Interactions between a half‐bridge MMC and a hybrid DC breaker during fault current interruption compared to a full‐bridge MMC

Abstract

The large‐scale integration of remote renewable energy resources into existing AC grids is one of the main drivers for the construction of multi‐terminal direct current (MTDC) systems. Today, a few MTDC systems are already in execution stage or even in operation. However, with a growing number of extended MTDC systems, the probability of high‐voltage DC (HVDC) insulation faults is likely to increase and therefore, reliable solutions for handling of such disturbances are crucial in order to maintain high availability. Since most HVDC converters are realised by modular multilevel converters (MMC) converters are based on half‐bridge (HB) modules, they lack an inherent capability to control DC fault currents. Therefore, additional equipment like HVDC breakers needs to be installed as well. Among the various breaker types, the so‐called ‘hybrid’ DC circuit breaker (DCCB) is considered most popular. This contribution provides a more holistic view on the overall system behaviour, represented AC grid, MMC HVDC converter, DC reactor and DCCB altogether. It discusses the physical interactions of these main components in MTDC systems. In particular, the influence of the electrical parameters of the hybrid DCCB and its required DC reactors on the MMC converter's internal current and voltage conditions is outlined.

Interdependencies in HVDC grid protection: Impact of converter parameters and controls on DC fault‐ride‐through capabilities and protection system design

“…The underlying assumptions (ideal DC voltage source, and symmetrical arm current distribution) are often found in literature that describes the fault current contribution during phase 1 (refer to Figure 3), and also in literature that analyses the design of MTDC protection systems [12,15,20,21,42]. To the authors' knowledge, existing analytical studies deriving the requirements on DCCBs and current-limiting inductors L DC solely rely on this idealised DC source model.…”

“…Thus, to derive a realistic worst-case ∆i arm /∆t, a threearm equivalent circuit-comparable to circuits considered in blocked-state analyses-is used [12,42]. For the derivation of ∆i arm /∆t, only the additional component of the arm currents is relevant.…”

Equal‐area criterion and analytical model for transient stability assessment of hybrid AC–DC transmission system using voltage sourced converter

“…Thus, a total of 54 flops, three square root functions, and three trigonometric functions are required by the proposed method for the system under the DC fault. In the calculation with an empirical method in [7], an empirical expression was introduced to replace the original integral calculus Equation (25). Then, the calculation of the critical clearing angle requires 14 flops and one trigonometric function.…”

“…Thus, during a DC fault, the effective reactance $$X^{{\rm dc,fault}}_{\rm T,pu}$$ depends on these two equivalent reactances that are influenced by the converter topologies. For example, in the case of the MMC based on full‐bridge submodules under the DC fault, the fault currents can be automatically interrupted by blocking IGBTs [25]. The transients of the HVDC system will then not affect the AC active power through the double‐circuit AC transmission line.…”

Third harmonic injection control for co‐phase traction power supply system using direct AC/AC full‐bridge modular multilevel converter

“…Modular multilevel converter (MMC) is widely used for railway power supply due to its scalable structure, low switching frequency, and good power quality [12][13][14]. To achieve the four-quadrant AC/AC conversion, two kinds of MMC structures are proposed.…”