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| Switching Transient Characteristics Analysis and Transition Resistance Selection Method for On-Load Tap-Changer of Converter Transformer |
| Chai Jianlong1, Li Xuan2, Yang Ming1, Sima Wenxia1, Liu Hui3, Li Gang4 |
1. State Key Laboratory of Power Transmission Equipment Technology Chongqing University Chongqing 400044 China;
2. State Grid Corporation of China Beijing 100031 China;
3. State Grid Lishui Power Supply Company Lishui 323000 China;
4. China Electric Power Research Institute Beijing 100192 China |
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Abstract The vacuum on-load tap-changer (OLTC) enables continuous voltage regulation in converter transformers. In HVDC systems with a high proportion of power electronic devices, the interaction of multiple factors—such as trigger angle, commutation angle, and power-factor angle—generates substantial harmonics, leading to complex transient behavior during vacuum OLTC operation. This results in high rates of change of breaking current and recovery voltage, complicating the switching process. Existing research lacks a systematic analysis of the transient characteristics during vacuum OLTC switching. Moreover, the appropriate selection of transition resistance is essential for reliable OLTC operation. The selection methods for transition resistance in converter transformer OLTCs follow traditional AC approaches, neglecting the influence of resistance on high switching electrical stress.
In this work, the coupling relationship among trigger angle, commutation angle, and power factor angle was investigated, and their collective impact on the transient characteristics of vacuum OLTC switching was revealed. First, based on an analysis method for electrical stresses during the switching process of vacuum on-load tap-changer in converter transformer, the load current of the converter transformer was accurately characterized. A vacuum OLTC with isolated contacts and a “three vacuum tubes, double resistance” configuration was used as the research object, and analytical expressions for breaking characteristic parameters—such as breaking current, recovery voltage, and their rates of change—were derived. Second, a transition resistance selection method tailored for converter transformer vacuum OLTCs was proposed, which specifically considered the influence of the transition resistance on the high rates of change of breaking current and recovery voltage. Based on these analytical expressions, a recommended range for transition resistance was determined, taking the rated step power and short-term emergency load current limit of the converter transformer considered as primary constraints. The transition resistance that minimized the total switching capacity was identified by calculating the switching capacity of main and transition contacts. Under the principle of balanced contact erosion loss, matching resistance values were obtained for different contact materials, allowing flexible selection according to engineering requirements. Finally, the selected resistance was validated using an electrical stress rate-of-change verification method.
Taking an actual ±800 kV UHVDC transmission project as an example, the variation patterns of OLTC breaking characteristics are quantitatively analyzed with trigger angle, commutation angle, and power-factor angle as variables. The coupling effects among these parameters on switching transients are further examined. Results indicate that the trigger angle α and commutation angle μ influence the transient characteristics through both harmonic content in the load current and the power-factor angle. The transition resistance is selected and verified accordingly. To ensure OLTC reliability, the recommended transition resistance range is 2.063 Ω<R<5.835 Ω. When minimizing total switching capacity, R=3.369 Ω is suitable; for E-Cu and Cu-W contact materials, the matching values are 2.895 Ω and 2.917 Ω, respectively. The proposed method offers broad applicability. By solving analytical expressions for OLTC breaking characteristic parameters and selecting the corresponding parameters according to different HVDC systems, quantitative analysis of switching transients and transition resistance selection can be performed for any topology.
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Received: 18 February 2025
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2026, Vol. 41 
