AC Bus Frequency Research Articles

Centralised Control and Energy Management of Multiple Interconnected Standalone AC Microgrids

When microgrids operate autonomously, they must curtail the surplus of renewable energy sources (RES) while minimising reliance on gas. However, when interconnected, microgrids can collaboratively minimise RES curtailment and gas consumption due to the ability of exchanging power. This paper presents a centralised controller and energy management of multiple standalone AC microgrids interconnected to a common AC bus using back-to-back converters. Each microgrid consists of RES, a battery, a gas-powered auxiliary unit, and a load. The battery’s state of charge (SOC) is controlled and is used in the AC bus frequency to indicate whether the microgrid has a surplus or shortage of power. High-level global droop control exchanges power between the microgrids. The optimisation problem for this interconnected system is modelled cooperatively to determine the optimal dispatch solution that minimises the energy cost from the auxiliary unit. The optimal dispatch is solved in three cases using the Nelder–Mead simplex algorithm under different settings: one-variable optimisation, three-variable optimisation with the standard droop equation, and three-variable optimisation with a modified droop equation. The optimised performance results are compared with those of the non-optimised benchmark to determine the percentage of optimal performance. The simulation results show that the total energy cost from the auxiliary unit is minimised by 8.98%.

A virtual inertial control strategy for bidirectional interface converters in hybrid microgrid

Insufficient inertia is one of the urgent problems to be solved in the stability of AC-DC hybrid microgrid. In order to improve AC bus frequency and DC bus voltage inertia in hybrid microgrid, a virtual inertia control strategy for bidirectional interface converter (BIC) based on virtual synchronous generator (VSG) is proposed. The virtual inertia equations on both sides of the AC and DC are used to slow down the variation of the DC voltage and the AC frequency. When the load fluctuates, the dynamic response of the DC voltage, the AC frequency and the power transmission between the AC and DC subnets is improved, thus improving the stability of the system. This control strategy enables BICs to share active power equally under stable conditions, prevents circulating power, and achieves accurate power sharing among parallel BICs. The small signal model of BIC under virtual inertia control is established, the dynamic characteristics of the system under different control parameters are analyzed, and the influence of the control parameters on the system is discussed. The simulation model is established in PSCAD/EMTDC software to verify the correctness of the proposed control strategy.

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.

Improving the stability of hybrid microgrids by nonlinear centralized control in island performance

In this paper, based on the nonlinear controller, a new structure is presented for centralized control of hybrid microgrids. In the proposed model, AC and DC buses are used to connect AC and DC sources and loads respectively. A bidirectional interface converter (BIC) is provided for the electrical and control connection of these two buses. Here, based on the generalized backstepping method (GBM), a new method is presented for integrated and simultaneous controlling of AC bus frequency and DC bus voltage. The advantages of this method are the centralized control of the hybrid microgrid, taking into account the dynamics of the whole system and the interaction of its state variables with each other. Harmonized and optimal exchange of power and precise regulation of control signals in this structure has led to improved stability and thus improved system performance. The comparative results of centralized and decentralized control show the efficiency of the proposed model to reduce fluctuations and improve the stability of the system.

Distributed control strategy for global economic operation and bus restorations in a hybrid AC/DC microgrid with interconnected subgrids

With the increasing penetration of distributed generators (DGs) in microgrid (MG), the economic power dispatch (EPD) is a fundamental optimization problem of MG systems. This paper investigates a distributed control strategy, which is consists of local economic control (LEC) and global economic control (GEC), for a hybrid MG with multiple interconnected ac and dc subgrids. The incremental cost (IC) of each DG is selected as a discrete consensus variable and the mismatch of bus parameters, such as dc bus voltage and ac bus frequency, between measured and reference values is assigned as a feedback variable. Thus, both the economic power allocation demand for all DGs and the bus restorations within each subgrid can be simultaneously realized by the proposed LEC. This reduces implementation cost, enhances the operation reliability, and accelerates the converging speed of algorithm. Furthermore, the interconnected subgrids are connected by a set of bidirectional interlinking converters (BILCs) that play a critical role in the power flow performance. Hence, the distributed GEC makes the whole MG system is in global economic operation in accordance with the equal IC principle. The impacts of communication time delays and communication link failures are also analyzed. Finally, the effectiveness of the proposed control strategy is verified by the real-time hardware-in-loop (HIL) tests.

An improved coordination control for a novel hybrid AC/DC microgrid architecture with combined energy storage system

This paper proposes a novel hybrid ac/dc microgrid (MG) architecture that integrates a combined energy storage system (ESS) for both ac and dc subgrids, which avoids the problems associated with the distributed ESSs with different state-of-charges (SoCs). To realize effective operation of the hybrid MG, this paper has also proposed an improved coordination control strategy between the combined ESS and the paralleled bidirectional interlinking converters (BILCs). First, a novel SoC-based adaptive droop control is designed for the combined ESS to provide dc bus voltage support and ac bus frequency support, and the lifetime of the combined ESS is extended because the overcharge and overdischarge can be avoided. Second, ac bus frequency and dc bus voltage are regarded as global parameters to enforce the power interaction between ac and dc subgrids. The power interactions between ac and dc subgrids can be proportionally allocated with the coordination coefficients to avoid an over-used BILC. Furthermore, the proposed distributed coordinated power control (DCPC) for BILCs network has plug-and-play capacity and allows an acceptable communication time delay and communication link failures. Finally, the effectiveness of the proposed coordination control strategy applied into the novel hybrid ac/dc MG system is verified by the real-time hardware-in-loop (HIL) tests.

