Protection and pole voltage rebalancing for pole-to-ground faults in symmetrical monopolar HVDC grids

Abstract

Pole rebalancing in symmetrical monopolar HVDC grids is necessary to remove pole imbalances resulting from pole-toground faults. For selective protection employing DC circuit breakers, pole rebalancing considering backup protection operation in case of breaker failure has until now not been studied. This paper proposes fault clearing and post-fault restoration sequences including pole rebalancing to deal with DC-side faults, considering both primary and backup protection operations. The performance of the proposed sequences and the impact of the key breaker parameters on pole rebalancing are investigated in a four-terminal test system using PSCAD/EMTDC.

An Approximated Analytical Model for Pole-to-Ground Faults in Symmetrical Monopole MMC-HVDC Systems

“…1 also shows the AC grounding, represented by the star-point-reactor (SPR), though other AC grounding schemes may also be used, e.g. a zig-zag transformer [23], [24]. The SPR is used to provide a ground path in the converter's AC side and to rebalance any voltage unbalances due to control or monopole faults that could cause DC current flow in the transformers [23].…”

“…To simplify the circuit, components that do not affect the fault behaviour can be neglected without compromising the model precision. Although some consequences of PG faults on DC systems are well known, e.g., the voltage drop in the faulted pole and voltage rise in the non-faulted pole [24], [26], better understanding is needed regarding the variables and system configurations that can affect the fault behaviour. Considering this, the influence of the grounding and fault time instant on the fault behaviour is analysed next.…”

Multi‐vendor interoperability in HVDC grid protection: State‐of‐the‐art and challenges ahead

“…Recent work focuses on (i) optimizing DCCB [104] aHSSes with small TIV capability is designed to speed up the fault current separation [45]. bDCCB at the converter terminal may also be used to provide selective breaker failure backup protection [27]. cMixed HB-MMC and FB-MMC is studied to analyze the impact of fault current control using FB-MMC in a fully selective strategy [104].…”

“…This section presents general fault clearing sequences of the three main protection philosophies based on protection studies presented in the literature [27,48,86,88,91,93].…”

“…Besides the interest from industry, multi-vendor interoperability of HVDC grids has also been studied in recent research projects. These projects focus on: (i) control interoperability of multi-vendor VSCs [7,20]; (ii) analysing adaptability of AC standards to HVDC grid protection [21]; (iii) interoperability analysis of protection equipment, such as DCCB technologies [22][23][24], stand-alone DC protection IEDs [25], and pole rebalancing technologies [26,27]; and (iv) multi-vendor protection design methodologies [28]. In summary, it is clear that recent works have made great progress in clarifying fundamental concepts for HVDC grid protection and specifying high-level requirements for the system behaviour and protection equipment.…”