A Communication-less Secondary Voltage Control Based on Small-AC-Signal Injection for DC Microgrids

This paper presents primary and secondary control of islanded microgrid. In conventional primary droop control, there is a problem of poor voltage regulation of microgrid. The drawback of conventional primary droop control is improved by voltage shifting based primary control. Voltage shifting based primary control is not completely satisfied by the voltage regulation of microgrid. Also, there is current sharing problem in both types of primary control. So, in secondary control, both current sharing and voltage regulation are achieved by slope adjusting and voltage shifting simultaneously. In this secondary control method, average value of current, voltage, and droop resistance of two neighboring converters is calculated then controlled by additional layer of distributed secondary control over both primary controls. By adjusting droop coefficient of different converters making same value of output impedance of converter, current sharing and voltage regulation are achieved. Also, in transient state, current sharing is achieved by using this secondary control technique. The performance of proposed microgrid is verified by MATLAB simulation.

A new robust decentralized approach based on adaptive fuzzy backstepping method for microgrid secondary voltage control

This paper presents techniques for the application of tertiary and secondary voltage control through the use of intelligent proportional integral derivative (PID) controllers and the wide area measurement system (WAMS) in the IEEE 39 bus system (New England system). The paper includes power system partitioning, pilot bus selection, phasor measurement unit (PMU) placement, and optimal secondary voltage control parameter calculations to enable the application of the proposed voltage control. The power system simulation and analyses were performed using the DIgSILENT and MATLAB software applications. The optimal PMU placement was performed in order to apply secondary voltage control. The tertiary voltage control was performed through an optimal power flow optimization process in order to minimize the active power losses. Two different methods were used to design the PID secondary voltage control, namely, genetic algorithm (GA) and neural network based on genetic algorithm (NNGA). A comparison of system performances using these two methods under different operating conditions is presented. The results show that NNGA secondary PID controllers are more robust than GA ones. The paper also presents a comparison between system performance with and without secondary voltage control, in terms of voltage deviation index and total active power losses. The graph theory is used in system partitioning, and sensitivity analysis is used in pilot bus selection, the results of which proved their effectiveness.

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The hierarchical control strategy enables seamless integration of power electronics-based distributed generators (DGs) into microgrids (MGs). This work presents a distributed coordinated secondary voltage and frequency control strategy of islanded MGs, utilizing distributed model predictive control (DMPC) approach. The Proposed approach is entirely distributed in nature, with plug and play features and requires minimal communication network with information exchange between immediate neighbors. This technique facilitates efficient information exchange and obviates the need for centralized control. Droop control is the primary controller for all the DGs, which leads to the deviations in DG voltage and operating frequency from standard nominal values. Secondary control enables the restoration of DG voltage and frequency to their nominal values. Input–output feedback linearization (IOFL) is used to transform nonlinear dynamics of inverter-interfaced DGs into first-order linear dynamics between input and output. Compared to some published works, the single-integrator dynamics eliminates the need to compute the derivative of the output voltage. Furthermore, DMPC has been used for all DG units as secondary controller, to achieve voltage and frequency regulation. This approach takes advantage of distributed architecture with optimal control actions and enhances reliability and flexibility of MG operations. Moreover, the proposed approach ensures that voltage and frequency of overall system are restored to its reference value while maintaining accuracy in real power sharing. Effectiveness of proposed control scheme is validated with simulations conducted on islanded MG test system using MATLAB/Simulink toolbox.

This Paper focuses on issues developed in parallel DC–DC converter in low-voltage isolated DC Microgrid. Two main issues are faced by DC Microgrid when two or more than two converters are connected in parallel, one is unequal load sharing and second circulating current. This occurs due to the output voltage of converter should be different. The main reason for the output voltage of both the converters is different due to change in load, input power, and also feedback voltage and current. If two or more than two sources are connected in parallel and there is any mismatch in output voltage will initiate circulating current and difference in current sharing which cause overload to the converters with variation in power sharing. Droop control is the popular method for load sharing in DC Microgrid but due to its disadvantages poor circulating current and voltage droop arise due to mismatch in the converters output voltages. Another method is virtual resistance introduced which gives the relationship between virtual resistance and current sharing as well as virtual resistance and bus voltage in DC microgrid. This proposed modified droop control technique try to minimize circulating current and losses in the output side of converters.

