LCI celebrates 50 years anniversary!
It is little bit hard to believe but the high-power drive champion LCI celebrates 50 years anniversary in 2024. We take this opportunity to look back at the history of this matador and maybe also a little outlook.
Birth of LCI in 1974
It was in 1974 when both ABB and Siemens introduced first load-commutated inverter (LCI) to the market. First variant was a starter but the full drive followed shortly after. 50 years later the product is still on the market, not just in service but still in active manufacturing phase as well.
LCI characteristics
LCI is a thyristor¹ based converter. It is primarily used as a soft starter, gas turbine starter or a high power drive. The thyristors are externally commutated by grid and machine voltage on the corresponding drive sides. LCI is a specific subset of current source inverters (character given by the dc reactor). The drive is inherently capable of 4-quadrant operation with bi-directional power flow. Unlike most other VFDs the power is inverted by inverting the voltage polarity while direction of current remains the same. LCI works together with synchronous machine only. It is because the commutation of the inverter requires an external source with leading power factor (overexcited machine).
With regards to motor friendliness the machine voltage is almost sinusoidal (good for the insulation system) while the stator current is closer to a rectangular shape (harmonics to be considered for thermal design of motor).
The converter is available either as air-cooled (soft starters, gas turbine starters, smaller drives) or liquid-cooled (some starters, large drives).
The technology inherently supports regenerative braking (if required/allowed).
¹ Also called ‘SCR’ = silicon controlled rectifier. The device is turned on by injecting a gate current pulse. The device turns off after loss of gate signal when reaching a zero crossover of current.
Configurations
The grid side and machine side converter consist of one or more 6-pulse thyristor bridges (in Figure 2 each thyristor block represents a 6-pulse bridge connection). The simplest configuration is thus a ‘6-6 pulse’ (6-pulse grid side and 6-pulse machine side). The pulse number at each side does not have to be the same. For example, there are starters in ’12-6 pulse’ configuration (12-pulse grid side and 6-pulse machine side). Large drives are typically 12-12 pulse with a dedicated three-winding transformer (four-winding design in case of filter winding) and dual three-phase synchronous machine.
Where to meet LCI
The product traditionally serves in industrial segments such as oil & gas, power generation, water or test stands. Applications include large compressor trains (multiple compressors in the shaft line), pumps (e.g. feedwater pumps or pumped storage power plants), fans, gas turbine starters, wind tunnels etc.
World record
The famous world record in terms of drive power is 101 MW drive driving a special fan as part of a wind tunnel application. The drive was delivered in 1997 and is in operation for more than 1/4 of a century.
Reasons for popularity
The factors that make LCI still very popular are:
- Simplicity: VFD topology is straight forward
- Robustness: combination of thyristors and reactor in dc link lead to a robust design
- Power scalability: power is scaled with voltage by putting several thyristors in series
- Maturity: some industries like the proven design and vast field experience
- Semiconductor redundancy: thyristors can be realized with n+1 redundancy
- High power density: this feature leads to comparably small footprint
- High efficiency: > 99% due to low-loss thyristors and fundamental switching frequency
- Regenerative operation: regenerative mode inherently available w/o extra hardware
- Engineered solution: product is typically customized for specific application/project
The motivation to use LCI may differ. For someone it is the vast experience of the manufacturers due to long presence on the market, for others it might be the footprint or the design flexibility.
New features
While the power hardware remains practically the same, the control hardware is upgraded to the state of the art. The control software allows implementation of new functions and features to improve the drive’s performance. Examples can be algorithms for torsional damping or enhanced robustness against voltage dips and grid disturbances in general (ride through capability).
Predictions about phase-out
First rumours about phasing out the product appeared already some two decades ago. However, despite the rapid progress of voltage source inverters (that dominate the VFD market today), LCI still has its place in the drive portfolio.
Outlook
We do not have any crystal ball to see in the future. However, based on the past experience and the perception of the product, it is expected that LCI will still defend its position in the VFD portfolio of global manufacturers. While majority of the drives is dominated by voltage source inverters (VSI) and their share is probably going to further increase, LCI likely remains popular in specific soft starter applications and very high power drives. The strengths of the product were already listed above. One particular item is the high level of engineering allowing quite some degree of freedom for tailor made solutions. Other drives, used in the mainstream applications and produced in higher quantity, are often much more standardized and potentially less flexible (although there are exceptions with highly engineered VSI drives as well).
Fit for next 50 years?
It seems that LCI will remain in active manufacturing phase for quite some time. Moreover, there is a large installed base of LCI drives. Thanks to long lifetime of the product and available control upgrades those drives will continue serving the industry for the years to come.
In any case, 50 years is an impressive age. There must be something good on the product when it was not pushed completely out by the younger VFD platforms.
References
[1] ABB medium voltage drives: MEGADRIVE-LCI, https://new.abb.com/drives/medium-voltage-ac-drives/megadrive-lci
[2] LCI versus VSI (article series), https://mb-drive-services.com/category/lci-versus-vsi/
