Frequency support characteristics of grid-interactive power converters based on the synchronous power controller

Grid-interactive converters with primary frequency control and inertia emulation have emerged and are promising for future renewable generation plants because of the contribution in power system stabilization. This paper gives a synchronous active power control solution for gridinteractive converters , as a way to emulate synchronous generators for inerita characteristics and load sharing. As design considerations, the virtual angle stability and transient response are both analyzed, and the detailed implementation structure is also given without entailing any difficulty in practice. The analytical and experimental validation of frequency support characteristics differentiates the work from other publications on generator emulation control. The 10 kW simulation and experimental frequency sweep tests on a regenerative source test bed present good performance of the proposed control in showing inertia and droop characteristics, as well as the controllable transient response.Peer ReviewedPostprint (author's final draft

Performance Guaranteed Inertia Emulation for Diesel-Wind System Feed Microgrid via Model Reference Control

In this paper, a model reference control based inertia emulation strategy is proposed. Desired inertia can be precisely emulated through this control strategy so that guaranteed performance is ensured. A typical frequency response model with parametrical inertia is set to be the reference model. A measurement at a specific location delivers the information of disturbance acting on the diesel-wind system to the reference model. The objective is for the speed of the diesel-wind system to track the reference model. Since active power variation is dominantly governed by mechanical dynamics and modes, only mechanical dynamics and states, i.e., a swing-engine-governor system plus a reduced-order wind turbine generator, are involved in the feedback control design. The controller is implemented in a three-phase diesel-wind system feed microgrid. The results show exact synthetic inertia is emulated, leading to guaranteed performance and safety bounds.Comment: 2017 IEEE PES Innovative Smart Grid Technologies Conferenc

The behaviour of virtual synchronous machine (VSM) based converters in front of non-saturable faults

Power converter penetration has increased substantially in the last 20 years bringing new challenges from the system protection perspective. The power network is undergoing a major transformation as the major part of new installed power comes from non-synchronous sources such as wind or solar. These changes might lead to malfunction of the conventional protection schemes such as overcurrent protection or distance protection relays. At the same time, the reduction of the system inertia might cause the tripping of the Loss of Main protection due to a very aggressive Rate of Change of Frequency. To enhance the grid voltage source characteristic and mitigate the loss of inertia, a new set of converter controllers known as Grid forming Converter or Virtual Synchronous Machine has been suggested in recent years. The performance of VSM could provide a potential advantage compared to traditional power converter controllers when a large frequency deviation occurs helping to keep the system stable. This article quantifies and compares the performance of different converter control algorithms including Current Vector Control, Virtual Synchronous Machine and Power Synchronisation Control in front of different frequency events

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

Virtual synchronous generator: an overview

The continuous increase in the penetration of renewable energy (RE) based distributed generations (DGs) in the power system network has created a great concern on the stability of the existing grid. Traditional bulk power plants, which are dominated by synchronous machines (SMs) can easily support system instability, due to the inherent rotor inertia and damping characteristic, as well as voltage (reactive power) control ability. Nevertheless, converter based RE has some special characteristics, such as stochastic real and reactive power output, quick active and reactive power response, small output impedance, and little or no inertia and damping property thereby causing frequency and voltage instability in the system. To solve this problem, virtual synchronous generator (VSG) concept was proposed to emulate some of the features of conventional SG through converter control strategy in order to provide additional inertia virtually. Different control schemes for VSG has been proposed in literature. Surprisingly, an overview of these schemes is yet to be efficiently presented. This paper presents an overview of the VSG control schemes. It provides the concepts, the features of the control schemes and the applications of VSG. Finally, the crucial issues regarding VSG control schemes and the necessary improvement that need to be addressed are highlighted.Keywords: Distributed generation, Synchronous generator, Virtual synchronous generator, Power electronicv converter, Energy storage system, Frequency contro

Power Sharing in Island Microgrids

The growth of local renewable energy sources and heavy loads in power distribution networks, such as the increasing electric vehicles charging stations, causes several issues with a direct impact on the stability of the electrical grid. An attempt to overcome such issues is the microgrid concept, which has the grid structured into local sub-grids that manage their power and energy balancing. A microgrid may operate connected or disconnected from the main grid, being dynamically necessary to guarantee a power balancing between local loads and sources. Furthermore, as several power units are connected to the same microgrid, equity is also required in terms of power sharing. Current work explores a scenario of an island operation of a microgrid with multiple sources, including battery storage systems and sharing power with multiple loads, including electric vehicle chargers, a scenario appropriated to a city grid. A local control solution for a stable operation of the microgrid in terms of both power balancing and power sharing is presented and validated through numerical and experimental results

Inertia effect and load sharing capability of grid forming converters connected to a transmission grid

The virtual synchronous machine concept (VSM) has been developed initially to reproduce the synchronous machine stabilizing effect by providing inertia with the emulation of swing equation, whereas droop control is developed initially to ensure load sharing and has no inertia. An introduction of a low pass filter to droop control has been motivated to filter the active power measurement and ensures a time decoupling with the inner control loops, whereas, this low-pass filter can also provide inertia to the system. This functionality is limited due to its negative impact on the active power dynamic. This paper proposes an analysis of the conventional droop control by showing its limitations and proposes an improved inertial droop control that allows providing the inertia to the system and ensures a good dynamic behavior of the active power at once in simple manner, and without modifying the load sharing capability. The results obtained are compared to the conventional method (Droop control and VSM) in various topologies in order to show the relevance of the proposed method

Equilibrium mechanism between dc voltage and ac frequency for ac–dc interlinking converters

The equilibrium between dc bus voltage and ac bus frequency (Udc-f equilibrium) is the algorithm core of unified control strategies for ac-dc interlinking converters (ILCs), because the equilibrium implements certain mechanism. However, what the mechanism is has not been explicitly explored, which hinders further studies on unified control. This paper reveals that the state-space model of a Udc-f equilibrium controlled ILC is highly similar to that of a shaft-to-shaft machines system. Hence a detailed mechanism is discovered and named “virtual shaft-to-shaft machine (VSSM)” mechanism. A significant feature of VSSM mechanism is self-synchronization without current sampling or ac voltage sampling