A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> Multilevel inverter technology has emerged recently as a very important alternative in the area of high-power medium-voltage energy control. This paper presents the most important topologies like diode-clamped inverter (neutral-point clamped), capacitor-clamped (flying capacitor), and cascaded multicell with separate DC sources. Emerging topologies like asymmetric hybrid cells and soft-switched multilevel inverters are also discussed. This paper also presents the most relevant control and modulation methods developed for this family of converters: multilevel sinusoidal pulsewidth modulation, multilevel selective harmonic elimination, and space-vector modulation. Special attention is dedicated to the latest and more relevant applications of these converters such as laminators, conveyor belts, and unified power-flow controllers. The need of an active front end at the input side for those inverters supplying regenerative loads is also discussed, and the circuit topology options are also presented. Finally, the peripherally developing areas such as high-voltage high-power devices and optical sensors and other opportunities for future development are addressed. <s> BIB001 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> In this work, a method is given to compute the switching angles in a multilevel converter to produce the required fundamental voltage while at the same time cancel out specified higher order harmonics. Specifically, a complete analysis is given for a seven-level converter (three dc sources), where it is shown that for a range of the modulation index m/sub I/, the switching angles can be chosen to produce the desired fundamental V/sub 1/=m/sub I/(s4V/sub dc///spl pi/) while making the fifth and seventh harmonics identically zero. <s> BIB002 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> The problem of eliminating harmonics in a switching converter is considered. That is, given a desired fundamental output voltage, the problem is to find the switching times (angles) that produce the fundamental while not generating specifically chosen harmonics. In contrast to the well known work of Patel and Hoft (1973, 1974) and others, here all possible solutions to the problem are found. This is done by first converting the transcendental equations that specify the harmonic elimination problem into an equivalent set of polynomial equations. Then, using the mathematical theory of resultants, all solutions to this equivalent problem can be found. In particular, it is shown that there are new solutions that have not been previously reported in the literature. The complete solutions for both unipolar and bipolar switching patterns to eliminate the 5th and 7th harmonics are given. Finally, the unipolar case is again considered where the 5th 7th, 11th, and 13th harmonics are eliminated along with corroborative experimental results. <s> BIB003 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> In this paper, the authors have developed a novel algorithm for selective harmonic elimination in a pulsewidth-modulated inverter based on the foraging behavior of a colony of ants. The problem of selective harmonic elimination, together with output voltage regulation, is redrafted as an optimization problem, and the solution is sought through the novel concept. The proposed scheme has several advantages, such as computational simplicity, derivative-free operation, and guaranteed near-optimal convergence with random initial guess. It is shown that a newly introduced parameter in the algorithm- quantum of the step movement of ants- largely influences the speed of convergence of the algorithm. Theoretical and practical results are included. Furthermore, the proposed algorithm is compared with the conventional Newton-Raphson method and genetic algorithm, and conclusions are presented. <s> BIB004 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> Due to the development of new grid codes, power converters' output signal harmonic control is currently becoming extremely important in medium- and high-power applications. By taking this new scenario into account, a new method to generate switching three-level pulsewidth-modulation (PWM) patterns to meet specific grid codes is presented. The proposed method, which is named selective harmonic mitigation PWM , generates switching three-level PWM patterns with high quality from the point of view of harmonic content, avoiding the elimination of some specific harmonics and studying all harmonics and the total harmonic distortion as a global problem by using a general-purpose random-search heuristic algorithm. This fact leads to a drastic reduction or even avoidance of the bulky and costly grid connection tuned filters of power systems. Any harmonic shaping can be considered due to the flexibility of the method. Power devices switching constraints are considered to obtain directly applicable results. As a practical example, limits from one actual grid code have been used to get the experimental results by means of a 150-kVA three-level diode-clamped converter test bench. Comparisons between the proposed technique, optimized PWM and Selective Harmonic Elimination methods have been carried out. The results obtained with this new method greatly improve previous ones. <s> BIB005 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> So far, several methods have been presented to eliminate and minimize the low order voltage harmonics at multi-level