Average Voltage Regulation Research Articles

Distributed Periodic Event-Triggered Algorithm for Current Sharing and Voltage Regulation in DC Microgrids

In modern control systems, most control algorithms, especially system-level ones, are implemented in discretetime (DT) with digital controllers and digital communications. In this paper, a distributed DT algorithm is developed to achieve proportional load current sharing and average bus voltage regulation in DC microgrids. In order to reduce the communication requirement among the controllers, a periodic event-triggered (PET) DT algorithm is proposed by introducing a novel PET condition. Since the PET condition is detected periodically, the Zeno phenomenon existing in continuous-time (CT) event-triggered algorithms can be avoided. The communication burden for detecting the PET condition is also relieved since only the local and neighbors' triggered information is required. Through the Lyapunov synthesis, the current sharing error is proved to converge to zero asymptotically. Finally, comparative results among different algorithms, based on a detailed switchlevel microgrid model, are given to validate the effectiveness of the proposed PET control design.

A novel cooperative distributed secondary controller for VSI and PQ inverters of AC microgrids

This paper proposes a novel cooperative secondary control strategy for microgrids which is fully distributed. There is a two-layered coordination, which exists between inverter based DGs of both types, i.e. Voltage Source Inverter (VSI) and Current Source Inverter (CSI), also called PQ inverter. In first layer of the proposed two-layered cooperative control strategy, VSIs will take care of the primary average voltage regulation by implementing the average consensus algorithm (ACA); then in the second layer of control, the PQ inverters will improve the voltage quality of the microgrid while maintaining the average voltage of buses at the same desired level. Zone dedication algorithm is utilized in the second layer for voltage quality purposes based on sensitivity analysis. The sensitivity analysis is based on Simplified Jacobian matrix and the result of that is used to define the zone related to each DG in the microgrid. The goal of this zone dedication is to assign loads to the DGs that can compensate their changes with less effort (generating less power) than the others. There are two major contributions in this paper; 1- PQ inverters are effectively involved to increase microgrids capacity for better power management by introducing sensitivity to the PQ inverters set-point. This is defined based on the structure of the microgrid and takes into account the location of load changes. 2- The proposed strategy not only focuses on transient response but also improves the steady state response which smooths the voltage profile of the system while keeping the average voltage at the same desired level.The algorithm has been applied to a 13 bus system with a fully distributed communication in which each VSI inverter only communicates with its immediate neighbors and each PQ inverter is only in touch with associated bordering agents. The conclusive results verify that the proposed control strategy is an effective way to control the microgrid's voltage to have a smoother and stable voltage profile. The analysis also confirms the robustness of the proposed cooperative control in presence of possible time delays.

A Novel Communication-Based Average Voltage Regulation Scheme for a Droop Controlled DC Microgrid

In a dc microgrid, enhanced voltage regulation (VR) and load sharing (LS) are necessary for the economic and reliable operation of the system. Traditionally, droop control is used for LS in microgrids. However, because of the nature of the droop control, good VR and accurate LS cannot be achieved simultaneously. To overcome this drawback, a low-bandwidth, communication-based average VR technique along with an algorithm for equal voltage correction factor is proposed in this paper. Here, precise LS is achieved via droop control using a high value of droop gain, while good VR is obtained through low-speed control area network (CAN) communication, which provides fault tolerance and expandability feature to the system. Only the local dc bus voltage information is exchanged over CAN by the grid connected converters, which ensure low communication traffic. Thus, sufficient bandwidth is available on the CAN bus that can be used for control and monitoring purpose—a highly desirable proposition for a smart grid application. Effect of communication delays on system stability has been modeled and analyzed. Simulation and experimental results are presented to validate the feasibility of the proposed scheme. Plug-n-play and fail-safe features of the proposed technique are highlighted.

Multitransformer Primary-Side Regulated Flyback Converter for Supplying Isolated IGBT and MOSFET Drivers

This paper presents primary-side voltage regulated multitransformer quasi-resonant flyback converter (MTFC) for supplying isolated power switch drivers. The proposed topology offers distinct advantages over frequently used flyback converter possessing one high frequency transformer with isolated multiple outputs. Particularly, when a large number of separate dc supply units is required, then MTFC enables improved regular distribution of magnetic coupling between the common primary and the multiple secondary transformers' windings providing a high degree of galvanic and electromagnetic isolation between multiple outputs. Primary side voltage regulation is based on the average output voltage estimation using auxiliary RDC circuit mounted across the primary windings. Operation principles of MTFC are enhanced with analytical study of cross regulation of multiple output voltages at unbalanced load conditions, indicating reduced voltage deviation of multiple outputs by applying the primary-side average voltage regulation. Experimental results of prototype 2-, 3-, and 6-transformer quasi-resonant flyback converters confirmed their cross regulation quality and application potential for independent multiple output supplies.

