LCI versus VSI:
60 MW case study
Introduction
In this post we share some results of a case study comparing load-commutated inverter (LCI) and voltage source inverter (VSI) for a 60 MW variable speed drive system. Due to confidentiality some quantities are only presented as relative values in percentage instead of absolute values.
Case description
The case study is based on a real project of a 60 MW drive that is currently in execution phase. The electric motor is a brushless-excited synchronous machine with rated speed approx. 3’000 rpm and speed range 2’100 rpm to 3’000 rpm (70% to 100%). Both LCI and VSI technologies are principally possible. LCI can be realized as one single bank, VSI solutions are mostly based on parallel connection (dual bank). The power flow is uni-directional. There is no need for active braking or power regeneration. Therefore, VSI solution consists of a diode rectifier (DFE) in order to achieve lower cost and higher efficiency. The supply grid is high voltage (> 35 kV) and therefore input isolation transformer is necessary regardless of selected technology.
Key criteria
From customer perspective following criteria were essential to succeed in the project:
– small footprint (brown field installation)
– high efficiency (power generation)
– input power factor (minimization of reactive power)
– grid compliance (strict harmonic limits)
VFD solutions for 60 MW
1. Variable Frequency Drive
Both solutions, LCI and VSI, had been originally proposed. VSI is a 6.9 kV multi-level inverter type with 36-pulse diode rectifier. Two VFDs need to be paralleled to the motor through output reactors (dual bank solution). The output reactors can be external components or can be integrated inside VFD lineup. LCI is a single bank VFD with 24-pulse thyristor rectifier and 12-pulse thyristor inverter. The output voltage is approx. 2 x 7.5 kV.
Both VFDs are liquid cooled which is clearly the preferred cooling method in this power range. Cooling pumps are either standard pumps with 2 x 100% redundancy or using a canned (“maintenance-free”) pump with superior availability. Liquid cooling leads to more compact design and overall higher efficiency. In addition, it minimizes the need for re-cooling of electric room.
Both VFD technologies have corresponding enclosures with protection class IP42 or higher (up to IP54). Arc resistant design with highest safety level is a standard feature of described VSI solution. LCI can be equipped with arc resistant option as well.
2. Motor
Motor type for LCI as well as VSI solution is a 2-pole synchronous machine with brushless excitation. Synchronous machine is the state of the art solution in this power range. It features higher efficiency compared to asynchronous machine.
For VSI solution the motor has conventional three phase stator, usually star connected (Y). For LCI solution the motor has dual three phase stator system. The winding systems have relative phase displacement of 30° electric (YY30). such method is commonly used for large LCI drives as it helps to significantly reduce the torque ripple.
Synchronous motor supplied from the VSI drive operates with unity power factor (1.0). In case of LCI the synchronous machine needs to provide reactive power for the commutation of the inverter and is therefore operated with nominal power factor 0.925 capacitive (-0.925).
3. Input isolation transformer
Proposed LCI features 24-pulse thyristor rectifier. Therefore, the input transformer has four phase shifted converter side (secondary) windings. The relative phase displacement between the windings is 15°. In addition, the transformer has a dedicated tertiary winding for connection of harmonic filter.
VSI solution features 36-pulse diode rectifier. The rectifier is supplied from an input transformer with corresponding six converter side (secondary) windings. The relative phase displacement between the windings is 10°. Transformer can be realized with single core or eventually with two active parts inside one common tank.
Rating of LCI input transformer is approx. 75 MVA plus tertiary winding of 30 MVAr. Rating of VSI input transformers is 2 x 34.5 MVA, i.e. 69 MVA in total.
Characteristics of VSI and LCI drive systems
For the end user key parameters are footprint, high overall efficiency, high input power factor and low harmonics. All these factors were carefully considered in the selection process of VFD drive system. Let’s look at each item.
1. Footprint
Footprint is critical indoors as it is a brownfield project with existing building. Maximum length of the VFD shall not exceed 10.5 meters and width is limited to approx. 2.5 meters. It is very challenging to fit a 60 MW VFD into such limited space.
LCI with its high power density makes it possible to install a 60 MW lineup (back to back) into the existing building, even with excitation, cooling and control panel in the main lineup. Further reduction of length is possible when separating excitation and control panels and placing them separately nearby the main power hardware.
Certain drawback of LCI is the harmonic filter system. In this case it is primarily needed for power factor correction. The harmonics are inherently low due to 24-pulse connection so the filter branches just help to reduce the harmonics even more resulting in superior grid side waveforms. The filter system requires significant amount of space (approx. 21 x 8 meters footprint). Fortunately, the filter system is installed outdoors (containerized solution) where the space is not so critical.
VSI solution consisting of two VFDs paralleled through output reactors has high power density as well. However, it is not as compact as LCI and does not fit into existing building. It would need to be installed outdoors inside suitable container. Footprint of each VSI bank is approx. 15 x 1.6 meters. On the other hand VSI does not require any harmonic filter/power factor correction system.
