The Role of Oxygen Content in Electronic Structures of LaBa2Cu3Oy

The electronic structures of the LaBa2CU30ysuperconductor were calculated using the band-structure treatment based on the EHMO approach and the role of the oxygen content was investigated. The results show that the oxygen content has a great influence on the band structures and the densities of states near the Fermi level Er- The lower or higher oxygen content results in the suppression of the transition temperature Te, which is in qualitative agreement with the behaviour of Te given by experiment. In addition, the study on the electronic charge of copper shows that the oxidation state of Cu is not the predominant factor for the supercondictivity of LaBa2CU30yand may just be a result of charge balance

The Role of Oxygen Content in Electronic Structures of LaBa2Cu3Oy

The electronic structures of the LaBa2CU30ysuperconductor were calculated using the band-structure treatment based on the EHMO approach and the role of the oxygen content was investigated. The results show that the oxygen content has a great influence on the band structures and the densities of states near the Fermi level Er- The lower or higher oxygen content results in the suppression of the transition temperature Te, which is in qualitative agreement with the behaviour of Te given by experiment. In addition, the study on the electronic charge of copper shows that the oxidation state of Cu is not the predominant factor for the supercondictivity of LaBa2CU30yand may just be a result of charge balance

Interlink hybrid DC circuit breaker

—To protect HVDC grids from DC faults, the concept of a hybrid DC circuit breaker is widely accepted due to its low conduction losses and fast interruption speed. For a well-built DC grid, a massive number of hybrid DC circuit breakers have to be installed. This will lead to high capital costs. An interlink DC circuit breaker based on an idea of sharing main breaker branch between two circuit breakers is proposed to reduce the overall costs of circuit breakers in a DC grid. Comparing with existing hybrid DC circuit breakers, the interlink hybrid DC circuit breaker can achieve the same DC fault interruption capability with fewer components. Novel structures of main breaker branches are designed and their parameters are determined to make the interlink hybrid DC circuit breakers be capable for both unidirectional and bidirectional interruption on demand. For a unidirectional interlink hybrid DC circuit breaker, the size of MOVs is reduced by 50%. For a bidirectional interlink hybrid HVDC circuit breaker, the number of IGBTs and MOVs are reduced by 25%. The interlink hybrid DC breakers are verified and compared to the hybrid DC circuit breaker via a three-terminal HVDC grid in PSCAD/EMTDC

The protection and interconnection of HVDC grids

It is cost-effective to construct a High Voltage Direct Current (HVDC) grid via interconnecting existing HVDCs. The grid protection and the interconnection between HVDCs are studied in the thesis. The future HVDC grid will be protected via the hybrid HVDC circuit breakers due to their high interrupting speed and low conduction losses. The hybrid DC circuit breakers have to be installed at each line end to fulfil the requirement of the grid protection. The overall cost of the breakers used in a HVDC grid will be extremely high as each breaker contains a large number of semiconductor switches. The research question in this part is how to reduce the cost of the hybrid DC circuit breakers. The novel interlink hybrid HVDC circuit breakers based on the concept of sharing main breaker branch will be proposed to reduce the size of the main breaker branch. An alternative method through coordination of converters and hybrid HVDC circuits breakers will be proposed to reduce the current rating of the breaker. Most of the commissioned HVDC projects are point-to-point Line Commutated Converter (LCC) HVDCs. The interconnection of LCC-HVDCs will achieve the benefits of the grid operation, such as highly efficient and flexible power transportation, and high security of power transportation. However, benefits of the interconnection are hindered by the disadvantages of the LCC-HVDC. The first one is that a LCC-HVDC has to reverse its voltage polarity in order to reverse its power. The second one is that a LCC-HVDC is at risk of the commutation failure, which will cause a DC fault when a commutation failure occurs. The research question in this part is how to achieve the interconnection of LCC-HVDCs with the benefits of the grid operation. Interconnection of LCC-HVDCs with the capability of power reversal and commutation failure mitigation will be studied, and the corresponding controls for the interconnection system will be proposes

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