| Operation & maintenance
Original system:
Updated system: Original vs updated Carsfad excitation system
the specialist engineering knowledge needed to support and upgrade them effectively. EES sees it as our duty, in the context of the UK’s engineering skills gap, to ensure that these capabilities are available domestically to British engineering firms.” EES is delivering the new equipment by designing around existing infrastructure, carefully managing installation constraints and using advanced modelling and simulation to achieve compliance. Once complete, the upgrade will reduce the risk of unplanned outages, improve overall reliability and ensure long- term supportability of critical systems. It also supports wider grid stability by maintaining dependable generation capacity while extending the operational life of existing assets.
Mechanical design changes to the Carsfad excitation system included: removal of DC exciter, along with brushes and commutator; removal of mechanical field switch; installation of brushless AC exciter; installation of rotor-mounted rotating diode bridge; and integration of new field connection arrangement.
Electrical design changes included: fuses replaced with MCBs for improved monitoring; ECB unit installation to improve 24 V DC distribution; 4 kVA excitation transformer installation; addition of diode failure detector for rectifier monitoring; and increased field current requirement
spare parts and restricted technical support, creating a significant risk of unplanned downtime and making long-term maintenance more challenging. This prompted the site’s electrical engineering team to push for an upgrade as part of a wider investment strategy to improve plant resilience and cost effectiveness. “The customer placed significant value on having direct access to UK-based excitation engineering expertise, particularly in those moments when rapid response is critical,” said Ryan Kavanagh, director at EES. “As many legacy excitation systems become obsolete, there is an increasing industry challenge around maintaining
Brushed to brushless upgrade for 90-year- old generator
The extension of service life has also been strikingly demonstrated by an EES excitation refurbishment project at the 90 year old Carsfad hydro plant in Scotland. The project was the subject of a presentation by EES graduate electrical engineer Deepal Trikha at the 2026 IMechE Steam Turbine and Generator User Group (STGUG) conference in March.
The presentation demonstrated how hardware and software upgrades to decades-old synchronous machines can extend their service lifetime to match the facilities they support. Commissioned in 1936 as part of the Galloway hydro-electric scheme, Carsfad was one of the few remaining UK power stations with an operating DC commutator-based excitation system. The system was designed for flexible, rapid-response operations with a 12 MW hydroelectric generating unit. Carsfad’s original excitation architecture had remained operational in the plant since it entered service 90 years ago. This meant it was vulnerable to brush and commutator wear, parts that are
Brushed DC exciter
Brushless AC exciter
AVR voltage step response improvement. Before (top graph) and after (lower graph) response time to a 5% voltage step
now obsolete, with the generator’s reliability depending on the mechanical condition of these almost century-old components.
In the original system, the automatic voltage regulator (AVR) controlled a shaft-mounted DC exciter, with DC transferred to the generator field through a mechanical field switch. “The excitation response was limited by the additional exciter stage between the AVR and the generator,” notes Deepal Trikha. “We observed a three second response time to a five per cent voltage step. This lag, along with the obsolescence threat, prompted the Carsfad engineering team to seek our support as part of an upgrade.” Electrical contractor Quartzelec removed the DC exciter and the mechanical field switch, and supplied the replacement brushless exciter, while EES was responsible for the control and integration aspects such as the AVR modelling and system retuning. AC excitation power is now generated by a brushless excitation system and rectified on the rotor using a rotating diode bridge. This upgrade ensures that DC is supplied directly to the generator field without the need for mechanical commutation or switching. The project eliminated mechanical components, and their maintenance requirements, from the excitation system. Following the changes, the voltage step response was noticeably faster, an improvement achieved without introducing instability or excessive overshoot. “The hydroelectric generation and long- term energy storage sectors in Scotland are growing, as public and private investment funds energy independence and flexibility,” said Ryan Kavanagh. “Excitation expertise is an essential ingredient in the skills mix needed to improve the infrastructure that this growth relies on.”
www.modernpowersystems.com | July/August 2026 | 19
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