Xiao Xiangning

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

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.

Super-capacitors energy storage system applied in the microgrid

In order to reduce the adverse effects of the distributed generation system to the main grid, a reasonable and effective solution is a systematic method, that is, microgrid. And the bus voltage of the micro grid will fluctuate when the main grid got failure or the micro grid is disconnected to the main grid. The paper proposed using super-capacitors energy storage system as part of the micro grids electrical energy storage to retrain the normal voltage level. It built the simulation models of the super-capacitors energy storage system on the PSCAD/EMTDC platform after analyzing the charge-discharge characteristics of the super-capacitor. And then it designed the control strategies as well. The correctness and the effectiveness of the designed models and the control strategies were verified in the end.

Study on the SVPWM algorithm of N-level inverter in the context of non-orthogonal coordinates

In this paper, the authors propose a new space vector pulse width modulation (SVPWM) algorithm based on non-orthogonal coordinates for N-level inverters. First, it is pointed out that classical aβ coordinates-based SVPWM has many shortcomings because of improper coordinate choice. Then, a non-orthogonal coordinates-based SVPWM is proposed to solve these problems. The proposed algorithm can easily identify which sector the reference space vector falls in and conduct simple operations to find the duty cycle of each vector. Finally, it is verified that the proposed SVPWM is actually a pulse—width modulation (PWM) technology based on line voltages.

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.

Simulation and study on mitigation measures of frequent subsynchronous oscillation with low amplitude at multi-power plants

Subsynchronous oscillation (SSO) with low amplitude that exceeds cumulative fatigue threshold of the generator shaft frequently could significantly reduce the shaft’s service life, which is a new SSO problem that emerges in recent years. According to the real recording oscillograph, the basic reason for frequently over-threshold SSO with low amplitude at multi-power plants was analyzed based on Hulunbuir League system. The sensitivities of the electrical damping to the main electrical parameters in the contributing loop of subsynchronous torsional interaction were calculated. Based on the sensitivities, a simulation method was presented, which was used to excite the same oscillation as the actual case by exerting disturbance on the firing angle. The limitation of wide-band and narrow-band supplementary subsynchronous damping controller (SSDC) for mitigating this kind of SSO was analyzed based on the electromagnetic transient simulation model of Hulunbuir League system. The difference of supplementary excitation damping controller (SEDC) and parallel-form FACTS connected to the generator terminal was compared from the aspects of response time and the ability of damping torque supplying. The analysis indicates that their response time is similar but FACTS has stronger ability of damping torque supplying than SEDC. Time-domain simulation method was used to compare the mitigation effects of SEDC, static var compensator (SVC) and static synchronous compensator (STATCOM). Considering the mitigation effect, the floor space limit of the power plant and so on, STATCOM was considered as the best mitigation measure. A control strategy of cascaded STATCOM for engineering application was presented and the capacity for SSO mitigation as well as output characteristics was analyzed. The analysis indicates that STATCOM using the proposed control strategy has better mitigation effect and output characteristics with smaller capacity.

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.

Design of Simulation Model of PV Cells Based on VSR PWM Rectifier Circuit

This paper designs the simulation model of the PV (photovoltaic) cells by using the mathematical model applying in engineering firstly. Then the output of the PV cells is simulated based on the PSCAD/EMTDC platform by using the single-phase VSR PWM rectifier circuit which uses the direct current control as well with the triangular wave current comparison as the control strategy. The results show that the model could simulate the output of the PV cells well and could also meet the V-I characteristics of the loads. This model will have good practicability for the further design of MPPT algorithm applying in the photovoltaic grid-connected system too.

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.