Decentralized Schemes For Grid Forming Inverter System Efficiency Improvement And Online Inverter Detection
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Decentralized Schemes For Grid Forming Inverter System Efficiency Improvement And Online Inverter Detection
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Author : Ali Sheykhi
language : en
Publisher:
Release Date : 2021
Decentralized Schemes For Grid Forming Inverter System Efficiency Improvement And Online Inverter Detection written by Ali Sheykhi and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2021 with Electric inverters categories.
Parallel grid-forming (GFM) inverters are used in many modern power system applications. Therefore, improving their system efficiency is of paramount importance for energy savings. The droop control method has been conventionally used to share the power among inverters proportional to their power ratings. However, the droop strategy does not guarantee an efficient power sharing especially at light loads, where the low power demand is divided among inverters, forcing them to supply a low power at a low efficiency according to their efficiency curve. Addressing the aforementioned issue while maintaining the advantages of parallel inverters used in residential areas is the focus of this thesis. A nested-loop control system for a single-phase inverter operating in standalone mode is introduced. The control system includes current, voltage, and harmonic control loops, and is designed based on the linear quadratic tracking (LQT) optimal control method. The designed controllers exhibit a satisfactory performance in tracking the reference signals and rejecting the unwanted harmonics in case of linear and nonlinear loads. Moreover, a total harmonic distortion (THD) of 1.65% is obtained when a nonlinear load is connected to the system. A decentralized modified droop strategy is also proposed to improve the system efficiency of parallel grid-forming inverters. The main idea is to control the power sharing among the inverters such that the output power of each inverter is maintained within a proper range with respect to the inverter efficiency curves. Hence, an optimal number of inverters supply the loads efficiently, and the unnecessary inverters do not share any power. In contrast to the existing methods, the proposed method does not have a single point of failure while it features modularity, flexibility, and reliability. Additionally, the reliability of the system is enhanced by extending the lifetime of inverters with higher power ratings which are considered as valuable assets of the system. Furthermore, a stability verification of the system within the designed operating range of the inverter no-load frequencies is conducted to ensure a stable operation. As in very-light load situations none of the inverters can operate within the proper power range with a high efficiency, an online-inverter detection (OID) method is proposed such that each inverter detects the online inverters and the unnecessary inverters stop injecting power to obtain a higher system efficiency. As the proposed OID is decentralized and modular, more inverters can be added to the system without adjusting the settings of installed inverters. Detailed derivations, simulations, and experimental results are presented to validate the effectiveness of the schemes.
Electrical Electronics Abstracts
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Author :
language : en
Publisher:
Release Date : 1997
Electrical Electronics Abstracts written by and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 1997 with Electrical engineering categories.
Science Abstracts
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Author :
language : en
Publisher:
Release Date : 1995
Science Abstracts written by and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 1995 with Electrical engineering categories.
Restoration And Universal Operation Of Grid Forming Inverters
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Author : Fahmid Sadeque
language : en
Publisher:
Release Date : 2022
Restoration And Universal Operation Of Grid Forming Inverters written by Fahmid Sadeque and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2022 with categories.
This dissertation presents novel control methods for frequency and voltage restoration, and universal operation of grid-forming (GFM) inverters. The developed controllers solve the technical challenges for a microgrid with multiple GFM inverters in synchronization and power-sharing, phase-angle detection under asymmetrical conditions, capability to operate in the grid-following (GFL) mode, and frequency and voltage restoration. First, two synchronization methods for GFM inverters are presented. In the output-sync method, an incoming GFM inverter synchronizes its output voltage parameters, i.e., amplitude, phase angle, and frequency, at its point of common coupling (PCC) before closing the circuit breaker. The task is uniquely performed by introducing two proportional-integral (PI) controllers that adjust the output voltage and the angular frequency of the inverter. In the controller-sync method, two sets of controller paths are run in parallel and kept synchronized to produce the same PWM reference. But, only one set of the controller is engaged at a time. The uniqueness of this method is that it allows an inverter to stay connected to the microgrid in a standby mode without injecting any power. Then, when enabled, the inverter gradually shifts to decentralized power-sharing mode without switching the controller. Finally, the controller seamlessly switches to GFM mode, where the inverters are completely independent but synchronized. Another noteworthy fact is that, unlike the output-sync method that used two sets of voltage measurements, the controller sync method only uses one set of voltage measurements. A state-space model of the GFM inverter with the controller-sync method is derived to show the stability at every mode. The efficacy of the developed methods is experimentally validated in a microgrid with both droop and virtual inertia for power-sharing. A new phase-angle detection method, called signal reformation-based direct phase-angle detection (DPD-SR), is developed that provides an enhanced phase-angle detection capability for the GFM inverter under asymmetrical conditions of the system. The developed DPD-SR can directly estimate the phase-angle by using a mathematical function. The unique signal-reformation technique used in this method can process the asymmetrical voltage measurements into a symmetrical form and extract the phase-angle of the system without using any complicated closed-loop control. The DPD-SR method is verified through hardware experiments and