Troll A provides a platform for HVDC Light and Motorformer

ABB's HVDC Light is intended to widen the applicability of HVDC, for example making it economically viable at lower power levels, and is specifically designed for underground and underwater transmission. It is more compact than "classical" HVDC and provides more control, including reactive power compensation. As the AC is completely synthesised from the power electronics, without the need for any auxiliary synchronous AC generation, HVDC Light can also work in "black start" conditions and can supply a passive load (such as an oil rig or island) without any other generators working.

The powering of Statoil's Troll A gas production platform via a DC link from the shore – the first time an offshore platform has been powered by such a link originating on land – provides a striking example of what can be achieved with this transistor based approach compared with the classical, thyristor based, technology.

Another key innovation in what is called the Troll A Precompression Electrical Drive System (EDS) project is that it combines HVDC Light with ABB's very-high-voltage (VHV) XLPE-cable-wound motor, aka Motorformer. This is a descendant of the Powerformer technology launched by ABB in 1998 as a high voltage generator (allowing transformers to be eliminated), which passed to Alstom when ABB sold its share in the shortlived ABB/Alstom power generation joint venture in 2000.

The Motorformer, which operates at 56 kV, features conventional rotor, exciter, control and protection technologies. Most of the stator technology is also conventional, with the exception of the winding. The cable slot design – the key to success for cable-wound machines – aims for low electrical losses, effective clamping, and efficient cooling. It is also completely oil free. A four-pole rotor is used, which is smaller, weighs less, has lower noise levels, and is more cost effective than a two-pole design.

HVDC Light is based on voltage source converters employing turn on/turn off insulated gate bipolar transistors (IGBTs), operating with high frequency pulse width modulation in order to achieve high speed. As a consequence filters can be smaller and both active and reactive power can be controlled independently.

A key feature of all HVDC Light installations to date is what ABB calls HVDC Light cable (manufactured at ABB's Karlskrona facility). This uses extruded polymer insulation, which minimises cable diameter and weight and maximises flexibility, facilitating installation both underground and undersea.

Troll Electrical Drive System

ABB was awarded the A250 million Troll A EDS contract by Statoil of Norway in 2002. Tests on the system were successfully completed in February 2005, while it entered commercial operation on 1 October 2005.

In the Troll A application two identical HVDC Light systems are used to power two new Dresser-Rand compressors on the platform, which constitute the first phase of the Troll precompression project. This project is envisaged to have three phases, with two complete precompressor systems to be installed in each phase (although at the time of writing no contracts had yet been placed for phases 2 and 3).

Phase 1 consists of two bipolar (HVDC Light is always bipolar) transmission systems, each serving one of the VHV motors, which are rated 40 MW mechanical at the shaft. The DC transmission voltage is ± 60kV.

The inverter station on the platform is directly connected to the motor with no transformer needed.

The rectifier at Kollsnes (onshore station) is connected through a standard power transformer to an existing 132 kV network with a breaker.

The connection between shore and platform consists of four 70 km lengths of HVDC Light cable, constituting the two bipolar systems.

The AC side of the inverter is connected to the VHV motor via a breaker and AC cables. The VHV motor’s outgoing shaft is directly coupled to a gearbox that drives the compressor.

As the compressor speed is variable, the motor is supplied with variable frequency and variable voltage. The operating window is 42-63 Hz (although the system can supply in the range 0-63 Hz). By means of modestly sized filters on the output of the converter, the motor winding stress is kept at a safe level.

The VHV motor is controlled by a MACH 2 system (already proven in a number of HVDC and SVC installations), which also governs the rectifier station at Kollsnes and the inverter on the platform.

Why precompression?

Precompression is required because the wellhead pressure at the 100 million m3/day Troll A platform is decreasing as a function of depletion and the delivery pressure from the platform needs to be boosted. The Troll field is the biggest gas find on the Norwegian continental shelf, accounting for 60% of Norway's offshore gas reserves and capable of supplying around 10% of Europe's gas needs. The Troll A platform, with a height of 472 m (standing in 302 m of water), is believed to be the largest structure ever transported on the surface of the earth.

The design of the original Troll gas plant envisaged the fall in wellhead pressure over time and assumed future installation of precompressors on the platform to maintain full capacity in the system, so adequate space was provided on the platform.

On most offshore installations, power supply generators and large compressors are driven by platform-mounted gas turbines or diesel engines. Many of these have total

efficiencies as low as 20-25%, under the best of conditions. The result is emission of large amounts of CO2 and unnecessarily high fuel consumption. On the Norwegian shelf, CO2 taxation is already in effect, ie emissions represent a substantial cost for the oil companies.

If hydro generated electric power can be supplied from shore – for power supply as well as compressor drives – CO2 emissions from offshore installations are eliminated.

It is estimated that the Troll A EDS project will eliminate annual emissions of 230 000 t of CO2 and 230 t of NOx.

This leads to a significant cost saving for oil companies. In addition, transmission of electrical energy from shore involves less maintenance, longer lifetime and higher availability than gas turbines and diesel engines. There are also health and safety benefits, as well as lower noise levels.

Although the HVDC Light option is higher in terms of initial capital cost than that for gas turbines, it is cost effective on a lifetime basis.

Space and weight are clearly major considerations in an offshore installation, so the HVDC Light/Motorformer combination is particularly attractive. Due to the smaller filters and elimination of synchronous condensers, HVDC Light is compact and lightweight compared with classical HVDC and use of Motorfomer eliminates the need for a large and heavy transformer.

Upgrade of the existing 20 MW AC connection to shore was another option considered in the Troll A case, but rejected because of the increased on-platform equipment requirements.

Offshore potential

ABB sees potential for HVDC Light power-from-shore transmission in a number of other offshore installations, with a window of opportunity where power requirements are roughly 70 MWe or more and distances are around 70 km and above. It is estimated that offshore platforms have average power requirements of about 80 MW, and the distance from the shore averages about 170 km.

Clearly, the environmental benefits of the power-from-shore approach are greatest where connection is to a grid with a large percentage of renewable generation (as is the case in Norway with its large hydro installed capacity).

A second HVDC Light based system serving a Norwegian offshore oil platform is planned. This is for BP's Valhall oil field, also in the North Sea, but 290 km from the shore. The link will replace 78 MW of offshore gas turbines, and once again the motivation is greenhouse gas emissions reduction, reduced maintenance requirements and less weight on the platform.

BP has awarded front end engineering design (FEED) contracts to ABB for the Valhall HVDC Light converters, but in this case it is Nexans which received the accompanying FEED contract for the DC power and fibre optic cables (from Lista to the Valhall field). Approval to go ahead is expected in mid 2006. The link will operate with a DC voltage of 150 kV, with 300 kV AC at Lista and 11 kV AC at the platform.

ABB argues that there are benefits to be had from the HVDC Light concept even in a case where, unlike Norway, the generation on the grid is predominantly fossil based. One consideration here is that on-platform gas turbines have to be operated at part load (and therefore low efficiency) to achieve redundancy.

Location of Troll, off Norway The Troll A platform HVDC Light cable, with triple-extruded-polymer insulation system Line diagram of a Troll A HVDC Light link. The installation consists of two of these links, one for each compressor. The rectifier building at Kollsnes The inverter module on the platform Semiconductor valves in the offshore inverter module. They are suspended from the ceiling to withstand constant platform movement, which has been described as creating conditions "like a constant earthquake" MW and distance from shore for offshore oil and gas platforms worldwide