Modelado en régimen permanente del Statcom en la red elétrica

Abstract

In the last decade, there has been an increased penetration of distributed generation and, therefore, the use of equipment to ensure voltage control and an ideal waveform has become necessary. The main reasons that make this control important are namely the following: - Optimize the equipment operation. - Reduce voltage drops, which may cause overheating in the electrical equipment. - Eliminate voltage rises, which lead to a deterioration of the insulation. - Minimize the reactive power flow through the network and the system losses, thanks to a proper voltage profile. With the development of power electronics, several solutions to compensate voltage fluctuations in distribution networks have appeared. Such solutions comprise the equipment used for reactive power compensation, which include the STATCOM (Static Synchronous Compensator). These devices consist on voltage converters connected to an energy storage element on one side and to the electrical system on the other, allowing to provide reactive power support at the nodes of the network. The load flow or power flow problem is to find the steady state operation point of an electrical system. More specifically, it means that given the required load on the consumption buses and the power supplied by the generators, all the voltages of the nodes and the power flowing through all the network components are obtained. For this study case, the Newton-Raphson method, one of the most popular iterative method for solving load flow, was used. These methods assign some initial values to variables and gradually set new values resulting from the iterative equations, repeating the same process until the value of the variables is within a certain range of error. This project presents a new model of STATCOM aimed at power flow solutions using the above mentioned Newton-Raphson method and, moreover, based on the paper "A New STATCOM Model for Power Flows Using the Newton-Raphson Method". In this device, the controller is not treated as an ideal controllable voltage source, but as a composite transformer that includes certain control properties based on PWM converters. The complex tap changer angle represents the phase shift that would exist in a PWM inverter, and coincides with the angle of the conventional VSC voltage source model. More specifically, it would be the angle required by the VSC to allow either the generation or absorption of purely reactive power thanks to the electronic processing of the voltage and current within the VSC. Both switching and resistive losses and also the LTC transformer connection are explicitly represented in this STATCOM model. The complex tap changer on the VSC model and the real tap changer in the LTC model allow an effective regulation of the voltage at the connection point with the network and in the AC node of the VSC. Taking this simple model of STATCOM as a start point, the objective is to extend its functionality by developing a similar but more complex model. Thus, it is possible to obtain a controller with the same advantages as the one above, but applicable to three-phase systems. Not only this, but its control capabilities can be extended, making it also useful to treat imbalanced cases where it is very beneficial to maintain the voltage of certain buses at a desired value. In order to achieve this, the same operating principles governing the simple model are used, but in this case all the system variables are triplicated to simulate the three phases, and also a fourth wire which performs the functions of the neutral is added in the rest of the network. The analysis of the results of the STATCOM operation consists of several simulations that allow us to appreciate both the improvements in its operation and the differences between the two models discussed throughout the document, i.e., the simple and the complex one (with and without voltage control, and with balanced or unbalanced network). As expected, one of the benefits of shunt compensation is to reduce system losses thanks to an improved voltage profile, which is possible because of the voltage regulation. The compensation provided by the STATCOM introduces an additional type of power loss that is associated with the high frequency switching of the PWM control used by the VSC technology, as well as resistive losses. However, STATCOM losses are relatively low, and both types are explicitly represented in the equivalent model. Regarding the Φ angle of the complex tap changer transformer of the VSC, we conclude that it represents the phase shift that would exist in a PWM inverter and that it coincides with the phase angle of the conventional VSC voltage source model, as it has already been explained above. The equivalent 𝐵 acts differently according to different study cases. If there is no voltage control in the STATCOM, the variable maintains its initial value given in the modeling, and it does not get involved in the load flow. In the event that the voltage control is active, the 𝐵 will behave differently depending on whether the setpoint control commands an increase or decrease in the voltage. Thus, it takes positive sign acting as inductance when a lower voltage is required, and it becomes negative acting as a condenser when a higher value of voltage is wanted. In case there is angle imbalance, it serves additionally to redirect power flow. It is found that the neutral wire, for his part, has a correct modeling. In the latter case study, in which a magnitude and angle imbalance is included in the line, it can be seen how the discharge occurs through the neutral phase. In the case where there is no imbalance of any kind, this cable is also present but only the remaining losses circulate through it. In the annex to the project the economic feasibility of the inclusion of a STATCOM at a load point exceeding 15 kW is analyzed. These charging points comprise any business, from large industries to local businesses, which include reactive compensation that aims to achieve greater efficiency in their electric bill. To perform this analysis, the ITC 1723/2009, included in the BOE in December 2009 is employed. The study methods used to evaluate the project are the NPV, the IRR and the Pay-Back methods. According to the results obtained from the cash flow, it is concluded that the annual utilization factor that makes it possible for the installation of the STATCOM to be economically viable is about 3%, being the Net Present Value the most restrictive method.