Detailed analysis of grid connected and islanded operation modes based on P/U and Q/f droop characteristics (original) (raw)
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IEEE Transactions on Power Delivery, 2012
For islanded microgrids, droop-based control methods are often used to achieve a reliable energy supply. However, in case of resistive microgrids, these control strategies can be rather different to what conventional grid control is accustomed to. Therefore in this paper, the theoretical analogy between conventional grid control by means of synchronous generators (SGs) and the control of converter-interfaced distributed generation (CIDG) units in microgrids is studied. The conventional grid control is based on the frequency as a global parameter showing differences between mechanical power and ac power. The SGs act on changes of frequency through their P /f droop controller, without inter-unit communication. For CIDG units, a difference between dc-side power and ac-side power is visible in the dc-link voltage of each unit. Opposed to grid frequency, this is not a global parameter. Thus, in order to make a theoretical analogy, a global measure of the dc-link voltages is required. A control strategy based on this global voltage is presented and the analogy with the conventional grid control is studied, with the emphasis on the need for interunit communication to achieve this analogy. A known control strategy in resistive microgrids, called the voltage-based droop control for CIDG units, approximates this analogy closely, but avoids inter-unit communication. Therefore, this control strategy is straightforward for implementation as it is close to what control engineers are used to. Also, it has some specific advantages for the integration of renewables in the network.
International Journal of Renewable Energy Research, 2019
Conventional droop-control scheme shares the load amongst energy sources in proportion to their ratings. The scheme suffers from the issue of ineffective utilization of the sources when performance of some of the sources is dependent on environmental conditions. Hence, a modified droop-control strategy is proposed for a microgrid comprising of photovoltaic (PV) based distributed generators (DG) operating in parallel with other DGs. Dynamic nature applied to the droop characteristic by the primary control unit (PCU) sets the frequency reference such that the PV sources operate at their maximum power point and the energy demanded from the auxiliary source is the minimum. The margin available after supplying the active power is used to allocate the references for reactive power sharing. The reactive power sharing algorithm employed in secondary control unit (SCU) ensures that the standard deviation of the percentage utilization of the inverters is kept the minimum. Even in case of the failure of the communication between the PCU and SCU, a reasonably good performance is ensured as the control shifts to the master-slave control having dynamic droop adjustment feature. The effectiveness of the proposed strategy against other approaches is justified through the simulation results obtained in MATLAB/Simulink.
Reactive Power Sharing Droop Control Strategy for DG Units in an Islanded Microgrid
The proposed method mainly includes two important operations: error reduction operation and voltage recovery operation. The sharing accuracy is improved by the sharing error reduction operation, which is activated by the low-bandwidth synchronization signals. However, the error reduction operation will result in a decrease in output voltage amplitude. Therefore, the voltage recovery operation is proposed to compensate the decrease., due to increasing the demand of electricity as well as rapid depletion of fossil fuels, and the government policies on reduction of greenhouse gas emissions , renewable energy technologies are more attractive and various types of distributed generation sources, such as wind turbine generators and solar photo voltaic panels are being connected to low-voltage distribution networks. Micro grid is an integrated system that contain in s distributed generation sources, control systems, load management, energy storage and communication infrastructure capability to work in both grid connected and island mode to optimize energy usage. The paper presents a advanced control technique for a micro grid system which works efficiently under a decentralized control system.
New Droop Control Technique for Reactive Power Sharing of Parallel Inverters in Islanded Microgrid
2021
This paper investigates a new droop control approach for accurate reactive power sharing of parallel inverters in islanded microgrid. The conventional active power-frequency droop and the reactive power-voltage magnitude droop control are widely used to share the total load demand among parallel distributed generators (DGs) according to their power ratings. However, even though the active power is properly shared, the reactive power sharing is inaccurate with the conventional droop control approach due to the power coupling of active-reactive power and the feeder impedance difference between DGs. In order to solve this problem, an improved droop method is proposed. The reactive power sharing error between DGs is estimated by injecting a small perturbation in the active power loop. Furthermore, an integrator is added to the classical reactive power droop controller to compensate reactive power sharing error. The effectiveness of the modified droop control strategy is verified by simulation, the results demonstrate the superior performances of the proposed control strategy.
Energy Conversion and Management, 2015
This paper describes a control technique for enhancing the stable operation of distributed generation (DG) units based on renewable energy sources, during islanding and grid-connected modes. The Passivity-based control technique is considered to analyse the dynamic and steady-state behaviours of DG units during integration and power sharing with loads and/or power grid, which is an appropriate tool to analyse and define a stable operating condition for DG units in microgrid technology. The compensation of instantaneous variations in the reference current components of DG units in ac-side, and dc-link voltage variations in dc-side of interfaced converters, are considered properly in the control loop of DG units, which is the main contribution and novelty of this control technique over other control strategies. By using the proposed control technique, DG units can provide the continuous injection of active power from DG sources to the local loads and/or utility grid. Moreover, by setting appropriate reference current components in the control loop of DG units, reactive power and harmonic current components of loads can be supplied during the islanding and grid-connected modes with a fast dynamic response. Simulation results confirm the performance of the control scheme within the microgrid during dynamic and steadystate operating conditions.