The paradigm shift in electrical power grids and the increased interest towards decentralisation has opened a new window in the design, control and theoretical analysis of small scale power systems, i.e. microgrids, which aim at the integration and utilisation of renewable energy sources, energy storage systems and responsive loads at a local scale. Given their DC nature, DC microgrids have attracted significant interest as they provide a natural interface to the main grid by avoiding additional conversion steps. The aim of this thesis is to design and analyse novel hierarchical control schemes, both at the primary and secondary control level, that guarantee tight voltage regulation, accurate power sharing, current or voltage limitation, and present a straightforward approach towards deriving stability conditions for DC microgrids that incorporate nonlinear loads. As the microgrid configuration is paramount in the theoretical analysis, a rigorous method of computing the admittance matrix is developed that facilitates the stability analysis of DC microgrid systems supplying a constant impedance (Z), constant current (I) or constant power (P) load. This method is particularly useful as it permits the factorisation of the admittance matrix, while separating the singular matrices. In this way, the closed-loop asymptotic stability proof can be more easily approached by isolating the singularities and then, employing straightforward linear algebra tools, such as quadratic eigenvalue problem (QEP) theory, to derive the stability conditions. As stated in the technical specifications of every source, a crucial issue is represented by the ability to protect itself and its interface device (power converter) during faults, transients and unrealistic power demands. That is why current-limiting control is often combined with the primary control, i.e. often droop control, in DC microgrids to ensure the desired unit protection. In this thesis, novel current-limiting droop controllers areformulated for DC microgrids consisting initially of multiple unidirectional DC/DC boost converters and, later on, bidirectional DC/DC boost and three-phase AC/DC converters, that integrate different distributed generation units with the local grid. In contrast to the traditional approaches that use small-signal modelling, here, the accurate nonlinear models of the converter units are taken into account to prove the boundedness and the current-limiting property, by employing Lyapunov methods and the ultimate boundedness theory. Exponential stability at the desired equilibrium point is mathematically guaranteed, and further analysed from a graphical perspective, providing insights of the load’s effect onto the system performance and stability. To address the shortcomings that conventional droop control approaches introduce, i.e. inaccurate power sharing and significant load voltage drop, a DC microgrid architecture that takes into account the power converter dynamics of distributed generation units is deployed under decentralised primary and distributed secondary control scheme, in a hierarchical control framework. At the primary control layer, a novel current-limiting droop control scheme is implemented in a decentralised manner, whereas at the secondary control layer a fully distributed controller that performs a voltage restoration and improves the power sharing is deployed using a nearest-neighbour coupling communication network. By investigating for the first time both the dynamics of the converters with the nonlinear load and the two-layer control, singular perturbation theory is applied to analytically guarantee the stability of the entire DC microgrid. Finally, apart from the desired overcurrent protection, since DC capacitors are customary used at the output of each converter unit to stabilise the output voltage, they also introduce a maximum voltage limit. Hence, as the need for protection against overvoltages has emerged, droop controllers with inherent overvoltage protection are also proposed for parallel and meshed configuration networks. The upper limit of the voltage of each source is rigorously proven using ultimate boundedness theory. Asymptotic stability to the desired equilibrium for the closed-loop system is analytically guaranteed, and detailed conditions are derived to guide the control design. In order to validate the effectiveness of the different control methodologies developed in this thesis, both simulation and experimental testing are performed for each one of the methods, and are also compared with the conventional approaches to highlight their superiority.

The islanded DC microgrid undertakes its voltage control and power management alone because of its independency from the grid. The line impedance brings difficulties for the droop control strategy to improve the control of the bus voltage and the power management of battery storage units (BSUs) at the same time. Therefore, a secondary coordinated control strategy, including the BSU power management and the bus voltage correction, is proposed in this paper, which improves the conventional droop control strategy. The droop control is the primary control, leading to the decline of bus voltages. The secondary control is a distributed control based on the consistency algorithm to correct the bus voltages, by increasing the system load until the BSUs reach the same state of charge (SoC). The effectiveness of the proposed strategy is verified by experiments in the Matlab/Simulink.

