Applied Mechanics and Materials Vols. 291-294

Abstract: Ultra-supercritical (USC) unit is more and more popular these years for its advantages. In this paper, a model predictive control (MPC) method is introduced for coordinated control of USC unit running in fixed pressure mode. Three inputs (i.e. valve opening, coal flow and feedwater flow) are employed to control three outputs (i.e. load, main steam temperature and main steam pressure). Piecewise models of the USC unit are obtained using the three inputs and the three outputs. In simulation, the output power follows load demand quickly and main steam temperature can be controlled around the setpoint closely in load tracking control. The simulation results show the effeteness of the proposed methods.

Abstract: Leakage demolition of temporary ground wires often leads to accidents named closing circuit breaker. For this, a detection device is designed in this paper, which is exclusive for the detection of temporary ground wires connected to the ground through towers. Traps present low impedance for 50Hz induced signal while they present high impedance for 200Hz sinusoidal signal, thus inducted current will flow into the earth through the path of traps and ground wires. The amplitude of 200kHz signal collected by the MCU is related to whether there are temporary ground wires connected to power lines. The MCU completes digital filtering work of the collected signal and counts the number that amplitude above the threshold voltage to determine whether there are any temporary ground wires connected to power lines.

Abstract: Frequency is an indicator of the balance between electricity generation and consumption. Maintaining the frequency stability is the first of all for a stable operation of power system. After analyzing the traditional power system’s primary frequency control, secondary frequency control and tertiary frequency control, the electric heater, water heater, refrigerator, air conditioner and other domestic electric appliances are tried to use to improve the frequency quality as the decentralized intelligent loads based on the thought of “demand following supply”. The statistics of the overall power of different domestic electric appliances of several countries are given. It indicates that the domestic electric appliances can be used to improve the frequency quality from the overall power. The operating characteristics and control requirements of different domestic electric appliances are analyzed. Their control strategies are put forward and model of decentralized intelligent loads is constructed. A series of questions of technology, economic and policy that need to be solved while utilizing the decentralized intelligent loads to control the power system frequency are put forward.

Abstract: In order to compute and analyze power flow of a distribution network with distributed generations(DGs) and eliminate various problems caused by placing of the DGs, several improved Newton methods were proposed successively, and initial value choice is one of the non negligible issues. The traditional initial value choice method also cannot ensure convergence under some circum stances, and the convergence problem is aggravated by the placing of the DGs. For the convergence problem in the computation for the distribution network with vast DGs accessed by the Newton method, this article proposes an approximation algorithm based on series expansion for computing voltage approximation as the computed initial value of power flow so as to improve algorithm convergence. Furthermore, effects of the number of the DGs, node types, their locations, etc. on error between the voltage approximation and the exact voltage value are analyzed through examples. Example verification shows that the error between the voltage approximation computed by the method described in this article and the obtained final value through calculation is small, and replacing the traditional initial value with the voltage approximation can effectively improve convergence of the power flow computation and reduce iterations.

Abstract: Conventional network partition and pilot nodes selection methods for reactive power / voltage control are mainly based on the reactive power - voltage sensitivity, however, it is hard to regulate the balance of the reactive power in partitions and pilot nodes may over-concentrate in some regions. According to the community detection algorithm in complex network theory, an improved community modular index is proposed with the consideration of the reactive power balance degree in partitions, while the power grid is modeled as the weighted network with similarity weight. By introducing the concept of vertex degree and betweenness, a novel pilot nodes selection index is presented , which is based on the ranking of observability and controllability sensitivity and can evaluate the centrality and connection density of node. Applying the proposed index and method to IEEE 39-bus system, simulation results show the effectiveness.

Abstract: In recent years, with the happening of large-scale power cut accidents, the electric network has been concerned widely increasingly. Pattern recognition which could be used in online analysis, can get rid of the mode limit of the operation characteristic of description system, and its counting volume is less influenced by system scale, also its general assessment is in a fast speed. In this text, it elaborates the conception of the power grid vulnerability and some kinds of available ways of assessing vulnerability, and it also tells us the target of electric network’s integrative vulnerability based on integrative vulnerability mode for electric power grid, and used in pattern recognition, which its the sensibility and necessity of pattern recognition has been verified by the IEEE-39 system.

Abstract: With integrated consideration of existing research on power grid development evaluation index system and requirements of power grid development diagnosis, a more comprehensive power grid development diagnostic index system is established from five aspects of power grid security and reliability, high efficiency, coordination, economics and social environment and more than 20 indicators. In addition, a new power grid development diagnostic evaluation method based on hierarchical analysis and multi-level fuzzy comprehensive evaluation is proposed in order to diagnose and assess power grid status quo and the actual level of development comprehensively, objectively and systematically ,which will help power grid enterprises conveniently to find the main reason of affecting the power grid and enterprise development and encourage enterprises to continuously improve the targeted improvement measures and the level of grid development.

Abstract: Ultrasonic positioning accuracy of transformer partial discharge is not high. The main reason is that it is difficult to obtain accurate time difference of the ultrasonic signals reaching two different sensors, especially when the signal-to-noise ratio is not high enough. To solve this problem, Hilbert-Huang transform (HHT) is applied in signal processing to extract the precise moment, and in this way the precise time difference is easily obtained. The signal-to-noise ratio (SNR) of received signal is low, and even worse PD signal is drowned in the strong interference signal. For this, the method named fast independent component analysis (FastICA) is applied in ultrasonic signal denoising before obtaining the time difference. FastICA is not effective in separating the ultrasonic signal from the separated components and in order to solve the problem, envelope identification method is proposed in this paper. In the method, the upper envelope of the signal waveform is sampled and the average of the sampling points is calculated to extract the ultrasonic signal. The numerical result confirmed the practicality of all the mentioned methods.

Abstract: Combined with the actual situation of transformer substation, PSO model is built to optimize the shunt capacitor series reactance rate in this paper. First, system harmonic impedance is analyzed and the harmonic impedance model is built. Second, the objective function and constraints of the optimization algorithm were set. The goal is to make the current distortion rate of high side smallest under the condition of the capacitor banks working safely, the output results will be the optimal allocation of the series reactance rate. Current-limiting reactor was also considered the influence to the design of series reactance rate. At last, testify the effectiveness with comparing the effect to the traditional way of designing the series reactance rate and prove the significant influence of the current-limiting reactor.

Abstract: The compensation principle of active power filter is analyzed，and the measurement method of harmonic and reactive current based on instantaneous reactive power theory is proposed．In the control methods of compensation current，aiming at the limitation of PI controller in compensation performance of active power filter，repetitive control is introduced in the tracking control of instruction current，and compound control strategy composed with PI controller and repetitive controller is adopted to affect the output of system．The system model based on compound control strategy is built and simulated by MATLAB/Simulink，The simulation results indicate that after compensation total harmonic distortion of the system is reduced to 1.33%， power factor is reached to 0.99，and the results demonstrate the validity and superiority of the compound control．