With the increasing integration of power electronic converters into the power system, the interactions between converters and their adjacent transmissions bring emerging oscillation issues. A new type of sub- and super-synchronous interactions (S2SI) between STATCOMs and the weak AC/DC grid were detected in China Southern Grid. Field measurements as well as simulation studies have revealed that the various factors contributing to the S2SI include the number of online STATCOMs, the strength of the AC system (in terms of effective short circuit ratio), the status of series compensation and the loading level of the HVDC. The mechanism of such oscillations have been clarified and it is caused by the S2SI between STATCOMs and weak AC grids, where sub- and super-synchronous components are involved in a coupled way. To fully represent such a coupling effect, the concept of sub- and super-synchronous coupled impedance model, or S2SC-IM, is proposed and developed in order to establish a stability criterion for the S2SI. The results show good consistence among theoretical analysis, electromagnetic simulations and field measurements. Therefore, the effectiveness of the developed models and the stability criterion has been well verified.

Electromagnetic transient (EMT) and transient stability hybrid simulations are predominantly used to analyze the interactions between HVDC systems and the AC grids accurately and efficiently. However, the dynamics of the converters will be affected by the waveforms of adjacent AC systems greatly. Waveform distortion as well as time delay caused by interfacing would significantly increase interface errors, resulting in the decrease of the overall accuracy of the simulations. To resolve such problems, a dynamic phasor based interface model (DPIM) is proposed in this paper to improve the accuracy of interfaces, especially when the fault occurs near the converters. In doing so, the whole system is partitioned into three parts: the transient stability (TS) subsystem, the EMT subsystem, and the DPIM. During each iteration, the interfaces between the TS subsystem and DPIM are represented by their Norton equivalents at fundamental frequency and Thevenin equivalents in the dynamic phasor form. Similarly, the interfaces between DPIM and EMT subsystems are represented by their three phase Norton equivalents and Thevenin equivalents in dynamic phasors, respectively. The effectiveness of the proposed method in terms of accuracy and efficiency is validated by simulating a practical HVDC Project.

When investigating voltage and current stresses in critical main-circuit components during fault near the converter, a detailed equivalent model capable of representing the balancing control strategies of the capacitor voltages on the sub-module level, along with blocking and de-locking, is always necessary. Among previously proposed equivalent models of the MMC, only sub-module averaged models (SAMs) can capture interested inner dynamics inside each arm. However, the simulation accuracy of SAMs is not always satisfactory, especially when the time step is larger than 10 μs. In order to further improve the simulation accuracy with guaranteed simulation efficiency, the shifted frequency modeling (SFM) of the half- and full-bridge hybrid MMC is proposed in this paper. Therein, each sub-module is represented by Thévenin equivalents derived by dynamic phasors. The arm of the MMC is represented by Norton equivalents to guarantee the efficiency, considering both normal and dc-blocking conditions. The effectiveness of the proposed method in terms of accuracy and efficiency is validated by simulating a MMC based HVDC transmission.

The interactions between PMSGs and weak grids may cause growing subsynchronous oscillation (SSO). The growing-SSO would evolve into a sustained-SSO when it is governed by the nonlinearities in the PMSG's converter control, such as current limiter. Most of the existing frequency-domain modeling and stability analysis methods are in the small-signal sense and thus unable to consider the impact of such nonlinearities. Besides, these methods can hardly give vital information to characterize the sustained-SSO, for instance, the magnitude. This paper presents a characteristic analysis of sustained-SSO while accounting for the effect of current limiter nonlinearity. First, a single-input-single-output (SISO) frequency-domain model of the PMSG is developed. This SISO model comprises the transfer functions of outer-/inner-loop controls and phase-locked loop. The nonlinear part, i.e., current limiter, is incorporated into the model using the describing function (DF) method. Then, the equivalent transformation on the system's closed-loop characteristic equation is used to separate the SISO model into independent linear and nonlinear parts. This separation enables the use of the DF-based Nyquist criterion for evaluating the system stability and determining oscillation attributes, namely the magnitude and frequency. The analytic and simulation results reveal that the current limiter has significant impacts on the characteristics of sustained-SSO

With the advancement of distributed generation (DG) technology, numerous grid-connected inverters are being integrated into the power system. The interaction between inverters and the power grid has triggered complex resonance and stability issues in the power system. Currently, the research models for LCL-type grid-connected inverters do not simultaneously consider the AC-side and AC-DC frequency coupling effect, which could lead to misjudgment in the system's stability. To address this deficiency, this paper formulates a third-order admittance matrix model for an LCL-type grid-connected inverter that accounts for both AC-side and AC-DC frequency coupling using the harmonic linearization. Furthermore, this paper presents a simplified modeling approach for the inverter. Taking a two-stage photovoltaic system as an example, this paper establishes the Generalized Nyquist Criterion (GNC) for the system, and employ GNC to perform stability analysis on the photovoltaic system. Results indicate that when the outer control proportional coefficient of the DC-bus voltage is relatively high, the AC-DC frequency coupling will significantly impact the system's stability. The proposed third-order admittance matrix model's effectiveness in stability analysis was verified through simulations on the PSCAD/EMTDC platform.

In this paper, a novel hybrid-arm modular multilevel converters (HA-MMCs) topology with DC low voltage operation and fault ride-through capability is proposed for unidirectional HVDC bulk power transmission. The HA-MMCs topology is a combination of two types of converter arms: the half-bridge sub-module based arm (HBSM-arm) and the diagonal bridge sub-module based arm (DBSM-arm). Replacing the traditional HBSM-MMC, the HA-MMCs are operated in current and reactive power control modes during normal conditions and can work as the rectifier or the inverter. Since the DBSM can provide bipolar voltages with unipolar currents, the HA-MMCs need not change its control strategy under DC low voltage conditions caused by the high-impedance faults. Under DC pole-to-pole or pole-to-ground faults, the HA-MMCs can both clear the DC fault current and work as the static synchronous compensator (STATCOM) to provide reactive power to support the AC grid. Compared with other MMC topologies with DC fault ride-through capability, the HA-MMCs topology is more economical for significantly decreasing the number of power devices. The operation states, control strategy and DC low voltage operation capability of the HA-MMCs are presented and simulation results of the LCC-HA-MMC hybrid system using PSCAD/EMTDC validate the characteristics and advantages of the HA-MMCs.