Variable Frequency Drives - A Comparison Of VSI Versus LCI .

Variable Frequency Drives - a Comparison of VSI versus LCI SystemsIntroductionTMEIC is a leader in the innovative design and manufacture of large ac variable f requency drivesystems. TMEIC has been very active in developing the most advanced high capacity VoltageSource Inverter (VSI) drives, large enough to compete with Load Commutated Inverter (LCI)drives. This study compares the strengths and weaknesses of TMEIC's VSI drives with competitiveLCI drives.LCI & VSI Comparison SummaryThe LCI drive has a long history and its advantages and limitations are well understood. It issimple and reliable, but requires careful attention to ac power system issues related to harmoniccurrents and reactive power. It also demands specially designed motors with low reactance, andmust work with harmonic heating and air gap torque harmonics.Figure 1. Simplified Block Diagram of LCI and VSI DrivesMedium voltage VSI drives have progressed significantly in the past 15 years, and are now thestandard practice in such demanding applications as main drives in metal processing, whichrequire high overload with very high performance up to about 20 MW. The development of highpower, controllable semiconductor devices a n d p u l s e w i d t h m o d u l a t e d ( P W M ) c o n t r o lhas made VSI drives both possible and suitable for broader applications such as high powerdrives for large compressors and blowers. The resulting VSI designs now provide manyadvantages over their LCI predecessors, including low power system harmonics, low reactivepower demand, and low torque pulsations in the motor air gap and shaft.1

1.Summary Comparison of VSI versus LCI Technology & SystemsThe following table summarizes the differences between VSI drives and LCI drives. These will bediscussed in later sections.VSI SystemLine Current Total 2 % THDHarmonicHarmonic filter is notDistortionrequiredLCI SystemUp to 12 % THDHarmonic filter is requiredAir gap torqueripple0.5 – 1 %Standard mechanicalsystem design is possibleUp to 7 %Special mechanical system designrequired for torque pulsationsInput PowerFactorGreater than 0.95PF correction equipment isnot necessary0.5 – 0.92Power factor equipment (capacitor)is necessaryPower systemstabilityStableDelicateAC powerdisturbanceRide through or AutomaticrestartCan ride throughReliability, MTBF28 yearsProven technologyGreater than 10 year MTBFProven technologyRepair time,MTTRAbout 0.5 hoursAbout 2 hoursEquipment sizeStandard VSI drive lineupMuch more space required for LCIlineup, Harmonic Filter, PFcorrection, and ReactorMotorIndependent fromconverterSpecial design.Strongly affected by converter andharmonic heatingMotor typesInduction or SynchronousmotorSynchronous motor onlyMotor power andDrive100 MW, 4 banksTMdrive-XL85120 MWDrive outputvoltageXL type VSI up to 7.2 kV;MVG type VSI up to 11 kVUp to 11 kV2

2.Supplier CapabilityRecent semiconductor and VSI developments have produced high power VSI drives, with a singlechannel power level up to 30 MVA, as shown in Figure 2. Multiple parallel channels can be employedas described in the table.Figure 2.Typical large VSITMEIC TMdrive-XL85TMEIC VSI 858/10MVA8 MVA20 MVA30 MVAVoltage of maximummotor size3.3 kV6.6 kV6.0 kV7.2 kVPower SemiconductortypeIEGTIGBTIEGTGCTMaximum number ofparallel VSI channels4244Maximum system power40 MVA16 MVA80 MVA120 MVALength of single channellineup126 inch221 inch370 inch319 inchVSI output power rangefor single channel3

LCIs have been used for many years for their high power and variable speed. A typical large LCI isshown in Figure 3.Figure 3.Typical large LCICapability of Competitive LCIsBrand A LCIBrand B LCIStandard 72 MWStandard 74 MWMotor voltage of maximum sizesynchronous motor10.2 kV11 kVPower Semiconductor switchingdevice typeSCRSCR3637 (120 max)Special, 101 MWSpecial, 120 MWNA433 inchLCI output range for synchronousmotorsMaximum MW rating of single LCIchannelMaximum system powerLength of single channel lineup, notincluding power factor correctionand harmonic filterLCIs are usually larger than the comparative power VSI. This is illustrated in Figure 4.4

Figure 4. Size Comparison of VSI System with LCI System3. Design ConsiderationsVSI Drive Designa. The only limitations in the designs are the power converter ratings. These ratings are based onthe capacities of the power semiconductor devices (IEGT and GCT) and the cooling systems.The nominal ratings are based on 40 C maximum ambient temperature and altitudes less than1OOOm. Applications outside these conditions require de-rating.b. There are no limitations on input voltage because the input transformer is separate from thedrive. The maximum motor voltage is inherent in the converter design depending on thevoltage margin in the devices and power circuit topology. TMEIC has used the highest motorvoltage possible consistent with adequate device voltage margin.c. The TMdrive-XL55, XL75, and XL85 drives have 36-pulse diode rectifier source converters.The likelihood of beat frequencies and inter-harmonics is very low. For compressor applications,the loading required to maintain continuous current in the source converter occurs well belowthe minimum continuous speed.d. The 36-pulse source connection requires six sets of 3-phase cables. To minimize cost, thedistance between the input transformer and the drive is made as small as possible. However,there is no electrical limit to the distance except for voltage drop in very long cables.e. The only limitation in cable length between the drive and motor is the voltage drop in thecables.5