inverter's output. Harmonics optimization aims to eliminate the low order harmonics, if it is possible, or at least to minimize them. Thus, harmonics optimization can be considered as a generalized form of harmonics elimination. In this way, this paper applies an advanced variation of Particle Swarm Optimization method to 7 to 17-level inverters. Results are compared with Continuous Genetic Algorithm, as a well-known intelligent method, and one of the most successful numerical methods, namely Sequential Quadratic Programming. Results confirm that PSO completely outperforms both CGA and SQP for all cases. <s> BIB006 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> A new family of DC to AC converters, referred to as multilevel inverter, has received much attention from industries and researchers for its high power and voltage applications. One of the conventional techniques for implementing the switching algorithm in these inverters is optimized harmonic stepped waveform (OHSW). However, the major problem in using this technique is eliminating low order harmonics by solving the nonlinear and complex equations. In this paper, a new approach called the "bacterial foraging algorithm" (BFA) is employed. This algorithm eliminates and optimizes the harmonics in a multilevel inverter. This method has higher speed, precision, and convergence power compared with the genetic algorithm (GA), a famous evolutionary algorithm. The proposed technique can be expanded in any number of levels. The purpose of optimization is to remove some low order harmonics, as well as to ensure the fundamental harmonic retained at the desired value. As a case study, a 13-level inverter is chosen. The comparison results by MATLAB software between the two optimization methods (BFA and GA) have shown the effectiveness and superiority of BFA over GA where convergence is desired to achieve global optimum. <s> BIB007 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> The selective harmonic elimination pulse width modulation (SHEPWM) switching strategy has been applied to multilevel inverters to remove low order harmonics. Naturally, the related equations do not have feasible solutions for some operating points associated with the modulation index (M). However, with these infeasible points, minimizing instead of eliminating harmonics is performed. Thus, harmful harmonics such as the 5 th harmonic still remains in the output waveform. Therefore, it is proposed in this paper to ignore solving the equation associated with the highest order harmonics. A reduction in the eliminated harmonics results in an increase in the degrees of freedom. As a result, the lower order harmonics are eliminated in more operating points. A 9-level inverter is chosen as a case study. The genetic algorithm (GA) for optimization purposes is used. Simulation results verify the proposed method. <s> BIB008 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> The design and implementation of a new control scheme for reactive power compensation, voltage regulation and transient stability enhancement for wind turbines equipped with fixed-speed induction generators (IGs) in large interconnected power systems is presented in this study. The low-voltage-ride-through (LVRT) capability is provided by extending the range of the operation of the controlled system to include typical post-fault conditions. A systematic procedure is proposed to design decentralised multi-variable controllers for large interconnected power systems using the linear quadratic (LQ) output-feedback control design method and the controller design procedure is formulated as an optimisation problem involving rank-constrained linear matrix inequality (LMI). In this study, it is shown that a static synchronous compensator (STATCOM) with energy storage system (ESS), controlled via robust control technique, is an effective device for improving the LVRT capability of fixed-speed wind turbines. <s> BIB009 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> Power converters offer a high capability to efficiently manage electrical energy flows. Until a few years ago, their primary use was in supplying motors in industrial applications and in electric traction systems. Nowadays, in addition to those fields they are employed in a very wide range of low, medium, and high power applications including residential applications, renewable energy systems, distributed generation, and automotive. Since digital control represents a key element of modern power converters, this paper presents a review on digital devices [microcontrollers, Field Programmable Gate Arrays (FPGA)], hardware and software design techniques as well as implementation issues useful for designing modern high-performance power converters. <s> BIB010 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> This paper investigates a plug-in repetitive control scheme for bridgeless power factor correction (PFC) converters to mitigate input current distortions under continuous conduction mode and discontinuous conduction mode operating conditions. From the PFC converter model and the fact that a type-II compensator is used, a design methodology to maximize the bandwidth of the feedback controller is suggested. After that, the error transfer function including the feedback controller is derived, and the stability of the repetitive control scheme is evaluated using the error transfer function. The implementation of the digital repetitive controller is also discussed. The simulation and