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The random fluctuation of renewable power generation output makes the frequency and voltage of distribution network fluctuate frequently. And the stable operation performance of the system is decreased. Therefore, the sliding mode control (SMC) strategy of grid-forming (GFM) energy storage converter with fast active support of frequency and voltage is proposed in this paper. Firstly, the virtual synchronous generator (VSG) control possessing the superior GFM performance is applied to single-stage neutral point clamped (NPC) converter of energy storage system. Meantime, the improved comprehensive equivalent circuit model of lithium iron phosphate battery and equivalent model of converter are constructed by means of ampere-hour method and Kirchhoff's law. Then, the SMC with fast response and strong robustness is utilized into the current inner-loop controller. Combined with VSG control, the SMC strategy of GFM energy storage converter is proposed, so that the converter could play an active supporting role by quickly adjusting the output power while the frequency and voltage are reduced. Finally, the simulation model of GFM energy storage converter SMC system is established. Through the simulation analyses, it can be seen that the response time of the proposed strategy to complete the active support is about 0.65 s. [ABSTRACT FROM AUTHOR]

Grid-forming (GFM) wind storage systems (WSSs) possess the capability of actively building frequency and phase, enabling faster frequency response. The frequency regulation power of GFM WSSs is provided by both the rotor of wind turbine and the battery storage (BS) in parallel with DC capacitor. However, with existing control strategies, the energy storage immediately responds to both small and large grid disturbances. The frequent responses significantly decrease the lifespan of energy storage. To address this issue, a cooperative strategy between rotor and energy storage is necessary. This paper proposes an advanced strategy of GFM WSSs for cooperative DC power support. The cooperative principle is that for small disturbances, the BS is disabled and total frequency regulation power is provided by the rotor, while for large disturbances, the BS is enabled to cooperatively provide power support with the rotor. The proposed cooperative strategy can decrease the charging and discharging times of BS with a small range of rotor speed fluctuation, and then the service life of BS can be significantly extended. Simulation results validate the effectiveness and superiority of the proposed strategy. [ABSTRACT FROM AUTHOR]

Grid forming control applied to power converters termed as grid forming converters (GFC) interfacing storage and renewable generation has been identified as a potential solution to facilitate a more significant penetration of converter-based renewable generation in the power system. The emulated electromechanical model in the GFC is also the synchronization unit which typically utilizes measured output active power from the GFC as the feedback variable. However, this feature has opened up a concern on the transient stability due to loss of synchronism under significant transient events in the power systems, which include frequency change, phase jump, and voltage dips. This paper presents a quantitative and illustrative analysis of the impact of the current limit in GFC on the transient stability of a system comprising of GFC. Furthermore, a solution based on virtual active power is proposed to improve the transient stability margin of the GFC when the GFC enters the current limit. Finally, the analysis and the proposed method to enhance the transient stability are verified by Power hardware in the loop (PHIL) experimental tests.

This paper presents an approach and a corresponding laboratory infrastructure for multi-hardware-in-the-loop (M-HiL) testing of power converters and intelligent electronic devices (IEDs) with wide area features for power system monitoring and control. The basis for the testing environment is a digital real-time simulation (RTS) platform which can be utilized for phasor-based or hybrid phasor and electromagnetic transient (EMT) power system simulation. A high-bandwidth power amplifier is used for interfacing the real-time simulated power system to the power hardware. The necessary elements and their interfaces required for realizing the M-HiL testing are highlighted in the paper. Finally, a complete example of an M-HiL test based on real-time simulation of the Nordic 44 (N44)-bus power system model is presented. The test setup includes two phasor measurement units (PMUs) with their corresponding communication layers and a converter configuration for scaled emulation of the HVDC transmission system of an offshore wind farm. The configuration also includes a grid forming converter unit operated in parallel to the grid-side HVDC terminal. The presented test demonstrates how a grid forming converter operated as a Virtual Synchronous Machine (VSM) can provide frequency support to the Nordic power system and how this support would be recorded by the PMUs.

The development of renewable energy resources has grown significantly in recent years. These distributed energy resources are typically connected to the grid through power converters, which can be operated in "grid-forming" or "grid-following" mode. But the large-scale integration of DG units has serious repercussions on the stability of the power system due to their low inertia compared to the synchronous generator. However, an appropriate control guarantees a dynamic, smooth, and fast converter behaviour in an island configuration and voltage-frequency support in grid-connected mode. This paper presents a control system for Grid-Following and Grid-Following converters for a grid-connected MG. The aim is to achieve a seamless transition between the Microgrid and the main grid, appropriate load sharing between distributed energy sources, and an improvement of system stability. The developed grid-connected battery storage system inverter has been designed to be able to operate in two different modes: grid formation mode and grid injection mode. The control strategy considers the control loops of Voltage/current, Frequency, and active/reactive power. The proposed control technique is presented and simulated under the MATLAB/Simulink tool. The simulation results demonstrate the efficiency and performance of the proposed control technique in eliminating voltage, current, and frequency fluctuations and also guarantee a seamless transition between the two modes. [ABSTRACT FROM AUTHOR]

