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Enhancing power electronics (PE) converter reliability is crucial for ensuring a reliable operation of current and future operating Wind Turbines (WTs). Achieving high reliability of variable speed WT PE systems requires careful consideration of their operation, and particularly their thermal cycling. This paper presents a methodology for evaluating and reconsidering operational strategies of WTs with relation to the thermal loading and lifetime consumption of the converter. The methodology is applied to compare control strategies for the WT generator and evaluate their impact on the converter reliability by observation of the thermal cycles and by calculating the resultant lifetime consumption of those stress cycles. The thermal stress on both the Machine Side Converter (MSC) and the Grid Side Converter (GSC) is examined and compared. It is shown that the least reliable of the three evaluated control strategies is the one that tracks the power curve below rated speed most closely. This paper suggests that dynamic transients associated with the WT control largely influence the IGBT module wear-out and their modelling needs to be prioritized for lifetime studies. These dynamic transients are captured by the improved model whose value is confirmed for the comparisons in the case study of the paper. [ABSTRACT FROM AUTHOR]

Compared to the AC microgrids, the advantages of DC microgrids are notable. However, the development of DC microgrids is being constrained by the lack of mature protection schemes. For the foundation of designing protection schemes, the existing fault analysis focused on the fault characteristics of voltage source converters and ignored other generators. However, in DC microgrids with high permeability of photovoltaic (PV) powers, the effects of PV powers should be paid considerable attention. In this paper, the cable fault characteristics of the PV powers are analyzed in detail, and both the pole-to-ground fault and the pole-to-pole fault are considered. The fault processes are divided into several stages, and the fault circuits and main equations are described for each stage. All of the analyses are verified by the simulation model based on the PSCAD/EMTDC, and with the help of simulation results, the two main challenges the protection system confronted with are emphasized and the impacts of fault distance are analyzed. Finally, this paper proposes to employ a more reasonable converter structure and a transverse differential protection to overcome the main challenges; the effectiveness of the methods is verified by the simulation. [ABSTRACT FROM AUTHOR]

The hard switching operation of the insulated gate bipolar transistor (IGBT)-based multilevel converters (MLCs) in the transformer less direct integration of renewable power plants produces a considerable amount of switching and conduction power loss. This eventually causes higher junction temperature and lower thermal stability. The symmetrically designed MLC using third harmonic sixty-degree bus-clamped pulse width modulation (BCPWM) technique demonstrates reduced switching losses and better harmonic spectra. However, the development of analytical power loss model is a complex and challenging issue due to the discontinuous switching mode of IGBTs for the BCPWM in MLC. In this regard, an analytical model for calculating the average and the rms currents for the hard switching operation of IGBTs and antiparallel diodes (APDs) with BCPWM is proposed. Furthermore, to calculate the device turn-on and turn-off losses, a discrete form of equation is presented to evaluate the transition times of switching more precisely. The numerical power loss values of IGBTs and APDs found from the derived analytical equations for MLC are compared with the simulation and experimental results. The numerical results agree well with the simulated and the experimental outcomes, which validate the proposed analytical loss model. Moreover, an advanced three dimensional thermal impedance model with chip-wise interdependent multilayer thermal coupling effect is also considered in this paper to realize the critical thermal loading condition of the switching devices. [ABSTRACT FROM AUTHOR]

This paper deals with the design of a converter-machine system considering a set of operating points (speed, torque) composing a routine cycle. From the 1-D analytical modelling of the Permanent Magnet Synchronous Machine (PMSM) and the power electronic converter, a design methodology minimizing the power losses-to-volume ratio is established where the flux weakening control structure is considered within the system sizing. Based on a deterministic approach developed for PMSM sizing only, we propose a methodology to include the converter losses in the machine design process. Hence, we demonstrate how the computing time of calculation can be significantly reduced limiting the calculation of the flux weakening mode to a few critical power points. This method is applied to a liquid piston mechanism as part of a Compressed Air Energy Storage (CAES) system. We then validate the converter-machine design method from a 2-D analysis applied to the most significant points. Finally, the PMSM design from both methodologies with and without considering the converter losses are compared. Results highlight how the converter affects the machine sizing and the distribution of power losses to reduce the total losses of the system. [ABSTRACT FROM AUTHOR]

