Variable Frequency Drives - a Comparison of VSI ... - Tmeic.com

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 and pulse width modulated (PWM) controlhas 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.Line Current TotalHarmonicDistortionAir gap torquerippleInput PowerFactorPower systemstabilityAC powerdisturbanceReliability, MTBFRepair time,MTTREquipment sizeMotorMotor typesMotor power andDriveDrive outputvoltageVSI System2 % THDHarmonic filter is notrequired0.5 – 1 %Standard mechanicalsystem design is possibleGreater than 0.95PF correction equipment isnot necessaryStableRide through or Automaticrestart28 yearsProven technologyAbout 0.5 hoursStandard VSI drive lineupIndependent fromconverterInduction or Synchronousmotor100 MW, 4 banksTMdrive-XL85XL type VSI up to 7.2 kV;MVG type VSI up to 11 kVLCI SystemUp to 12 % THDHarmonic filter is requiredUp to 7 %Special mechanical system designrequired for torque pulsations0.5 – 0.92Power factor equipment (capacitor)is necessaryDelicateCan ride throughGreater than 10 year MTBFProven technologyAbout 2 hoursMuch more space required for LCIlineup, Harmonic Filter, PFcorrection, and ReactorSpecial design.Strongly affected by converter andharmonic heatingSynchronous motor only120 MWUp to 11 kV________________________________________________________________________________________________________2

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 CapabilityVSI output power rangefor single channelVoltage of maximummotor sizePower SemiconductortypeMaximum number ofparallel VSI channelsTMdrive-708/10MVATMdrive-XL55TMdrive-XL75TMdrive-XL858 MVA 20 MVA 30 MVA3.3 kV 6.6 kV 6.0 kV 7.2 kVIEGT IGBT IEGT GCT4 2 4 4Maximum system power 40 MVA 16 MVA 80 MVA 120 MVALength of single channellineup126 inch 221 inch 370 inch 319 inch________________________________________________________________________________________________________3

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 LCILCI output range for synchronousmotorsMotor voltage of maximum sizesynchronous motorPower Semiconductor switchingdevice typeMaximum MW rating of single LCIchannelStandard 72 MW Standard 74 MW10.2 kV 11 kVSCRSCR36 37 (120 max)Maximum system power Special, 101 MW Special, 120 MWLength of single channel lineup, notincluding power factor correctionand harmonic filterNA433 inchLCIs are usually larger than the comparative power VSI. This is illustrated in Figure 4.________________________________________________________________________________________________________4

3. Design ConsiderationsVSI Drive DesignFigure 4. Size Comparison of VSI System with LCI Systema. 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. Since the induction motor has a lower 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 TypeSynchronous motorInduction motorMotor voltageLarge motor designNumber of 3-phasewinding setsTMEIC Motor CapabilityPower range up to approximately 100 MWSpeed range up to 6,500 rpm (2-pole motor)Power range up to 25 MWSpeed range up to 12,000 rpm (2-pole motor)Up to 13.8 kVCustom design1________________________________________________________________________________________________________6

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 driveshave 36-pulse diodesource 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 12-pulse 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 mustaccommodate the harmoniccurrents.________________________________________________________________________________________________________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 Pulsations________________________________________________________________________________________________________10

Figure 9. VSI Motor Current Waveforms causing Negligible Torque PulsationsVSI Torque RippleThe Pulse Width Modulation(PWM) drive control strategyhas been developed tominimize or eliminate lowfrequency m o t o r a i r g a ptorque pulsations whendriving a compressor. Thetarget value for motor air gaptorque harmonics atfrequencies less than 100 Hz(6,000 rpm) is less than 2%of rated motor torque.LCI Torque RippleAn LCI normally has large motor air gap torquepulsations that are proportional to the averagemotor torque. The amplitude depends on thenumber of converters connected to the motor.A 12-pulse motor (two converters) would haveabout 25% peak-to-peak torque ripple. If theripple frequency is close to the torsional shaftvibration frequency, resonance can occur.The possibility of modulation torques is alsopresent in an LCI drive. The source converterripple frequency and the load converter ripplecan interact across the dc link reactor to causelow frequency modulation of the motorcurrents. This can result in low frequencytorque pulsations at high motor speeds.________________________________________________________________________________________________________11

9. ReliabilityBoth the VSI and LCI have high reliability. Based on 12 years of field experience with over 1000medium voltage TMdrive-70 VSI drives in the steel industry, the MTBF is 37 years. The MTBF of LCIsis also believed to be high.VSI ReliabilityBased on field experience with similarmedium voltage VSI drives, the MTBF ofa single XL drive has been estimate tobe 28 years. These drives are designedand built to high quality standards andhave redundant w a t e r cooling waterpumps with automatic switchover.LCI ReliabilityThe MTBF of a typical LCI driveis more than 10 years, based ona large amount of fieldoperating experience over thepast 40 years.10. Drive RepairThrough careful design and modular construction, the repair times of both the VSI and LCI have beenreduced to just a few hours.VSI RepairThe repair time (MTTR) of the VSI drive isabout 0.5 hours for power units and control,assuming trained technicians and sparemodules are readily available. Completepower modules are replaced; faultymodules are repaired at a TMEIC servicecenter.LCI RepairThe estimated MTTR of anLCI drive is 2 hours fromidentification of the problemto replacement of the failedpart in the power section orcontrol system.________________________________________________________________________________________________________12

11. System EfficienciesThe motor efficiencies shown in the following table are based on 2-pole synchronous motors.The drive efficiencies a r e published data from TMEIC and two LCI vendors. The overall systemefficiency is the product of all the system components – Drive, Motor, Transformer, Power FactorCorrection, and Filter. The results show the overall VSI system efficiencies are about1% higher than LCI systems, mainly due to the required power factor correction and harmonicfilter.Figure 10. Comparison of Overall System Efficiency for different size systemsComponent and System EfficienciesPowerLevelDriveTypeDriveEffic.%MotorEffic.%TransformerEffic. %HarmonicFilter/PowerFactor correctionEffic. %EntireSystemEffic.%15 MWVSILCI98.699.096.696.298.598.5Not required99.093.892.925 MWVSILCI98.699.097.296.898.598.5Not required99.094.493.550 MWVSILCI98.699.098.097.699.099.0Not required99.095.794.7________________________________________________________________________________________________________13

SummaryThe latest h i g h - p o w e r VSI designs now provide many advantages over their LCI predecessors.These advantages include low power system harmonics, low reactive power demand, and lowtorque pulsations in the motor air gap and shaft. In addition, the ability to drive induction motorsinstead of more expensive synchronous motors provides for equipment cost savings.Paul BlaiklockTMEIC CorporationRoanoke, VirginiaFebruary, 2013________________________________________________________________________________________________________14