LCI versus VSI:
Part 3: Comparison
Figure 1: LCI vs VSI
In today’s column we continue our popular series ‘LCI vs VSI’ and look a bit closer at the differences between a load-commutated inverter (LCI) and voltage source inverter (VSI).
1. History and references
The history of LCI started in mid 1970s [1]. Therefore, LCI can look back at 45 years of successful operation in many industrial applications and covering all continents except Antarctic. For more details refer to LCI vs VSI: Part 1 of this series [2].
The history of medium voltage VSI started in late 1990s when the self-commutated semiconductors reached sufficient voltage and current rating. Nowadays VSI drives are dominant in low and medium power range.
2. Topology
LCI topology is simple and easily scalable. Basic building block is a 6-pulse thyristor bridge. The same bridge is used in the rectifier as well as in the inverter. Small LCI has a 6-pulse rectifier and 6-pulse inverter (in short ‘6-6 pulse’). Such topology is often used as a soft starter or gas turbine starter. Larger LCIs usually have 12-pulse rectifier and 12-pulse inverter (in short ’12-12 pulse’). Other combinations are principally also possible, e.g. ’12-6 pulse’ for lower line side harmonics. Between rectifier and inverter there is a DC reactor. The purpose is to smooth the current in the dc link (reduce its ripple) and enable reactive power exchange. This reactor gives the inverter the ‘current source’ type of behavior.
VSI cannot be classified as just one topology. In fact, there is a complete range of topologies. One of the oldiest and most common one is neutral point clamped (NPC). Low cost and wide spread topology is the multi-cell design (cascaded H-bridge) with a multi-winding integrated transformer and low voltage cells connected in series. Another topology is multilevel inverter with flying (floating) capacitors or emerging modular multilevel converter (MMC) topology. Each of the mentioned VSI topologies has its advantages and drawbacks. Some are better for high dynamic performance, others excell in motor friendliness etc.
3. Semiconductor type
LCI uses classic thyristors. The same semiconductor is used in the rectifier and in the inverter which makes it very simple from spare part point of view.
VSI uses self-commutated devices in the inverter. For high power VSI drives IGCT thyristors are widely used. Lower power drives use IGBT transistors – either medium voltage (in NPC, NPP or flying cap topology) or low voltage (in multi-cell and MMC topologies). The rectifier is mostly diode front end; usually 12-pulse or higher. In case of active front end rectifier the hardware is identical to the inverter.
4. Commutation
LCI uses thyristors that are externally commutated: on the line side by the grid voltage and on the motor side by the motor voltage. The thyristor is switched on by a current switching impulse into the gate and switches off when a blocking voltage appears. The commutation circuits are very simple.
VSI uses self-commutated semiconductors that can be turned on and off. IGCT is switched on and off by current impulse, IGBT is controlled by polarity of gate voltage. IEGT is basically an improved IGBT.
5. Switching frequency
LCI thyristors are externally commutated and switch with the fundamental frequency. Therefore, the rectifier switching frequency is either 50 Hz or 60 Hz. The inverter switching frequency is equal the stator electric frequency of the motor, typically also in the range 35 Hz to 70 Hz. Due to that, LCI has low switching losses and high efficiency.
VSI drives have self-commutated power semiconductors in the inverter. The switching frequency depends on the topology, motor data etc. Typical range is from 250 Hz up to 600 Hz or more per semiconductor. Note that the switching frequencies in medium voltage are lower than those in low voltage VFD. Diode rectifier is passive and the diodes commutate with the grid frequency, i.e. 50 Hz or 60 Hz. Active rectifier (AFE) has similar switching frequencies like the inverter.
6. Output frequency range
Standard output frequency of LCI is up to approx. 66 Hz, optionally up to 120 Hz with minor or no modifications. This corresponds to 6’000 rpm of a 2-pole turbomotor (semi high-speed).
Standard output frequency of VSI is up to 66 – 75 Hz. Optional output frequency is 120 Hz for most topologies. Some VSI drives are modified for high-speed applications with output frequencies up to 300 Hz (corresponding to almost 18’000 rpm motor speed).
7. Output power range
LCI is a champion in high power. Standard drives are suitable for up to approx. 60 MW. High power version reaches over 100 MW without the need of paralleling individual VFDs. Reference with 101 MW shaft power is in operation since 1997 [3, 4].