Two important and upcoming technologies, microgrids and electricity generation from wind resources, are increasingly being combined. Various control strategies can be implemented, and droop control provides a simple option without requiring communication between microgrid components. Eliminating the single source of potential failure around the communication system is especially important in remote, islanded microgrids, which are considered in this work. However, traditional droop control does not allow the microgrid to utilize much of the power available from the wind. This dissertation presents a novel droop control strategy, which implements a droop surface in higher dimension than the traditional strategy. The droop control relationship then depends on two variables: the dc microgrid bus voltage, and the wind speed at the current time. An approach for optimizing this droop control surface in order to meet a given objective, for example utilizing all of the power available from a wind resource, is proposed and demonstrated. Various cases are used to test the proposed optimal high dimension droop control method, and demonstrate its function. First, the use of linear multidimensional droop control without optimization is demonstrated through simulation. Next, an optimal high dimension droop control surface is implemented with a simple dc microgrid containing two sources and one load. Various cases for changing load and wind speed are investigated using simulation and hardware-in-the-loop techniques. Optimal multidimensional droop control is demonstrated with a wind resource in a full dc microgrid example, containing an energy storage device as well as multiple sources and loads. Finally, the optimal high dimension droop control method is applied with a solar resource, and using a load model developed for a military patrol base application. The operation of the proposed control is again investigated using simulation and hardware-in-the-loop techniques.

Direct current (DC) microgrid is being increasingly investigated in modern power grid. An important issue in DC microgrid operation is to ensure proper current sharing among converters. While this has been addressed through droop control, the resulting voltage deviation in DC bus has to be compensated. To solve this problem, a new distributed secondary control scheme is presented in this paper for both current sharing and voltage restoration. A key part of the presented scheme is the integration of a new parameter `virtual voltage drop' defined from droop gain and line resistance. Since the DC bus voltage is not required as a feedback signal, the proposed secondary control is simple and easy to design and implement. In addition, as the proposed scheme has no requirement for the loads, it can handle both resistance loads and constant power loads (CPLs). Simulations as well as experimental studies are carried out to demonstrate the effectiveness of the proposed scheme.

An adaptive droop control with constant voltage regulation is proposed for the power and voltage management of a DC microgrid (DCMG) with multiple power sources, such as a utility grid (UG), a distributed generator (DG), an energy storage system (ESS), and an electric vehicle (EV). In the proposed scheme, the droop characteristics for the UG, ESS, and EV are adaptively changed according to electricity price conditions and state-of-charge (SOC) levels in order to optimize the DCMG operation flexibility as well as electricity cost. The proposed control method not only ensures the power-sharing of DCMG reliably without the use of a communication link, but also regulates the DC bus voltage stably at the nominal value. To achieve this, the proposed scheme consists of primary control and secondary control. The primary control is used to achieve power-sharing in the decentralized DCMG, while the secondary control is used to overcome the disadvantage of conventional droop control, i.e., to remove DC bus voltage deviations. Decentralized power management is also presented to enhance the DCMG system reliability in the presence of uncertainties such as DG generation power, the ESS and EV SOC levels, the grid and EV availabilities, the load demand, and electricity price conditions. The effectiveness of the proposed scheme is demonstrated in a comprehensive simulation and experiment under various conditions. The test results clearly confirm the control flexibility and overall performance of the proposed control scheme for decentralized DCMG system.

Droop control represents the key grid-forming control strategy in modern micro-grids consisting of multiple distributed energy resources (DERs), implemented at the primary control layer of the hierarchical control architecture. In this paper, an enhanced droop control methodology for meshed DC micro-grids with constant power loads (CPLs) is proposed, which inherits the conventional droop control features and additionally guarantees a crucial overvoltage protection property of each DER unit, independently from each other or the loads. Since one of the main challenges in DC micro-grids is that CPLs introduce negative impedance characteristics that can lead to system instability, in this paper, based on the proposed novel control structure and using nonlinear ultimate boundedness theory, an upper limit for the output voltage is rigorously guaranteed. In addition, asymptotic stability to the desired equilibrium for the closed-loop system is analytically proven, and detailed conditions are derived to guide the control design. Simulation testing is performed for a meshed DC micro-grid to verify the theoretical contribution and the effectiveness of the proposed primary droop controller.

