Adrian Vasile Timbus

Overview of Control and Grid Synchronization for Distributed Power Generation Systems

Renewable energy sources like wind, sun, and hydro are seen as a reliable alternative to the traditional energy sources such as oil, natural gas, or coal. Distributed power generation systems (DPGSs) based on renewable energy sources experience a large development worldwide, with Germany, Denmark, Japan, and USA as leaders in the development in this field. Due to the increasing number of DPGSs connected to the utility network, new and stricter standards in respect to power quality, safe running, and islanding protection are issued. As a consequence, the control of distributed generation systems should be improved to meet the requirements for grid interconnection. This paper gives an overview of the structures for the DPGS based on fuel cell, photovoltaic, and wind turbines. In addition, control structures of the grid-side converter are presented, and the possibility of compensation for low-order harmonics is also discussed. Moreover, control strategies when running on grid faults are treated. This paper ends up with an overview of synchronization methods and a discussion about their importance in the control

Evaluation of Current Controllers for Distributed Power Generation Systems

This paper discusses the evaluation of different current controllers employed for grid-connected distributed power generation systems having variable input power, such as wind turbines and photovoltaic systems. The focus is mainly set on linear controllers such as proportional-integral, proportional-resonant, and deadbeat (DB) controllers. Additionally, an improved DB controller robust against grid impedance variation is also presented. Since the paper discusses the implementation of these controllers for grid-connected applications, their evaluation is made in three operating conditions. First, in steady-state conditions, the contribution of controllers to the total harmonic distortion of the grid current is pursued. Further on, the behavior of controllers in the case of transient conditions like input power variations and grid voltage faults is also examined. Experimental results in each case are presented in order to evaluate the performance of the controllers.

Flexible Active Power Control of Distributed Power Generation Systems During Grid Faults

The increasing penetration of distributed power generation into the power system leads to a continuous evolution of grid interconnection requirements. In particular, active power control will play an important role both during grid faults (low-voltage ride-through capability and controlled current injection) and in normal conditions (reserve function and frequency regulation). The aim of this paper is to propose a flexible active power control based on a fast current controller and a reconfigurable reference current selector. Several strategies to select the current reference are studied and compared using experimental results that are obtained during an unsymmetrical voltage fault. The results of the analysis allow selection of the best reference current in every condition. The proposed methods facilitate multiple choices for fault ride through by simply changing the reference selection criteria.

New Positive-sequence Voltage Detector for Grid Synchronization of Power Converters under Faulty Grid Conditions

This paper deals with a fundamental aspect in the control of grid-connected power converters, i.e., the detection of the positive-sequence component at fundamental frequency of the utility voltage under unbalanced and distorted conditions. Accurate and fast detection of this voltage component under grid faults is essential to keep the control over the power exchange with the grid avoiding to trip the converter protections and allowing the ride-through of the transient fault. In this paper, the systematic use of well known techniques conducts to a new positive-sequence voltage detection system which exhibits a fast, precise, and frequency-adaptive response under faulty grid conditions. Three fundamental functional blocks make up the proposed detector, these are: i) the quadrature-signals generator (QSG), ii) the positive-sequence calculator (PSC), and iii) the phase-locked loop (PLL). A key innovation of the proposed system is the use of a dual second order generalized integrator (DSOGI) to implement the QSG. For this reason, the proposed positive-sequence detector is called DSOGI-PLL. A detailed study of the DSOGI-PLL and verification by simulation are performed in this paper. From the obtained results, it can be concluded that the DSOGI-PLL is a very suitable technique for characterizing the positive-sequence voltage under grid faults.

Synchronization methods for three phase distributed power generation systems - An overview and evaluation

Nowadays, it is a general trend to increase the electricity production using distributed power generation systems (DPGS) based on renewable energy resources such as wind, sun or hydrogen. Jf these systems are not properly controlled, their connection to the utility network can generate problems on the grid side. Therefore, considerations about power generation, safe running and grid synchronization must be done before connecting these systems to the utility network. This paper is mainly dealing with the grid synchronization issues of distributed systems. An overview of the synchronization methods as well as their major characteristics is given. New solutions to optimize the synchronization methods when running on distorted grid conditions are discussed. Simulation and experimental results are used to evaluate the behavior of the synchronization methods under different kind of grid disturbances such as voltage dips, harmonics and notches.

