S. Conti

Adequacy Evaluation of Distribution System Including Wind/Solar DG During Different Modes of Operation

Keen interest in the development and utilization of renewable distributed generation (DG) has been currently observed worldwide. The reliability impact of this highly variable energy source is an important aspect that needs to be assessed as renewable power penetration becomes increasingly significant. Distribution system adequacy assessment including wind-based and solar DG units during different modes of operation is described in this paper. Monte Carlo simulation (MCS) and analytical technique are used in this work with a novel utilization of the clearness index probability density function (pdf) to model the solar irradiance using MCS. The results show that there is no significant difference between the outcomes of the two proposed techniques; however, MCS requires much longer computational time. The effect of islanding appears in the improvement of the loss of load expectation (LOLE) and loss of energy expectation (LOEE).

Generalized Systematic Approach to Assess Distribution System Reliability With Renewable Distributed Generators and Microgrids

This paper presents an innovative generalized systematic approach and related analytical formulation to evaluate distribution system reliability in smart grids where islanded operation of microgrids helps to improve local and overall reliability. In order to do this, the analytical formulation involves the adequacy calculation of conventional and renewable distributed generators supplying microgrids by using probabilistic models. Adequacy is computed by means of a new general analytical expression which takes into account load shedding (user load disconnection) and curtailment (user load reduction) policies.

Integration of multiple PV units in urban power distribution systems

Abstract The amount of power and energy that can be injected into the distribution system by dispersed generation (DG) is constrained by technical issues related to electrical components rating and distribution system operation. In this context, the aim of the paper is to study the voltage profile of a LV feeder in order to assess the maximum value of the power that can be injected into multiple load points of the feeder by PV units without violating the voltage constraints. To perform this study an analytical method with integral approach is used: the distribution of generators and loads along the feeder is represented by means of continuous functions and constant current model. The main result of the proposed study is that, with reference to a set of contiguous generation units, it is possible to derive analytical relationships between the position of the point of maximum/minimum voltage on the feeder and the characteristics of the distributed generation (e.g. length of the concerned feeder portion, position of the generators and overall current injected by the generators).

Study of the impact of PV generation on voltage profile in LV distribution networks

The present paper aims at assessing the effects produced on the distribution system by dispersed generators directly connected to LV networks. In particular, the introduction of photovoltaic (PV) generation systems has been studied, since such systems are the only ones having a natural inclination to be easily integrated into high density urban LV distribution networks. In the proposed study some aspects of the quality of power supplied to the customers will be dealt with taking into consideration slow voltage variations. In this regard the effect of dispersed generation (DG) on the voltage profile of a LV distribution feeder has been examined. With reference to a different kind of load distribution along the line, analytical expressions have been derived to determine the limit value of the power that can be injected into the distribution network without causing overvoltages. These expressions have been developed under some simplifying hypothesis related both to the DG unit and to the distribution network.

Innovative solutions for protection schemes in autonomous MV micro-grids

In this paper the development of protection schemes for multi-phase and phase-to-ground faults in autonomous micro-grids is dealt with, accounting for the fact that a distribution “island” can be intentionally intended to operate for a long time duration (e.g. during maintenance in the main upstream network or due to economic advantages for the micro-grid electrical customers). The definition of an adequate protection scheme has to account for the extremely variable micro-grids architecture, characteristics and fault conditions that make difficult to protect them by using traditional criteria and schemes. The work aims at finding innovative protection schemes that involve minimum implementation costs for the distribution network operator. For this reason the proposed solutions make use of traditional or commercially available protective, switching and sensing devices, as well as telecontrol/automation techniques currently used in italian distribution networks. Traditional technologies employed by innovative protection schemes allow also to find interesting practical solutions to protect micro-grid feeders in presence of a high number of inverted-interfaced generators resorting to differential protection schemes. This is especially important when a micro-grid has to accommodate a large amount of generators based on renewable energy sources.

Generators control systems in intentionally islanded MV microgrids

The aim of the present paper is to describe appropriate control systems for local generators able to correctly manage a microgrid during its transition from a grid-connected to an islanded operation, as well as during its autonomous operation. In order to study the dynamic behaviour of a microgrid including renewable energy-based generation units, control systems for both synchronous generators and inverter-interfaced generation units are developed and implemented in a software environment. The control schemes are designed to allow each type of generator to operate in both grid-connected and islanded modes. In a companion paper the software implementation of a test MV distribution network with a microgrid inside it and the developed generatorspsila control systems is described and simulation results are presented.

Reliability Assessment of Distribution Systems Considering Telecontrolled Switches and Microgrids

Telecontrol of distribution switching devices is recognized as a major means for reducing interruption duration after a fault occurrence in modern distribution systems, since it allows one to isolate the faulted sections and to restore the healthy ones as quickly as possible. In order to reduce interruption frequency as well, it is necessary to increase the number of series automatic circuit breakers. Further benefits in terms of service continuity improvement can be obtained by allowing islanded operation of relatively small portions of distribution network (microgrids) in the presence of distributed energy resources in order to supply the local load autonomously during upstream faults. This paper aims to describe an innovative systematic method and its related analytical formulation to evaluate distribution system reliability indices accounting for advanced operation practices such as those based on telecontrol and islanding.

Distributed generation in LV distribution networks: voltage and thermal constraints

The paper deals with the problem of assessing the amount of power that distributed generation (DG) can inject into LV distribution feeders without violating some technical constraints, such as the limitation posed by voltage constraints and lines thermal rating. The problem is studied following an analytical approach. The maximum value of the power that can be injected into the feeder by DG without violating the thermal constraints is analytically compared to the that which can be injected without violating the voltage constraints, in order to assess which of the two limitations is the most restrictive depending on the system operating conditions. The results obtained are also discussed by means of some numerical examples.

Modelling of Microgrid-Renewable Generators Accounting for Power-Output Correlation

The possibility to operate in islanding mode some portions of a distribution network, after fault occurrence, helps to improve system reliability. In this perspective, it is crucial to estimate the ability of distributed generators (DGs) to meet the local load. A major issue in the adequacy assessment is to take into account the correlation existing among the power outputs of DGs based on the same renewable intermittent primary energy source (sun, wind). This paper presents a new modelling approach to provide the hourly generation models for each type of renewable DGs, by taking into account both power correlation and hardware availability. An interesting aspect of the proposed approach is that it encompasses such correlation, avoiding the analytical calculation of its value. An innovative method to obtain hourly load models is also presented. Finally, a method to evaluate analytically loss of load probability by using the proposed generation and load models is also described. Both the presented method and Monte Carlo simulations (MCS) are applied to the IEEE RBTS-BUS6. Applying the proposed models and analytical method actually enables obtaining benchmark results to test approximated models and/or MCS results.

Voltage sensitivity analysis in radial MV distribution networks using constant current models

The paper presents some results of studies conducted on the development of analytical tools useful to allow real time control of voltages and power fluxes in active distribution networks in presence of distributed generation (DG) in the framework of Smart Grids implementation. The analytical tools can nevertheless be useful at the planning stage to guide some choices made by distribution planners. Specifically, a simple approach for calculating voltage sensitivity indexes with respect to active and reactive power changes at any node in a radial distribution network is investigated. The paper presents first stage results obtained by using two constant current models for loads and generators to obtain node voltages and their sensitivities. Subsequent stages of the study will perform a comparison with different models, such as the constant power one, and will assess the viability of the various approaches. Then, in this paper voltage sensitivity indexes are formulated with respect to active and reactive components of net node currents and the influence of some simplifying assumptions has been assessed. Numerical results are provided as the outcome of calculations performed on a test distribution network using MATLAB®.