Pole Voltage Balancing in HVDC Systems: Analysis and Technology Options

Abstract

Maintaining balanced voltage during normal operation and rebalancing pole voltages after a pole-to-ground fault are both necessary to prevent high insulation stresses on the DC cable in a HVDC system. The state-of-the-art solution requires two separate devices to deal with normal and post-fault operation separately. This paper proposes pole rebalancing solutions using a single device applicable to both normal and post-fault operations, thus leading to reduced cost and footprint for pole rebalancing. The proposed circuits use fast switches to dynamically adapt the connected impedance in order to fulfil the requirements for rebalancing voltages during both situations. The performance of the proposed circuits are verified in a four-terminal HVDC test network using EMTP simulations.

DC Powering Solutions for the Future Circular Collider: Converter Topologies, Protection, and Control

“…In the event of a pole-to-ground fault, the fault current is significantly reduced due to the system's grounding through a high-value impedance [28]. However, the main drawback is the occurrence of an overvoltage on the healthy pole, which must be discharged within a short time to avoid damage, either using the converter [29] or by means of a discharge resistor [30]. On the other hand, if the RF and the transmission network are combined, an asymmetric monopolar configuration is needed since the RF system requires one pole to be grounded.…”

Small-Signal Stability Analysis of the Asymmetrical Dc Operation in Hvdc Networks

“…Reference [11] proved that each pole of a symmetrical monopolar HVDC station can be controlled independently using its AC grounding system, but the study was addressed in a DC network with only an HVDC station. The DC voltage pole imbalance and different methods for rebalancing were analyzed in [12,13], but the focus was put on the transient DC asymmetry after a pole to ground fault in a network with a single HVDC station topology. Similarly, [7-9, 12,14] also described controllers aimed to avoid the asymmetrical DC operation but, again, they were only valid for the transient DC asymmetry after clearing a pole-to-ground fault and might cause undesired voltage levels in case of a permanent DC asymmetry.…”