The main goal of this paper is to get a simplified equivalent dynamic model of a co-generator when it is operated in the grid-connected mode. The gas-fueled co-generator installed at the PrInCE Lab microgrid is adopted and modelled in the real-time as a dynamic model of the second order. Parameters of the proposed model are derived through an input-output identification methodology based on the Lyapunov Theorem applied to the Sensitivity Theory (LTST). The correctness of proposed model has been tested experimentally.

The diffusion of distributed energy resources in distribution networks requires new approaches to exploit the users' capabilities of providing ancillary services. Of particular interest will be the coordination of microgrids operating as an aggregate of demand and supply units. This work reports a model predictive control (MPC) application in microgrids for the efficient energy management of energy storage systems and photovoltaic units. The MPC minimizes the economic cost of aggregate prosumers into a prediction horizon by forecasting generation and absorption profiles. The MPC is compared in realistic conditions with a heuristic strategy that acts in a instant manner, without taking into account signals prediction. The work aims at investigating the effect that different types of energy tariffs have in enhancing the end-users' flexibility, based on three examples of currently applied tariffs, comparing the two storage control modes. The MPC always achieves a better solution than the heuristic approach in all considered scenarios from the cost minimization point of view, with an improvement that is amplified by increasing the energy price variations between peak and off-peak periods. Furthermore, the MPC approach provides a cost saving when compared to the case considering a microgrid endowed with only photovoltaic units, in which no storage is installed. Findings in this work confirm that storage units better perform when some knowledge of future demand and supply trends is provided, ensuring an economic cost saving and an important service for the overall community.

This paper presents the "hardware in the loop" wind turbine emulator installed at the PrInCE Lab microgrid of the Polytechnic University of Bari. The emulator consists of a four-quadrant 60-kVA AC/AC converter that behaves as a controlled current source for the microgrid. The control of the power converter is performed by a local microcontroller, which enables the emulation of different kinds of horizontal as well as vertical axis wind turbines. The emulator includes also an HMI software that allows the user to directly interact with the emulator, providing him the ability to choose the wind turbine model and wind speed to adopt for testing purposes.

With the increase of the distributed generation, the development of distribution networks able to operate detached from the bulk grid has become possible. However, these islanded distribution networks lead to several issues to be dealt with, including the stability, the efficient generation profile scheduling and the procurement of balancing services. This paper proposes an optimal network management scheme capable of obtaining the power profiles of all users connected to the network for a given time frame. This scheme, based on a double negotiation phase, aims to minimize the social cost for the coverage of the overall demand, keeping the network within its technical operating limits. The impact of Multi-Energy Storage Systems (MESS) in the cost of providing these services is assessed by considering the technical limits of the specific users. Finally, the proposed control scheme has been tested on four different scenarios, with increasing MESS penetration level, demonstrating to be able to reduce the social cost associated with the re-dispatching by exploiting the services offered by the Battery and Thermal Energy Storage Systems.

The increasing focus on the active participation of low-voltage (LV) active distribution networks (DNs) in electricity markets requires the real-time optimal control of these DNs. To achieve this goal, a cheap semi-definite programming (SDP)-based optimal power flow (OPF) model for active neutral-equipped DNs, hosting both wye- and delta-connected loads, is proposed in this paper, aiming at overcoming the high computational requirement of the primal SDP-based OPF model. The coupled power injections between conductors are explicitly represented for each conductor by utilizing the network admittance matrix-based approach. Furthermore, three novel propositions (P1, P2 and P3) are proposed for the modelling of the constant current component of ZIP end-users in the context of the proposed OPF model. Moreover, the impact of the voltage-angle deviation on the exactness of the P1- and P2-based models is discussed. Simulations are carried out on several LV active DNs for various parameters of ZIP end-users, and the quality of the proposed OPF model is verified through the % optimality gap, power mismatch, voltage violation and root-mean-square error criteria. It is successfully shown that the proposed OPF model provides an optimal and feasible solution for all load types (wye, delta, mixed wye-delta) under a large range of ZIP load parameters. Furthermore, among the three propositions, the P3-based OPF model appears to be the most accurate in terms of determining an optimal and feasible solution. Finally, the reduced computational time of the cheap conic model allows its real-time implementation for medium- and large-sized DNs for which the primal multi-phase SDP-based model is practically difficulty to realize.

