Droop Characteristics Research Articles

Virtual oscillator‐based methods for grid‐forming inverter control: A review

In inverter-dominant power systems, grid-forming (GFM) inverters regulate voltage and frequency. To construct GFM inverters, conventionally, various control methods based on synchronous machine emulation or droop characteristics have been employed. However, recently, inverters are regulated by Virtual Oscillator Control (VOC) to emulate the dynamics of a weakly nonlinear oscillators. In contrast to droop control, VOC is a time-domain controller that enables interconnected inverters to stabilize arbitrary initial conditions to a synchronized sinusoidal limit cycle rapidly. The fast synchronization, accurate power sharing in grid-connected and islanded modes, simple and robust implementation make it a promising candidate for inverter-dominant power systems. Moreover, VOC removes the computational burden within machine emulation-based methods and provides an approach to measure the frequency without employing phase-locked loop (PLL). Hence, to leverage the advantages of this method and expand its application in power system control, this paper reviews different VOC implementations. Mainly, this paper focuses on the studies related to the fundamental theory of oscillators and design process. Additionally, there are discussions on the stability analysis of VOC-based systems, harmonics and pre-synchronization. Challenges on the compatibility with heterogeneous controllers, fault ride-through and other limitations of VOC including non-inertial response and coupled power control are investigated and solutions are outlined.

Tertiary Control for Energy Management of EV Charging Station Integrated With PV and Energy Storage

Along with the rapid increase in the number of electric vehicles, more and more EV charging stations tend to have charging infrastructure, rooftop photovoltaic and energy storage all together for energy saving and emission reduction. Compared with individual design for each of the components in such kind of systems, an integrated design can result in higher efficiency, increased reliability, and lower total capital cost. This paper mainly focuses on the tertiary control strategy for dynamic state operation, such as PV generation fluctuation and random arrival/leave of EVs. The tertiary control aims to achieve stable operation under dynamic states, as well as to optimize the energy flow in the station to realize maximal operational benefits with constraints such as peak/valley price of electricity, state of discharge limitation of battery, etc. In this paper, four energy management functions in tertiary control level are proposed, and their performance is verified by simulations. By using prediction of PV power and EV load in the following 72 h, a novel tertiary control logic is proposed to optimize PVC and ESC power flow by changing their droop characteristics, so that minimum operational cost for the station can be achieved. Furthermore, a sensitivity analysis is conducted for three parameters, including ES battery capacity, weather influence, and PV and EV load prediction error. The results from sensitivity analysis indicate that ES battery capacity and weather condition lead to a great impact on the operational cost of the integrated charging station, while a typical prediction error of PV power and EV load will not influence the optimization result significantly.

Integration of Renewable Energy Generating Sources with Micro-Grid

This paper shows the control of microgrids at isolated sites powered by hybrid wind and solar energy sources. The machine used for wind energy conversion is a double fed induction generator (DFIG) and the battery bank is connected to a common DC bus from them. Photovoltaic (PV) arrays are used to convert solar energy. Solar energy is consumed in a cost-effective manner with a DC-DC boost converter on DFIG's common DC bus. The voltage and frequency are adjusted by the indirect vector control of the line-side converter with drooping characteristics. It changes the frequency reference based on the energy level of the battery, slowing the overcharging or discharging of the battery. This system can be operated without wind power. Control algorithms for wind and solar systems include maximum power point tracking (MPPT). This system is designed for fully automated operation, taking into account all the actual conditions of the system also has external power support to charge the battery without additional requirements. The simulation model of the system is developed in the MATLAB environment, and the simulation results are displayed under various conditions. wind or sun impermeable, unbalanced load is equally non-linear, low battery charge state. Finally, the prototype of the system is implemented using a 5kW photovoltaic array simulator and a 3.7kW wound rotor asynchronous machine to generate experimental results to validate the theoretical model and design.

