Cristina Gonzalez-Moran

Scheduling of Droop Coefficients for Frequency and Voltage Regulation in Isolated Microgrids

This paper details a procedure based on bifurcation theory to evaluate the impact that droops and primary reserve scheduling have on the microgrid stability. The methodology is based on finding the worst primary reserve share-that is, the share closest to instability-that can be found after rescheduling the droops of selected generating units that support frequency (and voltage) regulation. The solution-which consists of a measure of the distance to instability in a given direction-is found in a multi-parameter space endowed with coordinates corresponding to the droop coefficients. Two stages are proposed to achieve the solution. First, an investigation of the distance to bifurcation is computed in a one-dimensional parameter space in a defined search direction. Then the direction of this search is updated by calculating the normal vector at the found bifurcation point. The procedure is iteratively repeated until the closest bifurcation is found. The proposed approach is analyzed in a 69-bus and 11-generation unit isolated microgrid. It is shown through the analysis of some scenarios how the distances and normal vectors provide valuable insight on the correct scheduling from the stability point of view, giving advice on how the primary reserve should be more reliably scheduled.

Complex-Valued State Matrices for Simple Representation of Large Autonomous Microgrids Supplied by

This paper focuses on representing the state space model of a microgrid in which power regulated (PQ ) and voltage/frequency regulated (Vf) generation units share a distribution system. The generation units considered in this paper are inverter interfaced. This introduces some interesting modeling problems which are treated in the paper, such as the decoupled cascaded control schemes or the non-negligible grid dynamics. A modeling approach is proposed based on four defined complex vectors. These vectors allow for complex-valued system matrices to be formed in a quite automated way. Moreover, a convenient partition of the system matrices is proposed, which in turn allows fast and easy modifications. Additionally, a multivariable methodology is proposed to simultaneously find the control system gains in an optimal sense. A 69-bus radial system, supplied by 20 generation units, is used to demonstrate how the proposal is of easy implementation to conduct small-signal stability analyses.

PQ

This paper details a modeling procedure that incorporates composite loads in stand-alone microgrids in which, because of the low system inertia provided by inverter-interfaced generation units, the grid dynamics is not neglected. The paper introduces a methodology based on 1) separately treating the plants (RL grid elements) from reference frames and control systems; and 2) establishing a vector valued function to methodologically describe all plants in a similar way. Induction motors equations are rearranged to be integrated within the model, giving as a result a highly structured, compact system model. Next, bifurcation theory is adapted to the problem to show that composite loads are a need in the microgrid modeling if more realistic results about oscillations and mainly about load margin are pursued. Thanks to the modeling procedure, this is proven by means of a series of analyses conducted in a microgrid of considerable larger dimensions than those presented to date in the literature.

and

In a smart micro-grid (MG) each generator or load has to take part in the network management, joining in reactive power supply/voltage control, active power supply/frequency control, fault ride-through capability, and power quality control. This paper includes a new concept for building integration in MGs with zero grid-impact so improving the MG efficiency. These aims are shown to be achievable with an intelligent system, based on a dc/ac converter connected to the building point of coupling with the main grid. This system can provide active and reactive power services also including a dc link where storage, generation, and loads can be installed. The system employed for validation is a prototype available at ENEA Laboratories (Italian National Agency for New Technologies). A complete and versatile model in MATLAB/SIMULINK is also presented. The simulations results and the experimental test validation are included. The trial confirms the model goodness and the system usefulness in MG applications.

Vf

This paper gives theoretical foundation to a procedure for modeling in a simply way the behavior of a droop-regulated islanded microgrid when, due to reserve scheduling considerations, the power reserves might be exhausted. The main problem observed for computing the operating point of those microgrids is that the droop formulation that is to be entered in the computation depends on the knowledge of the final result, which indeed means that the solution by means of Newton-Raphson-like methods is hindered. The proposed procedure reduces the complexity by formulating the problem as a complementarity problem. A discussion is offered then on the specific problem of droop formulation: two states are possible (with and without power limit reached), with the particularity that the power must be considered constant only when the power limit is reached, whereas the frequency can freely vary in both states, searching for an equilibrium in the load share among all the generation units endowed with droop regulation. Further, the problem is simplified by resorting to the use of Fischer-Burmeister NCP-functions (NCP for nonlinear complementarity problem), which substitutes the piecewise-defined droop function by an only scalar function (Fischer-Burmeister function) that makes the problem tractable to be solved by Newton-Raphson-like methods. The paper concludes with an exposition of numerical simulations in which the consequences of considering the power exhaustion on stability and operating points are demonstrated.