Adjustable inertia implemented by bidirectional power converter in hybrid AC/DC microgrid

Hybrid AC/DC microgrid is regarded as a low inertia system due to the extensive integration of renewable energy sources. Thus, the AC bus frequency and DC bus voltage are easily damaged when a step change or random fluctuation appears, which threatens the system power quality. Focusing on this problem, this study proposes an adjustable inertia implementing method for the two-stage bidirectional power converter (BPC) to enhance the dynamic performance of the hybrid power system. Concretely, the first stage of the BPC is controlled as a virtual synchronous generator to support the AC subgrid and plays the role of imitating the inertia frequency response in the AC bus. The second stage controls the built-in capacitor of the BPC to implement adjustable inertia for the DC subgrid and improves the DC bus voltage response. Besides, the terminal voltage of the built-in capacitor will be restricted to ensure the normal operation of the BPC. The small-signal analysis of the hybrid system is provided to illustrate the stability of the proposed control method. Finally, the experimental results based on the real-time hardware-in-loop platform demonstrate the effectiveness of the proposed method.

Comprehensive power management scheme for the intelligent operation of photovoltaic‐battery based hybrid microgrid system

This study proposes a comprehensive power management scheme (CPMS) for the economical and smart operation of a photovoltaic (PV) battery based Hybrid Microgrid (HMG) system. The solar-PV, DC-loads and battery storage are interfaced to DC-bus of the HMG. The battery is integrated into the DC-bus using a bidirectional energy storage converter (BESC). The voltage and frequency of AC-bus are regulated by utility grid and bidirectional interlinking converter (BIC) during grid-connected and islanded modes of operation, respectively. The proposed CPMS obviates high-speed communication link between converters for maintaining the system stability during power disturbances. Therefore, the vulnerability to collapse due to communication link failure is eliminated, thereby enhancing system reliability. The operational performance of HMG is improved with the proposed CPMS by providing additional features (minimisation of operating cost of the HMG, enhancement in voltage regulation of the DC-bus etc.) using low-speed communication network. The proposed CPMS can also be extended to a larger HMG system by utilising the full bandwidth of the communication network. Further, the proposed CPMS enables HMG to seamlessly switch from grid-connected to islanded mode and vice-versa. Experimental results validating the efficacy of the proposed CPMS are presented.

Coordinated Power Management of the Subgrids in a Hybrid AC–DC Microgrid with Multiple Renewable Sources

ABSTRACTThis paper presents the assessment of the operation and control of hybrid AC–DC microgrid (HMG) integrated with alternative energy sources. The system is analyzed by incorporating fuzzy logic and adaptive neuro-fuzzy inference system (ANFIS) based controllers in the control scheme of battery. Photovoltaic array and fuel cell system are coupled to the DC bus and an electrolyzer based hydrogen storage system is used for energy storage in the DC subgrid. The output of the wind energy system along with the battery is fed to the AC bus through an inverter. The AC and DC subgrids are integrated through an interlinking converter which is controlled to facilitate power exchange between the subgrids based on the frequency of AC bus and DC bus voltage. Appropriate power management is achieved within the subgrids and between the subgrids by the coordinated control schemes employed in the HMG. The main focus of this article is the analysis of the impact of incorporating conventional proportional–integral controller, fuzzy logic controller and ANFIS based controller in the control scheme of the battery when the subgrids are subjected to power fluctuations. By employing the ANFIS based controller, the transients in the voltage and power output of the battery system are minimized and deviations occurring in the AC bus voltage during load variations are reduced.

Bidirectional Droop Control of AC/DC Hybrid Microgrid Interlinking Converter

The interlinking converter in an AC/DC hybrid microgrid plays a crucial role in the stable operation and power allocation of power system. A bidirectional droop control method for the converter is proposed, which measures the power demand degree of AC subgrid and DC subgrid according to AC bus frequency and DC bus voltage respectively, and determines the direction and magnitude of the converter transmission power. Two droop control modules, DC voltage-active power and AC frequency-active power, are included in the control structure. The power reference value of the converter is achieved from the output subtraction of two modules. Meanwhile, to reduce the deviation of voltage and frequency caused by droop control, a recovery control is designed to improve the power quality and the reliability of the hybrid microgrid. The bidirectional droop control can accurately coordinate the power transmission between AC sub-grid and DC sub-grid and make full use of the distributed energy. The feasibility of the control method is verified by a simulation system built with DigSILENT.