Power Loss and Total Load Demand Coverage in Stand-Alone Microgrids: A Combined and Conventional Droop Control Perspectives

Conventional droop control plays an essential role in microgrids with distributed generators, DGs, and variable load demand. Despite its advantages, the conventional droop scheme does not take into account the minimization of power losses and the maximization of the total load coverage. Moreover, most of the nonconventional droop control studies that address power loss consider one variable load. This study proposes a combined droop scheme and inspects its effects on power loss minimization and total load coverage. The proposed scheme combines conventional and nonconventional droop schemes and constructs a 3D <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P-f</i> droop characteristic that takes into account the presence of two variable loads. Four and six-bus microgrids are adopted to simulate and illustrate the system behavior in MatLab. Results are compared to the standard fully conventional droop approach; results showed that the combined scheme has higher performance in terms of power loss gain and total load coverage.

Development of a Droop Control Strategy for Flexible Operation of Distributed Generators

Droop control is one of the control strategies utilized to establish a simple, effective, and a communication-less power sharing between several distributed generators (DGs) in power systems and microgrid networks. In this paper, the control of an inverter-based system operating with the proposed droop control is developed and analyzed. The proposed droop control with its adjustable nonlinearity level aims to flexibly control the DGs in order to fulfill multiple objectives that aim to stabilize the operation of the sources and optimize the power sharing. These objectives can be achieved, simultaneously, as the droop characteristics generated under the proposed control cover all the possible operating points in the frequency-active power (f-P) and voltage- reactive power (V-Q) planes. The capability of the proposed control strategy to construct highly non-linear characteristics enables the DG to effectively meet different complex technical and economic constraints. The work carried out in this paper focuses on the f-P droop relation, and investigates the system performance under different physical and control parameters. The obtained results show that the proposed droop control is able to perform well under different operating conditions, shape the power sharing of the running DGs and stabilize the system performance.

Control of single-phase photovoltaic H6 inverter in grid-connected and stand-alone modes of operation

This paper proposes a control strategy for single-phase transformerless photovoltaic H6 inverter with capability of operation in both grid connected and stand-alone modes. This control strategy enables seamless transition between two modes without any reconfiguration of control structure and need of any external communication. A multi-loop control structure including an inner voltage and current control with PR controller and outer power control with droop controller is utilised to make the inverter operate in both modes. In grid-connected mode, the power control loop regulates reference voltage and frequency for voltage control loop, to adjust power flow of inverter to the main grid while in islanded mode, reference voltage and frequency are determined according to droop characteristic of inverter to supply local load demand. The simulation and experimental result are used to verify the performance of the proposed control.

Decentralized Power Management of DC Microgrid Based on Adaptive Droop Control With Constant Voltage Regulation

An adaptive droop control with constant voltage regulation is proposed for the power and voltage management of a DC microgrid (DCMG) with multiple power sources, such as a utility grid (UG), a distributed generator (DG), an energy storage system (ESS), and an electric vehicle (EV). In the proposed scheme, the droop characteristics for the UG, ESS, and EV are adaptively changed according to electricity price conditions and state-of-charge (SOC) levels in order to optimize the DCMG operation flexibility as well as electricity cost. The proposed control method not only ensures the power-sharing of DCMG reliably without the use of a communication link, but also regulates the DC bus voltage stably at the nominal value. To achieve this, the proposed scheme consists of primary control and secondary control. The primary control is used to achieve power-sharing in the decentralized DCMG, while the secondary control is used to overcome the disadvantage of conventional droop control, i.e., to remove DC bus voltage deviations. Decentralized power management is also presented to enhance the DCMG system reliability in the presence of uncertainties such as DG generation power, the ESS and EV SOC levels, the grid and EV availabilities, the load demand, and electricity price conditions. The effectiveness of the proposed scheme is demonstrated in a comprehensive simulation and experiment under various conditions. The test results clearly confirm the control flexibility and overall performance of the proposed control scheme for decentralized DCMG system.

A Decentralized Passive Islanding Detection Method Based on the Variations of Estimated Droop Characteristics

A microgrid including distributed generators can operate connected to the main electrical network or in an isolated manner, referred to as island operation. The transition between both states can occur voluntarily, but a disconnection can also happen unexpectedly. The associated transients can be harmful to the grid, and compensating actions must be triggered to avoid service interruption, preserve power quality, and minimize the possibility of faults; island detection methods are essential to this end. Such techniques typically depend on communication networks or on the introduction of minor electrical disturbances to identify and broadcast unexpected islanding events. However, local energy resources are distributed, variable, and are expected to be integrated in a plug-and-play manner; then, conventional island detection strategies can be ineffective as they rely on specific infrastructure. To overcome those problems, this work proposes a straightforward, distributed island detection technique only relying on local electrical measurements, available at the output of each generating unit. The proposed method is based on the estimated power-frequency ratio, associated with the stiffness of the grid. A “stiffness change” effectively reveals island operating conditions, discards heavy load variations, and enables independent (distributed) operation. The proposal was validated through digital simulations and an experimental test-bed. Results showed that the proposed technique can effectively detect island operation at each generating unit interacting in the microgrid. Moreover, it was about three times faster than other reported techniques.