Note that the harmonic filter system requires roughly twice as much footprint as the total VSI solution (dual bank) and almost four times the footprint of LCI drive itself.
2. Efficiency
High efficiency is very important for customer and correspondingly evaluated in the tender. Lifetime of the plant is expected to be more than 30 years. Therefore, small difference is efficiency results in a substantial operating cost difference over the entire life cycle. The losses are penalized with approx. 4’000 EUR/kW.
Proposed LCI and VSI solutions reach very similar efficiency figures on the system level. The LCI has marginally higher efficiency than VSI (99.2% versus 98.9%) while the motor efficiency of VSI solution is slightly better due to lower harmonics and unity power factor (LCI operates with pf = -0.925 at rated load). Transformer efficiency and losses are comparable for both VFD technologies.
As mentioned, synchronous machines generally achieve higher efficiency compared to their asynchronous counterparts – see e.g. [3].
3. Input power factor
Input power factor shall be as high as possible and must remain inductive. Overcompensation is forbidden. VSI solution with multi-pulse diode rectifier reaches power factor approx. 0.95 at full load. At reduced load the input power factor further increase (for explanation see [4]). No reactive power compensation is required.
The input power factor of LCI corresponds to the firing angle α of the rectifier (pf = cosφ ∼ cosα). This fact leads to low power factor at low motor speeds. In order to compensate the reactive power and improve the power factor a filter system is included. The filter system consists of several L-C and R-L-C branches that are tuned for characteristic harmonics and provide also necessary reactive power. As mentioned, the LCI in our case study has 24-pulse rectifier and therefore produces quite low harmonics. Therefore, main purpose of the filter system is to compensate reactive power and keep the power factor > 0.95 throughout the operating range. The filter branches have separate breakers and are switched on and off sequentially. The multi-branch filter system is supplied from a dedicated tertiary winding of the input transformer (see [5] for more details on how to connect a grid-side filter, particularly Figure 6).
4. Grid harmonics
Injected grid harmonics and harmonic distortion shall be as low as possible and shall comply with national and international standards. Moreover, customer appointed a third party to elaborate a network study. The study revealed a potential parallel resonance in the range 1’800 -2’000 Hz. Therefore, special care shall be given to harmonics present in this frequency range. The VSI solution with 36-pulse rectifier and LCI solution with 24-pulse rectifier and harmonic filter are both very grid-friendly causing minimum voltage distortion. Since the VSI solution consists of dual channel system where both VFD have exact same load, a quasi 72-pulse system could theoretically be reached (if necessary).
Cost comparison
For cost/price comparison it only makes sense to compare total cost of the entire drive system. LCI in high power range (> 40 MVA) is usually very cost competitive. VSI drives for the same total power might be 40-60% more expensive. That sounds like a large difference. However, LCI requires the harmonic filter system with dedicated switchgear which is not needed for VSI. In our case study the harmonic filter is located outdoors in a container. Therefore, additional cost for the container apply. The input transformer of LCI is more expensive, mainly due to additional filter winding (30 MVAr in this case). LCI is also more strict on transformer impedances. VSI with diode front end is less sensitive to transformer impedance as long as it is in a specified range. Motor for LCI is also more expensive than comparable motor for VSI due to lower power factor (larger apparent power) and somewhat higher stator current harmonics.
Summary on cost is that LCI is cheaper than VSI on component level only. However, on system level the cost is very similar since VSI is less demanding towards transformer and motor.
Comparison in a nutshell
Below table summarizes selected parameters to be compared between LCI and VSI for our specific 60 MW drive system. Cost section only compares cost of specific component when used in one or the other VFD technology.
Final selection
Both described VFD technologies, LCI and VSI, provide a good technical solution. LCI is being used already for multiple decades and has many references in high-power range. VSI is obviously younger technology, but there are high power installations with over 10 years of operation.
LCI is the champion in power density due to its simplicity and high current rating of thyristors. VSI has larger footprint, but does not need the bulky harmonic filter system that is typically significantly larger in footprint than the VFD itself.
In this particular case of brownfield installation the space restrictions were very critical and therefore final decision was to use LCI. If the site would be a green field or the outdoor space would be more limited, the decision could look different. In any case, both technologies, when properly designed and integrated into the plant, shall serve reliably to customer’s full satisfaction.
Comments or questions?
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References
[1] LCI versus VSI series, https://mb-drive-services.com/category/lci-versus-vsi/
[2] Medium voltage AC drive portfolio, https://new.abb.com/drives/medium-voltage-ac-drives
[3] Synchronous motor efficiency record, https://www.abb-conversations.com/2017/07/abb-motor-sets-world-record-in-energy-efficiency/
[4] Input power factor and reactive power consumption, https://mb-drive-services.com/energy-efficiency-part-6-power-factor-and-reactive-power-consumption/
[5] Harmonic filters, https://mb-drive-services.com/network-harmonics-harmonic-filter/
