Microgrid Operating In Islanded Mode Research Articles

A new integrated fault detection and control scheme of islanded DC microgrids: A resilient approach

This paper presents a new resilient integrated fault detection and control module for a DC microgrid operating in islanded mode. The proposed design is unique in its ability to simultaneously improve microgrid reliability and mitigate variations through a multi-objective approach that considers both control and diagnosis objectives in the form of Linear Matrix Inequalities through a presented algorithm. The module is designed by setting out desired performance indices to ensure system stability, mitigate disturbances and uncertainties, and detect faults while controlling the DC-link voltage. Additionally, the module is robust to its own variations and capable of detecting non-compensable faults for further diagnosis. The efficacy of the proposed approach is demonstrated through a comparative study that highlights the vulnerability of robust design to parameter variations.

Improved Droop Control Strategy of Multiple Energy Storage Applications in an AC Microgrid Based on the State of Charge

Distributed energy storage technology is used to stabilize the frequency and voltage of the microgrid operating in islanded mode. However, due to the inconsistent state of charge (SoC) of the energy storage unit (ESU), the active power output of the ESU cannot be shared reasonably. On the basis of stabilizing voltage and frequency, this paper presents a power exponential function droop control (PEFDC) strategy considering the SoC. In this control strategy, the ESU is allowed to adjust the output power adaptively according to its own SoC level during discharge and reaches SoC equilibrium. Simulation models are built to compare the PEFDC strategy with conventional droop control (CDC) and power function droop control (PFDC) approaches. The simulation results illustrate the superiority of the proposed control strategy over the other two methods. Finally, the hardware-in-the-loop experiment is conducted to verify the effectiveness of the PEFDC strategy.

A Cyber-Secure Distributed Control Architecture for Autonomous AC Microgrid

Security of various cyber-physical systems is a major concern for researchers worldwide. Nowadays, microgrids also form such cyber-physical system to achieve a number of objectives such as enhanced frequency regulation, economic active power sharing, proper reactive power sharing, etc. Communication links are essential in such microgrids to ensure bilateral flow of data to be fed to the secondary controllers of the distributed generators integrated to the power network. This article, therefore, studies the impact of various kinds of cyberattacks viz., false data injection, denial-of-service, and replay attack on the performance of these communication enabled secondary controllers having potential vulnerabilities. Additionally, it presents a unique framework to ensure system stability when communication links are subjected to such attacks. The analysis has been carried out in a microgrid operating in islanded mode where the controller area network (CAN) bus communication network is utilized to form the cyber-physical system. Simulations are performed in MATLAB/Simulink platform and real-time evaluation of the proposed framework has been carried out in a test bed having OPAL-RT (OP5600) simulator and physical CAN devices to form the communication links.

Voltage Balance Control Strategy of Microgrid Inverter under Unbalanced Loads

微网孤岛运行时系统通过众多网内微网逆变器提供电压支撑，由于网内大量单相负载的存在使得负载难以保持三相平衡，而单相负载投切的随机性又使得负载表征为动态的不平衡。为控制电压的平衡，微网逆变器一般采用双DQ控制方式。这一控制方式可以实现对微网逆变器端电压不平衡控制，但在负载频繁随机投切情况下，电压不平衡控制需要一定动态调节时间。此外，需多个控制器且控制参数整定相对困难。为此，本文提出一种前馈控制策略，通过检测微网逆变器电流实时动态计算电压不平衡补偿量，实现对微网逆变器输出电压不平衡控制。通过前馈控制方式，即使负载存在频繁投切仍可快速跟踪负载变化，实现微网逆变器不平衡电压的补偿。基于该控制策略搭建了仿真模型，仿真结果表明即使在负载突变情况下微网逆变器仍可快速保持电压平衡。 The system voltage of microgrid operating in island mode is supported by numerous microgrid inverter switches in the network. Due to the existence of a large number of single-phase loads in the network, it is difficult to keep three-phase loads balance. Furthermore, the system loads represent dynamic unbalance because of the random switching in or out of single-phase loads. In order to control the voltage to be balance, the microgrid inverter usually adopts double DQ control mode. Although this control mode can realize the voltage unbalance control of the microgrid inverter terminal voltage, the voltage imbalance control needs a certain dynamic regulation period when frequent load switching occurs. In addition, several controllers are needed and its control parameters tuning is complex. To this end, this paper presents a feedforward control strategy to achieve imbalance control of microgrid inverter output voltage, by detecting the microgrid inverter real-time current and then calculate the compensation of voltage imbalance. Through the feedforward control method, the load change can be traced quickly and the unbalanced voltage of the microgrid inverter can be well compensated even if the system loads are switched frequently. Based on the control strategy, the system simulation model is built and resulting simulation results show that the microgrid inverter can maintain the voltage balance even under sudden load change.

