| The unbalance between economic development and energy distribution of China determines that it is necessary to build ultra-high voltage(UHV) power grids to optimally allocate energy resource across the whole country, also determines that the main task of state power grids of China is to transfer large-scale power from the Northwest and Southwest energy bases to receiving-end systems for a long time. With the development of energy bases and UHV, several huge receiving-end power systems are emerging. To ensure the security operation of such receiving-end systems is a challenging work, lots of works should be done on system planning, operation, dispatch management and realtime control decision. Finding out the exact total transfer capability(TTC) of interconnected power systems plays an important role in power system security operation and control. Conservative results will limit the full use of available resources of power grids. In contrast, too progressive results may lead to improper control measures, even lost stability when anticipated fault occurs. Analysing these various factors and their affects on TTC calculation, determining the accurate TTC of interconnected power system, and studying how to improve TTC by operation plan optimization and voltage control, are urgent and have theoretical and practical significance.Based on wide reading, analizing and summarizing, intensive studies are done on how to improve the UHV's TTC. Considering China's power grids actual planning and development, the validity of numerical simulation, the effects of UHV/EHV electromagnetic loop network on security operation and the application of static synchronous compensator(STATCOM) are investigated. Some key works and creative results are listed in the following:According to the issues of validity of numerical simulation when calculating TTC, Based on the various assessment demands, five indicators are proposed to assess the validity of numerical simulation, the validy assessement scheme is established. The five indicators include: (1) the stability deviation assessment indicator based on generator angle trajectory, which can quantitatively evaluate the deviations caused by various simulation facts by the maximum angle difference in the system; (2) the stability error assessment indicator based on the Extended Equal Area Criterion(EEAC), which can quantitatively evaluate the errors caused by numerical simulation from the macroscopic view using the stability margin provided by EEAC; (3) the extreme deviations evaluation indicator, which reflects the maximum deviation during the simulation time window; (4) the average deviation evaluation indicator, which calculates the average deviation within the entire simulation time window; (5) the cost-effective indicator, which can evaluate the effects of models, numerical methods and parameters on the cost of numerical simulation - consumed time. Based on the five proposed indicators, detail works are done with Fujian power system to study the impacts of generator models, load models and integral-step size on numerical simulation deviations. Results show that generator and load modeling may cause apparent deviation in simulation results, and these indexes can evaluate the validity of numerical simulation from different aspects. It's also found that when the deviations tendencies are the same, the effects on system properities, such as transient stability, may be very slight even if the effects on single variable are apparent. It is indicated that, according to power system, it is not sufficient and suitable to assess the simulation validty by only assessing the deviations of single variable. The assessment from system viewpoint is necessary. The results of detail analysis show that the proposed indicators and assessment scheme works well.The factors such as generator modeling, load modeling and integral-step size may bring simulation deviations to transient stability calculation. Therefore, they may also affect the results of TTC calculation when transient stability treated as constraint conditions. Respective works are done with one-machine system, Shandong power system and North-Central-East China interconnected UHV synchronous power system to analyzs the effects of modeling and parameter issues on TTC calculation. Analysis shows that generator models may bring apparent deviation to TTC calculation. Usually, the TTC got with second-order and transient generator models are larger than that with sub-transient generator models, and the deviations caused by generator modeling varies with different power systems and different faults. The maximum deviation caused by generator model is more than 18% in the studying cases, which can't be ignored. Load modeling also may influence the results of TTC calculation. Usually, constant impendance load model contributes more to transient stability than constant power load model, so the TTC calculated with constant impendance load model is bigger than that with constant power load model. The maximum deviation caused by different load models exceed 25 %. However, the results show that the effects of load model on simulation result varies according to different power system, the trend of deviation varing may be contrary according to different system. Integral-step size is another issue that can affect numerical simulation. Compared with the impacts caused by generator model and load model, the impacts caused by integral-step size are very small, and closely related to generator model. According to one-machine system, the effects are more apparently when sub-transient generator models used, but it almost has no impact when second-order generator model adopted. According to practical power systems, the impact of integral-step is much smaller than that of generator model and load model. Usually, the deviations are almost all less than 1.5%, which can be ignored in practice. In short, models and other factors may bring apparent deviations when calculating TTC, it's very important to choose proper models when performing power system numerical simulation and analysis. Therefore, models and parameters should be used carefully in TTC calculation and decision making to ensure the validity of numerical simulation.In steady state, more power can be transfered when the system is operating with the high-low-voltage electromagnetic loops closed, but serious potential safety issues may also exist. To open the electromagnetic loops or not depends on many factors, such as security issues, stability issues and economic issues. These factors must be detailedly assessed when related decisions making. Based on the characteristics of UHV-EHV power system, a fuzzy evaluating method of lOOOkV-500kV electromagnetic loop is presented. Based on analytic hierarchy process (AHP), the corresponding fuzzy comprehensive evaluation model is established, and its indicator system is defined. The static security indicators, including branch overloading and node voltage off-limit information, are used to reveal system static security. The transient stability indicators, including the maximum generator angle difference between any two machines and the transfer limit of selected cross-sections, are used to assess system transient stability. The operation economic indicators, including the changes in investment because of the increase of short circuit current and system total losses, are used to reflect system economic situation. The member-ship functions and the comment set are determinded. Based on fuzzy comprehensive evaluating method, the indicators are evaluated. Firstly, performing numerical simulation to calculate static security, transient stability and the economy of operation, and then calculate these proposed indicators. Finally, the method gives a quantitative and comprehensive evaluation of the electromagnetic loop opening schemes based on these indicators.A program is developed under Windows platform to do the presented fuzzy comprehensive evaluation of electromagnetic loop opening schemes. Working together with the Power System Analysis Software Package(PSASP), it reads the simulation results provided by PSASP, calculates the indicators, then analyse and perform fuzzy evaluation, which mainly includes fuzzy treating of indicators, comprehensive fuzzy evaluating of all the indicators, and outputs the evaluation results. The method is applied to North-Central-East China interconnected UHV synchronous power system. The results compliance well with practical experience, which verifies the validity of the presented method, and it will help a lot in the operation of UHV/EHV systems.STATCOM has characteristics of wide regulating range and fast regulating speed, which can be used to enhance the voltage bearing ability and improve voltage stability of the receiving-end power system. Based on detail analysis and summary of STATCOM static and dynamic characteristics, the static model of STATCOM based on injection power and the dynamic model of STATCOM based on controlled current source are established. These models are also implemented with PSASP by user-define modeling method. It is found that the gain K of the STATCOM model affects the convergence characteristic of power flow computation. The parameter, K, leads to the iteration number increase when it is too large or too small. The dynamic and static characteristics of user-defined STATCOM are detailedly analysed with Shandong power system. Simulation results show that the user-defined STATCOM models are convenient and efficient, and show that STATCOM can play a combined role in dynamic voltage regulation, strong reactive power supply and voltage drop suppression. Combined with the need of interconnection of Shandong power system with UHV system, using STATCOM to enhance the voltage support ability of Shandong power system is studied. The stability indicator based on power transmission path is used to find the weak nodes of voltage stability. The weak regions of voltage stability in Shandong power system are analysed, and the STATCOM locations are determined. Considering the power plant and power network development plans, Laoshan Station is the priority location to install STATCOM, which can improve the power transfer limit of the power corridor in Shandong power system, and enhance the power receiving ability of Shandong power system, which can also improve the transfer capacity of UHV power lines when the are connected with Shandong power grids.
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