A Distributed Finite-Time Secondary Average Voltage Regulation and Current Sharing Controller for DC Microgrids

This paper proposes a distributed finite-time secondary controller to achieve average voltage regulation and proportionate current sharing within a finite settling time for autonomous network of dc microgrids. It is employed by using a distributed finite-time control approach which maintains average voltage regulation of the system facilitating proportionate current sharing simultaneously by virtue of dynamic consensus between its neighbors. The proposed scheme ensures an improved performance over the conventional distributed secondary methods by reducing overshoots and chattering which is significant for critical operation of the loads. The proposed control strategy is simulated in MATLAB/Simulink environment to test link-failure resiliency, plug and play capability, and controller performance under communication delays within a tolerable upper bound on delay determined using time-delay analysis. Moreover the effect of variable time delays in different transmission medium is also simulated to test the practicality of the approach. This strategy is further tested on a 500-W FPGA-based experimental prototype to validate the control approach under different scenarios.

Distributed Averaging Control for Voltage Regulation and Current Sharing in DC Microgrids

In this letter we propose a new distributed control scheme, achieving current sharing and average voltage regulation in direct current (dc) microgrids. The considered dc microgrid is composed of several distributed generation units (DGUs) interconnected through resistive-inductive power lines. Each DGU includes a generic energy source that supplies a local current load through a dc-dc buck converter. The proposed distributed control scheme achieves current sharing and average voltage regulation, independently of the initial condition of the controlled microgrid. Moreover, the proposed solution requires only measurements of the generated currents, and is independent of the microgrid parameters and the topology of the used communication network, facilitating plug-and-play capabilities. Global convergence to a desired steady state is proven and simulations indicate a good performance.

A Stealth Cyber-Attack Detection Strategy for DC Microgrids

This paper proposes a cooperative mechanism for detecting potentially deceptive cyber attacks that attempt to disregard average voltage regulation and current sharing in cyber-physical dc microgrids. Considering a set of conventional cyber attacks, the detection becomes fairly easy for distributed observer based techniques. However, a well-planned set of balanced attacks, termed as the stealth attack , can bypass the conventional observer based detection theory as the control objectives are met without any physical error involved. In this paper, we discuss the formulation and associated scope of instability from stealth attacks to deceive distributed observers realizing the necessary and sufficient conditions to model such attacks. To address this issue, a novel cooperative vulnerability factor (CVF) framework for each agent is introduced, which accurately identifies the attacked agent(s) under various scenarios. To facilitate detection under worst cases, the CVFs from the secondary voltage control sublayer is strategically cross coupled to the current sublayer, which ultimately disorients the control objectives in the presence of stealth attacks and provides a clear norm for triggering defense mechanisms. Finally, the performance of the proposed detection strategy is simulated in MATLAB/SIMULINK environment and experimentally validated for false data injection and stealth attacks on sensors and communication links.

An Adaptive Event-Triggered Communication-Based Distributed Secondary Control for DC Microgrids

This paper proposes an adaptive event-triggered communication-assisted distributed secondary cooperative control strategy using a parameter projection law-based estimate of states to reduce communication burden. It overcomes the drawbacks of operating in an open loop manner between two triggering time instants in traditional zero-order hold-based event triggering schemes using a full state feedback control strategy to update the control input simultaneously. Moreover, real-time precision of information and model uncertainties is well dealt owing to the adaptive mechanism via an event-triggering condition designed using the Lyapunov technique to ensure the stability of the system. This strategy is used in tandem to achieve global average voltage regulation and proportionate load sharing in dc microgrids for various disturbances without sacrificing system performance. The proposed control strategy is simulated in MATLAB/Simulink environment and tested on a 500-W FPGA-based experimental prototype to validate the control approach under different scenarios.

