Single-phase Microgrid Research Articles

Slide mode control of microgrid using small hydro driven single‐phase SEIG integrated with solar PV array

This study presents a susceptance theory-based sliding mode control (STSMC) algorithm for an improved power quality voltage and frequency control of a single-phase microgrid. The proposed microgrid includes a governor-free small hydro turbine-driven single-phase two winding self-excited induction generator (SEIG), a solar PV array and a battery energy storage system (BESS). The non-linear relationship among magnetising reactance, frequency and speed of SEIG along with random fluctuation in active power output of the solar PV array create the major challenge in frequency and voltage control of such renewable energy-based microgrid. The STSMC algorithm is found suitable to control frequency and voltage of such non-linear and complex system. In this proposed control, the system frequency control in dynamics and steady-state conditions and balance of power among various energy sources and BESS are achieved using the sliding mode control. The STSMC eliminates all possibilities of overshoot and undershoot problem in DC-link voltage of the VSC, which in turn reduces the required size of DC-link capacitor and BESS.

An Islanding Detection Method by Using Frequency Positive Feedback Based on FLL for Single-Phase Microgrid

An active islanding detection method based on frequency-locked loop (FLL) for constant power controlled inverter in single-phase microgrid is proposed. This method generates a phase shift comparing the instantaneous frequency obtained from FLL unit with the nominal frequency to modify the reference phase angle. An initial low frequency variable triangular disturbance is added to the phase shift in order to reduce non-detection zone and accelerate the detection process especially in the case of power matching. With the modified phase angle, the frequency at point of common coupling will be drifted away from the nominal frequency until exceeding the threshold because of the frequency positive feedback after islanding. Besides, FLL is introduced to this method in order to lock frequency quickly considering that the frequency is time-varying during the islanding detection process. Simulation and experiment have been done to evaluate this method.

Instantaneous power calculation based on intrinsic frequency of single-phase virtual synchronous generator

In order to enhance the stability of single-phase microgrid, virtual synchronous generator (VSG) control method is investigated in this paper. Its electromagnetic model and electromechanical model are established to illustrate the performance of VSG. Considering the 2nd fluctuation of fundamental-frequency in the output power, an instantaneous power calculation strategy is proposed based on the intrinsic frequency of single-phase VSG. Besides, a virtual power calculation method is presented to achieve islanded/grid-connected seamless transition. Stability analysis and comparison simulation results demonstrate the correctness of the presented power calculation method. At last, the effectiveness of the proposed approach is verified by comparison experiments of islanded/grid-connected operations in a 500 VA single-phase inverter.

On the Interplay of Distributed Power Loss Reduction and Communication in Low Voltage Microgrids

Distributed generators (DGs), coupled with suitable control and communication infrastructures, are expected to play a key role in improving the efficiency of electricity grids. In this paper, we focus on low-voltage and single-phase microgrids exploring the interplay of distributed power loss reduction and communication. We select representative power-loss reduction algorithms from the state of the art and provide design rules for the required networking strategies in the presence of lossy communication links, assessing the impact of communication as well as electrical grid features. Toward this end, we devise a novel statistical cosimulation (electricity grid, communication, and control) framework that faithfully mimics the characteristics of real-world microgrids in terms of communication and grid topologies, power demand, and distributed generation from solar sources. Our numerical results highlight the role of communication procedures and the differences among the selected optimization techniques for power loss reduction, assessing their convergence rate and quantifying the impact of communication failures, line impedance estimation error, communication and electricity grid topologies, network size, and number of DGs.

Single-Phase Microgrid With Seamless Transition Capabilities Between Modes of Operation

Microgrids are an effective way to increase the penetration of distributed generation into the grid. They are capable of operating either in grid-connected or in islanded mode, thereby increasing the supply reliability for the end user. This paper focuses on achieving seamless transitions from islanded to grid-connected and vice versa for a single phase microgrid made up from voltage controlled voltage source inverters (VC-VSIs) and current controlled voltage source inverters (CC-VSIs) working together in both modes of operation. The primary control structures for the VC-VSIs and CC-VSIs is considered together with the secondary control loops that are used to synchronize the microgrid as a single unit to the grid. Simulation results are given that show the seamless transitions between the two modes without any disconnection times for the CC-VSIs and VC-VSIs connected to the microgrid.

