Studies On Overhead Transmission Line Power Transfer Capability

With the proposition of the energy-saving and the emission-reduction policies, energy problem has become the focus of research. The source has been rapidly developed. Renewable resources energy such as the recyclable and non-Polluting renewable wind energy has attracted the attention of the world. The wind power output is very dependent on the wind speed which is fluctuant, intermittent and stochastic. With the more and more wind power plants on power systems, it will bring challenges to the safety and stabilization of the power system. With the development of the economy and the progressing of the society, the contradiction between power supply and demand is becoming more and more obvious. Such as, in the real operating conditions of power system, how to accurately calculate the transmission capacity of overhead transmission line, how to develop the potential ability of overhead transmission lines, how to improve efficiency of the existing power system to alleviate the lack of transmission capacity. Above at all, these have become the focus study of the academic and engineering field.At present, the DTR technology has been more mature. DTR technology has achieved real-time tracking of the operating transmission line temperature and thermal rating through variable measuring, communication equipments fixed on transmission lines. DTR technology develops effectively the potential of the transmission capacity. Based on the transmission line dynamic thermal rating and large-scale wind plants in the power system, the study of the transmission capacity of overhead transmission lines puts forward some new problems in the depth and breadth. Such as:because the DTR technology depends on hardware and high cost of operation and maintenance, it is difficult to be spread. Based on the DTR technology and taking into account voltage level and steady-state-stability limitation, how to solve the online valuing loadability under operation. With the change of the wind speeds, how improve the forecasting accuracy of the wind speed and wind power. With wind power fluctuations about the large-scale wind plants on the power grid, how to affect the power transfer capacity and so on.Consequently, in the paper we make an in-depth study of the transmission capacity of overhead transmission lines based on the background of the dynamic thermal rating and wind plants on the power grid. The main contents and results are as follows:(1) For the DTR technology depending on the hardware and its expensive operation and maintenance costs, we give a method to realize DTR function in only measuring the carrying current and temperature of transmission line. Firstly, the calculation formula for the IEEE standard transmission line thermal balance equation of the coefficients and its physical significance are described in detail. On the basis of that, the discrete heat balance equation of transmission line are obtained, and the comprehensive effects of wind direction, wind speed, sunshine and environmental temperature are equivalent to the discrete parameters of the heat balance equation of transmission line. Secondly, limited memory generalized least squares estimation method is used to estimate the heat balance equation of discrete parameter online. This method overcomes the problem that the gain matrix of the classical recursive least square method with time gradually tends to zero and losing correction capability, obtained the consistent, unbiased estimator of discrete parameters of heat balance equation. In the process of continuous sampling which sampling time is short, the time-varying discrete parameters of heat balance equation are approximated continuous identified by a recursive process. So, the purpose that heat balance equation of transmission line is modeled online has been reached. Again, the discrete parameters of heat balance equation of transmission line are known, and real-time value on thermal loadability is determined online in transmission line of long-term and short-term. Finally, the example analysis shows that loadability of thermal balance equation of transmission line based on discrete parameters are determined online simplifies the meteorological conditions of measurement system. To determine the real-time value of long-term and short-term maximum permissible carrying current online, it is only need to measure carrying current and temperature of transmission line, which excavates the transmission potential of transmission line effectively.(2) Based on the DTR technology and taking into account thermal rating, voltage level and steady-state-stability limitation, the model and algorithm of online valuing loadability under operation are established. First, through a simplified transmission line equivalent model of the sending and receiving end system, we make a qualitative analysis of online loadability under an operating system. It shows that the loadability is reflected by sending and receiving end equivalent parameters on transmission lines in different voltage levels. The real-time system equivalent model is very necessary for us to analyze the loadability. Secondly, taking the system as a whole and considering the effect of parallel flow on the transmission line loadability in interconnected system, the dual-end resource equivalent model of the sending and receiving end system is proposed by the system node impedance matrix transform and equivalents simplification. The derivation of the model is simple, and it has a clear physical meaning. Finally, based on state estimation in power system, equivalent parameter on-line tracking is realized. Considering the thermal rating, voltage level and steady-state-stability limitation, the analysis and calculation of online valuing loadability of transmission line are implemented. The feasibility and effectiveness of the equivalent model and the method are confirmed by the analysis of the loadability of Shandong220kV transmission line under operation.(3) Accurate forecasting of wind speed is helpful to improve the accuracy of wind power forecasting. It can also effectively relieve or avoid the disadvantageous impact of wind power plants on power systems and enhance the competitive ability of the wind power plants against other power plants in electricity markets. First of all, without the need for a large number of historical data, the wind speed is predicted by the new generation mesoscale forecasting model of WRF. The WRF provides high resolution and priority design, locates effectively the wind plants, captures the characteristics of weather system and uses different physical options and advanced data assimilation system to establish various wind speed forecasting models. Secondly, according to various physical schemas, we forecast wind speed respectively aging for12hours and24hours. Finally, compared with the real measured wind speed data obtained from the SCADA system of Shandong Ancheng wind plant, the wind speed forecasting based on WRF is a new effective method to improve the forecasting accuracy by analysis of the error. In this paper, the target is a utility of display and design based on WRF’s the wind speed forecasting. The utility study of wind speed forecasting consists of input files based on GFS, the WRF core processing and the visualization output based on GrADS. It forms a new framework of three-dimensional wind speed forecasting.(4) The wind power forecasting of wind plants is one of the effective means to ease the peak pressure of the power system and to improve the wind power capacity. The paper studies wind power forecasting by organically combining the wind speed forecasting method and wind power curve forecasting method. First of all, the direct method, the Bean method, the maximum value method and the power function method are used to establish the total power curve of wind plants. Secondly, the wind power curves of four kinds of the established method are combined with the wind speed forecasting based on WRF to study wind power forecasting aging for12hours and24hours. Finally, forecasting power obtained with different methods is compared with the wind power actual output values from the Shandong Ancheng wind plant by average relative error, root-mean-square method, correlation coefficient method and wind power forecast accuracy method. Through comprehensive analysis and verification, nonlinear power function method is a simple and effective method of improving the wind power forecasting accuracy.(5) For large-scale wind plant on the power system, wind power output fluctuations will inevitably affect the power system transmission capacity. Transmission capacity is somewhat optimistic for some of the assumptions, which the terminal voltage (PV nodes) remains unchanged. First, under the thermal loadability of the transmission line based on the DTR, taking into account wind power plants and the generator units’operating restrictions on the power transmission capability, the power flow model and the corresponding algorithm based on expending power flow are established for the calculation of TTC. Extended power flow broke PV, PQ and the balance node hypothesis. After adding the component dynamic characteristics of the differential equation into the flow calculation, the steady-state solution is obtained by simultaneous calculating of the traditional flow equations and dynamic component equations, and we can solute each node voltage, phase angle, internal state variables of the dynamic element at the same time in the system. Comparing with the traditional flow calculation, we get more information and get a more comprehensive description of the model of the power system. Secondly, based on the models and methods of expanded power flow, we study the impact on the power transmission capacity through the analysis of Large-scale wind plants position, changing wind speed and wind power, and dynamic characteristics of operating synchronous generators and excitation system. Finally, the real example analysis results prove that the effects of the total transfer capability are accord with the actual situation. It is indicated that the impact on calculating the TTC of power system containing large changing wind speed, especially which is too low or too high to result that the wind turbine is removed from the power system, should be considered.