Modelling of Steam Turbine Generators from Heat Balance Diagram and Determination of Frequency Response

摘要

In the power system, apart from ensuring the availability of Power, maintaining the power system frequency is of utmost important. The intent is to ensure stabilized frequency to the consumers at all times and maintain load frequency control of the power grid which requires necessarily the power load operators and regulators to manage generation and distribution services efficiently to maintain reliability of the power system frequency. In an interconnected power system the power load demand varies randomly which impacts both the frequency and tie-line power interchange. Hence, it is necessary to develop a methodology to make decisions synchronously and automatically by all grid connected generating units. The load frequency control along with restricted governor mode control address this issue and minimizes the deviations in the power grid frequency and tie-line power interchange bringing the steady state errors to zero and maintaining the balance between demand and supply in real time. Restricted governor mode control is a primary frequency control but with inclusion of a dead band of governor not exceeding +/- 0.03 Hz where primary control is blocked by the governor dead band unlike free governor mode. This ripple factor of +/- 0.03 Hz prevents continuous hunting in the governor due to very small frequency variation. Restricted governor mode control does not act in proportion to the frequency deviation like free governor and is not strictly a frequency controlling mode, rather this mode restrict sudden and large frequency deviation with an additional step load disturbance during drop of normal running frequency under contingency control which operate along with load frequency controller enhancing the generation of power. In order to ensure the same, the precision Restricted Governor Mode Control is necessary simultaneously for all the power grid connected generating stations and to define the methodology close to accurate derivation of the various parameters for the modelling of turbine is necessary. This paper describe the procedure for deriving the parameters of a steam turbo generator model of a typical 660 MW Ultra-supercritical machine from heat and mass balance diagram and the conceptual load frequency control with restricted governor mode control. The main focus of the work is to determine the various time constants and finding the frequency response of a typical steam turbine generator based on a realistic mathematical model using the heat and mass balance data with some thermodynamic assumptions. The simulated model response for various scenarios are also presented in this paper.