Grid-Forming Control of Voltage Source Converters Based on the Virtual-Flux Orientation

Abstract

The increasing penetration of renewable generation in power systems is causing growing concern about system stability. Grid-forming (GFM) control has been appointed as the technology required for achieving a high penetration of renewables in the grid, as it successfully contributes to the power system stability. This paper proposes a novel grid-forming control scheme for voltage source converters (VSC) achieving the operational characteristics of a virtual synchronous machine (VSM). The proposed scheme is based on the orientation of a defined virtual flux to a reference axis obtained from the emulation of the synchronous generator swing equation. The virtual flux is obtained by the integration of the VSC internal voltage and it is oriented to the reference axis by means of a flux controller that also controls the flux magnitude. In this way, the VSC synchronism is maintained, operating as a voltage source connected to the grid. Moreover, the flux orientation in turn allows to control the VSC active power, while the flux magnitude control allows to regulate the VSC reactive power or terminal voltage in isolated mode. The paper also demonstrates that the stability of GFM converters is negatively affected when emulating high inertia values. So, a stabilizer is also proposed for damping these modes. Finally, due to fact that GFM converters behave as voltage sources, their low voltage ride through capability becomes an issue, because they naturally respond to faults with high short-circuit currents, that cannot be withstand by power electronics converters. Therefore, a method for limiting fault currents is proposed as well in the paper. The proposed grid-forming control scheme has been implemented in a real-time control platform and validated using comprehensive simulation models in a real-time simulator for assessing its grid-forming capability.INDEX TERMS Grid-forming power converter, virtual-flux orientation, virtual synchronous machines.

Sequence Control Strategy for Grid-Forming Voltage Source Converters Based on the Virtual-Flux Orientation under Balanced and Unbalanced Faults

“…First, the sequences extraction algorithm is explained. Next, the positive sequence control is summarized, which is quite similar to the one proposed in [43] but with some slight modifications to control the amount of active and reactive current setpoints during the fault.…”

“…ω + 2 is the output of the positive active current limiting (ACL + ) block, which ensures that the maximum active current value, I max act , is not exceeded. When this limit is approached, the ACL + block restricts the control angle θ, thereby limiting the active current module [43].…”

“…The inputs of this block, depending on whether the VSC is operating as a PQ node or a PV node, are the positive sequence reference and measured reactive power (Q + * and Q + , respectively) or voltage (v + * and v + ). As indicated in [43], the reactive power exchanged by the converter depends on the difference between the module of the internal voltage e and the grid voltage v, where e is proportional to ψ * v . So, the reactive power can be controlled by a droop control where the virtual-flux module reference is obtained as:…”

“…Considering their importance as an alternative to overcome the limitations of GFL converters in terms of instability, inertial response, and islanding capability, effective methodologies must be developed to comply with the guidelines established by the new grid codes. A recent publication [43] has introduced a novel control scheme for GFMs. This scheme presents an innovative solution that regulates a state variable, referred to as virtual flux, calculated as a function of the output current and the integral of the voltage measured at the converter's terminals.…”

A Flexible Virtual Inertia and Damping Control Strategy for Virtual Synchronous Generator for Effective Utilization of Energy Storage

“…A different approach is presented in [12] to mimic synchronous machines using voltage components called voltage-based virtual synchronous machines (VSM). A virtual flux orientation-based VSG is presented in [13] to decouple the active and reactive power. A VSM-based electric vehicle charger is presented in [14], which can supply households in the absence of a grid with the help of an electric vehicle battery.…”