Search

This paper proposes a fully autonomous controller that slides the droop curves of a Static Synchronous Generator, avoiding the need of hierarchical controllers to perform power sharing and frequency and voltage regulation. The aim of this controller is to enhance the primary controller in order to perform additional functionalities in a similar way as those performed in a secondary control level hierarchy, avoiding the need of a communication- system and without mischaracterizing the primary behavior as a virtual synchronous machine. The proposed seamless controller is still compatible with conventional communication-systems, and can receive inputs from hierarchical controllers. In grid-connected microgrids, distributed generation (DG) units perform active-power supply and adequate voltage regulation, whereas the system frequency is imposed by the grid. Contrarily, in islanded microgrids, DG units have to perform active-power sharing between all units, reactive-power sharing, as well as to ensure adequate frequency control and voltage regulation. When static droop curves are employed, power dispatch planning along with coordinated set of DG units is required to achieve accurate active-power sharing. This drawback is mitigated with the proposed fully autonomous controller. Simulation results were obtained in a microgrid scenario to demonstrate the effective approach for power sharing. Experimental results are also presented.

An experimental evaluation of a digital control system based on a dual Digital Signal Processor (DSP) architecture is proposed for a three phase Unified Power Quality Conditioner (UPQC). A classical UQPC is constituted by two power conditioners, connected in series and shunted with the power grid, and sharing a common DC-link. In a smart grid scenario of operation, a UPQC will be fundamental for compensating power quality problems, also contributing to improving the efficiency of the electrical grids from a global perspective. The UPQC operation requires a bidirectional energy transfer between the two power conditioners, however, respecting some constraints, they can be controlled independently. In order to take advantage of this characteristic, the control algorithms can be executed on two independent DSPs, without any communication between them and maintaining the operational characteristics of the UPQC. Comparing with the classical control architecture based on a single DSP, with the proposed dual DSP architecture, the computational effort of each DSP is decreased of about 35%, allowing to increase the sampling rate. Therefore, the main advantages of the proposed approach are the minimization of delays caused by the processing time, which are very common in digital control systems, as well as the increment of the UPQC performance. Along the paper, detailed analysis of the processing speed and memory requirements to implement the UPQC control algorithms in both DSPs is presented. The paper also presents a set of detailed experimental results, obtained with a developed 5 kVA laboratory prototype of UPQC, which was used to evaluate the performance of the proposed dual DSP architecture.

Este artigo apresenta uma proposta de controlador para o Transformador de Frequência Variável, ou VFT (Variable Frequency Transformer), aplicado na interligação de sistemas elétricos de mesma frequência nominal, mas sem a necessidade de estarem conectados de forma síncrona por meio de uma interligação CA. Foram desenvolvidos modelos lineares do VFT e dos sistemas de potência, através dos quais o controlador proposto para o VFT foi ajustado. A adequação das proposições foi comprovada através de simulações digitais utilizando modelo detalhado do VFT e da rede elétrica.

Com a crescente demanda por energia elétrica, os sistemas HVDC tendem a assumir um papel cada vez mais importante no cenário mundial. No entanto, a extração ou injeção de pequenos blocos de energia ao longo de linhas HVDC constitui-se em um problema bastante complexo, devido a sua característica de transmissão ponto-a-ponto. Este estudo apresenta uma nova topologia para o Tap HVDC, bidirecional em energia, permitindo a conexão tanto de consumidores quanto de geradores ao sistema HVDC, de forma a realizar intercâmbio de energia com este. A fim de comprovar a eficiência do equipamento no que se refere ao dreno e a injeção de potência ao longo da linha, modelou-se um sistema simplificado no qual o Tap terá seus terminais conectados a uma fonte de corrente e ao potencial de terra. A ferramenta computacional utilizada para simulação foi o software de transitórios eletromagnéticos PSCAD/EMTDC.

Problems related to harmonic propagation in transmission systems have been noticed in the last years. Passive filters, commonly used to attenuate noncharacteristic harmonics in HVDC systems, stay at risk of disconnection from the system due to overcurrents. One of the possible solutions to solve the problem of high harmonic levels would be the use of pure active filters. However, this alternative may be very expensive due to the high power of the transmission systems. This paper relates to a comparative study on two different topologies of hybrid filters to damp harmonic resonance in power transmission systems. The first one combines a smallrated active filter in series with passive filters, limiting overcurrents, when existent, or improving the quality factor of the filter when the passive filters work under its nominal current. The second one consists of an active filter in parallel with passive filters, compensating all eventual overcurrent in these. Simulation results with the two proposed topologies present the efficiency of, and the differences between, the hybrid filters in power transmission systems.

