Research On The Reliability Evaluation Method For AC/DC Power Grid

As the emergence of problems such as large-scale renewable energy and DC loads connected to power grid, the advantage of DC transmission system is more and more prominent. The new pattern of the power transmission system from west to east, contribute to the development of AC/DC parallel systems and AC/DC hybrid systems. According to the development of AC/DC power grid, the reliability problems caused more and more attention. At present, the reliability evaluation of AC grid and two-terminal DC transmission system is increasingly mature, and the reliability evaluation of DC grid and AC/DC hybrid grid has just started.In this paper, on the basis of comparative analyzing reliability evaluation methods, the FD method and minimum cut sets method are selected to comprehensive research the reliability evaluation model and evaluation method of DC grid and AC/DC hybrid grid. First, this paper proposes a suitable equivalent model for DC grid reliability assessment. This model avoids the dimension disaster and reduces the amount of calculation; can also calculate reliability indexes such as failure frequency, etc. A simple DC grid reliability calculation results prove the correctness and effectiveness of this model. The calculation of weakness in system and parameter sensitivity analysis show that the submarine cables and sea inverters are weak links affected the reliability of system. Based on FD method and the minimum cut sets method, this paper proposes a suitable reliability evaluation method for AC/DC hybrid gird. Models for evaluating DC system and AC system are set up respectively. The reliability indexes such as loss of load probability, loss of load frequency, loss of load duration, and power less than expectation of each node can be calculated. Through the analysis of the load point reliability indexes, the weak link of grid can be found. To contain two-terminal HVDC system and multi-terminal HVDC system as two examples, the calculation results can reflect the reliability level of each node in grid, which is good enough to prove the correctness and feasibility of the proposed method.