LCI versus VSI
Part 1: Introduction
LCI (load-commutated inverter) and VSI (voltage source inverter) are the dominant technologies when it comes to high power medium voltage drives. Each technology has certain advantages as well as drawbacks. Project specific conditions and requirements shall be analyzed before deciding for LCI or VSI.
Let’s repeat shortly the main characteristics of each technology:
The history of LCI started in mid. 1970s when the first units were manufactured (ABB manufactured first LCI in 1974). The product has more than 40 years of experience and is very mature. LCI is based on thyristor technology known for its robustness and short time overloadability. It is indirect converter consisting of rectifier, dc link and inverter. The dc link consists of a smoothing reactor which gives the LCI the character of a current source (LCI belongs to the family of current source inverters). The thyristors are commutated by external voltage. On the line side it is the grid voltage and on the motor side it is the machine voltage. This is the reason why LCI can only work with synchronous machine. The thyristors can inherently switch very high current and allow easy series connection. As the power goes up more thyristors are put in series and the motor nominal voltage increases correspondingly. LCI has therefore ideal scalability in voltage. For motors with low power and high voltage the product is usually not cost competitive, but the higher the power the better for LCI. The world record in terms of power is a MEGADRIVE-LCI for a wind tunnel with continuous rating of 101 MW. The low parts count (small number of semiconductors) and the simplicity of the circuitry provides superior reliability. On top, the current limiting reactor in the dc link provides the drive the ability of being short circuit proof. The topology is inherently suitable for 4-quadrant operation and allows regenerative braking without extra cost or complexity (the direction of current does not change, but the voltages are reversed). The thyristors are switching relatively slowly (50 or 60 Hz on the line side and usually similar frequency on the motor side) and have low switching losses. The drive achieves efficiency over 99% at full load. High speed drives are feasible with limitation to approx. 120 Hz (corresponding to 7’200 rpm with 2-pole turbomotor). The converter can be either air cooled or liquid cooled. High power drives with continuous duty are typically liquid cooled to minimize the losses into ambient. On the other hand as soft starter the converter can achieve very high power with air cooling benefiting from transient thermal behavior. One drawback of LCI is lower input power factor (around 0.87-0-92 at full load and usually dropping at partial load). This is usually compensated by installing an input filter. This filter has two functions: eliminate (absorb) current harmonics that would otherwise be injected into the grid and compensate the reactive power so that overall power factor seen from the grid is almost unity (1.0).
The history of VSI started much later when the semiconductors were ready. First medium voltage VSI drives were developed in 1990s and marketed since 1996. Unlike LCI, VSI is a whole family of several sub-topologies. They all have capacitors in dc link providing them the voltage source character (therefore the name VSI). The most common topology that every main manufacturer has in his portfolio is the neutral point clamped (NPC). In low voltage drives 2-level VSI is very common. In medium voltage this would be insufficient to achieve a good output waveform (close to a sine wave, i.e. without too much harmonic distortion). Therefore, 3-level NPC is used as a standard platform. While LCI is externally commutated, VSI drives are always self commutated. This means that the semiconductors can be switched on and off by the gate unit. IGCT thyristors are used for higher power and (lately) IGBT transistors for lower power. The rectifier can be either passive consisting of diode bridges (DFE = diode front end) or active with topology equal to the inverter (AFE = active front end). Diode rectifier is usually available as 12-pulse, 18-pulse, 24-pulse or 36-pulse. The higher the pulse number the less harmonics injected into the grid (cleaner waveform), but at the same time more complex input transformer is required (multi-winding design with phase shifting). DFE has slightly inductive power factor, usually 0.95-0.96 at full load and even higher at partial load. Active rectifier is more expensive. On the other hand, AFE reaches unity power factor as standard or even capacitive power factor as option (VAR compensation). It allows regenerative braking, but has higher losses in driving mode compared to DFE. Both DFE and AFE normally do not require any VAR compensation. The inverter output frequency is usually higher than for LCI and can reach up to 250-300 Hz. The motor friendliness depends on the output filter: 3-level NPC with output sine filter provide clean output waveform and does not stress motor insulation.
Therefore, a standard motor can be used and the drive is suitable also for retrofits of direct on-line motors (fix speed motors). 3-level NPC without sine filter at the output requires specially designed motor with increased insulation and thermal design accounting for harmonics. On the other hand these drives can be used in applications with highest dynamic requirements (e.g. rolling mill drives) that could not be reached by using a drive with sine filter. The scalability in power is achieved by paralleling of complete inverters (hard paralleling), the scalability in voltage is limited (however possible when connecting 3-level NPC modules into cascaded H-bridge). The efficiency of DFE drive is marginally lower than LCI, approx. 98.5-99.0%, the efficiency of AFE VSI drive is approx. 97.5-98.0%. The wolrd record in power is 100 MW per single converter and therefore comparable with LCI. The topology allows for multi-drive, i.e. drives with several inverters connected to a common dc bus and supplying different motors. Motors are controlled individually, can have different rating and can even be of different types (e.g. one induction and other synchronous). Besides described 3-level NPC there are several other topologies within the VSI family. Among other cascaded H-bridge with low voltage semiconductors (especially at low power) and increasingly also modular multilevel converter (MMC, M2C) topology. Each of them has some benefits are drawbacks (e.g. dynamic, but not motor friendly or motor friendly, but with larger footprint and not suitable for low-speed or constant torque apps) and therefore the main drive vendors have several VSI topologies in their active portfolio.
The LCI and VSI live together for about 20 years. While VSI is gaining more and more experience and continuously grows in power, LCI still has some goodies that make this technology attractive (technically as well as price-wise) for certain applications. You will learn about it while following this series # LCI versus VSI.
References
[1] ABB medium voltage drives overview, https://new.abb.com/drives/medium-voltage-ac-drives
[2] Entire blog series on LCI versus VSI, https://mb-drive-services.com/category/lci-versus-vsi/
