#LCI versus VSI:
Part 4: Comparison (continue)
Our series LCI versus VSI belongs to some of the most popular in this blog. We talk about the main medium voltage high power VFD technologies, Load-commutated Inverter (LCI) and Voltage Source Inverter (VSI), and compare them in various aspects. This post is a continuation of the previous one (you find all related posts in LCI versus VSI category) and therefore we continue the numbering starting with item no. 12.
Have you missed our previous post on comparison LCI versus VSI? You can read that article clicking on following link.
12. Cooling
LCI converter is available with both air cooling and liquid cooling. Air cooled LCI can reach impressively high power, up to approx. 30-40 MVA. While air cooling is not preferred for continuous duty VFDs (heat dissipation into ambient -> large HVAC unit) it is an advantage for large soft starters or gas turbine starters with short duty. Large LCI drives are liquid cooled.
VSI converters are also available as air cooled and liquid cooled. Air cooling is mostly used at lower power range. Maximum rating of air cooled VSI is about 12-15 MVA. High power VSI drives are liquid cooled. Recently, one manufacturer started to market air-cooled VSI for very high power. Principally it is possible, but then some kind of air to air recooler is needed. Otherwise the heat load would simply be too high.
Common for LCI and VSI: The cooling circuit can be designed with n+1 redundancy (typical for cooling fans) or 2×100% redundancy (typical for cooling pumps). Liquid cooled drives can be equipped with redundant sensors such as pressure sensor, conductivity sensor or a leakage detector.
Table 1: Comparison of air cooling and water cooling
13. Footprint
LCI is a drive predestined for high power. In lower power range it does not make sense economically except for starter applications. Therefore, if we consider LCI in the higher power range then the power density is superior. Opposite to that, lower power ratings, particularly in combination with high voltage are not the sweet spot of LCI drives.
VSI drive tends to be an opposite. The drive is very compact at lower power range. At high power the footprint of VSI is slightly larger than LCI. Above certain power, roughly 35 MVA, multiple VSI drives need to be paralleled to reach the required power. In this case the VSI total footprint is considerably larger than LCI.
14. Reliability
LCI reaches high reliability that is greatly proven in the field. The product allows N+1 thyristor redundancy as option, but majority of the LCI drives installed in the field do not have that redundancy.
VSI family consists of many sub-topologies. They might somewhat differ in the reliability figures as the type of components and their quantity are very different. Therefore, we are not able to provide a general statement for the entire VSI technology. Generally the reliability of VSI increases over time as the technology is getting more and more mature.
15. Torque ripple
LCI in its simplest configuration (6-pulse rectifier and 6-pulse inverter) has high torque ripple. Therefore, such configuration is typically used only for soft starters or gas turbine starters with short time duty. As a full drive the LCI typically uses 12-pulse inverter in combination with 6-phase motor (dual star stator). In this configuration the torque ripple is approx. 3-times lower than the ripple of 6-pulse inverter and therefore comparable with VSI.
VSI uses self-commutated semiconductors that can be turned on and off. Depending on the switching frequency and modulation scheme, relatively low torque ripple can be achieved. In general, high performance topologies with excellent dynamics tend to have higher torque ripple while topologies with lower dynamics usually have quite low torque ripple.
16. Torque spectrum
The spectrum of torque is more important than the torque ripple as such. It defines whether the VFD excites some of the mechanical resonances.
LCI contains characteristic harmonics that are multiples of the inverter pulse number (for 6-pulse inverter characteristic harmonics are 6th, 12th, 18th, 24th etc.; for 12-pulse inverter characteristic harmonics are 12th, 24th, 36th etc.). These characteristic harmonics have magnitude of 1%-2% each. In addition, there are also inter-harmonic frequencies as byproducts due to different frequency of line side converter and motor side converter and the pulse number of these converters.