VSI drives can reach approx. 35 – 40 MW as single bank (status 2019 – increasing trend). Higher power is achieved by paralleling of multiple VFDs through output reactors. Some manufacturers have scalable concept to parallel up to four VFDs in this way and reach 4 x 30 MVA = 120 MVA total power. There are also special VSI frequency converters that can reach more than 100 MW as single unit. However, these converters are not primarily developed for motor drive applications (HVDC transmission, grid interties etc).
8. Reversal of power flow
To reverse the power flow in LCI drive the voltage polarity is reversed while the direction of current remains unchanged. The LCI topology can inherently reverse the power flow as the thyristor bridge can have positive or negative voltage polarity depending on the control angle of firing pulses –> no changes in the power hardware are required.
To reverse the power flow in VSI drive the direction of the current must be reversed while the polarity of voltage in the dc bus remains unchanged. DFE drive does not allow reverse power flow. Instead, an AFE drive is required.
9. Regenerative braking
LCI allows regenerative braking as the topology inherently supports four-quadrant operation. The user gets this feature ‘for free’.
VSI with diode rectifier (DFE) cannot provide regenerative braking capability. Resistor braking can be provided at extra cost by adding a braking chopper. For applications with seldom braking this is a good solution. The braking resistor can be placed inside the VFD or externally.
VSI with active rectifier (AFE) is suitable for regenerative braking. For frequent and dynamic braking this is the preferred solution. However, AFE comes with extra price tag and with higher losses in driving mode [5].
10. Input power factor
The power factor of a thyristor rectifier is linked with the control firing angle. Power factor PF ~ cos φ ~ cos α (φ is the angle between fundamental voltage and current phasor; α is the control firing angle of thy thyristor bridge). At low speed the control firing angle is high (towards 90° el) and the power factor is low. To mitigate it, LCI drives are usually equipped with an input harmonic filter that fulfills two functions: (1) filtering of harmonics, (2) power factor correction. The input power factor with a filter can typically be controlled in the range 0.95 inductive to 1.0, regardless of the load point.
VSI with diode rectifier (DFE) has high power factor > 0.95 across the whole operation range. With partial load the power factor marginally improves (neglecting extreme cases at almost no load) while at full load the value is approx. 0.95-0.96 depending on system design and grid impedance [6].
VSI with active rectifier (AFE) can control the input power factor. Default setpoint is 1.0, but on demand a reactive power compensation can be used. In such case the power factor would typically be slightly capacitive to compensate other inductive loads within the same plant.
11. Efficiency
LCI reaches ‘true efficiency’ (incl. auxiliary consumption) up to approx. 99.3% due to low switching losses (low switching frequency) and low on state losses of power thyristors. In case of thyristor redundancy (N+1) the efficiency is slightly reduced (additional semiconductors in series generating additional losses).
VSI again depends on the specific topology, rectifier type, type of semiconductor, switching frequency and power rating. Generally, larger VSI drives tend to have a better efficiency (same trend as in transformers or motors). Larger VSI (> 10 MVA) with diode rectifier can reach almost as high efficiency as LCI, i.e. some 99.2%. VSI with active rectifier is roughly 0.6% – 1.2% lower due to higher losses in the rectifier’s active switches. The efficiency is higher with synchronous motor due to unity power factor (1.0) of the inverter and subsequently lower inverter current for the same shaft power and same motor voltage.
For more details please refer to our previous post in Energy efficiency category [5].
Hope you enjoyed this post. We will compare more items, such as control, reliability, footprint, motor compatibility or torque ripple in our next post. Stay with us to learn more! Next post on LCI vs VSI will follow soon.
LCI vs VSI – Follow the whole series in our blog: https://mb-drive-services.com/category/lci-versus-vsi/
References
[1] Static frequency converters for soft starting of synchronous machines in power plants, https://library.e.abb.com/public/b09e1b7fc1063305c12572490033a751/MEGADRIVE-LCI_for_SoftStarter_Rev0.pdf
[2] http://mb-drive-services.com/lcivsi-01/
[3] R. Bhatia et al., “Adjustable speed drive using single 135,000 HP synchronous motor”, IEEE International Electric Machines and Drives Conference, May 1997, pages TC1/9.1 – 9.3
[4] G. Sydnor, R. Bhatia, H. Krattiger, J. Mylius, D. Schafer and E. Carpenter, “Fifteen years of operation at NASA’s National Transonic Facility with the world’s largest adjustable speed drive”, 6th International Conference on Power Electronics, Machines and Drives (PEMD), 27-29 March 2012
[5] http://mb-drive-services.com/energy-efficiency-part-5/
[6] http://mb-drive-services.com/energy-efficiency-part-6-power-factor-and-reactive-power-consumption/
