Parameter Sensitivity Analysis of SPC-based Control Under Different Grid Conditions

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.

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.

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

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.

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.

Virtual Synchronous Generator technology improves the frequency dynamic performance of the system by introducing virtual inertia and damping coefficients into the power electronic inverter. In order to further suppress power oscillations and solve the problem that constant virtual parameters cannot take into account the maximum frequency overshoot and response speed, firstly, a small-signal model of active and reactive power rings of virtual synchronous machine is established to study the influence of virtual inertia on the frequency stability and power equalization effect of parallel connection of multiple machines. Secondly, based on the fuzzy control method in variable universe, the control strategy of coordinated adaptive adjustment of the virtual inertia of the parallel system according to the output angular frequency is proposed, and at the same time, the automatic adjustment of the virtual damping coefficient with the inertia is realized by combining with the appropriate damping ratio. Finally, the parallel parameter matching principle is designed so that the output power can be distributed according to the capacity ratio. Two different virtual synchronous machine control strategies are compared and analyzed by matlab/simulink to verify the correctness and feasibility of the proposed adaptive control strategy.

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.

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.

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.

This paper investigates the conventional droop control and the recently-developed droop control, like virtual synchronous machine (VSM) based control and Q - V control. The VSM based control can effectively smooth frequency dynamics by emulating the damping and the inertia of the synchronous machine, and Q - V droop control can enhance the more ideal results in the reactive power sharing among converters. Both controls bring their frequency and V to the initial values depending on the restoration mechanism. Such a mechanism is achieved by deviating their power reference, and it may lead to severe nonideal power-sharing when the converters asynchronously connect with the system. To improve this problem, this study presents a resetting mechanism realized through a central console. In the end, the laboratory test results are supported to validate their benefits.

The low‐frequency dynamics of virtual synchronous machine (VSM) depends on multiple factors. In this study, the oscillation mode of a single VSM is first identified by exploring the evolution of oscillation from synchronous mode to sub‐synchronous mode with the variation of structure and parameters. The inter‐oscillation modes among multiple VSMs modelled in quasi‐state phasor domain are then studied by decomposing into mean motion and relative motion. Inspired by and weak‐coupling‐based coherency identification, the associated VSM group can be fast identified and used for effective stabilizer location. The approximate eigenvalues can be fast obtained by system decomposition in uniform‐damping scenarios or by extended‐equal‐area‐criterion approach in non‐uniform‐damping scenario. An improved design for lead–lag compensation is proposed to damp both synchronous and sub‐synchronous oscillation of VSMs. Effectiveness of the proposed control strategy in grid‐connected/islanded mode is verified with real‐time simulation.

The development of the modular multilevel converter (MMC) enabled the efficient creation of high-power high-voltage direct current (HVdc) transmission systems. As a result, MMC-HVdc transmission systems became the main alternative to integrate remote renewable energy sources being deployed in accelerating rates to fight climate change. As the number of online classical synchronous generators (SGs) decreases while the one of converter-based power sources increases, power systems are suffering from lower inertia levels and from fewer providers of ancillary services. Therefore, new control strategies, such as the grid-forming (GFM) converter operation, were developed to address the ongoing power system transformation. The main contribution of this article is to propose a nonlinear (NL) control strategy compatible with GFM operation for an MMC-HVdc transmission system controlled as a virtual synchronous machine (VSM). The control strategy is developed using NL control tools, such as feedback linearization, dynamic feedback linearization, and backstepping. In addition, this article provides a rigorous mathematical stability analysis applying Lyapunov theory. The proposed control strategy is then validated by simulations using the MATLAB/Simscape Electrical package in three situations: active power tracking, converter energy tracking, and a frequency support scenario. Results show the good performance of the proposed NL controller for all situations considered, presenting a fast response and a faster disturbance rejection compared with the classical proportional integral (PI) controller.

Virtual inertia extraction from a DC bus capacitor in a three−phase DC/AC inverter-based microgrid with seamless synchronisation operation modes

In this article, the slip mechanism in the rotor-dynamics of the induction machine is emulated to improve the transient stability of grid-forming converter (or virtual synchronous machine). Different from the synchronous machine, the speeds of the rotor and of the rotating magnetic field in the stator are different in the induction machine. This slip frequency in the induction machine offers another degree-of-freedom to regulate the frequency, playing a similar role of the damper winding in the synchronous machine. It can inject transient damping to the system, and virtually increase the feedback power to smooth the power angle variations when severe fault occurs. The effectiveness of the proposed controller on improving both small-signal and transient stability (large-signal) is presented. The proposed algorithm is verified by experiments.

Transmission system operators (TSOs) are increasingly requesting frequency support, reactive power compensation, and grid-forming capability to power plants interfaced by power converters. In these plants, a large number of converters is operated in parallel and all of them should be adequately coordinated to provide these services. For that purpose, the virtual synchronous machine (VSM) control technique is commonly adopted. However, the dynamic characteristics of the whole plant depend on the number of converters connected, their size, control parameters, and electrical interconnections. This makes it difficult to know the aggregated dynamic characteristic of the plant and how each VSM contributes to the plant response. In this article, the operation of VSMs in parallel configuration for island and grid-connected applications is studied. The developments make it possible to adapt the VSMs parameters so that the grid sees the power plant as an equivalent generator whose low-frequency characteristics do not vary with the number of converters in operation. The developments were validated in a laboratory consisting of four 15 kVA converters operating as VSMs, and a grid emulator.