Research on Control Method of Microgrid Based on Multi Distributed Generation

Reducing the dependence of microgrid upon the communication system and realizing the efficient control of multiple distributed generation of the microgrid are problems that need to be solved urgently. Through the research, based on multiple microgrid operation modes, the peer-to-peer control strategy in microgrid is investigated, and the peer-to-peer control strategy method of microgrid is given for a variety of complex control problems of distributed power According to the peer-to-peer control strategy method, distributed power supply adopts droop control in adjusting distributed power supply in output voltage and frequency; the droop controller has P-f and Q-U droop characteristics. This paper establishes a peer-to-peer control microgrid simulation model, adopts the droop controller designed in this paper to island mode and grid-connected mode, and investigates how the microgrid switches between the two modes. In accordance with Matlab/Simulink simulation outcomes, the research examines frequency, voltage and power changes in distributed generation in the microgrid, and verifies the validity and feasibility of microgrid peer-to-peer control strategy.

Voltage-Based Segmented Control of Superconducting Magnetic Energy Storage for Transient Power Fluctuation Suppression in Island DC Microgrid

Voltage stability is one of the critical factors for the stable operation of DC microgrids (MG). For the communication free DC MG, the DC voltage is more vulnerable due to the DC voltage deviation caused by the droop characteristics. When facing the transient power fluctuation caused by multiple electric vehicles (EVs) connected to the grid, PV shedding, etc., the DC bus will have a large voltage sag. The voltage quality degradation will bring adverse effects to the DC MG. However, most energy storage systems like battery energy storage systems (BESS) are not suitable for suppressing transient large power fluctuation. In this paper, the SMES is introduced to a DC MG to form a power regulation system. A voltage-based segmented (VBS) control is designed for the SMES-based power regulation system to suppress transient large power fluctuation in the DC MG. The voltage deviation caused by power distribution can be tolerant by VBS controlled SMES. Meanwhile, the dynamic recovery capability of the DC voltage is improved. The control method of SMES is independent of other devices in DC MGs, hence it is not dependent on communication. The control strategy of the SMES is further verified by simulation results.

Double-Layer Feedback Control Method for Synchronized Frequency Regulation of PMSG-Based Wind Farm

With the rapid growth of wind power penetration in power grid, the system operators need to implement the grid specification of wind farm participating in primary frequency regulation (PFR). In this paper, a double-layer control framework based on permanent magnet synchronous generator (PMSG) is proposed, which can simulate the inertia and droop characteristics similar to thermal units. The top-layer control adopts centralized model predictive control (MPC), while the bottom layer carries out distributed real-time correction control in case of inaccurate MPC prediction. In order to realize fast distributed correction among wind turbines (WTs), a distributed Newton method is proposed, which only requires WTs to exchange limited information with their neighbors on sparse communication network, and has a super-linear convergence rate. By introducing an index of state of energy, this method adequately exploits the kinetic energy of all WTs without loss of security. Compared with the centralized schemes, the dependence on prediction accuracy and data synchronization is greatly reduced. Compared with the distributed schemes, the overall characteristics and computing efficiency can be guaranteed. The simulation results show that the method has satisfactory dynamic performance and reliability, and the system frequency stability is significantly improved under the synchronized control method.

