VFD versus gas turbine
In this post we are going to compare two variable speed drive solutions: an electric drive consisting of a VFD and a motor and a gas turbine.
Gas turbine is an internal combustion engine, i.e. an energy conversion device that converts energy stored in the fuel to useful mechanical energy in the form of rotational power.
The interest in all-electric compressor drive trains is going on for several years. One enabler is the fact that high-power variable frequency drives are already considered as mature and proven technology. In addition, there are many operational benefits as we will discuss in the next few paragraphs. And finally this trend is in line with the aims to minimize the emissions of carbon dioxide and to strive for carbon neutral future.
Benefits of electrically driven LNG plants
There are multiple benefits when using a full electric VFD fed motor drive instead of a gas turbine driver [1, 2]. Let’s explore some of them in following section:
1. Lower emissions and environmental impact
A full-electric drive train typically produces less pollution and noise. There are practically no emissions generated locally. In contrast, gas turbines are environmentally less acceptable and it becomes increasingly difficult to get a permission to install such a turbine. Noise level of an electric drive is also lower than noise produced by a gas turbine which is another environmental advantage.
2. Less intensive maintenance
An electric drive train requires less scheduled maintenance. Moreover, certain maintenance tasks can be performed safely while the VFD remains in operation, i.e. no shutdown is required. Less intensive maintenance results in higher availability of the system and more profitable operation and lower maintenance expenses.
3. Higher availability
Availability is a key performance indicator in Oil and Gas industry [3]. VFD has much shorter mean time to repair (MTTR) than a gas turbine and therefore features higher availability. That fact brings a commercial advantage in terms of higher productivity. [1] reports typical availability of gas turbine 93% to 94% whereas VFD driven motor reaches availability 98% to 99%.
4. Starting capability
Gas turbine cannot start on its own and need an additional starting equipment. In fact, VFD with an electric motor is often used as starter/helper along with a gas turbine (both LCI and VSI technology used for such purpose). The role of VFD supplied motor is to provide starting capability and improve dynamic response. VFD start is much faster while gas turbine start takes considerable time.
5. Dynamic performance
Electric drive has much better dynamic performance compared to gas turbine. A VFD driven motor may be used in addition to gas turbine as part of shaft train to improve the dynamic response.
6. Rated speed and maximum speed
Advantage of gas turbine is that it is inherently ‘high speed’ and can directly drive a turbo-compressor. An electric drive is either in conventional setup with a speed-increasing gear (4-pole motor with typical nominal speed 1’200 rpm to 2’000 rpm and a step-up gear) or a direct drive with high-speed electric motor (gearless solution).
7. Wider speed range
Gas turbine allows only limited speed variation. Typical speed range of a gas turbine driver is 96% to 102% of nominal speed (at lower turbine speeds the efficiency suffers significantly). In contrast, VFD supports much wider speed variation. Typical range is 70% to105% of nominal speed and further expansion is feasible (certain high-speed VFD driven gas compressors feature speed range 30% to 110%).
8. Energy efficiency
Besides lower emissions VFD based full electric drive system usually outperforms the gas turbine driver in terms of thermal energy efficiency (especially in co-generation plants).
Summary
There is plenty of benefits of VFD and electric motor versus gas turbine driver. In short, these advantages include lower environmental impact (emissions), reduced maintenance and higher availability, inherent starting capability and increased plant flexibility.
In many new projects a full electric drive is planned to be installed instead of a gas turbine. Also several brownfield projects consider replacement of existing gas turbine by large VFD fed electric motors. Replacement of a gas turbine inside an existing plant has specific challenges that need to be carefully evaluated.
There are even considerations to electrify entire oil&gas platforms with power from shore and decommission existing gas turbines located in these platforms.
References
[1] B. Martinez, C. B. Meher-Homji, J. Paschal, All Electric Motor Drives for LNG Plants, Gastech, Bilbao, 2005
[2] G. Schwarz, H. Keller, Turbine Replacement by Electric Drive Systems Provides Significant Benefits, PCIC Europe, Vienna, 2017
[3] VFD industry requirements: Oil and Gas, https://mb-drive-services.com/vfd-industry-requirements-oil-and-gas/
[4] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
2 Comments
Hemei Gu · June 13, 2021 at 4:19 pm
Dear Sir:
Two points :
(1) How about the reliability (MTBF) of large e-Drive ?
Less maintenance or less intensive maintenance only make sense when e-Drive is staying a comparable level in reliability Vs conventional gas ( or steam ) turbine driven motor.
(2) Would able to share the files of referrence 1&2? Thanks in advance
admin · June 14, 2021 at 10:30 am
Dear Mrs. Gu,
Great questions, many thanks!
(1) MTBF of electric drives is very high. The figures range from 5 years to 28 years. For me MTBF in the range 5-10 years is realistic and proven by the installed base. Unfortunately, there is usually no proper methodology how to determine MTBF. Manufacturers use different approaches, i.e. theoretical calculation versus field experience. One approach may be too pessimistic while the other one too optimistic. We have published articles on reliability, availability and MTBF as it is a topic on its own (see our blog –> “Reliability and availability” category).
MTBF and maintenance are two a bit different things. MTBF stands for “mean time between failures” and those failures happen due to different reasons (quality, aging, design issue etc). Maintenance work is normally something scheduled/predicted. It is not a failure in that sense but a regular replacement of specific components with shorter lifetime (e.g. gaskets of cooling pump, dust filters in air suction etc) or just a condition check. It is a known fact that gas turbine requires more frequent maintenance inspections than an electric drive (VFD plus motor).
You can imagine it on an example of a car. Failure is when a clutch or a damper or air conditioning gets broken. It may happen suddenly and unexpectedly. Maintenance is when you need to replace the oil after certain period of time or after certain distance.
(2) I will approach you separately.
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