VSI contains characteristic harmonics, too. Majority of VSI inverters have 6-pulse configuration so the characteristic harmonic orders are 6th, 12th, 18th, 24th etc. The inter-harmonics are usually also present, but their origin is different than in LCI drives. They are either part of random ‘noise’ in case of hysteresis based modulation or they appear as byproducts of switching frequency and sampling frequency.
17. Motor type
LCI requires a synchronous motor as load. That is because the inverter is load commutated (as the name says). Moreover, the motor must be overexcited (with capacitive power factor) since the inverter requires external source of reactive power. The line side converter gets the reactive power from the grid and the motor side converter receives the reactive power from the motor.
VSI supports all common type of industrial motors – induction machine (IM), synchronous machine with wound rotor (SM) or synchronous machine with permanent magnets (PMSM). In the lower power range induction motors are almost exclusively used and the capability of induction motor is constantly pushed towards higher power. However, special cases may benefit from synchronous machines, e.g. in low speed applications.
18. Motor friendliness
LCI drive has distorted motor current. The motor design needs to account for these harmonics and their thermal effect (harmonic heating). Insulation requirements are very low as the motor voltage is almost sinusoidal, apart from commutation notches.
VSI drive can reach quite clean waveform of motor current. One way to reach close to sine wave current is to use a multi-level inverter, the other way is to use an output sine filter. The voltage waveform is characterized by steep gradient (dv/dt) unless a sine filter is used. Therefore, motor insulation system frequently needs an insulation margin.
Figure 4 illustrates voltage and current waveforms of LCI and 3-level VSI (without sine filter). Note that the LCI voltage shown is the line side voltage. However, motor side voltage looks similar.
19. Control
The main brain of LCI is in the rectifier (line side converter) while the inverter just keeps synchronized with the motor, e.g. by a Phase Locked Loop (PLL). Both the speed controller and current (torque) controller are embedded in the rectifier. Normally the strategy is to maximize the inverter firing angle to keep torque harmonics as low as possible. Therefore, there is a specific inverter firing angle for given motor speed and current. Almost all dynamic changes are done by the rectifier control.
VSI control is almost completely an opposite. The motor control functionality, i.e. speed and/or torque control, is realized in the inverter. That makes sense because the rectifier is often just a passive diode bridge with no control possibility at all. Some VSI topologies are therefore perfectly predestined for a multi-drive concept.
20. System integration
Both technologies, especially in higher power range, require specific considerations regarding system integration. In case of LCI one obvious thing is the input filter used for filtering of harmonics and correction of power factor. In order to properly design such filter, the network data shall be known. VSI usually does not require an input filter due to higher rectifier pulse number. However, when it comes to resonances even 24-pulse or 36-pulse rectifier might cause an issue. This is the case when the frequency of one characteristic harmonics corresponds to the network resonance frequency. The probability is low, but Murphy’s law sometimes kicks in.
Another topic of system integration are harmonics on the motor side. We can talk about “motor friendliness” and “load friendliness”. Motor friendliness is linked with distorted motor current, voltage peaks, common mode and dv/dt. LCI has more distorted motor current. On the other hand, the output voltage is sinusoidal apart from commutation notches. VSI can reach motor current with low distortion (multi-level inverters and VFDs with output sine filter). However, voltage waveform is usually less motor friendly (with exception of output sine filter). Load friendliness has a lot to do with torque pulsations and oscillations/vibrations. It is not only about air gap torque ripple. Other factors, such as speed control algorithms, control dynamics, closed loop effects etc. will impact the torsional behavior.
System integration is a topic on its own. Both LCI and VSI have their own specific topics to consider. Experienced manufacturer and system integrator shall ideally work closely together to achieve a reliable and well performing system.
References
[1] How to choose a medium voltage VFD: Cooling, https://mb-drive-services.com/choose-mv-vfd-cooling/
[2] How to choose a medium voltage VFD: Motor compatibility, https://mb-drive-services.com/choosing-mv-vfd-motor-compatibility/
[3] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