compared with state-of-the-art method. Eventually, a universal controller is developed that combines the feature of the controller-sync and the DPD-SR methods. Furthermore, the developed universal controller enables an inverter to operate in GFL and GFM mode is developed. With the universal controller, the inverter can inject desired active and reactive power to the grid or microgrid in GFL mode and seamlessly transit to GFM mode if islanding is detected. The universal controller has two sets of parallel paths, where one path provide GFM control and the other path provides the GFL control and synchronization. A frequency-based islanding detection is added to the developed controller. The uniqueness of the universal controller is, the two paths are always synchronized and can provide seamless GFL to GFM transition when a fault occurs. The controller's stability is analyzed through a state-space model, and the performance of the developed controller is verified in a microgrid for grid-connected and islanded operation in this dissertation. Finally, a frequency and voltage restoration controller is developed to overcome the frequency and voltage deviation problem with the droop controller for power-sharing. In droop-controlled power-sharing, the frequency and voltage are required to deviate from their nominal values. On the contrary, the frequency and voltage of the system are expected to be regulated by the GFM inverters. This dissertation develops a threshold-based frequency and voltage restoration method for the GFM inverters without compromising the effectiveness of the decentralized power-sharing methods. The novelty of this method is that the restoration control paths are only enabled when a load change or plug-in of an inverter is identified and disabled when the frequency and voltage parameters are restored. A state-space model is developed to perform stability analysis. The developed method is compared with a timer-based method to demonstrate its superiority. The efficacy of the restoration method is verified through experiments.
Design Power Control Strategies Of Grid Forming Inverters For Microgrid Application
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Author : Jing Wang (Architectural engineer)
language : en
Publisher:
Release Date : 2021
Design Power Control Strategies Of Grid Forming Inverters For Microgrid Application written by Jing Wang (Architectural engineer) and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2021 with Electric inverters categories.
From Grid Following To Grid Forming
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Author : Debjyoti Chatterjee
language : en
Publisher:
Release Date : 2022
From Grid Following To Grid Forming written by Debjyoti Chatterjee and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2022 with categories.
Electrical power generation is drastically shifting from centralized power generation to decentralized distributed power generation as a result of the rising integration of renewable energy sources into the electrical grid. The primary challenge in this transition is replacing synchronous generators (SGs) with inverter-interfaced renewable generations. When there are one or more synchronous generators in the system, grid-connected inverters follow the voltage and frequency reference generated by the synchronous generator and act as a controlled current source to supply necessary quantity of active and reactive power. In the presence of one or more stiff voltage sources, such inverter operation has recently been labeled as ‘Grid-Following’ (GFL) mode of operation. If all synchronous machines are taken out of service, there will not be any voltage reference, rendering grid-following inverter operation infeasible. Hence, the way that the GFL inverters are controlled today results in the inability of the grid to operate 100% inverter-based resources (IBR). Therefore, in the absence of a synchronous generation as a stiff voltage source, the frequency and voltage of the grid must be controlled by some of the inverters. These inverters, referred to as "Grid-Forming" (GFM) inverters, are tasked with supporting a stable voltage and frequency in a variety of situations, including the connection or disconnection of a load or a generator, or the occurrence of a power system fault. Grid-forming inverters (GFMIs) will have a crucial role with the increase in renewable penetration during the coming years. This thesis aims to study the modeling approach and control technique of a GFM inverter in an islanded grid. The droop-based control of a GFL inverter is also studied and compared to that of a GFM inverter to understand the fundamental difference in their operation. As GFM inverters will gradually replace synchronous generators, GFM inverters are expected to behave very similarly to synchronous generators in a grid without a utility connection. Hence, the voltage balancing and short circuit behavior of GFM inverters are further compared to that of synchronous generators. Additional controller modifications are also proposed for the enhanced performance of the GFM inverter. Finally, GFM inverter-based virtually islanded Hybrid AC-DC microgrid architecture is proposed for the power distribution of future residential buildings
Improved Control Strategy Of Grid Forming Inverters For Fault Ride Through In A Microgrid System Preprint
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Author :
language : en
Publisher:
Release Date : 2022
Improved Control Strategy Of Grid Forming Inverters For Fault Ride Through In A Microgrid System Preprint written by and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2022 with categories.
This paper develops improved control strategy of GFM inverter with fault ride-through (FRT) capabilities to guarantee stable operation of microgrid under fault conditions especially islanded microgrid and asymmetrical faults. The proposed control strategy includes dual control of positive-sequence and negative-sequence control, and the adaptive virtual impedance (VI) control. Unlike the existing works, the proposed strategy only applies the VI control for the d-component of the positive-sequence control and leaves the q-component of positive-sequence control and dq-component of negative-sequence control zero, thus achieving the improved stability and balanced three-phase voltage under asymmetrical faults. The adaptive feature of the VI control guarantees the stability of the GFM inverter under severe faults which could cause saturated inner current loop and instability if the VI is not adaptive. Simulation results of a phase-to-phase-ground fault show that the proposed control strategy improves the stability of GFM and achieves the stable and balanced output voltages in islanded microgrid.
