A voltage-source inverter for microgrid applications with an inner current control loop and an outer voltage control loop

Distributed generation (DG) units are commonly inter-faced to the grid by using voltage-source inverters (VSI’s). Extension of the control of these inverters allows to improve the power quality if the main power grid is disturbed or disconnected. In this paper, a control technique is developed for a VSI working in island mode. The control technique is designed in the time domain, combining an inner current control loop with an outer voltage control loop. Voltage regulation under various linear and non-linear load disturbances is studied

Gather-and-broadcast frequency control in power systems

We propose a novel frequency control approach in between centralized and distributed architectures, that is a continuous-time feedback control version of the dual decomposition optimization method. Specifically, a convex combination of the frequency measurements is centrally aggregated, followed by an integral control and a broadcast signal, which is then optimally allocated at local generation units. We show that our gather-and-broadcast control architecture comprises many previously proposed strategies as special cases. We prove local asymptotic stability of the closed-loop equilibria of the considered power system model, which is a nonlinear differential-algebraic system that includes traditional generators, frequency-responsive devices, as well as passive loads, where the sources are already equipped with primary droop control. Our feedback control is designed such that the closed-loop equilibria of the power system solve the optimal economic dispatch problem

Distributed MPC for coordinated energy efficiency utilization in microgrid systems

To improve the renewable energy utilization of distributed microgrid systems, this paper presents an optimal distributed model predictive control strategy to coordinate energy management among microgrid systems. In particular, through information exchange among systems, each microgrid in the network, which includes renewable generation, storage systems, and some controllable loads, can maintain its own systemwide supply and demand balance. With our mechanism, the closed-loop stability of the distributed microgrid systems can be guaranteed. In addition, we provide evaluation criteria of renewable energy utilization to validate our proposed method. Simulations show that the supply demand balance in each microgrid is achieved while, at the same time, the system operation cost is reduced, which demonstrates the effectiveness and efficiency of our proposed policy.Accepted manuscrip

Distributed Model Predictive Consensus via the Alternating Direction Method of Multipliers

We propose a distributed optimization method for solving a distributed model predictive consensus problem. The goal is to design a distributed controller for a network of dynamical systems to optimize a coupled objective function while respecting state and input constraints. The distributed optimization method is an augmented Lagrangian method called the Alternating Direction Method of Multipliers (ADMM), which was introduced in the 1970s but has seen a recent resurgence in the context of dramatic increases in computing power and the development of widely available distributed computing platforms. The method is applied to position and velocity consensus in a network of double integrators. We find that a few tens of ADMM iterations yield closed-loop performance near what is achieved by solving the optimization problem centrally. Furthermore, the use of recent code generation techniques for solving local subproblems yields fast overall computation times.Comment: 7 pages, 5 figures, 50th Allerton Conference on Communication, Control, and Computing, Monticello, IL, USA, 201

Methodology for testing loss of mains detection algorithms for microgrids and distributed generation using real-time power hardware–in-the-loop based technique

The effective integration of distributed energy resources in distribution networks demands powerful simulation and test methods in order to determine both system and component behaviour, and understand their interaction. Unexpected disconnection of a significant volume of distributed generation (DG) could have potentially serious consequences for the entire system [1], this means DG sources can no longer be treated as purely negative load. This paper proposes a method of testing loss-of-mains (LOM) detection and protection schemes for distributed energy resources (DER) using real-time power hardware-in-the-loop (RT PHIL). The approach involves connecting the generator and interface under test (e.g. motor-generator set or inverter, controlled by an RTS – Real Time Station[3]) to a real-time simulator (an RTDS – Real Time Digital Simulator[2]) which simulates the local loads and upstream power system. This arrangement allows observation of the interaction with other controls in the network beyond the local microgrid area. These LOM schemes are of increasing importance because with growing penetration levels of distributed generation the network operator has less visibility and control of the connected generation. Furthermore when the generation and load in a particular network area are closely matched (e.g. a grid-connected microgrid), it becomes increasingly difficult to detect a loss of grid supply at the generator. This work builds upon the existing LOM testing methodology proposed in [4]. By utilising RT PHIL and a laboratory microgrid, the testing environment has been brought to a new level of functionality where system integrity can be more rigorously and realistically evaluated

Distributed Apportioning in a Power Network for providing Demand Response Services

Greater penetration of Distributed Energy Resources (DERs) in power networks requires coordination strategies that allow for self-adjustment of contributions in a network of DERs, owing to variability in generation and demand. In this article, a distributed scheme is proposed that enables a DER in a network to arrive at viable power reference commands that satisfies the DERs local constraints on its generation and loads it has to service, while, the aggregated behavior of multiple DERs in the network and their respective loads meet the ancillary services demanded by the grid. The Net-load Management system for a single unit is referred to as the Local Inverter System (LIS) in this article . A distinguishing feature of the proposed consensus based solution is the distributed finite time termination of the algorithm that allows each LIS unit in the network to determine power reference commands in the presence of communication delays in a distributed manner. The proposed scheme allows prioritization of Renewable Energy Sources (RES) in the network and also enables auto-adjustment of contributions from LIS units with lower priority resources (non-RES). The methods are validated using hardware-in-the-loop simulations with Raspberry PI devices as distributed control units, implementing the proposed distributed algorithm and responsible for determining and dispatching realtime power reference commands to simulated power electronics interface emulating LIS units for demand response.Comment: 7 pages, 11 Figures, IEEE International Conference on Smart Grid Communication

Evaluation of the effects of photovoltaic inverter controllers on grid injected power with local dynamic loads

In this paper, the most prominent inverter interface control techniques for the grid-connected photovoltaic (PV) systems; the constant-current control (CCC) and constant-power control (CPC) were evaluated based on the dynamics of the inverter local load. A fully controlled 100 kW grid-connected PV distributed generation (DG) with dynamic load is simulated in MATLAB Simulink. The CCC has one fast control loop while the CPC has two control loops, fast inner current loop and slow external power loop that sits on top of the inner loop. As loads on the DG may change dynamically due to demand variations, the effects due to these controllers on the grid-injected power are compared. The CCC is found to be more susceptible to THDi for larger load variation than the CPC although with better response to transients. From the results CPC can be considered best control option for grid-connected inverter

Voltage stabilization in DC microgrids: an approach based on line-independent plug-and-play controllers

We consider the problem of stabilizing voltages in DC microGrids (mGs) given by the interconnection of Distributed Generation Units (DGUs), power lines and loads. We propose a decentralized control architecture where the primary controller of each DGU can be designed in a Plug-and-Play (PnP) fashion, allowing the seamless addition of new DGUs. Differently from several other approaches to primary control, local design is independent of the parameters of power lines. Moreover, differently from the PnP control scheme in [1], the plug-in of a DGU does not require to update controllers of neighboring DGUs. Local control design is cast into a Linear Matrix Inequality (LMI) problem that, if unfeasible, allows one to deny plug-in requests that might be dangerous for mG stability. The proof of closed-loop stability of voltages exploits structured Lyapunov functions, the LaSalle invariance theorem and properties of graph Laplacians. Theoretical results are backed up by simulations in PSCAD