In order to minimize the footprint of human activities on the environment, technologies to reduce greenhouse gases while meeting constantly growing electricity demands are critical. Amongst the various sources of energy production, Gas Turbines (GT) are an efficient way to stabilize the grid with regards to renewable sources like wind and solar energies. The demand for higher efficiency, higher power output while reducing emission levels (especially NO and NO2) at high loads, and for higher flexibility within the H-class Gas Turbine market is thereby a natural consequence. The development and validation of a two-stage sequential combustor, so-called Constant Pressure Sequential Combustion (CPSC) system, to achieve these goals has been accomplished by Ansaldo Energia. The CPSC consists of a premix burner system (First Stage) and of a sequential burner (SB) in series within a can combustor. At the 2017 and 2019 ASME conferences, high pressure test rig validation results of the CPSC were introduced. The advantages with regards to fuel flexibility, hydrogen combustion and low emissions at high firing temperature were presented [1,2,3,4,5]. This article focuses on the validation of the combustor performance in Ansaldo Energia’s Validation Power Plant located in Birr, Switzerland, which includes detailed validation from ignition to full speed no load, part load operation and full load over various ambient and engine thermal state conditions. To allow for detailed validation, dedicated fully instrumented combustor cans were installed in the GT. Detailed validated air distribution and emission models support the results obtained on the engine. Ignition and ramps up to full speed no load have been validated with large variations of the first combustor stage firing temperature to minimize power consumption and start-up time. The potential of the CPSC with regards to turndown capability, with minimum environmental load (MEL) below 25% GT load while keeping CO levels low has been confirmed. The MEL can be kept low over a wide range of ambient temperature and fuel compositions by adjusting the inlet temperature of the sequential burner. Low NOx values were achieved at baseload and peak firing temperature. The operational flexibility and stability of the premixed first stage combustor over the load range and over a large variation of combustor inlet plenum pressures was as well validated along with the operation concept of the gas turbine.