A novel optimal PhotoVoltaic (PV) inverter dispatch scheme is proposed in this paper which combines the active power curtailment and reactive power control schemes in order to simultaneously determine the optimal active and reactive power set points of residential PVs. The non-convexity of resultant AC optimal power flow model, caused by the inherent power balance constraints and selective inclusion of PV systems through binary variables, is handled by leveraging the cheap semi-definiteprogramming and sparsity-promoting-regularization approaches. The application of proposed methodology on a low-voltage test distribution network shows that recurrent technical issues of these networks such as voltage rise and voltage unbalance can be successfully mitigated as well as significant reduction in active power losses can also be achieved.

As a consequence of the increasing renewable energy sources penetration, Transmission System Operators are facing critical stability challenges, e.g. lack of rotating inertia, frequency regulation and voltage control. In fact, majority of the recently installed renewable power plants are decoupled from the grid and are, therefore, unable to contribute to the grid stability in the case of network perturbations. In the world, wind energy represents one of the most interesting Renewable Energy Sources in terms of new installations. Two main categories of wind turbine exist: fixed speed and variable speed. In the second category, which is the market standard nowadays, Permanent Magnet Synchronous Generator and Doubly Fed Induction Generator are possible technologies. In particular, while the first one is fully decoupled from the grid, the second one is directly connected to the grid via the stator circuit and includes a power electronic converter to drive both magnitude and frequency of rotor currents. Differently from a fixed speed wind generator, the power converter enables the wind turbine to regulate the output power over a wide range of wind speeds to maximize the wind-to-electricity conversion efficiency. With the aim of preserving grid stability, recently connected wind turbines are required to contribute in the frequency and voltage regulation. In detail, the recently applied Italian connection standard for new type D wind power plants (i.e. rated power higher than 10 MW) imposes controllers for providing frequency and voltage support. The paper discusses the positive role that wind generators could have in terms of network stability if recent standards are correctly applied, making use of suitable simulations realized in the DIgSILENT PowerFactory environment. Doubly Fed Induction Generators have been considered, since this technology is currently the most widely used.

In this paper, a resynchronization control strategy for a master-slave controlled microgrid has been developed. In the proposed control scheme only the generator acting as the master for the isolated microgrid is responsible for its resynchronization. The derived controller consists of two different control loops: active and passive. The first one consists in forcing the master unit to adjust its active and reactive power outputs to rapidly adapt the overall system frequency and voltage magnitude to the reference signal. On the contrary, the passive method waits that the two sets of voltage phasors at both microgrid and utility grid sides are synchronized before enabling the master unit to start the resynchronization process. Experimental tests have been performed on the PrInCE Lab microgrid built at the Polytechnic University of Bari, Italy, in order to prove the effectiveness of the proposed controller.

Nowadays, wind turbines commonly installed in new plants include a power converter. This allows these generators to operate in a variable speed range with the aim of maximizing the primary source exploitation. On the other hand, the presence of a converter implies a partial decoupling from the grid. Several studies have been carried out to enhance the ability of wind turbines in supporting frequency stability in the case of network events, and some grid codes recently include these regulating functions as required specifications. This work presents a novel control system applied to a Doubly Fed Induction Generator to make it compliant with present Italian standards. In order to characterize the role of wind turbines in supporting network frequency, several simulations have been carried out in the DIgSILENT PowerFactory environment. Main advantages and disadvantages are identified and discussed, depending on both the wind turbine operating conditions and the characteristics of the required stabilizing contribution.

