According to GE, its Hydro division’s new variable frequency transformer (VFT) is to ‘revolutionise the world of transmission solutions, providing unparalleled flexibility for utilities and transmission system developers to create viable economic business models to meet ever-changing energy market needs’. What it actually does is avoid many of the problems of transferring power smoothly between systems at different frequencies. It provides a simple and controlled path between electrical grids, permitting power exchanges that couldn’t previously be accomplished owing to technical constraints such as asynchronous boundaries or congested systems.

The VFT system, based on a combination of hydro generator and transformer technologies, consists of a rotary transformer that provides a continuously controllable phase shift for any angle, together with a drive system and control that adjust the angle and speed of the rotary transformer to regulate power flow through the VFT. Smooth power control comes from regulating the torque through the drive system. Rotational speed is dictated by the difference in grid frequencies and will generally be below 3 rpm.

The VFT’s 100MW channels can be combined for up to 400MW in a single installation. The VFT rotary system has an efficiency in excess of 99% at full load, including cooling fans. This low cost design has a by product, lower than normal machine temperatures, which benefits reliability and maintenance issues.


Features designed to increase availability include:

• High reliability machine design, based on established performance.

• Low speed operation, resulting in low maintenance.

• Redundancy in auxiliary services, eg, cooling fans.

• Use of common substation components, eg, transformers, capacitors and breakers, allowing the opportunity for maintenance crews to be more proficient.

• The main components can be selected for low stress, resulting in higher reliability.

• Independent operation of channels within a station.

• Compact size.

The VFT provides self-standing plug and play capabilities to provide controlled transmission solutions independent of grid interconnection considerations. The low grid integration of the VFT in terms of harmonics, control interactions and impact on nearby generators allows the installation and operation to be de-coupled from other grid issues.

A unique benefit of the VFT is that it can be developed and installed with a minimum of delays and cost impacts on the affected parties.

A major economic and aesthetic advantage of the VFT is its smaller physical size. Because of its inherently good power quality, the need for harmonic filters and associated significant space requirements has been eliminated. In a comparison of a 200MW VFT with a 200MW HVDC installation, the VFT is significantly smaller. This allows a VFT installation to be located on sites where a conventional HVDC will not fit. In addition, the VFT provides two independent channels for improved reliability in comparison to the single channel outlet of the HVDC. There are pre-designed layouts for 100MW to 400MW installations; custom-designed, larger installations are also available.

First installation

TransÉnergie, Hydro-Québec’s transmission division, will be the first to install the VFT technology, at its Langlois substation in Québec, Canada. The project will enable TransÉnergie to interconnect with other power grids to gain greater transmission network operating flexibility.

As the project’s turnkey contractor, GE will supply all site construction, VFT transformer and balance of plant electrical equipment and installation services. Langlois substation is located in St Timothée, southwest of Montréal, near existing interconnection facilities close to Ontario and New York State. The substation will have an exchange capacity of 100MW, along with more flexibility in the way the power is transferred between these neighbouring power grids. The project is scheduled for commercial operation in May 2003.

This pilot project will allow the development of a new competitive technology to optimise utilisation of existing installations, said Jacques Regis, president of TransÉnergie. He also expects the technology to help TransÉnergie increase the reliability its power supply.