POWER Power on, risk off: Enhancing electrical


protection without disruption In June 2025, widespread power outages hit the Indian town of Madhubani after several overloaded 10 MVA transformers failed in quick succession; a reminder of the risks posed by ageing electrical infrastructure. Here, Oliver Carpenter, electrical engineer at Excitation & Engineering Services, shares lessons from a recent UK plant expansion project.


T


he project involved integrating a 50 MW peaking plant into an existing combined heat and power (CHP) facility. The new unit, designed for rapid-response generation, was connected to the site’s electrical infrastructure via a 325 MVA transformer, substantially larger than the existing 265 MVA system. This change introduced new risks. Higher fault levels, tighter trip margins and aging protection equipment meant the legacy scheme could no longer guarantee reliable operation. Relay settings designed for smaller loads no longer matched the site’s needs, while several components were approaching obsolescence.


Events elsewhere raised the stakes. A transformer protection failure at Heathrow’s North Hyde substation halted airport operations for hours. Different scale, same message: protection must reflect current demand, not past assumptions.


Parallel protection


Upgrading protection in a live plant requires foresight, coordination and an understanding of how old and new infrastructure interact under fault conditions.


The original protection panel, still functioning after nearly two decades, housed a patchwork of electromechanical units and early digital relays. Some wiring lacked documentation, while other connections had degraded physically or weren’t aligned with current site logic.


A complete replacement would have required isolating one of the site’s combined cycle gas turbine units, which was not an option as it would compromise the existing protection scheme. Instead, EES installed a new panel alongside the old one, with both operating in parallel during commissioning. This allowed staged testing without compromising the existing protection. Each change was carefully coordinated, down to the wiring routes. Marshalling had to be precise, avoiding interference with active


trip signals and preserving continuity across shared circuits.


The scheme incorporated differential, earth- fault and overcurrent protection, but more important than the functions themselves was their application. Each relay setting had to be validated against the real operating behaviour of the site, not a generic model.


Commissioning in real time Commissioning followed the same principle as the design: keep the system live, protect what’s already working and introduce change in stages.


With five relays installed in the new panel, EES brought three online first. These covered the new transformer and its associated feeder, allowing the team to test protection logic in isolation while the gas turbine remained on its existing scheme.


EES used an Omicron three-phase injection device to accurately and repeatably simulate a range of events, from phase-to-earth faults to differential current imbalances. These simulations were tailored to the site’s specific layout, transformer impedance and clearance expectations.


Engineers logged, evaluated and recalibrated every test where needed. The objective was to confirm, in measurable terms,


22 OCTOBER 2025 | ELECTRONICS FOR ENGINEERS


that the protection system would act exactly as expected when needed.


Time pressures made that process more complex. The protection upgrade formed part of a broader £120m expansion. With a tight commissioning window and no tolerance for rework, the team had to combine methodical testing with clear communication. That meant synchronising schedules not just for the engineering work, but for operations, safer systems and live equipment control.


Beyond the upgrade


Transformer failures, like those in Madhubani or at Heathrow’s North Hyde substation, highlight a broader reality: ageing infrastructure rarely fails dramatically all at once. It degrades gradually until a single fault becomes a system-wide issue. Protection must evolve alongside the plant and power system demands. That requires planning, sequencing and a clear understanding of how protection behaves under real conditions.


excitationengineering.co.uk/resource- centre.


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