The Optimal Power Flow (OPF) model for low voltage active Distribution Networks (DNs), which are equipped with neutral conductors, requires an explicit representation of both phases and neutral conductors in its formulation to obtain complete information about the state variables related to these conductors. In this regard, a centralized OPF relaxation based on semi-definite programming is presented in this paper for neutral-equipped DNs hosting ZIP loads and neutral-ground impedance, and contain a significant level of unbalance. The major restriction in the development of an OPF model for these networks is the coupled power injection across the conductors which is successfully handled by deriving the explicit active and reactive power injections for each conductor through a network admittance matrix-based approach. The shortcomings of existing voltage magnitude-based technique for the modelling of ZIP loads are comprehensively reported and a novel complex voltage variable-based approach is proposed which successfully incorporates ZIP loads in the developed multi-phase OPF relaxation. For the handling of constant current load, a modelling approach based on the first-order-Taylor series is introduced as well. Furthermore, the impact of the application of Kron reduction approach on the global optimal solution of single- and multiple-point grounded DNs is discussed in detail. Three metrics, eigenvalue ratio, power mismatch and cumulative normalized constraint violation, are utilized to evaluate the exactness of proposed relaxation. Simulations, carried out on several medium and low voltage DNs, show that the proposed relaxation is numerically exact under several combinations of ZIP load parameters and a reasonable range of grounding impedance value for both time-varying and extreme system loading scenarios irrespective of the degree of unbalance in a network.

Optimal power flow problem for low-voltage radial distribution networks including neutral conductor is considered in this paper. The standard semi-definite programming based relaxation, initially proposed for three-phase networks, is extended to such multi-phase networks by taking into account the coupled phase-neutral power injections. An approach based upon correction current injection is utilized which incorporates the network loads into the system admittance matrix by representing them through constant admittances. Furthermore, the impact of neutral-ground impedance has also been taken into account. Simulations are carried out on real low-voltage unbalanced distribution networks for minimization of slack-bus power injection and power losses objective functions. It has been shown that the proposed approach is numerically exact under a large range of the value of neutral-ground impedance and can be successfully solved by a generic optimization solver such as MOSEK. The exactness of the proposed scheme has also been verified for the time-varying load and generation profiles, whereas the accuracy of the recovered solution is also confirmed by root-mean-square-error parameter.

A microgrid is a localized group of electricity sources and loads that normally operates connected to the utility grid and acts as a single controllable entity. However, this system has the ability of operating disconnected from the main grid, according to physical and economic conditions. Several test beds all over the world have been installed with the aim of studying new control strategies and key technologies able to increase the reliability of these systems. In the case, innovative control strategies and/or devices are made available, their behavior in the test bed needs to be studied before field tests. Therefore, a suitable dynamic model of the microgrid is required. The purpose of this paper is to develop a dynamic model of the combined heat and power system installed in the PrInCE Lab Microgrid of the Polytechnic University of Bari. More specifically, a particular focus is put on the elements connected to the DC busbar of the combined and heat power system. The model has been implemented in DIgSILENT PowerFactory environment (EMT simulations) and then validated through real measurements in both grid-connected and islanded operating conditions.

The increasing integration of renewable energy sources in microgrids has introduced new challenges in their control strategy management. The lack of inertia, due to the large presence of power-electronic interfaced generators, worsens network dynamic performances in the case of sudden variations in its working conditions and, as a result, the stability of microgrid may no longer be guaranteed. In order to ensure the stable operation of microgrid, the use of suitable controllers, applied to static generators for providing a power contribution during network perturbations, is a concrete way. In this work, a virtual synchronous machine control is developed for a photovoltaic plant, without deload requirements. Unlike the swing equation based inertial response, this control strategy appears more suitable since it ensures grid stability even when no reference frequency is locally available. Different simulations, carried out in DIgSILENT PowerFactory environment, validate the proposed control scheme for a test microgrid. Finally, the controller operation during low solar radiation periods is also illustrated.