Control of a Three-Phase Four-Wire Modular Multilevel Converter as a Grid Emulator in Fault Scenarios

Testing Requirements and Control Strategies of Next-Generation Grid Emulator: A Review

“…However, the 130%~160% overvoltage and the multi-consecutive faults are not easy to achieve [7], [18]. Further, the impact of reactive current injection by DUT on the emulated voltage profile is overlooked in existing MMC-based grid emulators [4], [5].…”

“…It has been demonstrated that by controlling MMC internal energy, the grid-forming based MMC is identical to an equivalent 2-level VSI. Moreover, the feasibility of energy-based internal control used in the MMC-based grid emulator has been validated in [4], [5]. It is shown that steady-state and transient functionalities, i.e., voltage sag and harmonic generation can be emulated by the MMC connected with passive loads.…”

“…A scalable and versatile grid emulator is demanded to accommodate the increasing power and voltage levels of WTs. The modular multilevel converter (MMC) emerges as a promising approach to meet this demand [4], [5]. The replacement of LC filter by L filter in the MMC enables to widen the bandwidth of voltage control for emulating different grid voltage profiles.…”

Medium-Voltage Megawatt Power-Electronic-Based Grid Emulators: Testing Capability Requirements and Dynamics Challenges—A Review

“…1 shows a general diagram of power-electronic-based grid emulators, which is commonly built on a back-to-back power conversion system with an active-front end (AFE) and a controlled voltage generator (CVG) [3]. For scalability, AFE and CVG are generally composed by series/ parallel connected modular power converters, such as the cascaded or interleaved neutral point clamped (NPC) converter [6]- [10], the cascaded H-bridge (CHB) converter [11]- [13], and the modular multilevel converter (MMC) [14]- [17]. Those topologies typically employ several neutral-point (NP), cell or submodule (SM) capacitors as the energy buffer of the power conversion, where low-frequency fluctuations of capacitor voltages are actively regulated, introducing internal dynamics into grid emulators [13], [14], [18].…”

“…For scalability, AFE and CVG are generally composed by series/ parallel connected modular power converters, such as the cascaded or interleaved neutral point clamped (NPC) converter [6]- [10], the cascaded H-bridge (CHB) converter [11]- [13], and the modular multilevel converter (MMC) [14]- [17]. Those topologies typically employ several neutral-point (NP), cell or submodule (SM) capacitors as the energy buffer of the power conversion, where low-frequency fluctuations of capacitor voltages are actively regulated, introducing internal dynamics into grid emulators [13], [14], [18]. The internal dynamics can interact with the external dynamics of CVG when emulating transient events, causing voltage imbalance or even over-/under-voltage of the internal capacitors, which may, in turn, adversely affect the testing capabilities of CVG, such as the synthesis of harmonic voltages and the emulation of varying grid impedance during a fault ride-through (FRT) test [16].…”