Power requirement and control characteristics of energy storage equipment in a microgrid operating in island mode

When the microgrid works in island mode, the energy storage equipment needs to provide the reference voltage supply. The output power of the energy storage equipment will affect the point of common coupling (PCC) voltage. According to the single-phase equivalent circuit, analysing the phasor diagram of voltage and current vector, considering the rating parameter of the insulated gate bipolar transistor and different kinds of load, the power requirement of the energy storage equipment that can ensure the stability of the PCC voltage is obtained. The output power of energy storage can be smaller than the load power. If an switching virtual circuit (SVC) or a static var generator (SVG) is in parallel with the energy storage equipment, the power requirement and output characteristic are analysed further when the reactive power of load is compensated by the SVC or SVG. At last, the simulation verifies the analysis results.

A control method for parallel inverters in Microgrid based on sliding mode and droop controls

Robust control and flexible operation are the major objectives of islanded microgrid. A microgrid can have different configurations with linear loads and nonlinear loads. The harmonic current caused by nonlinear loads make Proportional Integral (PI) or Proportional Integral Derivative (PID) voltage controller far beyond excellent performance in case of microgrid operating in islanded mode. Additionally, the robustness of the PID closed loop system can not be guaranteed. The voltage control by using Proportional resonant (PR) controller are recommended. Although PR controller has the ability to sinusoidal signals. However, PR controller has a slight deviation of the frequency of selected harmonic component. This paper presents a Sliding Mode Control (SMC) for voltage control of parallel inverters operating in islanded microgrid. This controller can enhance the robustness of control system and reduced-state tracking error. The stability of the closed-system is verified by means of Lyapunov stability criterion. The control structure is based on the inner sliding mode closed-loop and the outer droop control loop. The main aim of this paper is to design inner controllers to enhance the dynamics of the microgrid. The results obtained from the simulation of Matlab.

Adaptive sliding-mode voltage control for inverter operating in islanded mode in microgrid

Abstract Harmonic current caused by nonlinear loads and parametric variations of output filter of inverters make popular proportional–integral–derivative (PID) voltage controller far beyond excellent performance in case of microgrid operating in islanded mode. Motivated by this limitation, this paper proposes an adaptive sliding-mode controller (ASMC) to enhance disturbance-rejection performance of control system of islanded parallel inverters. And adaptive algorithms are designed to observe external disturbances and internal perturbation so as to guarantee the globe robustness of control system of inverter. The switching gain of control input is designed to be a time-varying value which effectively reduces undesirable chattering of control input signal. Simulating and experimental results are presented that the total harmonic distortion, chattering, and steady-state error of output voltage of islanded parallel inverters are effectively reduced and the dynamic performances and the capability of perturbation rejection of control system of inverter are effectively enhanced.

Combining a dynamic battery model with high-resolution smart grid data to assess microgrid islanding lifetime

In this paper, we use experimental data collected from an Austin, Texas smart grid test bed with a system-level battery energy storage model to assess the lifetime of batteries in a microgrid operating in islanded mode during a distribution-level outage. We consider a hypothetical microgrid consisting of 21 single-family detached homes and three transformer-level community energy storage (CES) battery units ranging in size from 25kWh to 75kWh. To describe the performance of CES batteries, we implement a dynamic behavioral circuit model capable of describing voltage transients and rate-capacity effects. We use one-minute electricity production and consumption data collected from the smart grid test bed in 2012 to assess how the timing of an electric outage affects the islanding lifetime of a residential microgrid. We contrast our results with the average outage duration reported by U.S. electric utilities to quantify how often a residential microgrid could withstand a typical outage. Our results show that increasing the amount of rooftop PV in a residential microgrid does not significantly increase how often it can withstand an average-duration outage. However, combining PV with CES extends the median islanding lifetime by up to 11.6h during morning outages. Based on our results, 50kWh CES provides the best tradeoff between the cost of a CES system and its reliability benefit, allowing downstream loads to withstand an average-duration outage approximately 93% of the time.