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The revised electric power of TQNPC unit 2 was insufficient at the assessment test. The Hitachi supplier considered that the insufficient output was caused by the too large flow area of high pressure cylinder. After the nozzle modification the unit output had not been improved significantly. In this paper, the characteristic flow area (CFA) was used to evaluate the turbine flow capacity. The results showed that the CFA values of high pressure cylinder and low pressure cylinder were both too large, but also within the design margin (3-5%). The reason of insufficient output was not the mismatch with flow capacity. According to the nuclear power plant diagnosis and analysis models, the thermal performance of the hot side, the cold side and the thermal system was researched respectively. The performance of hot side and cold side were better than the design values, which was favor for the unit output. So the insufficient output was focus on the thermal system. Then the technical staffs took corresponding measures to transform the thermal system and then the unit output was improved finally.

There are significant differences between the nuclear power plant turbine and the coal-fired power plant turbine. So the characterization and monitoring models for the coal-fired power plant turbine can not be reused in the nuclear power plant second loop. The unknown wet steam enthalpy and the characteristic of the turbine flow capacity were the two important obstacles for the nuclear power plant wet steam turbine. Based on the accurately measured test data, the accurate thermal performance history files was constructed using the small error heat balance method and the characteristic flow area (CFA). The accurate thermal performance history files were the base for the performance analysis, variable condition calculation, monitoring and diagnosis. The accurate thermal performance history files of Ling Ao unit 3 showed that the unit performance almost was excellent. But the CFA values and the efficiency indicated that some stages of intermediate pressure cylinder (IPT) had leakage in diaphragm, which decreased the unit output. The characteristic parameters of heat regenerative system and moisture separator reheaters system were almost close to the design values, which denoted that the performance of the two systems was acceptable. But the condenser back pressure was larger than the design values, which suggest that the performance was worse than the design condition.

The turbine flow capacity and thermal economy of regenerative system are the difficulty and key points for the monitoring and analysis system in Daya Bay nuclear power plant. Characteristics of flow area are used as the parameters to characterize the turbine flow capacity in monitoring system. And the quantitative analysis models of secondary loop thermal system is proposed using the theory of equivalent enthalpy drop theory and cyclic functional method. These models effectively represented that the heater terminal temperature difference, system internal-leak, condensate sub-cooling and many other factors impact on the power. And then the structure and function of on-line monitoring and analysis system in Daya Bay nuclear power station is described. The monitor system can be applied for on-line monitoring operating state of the equipments and diagnosing economy of the second loop and guiding diagnosis report to operators, which ensure a safe and economical operation to a nuclear power plant. Key words-Daya Bay nuclear power plant; Thermal Performance; On-line Monitoring; Diagnosis