Grid Forming Control for Power Systems with up to 100% Inverter Based Generation

With the increase of distributed power generation integrated into the power grid through power electronic converters, the problem of inertia reduction in power system has been caused, a new control technology is needed to enable distributed power sources to actively participate in the voltage and frequency regulation of the grid. In this paper, a model predictive control (MPC) strategy based on virtual synchronous machine (VSM) for the grid-connected converter is proposed. The method does not require traditional pulse width modulation and current inner loop control. When the voltage and frequency of power grid change, the active power and reactive power can respond in time and simulate characteristics of traditional synchronous machine. The simulation results are given to verify the effectiveness of the control strategy presented above.

Traditional power systems based on synchronous generators often feature low frequency electromechanical oscillations. However, the integration of renewable energy sources through power converters can help tackling this issue. In fact, thanks to the concept of Virtual Synchronous Machine (VSM), it is possible to make the inverters behave as real synchronous machines (SMs). This way, the inverters can be integrated into the grid as traditional SMs and can even outperform them when it comes to damping low frequency oscillations in the power system. In order to do that, proper damping algorithms must be adopted in the VSM model. Therefore, this paper presents a simple and straighforward damping method for VSMs based on a single lead-lag filter acting on the VSM active power feedback. The proposed method and its integration in the VSM model are described. Then, the proposed solution has been experimentally compared to conventional methods, along with comparison metrics, to highlight its benefits.

Considering the increase in renewable generation and the consequent reduction in power system inertia, the Virtual Synchronous Machine (VSM) control method has been proposed to control power converters to emulate the inertia and other the characteristics of the synchronous machine. However, to achieve the function of VSM control, an extra energy base, typically storage, is required to connect to the controlled converter. In this work we investigate the application of the VSM control to the distribution system demand through the use of a VSM controlled smart transformer. Through control of the demand in this way, the demand itself can be used to emulate inertia and provide frequency support. This paper presents the details of the flexible demand control applied to a smart transformer supplying a low voltage distribution grid. The operation of the control is validated on scaled hardware using real time simulation with hardware in the loop. Simulations on a 400 kVA, 400 V distribution network are used to quantify the demand flexible. IEEE 39 bus is used to verify the benefit of the proposed control in terms of voltage and frequency in the power system.

The increasing penetration of converter‐based generation in many power systems around the world has sparked a discussion about how to operate these power systems with the usual levels of efficiency, reliability and cost‐effectiveness. Current grid‐following converter‐based generators have proven to run stably in parallel to one another, even if there are thousands of them connected in a power system, and even in very small isolated power systems with extremely low system inertia. Discussions around the necessity of additional converter performance, usually under the ‘grid‐forming’ and ‘Virtual Synchronous Machines’ concepts, have recently been transferred from the academic sphere to national and international industry fora. Formal discussions have started in Great Britain, in Germany and at ENTSO‐E level. However, there is still a lot of uncertainty about the real and not simulated performance of grid‐forming converters, whilst the needs case for requiring this radically different control method has not been adequately justified. With the present paper we raise key questions that will serve towards an objective discussion about power system needs, grid infeed technologies and their interaction.

Grid-connected power converters can be operated as virtual synchronous machines (VSMs) to mimic the dynamics of conventional synchronous machines. VSMs can provide inertial responses to the power grid, and present desired dynamics when connected to weak grids that featured by high grid impedances. However, when the VSM is connected to a strong grid with low grid impedance, its stability margin will be reduced, which is particularly analyzed in this paper. Moreover, we demonstrate that the VSM's stability can be improved by properly reducing the virtual inertia. Based on the stability analysis, an adaptive inertia control (AIC) is proposed, which can regulate the virtual inertia according to the identified grid impedance to ensure that the VSM has a desired stability margin under various grid strength. The grid impedance is identified in real time along the system trajectory, using the recursive least square algorithm to enable the implementation in the controller. Simulations verify the validity of the analysis and the effectiveness of the AIC on improving the VSM's stability when considering various power grid strength.

