Systematic evaluation for multi-rate simulation of DC Grids

With wide applications of power electronic devices in modern power systems, simulation using traditional electromechanical and electromagnetic tools suffers low speed and imprecision. Multi-rate methods can enhance efficiency of simulation by decreasing the scale of systems in small time-steps. However, the existing traditional methods for multi-rate simulation suffer the problems of instability and simulation errors. These have hindered the application of multi-rate simulation in power industry. Therefore theoretical evaluation on different multi-rate simulation methods is crucial to understand the feasibility and limitation of the methods, and to contribute to overcome the drawbacks of the traditional methods. In this paper, the multi-rate simulation performance based on two traditional technologies and a Modified Thevenin Interface are evaluated to provide an overall feasibility of multi-rate algorithms in the power simulation. The Modified Thevenin Interface is proposed to overcome the drawbacks in synchronization. Three theorems are proposed and proved for theoretically analyzing the stability of the simulation methods. Error analyses of the multi-rate methods are performed to identify the relationships between errors and simulation conditions. Besides, the accuracy and efficiency performance in a practical project of VSC-MTDC shows the feasibility and necessity by using multi-rate simulation. Through the theoretical analysis, the issues of stability and accuracy of multi-rate simulation for the DC grids have been better understood, based on which an improved simulation algorithm has been proposed to overcome these issues. Long-term system dynamics of large-scale systems containing DC grids and fast transients of HVDC converters can be investigated simultaneously with high speed and sufficient accuracy

Improved ADC model of voltage-source converters in DC grids

Due to a large number of converters in dc grids, the simulation speed using traditional electromagnetic simulation tools becomes very slow. An associated discrete circuit (ADC) switch model can improve the simulation efficiency greatly by avoiding the modification of system matrix during switching. However, existing ADC switches induce virtual power losses due to simulation errors during switching transients. The mechanism of the virtual power loss is analyzed, and a power loss model is established. An improved ADC switch model is designed by adding compensation sources to mitigate the simulation errors. Theoretical analyses are carried out to prove this improvement. A fast algorithm to obtain the initial values of the compensation sources is proposed by utilizing the complementary operation of IGBTs. The improved ADC switch provides fast simulation speed and high accuracy. The modeling is particularly suitable for investigating long term system dynamics of dc grids with multiple converters and fast converter transients at the same time

Investigation and analysis of psychological stress and professional identity of nursing students during COVID-19 pandemic

426-432Certain diseases or infections affect large number of people in short span of time. A local endemic disease can outbreak into an epidemic affecting the whole population or region which at times extend to other countries and continents and become pandemic. Pandemics results in loss of life as well as economy. Pooled efforts and resources, effective sharing of data, parallel multiple approaches as well as physical and mental state of front line staff influence management of pandemics. The coronavirus disease COVID-19 caused by SARS-CoV-2 started in December 2019 from Wuhan in China, is now a worldwide public health emergency affecting millions of people. It affects many frontline healthcare workers too. Here, we studied psychological stress and professional identity of nursing students for possible correlations, if any, and analyze influencing factors. We used purposive sampling technique with 415 nursing students in Nanjing, China through a general information questionnaire, perceived stress scale and nursing professional identity questionnaire. Students' origin, monthly living expenses and their knowledge on epidemic prevention and treatment have shown a significant impact on their psychological stress (P <0.01). Similarly, students' gender, origin, clinical practices and knowledge of prevention and treatment, and whether they actively learn such knowledge impact significantly on their professional identity (P <0.01). The overall score of psychological stress was (24.47±7.35) and professional identity had 72.47±8.07. The stress condition exhibited negative correlation with the degree of professional identity (P <0.01，r = -0.457). Increased psychological stress, had lower sense of professional identity. Overall, analysis of data on perceived stress and professional identity pandemic suggests that stress levels are inversely proportional to knowledge in effective ways of handling the pandemic. Students with clinical practice fared better in terms of professional identity. The study suggests nursing students to stay focused on studies, clinical practice and counselling, if required

An aggregated dynamic model of Photovoltaic units for large voltage disturbances

Many Photovoltaic (PV) units connected to the low-voltage level greatly impact the distribution network’s dynamic characteristics. However, the dynamic characteristics of PV units under large disturbances are widely different, which leads to the model being unable to effectively describe the dynamic performance of aggregated PV units in the low-voltage transient stability analysis. To overcome this issue, an aggregated dynamic model of PV units is proposed in this paper. First, a general dynamic model of a PV unit is proposed, focusing on the control triggered by voltage disturbances, and multiple PV units is established based on ADPSS. The simulation results show that the major factor which influences the transient characteristics of multiple distributed PV units is its Low Voltage Ride-Through (LVRT) control. Then, an aggregated dynamic model that can describe the multiple transient characteristics is proposed. The simulation verifies that the proposed model can effectively represent aggregated transient characteristics

Power flow and power reduction control using variable frequency of offshore AC grids

In the “integrated strategy” for connecting offshore wind farms to onshore grids, an offshore AC grid is formed by interconnecting offshore wind farms and point-to-point VSC-HVDC links using an AC network. The advantage of this strategy is that in the case of a DC fault, AC circuit breakers are used to isolate the faulted HVDC link without the need for DC circuit breakers. This paper presents a control system that regulates power sharing between the HVDC links by varying the operating frequency of the offshore AC grid. Primary and secondary power regulation controllers are designed to achieve automatic power flow coordination between HVDC links. Increase in the offshore AC grid frequency during onshore AC grid fault or DC fault signals power flow imbalance. This signal is used to suppress the transient DC over-voltage through the coordination control between the wind farm side converters of HVDC links and the power reduction control of wind turbines. Importantly, because the inputs of the controllers are from local frequency signals only, fast communication is not required under the proposed control strategy. Simulation results are presented to verify the functioning of the proposed control strategy and illustrate variable frequency operation of an offshore four-node AC grid