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Studies on the Theory of Power Transfer Capability Calculation of Power Grids Based on Expanded Power Flow

Author: WangYanLing
Tutor: HanXueShan
School: Shandong University
Course: Proceedings of the
Keywords: power system power grid dynamic thermal rating transmissionline loadability total transfer capability (TTC) expandedpower flow optimal power flow distributed generators master-slave decomposition and coordination method
CLC: TM744
Type: PhD thesis
Year: 2012
Downloads: 254
Quote: 2
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Abstract


With the social development and diversification, exhaustible resource shortage and the worsening of the ecological environment, clean energy use is imminent. Such, large-scale renewable energy generation connected to the power grid is imperative. For the power system, the concentration and distribution operation co-exist, the wind, light, reservoir, water, fire and other power co-exist, and power grid structure is more and more complex. The calculation of power transfer capability of power grid is also more and more complex. This complexity is mainly reflected as follows:(1) In order to excavate the maximum potential of the existing power transfer components, it is very necessary to online value the loadabilitiy of power transfer component based on dynamic thermal rating (DTR);(2)In the calculation of the power transfer capability of power grids, it is very essential to consider the voltage support capability of all kinds of power generators and the regulation characteristics and operational constraints of various types of power generators;(3) After the introduction of distributed power, power system from a centralized conformation into scattered conformation, the transmission and distribution network difficult to clear delimitation caused by backflow problem, it is particularly necessary to calculate the TTC based on the transmission-distribution joint systems.Therefore, this paper, in the scenarios of dynamic thermal rating, taking into account the generator units’regulation characteristics and operating restrictions, based on expanded power flow, thoroughly researches the online value of the loadability of transmission components under operation and the decision-making of the static TTC considering the influence of the distributed generators. The main works and achievements can be summarized as follows: (1) Based on the DTR technologies and taking into account voltage drop and steady-state-stability limitation, the model and algorithm of online valuing loadability under operation are established. Considering the generator units voltage regulation characteristics, the variable parameter dual-port Norton equivalent model based on expanded power flow is derived, and the online tracking of the equivalent parameters of the system is implemented. Based on the historical samples, time series method is used to analyze the variation law of the equivalent parameters to make the maximum loadability of transmission line under operation closer to the actual situation. Studies have shown that the dual-port Norton equivalence can very well reflect the effect of parallel flow on the transmission line loadability in interconnected systems, and the changes in the equivalent parameters reflect the non-linearity and time-varying properties of power systems, and by the equivalent parameters variation, the time-varying nature of the electromagnetic power is reflected. The feasibility and effectiveness of the equivalent model and the algorithm are testified by continuous power flow in IEEE-39system and are confirmed by the analysis of the loadability of Weihai220kV transmission line under operation.(2) Taking into account the generator units’voltage and frequency regulation characteristics and operating restrictions on the power transmission capability, the optimal power flow model and the corresponding algorithm based on expended power flow are established for the calculation of TTC. The objective of the model is the largest total number of active power transferred by branches of cross-section. The equality constraint of the model is no longer the traditional power flow, but the expended power flow. The expanded power flow adding the component dynamic characteristics differential equations to the power flow calculation, combining the traditional power flow equations with the state equations of the dynamic components to strike the steady-state solution, the system node voltage, phase angle, the state variables within dynamic components are solved, More information and more comprehensive description of power system model than traditional power flow, In the expanded power flow, the generators use biaxial fourth-order model, the excitations use the IEEE I-type excitation regulator model and the speed governors use the simplified typical model. The optimal power flow is solved by linear programming method based on trust region, and compared with the solution results of the total transfer capability based on the traditional power flow, due to the governor and excitation regulation characteristics considered in the expanded power flow, thus breaking the previous transfer capability assumption of constant terminal voltage of generator units to make the results more realistic.(3) Taking into account the transmission and distribution network difficult to clear delimitation caused by backflow problem after the introduction of distributed generators, the expended power flow model and algorithm of transmission-distribution joint system with distributed generators are established, which is the basis of integration TTC calculation of the transmission-distribution joint system. Based on the analysis of dynamic characteristics of distributed generators such as gas turbines and wind turbines, expanded power flow equation of transmission-distribution joint system is derived. The scale of the transmission-distribution joint system is extremely large. The number of nodes and branches of joint system is a lot more than the corresponding transmission system. The network structure, network parameters and the size of power flow are different in different regions of the joint system. Therefore, the unified calculation of joint system will not be able to assure the reliability and convergence. This paper uses the master-slave decomposition and coordination method. The transmission-distribution joint system with distributed power is divided into master-slave systems. The transmission system is defined as master system. The distribution system is divided into several slave systems. The connection nodes of master system and slave system are called as root nodes. The expanded power flow of master system and slave system is deduced based on the expanded power flow of transmission-distribution joint system. In the decomposition calculation, the master and slave system use their adaptive algorithms respectively for the different characteristics of their own. On the basis of the decomposition calculation, the coordination calculation eliminates the mismatch of root nodes, in order to achieve the integration power flow calculation of the transmission-distribution joint system. Furthermore, the transmission and distribution system do not exist the exact relationship of transmission and consumption power is determined by examples. So taking into account the distributed power, it is particularly necessary to calculate the TTC based on the transmission-distribution joint systems.(4) Based on the expanded power flow of transmission-distribution joint system with distributed generators, the integration continuous flow model and algorithm of transmission-distribution joint system are established. Continuous power flow is an important tool for TTC calculation. Continuous power flow calculation based on the expanded power flow of the transmission-distribution joint system, taking into account the influence of distributed generators, can effectively calculate the TTC of transmission system. Based on the expanded power flow of transmission-distribution joint system with distributed generators, taking full account of the synchronous generator and gas turbine active power and rotor current constraints, and the wind generator stator current constraints, the TTC is accurately analyzed by continuous power flow of the joint system. The continuous power flow of joint system is analyzed and is solved by master-slave decomposition and coordination method. The decomposition calculation is to predict, correct and compute the load growth. The coordination calculation is to eliminate the mismatches of root nodes. Such the continuous expended power flow calculation of master and slave system is achieved. The above treatment method is Applicable to solving a large system. The joint calculation, taking into account the interaction of main and slave system, realizes the integration calculation of power system and makes the results more realistic.

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CLC: > Industrial Technology > Electrotechnical > Transmission and distribution engineering, power network and power system > Power system simulation and calculation > The calculation of the power system
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