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Trends in offshore generation

Published:  27 February, 2015

It is normal practice to have at least two gas turbine generators on main platforms, with an additional standby generator for emergencies. While a typical gas turbine is rated up to 30 MW, the number and exact configuration depends on the degree of flexibility needed and how the platform operator plans to expand or further develop the facility. ODEE talks to ABB’s Anders Stiger, product manager, generators for steam and gas turbines, about what’s driving the development of offshore gas turbine based power generation and hear new ways to meet some perennial challenges.

What factors and trends are driving the development of offshore gas turbine generators?

There are several factors pushing the development of offshore gas generator sets. Of course, the harsh conditions and high winds, common on offshore facilities, means that generators need to be robust and with an appropriate IP rating. There is also the ever present danger posed by hazardous areas and the need to ensure that generator sets meet the appropriate Ex requirements and offshore marine certifications.

We see turbine OEMs, packagers and end users increasingly demanding higher generator powers. The trends driving this include a demand for more power using fewer larger turbines instead of many smaller ones. This helps keep capital expenditure (CAPEX) down, while ensuring the package has the smallest footprint to make the most efficient use of the minimal space available. This trend is leading to larger 4-pole generators, with powers reaching the 80 MVA class.

The desire to draw more power from the same size turbine, by cooling the gas turbine’s inlet air, or by replacing it with an upgraded engine design, is another trend, which, in turn, requires generators with higher powers.

Yet another issue is that of short circuit peak current increasing in importance with higher total powers. This causes a demand on higher reactance to limit the current, again resulting in the need for larger generator powers.

What do these trends imply for the generators employed?

Some of these trends will lead to dimensioning the generator for a higher power than is actually required. And when this de-rating is needed, such as for high power applications in hot environments using air-to-air cooling, a 2-pole configuration was previously the only technical solution available. Advanced 4-pole technology is now feasible to increase, not only the actual maximum power levels, but also the power in hot ambient environments. The lower unit numbers also demand high component reliability and safety to satisfy the need for redundancy and high availability.

You mentioned cooling, which is also a critical consideration. How is the technology of cooling developing?

Totally enclosed generators use either air-to-air (CACA) or water-to-air (CACW) type heat-exchangers for cooling. Water coolers, CACW, offer the best efficiency, but they can only be used if there is a separate cold water system available on site.

Previously, in places where cooling water supply was not available, less efficient and more expensive CACA had to be used. This also meant long lead times. A CACA system is much larger, up to three times the size of the generator itself.

To address this need, ABB has developed the CAWA cooler, which stands for cooling from air-to-water-to-air – that is, from primary internal air circuit to secondary closed water circuit to ambient air (IC 8A1W7 as detailed in IEC 60034-6).

This is a compact integrated water cooling package that offers over 60% weight reduction compared to CACA cooling. It uses standard components to build an independent, closed-circuit cooling system mounted on top of the generator. The CAWA cooler is delivered with a water mixture installed, giving a ‘plug and play’ solution similar to that offered by CACA.

What are the major benefits of this approach?

Placing a small-sized CAWA cooler on top of the generator offers many benefits over the much larger CACA air cooler. It enables higher maximum output powers from each generator frame size and provides a cost saving design with shorter manufacturing lead times. OEMs can use their standard turbine skids for higher powers, because the same generator size can be used for wider power levels. This boosts the turbine generator set’s performance and economy. To upgrade the power of the turbine, the CACA can be replaced with a CAWA to increase the power of the generator, avoiding the need to invest in new generators. Offshore, the compact CAWA cooler can easily cope with wind loads. It runs on low power, unlike the main water pump which an emergency generator cannot operate during a black start. It offers built-in redundancy for both blower and pump capacity for the highest availability.

Is there more scope now for 4-pole generators?

The above trends – the demand for fewer but larger turbine sets, turbine inlet cooling, higher reactance demand - give more reasons to select the newly available 4-pole configuration. Previously, the choice of 2-pole was dictated by the fact no 85 MVA size generators were available.

Both steam and gas turbine manufacturers are seeking to increase speed in order to operate their turbines in a more optimal range and boost efficiency. With a gearbox setup, OEMs can freely select the turbine speed and optimise the design for the lowest cost and highest efficiency. A 4-pole generator offers the best way to achieve these goals.

What are their advantages?

Compared to a 2-pole configuration, the main benefits of a 4-pole solution are compact size, lower cost of the generator-gear package, better efficiency and simplified maintenance. Overall length is reduced, offering a significantly smaller footprint, and weight is cut by approximately 20 percent.

For turbine OEMs, using a 4-pole generator solution can be very attractive, as it enables the speed range to be optimised to increase the system efficiency. This results in a very quiet and compact turbine package, with lower running and maintenance costs and up to a 30 percent reduction in the need for cooling water and lubrication media.

The higher power levels available mean that the number of turbine units needed can be reduced, meeting the need for minimal space requirements and lower total investment cost. Power plants are increasingly being run in a frequent start-stop mode, posing higher stresses on generators. This is a particular problem for 2-pole units, which need to run over the critical speed area. The more rigid 4-pole solution offers better reliability to cope with these stresses.

How can 4-pole solutions help turbine manufacturers develop their designs?

To meet the demand for larger 4-pole generators, ABB has developed the technology to manufacture even larger 4-pole generators up to 85 MVA.

The range makes use of a solid, salient 4-pole rotor design that operates below the first critical speed, as opposed to a cylindrical 2-pole rotor that has to successfully pass it. This means there are no forbidden speed areas in the operating range, giving better low vibration characteristics.

This type of rotor also exhibits a high inertia, ensuring excellent speed stability for the turbine drive train system. It also has no need for damper windings, allowing a high reliability as well as a long lifetime. Maximised rotor coil surfaces, together with symmetrical air flow that avoids hot spots, gives highly efficient and evenly distributed cooling.

What are the cost implications of using the 4-pole approach?

Long maintenance intervals mean that the generators can fit into the turbine service plan, therefore no extra stoppages are needed for separate generator servicing, avoiding down times. This is a real benefit in applications with continuous operation and long operating times.

The major benefit of ABB’s approach is the lower CAPEX investment needed, typically 20 – 30 percent lower investment costs. The smaller footprint provides a unit that is between 20 – 30 percent shorter than the alternative 2-pole approach, while it is also around 20 percent lighter.

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