Adaptive fuzzy sliding mode command-filtered backstepping control for islanded PV microgrid with energy storage system

This study focuses on the control of islanded photovoltaic (PV) microgrid and design of a controller for PV system. Because the system operates in islanded mode, the reference voltage and frequency of AC bus are provided by the energy storage system. We mainly designed the controller for PV system in this study, and the control objective is to control the DC bus voltage and output current of PV system. First, a mathematical model of the PV system was set up. In the design of PV system controller, command-filtered backstepping control method was used to construct the virtual controller, and the final controller was designed by using sliding mode control. Considering the uncertainty of circuit parameters in the mathematical model and the unmodeled part of PV system, we have integrated adaptive control in the controller to achieve the on-line identification of component parameters of PV system. Moreover, fuzzy control was used to approximate the unmodeled part of the system. In addition, the projection operator guarantees the boundedness of adaptive estimation. Finally, the control effect of designed controller was verified by MATLAB/Simulink software. By comparing with the control results of proportion-integral (PI) and other controllers, the advanced design of controller was verified.

Supervisory Control for Power Management of an Islanded AC Microgrid Using a Frequency Signalling-Based Fuzzy Logic Controller

In islanded ac microgrids consisting of renewable energy sources (RES), battery-based energy storage system (BESS), and loads, the BESS balances the difference between the RES power and loads by delivering/absorbing that difference. However, the state of charge and charging/discharging power of the battery should be kept within their design limits regardless of variations in the load demand or the intermittent power of the RES. In this paper, a supervisory controller-based on fuzzy logic is proposed to assure that the battery power and energy do not exceed their design limits and maintaining a stable power flow. The microgrid considered in this paper consists of a PV, battery, load and auxiliary supplementary unit. The fuzzy logic controller alters the ac-bus frequency, which is used by the local controllers of the parallel units to curtail the power generated by the PV or to supplement the power from the auxiliary unit. The proposed FLC performance is verified by simulation and experimental results.

Improved power flow control strategy of the hybrid AC/DC microgrid based on VSM

The hybrid AC/DC microgrid with different types of distributed generations (DGs) and load demands is considered to be the preferred microgrid mode in the future. For hybrid AC/DC microgrid, autonomous and bidirectional power flow between AC and DC subgrids with superior dynamic performance is the fundamental objective to realise proportional power sharing and robust operation. In this study, an improved active power control strategy of the bidirectional AC/DC main converter (BMC) based on virtual synchronisation machine (VSM) for inertia improvement of the AC bus frequency and DC bus voltage of the hybrid microgrid is proposed. The VSM control is derived through the virtual inertia equation and the virtual capacitance equation, which can manage the AC frequency and DC voltage simultaneously. Then, the small signal and large signal characteristics analysis of the BMC are conducted to analyse the VSM control feature with different control parameters. On the basis of the small signal analysis, a proportional power sharing principle to select the control parameters is given to make DGs share the power commensurate with their power ratings. Finally, simulation cases are carried out to show the validity and efficiency of the proposed control strategy.

Dynamic Analysis of Small Wind Turbines Frequency Support Capability in a Low-Power Wind-Diesel Microgrid

When wind power accounts for a large portion of the islanded microgrid power, it may need to support the ac bus frequency regulation. The increasing penetration of variable speed wind turbines (WTs) in microgrids leads to a lower inertia, as the rotational speed of the turbine and the grid is decoupled by power electronic converters. Lower system inertia results in a larger and faster frequency deviation after occurrence of abrupt variations on generation and load. It is possible to implement control loops in the WT converters to provide a virtual inertia and support frequency regulation in the microgrid. This paper investigates the variables related to the frequency compensation capability of WT, such as kinetic energy, dc-link capacitance, turbine size, wind penetration, number of turbines, operating region along the power curve, power reserve and droop control gain, etc. The analysis is structured as a design of experiment (DOE) to have a clear and organized comparison of multiple system configurations. An optimal control technique is applied to provide fair comparison among the system variables. A flowchart explaining how the DOE and controllers gains were defined is provided.

Power system stabilizer design via structurally constrained optimal control

This paper proposes an integrated method for power system stabilizer design applicable to multimachine systems. The parameters of all stabilizers are jointly determined, so that the dynamic interactions among the system machines are properly taken into account during the design procedure. By imposing output feedback and decentralization as structural constraints on the control problem, the method provides results which are in full agreement with PSS topologies usually employed in practice. Also, it allows the design of stabilizers derived from speed, electric power, AC bus frequency, etc., or dynamical combinations of these signals. To assess the performance of the proposed method, both numerical and simulation results of its application to two distinct power systems are presented.

Applying Power System Stabilizers Part I: General Concepts

The general concepts associated with applying power system stabilizers utilizing shaft speed, ac bus frequency, and electrical power inputs are developed in this first part of a three-part paper. This lays the foundation for discussion of the tuning concepts and practical aspects of stabilizer application in Parts II and III. The characteristics of the "plant" through which the power system stabilizer must operate are discussed and the implications upon stabilizer tuning and performance are noted. A general approach for analyzing stabilizers utilizing an arbitrary input signal is described and applied to the frequency and electrical power input signals.