Frequency control scheme with dynamic droop characteristics of a DFIG for mitigating the frequency fluctuations

The variations of wind generation result in the considerable fluctuations of the system frequency, which are difficult to be regulated in the acceptable range in electric power systems with highly penetrated wind generations. This paper suggests an enhanced frequency control scheme with dynamic droop coefficients of a doubly fed induction generator (DFIGs, type 3) that is able to mitigate the fluctuations of the system frequency with more efficiency. To this end, a supplementary control loop with system frequency-dependent droop control coefficients is employed based on the deviation of the system frequency. Simulation results clearly indicate that the suggested frequency control scheme is benefit to improving the frequency regulation capability under various wind speed patterns and penetration levels as energy storage systems do for frequency regulation, particular in highly wind generation penetrated power systems.

Energy loss evaluation of a battery buffered smart load controller

This paper aims to evaluate the energy and power loss of a single-phase battery-buffered smart load (BBSL) under demand-side primary frequency control (PFC). The BBSL consists of a battery and a power & energy management system (PEMS), the evaluation in this work entails the experimental measurement of the energy lost in the battery during the charge and discharge procedures dictated by the PFC and the measurement of the power lost in the PEMS. A method is developed to measure the battery energy loss using the smart load strategy by maintaining the battery state of charge (SOC) constant over the PFC regulation period. The effects of the PFC droop characteristics and dead-bands on battery energy loss are studied. The experimental measurements of the energy loss are confirmed by simulation of a battery model in which the total voltage drop of the battery is represented by a nonlinear current-dependent resistance. Then, the power loss associated with the power electronic converters of the PEMS is experimentally determined under different PFC droop and dead-band settings.

Intelligent control of islanded AC microgrids based on adaptive neuro-fuzzy inference system

In inverter-based AC microgrids, active power sharing between Distributed Generation (DG) units is achieved by introducing a droop frequency control for the inverter. The purpose of the secondary controller is to regulate the frequency deviation caused by the primary controller while maintaining the droop characteristic of active power sharing among DG units. In this paper, Adaptive Neuro-Fuzzy Inference System (ANFIS) is employed to effectively control frequency and power sharing among DG units in a 100% renewable based microgrid while working in different areas of the operating points. Rules and configuration of ANFIS are introduced to handle the complexity of larger microgrids. The effectiveness of the proposed method is demonstrated in two case studies. The simulation results suggest that the proposed ANFIS controller, compared to the conventional controllers, is able to satisfy the power sharing among DG units while better regulating the frequency.

Detailed analysis of grid connected and islanded operation modes based on P/U and Q/f droop characteristics

This paper presents a thorough control structure of the distributed generators inside the microgrid during both grid-connected and islanded operation modes. These control structures of the DGs voltage source inverters are implemented in synchronous reference frame (SRF) and controlled using linear PI controllers. By implementing the control structures, the desired real and reactive power can be efficiently transferred to the local loads and the utility load by the microgrid generating units. A modified droop control technique is introduced to facilitate the microgrid performance during both modes of operation. The active and reactive power sharing of the load demand between the utility grid and the microgrid can be performed by this drop control technique during the islanded mode. The system performance during intentional islanding event and utility load increase is investigated. The effectiveness of the offered control structures is confirmed through simulation results during both modes of operation.

Implementation of Multi-Modular Converter with Droop characteristics for High Speed Railway Traction System

Multi Modular Converters (MMC) plays a significant role in high voltage power transmission networks due to its high scalability and modularity characteristics. MMC converters produce better efficiency & show superiority in harmonic performance in high voltage applications like HVDC and high speed AC-Traction power supply systems. Its bidirectional operation also aided to get high installation reliability comparatively. The complicated power quality problems such as voltage fluctuations, Harmonic issues and reactive power problems of railway tracks due to the rapid movement of the locomotives need to be mitigated. The loss of speed is caused due to the neutral sections (NS) which are present throughout the power line. It is necessary that the neutral sections should be eradicated especially for high speed railway networks. In this paper, a new traction power transmission network which comprises zero NS and comparatively low power quality problems is designed. Droop control strategy is implemented to improve power transfer capability between the two adjacent traction substations. The implementation of MMC with parallel converter concept using the DSP processor is executed for low voltage as a prototype model. The results obtained are confirmed that the proposed system is very much suitable for high speed railway networks. Index Terms: High speed traction, Parallel inverters, Droop characteristics, Indian traction power supply system.