In recent years, interest in the distributed energy sources is increasing because of the advancement of research in renewable energy sources (RESs). The rotating mass and the damping property of a traditional synchronous machine (SM) has a significant role to play in its dynamic performance as these properties ensure a slow rate of change of frequency in the system for nominal load variation. In RESs and distributed generator (DG) units like photovoltaics and fuel cells, there is neither damping property nor rotating mass. So, due to the irregular nature of the generation of RESs, it is very difficult to maintain the grid stability and power balance in the system. A control scheme is applied to the inverter such that the inverter acts like a virtual synchronous machine (VSM). VSM is inspired by the droop characteristics, swing equation and damping of the conventional SM. The main objective of this work is to design and implementation of the VSM in microgrid (MG). The simulation results show the efficacy of the suggested controller.KeywordsVirtual synchronous machineDistributed generatorsMicrogridSynchronous machine

Combustion of fossil fuel by single-cycle gas turbines in offshore oil and gas (O&G) facilities on the Norwegian continental shelf contribute significantly to Norway’s total greenhouse gas emissions. The CleanOFF Hub Research Project aims to reduce those emissions through innovative solutions, such as large-scale offshore hubs delivering low-carbon power to O&G platforms. Within this context, this paper presents an advanced electromagnetic transient (EMT) model for a hydrogen-based energy hub connected to a gigawatt-scale wind farm, designed to power six isolated O&G platforms. Building on previous research, this paper introduces an EMT model that addresses earlier limitations in simulating fast electrical transients and incorporates a more sophisticated virtual synchronous machine (VSM) architecture enabling the study of critical millisecond scale transient phenomena. Key contributions include the development of a publicly available EMT model of a gigawatt-scale wind-powered system with primary and inertial frequency control reserves provided by four VSMs, and the integration of secondary reserves from large grid-following electrolyser and fuel cell systems. These advancements facilitate robust analyses of dynamic behaviors and stability challenges in large-scale, isolated offshore power systems, contributing to the goal of decarbonizing the Norwegian continental shelf.

This paper evaluates the operation of an HVDC interconnection for providing virtual inertia and the impact on the power system frequency dynamics when one or both the converter terminals are controlled as a virtual synchronous machine (VSM). The analysis aims at assessing the performance of the inertia support from a power system perspective by studying the impact on the frequency nadir and the Rate-of Change-of-Frequency (RoCoF) of two asynchronous power system equivalents. It is demonstrated how the dc voltage control can be integrated with the VSM-based control when both converter terminals should be controlled to provide inertia support, but it is also found that this may require additional capacitance to ensure sufficient stability margins. Simulations in PowerFactory are presented for a case with two asynchronous power system equivalents connected by an HVDC transmission scheme modelled with parameters based on the planned Nordlink connection between Norway and Germany. The results confirm the beneficial effects of the inertia support on the frequency dynamics and demonstrate how the VSM-based control provides a clear improvement of the frequency nadir compared to traditional power-frequency droop control.

A virtual synchronous machine (VSM) is a converter which, compared to other types of converters, has more friendly interactions with the power grid because it is able to simulate the external characteristics of a synchronous machine, which can provide virtual inertia and damping. When the grid voltage is unbalanced, there will be negative sequence current and power oscillations. There will also be double-frequency ripples on the DC bus, which affect the normal operation of the DC power source or load. In order to solve these problems, a comprehensive control strategy is proposed in this paper. The principle of a VSM operated as a current source converter, also called VISMA, is used in the design. A complex coefficient filter is applied to separate the positive and negative sequence components of the grid voltage. By analyzing the reasons of power oscillations under unbalanced voltage, the electrical simulation part of the VSM is improved to achieve several objectives: to suppress negative sequence current and DC voltage ripples. Additionally, the rated voltage in the reactive control part is adaptively adjusted to stabilize the system. The validity of the proposed control strategy is verified by simulation and experiment.

