Yin Xianggen

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.

On-site safety evaluation for earth fault in mining power systems

Many mining power systems utilize ineffectively grounded sources to restrict the residual current of single-phase earth fault in order to reduce outage and shock hazard. In practice, with the system expansion, topology changing and insulation aging, the potential residual current and zero sequence voltage for earth fault vary dynamically and some arcing earth faults can easily cause overvoltage and induce multiple faults. In order to improve the system safety, on-site condition monitoring, safety evaluation and preventive maintenance for an earth fault are proposed in this paper. Some techniques for on-line parameters-measurement based on the resonance-measurement principle are presented. Power system operation states are classified into normal secure state, alert state, incipient fault state and fault state. Some security enhancement methods (preventive action and remedial action) for each state are implemented. The prototype for safety evaluation has been developed and installed in industry power systems for several years.

The algorithm of probabilistic load flow retaining nonlinearity

In this paper, a novel method was presented for obtaining the probabilistic load flow (PLF) solution, with which we can gain the probabilistic nature (such as expectation and variance) of variables and line flows in networks. The method expands the mathematical model of calculation of load flow in a Taylor Series about mean value, and retaining nonlinear terms of the load flow equations, so improving the precision of the mathematical model; at the same time, it also considers linear factors by using a corresponding linear solution to replace higher order terms, so simplifying the mathematical model and making the method more practical; and more, the method applies the technique of probability and statistics, and that we only calculate load flow once to finish calculation of PLF, so enhancing speed of calculation. The method has good adaptability and can apply in a majority of instances. The performance of the proposed method is illustrated through the IEEE 14-busbar test system, and also compared with Monte Carlo simulation techniques, the result has shown that the method is available for calculation of PLF.

Wavelet-based ground fault protection scheme for generator stator winding

Abstract A new stator ground fault protection scheme is introduced in this paper. It is based on wavelet transform, particularly the multiresolution analysis (MRA) technique. The effectiveness of the proposed scheme was verified both in the experiment and in the field. Investigation results show that the scheme can detect the ground fault with high sensitivity and selectivity during all operating conditions.

Research on protective relaying systems based on multi-agent system

Improving selectivity, celerity, sensitivity and reliability is the purpose of research in protection. Although principles and performance of protective relaying have been improved, their operation is still unsatisfied, due to the difficulty in cooperation work. As a branch of distributed artificial intelligence (DAI), agent technology has become a hot spot and researches have been done. But the coordination and collaboration of protection has not been well considered in those works. In this paper, agent technology and the up-to-date research works on its application in protection systems are reviewed. The application of agent in protection system is considered in details from three different hierarchies. Then a protection system based on multi-agent system (MAS) is proposed. The characteristics, structure, classification and decision-making process of this system is elaborated. Served as example, two designs are outlined. One is backup over-current relaying for a power line, the other is a grounding fault detector for a substation. Using the Alternative Transients Program (ATP) as the simulation tool, these systems are simulated. The result shows that through the coordination among distributed protection agents, the process of relay tripping and fault detection can be accelerated and the accuracy of grounding fault detection can be improved. Through cooperation and consideration of protection agents based on different principles, the systems ability of fault-tolerance can be enhanced. Consequently trip failure and malfunction can be avoid effectively and the time for operation can be curtailed effectively. Compared with a conventional protection system, its performance is more satisfactory.

Improvement of subharmonic injection schemes for huge hydro-generator stator ground fault protection

The stator winding of huge hydrogenerator unit has large leakage capacitance to earth. Traditional subharmonic injection schemes for generator stator grounding fault protection have poor performance in detecting high impedance grounding faults (HIGF). In order to improve them, a reactor added in parallel with the resistance in the secondary of a neutral grounding distribution transformer is applied to compensate the leakage capacitance in the paper. By measuring the voltage and current of the resonant signal injected from the additional reactor, grounding fault resistance can be calculated directly and HIGF protection for 100% generator stator windings is proposed. Simulation results show that the improved subharmonic injection scheme can detect HIGF with fault resistance higher than 8 k/spl Omega/.

Study on the operating characteristic of sampled value differential protection

The sampled value differential protection is described in this paper. The selections of S, R are investigated and the basic selective rule is to ensure that the restraint effectiveness of the sampled value differential protection be equal or superior to the corresponding phasor differential protection during external faults. The particular fuzzy operating zone of the proposed protection is also discussed and its expressions are presented. In addition, the phasor analyzing method, in which the phasor relationships of input currents of the protected system are used to represent the operating zone, fuzzy operating zone and restraint zone, is proposed to analyze the operating characteristic. The engineering applications and test results are also introduced.

Novel methods for high-impedance ground-fault protection in low-voltage supply systems

Many high-impedance ground faults (HIGFs) that happen in low-voltage (LV) systems often cause customer supply loss, fire, and human safety hazards. Traditional ground-fault protection is provided by the residual current circuit breaker (RCCB). The RCCB is usually employed in an analogue measuring circuit and often causes nuisance tripping due to capacitive leakage current and load-switching operations. It offers only ground fault protection to a certain extent and has difficulty in detecting HIGFs associated with a dielectric material defect. In this paper active power variation–based protection for HIGFs is developed, and a dissipation factor (DF)–based criterion for identifying load-switching operation is proposed. They are implemented by cross-correlation analysis between phase voltage and residual current in single-phase networks. A digital protection scheme is also designed. EMTP simulation results show that the new protection can remove the influences of capacitive leakage current and load-switching operations and is able to detect HIGFs and prevent electric shock with high sensitivity and robustness.

A new on-load tap changing system with power electronic elements for power transformers

In order to avoid the disadvantages of conventional mechanical tap changing system and its influence on power system, a new on-load tap changing system with power electronic elements for power transformers is presented in this paper. The system is composed of main circuit, signal detection and triggering circuit design, choice of the thyristor switching, protecting transformer from overvoltage both in case of switching on and switching off no-load transformer. It possesses the inherent attributes of greater efficiency and less maintenance.

A new three-level NPC inverter based on phase individual DC-link circuit and high quality digital SPWM control technology

Due to the existence of reverse recovery time of power diodes, the clamp diodes and di/dt-chokes diodes of three-level NPC inverter will enter the reverse recovery region at the same time, and the partial over-circuit phenomena will occur. This problem limits the output power of inverter system and do harm to the devices. To avoid this over-current problem, a new circuit implementation of 6kV three-level NPC inverter based on three phase individual DC-link is presented. Using this new circuit the partial over-current problem of traditional three-level NPC inverter can be solved. On the other hand, the security, stability and flexibility of three-level NPC inverter control system are much more complicated than traditional two-level system and low-voltage inverter. So the high reliability hardware and software based on layer organization structure has been put forward. This control scheme is realized with only a single FPGA chip and can provide an effective, flexible, and safe solution for high-power three-level NPC inverters. At last, some simulation and experimental results are presented to verify the advantages of the 6kV three-level NPC inverters.