experimental results show that the input current THD is significantly improved by using the proposed control scheme for a 1-kW single-phase bridgeless PFC converter prototype. <s> BIB011 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> This paper presents the hybrid seven-level cascaded active neutral-point-clamped (ANPC)-based multilevel converter. The converter topology is the cascaded connection of a three-level ANPC converter and an H-bridge per phase. The voltage of the H-bridge is actively maintained to the required level through selection of the switching states of the converter. The topology is operated under selective harmonic elimination pulsewidth modulation (SHE-PWM), maintaining the switching frequency of the converter to a minimum. The operating principles, voltage balancing methods, and limitations of the converter are analyzed together with extensive simulation results of the topology. Experimental results from a low-power laboratory prototype are presented that verify the operation of the hybrid converter under SHE-PWM. <s> BIB012 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> Modular multilevel converters (MMC) are considered a top converter alternative for voltage-source converter (VSC) high-voltage, direct current (HVDC) applications. Main circuit design and converter performance evaluation are always important issues to consider before installing a VSC-HVDC system. Investigation into a steady-state analysis method for an MMC-based VSC-HVDC system is necessary. This paper finds a circular interaction among the electrical quantities in an MMC. Through this circular interaction, a key equation can be established to solve the unknown circulating current. A new steady-state model is developed to simply and accurately describe the explicit analytical expressions for various voltage and current quantities in an MMC. The accuracy of the expressions is improved by the consideration of the circulating current when deriving all the analytical expressions. The model's simplicity is demonstrated by having only one key equation to solve. Based on the analytical expressions for the arm voltages, the equivalent circuits for MMC are proposed to improve the current understanding of the operation of MMC. The feasibility and accuracy of the proposed method are verified by comparing its results with the simulation and experimental results. <s> BIB013 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> The solid-state transformer (SST), which has been regarded as one of the 10 most emerging technologies by Massachusetts Institute of Technology (MIT) Technology Review in 2010, has gained increasing importance in the future power distribution system. This paper presents a systematical technology review essential for the development and application of SST in the distribution system. The state-of-the-art technologies of four critical areas are reviewed, including high-voltage power devices, high-power and high-frequency transformers, ac/ac converter topologies, and applications of SST in the distribution system. In addition, future research directions are presented. It is concluded that the SST is an emerging technology for the future distribution system. <s> BIB014 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> This paper proposes a distributed digital control architecture of a modular-based solid-state transformer (SST) using a digital signal processor (DSP) and a field-programmable gate array (FPGA). In particular, the three-stage SST based on a modular structure is the topology of most interest because of its superior controllability. In order to make the modular-based SST, the digital implementation is inevitable to achieve higher performances, improved reliability, and an easy development. In addition, the modular-based SST requires enough capacity for implementing complex control algorithms, multiple interfaces, and a large number of internal variables. In this paper, a digital control platform for the modular-based SST is built using a floating-point DSP and an FPGA that operate cooperatively. As a result, the main control algorithms are performed by the DSP, and the simple logical processes are implemented in the FPGA to synthesize the suitable gating signals and control external devices, respectively. The proposed implementation method enables high-switching-frequency operation, multitasking, and flexible design for the modular-based SST. Experimental results are presented to verify the practical feasibility of the proposed technique for the modular-based SST. <s> BIB015 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> Multilevel converters have received increased interest recently as a result of their ability to generate high quality output waveforms with a low switching frequency. This makes them very attractive for high-power applications. A cascaded H-bridge converter (CHB) is a multilevel topology which is formed from the series connection of H-bridge cells. Optimized pulse width modulation techniques such as selective harmonic elimination or selective harmonic mitigation (SHM-PWM) are capable of preprogramming the harmonic profile of the output waveform over a range of modulation indices. Such modulation methods may, however, not perform optimally if the dc links of the CHB are not balanced. This paper presents a new SHM-PWM control strategy which is capable of meeting grid codes even under nonequal dc link voltages. The method is based on the interpolation of different sets of angles obtained for