Anna Rita Di Fazio

Decentralized Control of Distributed Generation for Voltage Profile Optimization in Smart Feeders

Distributed generation, which is installed to exploit renewable energy sources, can also be used as a reactive power resource and contribute to tackle the voltage regulation problem in smart distribution grids. Adopting a decentralized approach with off-line coordination, the paper proposes an optimal set-point design for the voltage/reactive power control scheme of distributed generation. The objective is to improve the voltage profile along the feeders of a distribution system. Using only local measurements, the actual operating conditions of the feeder are firstly estimated; then, the set-point is evaluated by solving an optimal voltage profile problem. The off-line coordination avoids any modification to the architecture of existing control systems and requires communication only in the case of significant changes of the distribution system topology. The results of numerical simulations are presented for MV feeders with wind and photovoltaic generations. A comparison with standard control schemes is reported, as well as considerations about the off-line coordination among the distributed generations and the tap-changer of the HV/MV transformer.

A Bayesian-Based Approach for a Short-Term Steady-State Forecast of a Smart Grid

Future distribution networks are undergoing radical changes, due to the high level of penetration of dispersed generation and information/communication technologies, evolving into the new concept of the Smart Grid. Dispersed generation systems, such as wind farms and photovoltaic power plants, require particular attention due to their incorporation of uncertain energy sources. Further and significant well-known uncertainties are introduced by the load demands. In this case, many new technical considerations must be addressed to take into account the impacts of these uncertainties on the planning and operation of distribution networks. This paper proposes novel Bayesian-based approaches to forecast the power production of wind and photovoltaic generators and phase load demands. These approaches are used in a probabilistic short-term steady-state analysis of a Smart Grid obtained by means of a probabilistic load flow performed using the Point Estimate Method. Numerical applications on a 30-busbar, low-voltage distribution test system with wind farms and photovoltaic power plants connected at different busbars are presented and discussed.

Enhancing distribution networks to evolve toward smart grids: The voltage control problem

The evolution of existing distribution systems toward smart grids requires the exploitation of all the capabilities of the new Distributed Generation (DG). To this aim, the present paper addresses the problem of improving the voltage profile regulation in distribution networks with DG. According to a decentralized approach, the set-point of a reactive power regulation scheme of the DG is designed in an optimal sense by solving a constrained minimization problem. Since the proposed design uses only local measurements new investments on the existing networks are limited. The results of numerical simulations are provided for a MV multiple-feeder distribution system with multiple DGs with reference to two different optimization objectives.

Testing New Reactive Power Control of DERs by Real-Time Simulation

Abstract In the smart grid paradigm, the reactive power control of distributed energy resources (DERs) plays a key role improving the voltage profile in the distribution systems. This topic has been addressed by previous papers in which the Optimal Set-Point Design (OSPD) of DER reactive control, based on a decentralized approach, has been developed. The OSPD determines the set point of a reactive power closed-loop regulation scheme according to an optimization strategy. After briefly recalling the OSPD procedure, the article presents validation studies aiming at testing the effectiveness of the OSPD. The validation is based on a hardware-in-the-loop real-time simulation facility. In particular, an experimental setup has been arranged and presented, in which the system is simulated using the real-time digital simulator (RTDS), while the OSPD has been implemented on a PC in the LabView environment. The OSPD has been developed by considering two different optimization objectives, namely the feeder voltage profile optimization and the distribution losses minimization. The achieved results are then presented and also compared with the ones obtained a classical regulation scheme.

Estimating Distributed Generation and Loads for Voltage Profile Optimal Regulation

Abstract In MV distribution systems it is very important to account for the impact of Distributed Generation (DG) on the nodal voltage profile. A key issue is to guarantee adequate performance of the voltage control system acting on the On-Load Tap Changer (OLTC) of the HV/MV transformer. Referring to the case of a distribution system with the DG connected at a single node, a simple but effective OLTC control scheme is proposed to account for both load and DG variations. It keeps the classical structure of the OLTC control system, using only local measurements of voltage and current at transformer MV terminals, and avoiding data exchange with DG. To this aim, the effects of the changes of loads and of DG power injection must be distinguished and separately evaluated. The present paper focuses on the problem of estimating distributed generation and loads and proposes a specific procedure. Numerical simulations of a basic case study are presented to give evidence of the performance of the estimating procedure.

Performance evaluation of a DG voltage controller for smart grids

Abstract In a previous paper a DG reactive power control scheme aimed at improving the voltage profile in a smart grid has been proposed. In this paper the control scheme is recalled and validated by detailed numerical simulations. In particular, the performance is tested in presence of uncertainties of the load variations, interactions of the proposed control scheme with the control system in the primary substation and large random variability of the DG primary source. The analysis of the results and a comparison with classical DG reactive power regulation give evidence of the obtained improvements of the voltage profile.

MIMO design of voltage controllers for distributed generators

A large penetration of Distributed Generation (DG) may severely impact on the voltage profiles along LV distribution feeders. The first, cheapest and simplest step that can be undertaken is to involve DG units in the voltage control of the LV distribution networks, adopting a fully-decentralized architecture and avoiding any additional communication infrastructure. The paper proposes a design methodology for voltage control of DG units: a local controller measures and regulates the voltage at the node to which each DG unit is connected by acting on the DG reactive power injections and without any need for data or measurement exchange with other controllers. The controller design is based on a structural MIMO model of the distribution system and ensures a satisfactorily regulation, while avoiding any operation conflict among DG units. Stability analysis is also presented. The results of numerical simulations in PSCAD/EMTDC environment are discussed to validate the proposed approach.

LV distribution system modelling for distributed energy resources

A key issue in LV distribution system management and control is to increase the flexibility of the existing networks so as to allow a large spread of Distributed Energy Resources (DERs). In this perspective, virtual levels of aggregation, called Virtual Microgrids (VMs), are introduced, which are composed of parts of the distribution networks including different types of DERs and their control systems. To identify the VMs it is necessary to model the impact of DERs on the voltages and power flows of the LV distribution networks and to aggregate them on the basis of their impact on specific control actions. This paper proposes, as the first step, a new model of the LV distribution system derived from the DistFlow equations. The model expresses the variations of the electrical variables of the LV network as linear functions of the DERs injections, thus proving to be suitable for the VM definition. In a case study, a comparison of the results given by the proposed model with the load flow solutions is discussed and the evaluation of the impact of the DERs on the nodal voltages of a LV distribution system is analyzed.

Optimal reactive power control of distribution feeders with distributed energy resources: Interaction analysis and validation

In the voltage regulation problem of MV distribution systems with Distributed Generation (DG), the regulator of the On Load Tap Changer (OLTC) equipped with Line Drop Compensator (LDC) interferes with the DG voltage regulator. This interaction can significantly changes the voltage profile along the feeders. With the aim to propose a solution to this problem, the present paper firstly models this interaction and subsequently defines a sufficient condition which sets an upper bound on the reactance of the LDC. According to the defined condition it is possible to define two separate areas of MV distribution system. The first area collects the nodes controlled by the LDC and the second one the nodes controlled by the DG. In this way the OLTC with LDC and the DG voltage regulator both improve the voltage profile without generating mutual interactions. The study is developed with reference to the DG voltage regulator designed according to an optimization procedure named Optimal Set Point Design (OSPD), previously proposed by the same authors. The validity of the presented analysis is finally validated by numerical simulations.