LCI versus VSI Technology: Commercial Comparison
In our LCI versus VSI series of articles we have intensively discussed the technical aspects and performance of both high-power drive technologies. Based on your feedback you are interested in a commercial comparison as well. Therefore we decided to write this post on LCI versus VSI: Commercial comparison.
Besides the technical advantages and drawbacks of two main high power VFD topologies – LCI and VSI – the price tag is important as well. In fact, price might be the decisive criteria to go for one or the other solution.
When reading some marketing and techno-commercial material of the VFD manufacturers you will come across the price comparison of LCI and VSI drive technology. And you will most likely find both type of statements: one claiming that LCI solution is more cost-effective as well as the other stating that VSI is commercially more attractive. Quite often these statements are not supported by any specific figures so the reader has practically no chance to verify whether it is true or false. So who is right? Does LCI require less investment? Or vice versa? Is that the “million dollar question”?
If you are interested in a surprisingly simple answer please read till the end.
Price or cost?
First of all, there is cost and price – two pairs of shoes. Price is what the customer pays and (hopefully) includes certain margin for the seller. Therefore, to be more transparent we would have to talk about cost comparison for the VFD manufacturer (material, development, engineering, assembly and testing etc). However, the end user does not see the internal cost. What matters for him is the final price. To keep it simple let’s just assume that the VFD manufacturer has exact same margin on both drive technologies.
Second point: VSI is not a unique definition. While the LCI is basically almost the same regardless of the manufacturer, VSI contains lot of specific topologies. They differ in the number of voltage levels, power quality and motor friendliness, type of semiconductor, dynamics etc. Let’s assume a diode front end (DFE) type of VSI. Moreover, we focus on medium and high power range and therefore certain low power VSI topologies are automatically disregarded.
LCI versus VSI: Commercial comparison
So let’s compare the prices of medium and high-power medium voltage drives. We will assume 12-12 pulse LCI drive versus 24-pulse or 36-pulse diode front end VSI.
Obviously, the price comparison depends on the specific power as well as on the specific product. As already stated: LCI drives are comparable among the manufacturers with minor differences. However, the VSI drive portfolio differs significantly.
The first figure is a general price comparison between LCI and VSI. Our aim is not to focus on exact numbers. Instead, we want to show a trend.
Why do the curves have such jumps? That is simple. Drives are standardized into certain frame sizes (here we have assumed 4 frame sizes of a single channel VSI drive, afterwards a paralleling of entire drives is necessary). It is much more cost effective and more practical than to design each drive entirely for specific project. Well, it does not mean that the design has no flexibility. However, the frame sizes allow better serialization. But back to these price jumps: Those are exactly the transitions from one frame size to another one.
Transition from single channel to dual channel VSI
One jump is bigger than the others. That is the case where maximum capacity of a single VSI bank is reached. Above this limit, two or more banks need to be paralleled. For most manufacturers the maximum power rating of a single VSI drive is somewhere between 30 MVA and 40 MVA. In this regard, LCI can reach higher power per converter without the need for paralleling. That is one of the main reasons why LCI is commercially so attractive at very high power. The steps in the LCI prices are mainly linked with the number of thyristors. Compared to VSI, there are more steps and each step is relatively small.
Trend in Figure 1 shows that VSI is tendentiously cheaper at the lower power range. As the power increases LCI is becoming more and more cost effective solution. At very high power, where multiple VSI banks would need to be paralleled, LCI has a clear cost advantage.
Consideration of harmonic filter
Now you may argue that such comparison is not entirely correct and I actually agree with you. So, to make the comparison more realistic, let’s compare LCI plus a harmonic filter with VSI that does not need any filter. As expected, the presence of filter increases the cost of the LCI solution. Especially at lower power range the harmonic filter increases the total cost of LCI solution. Remember that the filter is partly pre-engineered but at the same time tailor made for the project. Therefore, besides the material cost there is also associated engineering cost. While the material cost scales with nominal power of the filter the engineering cost is pretty much constant.
Remark: By “harmonic filter” for LCI we mean a filter system consisting of several branches that absorb characteristic harmonics of LCI and inherently also provide reactive power for power factor correction.
Comparison of LCI plus an input harmonic filter versus VSI is shown in Figure 2. The filter cost basically shifts the threshold where LCI becomes more cost effective solution.
Also this comparison is not 100% correct. The transformer and motor cost may somewhat differ for LCI and VSI. However, this is not so easy to quantify. In medium power range VSI can operate with asynchronous machine that is cheaper than synchronous. In truly high power range both VFD technologies will supply synchronous motor.
What power to select for benchmarking?
Now when you look at Figure 2, which VFD technology is actually cheaper? LCI or VSI? It depends what power we compare, right?
At P1 one can claim that VSI is more cost effective. At P2 we would consider LCI to be cheaper. And at P3 LCI clearly seems to be more attractive from commercial standpoint.
And this is the whole “problem” in this topic. If you want to present VSI as commercially better solution you just need to pick a power corresponding to the top of frame and focus on medium power. If a VSI frame is rated 15 MVA then maybe 15 MVA VSI looks somewhat better than 15 MVA LCI. However, if the intention is to present LCI as a very attractive solution compared to VSI the best thing is to pick a power rating where the VSI jumps to the bigger frame. or even better: spot the rating where VSI changes from a single bank to double bank (paralleled VSI drives). This lowest power of a double bank (dual channel) solution will probably not look very good for VSI.
Commercial comparison on a system level
In fact, the most meaningful and transparent price comparison would be on the system level, i.e. considering total investment cost of a drive system (transformer, VFD, motor and harmonic filter if needed).
In the lower end of considered power range the VSI solution would look once again slightly better as there induction machine could be used instead of synchronous. However, if the operation cost is evaluated as well, better efficiency of synchronous machine compared to induction machine will almost certainly pay off.
At high or very high power we can assume synchronous machine as a logical choice for both LCI and VSI. Still, VSI typically operates with motor power factor 1 while LCI requires an overexcited machine. For certain power rating the LCI may eventually require more costly motor whereas for other power rating the machine cost in case of LCI and VSI would be quite similar.
Summary
As you can see and read, there is no simple answer whether LCI or VSI technology is commercially more attractive. It is not black and white. Instead, there are many shades.
Unfortunately, it is often misused to present one technology more cost effective than the other. When selecting a specific power rating and other boundary conditions one can claim that VSI is cheaper than LCI or vice versa. However, making a general statement out of it is not correct and really misleading. Better to take such statements with a bit of salt and review them critically.
References
[1] LCI versus VSI: entire series of articles, Link to article series
[2] Current source and voltage source inverter, Link to article
[3] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
Do you want to learn more about variable frequency drives and drive systems? Is your aim to increase drives knowledge step by step?
Sign up to our premium service and get access to additional resources.
