Wenxuan Lv

The hybrid power flow controller based on cascade H-bridge (HPFC) technology represents a versatile and cost-effective solution for power flow control. This device capitalizes on the strengths of phase-shifting transformers and power electronic devices, offering benefits for power grid optimization, renewable energy transmission, and flexible interconnection of regions. However, in the event of a permanent fault on the line with HPFC, conventional three-phase reclosing can lead to increased secondary short-circuit currents, posing significant risks to the power grid. The operational characteristics of HPFC render passive fault location methods less effective, thereby affecting fault localization accuracy. This paper presents a three-phase adaptive reclosing scheme for the line based on active injection of characteristic voltage by HPFC. By leveraging the unique features of HPFC, the scheme proposes a novel method for generating and injecting characteristic voltage. Considering factors such as current transformer measurements and transition resistance, the scheme introduces a fault nature criterion using staged characteristic voltage injection. This approach enhances the success rate of three-phase reclosing and ensures fault recovery capability. The feasibility of the proposed scheme is validated through a PSCAD/EMTDC simulation model of the line with HPFC.

Modular multilevel converters (MMC) with fault control capability, such as full-bridge and hybrid MMC, can maintain the operation of DC system by actively limiting the fault current. However, converters with these capabilities present complex fault characteristics, complicating the DC protection design. Although fault features can be analyzed by simulations, short-circuit current calculations are desirable as they give analytical expressions, which simplify the analysis of factors affecting the fault response and are suitable for large-scale applications. However, calculations become challenging when MMC arm voltage reaches the limitation sum sub-module capacitor voltage, a condition known as converter saturation, which is hard to represent with transfer functions. To clarify the fault characteristics during converter saturation, a DC fault current calculation method for converters with fault control capability is proposed. The features under fault control are analyzed and divided into different stages. Then the basic analytical equation based on control and fault circuit is deduced and the saturation impact is considered by adopting a short-time numerical calculation. The proposed method is validated in Simulink and a statistical similarity check shows 91% similarity between the proposed model and simulation results, versus 23% similarity when using a transfer function model that does not consider saturation effects.

To improve the reliability of power supply, reclosing schemes are required after transient faults, which commonly occur in overhead line based high voltage DC (HVDC) systems. However, in the event of permanent faults, the auto-reclosing scheme may cause a severe strike. To avoid the severe impacts caused by permanent faults, the fault type should be discriminated before activating the reclosing scheme. Therefore, an adaptive reclosing scheme based on phase characteristics is proposed in this paper. Firstly, the modulation of a periodic voltage by actively controlling the hybrid DC circuit breaker (DCCB) is introduced. Then, a cascaded π equivalent model and its decoupling algorithm are presented to analyze the frequency-domain characteristics of the measured impedance of the coupled overhead lines. From the frequency-domain characteristics, the frequency of the periodic detecting voltage is determined to analyze the phase features of the measured impedance at primary frequency. The permanent or transient faults can thus be accurately identified by using these different phase characteristics, with negligible influence on the healthy lines. In addition, the proposed scheme is robust to various fault resistances, leading to improved reliability. The effectiveness of the proposed scheme is verified in PSCAD/EMTDC.