchronous motor of this size and type has anexcellent electrical efficiency of 97.6% at ratedpower, but this still amounts to almost a megawattof heat losses in the machine. These losses are takenup by an internal closed-loop cooling air streamand transported by shaft-mounted axial fan bladesto four side-mounted tube-type water-air heatexchangers. From these coolers the losses are trans-ferred to the plant’s water-glycol closed-loop cool-ing system. For an illustration of the initial motordesign chosen, see Fig. 6; however, the electric mo-

tor driven fans shown for the internal air circula-tion have, in the final design, been replaced byshaft-mounted fan blades to increase the opera-tional reliability of the machine.

In any high-speed drive application, the rotordynamic considerations are of paramount impor-tance to assure a smooth running rotating string


0 1000 2000 3000 4000 5000 6000 7000 8000 10.00012.000

80 Synchronous MotorSolid Rotor

Norg Motor

GLT Motor(Mag. Bearings)

Induction Motor

40[MW]

30

20

10

15

5

09000 [rpm]

Fig. 4. Present day output limitations of VSDS using synchronous and asynchronous motors.

Fig. 5. Field and damper windings will be insertedinto the solid rotor body of the high speedsynchronous motor after finish machining.

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Fig. 6. Drawing of air-water cooled 50,000 hptwo-pole brushless synchronous motor for variablespeed operation from 2,800 to 4,200 rpm.

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over the entire speed range and during all transientoperations. Also, the behavior of the string duringfault conditions, the most severe one—as unlikelyas it is—being a short circuit at the motor termi-nals, must be known prior to the final design of therotating equipment to allow for possible modifica-tions. LCI supplied synchronous motors, unlikefixed-speed line-supplied ones, show a soft,smooth, and fully controlled starting behaviorwithout noticable mechanical and thermal stressesimposed on the rotating string. On the other hand,they continuously produce small torque oscilla-tions, typically below 1% of rated torque-over theentire speed range. The effects of such torque pul-sations must be analyzed carefully, along withother torsional excitations from the gear and thecompressor, during the design phase.

The operating regime of the compressor in thisapplication is characterized by the frequentstart-stop cycles that are typical for peak-shavingduty. To assure smooth running of the rotatingstring, even when started during the cool-downperiod of the system and after long shut-downperiods—when differential temperature distribu-tions can cause thermally induced asymmetries andvibrations, an idling speed of around 680 rpm hasbeen introduced to stabilize the system.

Frequency ConverterExternally commutated (versus self-commutated)thyristor-frequency converters are among the mostsimple and reliable power electronic systems on themarket. In combination with synchronous motorsthe back-electromotive force (EMF) of the machineprovides both the actual motor frequency and thecommutating voltage, and reactive power, for themotor-s ide thyr i s tors , thus , the termload-commutated inverter, or LCI drive. In identi-cal basic configuration, and with the same powerelectronic devices, such converters are employed byelectric utilities in ratings up to several thousandmegawatts in high-voltage dc transmission sys-tems (HVDC); thus, the 38 MW required here did,in general, not require a special design (see Fig. 7).

An exception are the dc link reactors. To avoidcostly and space consuming externally installed

air-cored solutions, the directly water-cooled andiron-cored reactor of smaller similar convertersneeded to be uprated to the dc link voltage of eightkV. Comprehensive type tests confirmed the suit-ability of the higher rated design, and proven fea-tures of the previous model were maintained. Bycross linking the dc currents of the two parallelsix-pulse systems, a negative-feedback system iscreated. There are only unidirectional fluxes cre-ated in the reactors, stressing the cores just withthe residual ripple of the dc current, reducing theirlosses correspondingly, and producing practicallyno stray fields.

Line- and load-side thyristor bridges areequipped with four disk-type silicon-controlledrectifiers (SCRs) in series without redundancy, butwith relatively large voltage and current safetymargins. The originally specified n + 1redundancyhas been eliminated following standard manufac-turer’s advice that modern power thyristors are nomore likely to fail than any other component in thepower circuit and that a measurable increase in re-liability cannot be guaranteed by this expensivescheme. Operating experience over two decadesshows that single-thyristor failures, and n + 1safe-guards only against those, are the rare exception.

Compared to previous models, the optical firingand checkback signals to and from the thyristorsare handled by faster microprocessor equipmentthat allows both a serial data communication withthe distant control and monitoring cubicles, andthe exact identification of a faulty thyristor.

To prevent impermissible di dt values from en-dangering the thyristors, the cable capacities to themotor and to the converter transformer are com-pensated by toroidal cores slipped onto internalwater-cooled busbars in the converter whereby thepreviously used ferrite “donuts” are replaced bymore efficient wound amorphous metal coils. Theresulting di dt values were confirmed to be safe bysubsequent measurements on site.