In today’s world, where the grid structure becomes smarter day by day, microgrid, one of the fundamental methods for the integration of renewable energy, are taking firm steps to be building blocks of the modern electrical grid. Along with its advantages, the microgrid brings extra challenges which attracts the attention of many researchers in these days. This paper focuses on addressing two of the main challenges: voltage stability and reactive power sharing in islanded mode. Contents of this paper include: first, elaboration of general hierarchical control structures of inverter-based distributed generators (DGs), second, review of primary droop control methods and analysis of their drawbacks, third, review and comparison of current secondary reactive power control methods to find the gap of more practical secondary control method. In the paper, a centralized secondary controller based on intermittence level, effective-impedance to load and severity of voltage deviation is proposed to overcome the disadvantage of current secondary reactive power control methods. The proposed method takes practical aspects into consideration and prioritizes DGs in reactive power balancing and voltage stability. Performance of proposed method is illustrated by a simulation study.

The primary goal of the research presented in this dissertation is the development of a decentralized control method for both dc and ac inverter islanded microgrids with high penetration of stochastic renewable sources that is an effective alternative to a centralized, optimal solution. One of the contributions of this work is the development of the Decentralized Mode Adaptive (DMA) guidance law for a Hamiltonian-based controller of an islanded, N-source, dc microgrid. The DMA guidance law reduces the gap between droop control and centralized control by providing a method that can operate autonomously in the event of source and bus load fluctuations as well as reduce communication requirements of centralized controllers, and thus increase resiliency and scalability. This is a significant trait, as stochastic renewable sources are prone to sizable fluctuations or even suddenly dropping offline. The high penetration of stochastic renewable sources mandates controls that can handle such swings. The proposed DMA guidance law is compared to the centrally-controlled optimal exergy destruction (OXD) power apportionment strategy, as well as a Hamiltonian-based droop control strategy. After adding a guidance law to the Hamiltonian-based droop controller to allow it to handle the removal of a source, both the DMA and droop controller can maintain a constant bus voltage. The DMA guidance law makes an improvement over droop control in that it is able to track OXD steady state performance when droop control cannot. Finally, it is shown that for a sufficiently large number of converters supplying a microgrid, the presented decentralized, mode-adaptive strategy provides an efficient and practical alternative to both droop control and centralized control schemes. Another contribution of this work is development of the Optimal Exergy Destruction (OXD) and Decentralized Power Apportionment (DPA) feedforward control law for a Hamiltonian-based controller for islanded, N-source ac inverter microgrids. The importance of the OXD guidance law is that it is an analytical, closed-loop optimal feedforward controller that was developed utilizing exergy analysis to minimize the exergy destruction in the microgrid. As in the dc microgrid case, the OXD outer loop controller requires a star or fully connected topology whereas the DPA operates with no communication among the inverters. The decentralized power apportionment feedforward controller eliminates some critical drawbacks to conventional P-ω/Q-V droop control. Unlike conventional P-ω/Q-V droop control, the DPA control scheme holds the bus frequency constant and is able to return the bus voltage to the reference value after large load or

This paper presents a distributed voltage unbalance compensation method for an islanded microgrid (MG) system. By using hierarchical control concept, we design a distributed compensation scheme in the secondary control layer, while the traditional inverter control methods including voltage and current PR control, droop control, virtual impedance are implemented in the primary layer. Different from most existing centralized compensation methods, no specified individual centralized secondary controller exists. We decompose the traditional centralized controller into several local secondary controllers. A novel distributed PI controller with negative sequence current feedback is designed in each secondary controller. By allowing them to exchange information with their neighboring controllers respectively, the unbalanced voltage in the sensitive load bus (SLB) can be compensated by all the distributed generators (DGs) cooperatively. In addition, the compensation effort of each DG is designed to be proportional to their power ratings respectively. An islanded microgrid system consisting of 4 DGs is built in MATLAB to validate the proposed method.