Advanced Materials Research Vols. 732-733

Abstract: In order to further improve the precision of harmonics measurement, the paper provides a new interpolation fast Fourier transform (FFT) algorithm based on RifeVincent (I) window. By using the cubic spline function to approach the ninth multinomial of frequency modification coefficient and the function of harmonic amplitude correction of the interpolation FFT algorithm based on RifeVincent (I) window, the computing speed has been improved. It can solve the contradiction between calculation accuracy and computational speed of windowed and interpolated FFT algorithm. The simulative results show that RifeVincent (I) window interpolation algorithm by using cubic spline function has the amplitude error less than 0.00001 %, the frequency error less than 0.000001 Hz, and the phase error less than 0.01%.

Abstract: Developing the Zhengzhou rail transit as a new transportation plays an important role in easing traffic congestion and promoting the economic development. The principle, connection mode and harmonic generation mechanism of the rectifier unit of Zhengzhou rail transit is analysed in theoretical. 12-pulse and 24-pulse rectifier unit model is simulated in MATLAB/Simulink environment. The simulation result shows that the 24-pulse rectifier unit can reduce the harmonic current injected to the power system greatly, and abate the effects on the utility grid compared with the 12-pulse rectifier unit. The 24-pulse rectifier can smoothen the output voltage, reduce the harmonic content and improve the quality of DC power supply. So it can obviously reduce the adverse effects on public power grid.

Abstract: A transformer model was built using PSCAD. The generation mechanism, waveform characteristics and influence factors of inrush current were simulated and analyzed. Combined with transformer differential protection, this paper discussed the conventional methods to identify inrush current and the operation logic to prevent mal-operation caused by inrush current. The typical transformer differential protection operating criteria were also simulated under different fault conditions. The results show that digital simulation can properly present inrush current waveform characteristics, different kinds of transformer fault status and inrush current influence on differential protection.

Abstract: Emergency HVDC power support (EDCPS) can be used for improving the transient stability of AC/DC power systems, but the effect is directly determined by the quantity, the starting time, the speed and other factors of the power support. In this article, the dynamic characteristics of AC/DC power systems after a fault are analyzed, generalized dynamical models of the AC/DC power system are established, the relationship between the duration and the quantity of the EDCPS and the dynamic characteristics of the system are established based on the impulse theorem, then the EDCPS strategy besed on the principles of the generalized dynamics is proposed. At last, it is verified by examples of simulation that the strategy can effectively improve the stability of the AC/DC power systems after a fault.

Abstract: Geomagnetically induced currents (GIC) are very harmful to the power system and caused by the changes of geomagnetic field. The changing magnetic fields originate from the interplanetary space. Therefore, recognizing the relationship between GIC and interplanetary disturbances is significant for the prediction of GIC. In this paper, we collected 35 GIC events during 1999-2005 and the corresponding interplanetary data observed by ACE satellite to analyze the correlations of various interplanetary parameters with GIC. The results show that there is positive correlation between GIC amplitudes and interplanetary parameters. As a single factor, solar wind dynamic pressure (Pk) correlates best with GIC, while interplanetary electric field (Ey) and interplanetary magnetic field (|B|) also correlate well with GIC. In addition, we investigated the relationships between interplanetary parameters and GIC amplitudes for different intensities (GIC>10A, GIC>20A). Comprehensive analysis shows that Pk and Ey are the most closely parameters related to GIC.

Abstract: Processing the parameters overly simplistic will leave space for overestimating or underestimating artificially when we assess the assets of medium and small hydropower stations using present earning value method. And absolute assumptions often affect the adequacy of the assessment results, also the persuasive and credibility. Monte Carlo simulation can determine the wave of parameters in present earning value method, so as to improve the persuasion and credibility of assessment results and reduce the personnel control.

Abstract: The noise reduction and ventilation cooling is contradictory each other for urban indoor substation. Our efforts were focused on the problems of the excessive noise and poor ventilation of indoor substation in this paper, the existing shortages of the primary ventilation cooling program and the noise source characteristics of the indoor substation were analyzed, and the renovation of noise reduction and ventilation cooling was proposed. The measurement data of before and after renovation demonstrated that this renovation on noise reduction and ventilation cooling has a visible effect in meeting the national standard for noise level of the substation surrounding as well as in improving the cooling effect significantly.

Abstract: Dynamic voltage instability is one of the new problems with the fast development of wind farm clusters. This text is about the improvement of voltage stability of weak grid connected with wind turbine generators (WTGs). It focuses on the control strategy and mode of WTGs and dynamic reactive compensators during disturbance. Firstly, in order to propel WTGs outputting reactive power, the active control mode regarding both doubly fed induction generators (DFIGs) and permanent magnetic synchronous generators (PMSGs) is given. Secondly, the method of installing SVC at the terminal of part of WTGs is proposed. The selection of WTGs to install SVC is based on the sensitivity analysis. Thirdly, a coordinating method of various reactive sources is discussed. Finally, the comprehensive control logic under time series is listed. The simulation validates the methods above.

Abstract: In this paper, a new approach to AC/DC power flow is proposed on the basis of uniform methods and control logic conversion of operation state. This method can take the action of the converter transformer tap into consideration and effectively deal with the conversion of HVDC system operation modes in power flow calculation. According to judgment logic of different operation modes, independent subroutines which are programmed in advance are employed to complete the AC/DC power flow calculation. The programming and simulation results demonstrate the approach is clear and easy to program and the transformer tap adjustment and the transformation of operation mode in HVDC system can be solved effectively in power flow calculation.

Abstract: A new topology for simultaneous AC-DC distribution along the same line is proposed in this paper to integrate DC power with the existing alternating current (AC) distribution, which incorporates a zigzag transformer and a boost chopper. The zigzag transformer is utilized to achieve either coupling or decoupling of the AC and DC power without sensible iron-core saturation due to voltage canceling out on one phase. The boost chopper is used to achieve DC power flow and keep the DC ratings at each node. A simulation model of the novel topology is established with PSCAD software, and detailed comparison verifies that the new topology with a zigzag transformer is preferable to that with a common transformer for simultaneous AC-DC distribution scheme, indicating a promising future of the proposed technology.