Comparing LCI and VSI Technologies: Is There a Winner?

This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.

The urgent need to decarbonize is causing many operators of large rotating equipment, such as gas compressor stations, to switch to electrified drive trains. This trend is driven largely by the imposition of carbon taxes or the availability of credits, as well as the need to meet the ESG demands of investors for sustainable operations. There are also considerable gains in efficiency to be made by using an electric drive train rather than a gas or steam turbine. This translates directly into a reduction in operational expenditure (OPEX). Electrification is aided by the growing availability of suitable, stable electric power connections, even in remote locations.

Operators face a challenge because a typical 10 million tonnes per annum (MPTA) plant, such as a liquid natural gas (LNG) facility, might impose total loads of some 450 to 550 megawatts (MW) on the variable speed drives (VSDs) that control the rotating equipment. That calls for individual medium voltage (MV) drive systems rated from around 30 MW to 100 MW, a level for which there are currently relatively few reference cases.

A key question is what is the best drive technology to use? LCI (load-commutated inverter) is regarded as state-of-the-art for high-power applications, while VSI (voltage source inverter) has grown in popularity in recent years. Below 20 MW, almost all new applications use VSI. Above 35 MW, LCI still predominates, however, VCI can reach higher powers with parallel configurations. Between these two powers, there is a gray area. It is useful to explore the different characteristics of the technologies to understand why the choice is often not clear-cut.

 

Image used courtesy of Adobe Stock

 

A Tale of Two Technologies

LCI drives are well-proven, having been on the market since the mid-1970s. They are based on phase-controlled thyristors (PCTs) and feature an indirect converter comprising a rectifier, DC link, and inverter. The smoothing reactor that provides the DC link gives the LCI its current source characteristics, making it part of the broader family of current source inverters (CSIs). The power capability of the LCI is very scalable by putting more thyristors in series. This is illustrated by the MEGADRIVE LCI installed at a NASA wind tunnel in 1997 that still represents the world’s largest horizontal single motor/drive combination at 101 MW.

The development of power semiconductors enabled the first MV VSI drives to come to market in the mid-1990s. They are based on high-voltage integrated gate-commutated thyristors (IGCT). In contrast with the singular nature of LCIs, VSI drives cover a range of sub-topologies. However, they all feature capacitors in the DC li, giving them their voltage source characteristics.

The power semiconductors used in both LCI and VSI are press-pack (hockey puck) type devices that provide arc-flash safety due to their robust ceramic housing and installation in a stack under pressure.

These semiconductors enable the highest power per single device, which results in a low component count and high reliability. A further benefit is low conduction losses, which result in high efficiency. Low-voltage IGBT module-based topologies such as the modular multi-level converter (MMC) can be used for the highest power segment (above 20 MW).

The basic LCI topology is similar for all manufacturers. That makes it possible to change out an LCI from one manufacturer and replace it with another brand while continuing to use the existing input transformer, power cable, and motor. This flexibility is advantageous for LCIs as it is very difficult to swap VSIs from one manufacturer to another due to the variety of topologies. It could be an important factor considering the shorter life cycle of the VSD compared to the other system components.

 

Motor and Load Considerations

Depending on requirements like shaft power, speed, and efficiency, the application might require a synchronous or induction motor. Both types can be used in high-power VSD applications and are available across a wide range of power ratings. In general, synchronous motors are selected for better efficiency, while induction motors are the simpler solution with a lower investment cost.

This is where we find an important difference between the two drive technologies: LCI can only drive synchronous motors since the back-electromotive force (EMF) is used to commutate the inverter, while VSI can drive both types of motors.

Current harmonics produced by the VSD create extra losses in the motor, which result in an increased temperature rise. For LCI operation, standard motors are normally not used, and the motor will need to be designed to account for the harmonic heating. It is possible to achieve a relatively clean waveform for the motor current with a VSI drive, either by using an output sine filter or a multi-level inverter.

Sine filters are normally not used for high-power applications because the motors are built for operation with a specific VSI and during the design process, the possible current harmonics are considered. If a sine filter is used, there is no additional temperature rise. A sine filter may be needed for a very long power cable (e.g., 10 km) between the drive and motor.

Furthermore, torsional oscillation of the driven shaft has always been a concern with a VSD for LCI and VSI. Although the amplitudes of pulsating torques may be small compared to the driving torque, they can excite resonances when their frequency coincides with the natural frequency of the shaft. Demanding systems with high torsional stress on the shaft may require an advanced design process. Technical risk can be eliminated by designing mechanical and electrical components as a string to ensure torsional-optimized VSD control.

The converters are different in the way in which they produce harmonics and how they directly excite resonances. In general, VSI has smaller amplitudes of 6th and 12th harmonic than the LCI, but they are still present and have to be dealt with. To reduce the pulsating torques for LCI-driven equipment, a motor with two winding systems has been used since the beginning of VSD development.

