Xu Yonghai

Study on Distribution Reliability Considering Voltage Sags and Acceptable Indices

In modern power systems, more sensitive loads appear. The impact of voltage sags on customers cannot be ignored. With the opening of electricity markets, it is necessary to modify and supplement the reliability indices to consider the influences of voltage sags. In this paper, three supplementary indices are proposed, that is: 1) event time; 2) sag energy; and 3) cost of events. Furthermore, we try to redefine the traditional reliability indices on service. These indices may be useful today and in the future as well. Computer simulation results show that voltage sags do affect reliability indices, which change the value of the third digit of the decimal part of reliability indices. The higher the reliability level is, the larger the impact of sags on reliability is

Analysis of adverse effects on the public power grid brought by traction power-supply system

With the rapidly developing of electrified railways, the adverse effects on the public power grid brought by traction power-supply system have been increasingly taken into account by railway and electricity department. In order to analyze the harmonics influence caused by traction power-supply system, the models based on PSCAD/EMTDC of typical traction power-supply system and public power grid have been established. Impacts of the harmonics and negative sequence under different operating modes and different wiring traction transformers have been detailed analysis by the model based on PSCAD. The comprehensive performance of Scott connection traction transformer is very well. The total harmonic distortions and the non-equilibriums among three phase voltages are less than other three traction transformers. The dynamic process during electric locomotives launch into or drop out has been simulated. The effects on the public power grid caused by traction load have been analyzed. The result of simulation analysis is credible and valuable in practical project.

Partial Least-Squares Regression Based Harmonic Emission Level Assessing at the Point of Common Coupling

This paper proposes a new method, called PLSR (partial least-squares regression), which is used for assessing harmonic impedance and the harmonic emission level at the point of common coupling (PCC). Utilizing the signals of harmonic voltage and harmonic current measured synchronously at PCC, regression coefficients are worked out through partial least-squares algorithm. By separating and filtering the measured data, the comprehensive variables that can explain dependent variables most effectively are distilled, and then the useful information and noise are identified from the measured data. Therefore, the disadvantage of variable dependence in establishing of the system model is overcome. After that, the supply harmonic impedance is got, consequently the harmonic emission level of customer is calculated. The simulation analysis and the analytic result of practical engineering road-testing data indicate that the proposed method is valid and feasible.

Simulation and analysis of voltage sag mitigation using active series voltage injection

The analyses of voltage sag and its mitigation techniques have received a lot of attention in recent years. Through the fictitious three-phase voltages and the voltage conversion, a new method that can be used to instantaneously quantify the magnitude, duration and phase-angle jump of voltage sag is described in this paper. Considering the actual power quality disturbances, a low pass filter is used to obtain the RMS value of the voltage. The deficiencies of the classical RMS computations for classifying voltage sags are overcome with the proposed method. The method is efficient for the evaluation of voltage sags and the injection for correcting voltage sags in real time on series compensation devices. Taking into account the actual disturbances and the injection capability, the control strategy used in series device is given. Finally, simulation results ate presented to demonstrate the validity of the proposed method.

Study on the simplified algorithm of space vector PWM

This paper, in accordance with the deficiency that the space vector PWM is complicated in calculation and difficult in engineering application, conducts detailed analyses of the classical solution algorithm for it, and proposes a simplified algorithm based on this. This algorithm simplifies the solution of the space vector PWM to the least degree; it can judge which sector the reference space vector V/sub r/ is located in very conveniently, and, based on this, find the solutions of the space vector PWM algorithm very easily.

Control strategy of PV inverter under unbalanced grid voltage sag

The presence of unbalanced voltage sag in grid actual operation gives rise to the output power fluctuation of PV inverter, the increasing of output current harmonics and unbalanced current. The calculation of the output current reference is a key technology in the control of PV inverter and has important influence on the output of the PV inverter under unbalanced grid voltage. Base on the instantaneous power theory, this paper proposed a calculation method of PV inverter current reference under unbalanced voltage sag. The proposed method will not bring harmonic current problems, with the benefits of selectively elimination of active power fluctuations, reactive power fluctuations or negative sequence component. In addition, the inverter output active and reactive power fluctuation can be flexibly adjusted by changing the value of the variable given in the proposed calculation method. Theoretical analysis and simulation results verify the validity of the proposed calculation method of current reference.

A new analysis method for compensation strategy of DVR and minimum energy control

The compensation strategy of the dynamic voltage restorer (DVR) has significant impact on the compensation result. A new analysis method which is based on pre-sag load voltage is proposed in the paper. The new method chooses the pre-sag load voltage phasor as the reference phasor and put the centre of the limit compensation voltage circle at the terminal of the system voltage phasor. It not only has definite physical concept but also is convenient to determine the compensation range. More importantly, it could be applied to unbalanced load. The basic theory and process of the new method was explained and compensation strategy of the three-phase DVR was analyzed. Using the new method, the minimum energy compensation strategy was realized through the simulation when unbalanced voltage sag happened in the case of the unbalanced load. The conditions for zero active power compensation was also analyzed which could prolong the compensation time of DVR. The simulation result shows the correctness of the theoretical analysis.

Study of Nonlinear Model of Shunt Reactor in 1000kV AC Transmission System

Modeling of accurate nonlinear shunt reactor is basis of harmonic analysis of 1000kV AC transmission system. Based on the electromagnetic relations of single-phase shunt reactor and the effect of iron core nonlinearity, a new model of single-phase shunt reactor used for harmonic analysis and computer simulation is presented. It presented shunt reactor by a simple equivalent circuit consisting of an impedance representing the leakage flux losses in series with a parallel combination of a nonlinear inductance (accounting for the saturation of the iron core) and a nonlinear resistance (accounting for the iron loss).The nonlinear impedance is calculated by the and curves which had been used for modeling of transformer excitation impedance. The result of simulation in PSCAD/EMTDC using the parameters of shunt reactor in Chinas UHV AC Pilot Project have proved that this model accurately reflect the nonlinear characteristics of shunt reactor, which is the prerequisite to complete the simulation model of UHV AC transmission system and analyze the harmonic characteristic of the system.

Experimental study and comparison of hybrid active filter with parallel active filter

The characteristics of the hybrid active filter, in which the L-C circuit is connected in series to an active filter, are analyzed. The VA rating of high switching frequency converter is notably reduced because of the L-C circuit used in the proposed hybrid active filter. An analytical method about the DC capacitor voltage, converter-rating etc. of hybrid active filter and pure parallel active power filter is presented in this paper, which shows the prominent advantages of hybrid active filter. The simulation results of the two kinds active filter are coincident with the theoretical analysis. Finally, results from a 50 kVA and a 100 kVA hardware prototype are presented to verify the prominent advantages of hybrid active filter and its good compensation characteristics.

Research of method for voltage sag source detection in power distribution network

Voltage sag is one of the most concerned power quality disturbances. The detection of voltage sag source is necessary for power quality troubleshooting, diagnosis, and mitigation strategy development. Moreover, the disputes about the major responsible party can be resolved fairly. This paper proposes an improved distance impedance method for voltage sag detection. By judging the rate of change of magnitude and angle of distance impedance before and after the sag event, the location of voltage sag source could be correctly determined in power distribution network.