Contribution to Management of Microgrids through the Integration of Renewable Energy Sources and Co-generation Systems of Heat and Power
29/11/2005 – 09/12/2014
The installation of more and more units of dispersed generation (DG) close to consumers leads to a new era for the Electrical Energy Systems. This doctoral thesis aims to develop proper methodologies, tools, software and analysis techniques for the quantification of the dispersed generation’s advantages, giving emphasis in Combined Heat and Power production (CHP) and Renewable Energy Sources (RES), whether they operate independently or in a coordinated way, thus forming a Microgrid. The transformation of the deterministic equations that govern these studies to their stochastic form as well as the development of methods to solve the probabilistic load flow in order to manage the uncertainty isnecessary.
More specifically, a methodology was developed, with the assistance of the sensitivity analysis method, for determining the sensitivity coefficients, for those quantities of the grid that are used for the quantification of the main economic benefits because of the augmented DG penetration. Those economic benefits, through an implementation in a specific grid, arise from postponing the investments in new grid components such as transformers and cables, from the reduction of the total active power loss, from avoiding buying energy based on the system’s marginal price during peak hours and from the potential reduction in electricity price. The improvement of the voltage level on the system’s buses was also examined.
Moreover, suitable calculation software for minimizing the operational cost in the Microgrid in which the energy and pricing policies are applied was also developed. The studied low voltage (LV) grid consists of consumers (loads) and energy microsources such as photovoltaics (PV), wind turbines (WT), microturbines (MT), fuel cells (FC), micro_combined heat and power units (Micro_CHP), electrical vehicles (EV). Policies that regard the augmented penetration of DG units were also applied. Furthermore, the Microgrid was sufficiently studied with variations in the pricing policies relevant to its operation, where the variable of energy demand elasticity as to the price was introduced. A number of logistic pointers, which are extracted for each one of the operational scenarios, were created, in order to manage the extracted information, so as to compare the scenarios and, consequently, the policies that can be implemented in a LV network, between them. So, by evaluating comparable scenarios, the policies that contain greater viability and realization potentiality are presented. The modeling was done in such as way as to ensure the maximum possible autonomous operation from the upstream network.
In addition, a viability study of a Micro_CHP investment for a residential complex compared to the conventional coverage of its energy needs was conducted. Then, a probabilistic model of a Micro_CHP unit in association with the external ambient temperature and the characteristics of the area to be thermally covered, so as to find the distributions of both the electrical output and the thermal power using the HPR ratio were examined. The proposed probabilistic model was used to find the hourly (or the aliquot of the hour) average thermal and electrical demand of the buildings. In larger electrical systems, the procedure to find the optimal load flow with environmental constraints and with CHP extraction units in Heat Match operation was successfully developed, highlighting these units as main players.
Then, finding the solution for the stochastic multi objective optimization problem in a Microgrid is achieved by taking into consideration that the functions of the expected operational cost and the risk functions (expected deviations of the electric and thermal power production) that will be minimized using the weighted sum are conflicting. Expanding and solving the known stochastic model of economical distribution using CHP extraction units so as to include wind parks, air pollutants and electrical system’s safety function due to augmented wind penetration is sufficiently dealt. This is how the operator has a strong decision making program so as to achieve more accurate solutions and estimations for a power system with RES penetration, under environmental constraints.
All in all, flexible computational software was developed for most of the arithmetic and analytical solution methods of the probabilistic load flow in electrical systems with RES penetration in order to make possible the comparison of the results and their efficiencies having Monte Carlo method as comparison base. In particular, the implementation of linearization of the AC load flow equations combined with the Gram-Charlier expansion or Cornish-Fisher method in the grid of Crete extracted reliable results in a very short time frame compared with the other examined methods. Moreover, the study of probabilistic load flow in a LV network with a Micro_CHP unit whose output (electrical and thermal power) is derived from the probabilistic model is fully conducted. The study of the effect of the external ambient temperature in voltage and power flow in the grid’s lines can be useful in cases when there many Micro_CHP units that operate similarly have penetrated.