Modular multilevel converters have become the technology of choice in high voltage direct current systems. They are composed of a large number of submodules, which poses significant computational challenges to electromagnetic transient simulations. To address this issue, scholars proposed several approaches that are a trade-off between simulation accuracy and computational burden, which mainly achieve numerical efficiency by adopting reduced and simplified models of the converter arm submodules. This paper proposes a different simulation paradigm based on sub-circuit isomorphism. It is suited for the analysis of intrinsically modular electronic circuits, as it exploits the common behavior of structurally identical submodules by clustering them together. This method does not require any simplification of the submodule electrical model and minimizes the number of equations to be solved at each time step of the time domain analysis, thereby significantly reducing the computational effort. Thus, this method is suitable for thorough simulations of ac/dc networks, which may require the implementation of detailed transistor-level representations of the converter submodules. [ABSTRACT FROM AUTHOR]

Semiconductor devices in power electronic converters of the doubly-fed induction generator (DFIG) are susceptible to significant junction temperature variations when operating around synchronous speed, thereby reducing the lifetime of the converters. This is due to the fact that the frequency of the rotor current in a DFIG is determined by the stator flux frequency and rotor speed, and hence will lead to low rotor current frequency when operating closer to the synchronous speed, and ultimately result in significant thermal stress on semiconductor devices. In this paper, a multimode operation control strategy is proposed for the DFIG to prevent operating around the synchronous speed (within a predefined deadband); thus, the proposed control strategy can avoid the thermal stress problem. The proposed strategy engages the existing crowbar scheme for DFIG-based wind energy conversion system to intentionally alter the operating mode of the generator between DFIG and induction generator (IG). Smooth transition between the two operating modes can be achieved with the supplementary control strategies. Unity power factor can also be maintained in both operating modes by using the grid side converter as a static synchronous compensator (STATCOM) to fulfill the reactive power requirement of the DFIG in IG mode. [ABSTRACT FROM PUBLISHER]

In future power systems, reduced overall inertia caused by an increased dominance of asynchronous generation and interconnections would make frequency control particularly challenging. As the number and power rating of voltage source converter (VSC) HVDC systems increases, network service provision would be expected from such systems and to do so would require overload capacity to be included in the converter specifications. This paper studies the provision of frequency services from modular multilevel converter (MMC)-based VSC HVDC interconnections using temperature-constrained overload capability. Overload of the MMC-based HVDC system is achieved through controlled circulating currents, at the expense of higher losses, and subject to a control scheme that dynamically limits the overload available in order to keep the semiconductor junction temperatures within operational limits. Two frequency control schemes that use the obtained overload capacity to provide frequency response during emergency conditions are investigated. The controllers’ performance is demonstrated in the context of the future Great Britain transmission grid through a reduced equivalent test system. Simulation results show that even modest temperature margins which allow overload of MMC-based HVDC systems for a few seconds are effective as a primary frequency reserve and also reduce the loss of infeed requirements of such interconnections. [ABSTRACT FROM AUTHOR]

In this paper, a new topology of two-stage cascaded switched-diode (CSD) multilevel inverter is proposed for medium-voltage renewable energy integration. First, it aims to reduce the number of switches along with its gate drivers. Thus, the installation space and cost of a multilevel inverter are reduced. The spike removal switch added in the first stage of the inverter provides a flowing path for the reverse load current, and as a result, high voltage spikes occurring at the base of the stepped output voltage based upon conventional CSD multilevel inverter topologies are removed. Moreover, to resolve the problems related to dc source fluctuations of multilevel inverter used for renewable energy integration, the clock phase-shifting (CPS) one-cycle control (OCC) is developed to control the two-stage CSD multilevel inverter. By shifting the clock pulse phase of every cascaded unit, the staircase-like output voltage waveforms are obtained and a strong suppression ability against fluctuations in dc sources is achieved. Simulation and experimental results are discussed to verify the feasibility and performances of the two-stage CSD multilevel inverter controlled by the CPS OCC method. [ABSTRACT FROM AUTHOR]