Converter Transformer andInrush Limiting ResistorOil-cooled outdoor converter transformers of thethree-winding two-tier design are pretty muchstandard. Special measures were taken in this projectto acoustically isolate the coil and core assemblyfrom the tank to reduce structure-born noise, tolower the induction value for noise reduction, and toincrease the impedance of the secondary windings asrequired to limit the possible short circuit current inthe fuseless thyristor frequency converter.

The voltage drop at the plant’s 30 kV distribu-tion system during switching on of the 50 MVAconverter transformers was of concern to the clientand had to be limited to 4%. Series-connected in-rush limiting resistors were thus installed on the30 kV side. These are momentarily switched in bya separate circuit breaker for about one sec duringtransformer magnetization.


Fig. 7. This compact 38 MW water-cooled 12-pulseLCI frequency converter with integrated dc linkreactors supplies the 50,000 hp motor.

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Auxiliary EquipmentSwitchgearsThe two incoming 110 kV underground cable feed-ers are switched and protected by their own breakersin the 14 km distant substation of the utility com-pany. All 20 and 30 kV switchgears in the UGSplant are of the metal-enclosed, air-insulated,arc-fault tested, draw-out vacuum circuit breakertype with fully digital protection and interfacing toa central control and energy management systemprovided by the utility company. Draw-out lowvoltage switchgear and motor control centers(MCCs) provide the local power distribution and thelow-voltage motor starter and protection functions.

Cooling SystemsThe combined heat losses of the turbocompressorsystems, including the motors, lube oil systems,and frequency converters, are transferred to the at-mosphere by a closed-loop water-glycol systemwith cooling banks. Except for the frequency con-verters with their integrated dc link reactors, allequipment is directly cooled by the water-glycolmixture. High voltage potential inside the fre-quency converter requires a controlled cleanliness(conductivity) of the cooling medium. This is pro-vided by another closed-loop deionized water sys-tem which is linked to the primary cooling systemvia heat exchangers. To maintain the availability ofthese cooling systems, their circulating pumps areredundant with automatic change-over.

Harmonic Filter PlantsTypical industrial LCI drive installations do not au-tomatically require harmonic filters; in most cases,the 12-pulse configuration will suffice to meet theutility’s requirements. For this project, unusuallystiff demands were posed by the responsible utilitycompany, and additional circumstances called forthe installation of an extensive harmonic filter plant.Connecting the plant via a long underground 110kV cable, and having a ripple control system operat-ing on that system, required the installation of se-ries-resonant filters to guarantee a safe operation ofboth the UGS plant, and the transmission grid (seethe Power Systems Study section).

Construction of the various outdoor air insu-lated reactors, capacitor banks, and damping resis-tors and their protection is pretty much standard,but their operating noise emissions gave lots ofheadaches and caused the owner to install silencinghoods and noise barriers as mentioned earlier.

Monitoring and Diagnostic Aid SystemThe availability of the electric drive plant is of primeimportance to many customers. An added criteriafor this project, however, is the eventual unmannedremote operation of the system from the central dis-patch center about 30 km from the UGS plants.Roving maintenance engineers with basic knowl-edge of the electrical system are sent to the site when

irregularities occur. These people are greatly as-sisted in their work by a dedicated PC based onlinegraphics monitoring and diagnostic aid systemwhich also serves as a living service logbook.

Visualization of the plant’s configuration andits momentary condition on 18 staggered displaypages is just the beginning of any analysis to gainan overview of the plant’s status. Continuousreal-time background recording of vital operatingdata; tracing functions for a post-mortem review ofselected conditions prior to, during, and followinga shut-down of the system; and an elaborate faultmessage system bring the service engineer closer tothe source of a problem, or, more important, indi-cate adverse trends that eventually can lead to ashutdown of the plant.

The real heart of the diagnostic aid system,however, is an extensive library of help texts in theDutch and English languages with extensive de-scriptions of the possible reasons for an alarm andhints on how to restore the operating condition, in-cluding the necessary cross references into theplant’s paper documentation. These help textshave been written, and subsequently modified, bythe commissioning engineers themselves, with ac-tive participation of the client’s service engineers,an invaluable training exercise for them as well.

TestingBack-to-Back Test in Motor FactoryNeither vsd systems nor compressors of exactly thesize in this project had previously been built. Ex-haustive type and performance testing of the com-pression systems were thus required from the verybeginning. The biggest obstacles in conductingtests with such enormous powers are in the avail-ability of the following:

� Space to set up the complete drive systemplus the compression and gas circulation andcooling system including a re-cooling plant

� Electrical power at the frequency, voltage,and short circuit capacity that closelymatches the project

� Cooling water supply and discharge possibil-ities, whereby the latter one is the more criti-cal, at least in Europe, due to environmentalrestrictions, such as the level of harm to fishfrom the heated cooling water

� Time to conduct such tests in respect of theproject schedule; testing practically amountsto a complete installation and commission-ing process to set up temporary rigs.

