ESB Independent Generation (ESBIGT) and Carrington Power have been fined £6 million by Ofgem after submitting inaccurate data to National Grid ESO.
The two companies said they had inadvertently breached the rules and have agreed to pay the penalty to the energy redress fund.
They regularly submitted misleading data about the minimum amount of energy the 884MW gas fired Carrington power station could supply between March 2019 and September 2020.
This caused National Grid ESO to purchase more energy from the plant than needed, and at times spend more money than it needed to, to help it balance supply and demand on the system.
While the companies considered their approach compliant with their obligations and believed it would benefit the ESO, Ofgem found that they didn’t have the internal processes in place to ensure staff understood and applied the rules correctly.
The companies subsequently took corrective action, and have since confirmed that they have improved their compliance processes and training around market manipulation and submitting data to the ESO.
Cathryn Scott, regulatory director at Ofgem, said this if a further instance of the regulator taking "strong action" against a generator for submitting inaccurate data.
It follows Ofgem fining EDF £6 million late last year after the French energy giant sent signals that inflated the minimum amount of power its West Burton B generator plant could supply between September 2017 and March 2020.
Earlier the same year, Ofgem fined InterGen £37 million for sending National Grid misleading signals to make what the regulator described as a “substantial profit”.
A spokesperson for ESB said: "Compliance with its regulatory obligations is a priority for ESBIGT. We take this breach extremely seriously and apologise for the inadvertent breach of our obligations. We were disappointed not to have met our own high standards and took immediate steps to comply with Ofgem’s guidance.
"Corrective actions have been taken to prevent reoccurrence with new governance arrangements in place to ensure ongoing compliance.
"Although this breach was inadvertent and ESBIGT believed the dynamic parameters it submitted would contribute to lower costs to balance the system, we have taken the engagement with Ofgem extremely seriously and cooperated fully with Ofgem throughout this process."