Design, Operation, and Control of S3 Inverter for Single-Phase Microgrid Applications

A single-phase voltage source inverter with a front-end dc–dc conversion stage followed by a synchronized push–pull configuration operating at a desired fundamental frequency (FF) is presented. The duty cycle of the dc–dc conversion stage is varied in the form of a unidirectional sine wave to produce a similar output voltage across the dc-link capacitor. The unidirectional voltage is made into an alternating voltage by the synchronized push–pull configuration. This inverter employs three semiconductor switches, in which one is operating at a high frequency and the rest are operating at an FF. Hence, it is named as the S3 inverter. Furthermore, simple and cost-effective analog circuits are presented for the generation of switching pulses and the control of the amount of power fed to the grid. The hardware prototype of the S3 inverter has been built in a laboratory, and its performance during the stand-alone and grid-connected modes of operation is validated.

A Novel Method of Fault Location for Single-Phase Microgrids

This paper presents a novel fault location method for single-phase microgrids. In order to locate a fault, a feature specific to the fault location is found, namely the maximum oscillation magnitude of the transient voltage signal induced by the fault. Our theoretical study and extensive simulations demonstrate that there is an approximated linear relationship between the maximum magnitude of the transient signal observed by a sensor, and the distance between the sensor and the fault location. Based on the discovered relationship, microgrid topology and sensor location information, we have designed an algorithm capable of locating the fault in the single-phase microgrids. The proposed fault location method has been implemented and validated through simulations in Electro-Magnetic Transients Program and MATLAB. The average localization error is less than 10% in the evaluation results, which manifests the significance of the novel method, as there is little research done for fault location in single-phase microgrids.

Mitigation of Harmonics in Grid-Connected and Islanded Microgrids Via Virtual Admittances and Impedances

Optimization of the islanded and grid-connected operation of microgrids is important to achieve a high degree of reliability. In this paper, the authors consider the effect of current harmonics in single phase microgrids during both modes of operation. A detailed analysis of the effect of the output impedance of the considered primary control loops on the harmonic output of the considered voltage source inverters is initially carried out. A virtual admittance loop is proposed to attenuate the current harmonic output in grid-connected operation that is generated due to the grid voltage distortion present at the point of common coupling (PCC) and due to local non-linear loads. This paper also considers the harmonic current sharing and resulting voltage harmonics at the PCC during islanded operation of the microgrid. A capacitive virtual impedance loop was implemented to improve the harmonic current sharing and attenuate the voltage harmonics at the PCC. Experimental results are given to validate the operation of the proposed algorithms.

Seamless Transfer Control Strategy for Grid-Interactive Inverters

—Single-phase microgrids are widely used in residential houses. The energy storage system with grid-interactive inverters in these microgrids should operate in both grid-tied and standalone modes. In this article, proportional resonant controller for the single-phase grid-interactive inverter with LCL-filter is designed and implimented in the both operation modes. Also, a novel seamless transfer strategy based on hysteresis current control is proposed when off-grid transfer operation is needed. With the proposed method, the inverter is able to provide voltage support to the local critical loads during the whole transfer process when the grid fails. For the grid-tied transfer process, a phase difference eliminating method based on inverter frequency regulation is introduced in this article to eliminate the phase difference between the grid voltage and the inverter voltage when the grid recovers. Simulation and experiments are carried out and the results verify the effectiveness of the proposed control method.

Flexible Microgrid Power Quality Enhancement Using Adaptive Hybrid Voltage and Current Controller

To accomplish superior harmonic compensation performance using distributed generation (DG) unit power electronics interfaces, an adaptive hybrid voltage and current controlled method (HCM) is proposed in this paper. It shows that the proposed adaptive HCM can reduce the numbers of low-pass/bandpass filters in the DG unit digital controller. Moreover, phase-locked loops are not necessary as the microgrid frequency deviation can be automatically identified by the power control loop. Consequently, the proposed control method provides opportunities to reduce DG control complexity, without affecting the harmonic compensation performance. Comprehensive simulated and experimental results from a single-phase microgrid are provided to verify the feasibility of the proposed adaptive HCM approach.

