#LCI versus VSI
Part 2: Truth and lies about LCI and VSI
We continue our series about the high power drive champions LCI and VSI. The introduction [1] last time was a bit longer, but the goal was to provide a good and unbiased overview. There is quite some material regarding this topic on the Internet, but unfortunately some content is very questionable and misses a solid base (diplomatically said; in other words there is bunch of semi-truths or even lies). Let’s look closer at some of these statements:
# 1: LCI requires specially designed motors with low reactance.
- This is a semi-true. Yes, the motors for LCI drives are usually specially designed (e.g. dual star stator, capacitive power factor). Lower stray reactance is an advantage as the commutation notches are smaller. However, LCI also works as a soft starter where the synchronous machine is a normal direct on-line type. LCI prefers lower reactance of motor while VSI prefers higher reactance. This is due to the different nature of both technologies: LCI with current source behavior and VSI with voltage source behavior. However, LCI can drive a motor with higher reactance same as VSI can drive a motor with lower reactance. In both cases the system is sub-optimal, but it is feasible.
# 2: LCI has higher air gap torque harmonics.
- This is generally not true. LCI with 6-pulse inverter has quite high pulsations of air gap torque. It is frequently used in applications such as soft starters or gas turbine starters, i.e. not for continuous operation. LCI drives almost always have 12-pulse output consisting of two 6-pulse bridges. The motor has dual three-phase stator winding with 30 deg relative displacement between windings (Y0Y30). This state of the art arrangement drastically reduces the torque ripple. 6-pulse inverter working with firing angle 165 deg has min. torque 70.7%, max. torque 100%, average torque over the period 92.2% and torque ripple (max-min) 29.3%. The 12-pulse configuration has about 3-times lower ripple, i.e. value comparable with VSI drives.
# 3: VSI has air gap torque ripple 0.5-1%.
- Seriously? I would love to see such VSI drive along with some measurements to prove this optimistic statement. 0.5-1% may be true for magnitude of individual harmonics, but not for overall torque ripple (unless the switching frequency is dramatically boosted which goes in hand with significant power derating of the inverter). Even VSI drives with output sine filter would usually have a bit more than 1% and in high power range the VSI drives seldom have a sine filter. Normally the magnitude of torque ripple is not the main concern. What matters is what frequencies are in the spectrum of air gap torque and if they can potentially interact with torsional natural frequencies of the shaft string.
# 4: LCI is more sensitive to AC power disturbances.
- This is not true. LCI with proper control algorithms has the same or better capability to ride through voltage dips in the supply. While the VSI typically uses “zero-torque ride through” strategy, LCI can offer “partial-torque ride through” strategy. For loads such as compressors this feature will extend time to surge and will allow to survive longer voltage dips resulting in less trips and higher availability. Such behavior is proven in the field (harsh conditions offshore Norway); it is not just a theory.
# 5: LCI is less reliable and has shorter MTBF than VSI.
- This is obviously not true. LCI uses classic thyristors that are known for their robustness and high short-term overloadability. In the high power range the LCI has lower parts count than VSI. It also allows for N+1 semiconductor redundancy (if requested). The design is inherently short circuit proof. The excellent reliability track is actually one of the main reasons why the product is still popular after so many years on the market.
# 6: LCI requires much more space.
- This is not really true. The converter itself is usually more compact than VSI. 25 MW LCI has footprint of approx. 16 m2 while VSI of same capacity requires approx. 20 m2 or more. Of course, LCI typically comes along with an input power factor compensation. It is realized by several filter branches. However, the power factor compensation is usually located outdoors so the space concern might not be so severe.
# 7: LCI has much higher line current total harmonic distortion, up to 12% THDi.
- Well, this sounds not quite correct. Yes, without input harmonic filter the current distortion would be normally higher than VSI. But the LCI can have 24-pulse rectifier with harmonic footprint fully comparable to VSI. Or the LCI is realized as 12-pulse with input harmonic filter (most common) and then the harmonics are almost entirely eliminated. One cannot use higher line harmonics and footprint of harmonic filter as two disadvantages at the same time. It is either one or the other.
# 8: LCI and VSI reach approx. same output power.
- Basically true. Both technologies have references with installed power of 100 MW per system. The difference is that LCI can reach this high power as single drive while VSI usually uses the concept of paralleling several drive banks through output reactors, e.g. 4 x 25 MW quadruple channel. Such solution has larger footprint and is more complex.
This post was supposed to correct some statements when comparing LCI and VSI technology. There is obviously lot of development in VSI technology and the limits are pushed even further. Every leading drive manufacturer has one or more (usually more) VSI drive platforms in his current portfolio. In some cases VSI is the preferred technology and makes absolutely sense, both technically and commercially. We wanted to remind the community that LCI also has some goodies and shall not be written off. Being open for both allows you to explore what is the best for you specific project.
By the way, there is still a development done for LCI as well! Unbelievable?! 🙂 Sophisticated control methods in combination with proven hardware platform open up new opportunities [3-4].
References
[1] LCI versus VSI: Introduction, https://mb-drive-services.com/lcivsi-01/
[2] ABB medium voltage drives portfolio, https://new.abb.com/drives/medium-voltage-ac-drives
[3] Advanced control performance optimization for LCI, ACPO for MEGADRIVE-LCI
[4] Model predictive control for LCI, MPC of LCI fed synchronous machines (IEEE Trans. on Power Electronics)