This paper presents a single-phase virtual synchronous machine (VSM) and its possible application for providing vehicle-to-grid (V2G) services from domestic battery chargers of electric vehicles (EVs). In a VSM, the power converter is controlled to emulate the inertia and damping effect of a synchronous machine. Thus, a VSM-based EV charger can contribute to the spinning reserve and the frequency regulation of the power system. In case of grid outages, the VSM can seamlessly establish an islanded grid and supply local loads from the battery onboard the EV. In order to avoid the influence on the virtual inertia from power oscillations associated with a single-phase circuit, the proposed control scheme relies on a virtual two-phase system for calculating active and reactive powers. The proposed VSM implementation is described in detail, and its dynamic performances in grid-connected as well as stand-alone operation are demonstrated by numerical simulations and by laboratory experiments.

The wavelet transform is introduced as a method for analyzing electromagnetic transients associated with power system faults and switching. This method, like the Fourier transform, provides information related to the frequency composition of a waveform, but it is more appropriate than the familiar Fourier methods for the nonperiodic, wide-band signals associated with electromagnetic transients. It appears that the frequency domain data produced by the wavelet transform may be useful for analyzing the sources of transients through manual or automated feature detection schemes. The basic principles of wavelet analysis are set forth, and examples showing the application of the wavelet transform to actual power system transients are presented.

This paper discusses the effectiveness of a Battery Energy Storage System (BESS) when it is controlled as a Virtual Synchronous Machine (VSM) to mitigate stability issues in systems with low short circuit capacity and inertia. In the proposed approach, a detailed mathematical model that represents both the mechanical response as well as the electrical characteristics of a synchronous machine is used to derive the reference signals for a battery-inverter model. The results confirm that this approach is able to capture transient and sub-transient impacts of the synchronous machine and also the impact of the exciter and governor response following disturbances. The proposed approach is tested using a simplified circuit as well as a practical network model example.

The global shift toward clean production, like using renewable energy, has significantly decreased the use of synchronous machines (SMs), which help maintain stability and control, causing serious frequency stability issues in power systems with low inertia. Fractional order controller-based virtual synchronous machines (FOC-VSMs) have become a promising option, but they rely on communication networks to work together in real time, causing them to be at risk of cyberattacks, especially from false data injection attacks (FDIAs). This paper suggests a new way to detect FDI attacks using a federated physics-informed long short-term memory (PI-LSTM) network. Each FOC-VSM uses its data to train a PI-LSTM, which keeps the information private but still helps it learn from a common model that understands various operating conditions. The PI-LSTM incorporates physical constraints derived from the FOC-VSM swing equation, facilitating residual-based anomaly detection that is sensitive to minor deviations in control dynamics, such as altered inertia or falsified frequency signals. Unlike traditional LSTMs, the physics-informed architecture minimizes false positives arising from benign disturbances. We assessed the proposed method on an IEEE 9-bus test system featuring two FOC-VSMs. The results show that our method can successfully detect FDI attacks while handling regular changes, proving it could be a strong solution.

We propose a modification of the virtual synchronous machine (VSM) control algorithm in order to better dampen oscillations in a power system comprising VSMs. The change is to add a virtual damper winding (VDW) to the control algorithm. The effect of the damper winding on the rotor of a real synchronous generator is similar to the presence of an asynchronous machine on the same axis. Its full theoretical analysis is complicated and requires several additional state variables. Instead, we derive the damper winding torque Tw from the existing state variables of the fourth-order VSM model and its terminal voltages. We assume that the damping torque is proportional to the power angle derivative. The decrease of the oscillations is verified by simulations.

Transient stability of photovoltaic-hydro microgrid systems is poor due to lack of inertia and the intermittent nature of photovoltaic systems. The stability of such systems can be improved by using virtual synchronous machines which add inertia into the system to allow for higher PV penetration without losing stability. A PV-hydro system was simulated in MATLAB/Simulink to analyze the transient stability problems due to PV fluctuations and/or load or generation changes. Virtual synchronous machine was added which reduced the frequency deviations and the high rate of change of frequency. The results indicate that the transient stability of photovoltaic-hydro microgrid systems can be improved by using virtual synchronous machines using a minimum amount of energy storage. However, the power requirements of the virtual synchronous machine's converter was found to be high compared to the overall system size. Furthermore, high PV penetration levels were achieved by adding virtual synchronous machine to the system.