Analysis On Power System Frequency Dynamics And Study On Auto Load-Shedding Control

Power system frequency is the index of the balance between generated and consumed electric power and the quality index of power system operation. If the EHV AC transmission lines or large-capacity generator units are set down or the power system is islanded because of some faults, it would induce a large imbalance between generation and consume and even causes the frequency instability in some seriously cases. The power system frequency always collapses during a power system blackout. The power system frequency analysis and control are on the base of power system frequency dynamic analysis after disturbance. The common methods used in power system frequency stability analysis are equivalent single-machine method and time-domain simulation method. The equivalent single-machine method can not consider the influences on the power system frequency dynamic caused by the system network and loads. The calculation complexity of time-domain simulation is so large that the method can not be used for power system analysis on-line. The WAMS monitors the power system in real time and provides new means for power system analysis and control. Using the power system data provided by WAMS, the main researches in this thesis are focused on the power system analysis, power system security and stability on-line assessment, auto load shedding of power syste frequency stability. The main results are as follows.(1) The study about the influences of coal fired power plant’s boiler and auxiliaries on power system frequency dynamic is made in this thesis. The common methods used for the analysis of power system frequency dynamic ignore the influences on the power system frequency dynamic caused by power plant boiler and auxiliaries. A simplified close-loop dynamic model based on SFR model of coal fired power plant for power system dynamic frequency study is proposed in this paper. The boiler, auxiliaries and unit control system are included in this moedel. The induction motors’frequency characteristic of auxiliaries powered by plant power is also considered in the close loop model. The researche about the effect of boiler and auxiliaries on power system dynamic frequency is done by system dynamic frequency simulations and comparations under different disturbances. The investigation of how to think about the effect of boiler and auxlieries on frequency dynamics is done in this paper.(2) About the method of power system frequency analysis, an algorithm using the WAMS data befor and right after disturbance for the analysis of power system frequency dynamic is provided. Firstly, the liaison nodes are eliminated and the network is simplified. With the linearization of the network equations, load equations, generator motion equations and turbine-governor motion equations, the state equation and output equation after disturbance are formulated using the WAMS data before and right after disturbance. With the solving of the state equation and output equation, the frequency after disturbance can be calculated. Compared with the simulation results of PSS/E, the algorithm can describe the power system frequency dynamic after disturbance accurately.(3) About the power system frequency security and stability on-line assessment, with the system data acquired by WAMS and the Arnoldi model order-reduction methods, an algorithm fastly predicting the system minimum frequency after disturbance is proposed. With the elementary transformation of state equation and output equation, Arnoldi model order-reduction method is used to simplify the state equation and output equation. With the solving of the order reduced state equation directly, the expression of the system frequency is got. Using the differential coefficient of the system frequency, the minimum frequency after disturbance can be calculated. Compared with the simulation results of PSS/E, it shows that the algorithm proposed in this paper can calculate the minimum frequency quickly and accurately. Using the data of 25 ms and 50 ms after disturbance respectively, the algorithm could also predict the minimum frequency well. It shows that the algorithm is rather robust. The algorithm can be used in the field of power system frequency security and stability on-line assessment.(4) About the auto load shedding for power system frequency stability, an algorithm of auto load shedding is proposed to control the minimum frequency after disturbance. It assumes that load shedding time and distribution proportion among the load buses are preknown. According to the minimum frequency predicted after the disturbance and the setting value of minimum frequency, a rude amount of load to shed is calculated. Then, the state equation and output equation after load shedding are formulated. Using Arnoldi model order-reduction method to reduce the order of state equation and output equation after rude load shedding, the minimum frequency after the load shedding can be calculated. Compared with the setting value of minimum frequency, the amount of load to shed is adjusted using bisection method. With the analysis of examples, the algorithm of auto load shedding can calculate the amount of load to shed quickly and accurately and it realize the on-line setting of auto load shedding for power system frequency stability.