Generation

This paper analyzes the effects of the decoupling of the cross-coupling terms when PI synchronous regulators are used in droop characteristic converter. The PI synchronous regulators are implemented for both voltage and current control. An analysis of the different working modes of the converter are presented. Three sub-modes for the island model, Conventional droop mode, power quality mode and Sync mode are analyzed. One mode for grid connected mode, Grid supporting mode is analyzed.a

Composite Loads in Stand-Alone Inverter-Based Microgrids—Modeling Procedure and Effects on Load Margin

We propose an approach to allocate and delimit a region in which the rotational losses are of most importance in the stator core of induction motors. The delimitation is based on the analysis of points at which the minimum flux density is not null. The analysis of flux paths and values of flux density over a number of motors allows a model of flux density to be proposed for the chosen rotational region. We conducted the process by post-processing finite-element results. A comparison with bench test results shows that the approach can confine the effects of rotational losses within a region allocated in the tooth roots without significant loss of accuracy. We give analytical expressions based on geometrical data. The approach provides a quick method to evaluate the rotational losses by analytical means, bypassing the use of numerical methods at those design stages at which is preferable to reduce the accuracy in favor of computational speed.

Efficient Energy Management in Smart Micro-Grids: ZERO Grid Impact Buildings

The aim of the present work is to propose a complete Photovoltaic (PV) Direct Current (DC) source model, considering non linear effects with ambient temperature and solar irradiance. The main advantage of this model is that all non ideal characteristics of the PV source are taking into account, and complex weather conditions patterns can be considered. This model includes: The PV array and boost-buck DC-DC converter that operates to assure maximum power extraction. The Maximum Power Point Tracking (MPPT) algorithm, which is based on the incremental conductance method, is also described. The model can be used for example, as a DC source to supply grid connected or islanded inverters, to study the interaction of PV generators with the power system.

Fischer-Burmeister-Based Method for Calculating Equilibrium Points of Droop-Regulated Microgrids

This paper presents three new contributions to power flow analysis of unbalanced three-phase distribution systems. First, a complex vector based model in αβ0 stationary reference frame is developed to state the power flow equations using a compact matrix formulation. The proposed model is based on Kirchhoffs current law (KCL) and Kirchhoffs voltage law (KVL). Then, a general and exact power transformer model in the αβ0 reference frame is proposed. Finally, this transformer model is incorporated into the power flow problem. It will be shown that the use of an orthogonal reference frame simplifies the modeling of the distribution network components. In this work, both the network and the power transformer, as well as PQ type loads, PQ and PV type generators and a slack bus are modeled. By using the node incidence matrix instead of the admittance matrix, the information about the grid topology and the grid parameters (including power transformers) is separately organized. As it will be demonstrated, the proposed formulation is ready to incorporate other complex models of loads, generators or storage devices. The model is tested by using the IEEE 4-Node and the IEEE 123-Node Test Feeders with different transformer connections and balanced and unbalanced lines and loads.

An improved control scheme based in droop characteristic control for microgrid converters

In this article, multiresolution analysis (MRA) and wavelets are applied to the study of the existence of an internal short circuit in a transformer. An analytical support is given to the proposal of a relaying technique based on the derivation of the existence of specific singularities in the modulus of the space vector as obtained from the three differential signals. The proposal stems from the quite different travel angles of the space vector: maximum 120 degrees for healthy inrush and 360 degrees for fault. The sharp notches in the modulus of the space-vector are then detected using de first detail in MRA.