f. An output filter has not been used in the past for any VSI drives.g. The motors for VSI drives are custom built with appropriate motor insulation systems. Thisusually means a higher voltage insulation system than for a standard utility-fed motor.LCI Drive Designa. The input transformer is separate from the LCI drive, so there are no limits on the inputtransformer primary voltage. The motor voltage can be as high as 11 kV (with the motorconnected directly to the drive) based on the rating of the SCRs and the components in thepower circuit.b. The LCI drive always operates at continuous current by design. Inter-harmonics in the powersystem are minimized by having sufficient inductance in the dc link inductor that decouples thesource and load converters.c. The cable limits for the LCI system are established by the capacitance of the cables. There is acapacitance limit that should not be exceeded to prevent di/dt failures in the SCRs; this limit isusually about 300 m.d. Cable length limits to the motor are subject to the same considerations for capacitance andvoltage drop as for the cabling between the transformer and source converter.e. Output filters are not applicable to LCI drives.4. Supplier Capability and Experience with VFD-driven MotorsThe LCI can only operate with a synchronous motor, but the VSI can operate with a synchronous oran induction motor. S i nc e the i nductio n mo tor h as a lowe r cost, in many medium powerapplications this is an advantage to the VSI system. TMEIC has extensive experience building bothlarge induction and synchronous variable speed motors, as summarized in the table below.Motor TypeTMEIC Motor CapabilitySynchronous motorPower range up to approximately 100 MWSpeed range up to 6,500 rpm (2-pole motor)Induction motorPower range up to 25 MWSpeed range up to 12,000 rpm (2-pole motor)Motor voltageUp to 13.8 kVLarge motor designCustom designNumber of 3-phasewinding sets16

5. Power System Harmonics and need for FiltersLCIs still develop considerable input current harmonics, although using multiple converters improvesthe situation. VSI development has reduced the effect of the converter on the power supply to levelswell below the IEEE519 standard. This is illustrated in Figure 5.Figure 5. Total Harmonic Distortion for large VSI and LCIVSI Harmonic DistortionTMEIC's large XL drives36-pulsediodehavesource converters, resultingin very low input currentharmonics. As shown inFigure 5, the total harmonicdistortion (THD) is 2.2 %,well below the IEEE519standard.LCI Harmonic DistortionLCI total harmonic current distortion (THD)can be as high as 10-12 % for 12-pulsesystems, requiring harmonic filters on theinput. LCI drives have either 6-pulse or 12pulse source converters.If multipleconverters are used in a system, thetransformers for the system can bedesigned with secondary phase shifts thatresult in low harmonics on the ac system.For example, a dual channel system withtwo 12-pulse source converters can beconnected for 24-pulse harmonic currentsthat need no harmonic filters.7

6. Power Factor versus LoadLCI drives have low input power factor, especially at lower speeds, and often additional power factorcorrection equipment is required. VSI drives on the other hand operate at close to unity power factor.Figure 6. Power Factor Variation with SpeedVSI Power FactorThe power factor of a VSI with dioderectifiers is essentially constant at 0.95 orbetter over the normal operating range.The diode converters only draw kW fromthe ac supply and no phase delay ispossible.Regenerative source converters (activefront ends) operate at unity power factor,or can operate to provide VARs to thepower system (leading power factor). Thischaracteristic can be used on isolateddrives supplied by weak power systemsthat may require voltage support. Thereactive power can be made withoutexternal capacitors and is controllablewithin the rating of the source converter.The TMdrive-70 and TMdrive-80 haveregenerative source converters.LCI Power factorThe input power factor of LCIdrives varies with the motorvoltage,which is usuallyproportional to motor speedwhen the motor is operatingbelow rated voltage. For avariable torque load, the inputpower factor varies from 0.4 at50% speed to 0.92 at ratedmotor voltage and speed.Power Factor variation withspeed is shown in Figure 6.This reactive power demandusually requires addition ofcapacitors to support voltage atthe supply bus.8

7. Machine side Harmonics and their Effect on Motor DesignVSIs have three or five switching levels producing output harmonics much lower than those from anLCI. Motors used with an LCI require special design to accommodate these harmonic currents andresulting heat.Figure 7. LCI Voltage and Current WaveformsVSI Output HarmonicsData on the VSI output harmoniccurrents are used in designing themotor and accounting for the motortemperature. Generally, the currentharmonics from a PWM drive arelow because the voltage harmonicsare reduced by effective waveformshaping and the higher number ofswitching levels in the drive, fivewith TMEIC XL drivesLCI Output HarmonicsThe motor is designed for theharmonic currents it will experiencein operation. Depending on thenumber of windings, the harmoniccurrents may be 6-pulse (25 %THD) or 12-pulse (10 % THD).Even with a 12-pulse connection,the current harmonics are likely tobe much higher than a PWM-fedmotor. Proper motor design mustaccommodatetheharmoniccurrents.9

8. Torque Ripple Produced in the MotorThe main risk with output torque pulsations is they may be close to a mechanical resonant frequencyand excite powerful torsional vibrations, which can lead to shaft or coupling cracks and failure. Pooroutput waveforms from the LCI can produce such torque pulsations or ripples. There are not manystrategies for improving the motor air gap torque pulsations, usually modifications to the mechanicaldrive train are the best way to avoid resonances, for example a damped coupling can be inserted.Figure 8. LCI Motor Starting Current Waveforms causing Torque Pulsations10