For time and cost reasons the customer ultimatelysettled for a no-load compressor/gear-string test atthe compressor manufacturer’s plant in Switzerlandand a near full-load back-to-back test of the first twovsd systems in the motor plant in Germany. By cou-pling two identical vsd motors back-to-back and op-erating one LCI drive in motoring, and the other onein generating mode, one must theoretically just sup-


ply the losses in the two systems when running such atest. For the active power balance this is true, but thenetwork to which the test setup is connected mustsupply the reactive power demand of two systems.This approached even the limits of a temporarily ded-icated feeder of the 110 kV Berlin city network. Withkilometers of additionally installed power cables, andspecial permits of the municipal water board to useand discharge the enormous cooling water quantities,the client-witnessed back-to-back tests were success-fully conducted on two weekend nights at the end of1996. With a torsional measuring shaft inserted be-tween the two machines, valuable pulsating torquerecordings could be obtained that were subsequentlyused to optimize the closed-loop speed control systemof the VSDS to further reduce the exciting momentsoriginating in the frequency converter.

The tests followed a structured and well-docu-mented step-by-step procedure, and the resultsformed the basis for the subsequent commission-ing on-site. Service engineers from the manufac-turer’s local office attended the entire installationand testing period and familiarized themselveswith the systems, ensuring a quick and prob-lem-free final installation and preparing thegrounds for an efficient after-sales service.

Site TestingAfter installing the compression systems at their re-spective sites near Norg (2 units) and Grijpskerk (1unit) in Northern Holland in spring of 1997, the pre-

planned and documented commissioning proceduresran relatively fast and with few problems, and perfor-mance testing could begin in summer of that year.

Commissioning and site testing lasted almostsix months for reasons not attributable to the rotat-ing and electrical equipment. The test programwas based on Shell Design Engineering Practice(DEP) requirements and included

� Routine tests� No load tests� Functional tests at 100% supply voltage� Fault condition tests according to cause and

effect diagrams� Load and heat run tests� Special tests.The exact and comprehensive measurement of all

oscillating torques at the coupling between motorand gearbox was of great concern to the client andwarranted the installation of a calibrated torqueshaft and its related analyzer equipment. Measure-ments were taken within the entire operating speedrange (2800 to 4200 rpm) at various load conditionsof the compressor. Specifically, readings were takenaround all torsional critical speed points.

At the end of March, 1997, the harmonic dis-tortion inside the plant distribution system and inthe 110 kV substation of the utility company weremeasured with the results shown in Table III.

In mid April, 1997, a 72-hour acceptance endur-ance test was successfully conducted to prove theperformance and reliability of the compression sys-tem. Since then the plants are operating as intended.

ConclusionHigh-speed electric motor variable speed drivesused to drive centrifugal compressors of any size areproven technology, and the examples shown hereare exemplary for many other successful installa-tions. Economics will not always justify their use,particularly if there is no reliable and economicalsource of electric power available at site, but mostapplications can ill-afford not to evaluate them.There are a number of success factors in the properdesign and use of very large vsd systems that willmake such systems a success:

� Thoroughly match the compressor’storque-speed envelope with available drivedesigns.

� Consider all system components combined.Their characteristics and capabilities must becarefully matched.

� Close cooperation of the compressor and mo-tor anufacturer is of paramount importance.

� Vibration and harmonic analyses prior tomanufacturing are mandatory and may haveconsequences on the final electrical and me-chanical design.

� An integrated and coordinated protection andcontrol system increases system transperencyand reliability.


Table III. Allowable versus MeasuredHarmonics

30 kV plant distribution system

VSDSoperatingpoint

THDvoltageresults

THDvoltagelimit

THDcurrentresults

THDcurrentlimit

Standstill 1.88% 7.7% 6.86% 15.29%

Idling 2.69% 7.7% 19.64% 15.29%

Min. speed 0.9% 7.7% 8.2% 15.29%

Nom. speed 0.92% 7.7% 2.86% 15.29%

Maximumspeed

0.89% 7.7% 2.29% 15.29%

110KV point of common coupling

VSDSoperatingpoint

THDvoltageresults

THDvoltagelimit

THDcurrentresults

Standstill 1.71% 3% 9.27%

Idling 1.72% 3% 13.23%

Min. speed 1.35% 3% 8.06%

Nom. speed 1.42% 3% 2.87%

Maximum speed 1.41% 3% 2.43%

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Choosing electric turbocompressor drivers