An Islanding Microgrid Power Sharing Approach Using Enhanced Virtual Impedance Control Scheme

In order to address the load sharing problem in islanding microgrids, this paper proposes an enhanced distributed generation (DG) unit virtual impedance control approach. The proposed method can realize accurate regulation of DG unit equivalent impedance at both fundamental and selected harmonic frequencies. In contrast to conventional virtual impedance control methods, where only a line current feed-forward term is added to the DG voltage reference, the proposed virtual impedance at fundamental and harmonic frequencies is regulated using DG line current and point of common coupling (PCC) voltage feed-forward terms, respectively. With this modification, the impacts of mismatched physical feeder impedances are compensated. Thus, better reactive and harmonic power sharing can be realized. Additionally, this paper also demonstrates that PCC harmonic voltages can be mitigated by reducing the magnitude of DG unit equivalent harmonic impedance. Finally, in order to alleviate the computing load at DG unit local controller, this paper further exploits the band-pass capability of conventionally resonant controllers. With the implementation of proposed resonant controller, accurate power sharing and PCC harmonic voltage compensation are achieved without using any fundamental and harmonic components extractions. Experimental results from a scaled single-phase microgrid prototype are provided to validate the feasibility of the proposed virtual impedance control approach.

Control Strategy of Microgrid Based on Distributed Generation

Upon the traditional wind-solar hybrid distributed generation system,a single-phase microgrid experimental system platform has been built based on the DC bus,Z topology was used to increase the energy utilization ratio. On the basic of realizing the maximum power point tracker of the new energy, different microgrid controlling strategy has been designed to apply to the island and grid connected operation mode: in the cases of island running, master slave control mode will be utilized in the system and the battery will be used as the main control unit to provide the reference voltage; on the other hand, a control method which can merge the generation into the grid as much as possible will be adopted in the case of grid connected. By the use of system model, the simulation of the two operation mode switch state and power flow has been done and the feasibility and effectiveness of the control strategy can be guaranteed,it also has been verified by the microgrid experimental platform.

A Novel Fast-Tracking D-Estimation Method for Single-Phase Signals

The phase, frequency, and amplitude of single-phase voltages are the most important and basic information required for single-phase microgrid connected applications. This paper proposes a new “D-estimation method” for instantly estimating the phase, frequency, and amplitude of frequency-varying single-phase signals in such applications. The D-estimation method has the following attractive features: 1) it is a new filtering method using the D-filters; 2) it involves the use of only filters and can be easily designed; 3) it is inherently stable and robust in hostile envelopments caused by a frequency variation, a phase jump, amplitude sag/swell, harmonic distortion, and/or contaminated noise; 4) even in the hostile envelopments, good instant estimates of the phase, frequency, and amplitude can be obtained; 5) it is simple, but can exhibit fast-tracking performance comparable to the “robust PLL method.” This paper presents the D-estimation method together with the design rules in detail; the usefulness of the method is verified by performing extensive numerical experiments.

Single-Phase Inverter Control Techniques for Interfacing Renewable Energy Sources With Microgrid—Part I: Parallel-Connected Inverter Topology With Active and Reactive Power Flow Control Along With Grid Current Shaping

In this paper, a novel current control technique is proposed to control both active and reactive power flow from a renewable energy source feeding a microgrid system through a single-phase parallel-connected inverter. The parallel-connected inverter ensures active and reactive power flow from the grid with low-current total harmonic distortion even in the presence of nonlinear load. A p-q theory-based approach is used to find the reference current of the parallel-connected converter to ensure desired operating conditions at the grid terminal. The proposed current controller is simple to implement and gives superior performance over the conventional current controllers, such as rotating frame proportional-integral controller or stationary frame proportional resonant controller. The stability of the proposed controller is ensured by direct Lyapunov method. A new technique based on the spatial repetitive controller is also proposed to improve the performance of the current controller by estimating the grid and other periodic disturbances. Detailed experimental results are presented to show the efficacy of the proposed current control scheme along with the proposed nonlinear controller to control the active and reactive power flow in a single-phase microgrid under different operating conditions.

Improved Load Sharing Control Techniques for Inverters used in a Microgrid

In this paper, an improved load sharing control scheme is presented, which is able to improve the transient response and power sharing accuracy of parallel-connected inverters used in microgrid. It also shows how the improved droop method can be easily adapted to account for the operation of parallel-connected inverters, providing good performance under the variation and disturbance of loads, as well as achieving good steady-state objectives and transient performance. Two DSP-based single-phase Microgrid inverters are designed and implemented. Simulation and experimental results are all reported, confirming the validity of the proposed control technique.