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| Real-Time Calculation Method of Converter Valve Current Based on Temporal Features of Valve-Side Current |
| Li Xiaohua1, Yin Shanshan2, Li Hao1, Cai Zexiang1 |
1. School of Electric Power South China University of Technology Guangzhou 510640 China;
2. College of Information Engineering Henan University of Science and Technology Luoyang 471023 China |
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Abstract As the essential element for electronic power conversion, the line-commutated converter is the core equipment in high-voltage direct current (HVDC) transmission systems. The safety of the converter equipment is critical to the stable operation of an AC/DC hybrid power grid after AC faults. When commutation failure occurs in the converter, the valve that experiences commutation failure is conducted for a long time with a high current, which may cause the valve to be damaged or burned, thereby exacerbating the security complexity between the converter and the grid. The valve current is a direct manifestation of the thyristor converter operating state. Recently, the existing studies on the converter thyristor valve current are mainly conducted through converter modeling, which involves the calculation of thyristor valve current. However, this method of valve current calculation must first assume the valve state. Nevertheless, especially during commutation failure and fault recovery, the assumed valve state may be wrong, resulting in incorrect valve currents. Therefore, a real-time calculation method of the valve current is proposed based on the temporal features of the converter terminal AC and DC currents. The protection and control capabilities of the equipment are optimized, and the reliability of the converter technology is improved by learning the accurate calculation of the thyristor valve current.
According to the circuit constraints of the converter topology, the vector sum of the three-phase AC current is equal to 0 under the condition that there is no fault inside the converter. The KCL equations of the five nodes in the converter topology are combined to establish the correlation equation between the converter terminal current and the thyristor valve current. In order to determine the current distribution, the valve state should be introduced based on the topological equation. Firstly, the polarity characteristics of the converter terminal AC current and temporal features between the valves are utilized to estimate the valve state. When the bypass group occurs, the AC and DC currents of the converter are isolated, and the polarity characteristics of the three-phase AC current cannot estimate the valve conducting state of the bypass pair. The valve conducting state of the bypass group is necessary to revise. Secondly, the bypass number of the converter is calculated based on the valve state, the parallel relationship between the bypass phases during multiphase bypasses is established, and the supplementary bypass loop equation is constructed. Finally, the supplementary bypass loop equation and the associated equation are combined to calculate the thyristor valve current. When the thyristor valve current is a negative value, the blocking state of the valve is corrected in real-time, and the valve current is recalculated.
The simulation results show that the calculated value of the thyristor valve current before the converter bypass equals the actual value. Scheme 1, based on revising the valve conducting state, can calculate the thyristor valve current in the bypass operating state of the converter. However, starting from the moment the converter exits the bypass operation, the thyristor valve current calculated by scheme 1 appears to be different from the actual value, indicating that the calculated value is incorrect. Scheme 1 addresses the conduction judgment of bypass operation after commutation failure, but the valve blocking state needs to be corrected after commutation failure. According to scheme 1, scheme 2, based on correcting the valve blocking state, addresses the correction of the valve blocking state after commutation failure by constraining the unidirectional conductance characteristics of the valve. Under different valve states and bypass groups, the proposed real-time calculation method can accurately calculate the valve current in the transient process of commutation failure and its fault recovery, which verifies the effectiveness and reliability of the valve current calculation method.
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Received: 08 January 2022
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2023, Vol. 38 