This paper presents a grid-forming control (GFC) scheme for two-stage photovoltaic (PV) systems that maintains power reserves by operating below the maximum power point (MPP). The PV plant in GFC mode behaves like a voltage source that supports the grid during disturbances in full or limited grid-forming mode as per the reserve availability. This is a model-free method that avoids the estimation of MPP power in real-time commonly done in the literature, which makes it simpler and more reliable. The proposed control also features an enhanced current limitation scheme that guarantees containment of the current overshoots during faults, which is not trivial in voltage-sourced GFC inverters. A thorough investigation is done, exploring various generation mixtures of synchronous machines (SM), GFC and grid-following (GFL) inverters, and all common disturbances, e.g., load change, faults and irradiance transients. The results show very favorable dynamic performance by the GFC inverters, far superior to GFL inverters and directly comparable to SMs. It is found that replacing SMs with GFC inverters may improve the frequency profile and terminal voltage during disturbances, despite losing out in the mechanical inertia and the strict inverter overcurrent limits. [ABSTRACT FROM AUTHOR]

The grid integration of renewable energy sources interfaced through power electronic converters is undergoing a significant acceleration to meet environmental and political targets. The rapid deployment of converters brings new challenges in ensuring robustness, transient stability, among others. In order to enhance transient stability, transmission system operators established network grid code requirements for converter-based generators to support the primary control task during faults. A critical factor in terms of implementing grid codes is the control strategy of the grid-side converters. Grid-forming converters are a promising solution which could perform properly in a weak-grid condition as well as in an islanded operation. In order to ensure grid code compliance, a wide range of transient stability studies is required. Time-domain simulations are common practice for that purpose. However, performing traditional monolithic time domain simulations (single solver, single domain) on a converter-dominated power system is a very complex and computationally intensive task. In this paper, a co-simulation approach using the mosaik framework is applied on a power system with grid-forming converters. A validation workflow is proposed to verify the co-simulation framework. The results of comprehensive simulation studies show a proof of concept for the applicability of this co-simulation approach to evaluate the transient stability of a dominant grid-forming converter-based power system. [ABSTRACT FROM AUTHOR]

• A novel grid forming current source inverter (CSI) is proposed and validated. • CSI is controlled to operate as a voltage-controlled source with grid forming characteristics. • CSI can limit inverter current without advanced control techniques such as virtual impedance. • CSI can still exhibit other useful characteristics of a GFM VSI and comparisons are presented. Grid forming (GFM) inverter control has received increasing attention in recent times due to the increasing penetration of Inverter-based-resources (IBR) in the electric grids across the world. Existing research on GFM control approaches is based on a pulse width modulated (PWM) voltage source inverter (VSI) interface to the network. In recent times, PWM current source inverter topologies have received increased attention in literature and recent research has shown some advantages over the traditional PWM VSIs. In this work, grid forming control using a PWM-CSI is proposed and the control structure is presented. The inverter possesses excellent current limiting characteristics since the inverter modulates the output current at the gating level. The use of advanced control schemes such as virtual impedance used in VSIs are not required. Further, the inverter still behaves as a voltage-controlled source and is similar in other aspects to a traditional VSI based GFM control. Simulation results for a grid connected droop controlled GFM CSI are presented. Performance during fault ride-through events is also studied for balanced and unbalanced faults and the transient stability is studied. The results validate the performance of the proposed controller and the current limiting properties of the CSI. [ABSTRACT FROM AUTHOR]

The grid integration of renewable energy sources interfaced through power electronic converters is undergoing a significant acceleration to meet environmental and political targets. The rapid deployment of converters brings new challenges in ensuring robustness, transient stability, among others. In order to enhance transient stability, transmission system operators established network grid code requirements for converter-based generators to support the primary control task during faults. A critical factor in terms of implementing grid codes is the control strategy of the grid-side converters. Grid-forming converters are a promising solution which could perform properly in a weak-grid condition as well as in an islanded operation. In order to ensure grid code compliance, a wide range of transient stability studies is required. Time-domain simulations are common practice for that purpose. However, performing traditional monolithic time domain simulations (single solver, single domain) on a converter-dominated power system is a very complex and computationally intensive task. In this paper, a co-simulation approach using the mosaik framework is applied on a power system with grid-forming converters. A validation workflow is proposed to verify the co-simulation framework. The results of comprehensive simulation studies show a proof of concept for the applicability of this co-simulation approach to evaluate the transient stability of a dominant grid-forming converter-based power system.

• Stochastic renewable generation results in the frequency variance. • Energy storage can alleviate this frequency variance but depends on the location. • A control strategy for the AC side energy storage system is proposed. • A small capacity energy storage system can reduce the frequency variance. Grid forming control of converter interfaced generation (CIG) requires some form of energy storage to be coupled with the generation. Energy storage systems (ESSs) can be coupled to the CIG either on the DC or the AC side of the power converter. When placed on the DC side, the ESS can provide damping of the variability in the generation but would require significant modification to the wind turbine hardware. The solution with an ESS connected to the AC side is simpler to implement with existing wind turbines but fails to provide damping of the CIG generation. This paper proposes a grid forming control strategy, based on virtual synchronous generator (VSG) control, which allows the ESS installed at the AC-side of the converter to have the same features and dynamic behaviour as those obtained from placement on the DC-side of the converter. In addition, the proposed control can also limit the exchanged power of the ESS within its rating for a safe operation. The proposed control is validated via a detailed Electro-Magnetic Transient (EMT) model and its impact on the grid is quantified via the case study of the All-Island Irish transmission system. Simulation results show that only a small ESS capacity can ensure that the frequency variance satisfies the grid code requirement even in the situation of a very high CIG penetration. [ABSTRACT FROM AUTHOR]