Yao Zhang

Study of HVDC Light for its enhancement of AC/DC interconnected transmission systems

This paper illustrates a study on HVDC light for its enhancement of AC/DC hybrid transmission system and multi-infeed HVDC system. In the sense of the realizing independent controlling of active and reactive power, HVDC light is more effective compared to the conventional HVDC system. An AC/DC parallel transmission system and a three-infeed HVDC system are modeled using PSCAD/EMTDC. Simulations corresponding to different situations are performed. From the simulation results, HVDC light is proven to be able to improve the rotor angle stability and dynamic performance of AC/DC interconnected transmission system, and support the AC voltage during the system fault to enlarge the margin of the voltage stability which is the basic difference from conventional HVDC. HVDC light can also prevent commutation failure in multi-infeed HVDC system and help the recovery of HVDC system too.

A modified pattern recognition algorithm and its application in power system transient stability assessment

A modified pattern recognition algorithm based on the residual analysis and the recursive partition is proposed in this paper, which is more effective than the original algorithm in cases of high-dimension finite-sample practical engineering problems. The proposed approach is first verified with artificial testing data and then applied to power system transient stability assessment (TSA). Case studies show that the proposed pattern recognition scheme can successfully find out the relationships between the pre-contingency steady state quantities and the stability indices under each specified fault. According to the spatial description of the obtained patterns in the feature space, the transient stability levels can be predicted according to the system operation states. Thus the TSA scheme obtained can not only predict the stability index, such as the CCT value, of faults but also offer instability preventive control strategies. Simulation results on the IEEE New-England test system verifies the feasibility and good performance of the proposed algorithm. As a pattern recognition technique, it can be widely applied in different engineering domains to realize knowledge acquisition.

Exploiting cloud computing for power system analysis

Cloud computing provides a new paradigm for easy access to larger scale computing resources over the Internet, thus offering an alternative solution to huge data processing and heavy computational work in power system. In this paper, InterPSS Cloud Edition, a cloud computing based platform for power system analysis, is set up by InterPSS development team using Google App Engine (GAE). At the current release, it provides mainly three functions including load flow, contingency analysis and ODM (Open Data Model) power system simulation data transformation services, and it is accessible anywhere around the world, 24×7, via internet. An overview of this project is given and tests regarding its functionalities are presented. Finally, the future applications and challenges of exploiting cloud computing for power system analysis are discussed.

Studies of Commutation Failures in HVDC System Based on Hypersim

The study of commutation failures requires more detailed and accurate simulation of HVDC systems. Hypresim is an integrated digital simulator, which could provide reliable, extensible and high-speed electromagnetic transient simulation for power systems. Its special characteristics are introduced in this paper. A HVDC system is established based on Hypersim. Modeling details of the ac system, dc lines, dc converters and their control systems are presented. Commutation failures are triggered by external disturbances on the ac bus and internal faults on the thyristor valve; the dynamic behavior of the HVDC system is demonstrated; detailed analysis is also given. A protection method against commutation failures is proposed. Simulation result indicates that the probability of failure could be reduced.

The fuzzy logic clustering neural network approach for middle and long term load forecasting

Middle and long term load forecasting of power system is affected by various uncertain factors. Using clustering method numerous relative factors can be synthesized for the forecasting model so that the accuracy of the load forecasting would be improved significantly. A clustering neural network consisting of logic operators is quoted in this paper, which can be used in mid-long term load forecasting Applying logic operators and in the fuzzy theory, the algorithm speed of the clustering network will be increased. Although competitive learning algorithm is used here for the network, it solves the dead unit problem and gives more room to select the initial values of the clustering center in the clustering analysis of the history data. The proposed model considers the influences of both history and future uncertain factors. Compared with the traditional methods, the results show that the new algorithm improves the accuracy of load forecasting considerably.

