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Medium-voltage power savings


Fluxys LNG in Zeebrugge, Belgium, is a major European hub for the transportation and storage of liquefied natural gas (LNG). Taking advantage of new drive technology from Rockwell Automation, Fluxys LNG recently became one of the first LNG plants to employ a variable speed drive to control a high-pressure LNG pump.

Fluxys transports gas from Norway to Zeebrugge by submarine pipeline, where it joins an interconnecting link with the United Kingdom that enables natural gas to be traded via the “Huberator” hub. There are two further LNG plants at the Zeebrugge peak-shaving site, which operate on standby and whose task is to balance out any consumption peaks in winter.

The LNG is moved from low-pressure tanks through high-pressure pumps
The LNG storage terminal receives liquefied natural gas by tanker ship at -160°C, and at an operating pressure of 110-150mbarg. Currently, the terminal receives some 60 tanker deliveries per year, but a planned expansion will see this figure rise to 110. The LNG is pumped from the tankers into one of three storage tanks, each capable of holding 87,000m3 – a fourth tank with a capacity of 140,000m3 is currently under construction. While it is being pumped, return gas or volume displacement gas is sent to the tanker to equalise pressure in the storage tanks and in the vessel’s tanks.
The LNG is moved from low-pressure tanks through high-pressure pumps, which increase the pressure to 80barg. It is then vaporised in evaporators that raise its temperature from -150 to 3°C. The heat required for this process comes initially from burners immersed in the water bath of each evaporator and finally, from a co-generation plant consisting of a 40MW gas turbine with 72MW heat recovery. The hot water generated by this process is re-injected into the evaporator water bath.

“Fluxys LNG achieved a 37% reduction of energy expenditure on its high pressure pumps and installation costs were minimised.”
As part of a schedule of upgrades – intended to improve plant effectiveness and reduce maintenance costs – Fluxys LNG decided to examine the feasibility of using a medium-voltage drive on one of the high-pressure pumps. The aim was to obtain the desired flow rate by controlling pump speed rather than the existing method of valve regulation. The use of a variable-speed drive in this way would afford considerable power savings and, thanks to lower wear on the pump, reduce maintenance costs. An implementation plan was therefore set up to replace the inefficient “pressure control/regulation valve” with a 6kV, 1.4MW medium-voltage drive to control pump speed directly.

A market study carried out as part of the tender process revealed that most drives employ a step down/step up of the drive voltage, requiring a 6kV transformer at both input and output. Transformers take up more space than the drive itself and require the additional capital and running costs of air conditioning due to heat losses. However, one supplier was able to offer an alternative; thanks to the latest technological developments, our Allen-Bradley PowerFlex 7000 Direct-to-Drive requires no input or output transformers allowing greater operating efficiency while enabling considerable space savings.
In view of the environmental hazards, an isolation transformer was required to limit contamination on the network. However, since only one such transformer was required, the overall cost of the PowerFlex solution was comparable with that of less advanced ‘traditional’ drives. Installation of the first PowerFlex 7000 18-pulse drive – designed to eliminate 5th, 7th and 11th harmonics – was completed in 2004, making Fluxys LNG one of the first companies to implement a variable-frequency drive on a high-pressure LNG pump.

Following satisfactory results, it was decided to fit two of the five pumps for installation with a medium voltage drive and to install a medium voltage drive on the compressor providing the return gas to the ships’ tanks. The compressor and the ice on the unit can be seen in the centre of the photograph
When the Zeebrugge LNG terminal was expanded in 2005, it was decided to extend the use of medium-voltage drives to include the gas compressors which send the return gas to the ships’ tanks. The same compressors are also used to treat gas in the holding tanks at the terminal. The existing regulation system provided flow control between 10,000 and 24,000m3/hour, and so its replacement with a medium-voltage drive solution was a logical choice. Once again, market analysis resulted in the PowerFlex 7000 Direct-to-Drive medium-voltage technology coming out as first choice.
The PowerFlex 7000 Direct-to-Drive feature combines three innovations designed to eliminate isolation transformers, using either new or existing motors. The first is the use of a Symmetrical Gate Commutated Thyristor (SGCT), which replaces the thyristor found in a traditional drive’s rectifier bridge. Delivering a higher voltage rating and the ability to be connected in series, the SGCT allows the PowerFlex 7000 to be rated up to 6.6kV without adding complexity. PowerFlex 7000’s Active Front End Rectifier uses active switching and Selective Harmonic Elimination to reduce total harmonic distortion to less than 5% and achieve compliance with IEEE 519, EN61000-2-4 and G5/4.

PowerFlex also features Common Mode Voltage Protection technology, designed to mitigate common mode voltage stress generated by variable-frequency drives without using an isolation transformer. Since no Reflected Wave Voltage Stress is generated in the motor, the PowerFlex 7000 can be used with older motors, so it is ideal for retrofit applications. Thanks to the high quality sinusoidal voltage and current at the drive’s output, the motor and the drive can be located up to 15km apart without the need for extra motor and cable insulation.
Using the PowerFlex 7000 Direct-to-Drive technology, Fluxys LNG achieved a 37% reduction of energy expenditure on its high-pressure pumps and installation costs were minimised thanks to being able to use the existing motor and cabling, and eliminating the need for unnecessary transformer and related infrastructure.

Author: Alfons Calders, ‘Industrie Technique & Management’