The interruption of high direct currents, required in fusion experiments both for protection and energy transfer systems, is a challenging task to be managed safely for the whole plant. Depending on the technology of the circuit breakers the resulting current derivative may be high, leading to dangerous transient overvoltages causing stress on insulations of the connected devices. This aspect has been analyzed for JT-60SA Quench Protection Circuits (QPC), the system devoted to the protection of poloidal (PF) and toroidal (TF) field superconducting coils. The QPC adopts edge technology solutions for current interruption: a Hybrid mechanical-static Circuit Breaker (HCB) as main circuit breaker in series with a PyroBreaker (PB) as backup protection; both impose high current derivative and relevant fast transient voltages when operated. Snubbers or clamp networks can be provided in parallel to the breakers to smooth the voltage waveform; their effectiveness depends not only on their design but also on their location in the circuit and on the stray impedance of their connections. Dedicated clamp networks for the HCB of PF and TF QPC have been designed and tested during the qualification of the QPC prototypes. On the contrary, it was preferred not to apply any component in parallel to the PB, the ultimate protection of the coils, to avoid reducing its reliability. For PB a different approach has been worked out, based on the optimization of the layout of the QPC connections. Analyses have been performed to reproduce the transient voltage across the TF coils at the PB intervention so as to highlight the impact of different busbar routes on the surge voltage. The results indeed showed a variation of the peak voltage in between ± 30% of the maximum allowed value. The reliability of the results of such analyses is strongly affected by the correctness of the QPC model, which has been validated against experimental results of the test campaigns on QPC prototypes and by the accuracy of the estimations of the busbars' stray impedances as in the final installation arrangement. The paper will present the analyses carried out and will discuss the results.
Analyses of the impact of connections layout on the coil transient voltage at the Quench Protection Circuit intervention in JT-60SA
Gaio E;
2014
Abstract
The interruption of high direct currents, required in fusion experiments both for protection and energy transfer systems, is a challenging task to be managed safely for the whole plant. Depending on the technology of the circuit breakers the resulting current derivative may be high, leading to dangerous transient overvoltages causing stress on insulations of the connected devices. This aspect has been analyzed for JT-60SA Quench Protection Circuits (QPC), the system devoted to the protection of poloidal (PF) and toroidal (TF) field superconducting coils. The QPC adopts edge technology solutions for current interruption: a Hybrid mechanical-static Circuit Breaker (HCB) as main circuit breaker in series with a PyroBreaker (PB) as backup protection; both impose high current derivative and relevant fast transient voltages when operated. Snubbers or clamp networks can be provided in parallel to the breakers to smooth the voltage waveform; their effectiveness depends not only on their design but also on their location in the circuit and on the stray impedance of their connections. Dedicated clamp networks for the HCB of PF and TF QPC have been designed and tested during the qualification of the QPC prototypes. On the contrary, it was preferred not to apply any component in parallel to the PB, the ultimate protection of the coils, to avoid reducing its reliability. For PB a different approach has been worked out, based on the optimization of the layout of the QPC connections. Analyses have been performed to reproduce the transient voltage across the TF coils at the PB intervention so as to highlight the impact of different busbar routes on the surge voltage. The results indeed showed a variation of the peak voltage in between ± 30% of the maximum allowed value. The reliability of the results of such analyses is strongly affected by the correctness of the QPC model, which has been validated against experimental results of the test campaigns on QPC prototypes and by the accuracy of the estimations of the busbars' stray impedances as in the final installation arrangement. The paper will present the analyses carried out and will discuss the results.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
