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Jinan, China

Li C.-Y.,Shandong University | Li Q.-M.,Shandong University | Li Z.,Shandong University | Yao J.-X.,EPRI of Shandong | Liu M.,EPRI of Shandong
Dianli Xitong Baohu yu Kongzhi/Power System Protection and Control | Year: 2010

The transient characteristics of protective electromagnetic current transformers are pivotal factors to ensure the stability of power systems. Equivalent circuit models of electromagnetic type protective CTs are established with DC bias. Combined with electromagnetic equations, the co-impacts from both DC bias and remnant flux on the time-to-saturation and the transient factor of protective CTs are deduced with quantitatively numerical expressions being given. The analysis indicates that the DC bias current will accelerate saturation of the protective CTs up to several milliseconds, and the CTs will saturate more quickly while the DC current becomes larger. In the meanwhile, the transient factor will be enlarged by the co-impacts, which results in that the selected CT won't fulfill the system demands. Simulations are presented to verify the above analyzed results. The proposed research presents useful reference for proper selection of protective CTs as well as reliable setting of corresponding relay device in power systems.

Li Z.,Shandong University | Li Q.-M.,Shandong University | Li C.-Y.,Shandong University | Yao J.-X.,EPRI of Shandong | Liu M.,EPRI of Shandong
Dianli Xitong Baohu yu Kongzhi/Power System Protection and Control | Year: 2010

Combined with the undergoing ± 660 kV HVDC transmission line from Yinchuan to Qingdao, quantitative harmonic analysis of the No. 1 main transformer of Kuangzheng substation which is 30 km away from the Qingdao converter station is carried out based on an established nonlinear magnetic model by PSCAD/EMTDC. The results indicate that, both the odd and even harmonic components are increasing significantly under DC bias than that without DC bias, which thereby makes the power transformers become the severe harmonic sources. Related factors of DC bias impact on the power transformers are discussed, and the relationship between the injected neural DC currents and the even harmonic ratio is established, which presents further potential for developing inverse-current-injection based DC bias suppression scheme.

Li C.,Shandong Polytechnic University | Li C.,Shandong University | Li Q.,Shandong University | Li Z.,Shandong University | And 2 more authors.
Dianli Zidonghua Shebei/Electric Power Automation Equipment | Year: 2012

The single-pole-to-ground operation mode of UHV/HV DC transmission system during system commissioning or fault causes DC bias phenomenon in AC power grid, which will be strengthened under the condition of peninsular geology. According to the specific grid structure and parameters of Shandong peninsula, the DC bias suppression scheme based on the serial neutral-point capacitor of transformer is analyzed and the dual protection topology of paralleled lightning arrester and spark gap is proposed for optimization. The selection of relevant devices, such as neutral-point capacitor, bypass switch, electronic switch, current-limiting reactor, etc., and the cooperation between lightning arrester and spark gap are given. The simulative results indicate that, the proposed dual protection topology can effectively protect the DC bias suppression equipment during system fault.

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