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This article describes design, operation and experimental testing of a mechanical DC CB (Circuit Breaker) with parallel capacitors. The topology resembles hybrid DC CB but there are possible advantages in the costs since the main semiconductor valve is replaced with capacitors, and in performance since this breaker inserts counter voltage earlier. A detailed PSCAD model is employed to support DC CB design and to analyse operating principles. A 5 kV, 2 kA hardware demonstrator with 1.5 ms disconnector opening time is developed in the university laboratory. The test results demonstrate successful breaking of DC currents, with the measured time for insertion of capacitor voltage of around 290 µs. Further experimental analysis evaluates stresses on the key components, optimal timing for opening of LCS (Load Commutation Switch) and the margins for successful current interruption.

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This paper presents a comprehensive analytical eigenvalue stability study of subsynchronous torsional interactions between the 1GW Eleclink MMC HVDC and the 1.12GW Gravelines turbine-generator in north France. It shows that when the generator and HVDC have similar power rating and the generator has low frequency torsional mode, the level of adverse interactions can be of significance. The worst case-scenario is when the generator and HVDC deliver rated power from the French to the English grid and the French grid is weak, and in extreme the HVDC could even destabilize the dominant 6.3 Hz mode. The results also show a noticeable effect of PLL, namely, increasing the gains can improve the stability margins. Participation factors are also analyzed for the 6.3 Hz mode indicating that the main cause of the torsional interactions is the HVDC power control loop.The scenario of the HVDC importing power from the English grid shows that the HVDC could deteriorate damping of the 6.3 Hz mode but only for high AC voltage controller gains. Reducing PLL gains in this case improves the damping which also suggests adopting different PLL gains on the rectifier and the inverter. The main findings are verified on the detailed EMTP model.

This paper proposes a new method to represent transmission lines and cables in state space. The method accounts for the frequency dependency of line parameters and essentially translates the state of the art time-domain universal line model into state space. The proposed model is compared against the latest state space model for single DC cable and a bipolar DC cable and it shows certain advantages. DC cables are represented without applying Kron reduction which means that a higher accuracy can be achieved. Single DC cable is represented with a lower model order and higher accuracy for a wide spectre of upper frequency limits and cable lengths. The longer the cable and the higher the upper frequency limit, the more pronounced is the model order reduction. The two state space models are also compared for a 70km bipolar DC cable in the frequency range up to 1 kHz and the proposed model achieves higher accuracy with a slightly lower model order. More importantly, the proposed model succeeds at representing the effect of mutual coupling within the desired accuracy criteria. Furthermore, based on the analysis of the two-conductor DC cable (bipolar cable), it is shown that the proposed method overcomes the issue to represent unbalanced multi-conductor lines and cables in state space. The reason is that it avoids modal transformation. Moreover, in case of multi-conductor lines and cables, it is expected that the proposed method is beneficial for longer systems, higher upper frequency limits and for more mutually coupled conductors.