COI-based backstepping sliding-mode emergency frequency control for interconnected AC/DC power systems

A new COI-based backstepping sliding-mode method for emergency frequency control of interconnected AC/DC power systems is proposed in this paper. It aims to provide inter-area power support to the disturbed control area(s) through the intact interconnecting structure under emergency conditions, when the normal Automatic Generation Control (AGC) is suspended to avoid worsening frequency stability. With the assumption of area Center of Inertia (COI) frequency and the employment of Wide-Area Measurement System (WAMS), the control strategy is driving the area COI frequency to track the overall system COI frequency by virtue of coordination control of emergency generation and HVDC power such that the long-term frequency stability can be promisingly improved. The combined approach holds desired robustness to area load disturbances. Simulations have been conducted on a two-area interconnected AC/DC system in computer and the results demonstrate the feasibility and effectiveness of the proposed design method.

Study on the Problem of Lightning Strike OPGW

When an OPGW is used as a lightning conductor for electric power transmission lines, lightning strikes are unavoidable, and often cause the OPGW strands to break. These accidents are serious threats to normal information transmission. Based on the phenomenon of lightning strike in OPGW application and the processing of the wire breakage, this paper confirms that the high temperature of the lighting arc which the thunder forms is the primary reason for wire breakages, than discusses the effect factors of wire breakage in different conditions and the measures to protect electric wires from lighting strike. Furthermore it also points out the economical and effective instrumentalities are to adopt high conductive aluminum clad steel wire, and the cable diameter and single line diameter of OPGW should be as larger as possible under design conditions. In the end, the paper puts forward some suggestions of improving anti-lightning strike level.

A real-time reactive power/voltage control system for regional power grid based on hierarchical coordination

A practical reactive power/voltage control system for regional power grid is introduced in this paper. Two-level hierarchy strategy is adopted in the control system. In the regional-level control, one day is divided into 24 time intervals, in each hour, family eugenics based on evolution (FEBE) theory is used in genetic algorithm (GA) to carry out optimization calculation. Compared with convenient SGA, FEBE can speed up convergence in large-scale reactive power/voltage optimization, which is very important in real-time optimization control. Substation-level control, which is based on an improved nine- area-graph control strategy, is implemented in each hour when the regional-level control does not work. The control system presented in this paper has been put into the practical operation and the results show that the control system is robust and effective.

An optimization model of under frequency load shedding in interconnected AC/DC power system considering emergency generation control

When a large disturbance resulting in large active power deficiency occurs, emergency automatic generation control (EAGC) and under frequency load shedding (UFLS) are the effective means to maintain frequency stability. In order to coordinate with UFLS, this paper considers the coordinated control of increasing the generators output and emergency power support of HVDC to enhance the system frequency stability in interconnected AC/DC power system. On this basis, an optimization model that concerns both frequency recovery performance and minimum amount of load shedding has been proposed, and the setting of action frequency and load shedding amount of UFLS scheme is designed. A corrected algorithm, named cloud adaptive particle swarm optimization with dimension mutation operator (CAPSO-DMO) is used for the above problem. The control and optimization algorithm is programmed in MATLAB. The simulation result verified the feasibility and effectiveness of the proposed model and method.

Influence of HVDC control on transient voltage security of the China southern hybrid AC/DC systems

The influence of HVDC control on transient voltage security of the China southern hybrid AC/DC systems following a large disturbance is researched using the transient security assessment tool (TSAT). The differences in the influence of different HVDC control modes on transient voltage security consist in increasing reactive power consumption along with quick recovery of DC active power. Its suggested that transient voltage stability of hybrid AC/DC systems following a large disturbance should be studied in company with synchronous stability concern. The research results show that the constant current control at the rectifier side and the constant DC voltage control at the inverter side are more effective for the recovery of converter AC bus voltage following a large disturbance than the constant power control at the rectifier side and the constant extinction angle control at the inverter side. To mitigate the unbalance of both active power and reactive power following a large disturbance, the parameters of the voltage dependent current order limiter (VDCOL) should be chosen as their compromise values. (5 pages)