Advanced Materials Research Vols. 732-733

Abstract: The reliability indexes and evaluation methods of AC transmission system and high voltage direct current (HVDC) transmission system were introduced. According to the characteristics of the DC grid, whether the above methods could be applied to DC grid reliability evaluation was analyzed, and the reasons and applications were also presented. The results show that state enumeration method, Monte Carlo method, and hybrid method can be applied to the large scale DC grid reliability assessment, based on either the grid frame structure or the power flow calculation; and the FD (frequency and duration)-FTA (fault tree analysis) method and FD-model combination method are more suitable for simple DC grid reliability assessment based on grid frame structure.

Abstract: By means of computer and network communication techniques, this system analyzes and calculates the real-time data, which is collected by telemetry and telecommand at every node in the whole grid via SCADA system. On the premise of ensuring the safe operation of the power grid and equipments, under the constraint condition of qualified voltage at every node and power factor of province gate way, and in accordance with power flow calculation, improved genetic algorithm and fuzzy expert system, this system is able to carry out online voltage reactive optimizing closed-loop control from the angle of the whole power grid. Thus it makes the investment of reactive-load compensation equipments and the adjustment of main transformers tap-changer more reasonable. Therefore, the system achieves the goal of local balance in reactive power stratification.

Abstract: According to the process characteristics and control methods of different voltage stability problems, the paper researches control rules and strategies of the voltage stability, then sets up controllers of FACTS devices on a simulation platform, and puts forward the coordinated control strategies of voltage stability problems based on expert rules. The simulation studies have shown that control effects of multiple FACTS devices are significantly better than a single device, and the coordinated control methods based on the expert rules is more conducive to improving voltage stability.

Abstract: Permanent-Magnet-Biased Saturation Based Fault Current Limiter (PMFCL) is a preferable solution to suppress the fault current, and may find early applications in large capacity power systems due to prominent advantages. Firstly, the topologies and the operating principles of the PMFCLs are elucidated in this paper. Then a new concept namely effective magnetic flux density is proposed in order to describe the overall impacts of the magnetic field on the current limiting capacity of the PMFCLs. Lastly, theoretical analysis and mathematical simulation on the dynamic characteristics of different modes of PMFCLs are carried out comparatively. All the results present invaluable fundamentals for further study on the operating mechanism of different modes of PMFCLs as well as optimization of the fault current limiting topology design.

Abstract: A model for optimally evaluating grid short-circuit current limitation measures is proposed. Based on the fuzzy comprehensive evaluation model, a comprehensive evaluation system is introduced to consider the limiting effect and economy of the limiting measures configure program, also the program's impact on system security is involved. A judgment matrix is constructed to determine the weights of different system layers. With fuzzy behavior indices and membership function being led into, the evaluation of configuration program is given. Taking Gansu power grid for example, the feasibility and practicality of the proposed model are verified by simulation results.

Abstract: Power flow transferring random occurred in power grid, so how to quickly identify overload line is a key problem to prevent cascading trips. This paper according to the bus self-impedance change rate rapid search the area which possible happened power flow transferring, and through the correction method for network equations checked the lines in this area whether overload or not, then selected the overload line caused by power flow transferring. Under the condition of known initial network parameters, this scheme according to the local network parameters modified to related calculation, improves the calculation speed, and has advantages for multipoint flow transferring recognition in complex power grid. Taking the New England 10-unit 39-bus system which also used this method as the example, in this case, this method is confirmed by the BPA simulation.

Abstract: Electric power network components can be simulated with equivalent circuit and establish the hybrid node voltage and branch current electricity network analysis model based on it. We form the equations reflected the voltage collapse conditions, and establish the extended power flow equations with the hybrid equations, the static voltage collapse point can be calculated by solving them. The advantage of this method is that the power flow calculation can be convergence smoothly near the collapse point because of the extended power flow equations when the Jacobian matrix tends to be singular. The convergence can be achievable just because the joining of the equation reflecting the voltage collapse conditions has changed the structure of the Jacobian matrix. The simulation results show that this method is very effective.

Abstract: In order to eliminate the adverse effects of coal-bed gas concentration disturbance to coal-bed gas engine air-fuel ratio control and the large pure time-delay existed in signal response of methane transducer, a fuzzy PID feedforward flow control plan was proposed for gas supply system. Fast and efficient flow control was performed in advance according to air-fuel ratio control requirements to eliminate air-fuel ratio control delays caused by the coal bed methane concentration disturbance. Based on the mentioned feedforward flow control plan, considering the premix coal-bed gas engine air-fuel ratio fluctuations arising from engine speed disturbances, a fuzzy PID closed-loop control scheme was proposed. By analyzing different air-fuel ratio control characteristics in transient condition and steady condition, the different closed-loop control of the air-fuel ratio was used to suppress air-fuel ratio fluctuations caused by engine condition changes. Compared to traditional PID control, the simulation results show that the air-fuel ratio fuzzy PID closed control scheme based on fuzzy PID feedforward flow control has ideal control effect, preferable stability and robustness. The air-fuel ratio control delays caused by the coal-bed methane concentration disturbance is well solved, and the problem of air-fuel ratio disturbance is well solved when the engines working condition changes, meeting the pre-mixed coal-bed gas engine air-fuel ratio control requirements.

Abstract: This paper proposes an equalizer for serially connected Lithium-ion battery cells. The battery cell with the lowest state of charge (SOC) is charged by the equalizer during the process of charging and discharging, and the balancing current is constant and controllable. Three unbalanced lithium-ion battery cells in series are selected as the experimental object by this paper. The discharging current under a certain UDDS and 20A charging current are used to complete respectively one time balancing experiment of discharging and charging to the three lithium-ion battery cells. The validity of the balancing strategy is confirmed in this paper according to the experimental results.

Abstract: Fast and accurate measurement of interharmonic is a difficult problem. Fourier transform is widely used but it is not suitable for real-time application. Wavelet transform has better real-time performance, but its accuracy needs to be improved, so the paper studies the performance of wavelet transform for interharmonic measurement. By software simulation, it evaluates the impact of interharmonic frequency, sampling window and sampling frequency on the accuracy of interharmonic frequency measurement. Results show that wavelet method doesn’t require long sampling window or high sampling frequency, which makes it easy to be applied, and it can provide better measurement accuracy for high frequency interharmonics than low frequency interharmonics.