specific situations of imbalance. Both simulation and experimental results are presented to validate the proposed control method. <s> BIB016 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> This paper proposes an analytical procedure for computation of all pairs of valid switching angles used in pattern generation in five-level H-bridge cascaded inverters. The proposed procedure eliminates harmonic components from inverter output voltage and, for each harmonic, returns the exact boundaries of all valid modulation index intervals. Due to its simple mathematical formulation, it can be easily implemented in real time using a digital signal processor or a field-programmable gate array. In this paper, after a detailed description of the method, simulation and experimental results demonstrate the high quality of achievable results. <s> BIB017 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> The parameters of electrical energy such as voltage amplitude are very important, particularly from the viewpoint of the final consumer and sensitive loads connected to the grid. The dynamic states in the power grid-deep voltage sags and swells-might cause faults and defects in sensitive loads. This paper deals with a three-phase hybrid transformer (HT) without dc energy storage to compensate voltage sags and swells and to protect sensitive loads against the rapid and extensive changes in supply voltage amplitude. The analyzed HT contains two main units: the first one is the conventional electromagnetic transformer, realizing an electromagnetic coupling, and the second one is the buck-boost matrix-reactance chopper, realizing an electrical coupling in the HT unit. In the presented solution, output voltage is transformed in two ways-electromagnetically and electrically. This paper presents an operational description, the theoretical analysis, and the experimental test results from a 2-kVA laboratory model. On the basis of the authors' research, it can be stated that the HT makes it possible to compensate deep voltage sags (deeper than 50% of nominal source voltage) and overvoltages (up to 140% of nominal source voltage) while maintaining good dynamic properties. The main advantages of the proposed solution, in comparison to other conventional solutions, are the ability to control the output voltage in the range of 0.66-3.5 US, good dynamics (transient state during source voltage uS amplitude change is shorter than 10 ms), and galvanic separation between source and load (such as in the case of the conventional electromagnetic transformer). <s> BIB018 </s> A Review of Architectures and Concepts for Intelligence in Future Electric Energy Systems <s> III. INTEGRATION OF RENEWABLE ENERGY RESOURCES <s> A new procedure to mitigate some harmonics is presented for a 5-level cascaded inverter, varying the modulation index. The two switching angles α ::: 1 ::: and α ::: 2 ::: are computed by using a procedure based on graphycal analysis by using Chebyshev polynomials and the Waring's formulae. The proposed mitigation technique is applied to reduce a pair of harmonics among third, fifth and seventh in inverter output voltage according to standard code requirements. Experimental results are also extracted by the prototype built at the laboratory. The measured results confirm the quality of the presented procedure. <s> BIB019
The integration of DER with the electrical power system requires the usage of PCs, necessary for adapting fluctuating produced energy with grid requirements; they are also necessary for efficient operation of ESS. The most appropriate topologies, devices, control, and modulation techniques are fundamental in order to fulfill grid codes/interconnection rules and for avoiding the occurrence of instability or synchronization problems, faults, and for obtaining high-efficiency operations. However, the nonlinear behavior of PC causes the injection of harmonic distortions at the point of common coupling (PCC). Moreover, stable outputs could not be ensured in every condition, such as during deep voltage sags. The subsequent generated electromagnetic interferences may lead to undesired situations, loss of synchronization of DERs, grid instability, and faults. These situations can be mitigated by insertion of special converters, namely, power factor controllers, static compensators, and ESS, as well. In addition, the implementation of ICT technologies and concepts allows prevention of abnormal situations BIB011 - BIB009 . To guarantee power quality at the input ac mains, harmonic standards and engineering recommendations must be also adopted to limit the level of distortion at the PCC  ,  . PC topology plays a significant role in full integration of renewable energies with the electrical power grid. Nowadays, there is a significant trend of considering multilevel converter (MLC) topologies for new smart-grid-related projects. They basically consist of arrays of power semiconductors in series and/ or in parallel, producing output voltage with discrete stairstepped waveforms. Typical topologies of MLC are diode clamped or neutral point clamped (NPC), active NPC, capacitor clamped or flying capacitor, and cascaded MLCs BIB001 - BIB010 . A significant part of their behavior (namely, the quality of output waveforms) depends on control and modulation methods. Popular methods are multilevel sinusoidal pulsewidth modulation (PWM), phase disposition, multilevel selective harmonic elimination, harmonic mitigation, and space vector modulation BIB012 - BIB017 . The higher the number of levels, the higher the quality of the output voltages and currents due to reduced electromagnetic emissions determined by chosen modulation technique is. Due to their intrinsic characteristics, MLCs operate at low frequency and at medium voltage; moreover, they eliminate common-mode currents. Additionally, they do not require expensive passive filters, while achieving significant improvements in terms of efficiency and cost. For such reasons, their usage simplifies DER design and allows direct connection of PC with power distribution lines. Another emerging topology in DER is the so-called modular MLC (MMC) BIB013 . This can be considered a new and promising design approach consisting in connecting basic PC modules in series and/or in parallel. Typically, each module consists of a half-bridge or a full H-bridge with a dc capacitor for energy storage and all the necessary sensors and control circuits. With this approach design, realization and maintenance of highpower and/or medium-voltage converters are simpler because hardware can fully exploit modularity. Usually, the design of the single modules is often very complex due to the numerous constraints. Moreover, software and control become more complex, too, due to the additional intelligence and communication capability needed by interactions among the modules. One of the early applications of MMC was in high-voltage direct current systems, but due to their significant advantages in terms of flexibility, scalability, maintenance, and costs, their usage has been extended to DER, e.g., megawatt-range wind turbines and to other high-power applications as well. Of course, there are also some drawbacks, mainly caused by the intrinsic nature of modular systems, namely, the use of connectors with potential interconnection problems, the potential instabilities due to interaction among modules, the increased distances among critical components, and the additional design complexity, due to the modularity. Comparison among MLC and MMC is not simple; moreover, MLC can be designed with a modular approach. However, in the authors' opinion, it should be expected a growth of both technologies and the diffusion of modular design approaches, both in hardware and in software as well. In modern power systems, PCs are also providing ancillary services (e.g., local voltage or frequency control) using reactive and active power management functions and therefore providing additional possibilities for the power grid operation. Examples are local voltage control possibilities at the PCC, which can be also used in a coordinated way to maintain power quality due to a high share of distributed generation. It is expected that, in the future, electromagnetic transformers will be replaced by electronic counterparts, the so-called "solidstate transformers (SSTs)," based on previous technologies BIB018 (an application example for SST is provided in Section VI-A). In fact, they can be easily integrated within the smart grid, interconnecting distinct electrical power grids operating with different characteristics (e.g., voltage and phase). They can simplify grid management during normal operating conditions and during faults. Moreover, they could simplify power factor control and harmonic reduction as well. In general, they are more compact in size and are expected to become cheaper than traditional transformers, penalized by the high cost of copper and of magnetic materials BIB014 - BIB015 . Since power levels become higher, optimum modulation of PC is fundamental for satisfying power quality demand and for improving design, performances, and efficiency. Elimination or mitigation of one or more harmonics from output without the use of passive filters is very important for achieving significant improvement in terms of cost, reliability, and space. Genetic algorithms and particle swarm optimization BIB008 - BIB006 , AI methods based on bacterial foraging algorithms, ant colony methods, and homotopy and continuation theory BIB007 - BIB004 are typical methods capable of finding the converter switching angles necessary for modulation. However, all these methods require offline calculations, hence offer limited flexibility. Recently, some analytical methods capable of identifying all possible switching angles eliminating a desired harmonic, for a fixed modulation index, in a five-level cascaded inverter have been successfully developed, allowing real-time implementation without the use of lookup tables BIB017 . The transcendental equations characterizing the harmonic content can be converted in polynomial equations, which can be then solved using the method of resultants from elimination theory BIB002 , BIB003 . It has been shown that a pair of harmonics chosen among third, fifth, and seventh can be eliminated in a five-level inverter output voltage using the method described in BIB019 , based on graphical analysis by using Chebyshev polynomials and the Waring's formulas. In order to generate switching PWM patterns with low harmonic content, avoiding the elimination of some specific harmonics, the total harmonic distortion can be treated as a global problem by using a general-purpose random-search heuristic algorithm BIB016 - BIB005 . In summary, AI techniques and concepts play an important role for controlling and optimizing elements on the subcomponent and component levels. It is expected by the authors that, in the future, more intelligent algorithms and concepts are being used on this level.