Conference Agenda

The Delta-connected STATCOM is regarded as the most advantageous topology for STATCOMs based on the Modular Multilevel Converter (MMC) technology. Embedding energy storage devices into the MMCs has gained significant research interest in recent years. This paper focuses on modeling of MMC-based Delta-STATCOMs with embedded energy storage. A flexible modeling approach is proposed, which allows easy interfacing of various converter models with various energy storage device models. Four commonly used types of MMC models are applied to STATCOM modeling: detailed, detailed equivalent, arm equivalent, and average value. Supercapacitors and batteries are used as energy storage devices. Dynamic performances of the models are compared in transient simulation cases using EMTP.

Power-electronic converters are essential elements for the effective interconnection of renewable energy sources to the power grid, as well as to include energy storage units, vehicle charging stations, microgrids, etc. Converter models that provide an accurate representation of their wideband operation and interconnection with other active and passive grid components and systems are necessary for reliable steady state and transient analyses during normal or abnormal grid operating conditions. This paper introduces two Laplace domain-based approaches to model buck and boost DC-DC converters for electromagnetic transient studies. The first approach is an analytical one, where the converter is represented by a two-port admittance model via mode averaging and inclusion of switching effects. The second approach consists of reconstructing the two-port admittance model of the converter from terminal measurements for a series of tests. The performance of both approaches is evaluated against EMTP simulations, with very close results.

Parallel operation of grid-forming inverters (GFMIs) is often achieved using droop characteristics implemented in converter controllers. Converters’ recovery after a disturbance depends on the dynamics of each individual GFMI, and the droop characteristic alone is unable to ensure successful parallel operation. This work proposes a dynamic-phasor based modeling approach that enables eigenvalue analysis of multi-converter systems to identify the underlying factors that affect the interactions among parallel GFMIs. Network dynamics are included through dynamic phasor modeling of its elements, and controller dynamics are fully included. Modeling modularity is preserved, which allows to easily extend the test system to any topology of interest. The results presented for an exemplar two-converter system prove that the virtual inertia time-constant plays a significant role in exciting interactions, and that network and control system parameters are vital in extending the stability margins of the systems. EMT simulation results from PSCAD/EMTDC are included to verify the validity of the predictions of the dynamic-phasor based model.

Weak grid sub-synchronous oscillation (SSO) is an important research topic. Although some analysis and mitigation schemes have been conducted, misperceptions still exist. This paper first aims to update the understanding of weak grid instability of voltage source converters (VSCs) and then points out that instability risks originate in the super-synchronous frequency range rather than in the sub-synchronous frequency range. Moreover, the resonance frequency can be even larger than the double fundamental frequency under certain parameter conditions. This paper also refines the impacts of VSC control parameters on weak grid instability mechanism andresonance frequency. Based on the results, the classification of this phenomenon should be revised.

In this paper, the behavior of traveling waves (TWs) on transmission lines which interconnect inverter-based resources (IBRs) is investigated, assessing the performance of well-known TW functions applied in protection and fault location schemes. To do so, traditional synchronous generators and wind turbine-based IBRs of types III and IV are investigated, allowing comparative studies regarding the shape of fault-induced transients measured at the monitored line terminals. Then, the impacts of typical terminations of IBR-interconnecting lines on the classical double-ended TW-based fault location method, as well as on directional, overcurrent and differential TW-based protections are analyzed, highlighting the effects of busbar and transformer stray capacitances on the performance of these functions. The obtained results reveal that TW solutions are promising for IBR-interconnecting lines. However, a significant influence of busbar and transformer stray capacitances on the performance of TW functions is also identified, revealing the need for taking these capacitances into account during the definition of settings used in TW protection and fault location schemes.

Electromagnetic transient (EMT) simulations are often performed to analyze disturbances which occur during a steady-state operation of the power grid. In modern transmission and distribution power grids, a number of voltage-source converters (VSCs) are used for renewable energy interconnections and system control. To perform EMT simulations with such VSCs, a time step of the order of microseconds is used to represent the switching operations of the VSCs. In order to avoid a prohibitively-long computation time, a steady-state initialization method is required to directly start from a steady state. This paper proposes a systematic and heuristic procedure for the steady-state initialization of generic VSCs. Using an AC steady-state solution, detailed portions in the circuit part and the control-system part of a VSC are systematically initialized. For validation, EMT simulations of a 6.6-kV distribution grid with two VSCs are performed with and without the proposed initialization procedure in this paper. Practically no transient is observed in the result with the proposed procedure, and therefore it is confirmed that directly starting from a steady state is made possible. On the other hand, the result without the proposed procedure does not reach the steady state even after continuing the EMT simulation for 300 ms.