Jinwen Sun

Study on the influence of large-scaled electric vehicle dispersed charging on transient voltage stability of distribution systems

As large-scaled electric vehicles (EVs) connecting to power distribution system via charging devices, the load of power system will get heaver in the foreseeable future. This will lead to an increasing risk for the security and stability of operation. Studying the quantitative influence that EV charging loads bring to the transient voltage stability of distribution system is of vital significance to analyze the security and stability status of power system after large-scaled EV loads are connected. Considering the torque-slip property of induction motor load, an index represented by relative value of slip considering induction motor loads is proposed in this paper, aiming at indicating the margin of transient voltage stability of distribution network. Furthermore, simulation analysis and calculations are carried out to study the transient voltage instability condition, and the focus of this paper is on the dispersed charging scenario of large-scaled EVs. The result shows that the index proposed is capable of reflecting the profound effect of large-scaled EV loads on the transient voltage security of distribution network.

Field studies on the Inrush Current for 33/11kV transformer and the effect on voltage interruption during switching operation of 10MW wind farm

The Inrush Current for transformer switching operation study is an electromagnetic transient analysis (EMT) and are performed in order to investigate the network response after the energized the two 33/11 kV, 7.75MVA transformers in the Layafa S/S. The Inrush current associated with transformer energizing can cause some reactions that may adversely affect other loads in the power networks system, for instance voltage sags or over-voltage phenomena which could cause nuisance tripping for protective devices or loads. Protection devices may mis-interpret these events as fault currents, if the protection devices are not properly coordinated well. In this paper, the inrush current analysis was performed for the generators-transformers of the wind farm switching operation. In the study analysis, the 33/11 kV transformers will not be energized at the same time. Each transformer will be energized at no load condition, with the tap changer set to the maximum step of the HV side (higher voltage) in order to minimize the inrush current values. The simulation analysis was performed which the aid of DigSilent and EMT computer simulation to establish this study while considering the minimum short circuit power in the grid and the Wind Farm disconnection from the system. This paper is based on the field results test of the newly installed wind farm generator-transformer in relation to network interruption on the 33 kV and 11 kV systems during the switching on operation especially transient voltage phenomena.

Last breaker protection strategy based on arrester energy consumption characteristics in light loaded system

Original last breaker protection based on arrester energy consumption characteristics might malfunction in light loaded power system. In this paper, we analyzes the relationship between the high side inlet voltage of converter transformer and the AC load, pointing out that the main reason of the arrester based last breaker protection maloperation is that the original protection strategy cannot distinguish light load state from isolation state of the AC line. This paper adds outlet head end voltage and output power to the original criterion, designs an improved one so that it can correctly distinguish what state the AC line and send the right signals for different action status. Simulate and analyze the act of original criterion and improved one in light loaded power system by using the simulation model which is built based on the actual operating parameters of Yunnan-Guangdong UHVDC project, validating the improvement of novel criterion.

Impacts of electric vehicles on the transient voltage stability of distribution network and the study of improvement measures

Electric vehicles connected directly to the distribution network are expected to increase the base electric loads and reduce the stability of a power system. Meanwhile, equipped with bidirectional battery chargers, electric vehicles can work in V2G mode, as spinning reserve, and enhance grid stability. This paper discusses the charger technologies, requirements and proposes transient voltage stability margin index considering the impacts of dynamic loads on the transient voltage stability grid. Based on the index, the paper evaluates and compares the impacts of electric vehicles charging loads, static loads and combined loads on the transient voltage stability of distribution network and proposes the improvement of considering the V2G mode for the transient voltage instability situation of electric vehicle charging. The model of bidirectional battery chargers and IEEE16 bus test systems are performed by MATLAB & SimPowerSystems, and the availability of the index is confirmed.

Field studies for transient stability in continuous operation and contingency condition during 3 − Φ short circuit at PCC for grid connected 10MW Kastina wind farm

As a result of worlds energy resources crisis and environment pollution increasingly aggravating, distributed generation (DG) based on renewable energy has become a developmental trend for electric power industry in 21 century. However, DGs are usually affected by natural conditions being not able to output power continuously and steadily, so when large scale wind turbine generators are incorporated into the grid there is likely attendance impact it would bring on electric power system stability. In this investigation, the wind farm connection was done at 33/1 1kV level at Kastina substation located at Northern part of Nigeria. This paper is to establish voltage instability and frequency fluctuation that might be associated with the connection of the wind farm to the existing grid. The DigSILENT Power Factory software was used for modeling and simulation analysis of the studies and the results shows that when wind farm is connected to the grid, large clearance time is required for the fault to be cleared for voltage and frequency to become normalize again as compare with normal grid system. The studies has verified transient stability associated problem due to voltage and frequency stability variation phenomenon during the disconection of both windfarm and the load from the existing grid and is easier to predict in future the setting of protective devices and clearance time.