High and low pressure cylinder bypass heat supply unit operation characteristics with coordinated intermediate pressure valve control

Under China's electricity spot market, high-pressure and low-pressure cylinder (HP-LP) bypass heat supply retrofitting enhances the deep peak-shaving capacity and operational economy of combined heat and power (CHP) units. However, adjusting the intermediate-pressure (IP) cylinder valve during heating significantly affects unit safety and performance. This study presents the HP-LP bypass heat supply scheme, identifies existing issues, and proposes a method for determining IP valve participation using MATLAB and EBSILON simulations. Operational characteristics are analyzed based on a case study. Results show that the method can determine IP valve operating modes under all HP-LP bypass conditions. Following the retrofit, the IP valve may only engage in regulation during heat supply when the main steam flow rate of the case unit exceeds 1433 t/h. Consequently, the thermoelectric characteristic region is divided into three zones: constant-pressure regulation, sliding-pressure regulation with IP valve involvement, and non-involvement scenarios. During deep peak shaving, the unit's thermal load varies between 0 and 1079.9 MW, while the electrical load remains below 30 % of turbine heat acceptance capacity. IP valve participation has a greater impact on heat rate than on energy efficiency. Notably, exergy destruction caused by high-temperature-difference heat transfer has a more significant effect on system performance degradation than IP valve operations. This study offers theoretical and practical guidance for HP-LP bypass system retrofits and safe, efficient operation strategy development in thermal power plants.