Efficiency Improvement For Small Scale Single Phase Grid Connected Inverters
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Author : Bo Cao
language : en
Publisher:
Release Date : 2015
Efficiency Improvement For Small Scale Single Phase Grid Connected Inverters written by Bo Cao and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2015 with Electric inverters categories.
This Ph.D. research focuses on efficiency improvement methods for small-scale single-phase grid-connected inverters. Many attempts applied to increase inverters’ efficiency found in the recent literature can be primarily summarized into two categories: specific topologies and complicated control algorithms, which result in a considerable increase in system cost and control complexity. Meanwhile, the power quality is another important issue for grid-connected inverters to comply with the utility standards. To overcome the drawbacks of the traditional methods and satisfy the utility requirement, a novel control algorithm called “variable switching frequency control (VSFC)” is developed as a major part of this thesis work to increase the overall efficiency of the inverter through selecting optimal switching frequencies of pulse width modulation (PWM) in real time while meeting requirements of grid interconnection standards. According to the inverter loss analysis and current harmonic estimation model presented in this dissertation, the selection of switching frequencies for the grid-connected inverter optimized by the proposed “VSFC” ensures that the inverter operates with maximum efficiencies at different output levels. In addition, as the operational switching frequency applied to the inverter under “VSFC” changes along with variations of work conditions, when the inverter operates at a low or medium switching frequency, the time-delay effect caused by sampling distribution, computation of the control program in DSP and inherent PWM generator update is amplified and can severely degrade the system stability and performance. Thus, a robust current control scheme featuring high adaptability to time delays and system uncertainties and high robustness to parameter mismatch is developed in this research. The proposed scheme is built on a structure of the predictive current controller and developed with an improved time-delay compensation technique which greatly reduces the current tracking errors through a simple weighted filter predictor (WFP) and completely eliminates static voltage errors introduced by uncertain system disturbances through a robust adaptive voltage compensator (AVC). The developed new control methods for grid-connected inverters have been verified through computer simulations and laboratory experiments. The results of simulation and experiment investigation have demonstrated the improvements of these methods in overall inverter efficiency and performance.
Design Power Control Strategies Of Grid Forming Inverters For Microgrid Application Preprint
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Author :
language : en
Publisher:
Release Date : 2021
Design Power Control Strategies Of Grid Forming Inverters For Microgrid Application Preprint written by and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2021 with categories.
This paper compares two control schemes in the application control layer of a non-PLL grid-forming (GFM) inverter to gain the insights and understanding of how the two schemes affect the dynamic responses of the GFM inverters and the transition operation of microgrids. Both schemes can achieve smooth microgrid transition operation and power tracking, however, the first scheme expects to exhibit more transients due to the feedforward of the inverse dynamic model of the output filter. Simulation results show that both control schemes achieve successful transition operation with smooth transients, and the second control scheme exhibits slightly better transients in the system frequency during islanding operation and the first one has slightly superior transients in the system frequency during reconnection operation. The power tracking performance in the grid-connected mode is evaluated, which shows that the first scheme has better active power tracking and the second one has better reactive power tracking. The analysis and results are useful to develop reliable control schemes for non-PLL GFM inverters because more and more inverters will work as non-PLL grid-forming sources in the future grids due to better stability and reliability.
A Novel Power Hardware In The Loop Interface Method For Grid Forming Inverter Systems Preprint
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Author :
language : en
Publisher:
Release Date : 2022
A Novel Power Hardware In The Loop Interface Method For Grid Forming Inverter Systems Preprint written by and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2022 with categories.
Power Hardware-in-the-Loop (PHIL) simulation of grid-forming (GFM) inverter systems facilitates the testing of drastic scenarios like on-grid to off-grid transition, islanded microgrid operation without stiff grid etc. To the authors best knowledge, most of studies in literature are focused on PHIL simulation for grid-following inverter systems and only few studies are focused on GFM inverters and those are challenging and problematic especially for high-power applications. In this article, a novel PHIL simulation platform is proposed that enables interfacing of high-power GFM inverter systems. It proposes the concept of a virtual GFM inverter as a part of the proposed PHIL interface for GFM inverter. This addition of virtual GFM inverter in the PHIL interface expands the conventional Ideal Transformer Model (ITM) method and enables it to overcome the issues of instability of existing ITM methods. In the validation stage, a PHIL experiment is conducted on a 3-phase 480 V, 125 kVA GFM inverter system with proposed interfacing method. The results corroborates the fact that the proposed PHIL simulation method performs well and stable for GFM inverter system.