Low voltage direct current (LVDC) distribution has gained the significant interest of research due to the advancements in power conversion technologies. However, the use of converters has given rise to several technical issues regarding their protections and controls of such devices under faulty conditions. Post-fault behaviour of converter-fed LVDC system involves both active converter control and passive circuit transient of similar time scale, which makes the protection for LVDC distribution significantly different and more challenging than low voltage AC. These protection and operational issues have handicapped the practical applications of DC distribution. This paper presents state-of-the-art protection schemes developed for DC Microgrids. With a close look at practical limitations such as the dependency on modelling accuracy, requirement on communications and so forth, a comprehensive evaluation is carried out on those system approaches in terms of system configurations, fault detection, location, isolation and restoration.

A novel current control strategy is proposed for voltage source converter connecting to weak grid using conventional current vector control with additional current error based voltage angle and magnitude compensations. For connecting to very weak AC network, conventional vector control is proved to be unstable, whereas the proposed current error based compensations can significantly improve system stability. In this way, the proposed control can still benefit from the presence of current closed-loop control without the need for control switching during large AC voltage variations. Comprehensive frequency domain model is established to analyze stability performance. Comprehensive time domain simulations are further carried out to validate its effectiveness and robustness by demonstrating its current control performance during a three-phase fault, multiple-converter situation and various grid strength conditions.

Deployment of Synchronous Condensers (SynCons) has become one popular solution in accommodating large-scale renewable energy source (RES) for existing power systems. However, as a rotating device, SynCon may lose transient rotor angle stability caused by injected active power from nearby RES after a fault. To reveal this phenomenon, the transient stability of SynCon is investigated in this article. To derive intuitive mechanism and assist quantitative analysis, the equivalent synchronous generators (ESG) model is proposed to characterize the transient behavior of SynCon and co-located RES. The Lyapunov's direct method is employed for the assessment of transient stability by analyzing attraction domain for a typical benchmark system. On this basis, an index of transient stability margin against grid strength and an adaptive low voltage ride through (LVRT) control strategy are proposed for further assessment and enhancement of transient stability. The accuracy and effectiveness of the proposed methods are verified with EMT simulations of a typical windfarm configuration.

Temporary overvoltage (TOV) is becoming one challenge for the integration of significant power electronics resources into a power system. To assess the risk of TOV in a Multi-Infeed Power Electronic System (MIPES), a worst-case estimation approach is proposed to assess TOV risk. The paper starts with analyzing the mechanism of TOV, which reveals that TOV is dominated by excessive reactive power resulting from comprehensive control laggings. Then, the principle of approximating the TOV for a Voltage Source Converter (VSC) connecting to an infinite bus is introduced within the context of the RMS model. After that, a method based on the concept of generalized short-circuit ratio for overvoltage (gSCR-TOV) is introduced to quantify the risk of TOV in a multi-bus system. This concept intends to permit screening to identify those nodes at greatest need for further EMT assessment. To localize these nodes, a concept of weighted sensitivity matrix is further proposed to pinpoint the bus subject to highest TOV in a MIPES. Finally, case studies of EMT time-domain simulations are carried out and verifies the effectiveness of the proposed methods.

This paper takes a systematic view on the control and protection of medium power DC networks in an active distribution power system considering fault current limiting, system control, and converter design. Reduced terminal capacitance and extra DC impedance are used to limit DC fault current and reduce the required converter current rating for medium power DC networks. An adaptive DC power stabilizer is proposed to alleviate possible system instability brought by the fault current limiting settings in the presence of constant power load. The effect of the current limiting method and the proposed stabilizer on DC fault current and stability enhancement are validated by simulation studies using a simple two-converter DC network and a multiterminal DC network in an active distribution power system.