This paper proposes a sufficient condition to assess linearized Bounded-Input-Bounded-Output (BIBO) stability of Multi-Vendor-Multi-Terminal (MVMT) HVDC grid over broad frequencies. Without madatory utilization of Nyquist Criteria or passivity of impedance, this paper provides a guidance on how the stability of an HVDC grid can be predicted based on measurements during the tests on disconnected components without access to internal design of HVDC converters. Assisted by realizing positive realness of an auxiliary transfer matrix, defined as current gain matrix, in frequency domain, a set of sufficient criteria are formulated. They include one global criterion, i.e. positive realness of every current (voltage) gain and two decentralized criteria, i.e. BIBO stability of every nodal impedance (and admittance), and BIBO stability of every transfer admittance (and impedance). The sufficiency of the stability criteria is derived via nodal analysis in s-domain. The paper also presents the schemes of testing stability of nodal admittance/impedance in time-domain to cover converters controlled as constant power loads. The overall methodologies are further generalized to bipolar HVDC grids including unbalanced conditions and transfer network with low-impedance branches. The achievability of positive real voltage/current gains is compared with passive nodal admittance in case studies assisted by time-domain simulations using Matlab/simulink.

Deployment of Synchronous Condensers (SynCons) has become one popular solution in accommodating large-scale renewable energy resource (RER) for existing power systems. In  this paper, rotor angle instability of SynCon is investigated. To  derive intuitive understandings and quantitative analysis, the  swing equations of the equivalent synchronous generators (ESG)  are derived. The Lyapunov’s direct method is employed to analyze  the attraction domain for a typical benchmark system. Based on  that, a rotor angle stability margin index is developed and an  adaptive low voltage ride through (LVRT) control strategy is  proposed for assessment and enhancement of rotor angle stability.  The criterion and effectiveness are verified with on EMT  simulations, respectively.

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 control lagging of converters. 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 methodology 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. The intent of this concept is 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 methodologies.