Study Of Harmonic Characteristics In Ningdong-Shandong±660KV HVDC

Ningdong-Shandong±660kV HVDC transmission demonstration project is the first DC transmission project of±660kV voltage grade all over the world. It could bring into play the coal and power resources advantage of the west and the economic advantage of the east coasts, achieve the energy saving and mitigation and promote the strategy of harmonious society and sustainable development. So it has important meaning and demonstrative effect on technique innovation. In Ningdong-Shandong±660kV HVDC system, due to nonlinear characteristics of converters, lots of harmonics is produced in the process of rectification and converter which could cause serious harmonic pollution increasingly and much harm to power system itself.This paper is based on Equipment parameters provided by Ningdong-Shandong±660kV HVDC project manufacturers, working summary of transmission project and the previous research and design results of each voltage grade. The simulation model of Ningdong-Shandong±660kV HVDC system is built using PSCAD/EMTDC software. It researches harmonics characteristics of±660kV HVDC system, analyses the characteristics of every time harmonic in various operation modes and studies the effect of harmonics to transformers protection.This paper researches the harmonic characteristics of AC bus current in the rectifier and converter stations of±660kV HVDC system. Then it simulates and calculates characteristic harmonics of both unipolar and bipolar AC bus current and DC voltage in the rectifier and converter stations. It draws to conclusions that characteristic harmonics of±660kV HVDC system AC bus give priority to12k±1time harmonics, harmonics distortion rate of the bus not adding filters is above4%and brings serious influence to the AC system of the rectifier and converter station. The calculation and simulation results are mainly about12k time characteristic harmonics and the voltage ripple factor is in the specified scope. So harmonics have no influence on the DC lines. The calculation results conform to simulation results during both unipolar and bipolar runtime, so the simulation model can be used for harmonics analysis of±660kV HVDC transmission system.By the simulation of harmonic characteristics in different operation modes of Ningdong-Shandong±660kV HVDC, such as dropping-voltage operation mode, bipolar voltage asymmetric operation mode and so on, this paper analyses the ripple factor of DC voltage and harmonic distortion rate of AC current. When the system is running in rated voltage mode, the amplitude of12times characteristic harmonic is the most. When the system is running in80%and70%dropping-voltage operation mode, The harmonic characteristics of DC voltage and AC current of the rectifier side is similar to that of inverter side. The characteristic harmonics’amplitude of DC voltage in both sides is more than the normal operation model, and amp-litude of24th characteristic harmonic outweighes other characteristic harmonics; the different polar show theirself dc voltage harmonic characteristics. Because the bus current harmonic distortion rate in the bipolar voltage asymmetric operation mode is obviously less than that in the dropping-voltage operation mode, this paper recommends the bipolar voltage asymmetric operation mode instead of the bipolar dropping-voltage operation mode.Combined with Ningdong-Shandong±660kV HVDC system relay protection configuration, this paper establishes simulation models of converter transformers relay protection. Under steady-state operation conditions, every time characteristic harmonic is filtered by filters, so the influence to the relay protection can be ignored. When faults happen in the area of converter transformers of Ningdong-Shandong±660kV HVDC systems, the maximum transient current of converter transformers in Qingdao station can be up to250kA. It also causes serious waveform distortion of three-phase current of transformers, the increase of harmonic ratio and large excitation inrush current. The second, third and fifth harmonic content is too high which could cause the atresia refusing action of converter transformers differential protection based on, so self-adaptive relay protection should be adopted to regulate the setting value of converter transformers differential protection.