Reactive Power Sharing in Islanded Microgrid by Droop Control Method
The proposed method mainly includes two important operations: error reduction operation and voltage recovery operation. The sharing accuracy is improved by the sharing error reduction operation, which is activated by the low-bandwidth synchronization signals. However, the error reduction operation will result in a decrease in output voltage amplitude. Therefore, the voltage recovery operation is proposed to compensate the decrease., due to increasing the demand of electricity as well as rapid depletion of fossil fuels, and the government policies on reduction of greenhouse gas emissions , renewable energy technologies are more attractive and various types of distributed generation sources, such as wind turbine generators and solar photo voltaic panels are being connected to low-voltage distribution networks. Micro grid is an integrated system that contain in s distributed generation sources, control systems, load management, energy storage and communication infrastructure capability to work in both grid connected and island mode to optimize energy usage. The paper presents a advanced control technique for a micro grid system which works efficiently under a decentralized control system.
New droop-based control of parallel voltage source inverters in isolated microgrid
International Journal of Electrical and Computer Engineering (IJECE), 2024
Microgrids, featuring distributed generators like solar energy and hybrid energy storage systems, represent a significant step in addressing challenges related to the greenhouse effect and outdated transmission infrastructures. The operation and control of islanded microgrids, particularly in terms of grid voltage and frequency, rely on the synchronization of multiple parallel inverters connected to the distributed generators. However, to determine the necessary grid parameters for effective control, the presence of circulating currents from unbalanced grid voltages arises as a challenge. This situation necessitates the development of a new approach to achieve phase angle locking for grid synchronization, with the aim of maintaining the voltage within acceptable limits in islanded microgrids. This objective is realized through the creation of a microgrid network model, design of an adaptive filter, utilizing the double second-order generalized integrator–phase-locked loop (DSOGI-PLL), for dynamic voltage transformation. The design is evaluated by simulation using MATLAB/Simulink. The primary goal is to investigate the DSOGI-PLL-based droop control and compare its performance with the conventional synchronous reference frame–phaselocked loop (SRF-PLL) control approach. Notably, the DSOGI-PLL successfully eliminates the ripples in phase angle estimation, consequently enhancing the quality of voltage output in the microgrid.
Transition From Islanded to Grid-Connected Mode of Microgrids With Voltage-Based Droop Control
Microgrids are able to provide a coordinated integration of the increasing share of distributed generation (DG) units in the network. The primary control of the DG units is generally performed by droop-based control algorithms that avoid communication. The voltage-based droop (VBD) control is developed for islanded low-voltage microgrids with a high share of renewable energy sources. With VBD control, both dispatchable and less-dispatchable units will contribute in the power sharing and balancing. The priority for power changes is automatically set dependent on the terminal voltages. In this way, the renewables change their output power in more extreme voltage conditions compared to the dispatchable units, hence, only when necessary for the reliability of the network. This facilitates the integration of renewable units and improves the reliability of the network. This paper focusses on modifying the VBD control strategy to enable a smooth transition between the islanded and the grid-connected mode of the microgrid. The VBD control can operate in both modes. Therefore, for islanding, no specific measures are required. To reconnect the microgrid to the utility network, the modified VBD control synchronizes the voltage of a specified DG unit with the utility voltage. It is shown that this synchronization procedure significantly limits the switching transient and enables a smooth mode transfer.
An overview of control approaches of inverter-based microgrids in islanding mode of operation
Renewable and Sustainable Energy Reviews, 2017
Increased penetration of distributed generation (DG) into the power systems has created fundamental challenges from the viewpoints of control and reliable operation of systems. Microgrids (an aggregation of DG units, loads, and storage elements) with proper control strategies can be a good solution for removing or facilitating these challenges. The introduction of inverter-based microgrid in a distribution network has facilitated the utilization of renewable energy resources, distributed generations, and storage resources; furthermore, it has improved power quality and reduced losses, thus improving the efficiency and the reliability of the system. As most DG units are connected via a power electronic interface to the grid, special control strategies have been developed for inverter interfaces of DG units in islanded microgrids. This paper presents an overview of advanced control methods for microgrids, especially the islanded and inverter-based. Moreover, various control methods are compared and categorized in terms of their respective features. It also summarizes microgrid control objectives with their most problematic solutions as well as their potential advantages and/or disadvantages. Finally, some suggestions are put forward for the future research.
IRJET, 2021
Recent climatic changes have led to the increase in utilization of sustainable Distributed Generation (DG) resources such as fuel cells using natural or biogas, wind, solar etc. To meet the ever increasing energy requirement, society is moving to renewable sources like solar energy, wind energy etc, as it is pollution free, clean and available in plenty. In this paper , a micro grid model is designed using PV cell by using two different control strategies have been analyased. Droop control and advanced dynamic droop control methods have been explained in this paper In both these techniques, there is no need of communication line and maximum power is extracted from solar panel using Perturb and Observe method. The LC filters are designed to eliminate harmonic current. The conventional droop control has poor voltage regulation at heavy load condition and poor power sharing performance at light load condition. The conventional P-Q droop methods shares the active power based on fixed droop coefficient irrespective of available energy from non conventional energy source. The disadvantages of the conventional droop control can be reduced with the use of advanced dynamic droop control. PV models with droop control and advanced dynamic droop control have been designed and compared using matlab/simulink models