 

Supply Network Considerations

 

Table 1. Comparison of LCI and VCI for supply network considerations.

Performance Item	LCI	VSI
Line side harmonics	Engineered Solution
Supply power loss ride-through	++	+
Power factor correction	Required	Not Required

 

Power factor: For high-power VSD applications, a diode bridge is considered for most VSI topologies. For a diode rectifier, the power factor is given by the inductive direct voltage regulation that depends on the commutation time, which depends on the actual commutation voltage, load current, and commutation impedance. A small network/transformer impedance, high voltage level, and small currents lead to a better power factor. The power factor also depends on the pulse number of the rectifier since the impedances are changing. The higher the pulse number,r the better the power factor. In general, the power factor is higher than 0.95 for diode rectifiers in a properly designed system for any operating point.

Active rectifiers can also be used on the network side for VSI drive systems. They are needed if the process requires regenerative operation. The power factor can be controlled to any value with active rectifiers, but its reference is usually set to unity. Typically, power factor correction equipment is not required for VSI systems.

For LCI drives, the power factor is dependent on the operation point and how the system is designed. The network power factor is proportional to the ratio of motor and network voltage. The motor voltage increases proportionally to the speed, and the network power factor increases accordingly. Due to network power factor requirements and speed range, a compensation system is often required for the LCI. Measurements of an LCI compressor system in commercial operation using modern control algorithms show massive improvements compared to traditional methods. With 92% speed, 82% torque, and 100% network voltage, a power factor improvement from 0.83 to 0.93 was achieved. Figure 1 shows that with compensation, it is possible for both VSI and LCI drives to achieve the same power factor.

 

Figure 1. Network power factor for VSI and LCI drives in a gas turbine replacement project. Image used courtesy of Bodo’s Power Systems [PDF]

 

Harmonic distortion: Generally, VSDs are non-linear loads that create current harmonic distortion. This distortion depends on the inductive direct voltage regulation for VSI and LCI. It also depends on the firing angle of the rectifier, which can be considered zero (α = 0°) for diode bridges. Current harmonics are calculated as defined in IEC/TR 60146-1-2. There is no linear relationship between the firing angle and amplitude of the harmonic current. Nevertheless, the amplitudes tend to get bigger by increasing the firing angle. These calculations are supplier-independent. Irrespective of VSI or LCI technology, the harmonic distortion can be decreased to a certain extent by using a higher pulse number for the rectifier. Practically, a 24-pulse or 36-pulse solution is feasible. But this depends on the accuracy of the transformer phase shift (half a winding cannot be done). Higher pulse numbers are not practical because the transformer vector group cannot be realized with the required tolerances.

For VSI and LCI, pulse numbers of 24 and higher are possible. To reduce complexity, 24 and 36-pulse rectifiers are preferred. Figure 2 shows common network configurations. Configuration 1 is the solution when harmonic filtering and power factor compensation are not required. This configuration is used for both VSIs and LCIs. For LCIs, the operation range must be small and close to the nominal speed, or the power factor is compensated by other means. Concerning harmonic distortion, the performance of a 24-pulse solution in a standard environment is sufficient to comply with international standards. If power factor compensation for the LCI is required, the filter system can also mitigate harmonic distortion. In this case, a 12-pulse solution is preferred to reduce complexity. The filter size (Mvar rating) is determined by the required network power factor and the operation points of the process. This filter can easily cope with the harmonic distortion created by a 12-pulse rectifier to ensure compliance with the previously discussed grid standards. Configuration 2 or 3 cacompensate forte the power factor and reduce harmonics for an LCI system.

 

Figure 2. Network configurations of VSI and LCI to reduce harmonic distortion and compensate the network power factor of the rectifier. Image used courtesy of Bodo’s Power Systems [PDF]

 

Supply power loss ride-through VSI and LCI-based ASDs can ride-through supply voltage disturbances using different methods. A drive shutdown can be prevented even during main power outages of multiple seconds by implementing advanced drive control software. The maximum sustainable duration of a power outage depends on the load system, the machine, and the actual operating point before the power outage. A VSI can provide partial torque when the network voltage is 70- 80%. When the DC bus voltage becomes too low, the drive enters the ride-through mode, in which the motor is used as a generator to compensate for converter internal losses. The energy stored in the rotating system of the motor and the load is used to keep the DC bus charged. Figure 3 shows the ride-through behavior.

 

Figure 3. VSI drive system behavior when entering and leaving ride-through mode. Image used courtesy of Bodo’s Power Systems [PDF]

 

The LCI has superior ride-through capability to a VSI. It is more robust due to its active front end and because the decoupling of the motor side does not rely on a capacitor bank. An LCI can provide partial torque down to a network voltage below 50% using state-of-the-art control methods. Model Predictive Control, an optimization-based control method, keeps the driven equipment in a healthy operation range even longer. An estimation of residual power during main supply power dips of a 41.2 MW export compressor system driven by an LCI is presented in Figure 4.