Laboratory Prototype Model of a High Speed Railway Supply System for Performance Enhancement with Multi Module Converter in Indian Railways

The single-phase traction system needs a performance enhancement due to the issues that involved with the single-phase transformer-based control in traction substation. Serious power quality issues such as imbalance in current and voltage at the single-phase transformer may arise during interfacing of the single phase traction system with three phase grid. Harmonic distortion, Negative sequence currents and reactive power problems are also produces disturbance in present existing traction system. This paper proposes a solution to the power quality issues due to intermittent switching (disengage and engage) of the drivetrain load. An alternate methodology to supply power to the AC drivetrain system is detailed. On AC traction side MMC (multi module converter) based converter is connected to self-synchronizing inverters. Simulation is performed in MATLAB Simulink. The drivetrain side inverter is developed based on the droop characteristics and results are verified. Simulation is carried out keeping in mind the perimeters of Indian railway. To verify the simulation results a low voltage laboratory experimental set up has been done and the results obtained are discussed.

The Strategy of Active Grid Frequency Support for Virtual Synchronous Generator

Virtual synchronous generator (VSG) control is a promising control approach for voltage source converters as an interface between new energy sources and the power grid. VSG is a grid-friendly control scheme, which can imitate the mechanical inertia of the synchronous generator (SG) and the power droop characteristics. Yet, the droop characteristics imitation of SG induces the frequency variation of the grid-connected inverter along with the droop characteristic curve, which will deteriorate the performance of the grid frequency support during the transient process. In this paper, a control scheme, which shapes the droop curve during the disturbance, is proposed for active grid frequency support. First, a load disturbance extraction strategy with a high-pass filter is applied in the proposed method, and the disturbance component is effectively extracted to compensate for the frequency reference variation in traditional VSG control. The grid frequency is actively supported by shaping the droop curve of active power to the frequency of VSG during the disturbance. Afterward, H∞ and H2 norms are used as the objective function to quantify the control performance of the proposed method, and the particle swarm optimization (PSO) algorithm is applied to optimize the control parameters of the proposed method. With a well-optimized high-pass filter, the active support performance is further improved. Finally, the simulation results and hardware in the loop (HIL) tests verify the effectiveness of the proposed method.

Design and Control of Micro-Grid fed by Renewable Energy Generating Sources

This paper presents a control of a micro-grid at an isolated location fed from wind and solar based hybrid energy sources. The machine used for wind energy conversion is doubly fed induction generator (DFIG) and a battery bank is connected to a common DC bus of them. A solar photovoltaic (PV) array is used to convert solar power, which is evacuated at the common DC bus of DFIG using a DC-DC boost converter in a cost effective way. The voltage and frequency are controlled through an indirect vector control of the line side converter, which is incorporated with droop characteristics. It alters the frequency set point based on the energy level of the battery, which slows down over charging or discharging of the battery. The system is also able to work when wind power source is unavailable. Both wind and solar energy blocks, have maximum power point tracking (MPPT) in their control algorithm. The system is designed for complete automatic operation taking consideration of all the practical conditions. The system is also provided with a provision of external power support for the battery charging without any additional requirement. A simulation model of system is developed in Matlab environment and simulation results are presented for various conditions e.g. unviability of wind or solar energies, unbalanced and nonlinear loads, low state of charge of the battery. Finally a prototype of the system is implemented using a 5 kW solar PV array simulator and a 3.7 kW wound rotor induction machine and experimental results are produced to reaffirm the theoretical model and design.

An Active/Reactive Power Control Strategy for Renewable Generation Systems

The development of distributed generation, mainly based on renewable energies, requires the design of control strategies to allow the regulation of electrical variables, such as power, voltage (V), and frequency (f), and the coordination of multiple generation units in microgrids or islanded systems. This paper presents a strategy to control the active and reactive power flow in the Point of Common Connection (PCC) of a renewable generation system operating in islanded mode. Voltage Source Converters (VSCs) are connected between individual generation units and the PCC to control the voltage and frequency. The voltage and frequency reference values are obtained from the P–V and Q–f droop characteristics curves, where P and Q are the active and reactive power supplied to the load, respectively. Proportional–Integral (PI) controllers process the voltage and frequency errors and set the reference currents (in the dq frame) to be imposed by each VSC. Simulation results considering high-power solar and wind generation systems are presented to validate the proposed control strategy.