The Provision of Frequency Control Reserves From Multiple Microgrids

The concept of microgrid is invented to harmonize local electricity production and consumption. The increasing interest on microgrids is caused by the considerable growth of renewable energy sources (RES) and distributed generation (DG). In a large geographical region, there can exist multiple microgrids. Many academic articles have reported the technicalities of microgrids operating in islanded mode, and few have analyzed their aggregated technical/economic impacts or benefits on the network in a larger region. Deregulation encourages RES to participate in energy markets to facilitate competition among different energy providers. Apart from participating in energy markets, an alternative way of making use of and making profit out of multiple microgrids is to take part in the ancillary-service market. This paper investigates the technical aspects of providing frequency control reserves (FCRs) and the potential economic profitability of participating in FCR markets based on a setup of multiple microgrids. In particular, it has a focus on the communication aspects under different market scenarios, i.e., decentralized or centralized coordination approach, for FCRs with the assumption of the possible aggregation of reserve provision from multiple microgrids (microsources and flexible loads).

Adaptive resonant controller for grid-connected converters in distributed power generation systems

Due to its superior performance when regulating sinusoidal waveforms and the possibility to compensate for low order harmonics by means of harmonic compensator (HC), proportional resonant (PR) controller is a real alternative to the conventional proportional integral (PI) controller, when implemented in a grid connected system like distributed power generation systems (DPGS). Anyway, both PR and HC necessitate the resonant frequency value inside their internal model. Normally, the nominal value of the grid frequency and its multiples are used, but in the case when the grid frequency experiences fluctuations, the performance of both PR and HC is diminished. This paper discuss the possibility of improving the behavior of resonant controller and harmonic compensator in the case of grid frequency variations. The proposed solution makes use of the frequency information provided by the phase-locked loop (PLL) system already used in most of DPGS today. Experimental results are presented in order to validate the proposed solution and it shows to work very well.

Active Management of Distributed Energy Resources Using Standardized Communications and Modern Information Technologies

Due to high penetration level in some regions of distributed generators (DGs) based on renewable energy resources, information about their power delivery capabilities is becoming essential for planning and allocating resources and reserves in power system. This paper discusses the modeling of DGs in the utility control and information technology infrastructures of power system operators. The communication standards International Electrotechnical Commission (IEC) 61400-25 and IEC 61850 and their extensions for DGs are first introduced, followed by an example of mapping wind turbine components onto a communication data model. A distribution network comprising several distributed-generation sources is then described from the control and communication point of view. Simulation results of production cost minimization and network constraint management are presented at the end, illustrating the new possibilities that a system-wide approach can provide for distributed generation.

Online grid impedance estimation for single-phase grid-connected systems using PQ variations

This paper presents an online grid impedance estimation method for single-phase grid-connected systems, such as photovoltaic systems, small wind turbines, fuel-cells power systems. The method is based on producing a small perturbation on the output of the power converter that is in the form of periodical variations of active and reactive power (PQ variations). The main idea is to make the power converter working in two operation points in order to solve the equation of the equivalent grid impedance. During the perturbation, measurements of voltage and current are performed and signal processing algorithms are used in order to estimate the value of the grid impedance. The online grid impedance estimation method can be used for compliance with the anti-islanding standard requirements (IEEE1574, IEEE929 and VDE0126) and for adaptive control of the grid-connected converters. The proposed method is embedded in the existing power converter control. The selected results validate the effectiveness of the proposed method.

PLL Algorithm for Power Generation Systems Robust to Grid Voltage Faults

In the case of unbalanced grid faults, one of the main problems for the Distributed Power Generation Systems (DPGSs) controller is the second harmonic ripple in the control, consequence of the phase unbalance caused by the fault. The second harmonic propagates into different sections of the controller and can have a negative influence on the controller even leading to trip out of the system. This paper proposes a novel Phase-Looked Loop (PLL) algorithm which is able to filter out the negative sequence and to provide a clean synchronization signal. Along with the conventional PI controller in the PLL structure, the proposed algorithm employs a repetitive controller to deal with the second harmonic. The purpose of the repetitive controller is to amplify the second harmonic in the reference of the PI controller, so that this can be better rejected. Simulation and experimental results are used to validate the robustness of the proposed system when running on grid faults.