Event-Triggered Communication Based Distributed Control Scheme for DC Microgrid

Development of microgrids and wide spread deployment of renewable generation and energy storage systems have highlighted the need for distributed control schemes to achieve maximum utilization and coordination among all the connected resources. In DC microgrids, the key goals of system-wide control are droop-based current sharing and network dc-voltage regulation. These may be achieved using a consensus algorithm secondary control that a) averages all the node voltages dynamically and maintains them at a nominal value and b) based on information from neighboring source currents to converge to a consensus in current sharing. Achieving the dynamic consensus may impose significant communication burden when implemented using conventional periodic communication methods. In this paper, an event-triggered communication based dynamic consensus algorithm is proposed that helps achieve both the stated goals of proportional current sharing along with average DC voltage regulation in a DC microgrid. The convergence and stability of the modified dynamic consensus algorithm and event triggering condition is proven using Lyapunov's stability criterion. The proposed control is applied to a DC microgrid system for secondary control of voltage and current regulation, and the performance of the proposed technique is validated under various operating conditions and disturbance in the communication system in both simulation and hardware environment.

Quality-Index Based Distributed Secondary Controller for a Low-Voltage DC Microgrid

Sources in a dc microgrid are required to have a proportional current sharing and an improved voltage regulation. Conventional droop-based control techniques achieve the proportional current sharing, but the voltage regulation of an individual bus may be large due to a high value of droop gains. In the literature, secondary controllers are discussed to improve either the voltage regulation of one bus or the average voltage regulation of the entire system. Most of these techniques rely on the communicated values of both the terminal voltage and the output current of individual sources. This paper puts forth a new approach to address the aforementioned issues. A figure of merit called quality index is introduced, which constitutes a weighted average of terms representing the voltage regulation and the power sharing at each source bus/node. An online algorithm to find the optimal droop coefficient based on the quality index is suggested. It does not require the knowledge of the system parameters. Furthermore, it reduces congestion in the communication network, as only output current information is exchanged among sources. Equations for the current sharing error and the voltage regulation in a microgrid system are derived based on droop gain values of the sources. Operation of the proposed algorithm is verified using a computer simulation. The method is validated on a scaled-down dc microgrid prototype.

A Consensus-Based Controller for DC Power Networks

In this paper we propose a new distributed passivity-based control scheme, achieving proportional (fair) current sharing and average voltage regulation in Direct Current (DC) power networks, with an arbitrary topology. The considered DC network is composed of several Distributed Generation Units (DGUs) interconnected through resistive-inductive power lines. Each DGU includes a generic energy source that supplies an unknown constant impedance load through a DC-DC buck converter. The proposed distributed control scheme achieves current sharing and average voltage regulation, independently of the initial condition of the controlled network, facilitating Plug-and-Play capabilities. Moreover, the proposed control strategy exploits a communication network to achieve current sharing using a consensuslike protocol. Global convergence to a desired steady state is proven and simulations show satisfactory performance.

Distributed Averaging Control for Voltage Regulation and Current Sharing in DC Microgrids: Modelling and Experimental Validation

Abstract In this paper we study a distributed control scheme, achieving current sharing and average voltage regulation in Direct Current (DC) microgrids. The considered DC microgrid is composed of several Distributed Generation Units (DGUs) interconnected, through resistive-inductive power lines, with loads. Each DGU includes a generic energy source that supplies the loads through a DC-DC buck converter. Remarkably, the proposed control scheme achieves average voltage regulation without the need of voltage measurements. An experimental validation is performed to assess the capabilities of the solution in a real network. The initial results are promising and show an additional need to compensate for partly unknown resistances present at the DGUs.

Distributed Primary and Secondary Power Sharing in a Droop-Controlled LVDC Microgrid With Merged AC and DC Characteristics

In an ac microgrid, a common frequency exists for coordinating active power sharing among droop-controlled sources. A common frequency is absent in a dc microgrid, leaving only the dc source voltages for coordinating active power sharing. That causes sharing error and poorer voltage regulation in dc microgrids, which in most cases, are solved by a secondary control layer reinforced by an extensive communication network. To avoid such an infrastructure and its accompanied complications, this paper proposes an alternative droop scheme for low-voltage dc microgrid with both primary power sharing and secondary voltage regulation merged. The main idea is to introduce a non-zero unifying frequency and a second power term to each dc source by modulating its converter with both a dc and a small ac signal. Two droop expressions can then be written for the proposed scheme, instead of the single expression found in the conventional droop scheme. The first expression is for regulating the ac frequency and active power generated, while the second is for relating the dc voltage to the second power term. The outcomes are better active power sharing and average voltage regulation in the dc microgrid, coordinated by the common injected ac frequency. These expectations have been validated by results obtained from simulations.