Showing papers by "V. Fernao Pires published in 2017"

Cascaded H-bridge multilevel inverter with a fault detection scheme based on the statistic moments indexes

Abstract: Multilevel inverters allow to generate AC voltages with low total harmonic distortion (THD) but requires an increased number of power switches. One of the disadvantages of that is the increased probability of a fault in one of the power switches. Thus in order to improve the reliability of the converter a fast and robust fault detection scheme must be used. In this context this paper presents a new fault detection scheme based on the AC voltages of the inverter. The proposed scheme uses fault factors that are based on the statistic moment method. This method was used for the identification of image patterns. Due to this the proposed scheme allows for a robust detection to load or voltage variations. To test the method was used a cascaded H-bridge inverter with five levels. Several tests were performed through numerical results. The use of a laboratory prototype was also used to confirm the proposed scheme.

Three-phase T-type qZ source inverter with control current associated to a vectorial modulator for photovoltaic applications

Abstract: This paper presents a fast and robust control system for a three-phase quasi-Z-source inverter (qZSI) connected to the grid. The topology combines two quasi-Z-source networks with a T-type inverter allowing to obtain AC voltages with multilevel characteristics and properties of the referred networks. To control this system a closed-loop current controller for the AC currents is proposed. The controller is based on a vectorial modulator associated to the shoot-through states. The DC-link voltage is maintained stable at the reference value by adjusting the shoot-through duty cycle. The controller is characterized by fast transient response and robustness to parameter and load variations. In order to confirm the desired characteristics and performance of the converter and control system several simulation tests were performed.

PV generators combined with UPQC based on a dual converter structure

Abstract: In this paper a power converter topology where an Unified Power Quality Conditioner (UPQC) is combined with the photovoltaic generators is presented. This topology is based on a dual structure of the classical three-phase two level inverters. Thus, multilevel operation is achieved with the advantages of this type of inverters. Another feature of this structure is the higher reliability of the combined system. In fact, even with a fault in one of the inverters the system can still operate. Voltage and current controllers are also described. The current and voltage controllers are implemented in the synchronous rotating reference frame (dq0-axis). Several results are presented in order to verify the operation of the system under various conditions.

Distributed energy resources network connection considering reliability optimization using a NSGA-II algorithm

Abstract: Distributed Energy Resources (DER) has been widely introduced in distribution networks in response to the increase of environmental awareness of the consumers. The benefits with the use of DER are increased with network reconfiguration, but in some countries exists the impossibility of island operation as well as the obligation to buy all the electricity generated. These limitations, in a network fault situation, causes a waste of resources, because of the unavailability of the DER. On other hand, the location of DER connection under the context of the improvement of the reliability indexes has not been studied. Thus, this paper will propose a multi objective optimization of the location of DER connection considering the switching devices placement to increase network reliability and availability of DER, minimizing at the same time the investment in equipment in a no island operation environment. For the resolution of the proposed formulation, it will be used the multi objective algorithm NSGA-II (Fast Non-dominated Sorting Genetic Algorithm). The formulation of the problem also considers a composite index made up of DER and interruption duration. This approach is applied to a real utility distribution network, with the results presented and discussed.

Teaching distribution network protection through the support of a software tool

Abstract: This paper presents a method to motivate and improve the study of distribution networks protection. The activities associated to this method are based on a software tool that was developed with the equations that are presented in the classes. Due to this, is possible to the students to first calculate the settings for the relays and then compare with the values obtained with the software tool. Another advantage is to verify in a graphic way the several results obtained by the tests. On other hand, some real case studies are used in order to improve their motivation and show that programs that may be developed by them could be used in real applications. Through the proposed educational method students can obtain an intuitive way of grasping the main concepts as well to explore their problemsolving capabilities. The positive response from students indicates that the proposed methodology is useful for the learning of distribution networks protection.

Differential line protection using dqo components for the communication channel

Abstract: Differential line protection has been playing a major role in power transmission line protection due to its simplicity, operation speed, and high sensitivity. However, problems with communication channel make current comparisons difficult to achieve. To improve the insensibility to this problem, this paper proposes a new approach. First, the three currents are acquired. Then these currents are transmitted to the other relay. However, instead of sending directly these currents to the other relay, first a Park transformation or ABC-dqo transformation is performed. The received currents are in dqo components. In this way, the transmitted signals are DC components. Before calculating the difference between the currents entering and leaving the protected zone an inverse transformation of the received currents (dqo-ABC) is made. Using this approach, immunity to problems such as sampling misalignment and time delay of the communication channel will be improved. A new methodology associated to this approach was also proposed. The obtained advantage is that it allows the reduction of the number of signals that must be transmitted between relays (using only do components). This methodology also improves the relay sensitivity. Several test results are presented in order to show the effectiveness of the proposed approach.

Controlling a battery energy storage system to support residential photovoltaic installations

Abstract: The penetration of PV systems in residential installations is now a reality in many countries. In order to encourage the introduction of those systems in the residential installations, one of the measures was through the feed-in tariff of the produced PV energy. However, that paradigm is now changing by forcing to residential owners to introduce storage systems. Nevertheless, normally the loads are not balanced, which may imply that some of the produced energy will flow to the grid. Thus, in order to improve the all system, a storage system that also gives support to balance the energy produced by the PV and loads is proposed. To achieve that goal a control system for the power converter associated to the batteries will be developed. To control the AC currents it will be used a space vector based hysteresis controller. An algorithm to generate the current references is also presented. Several simulation tests will be presented in order to verify the effectiveness of the proposed system.