 

Figure 4. Estimate of the residual power during voltage dips with LCI. Image used courtesy of Bodo’s Power Systems [PDF]

 

A Matter of Scale

Upscaling is necessary to reach high power. There are, in general, four directions to upscale a VSD by series or parallel connection of the following components:

1) Semiconductors: A series connection of semiconductors means that there are no changes in the topology on the control scheme. To increase the power and voltage for an LCI, semiconductors (thyristors) are connected in series. Thyristors are extremely robust; therefore, an “n + 1” redundancy is not required but can be easily provided.

2,3) Inverter: Series or parallel connection of inverters changes the topology. A multi-level inverter is introduced for series connection, which requires complex input transformers. However, the output waveform can be optimized by using control schemes with higher complexity. VSI-based topologies use parallel-connected inverters to increase the power and current rating.

4) VSD system: Parallel connection of VSD systems means standard-size products can be used. However, it increases the complexity of the installation, control, and protection scheme. This approach can offer redundancy on the VSD system level, which is only useful when the systems are oversized or the process tolerates reduced power. LCI and VSI-based topologies use parallel-connected ASD systems to increase power.

 

Reliability and Availability

 

Table 2. Comparison of the relative reliability of VSI and LCI at different shaft powers

Shaft power	20 MW		50 MW
VSD topology	VSI	LCI	VSI	LCI
Reliability	1.0	1.1	1.0	1.4

 

When reviewing reliability, Mean Time Between Failure (MTBF) and similar figures should be used with care. They are relevant when comparing different layouts in the same drive. However, they are not useful when ranking the reliability between different suppliers because of the different approaches generally taken to obtain the numbers. They can be used to compare two different systems if the same supplier or operator has adopted a uniform approach to make the comparison in both cases makes the comparison. While it is impossible to deliver a supplier-independent comparison of reliability figures between VSI and LCI-based system topology, the VSD is the most complex system component. It, therefore, makes sense to focus on this element.

Table 3 shows ABB’s comparison of the expected failure rate of the topologies in two independent systems (20 and 50 MW), with the VSI reliability used as a reference for both (one per unit). The LCI has the advantage at 20 MW because of its simple topology. The LCI maintains its advantage at higher power because its scalability requires fewer additional components.

Based on field experience, only about 25% of trips are related to the drive. Out of this, 90% are related to the implementation and design of the converter and the remaining 10% are related to the MTBF of the components. That means only about 2.5% of all trips are related to component issues. The most important factor is good system engineering, including network integration, mechanical-electrical interaction, and system protection. In addition, continuous improvements and the high maturity of equipment lead to high availability.

If the VSD must operate for years without a shutdown, be it scheduled or un-scheduled, then it needs to be configured for redundancy on the macro level – with either a complete VSD or a redundant converter. There are many possible ways to realize full drive redundancy. However, one possibility is based on these boundary conditions:

• The process allows zero torque for < 150 ms in case of a drive failure
• This requires VSD and, if applicable, excitation system (exciter converter) redundancy
• Both VSDs, including control and cooling, are totally independent and are rated to run 100% motor power.

Due to the differences in the protection concepts of VSI and LCI, the LCI can have the main circuit breaker on the secondary side of the input transformer. This means VSD input transformer redundancy is optional and does not influence the switchover performance for LCI. Even so, we can assume the system would be as shown in Figure 5. A network filter is not needed as critical VSD systems are designed to provide full torque without it. One filter system is sufficient.

The LCI topology is superior because the decoupling of systems 1 and 2 is achieved by thyristors in a blocking sta. Decoupling is independent of the main circuit breaker (MCB) and output switches (S1/2). In addition, charging of the DC-link is not required and the MCB can be switched on anytime. With S1/2 closed and the inherent line and motor voltage measurements, the LCI is always aware of the status of the motor (and network). With the zero-current feedback of the faulty system, the control can be released immediately.

 

Figure 5. Overview of a redundant VSD system,m including redundant input transformers. Image used courtesy of Bodo’s Power Systems [PDF]

 

The Field Is Open to Both VSI and LCI Solutions

This article overviews some of the most important differences between VSI and LCI technology. However, there are further project-specific aspects that should be considered, such as safety, cost, efficiency, and installation footprint. However, to summarize the current situation, despite the availability of more variants of VSI solutions, neither VSI nor LCI can be shown as conclusively superior to the other for high-power applications in general. The well-proven and simple LCI topology leads to the highest reliability, while VSI-based drives can lead to more flexibility in terms of system integration. With the development of a Model Predictive Control algorithm for the LCI, the gap regarding dynamic performance is closing.

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine.