Duan Xianzhong

Agent-based self-healing protection system

This paper proposes an agent-based paradigm for self-healing protection systems. Numerical relays implemented with intelligent electronic devices are designed as a relay agent to perform a protective relaying function in cooperation with other relay agents. A graph-theory-based expert system, which can be integrated with supervisory control and a data acquisition system, has been developed to divide the power grid into primary and backup protection zones online and all relay agents are assigned to specific zones according to system topological configuration. In order to facilitate a more robust, less vulnerable protection system, predictive and corrective self-healing strategies are implemented as guideline regulations of the relay agent, and the relay agents within the same protection zone communicate and cooperate to detect, locate, and trip fault precisely with primary and backup protection. Performance of the proposed protection system has been simulated with cascading fault, failures in communication and protection units, and compared with a coordinated directional overcurrent protection system.

The impact of TCSC on distance protection relay

When a fault occurs in thyristor controlled series compensation (TCSC) transmission lines, TCSC presents a very complicated impedance characteristic and exerts influence on the power systems distance relays. Under ground faults, the complex changes of sequence currents exert influences on the mho characteristic, reactance characteristic and directional characteristic and makes the protection region unstable. Longitude distance protection with an overreaching setting value may be a solution to TCSC lines protection, but it has difficulty in setting the relay on the reverse directional line to avoid misoperation. Simulation shows that sampling frequency has an influence on distance protection performance in TCSC transmission lines.

Context Information-Based Cyber Security Defense of Protection System

With the development of a substation automation system, it is now feasible for the intelligent electronic devices-based protection system to collect its measurements from electronic instrument transformers through a communication network such as an Ethernet-based local-area network. The application of a wide-area broadband network in power utilities introduces underlying danger for protection systems to maloperate when receiving fake data packages from an intruder. This paper proposes identifying a vicious fault by using context information, such as voltage and current, of the same substation. When the protection system detects a fault based on measurements transmitted through a network, it collects all measurements of the substation and feeds these data to a probabilistic neural network. Thereafter, the fault caused by fake data that differs from the known fault pattern can be identified and blocked.

A novel dynamic voltage restorer and its unbalanced control strategy based on space vector PWM

A novel dynamic voltage restorer which is composed of a conventional three-phase voltage source inverter and an emitter follower is presented in this paper to reduce the adverse effects on sensitive loads caused by three-phase supply voltage unbalance in distribution systems. The adverse effect caused by the negative sequence component of the supply voltage can be eliminated and the load voltage magnitude can be restored to specified value by using the three-phase voltage-source inverter. The emitter follower is capable of eliminating the adverse effect caused by the zero sequence component of the supply voltage. An unbalanced control strategy based on space vector PWM is presented to realize the fast dynamic response of the dynamic voltage restorer. Feasibility of this method is verified by digital simulation.

Comparison of superconducting fault current limiter in power system

Due to its ideal electrical behavior, the superconducting fault current limiter (SFCL) has become one of the most important development trends of limiters in power systems. This paper summarizes the main electrical behaviors of SFCL, including the relationship between SFCLs working current and the quench current of the superconducting coil (SC), and the relationship between equivalent fundamental frequency impedance of SFCL and normal state resistance of SC. Operation losses and response time of SFCL are also discussed. The paper analyzes different influences of quench type SFCL and nonquench type SFCL on waveforms and total harmonic distortion (THD) of line current and node measurement impedance. Electromagnetic simulation is adopted to analyze the characteristics of the SFCL.

Self-Organized Criticality in Time Series of Power Systems Fault, Its Mechanism, and Potential Application

The paper analyzed time series of fault in four transmission and distribution systems for evidence of self-organized criticality (SOC). The detrended fluctuation analysis (DFA) of the time series shows prominent long-time correlations. Furthermore, the probability distribution of number of faults per day has a power law tail. Thus, the time series of power system fault seem consistent with SOC. Detailed analysis showed that time series of fault in transmission systems derives its SOC from cascading outage within power system and SOC of atmosphere system, while the time series of fault in distribution systems derive its SOC from that of atmosphere system. The roles that cascade outage play in SOC of the time series of fault in distribution systems remain unknown. Since extreme climate will be more intense and frequent with climate variation, the skewed distribution of power systems fault will be intensified as a consequence. Potential application using SOC feature of time series of fault can be developed to facilitate power systems adaptation to climate change in an economical way.

New concepts of dynamic voltage restoration for three-phase distribution systems

In three-phase AC power distribution systems, a dynamic voltage restorer (DVR) can be used to protect sensitive power customers from voltage disturbance. The outputs of the DVR relates to the disturbance level of supply voltages, the required restoration level of load voltages, and the limitation of energy exchange between the DVR and the transmission line. In this paper, the influences of these factors on the DVR outputs are analyzed in detail. A generalized DVR with multiple voltage source inverters that are connected to a common DC energy storage device and inserted into the different sensitive load branches respectively is presented. It has the potential to reduce the rating of DVR and its DC energy storage device, and the adverse effects of the DVR fault will be limited. The analysis results and the feasibility of the generalized DVR are verified by digital simulation.

Probability Distribution of Fault in Distribution System

This letter analyses time series of two distribution system (DS) fault records to find their probability distribution. Observation indicates that the complementary cumulative distribution function (CCDF) of number of fault per day has a power law tail and that of location of fault has an exponential tail. The data add to the knowledge that is a fundamental basis for developing methods for power system maintenance and reliability.

Fundamental frequency model of thyristor-controlled series capacitor for transient stability studies

This paper presents a fundamental frequency stability model for a thyristor-controlled series capacitor (TCSC). The paper first analyses the factors that influence the transients of TCSC when the operation condition changes, and time domain simulations are performed. Assuming that the dynamics of TCSC can be predicted, a simplified fundamental frequency model is derived on the basis of simulation results. The model inherently incorporates the thyristor triggering logic and the synchronization system. In transient stability programs, the TCSC is thought of as a variable impedance. Therefore the computation burden does not increase too much and the results are accurate. However, the presented model is only suitable for TCSC in capacitive zones. For the transition from a capacitive vernier mode to bypass mode, the TCSC is modeled with its detailed dynamics.

A new expression-analysis-based losses allocation method

How to allocate transmission losses equally and reasonably is an important issue in the context of competitive electric power markets. A new expression-analysis-based power allocation method, which can allocate losses to all the generators and loads, is advanced in this paper. Generators (loads) are replaced with current sources while loads (generators) are replaced with shunt branches. Transmission losses are expressed as function of current sources and elements of the Z-bus matrix of the equivalent network. Interaction terms are allocated to related current sources averagely. Test results show that this method can provide effective economic signals for both generators and loads.