Emulation Of Synchronous Machines Research Articles

Small signal analysis of a grid-forming modular multilevel converter with a novel virtual synchronous generator control

This paper introduces a grid forming (GFM) control method – detailed synchronous machine emulation virtual synchronous generator (VSG). The proposed method makes a voltage source converter exactly emulate a synchronous generator (SG), using a current source interface. The precise emulation of an SG gives tighter control over overcurrent and improved transient damping. Electromagnetic Transients (EMT) simulation is used to demonstrate the operation, and small signal model is used to assess the stability performance. Small signal analysis shows that the resulting VSG operates stably, and oscillatory modes can be damped by appropriate optimization of the virtual damping windings resistances.

Analysis of multi-converter network impedance using MIMO stability criterion for multi-loop systems

This work presents a novel method of simplifying multi-converter systems containing complex grid-forming and grid-following controllers for stability analyses. Analysis is conducted via the equivalent converter output impedance. Important points for impedance-based stability analysis of larger networks are considered, including impedance rotations, proper combination of sources and correctly obtaining a minimum system realisation with appropriate tolerances. The proposed method is used to conduct a sensitivity analysis of different grid-forming and grid-following converter control parameters to determine tuning recommendations for the modern electricity network. To achieve this, complex control structures including power/voltage control, negative sequence regulation and synchronous machine emulation are included. Network stability is found to be sensitive to current control, active power control and frequency-based components such as PLLs. However, voltage control exhibits a smaller effect on stability for a range of short circuit ratios. Some recommendations are provided on when the simplified method can be utilised and how to ensure correct application.

Virtual oscillator‐based methods for grid‐forming inverter control: A review

In inverter-dominant power systems, grid-forming (GFM) inverters regulate voltage and frequency. To construct GFM inverters, conventionally, various control methods based on synchronous machine emulation or droop characteristics have been employed. However, recently, inverters are regulated by Virtual Oscillator Control (VOC) to emulate the dynamics of a weakly nonlinear oscillators. In contrast to droop control, VOC is a time-domain controller that enables interconnected inverters to stabilize arbitrary initial conditions to a synchronized sinusoidal limit cycle rapidly. The fast synchronization, accurate power sharing in grid-connected and islanded modes, simple and robust implementation make it a promising candidate for inverter-dominant power systems. Moreover, VOC removes the computational burden within machine emulation-based methods and provides an approach to measure the frequency without employing phase-locked loop (PLL). Hence, to leverage the advantages of this method and expand its application in power system control, this paper reviews different VOC implementations. Mainly, this paper focuses on the studies related to the fundamental theory of oscillators and design process. Additionally, there are discussions on the stability analysis of VOC-based systems, harmonics and pre-synchronization. Challenges on the compatibility with heterogeneous controllers, fault ride-through and other limitations of VOC including non-inertial response and coupled power control are investigated and solutions are outlined.

Synchronous Voltage Controllers: Voltage-Based Emulation of Synchronous Machines for the Integration of Renewable Energy Sources

Recently, significant attention has been paid to the integration of renewable energy sources into power systems by similar characteristics of synchronous machines (SMs). The conventional method to emulate SMs is to use power components, typically known as power-based virtual synchronous machines (PB-VSMs). This paper looks at the development of a new approach to mimic SMs using voltage components called voltage-based virtual synchronous machines (VB-VSMs). A VB-VSM is a cascaded double-loop controller with current and voltage regulation as opposed to a single-loop PB-VSM. Further, the dynamic model of an automatic voltage regulator (AVR) and a physical rotor field circuit are integrated into the controller. In addition to grid-responsive features, the VB-VSM can also eliminate an extra synchronization unit, typically a phase-locked loop (PLL), as the internal controller acts in a similar fashion to regular PLLs in research literature. Since the controller adopts voltage components for synchronization, control, and emulation purposes, it can be regarded as a synchronous voltage controller (SynVC). Extensive case studies under various conditions are carried out to demonstrate the performance of a SynVC.

Parallel current‐controlled synchronverters for voltage and frequency regulation in weak grids

The emulation of synchronous machines is becoming an attractive solution to provide virtual inertia and voltage support to weak grids and microgrids, representing an alternative to conventional current-injection control strategies. However, synchronverters operate as voltage sources and this makes it difficult to limit the current when an event like voltage sag takes place. Here, a method to limit the output current of synchronverters in parallel connection is proposed. First of all, a fictitious reference current is generated, and its value is then followed by an internal current controller. A method to avoid the loss of synchronism when the current saturates is proposed and studied in detail. Also, the implementation of the controller is addressed carefully. All the control system improvements are tested on a prototype consisting of two 15 kVA synchronverters connected in parallel to a configurable weak grid.