Background: The use of renewable energies is extended due to their valuable features such as abundant and clarity. The microgrids that include the renewable energies are widely used in various applications such as power supplying of remote areas, increasing the network reliability, reducing the greenhouse gas emission, reducing the consumption demand, eliminating the consumption peaks, and so on. But, energy management in the these systems in an challenging problem. Because, there are some natural perturbations such as variation output load, grid-side faults and changes of irradiation and temperature. Aim and Objective: The problem is to design a controller to regulate the output voltage/energy under aforementioned disturbances. Methods: The paper presents a new approach for energy management in Photovoltaic (PV)/Battery/Fuel Cells (FC) systems. The uncertainties are compensated by the new optimization rules based on Immersion and Invariance (I&I) theorem and proposed deep learning type-2 fuzzy logic compensator (T2FLC). The objective function of T2FLC is to minimize the tracking error in presence of perturbations. The adaptation rules are derived such that the I&I stabilization criterions are satisfied. Both rules and fuzzy sets (FSs) of T2FLCs are optimized by guaranteed stability rules to tackle the effect of perturbations and estimation errors. Results and Discussion: It is shown that a well voltage/energy regulation performance is achieved under variation of temperature, suddenly changes of load and variation of irradiation. A comparison with similar controllers demonstrates the superiority of the suggested approach. Conclusion: The suggested regulator do not depend on the mathematical models, and results in good accuracy under difficult conditions, then it can be used in various applications.

This work proposes a control solution based on finite control set model predictive control (FCS-MPC) for interleaved converters with the features of maintaining phase currents balanced and reducing the output current ripple while presenting a voltage regulation with a high disturbance rejection capability even under constant power loads. Moreover, the proposed solution adds a penalty strategy in the cost function to prevent overcurrent. Further, this work provides a secondary contribution, consisting of a method to estimate the predictive control bandwidth based on the disturbance rejection capability. The control analysis and the simulation results carried out throughout the work confirm that the proposed strategy solves the main issues of the interleaved converter and ensures the operation for applications in power flow interfacing between dc links.

With the advent of smart grid theory, distribution networks can include different microgrids (MGs). Therefore, to achieve the desired technical and economic objectives in these networks, there is a need for bilateral coordination between their operators. In the following, by defining an energy management problem for them, it is predicted that the mentioned goals can be achieved. Therefore, this paper presents the hybrid flexible-securable operation (HFSO) of a smart distribution network (SDN) with grid-connected multi-microgrids using a two-layer coordinated energy management strategy. In the first layer, the microgrid (MG) operator is coordinated with sources, storages, and demand response operators. This layer models the HFSO method in the grid-connected MGs, which is based on minimizing the difference between the sum of operating cost of nonrenewable distributed generations and cost of energy received from the SDN, and the sum of flexibility and security benefits. It is constrained to AC optimal power flow, flexibility and voltage security constraints, operation model of sources and storages, and demand response. The second layer concerns coordination between the MG operators and the SDN operator. Its formulation is the same as that of the first layer, except that the HFSO model is used in the SDN according to MGs power daily data obtained from the first layer problem. The strategy converts the mixed-integer nonlinear programming to linear one to obtain the optimal solution with low calculation time and error. Moreover, stochastic programming models the uncertainties of load, energy price, and renewable power. Eventually, numerical results confirm the capability of the scheme to improve technical and economic indices simultaneously. As a result, by expecting the optimal operation for sources, storage, and responsive loads, it succeeded to enhance energy loss, voltage profile, and voltage security of the mentioned networks by up to 30%, 22%, and 5%, respectively, compared to power flow studies. In addition, there was enhancement in economic and flexibility status of the SDN and MGs.

Over the years, high voltage transmission of large energy blocks over long distances has widely developed from the Line Commutated Converter (LCC) technology. However, the continuous expansion of the AC network and the increase in renewable energy penetration leads to the weakening of Electric Power Systems (EPS), causing operational problems for the LCC-HVDC. This paper evaluates the degree of contribution of the VSC-HVDC on the LCC-HVDC dynamic performance when the infeed is in a hybrid multi-infeed HVDC system. Through a steady-state mathematical framework, the new concept of Hybrid Power Voltage Interaction Factor (hPVIF) is proposed to assess the transient stability improvement of LCC. Additionally, this article introduces two key contributions from hPVIF: the complementary Improved Commutation Failure Immunity Index (iCFII), as a measure of the effective short circuit enhancement, as well as the apparent line length, which emulates the line lengths adjusting the power dispatch in the VSC-HVDC. PSCAD/EMTDCTM time-domain dynamic simulations are performed to assess the indexes, and the Matlab® software will be used to assist in mathematical operations.

Diode-based HVDC link technology is considered an alternative to reduce the cost and complexity of offshore HVDC platforms. When this technology is used, the AC grid of the wind farm must be created artificially. This paper proposes an advanced frequency control method that permits forming an AC grid voltage system to connect offshore wind turbines to a diode-based HVDC link rectifier. The proposed algorithm can be easily implemented in the wind farm’s overall Power Plant Controller (PPC) without any change in the commercial wind turbine firmware. All wind turbines receive reactive power targets from the PPC to maintain the frequency and amplitude of the offshore AC line, delivering the maximum active power generated by the wind. A novel black start method is proposed to establish the wind farm’s local AC grid voltage system. The control method has been implemented and proved in an experimental setting. The black start has been successfully verified, and the frequency control algorithm shows excellent experimental results.

