Virtual Induction Machine Strategy for Converters in Power Systems with Low Rotational Inertia

This paper presents a novel comprehensive control strategy for grid-connected Voltage Source Converters (VSCs) in power systems with low rotational inertia. The proposed model is based on emulating the physical properties of an Induction Machine (IM) and taking advantage of its inherent grid-friendly properties, i.e. self-synchronization, virtual inertia, power and frequency oscillation damping. For that purpose, a detailed mathematical model of the IMs working principles is derived, which includes the possibility of obtaining the unknown grid frequency without a dedicated synchronization unit, but rather via processing the voltage and current magnitude measurements at the converter output. This eliminates the need for an inherently nonlinear phase-locked loop, characteristic for virtual synchronous machines, while simultaneously preserving the synchronization and damping properties of a conventional electrical machine. Several case studies are presented that validate the mathematical principles of the proposed model and conclusions on VSC performance are drawn

Stability Performance of Power Electronic Deviceswith Time Delays

This paper deals with the impact of time delays on small-signal stability of power systems with an all converter-interfaced generation. For this purpose, a delay differential algebraic equation model of the voltage source converter and its control scheme is developed. The regulation is based on replicating the dynamical properties of a synchronous machine through appropriate controller configuration. Therefore, a virtual inertia emulation is included in the active power control loop. A transcedental nature of the characteristic equation is resolved by implementing the Chebyshev's discretization method and observing a finite number of critical, low-frequency eigenvalues. Based on the proposed approach, a critical measurement delay is evaluated. Furthermore, a bifurcation analysis of the droop gains and inertia constant is conducted. Stability regions and optimal parametrization of the voltage source converter controls are evaluated and discussed

Fast Frequency Control Scheme through Adaptive Virtual Inertia Emulation

This paper presents a novel virtual inertia controller for converters in power systems with high share of renewable resources. By combining the analytical study of system dynamics and a Linear-Quadratic Regulator (LQR)-based optimization technique, the optimal state feedback gain is determined, adapting the emulated inertia constant according to the frequency disturbance in the system. The optimality is achieved through trade-off between the critical frequency limits and the required control effort, i.e. utilization of the internal energy storage. The proposed controller is integrated into a state-of-the-art converter control scheme and verified through EMT simulations. The results show a significant improvement in the frequency response compared to an open-loop system, while also preserving drastically more DC-side energy than a non-adaptive controller

Droop vs. virtual inertia: Comparison from the perspective of converter operation mode

Virtual Inertia Emulation (VIE) and traditional Active Power Droop Control (APDC) are among the most common approaches for regulating the active power output of inverter-based generators. Furthermore, it has been shown that, under certain conditions, these two methods can be equivalent. However, neither those studies, nor the analyses comparing the two control schemes with respect to their dynamical properties, have investigated the impact of the converter operation mode. This paper explores the subject by investigating the two control approaches under such conditions, and determining when this assumption does not hold. Using time-domain simulations with a detailed Voltage Source Converter model, we compare VIE and APDC qualitatively and reformulate the respective conditions for equivalence

Stability Analysis of Converter Control Modes in Low-Inertia Power Systems

This paper deals with the small-signal stability analysis of converter control modes in low-inertia power systems. For this purpose, a detailed differential-algebraic equation model of the voltage source converter and its control scheme is developed. Both grid-forming and grid-feeding concepts have been considered, as well as different active power controllers based on traditional droop and virtual inertia emulation. An eigenvalue analysis of the linearized state-space system is conducted and the performance of different control configurations is compared. Furthermore, various bifurcation studies have been completed and conclusions on stability margins have been drawn with respect to control sensitivity and robustness

Interval-Based Adaptive Inertia and Damping Control of a Virtual Synchronous Machine

This paper presents a novel virtual synchronous machine controller for converters in power systems with a high share of renewable resources. Using an interval-based approach, the emulated inertia and damping constants are adaptively adjusted according to the frequency disturbance in the system, while simultaneously keeping the frequency within prescribed limits. Furthermore, the sufficient stability conditions for control tuning are derived. The proposed design is integrated into a state-of-the-art converter control scheme and tested through time-domain simulations. A comparative study against the existing approaches in the literature verifies the control effectiveness

Robust Converter Control Design under Time-Delay Uncertainty

This paper deals with the converter control design under time delay uncertainty in power systems with high share of converter-based generation. Two approaches for time delay modeling are proposed using linear fractional transformations and linear parameter-varying systems, respectively. Subsequently, two output-feedback synthesis methods are implemented based on H∞ control theory, and formulated using linear matrix inequalities: (i) a norm-bounded parametric H∞ controller; and (ii) a gain-scheduled H∞ control. These robust control principles are then employed to improve the performance of Voltage Source Converters (VSCs) under varying measurement delays. Three novel control strategies are proposed in order to redesign the conventional inner control loop and improve converter performance when dealing with measurement uncertainty. Finally, the controllers are integrated into a state-of-the-art VSC model and compared using time-domain simulations

Partial Grid Forming Concept for 100% Inverter-Based Transmission Systems

With the current trends in renewable energy integration, the concept of a 100% inverter-based power system is becoming more of a reality. However, the existing Voltage Source Converter (VSC) control schemes for such systems focus mostly on the operation of low-voltage microgrids, which have different requirements from the transmission system perspective. This paper proposes a new classification of VSC control strategies depending on their mode of operation. Then, the concept of partial grid forming VSC is introduced and it is shown that a system with zero rotational inertia can operate without a dedicated grid-forming VSC unit, but rather with partial forming of key system characteristics distributed across different VSC units. The performance of this approach is tested on detailed VSC models developed in both MATLAB Simulink and virtual Hardware-In-the-Loop (vHIL) platforms. Furthermore, an investigation towards necessary converter and network criteria for providing a stable system under the proposed control concepts is presented

LQR-Based Adaptive Virtual Synchronous Machine for Power Systems with High Inverter Penetration

This paper presents a novel virtual synchronous machine controller for converters in power systems with a high share of renewable resources. Using a linear quadratic regulator-based optimization technique, the optimal state feedback gain is determined to adaptively adjust the emulated inertia and damping constants according to the frequency disturbance in the system, while simultaneously preserving a tradeoff between the critical frequency limits and the required control effort. Two control designs are presented and compared against the open-loop model. The proposed controllers are integrated into a state-of-the-art converter control scheme and verified through electromagnetic transient (EMT) simulations

A Stochastic-Robust Approach for Resilient Microgrid Investment Planning Under Static and Transient Islanding Security Constraints

When planning the investment in Microgrids (MGs), usually static security constraints are included to ensure their resilience and ability to operate in islanded mode. However, unscheduled islanding events may trigger cascading disconnections of Distributed Energy Resources (DERs) inside the MG due to the transient response, leading to a partial or full loss of load. In this paper, a min-max-min, hybrid, stochastic-robust investment planning model is proposed to obtain a resilient MG considering both High-Impact-Low-Frequency (HILF) and Low-Impact-High-Frequency (LIHF) uncertainties. The HILF uncertainty pertains to the unscheduled islanding of the MG after a disastrous event, and the LIHF uncertainty relates to correlated loads and DER generation, characterized by a set of scenarios. The MG resilience under both types of uncertainty is ensured by incorporating static and transient islanding constraints into the proposed investment model. The inclusion of transient response constraints leads to a min-max-min problem with a non-linear dynamic frequency response model that cannot be solved directly by available optimization tools. Thus, in this paper, a three-stage solution approach is proposed to find the optimal investment plan. The performance of the proposed algorithm is tested on the CIGRE 18-node distribution network