This work proposes a metric to assess the spread of the switching frequency profile of power converters, focusing on Finite Control Set-Model-Predictive-Control (FCS-MPC)-based converters, and analyses the effect of the filter sizing, sampling period, and cost function on the switching frequency spread. An algorithm that estimates the instantaneous switching frequency is the basis of the proposed metric. The literature review shows that only two metrics, THD and average switching frequency, are used to assess the switching frequency of FCS-MPC, but the findings of this research show that both are insufficient to characterize FCS-MPC. The main contribution of this paper is that now researchers can quantitatively assess the switching frequency spread of FCS-MPC through the proposed metric.

In this study, an intelligent control scheme is developed for induction motors (IMs). The dynamics of IMs are unknown and are perturbed by the variation of rotor resistance and load changes. The control system has two stages. In the identification stage, the group method of data-handling (GMDH) neural network (NN) was designed for online modeling of the IM. In the control stage, the GMDH-NN was applied to compensate for the impacts of disturbances and uncertainties. The stability is shown by the Lyapunov approach. Simulations demonstrated the good accuracy of the suggested new control approach under disturbances and unknown dynamics.

This paper presents an analysis of a new application of different direct matrix converter topologies used as power interfaces in AC, DC, and hybrid microgrids, with model predictive current control. Such a combination of a converter and control strategy leads to a high power quality microgrid voltage, even with a low power quality main grid voltage and even during the connection and disconnection of a variety of loads and generation sources to the microgrids. These robust systems are suitable for applications in which sensitive loads are to be supplied and these loads are connected close to distributed-generation sources with inherent intermittent behavior. The authors also propose the use of new direct matrix converter configurations with a reduced number of switches in order to achieve reduced cost, reduced failure rate, and higher reliability, which are very desirable in microgrids. Finally, the authors also introduce new hybrid direct matrix converter topologies that provide interesting options for the islanded operation of the microgrids with the use of a battery system. In other words, the proposed hybrid direct matrix converters result in flexible hybrid microgrid configurations integrating DC and AC devices with high power quality and high power supply reliability.

This paper presents a new and specific use of a bidirectional interleaved converter to perform a power interface in hybrid microgrids. The converter is responsible for regulating the power flow between the direct-current (DC) microgrid and the rest of the hybrid microgrid by controlling the DC microgrid voltage. The authors present a detailed modeling of the mentioned system in order to develop the system control design and a stability analysis. In addition, the authors propose a new control design strategy aiming at improving the voltage control disturbance rejection characteristic, while maintaining a good dynamic behavior regarding the reference tracking functionality. In this hybrid microgrid topology, a back-to-back converter connects the main grid to the AC microgrid. The main objective of this converter is to provide a high-power-quality voltage to critical and sensitive loads connected to the microgrid. The interleaved converter adjusts the DC microgrid voltage according to the operational voltage of the back-to-back converter DC link. In the DC microgrid case, the variation of load and generation connection could lead to serious voltage sag and oscillations that could be harmful to the sensitive loads. The voltage controller must be capable of rejecting these disturbances in order to maintain a high-power-quality voltage. Furthermore, experimental results are provided in order to validate this specific application of the interleaved converter and the presented control design strategy.

Este artigo apresenta uma proposta de modulação híbrida, combinando a modulação em degrau convencional com a modulação PWM (Pulse Width Modulation) descontínua, aplicada aos conversores multiníveis em cascata assimétricos. O desempenho da modulação híbrida proposta neste trabalho será comparada com duas estratégias híbridas convencionais por meio de resultados de simulação e experimentais. O conversor multinível implementado é composto por três inversores monofásicos, em ponte completa, por fase. Por fim, um compensador síncrono estático aplicado a redes de distribuição de média tensão (DSTATCOM) é implementado no intuito de validar experimentalmente a estratégia de modulação proposta.

O D-STATCOM (Distribution Static Synchronous Compensator) é em sua essência uma fonte de tensão ac onde a magnitude, o ângulo de fase e a freqüência da tensão de saída podem ser controlados. Entretanto, devido à sua conexão paralela, é um equipamento de injeção de corrente. Visando explorar esta dualidade, este artigo apresenta a comparação de dois modos distintos de controle realizados em um D-STATCOM baseado em um conversor fonte de tensão (VSC). O primeiro modo de controle atua na tensão de saída do conversor e o segundo age diretamente sobre a corrente a ser injetada. Nesse contexto, foi simulado um sistema elétrico onde o equipamento é inserido em paralelo com uma carga de 50 kVA a fim de regular a tensão na barra de carga ou corrigir o fator de potência. Foram inseridos componentes de quinto harmônico na corrente da carga, de forma a analisar o comportamento do equipamento.