The large amount of power electronic submodules and semiconductor switching events in the Modular Multilevel Converter (MMC) introduces several challenges for efficient and accurate Electro-Magnetic Transient (EMT) simulation. Research efforts have focused on developing Average Value Models (AVMs) of MMC that enable fast and accurate dynamic simulation of the converter. This paper proposes a universal blocking-module-based AVM, which can simulate the MMC of different submodule types and provide accurate results for the MMC operating in both blocking and de-blocking modes. An analytical approach is included in the model to calculate the semiconductor losses of different submodule types in the MMC. The proposed model is validated against a detailed switching-based model and the state-of-the-art AVMs in a 41-level two-terminal MMC-HVDC system. [ABSTRACT FROM AUTHOR]

Modeling and simulation play a vital role in the design and testing of modular multilevel converter (MMC) high voltage direct current (HVDC) systems. Detailed equivalent model (DEM) and switching-function-based average value model (SFB-AVM) are two major types of accurate and efficient models to represent the dynamic response of the MMCs. However, the DEM and the SFB-AVM possess unique benefits depending on the purpose of the simulation studies. The DEM provides a detailed representation of submodule (SM) switching events and individual capacitor ripples. The SFB-AVM provides faster simulation speed by using arm equivalent capacitance. Combining both models in a universal arm equivalent circuit gives the users the choice of selecting the most appropriate modeling method during dynamic simulation. This paper proposes a universal modeling framework combining the DEM with the SFB-AVM which allows the DEM and the SFB-AVM smoothly switch from one to the other during dynamic simulation. The proposed SFB-AVM can accurately represent the MMCs with different SM types. The proposed models are validated in offline and real-time simulation studies which demonstrate the improved simulation speeds of the proposed SFB-AVM over the DEM especially for large numbers of SMs. [ABSTRACT FROM AUTHOR]

The flying-capacitor-based multilevel converter is one of the well-known breeds of the multilevel power converters. This paper proposes a new flying-capacitor-based multilevel converter to minimize the number of flying capacitors (FCs) and power switches. The advantage of the proposed FC-based multilevel converter in comparison with the conventional flying-capacitor multicell converter is that it needs fewer FCs. Also, in comparison with the stacked multicell converter, the proposed multilevel converter requires fewer semiconductor switches. In order to balance the voltage of the FCs in proposed multilevel converter, a new active voltage balancing method which is fully implemented using logic-form equations is presented. The proposed voltage balancing method measures output current and FC voltages to generate switching states to produce the required output voltage level, as well as balance the FCs voltages at their reference values. The output voltage of the proposed multilevel converter controlled with suggested active voltage balancing method can be modulated with any pulse-width-modulation (PWM) method, such as phase-shifted-carrier PWM or level-shifted-carrier PWM. Simulation results and experimental measurements of proposed FC-based multilevel converter are presented to verify the performance of the proposed converter, and its novel switching and modulation strategy, which is based on the active voltage balancing method. [ABSTRACT FROM PUBLISHER]

Improving offshore wind turbine reliability is a key industry goal to improve the availability of this renewable energy generation source. The semiconductor devices in the wind turbine power converter are traditionally considered as the most sensitive and important components to achieve this and managing their thermomechanical stressing is vital, since this is one of their principal long-term aging mechanisms. Conventional deterministic reliability prediction methods used in industrial applications are not suitable for wind turbine applications, due to the stochastic nature of the wind speed. This paper develops an electrothermal model of the power devices, which is integrated with a wind turbine system model for the investigation of power converter thermal cycling under various operating conditions. The model has been developed to eliminate the problems of pulse width modulation switching, substantially reducing simulation time. The model is used to improve the current controller tuning method to reduce thermal stresses suffered by the converter during a grid fault. The model is finally used to design a control method to alleviate a key problem of the doubly fed induction generator—severe thermal cycling caused during operation near synchronous speed. [ABSTRACT FROM AUTHOR]

Fault ride-through (FRT) is a common requirement to abide by grid code all over the world. In this paper, to enhance the FRT capability of fixed speed wind generator system, a modified configuration of bridge-type fault current limiter (BFCL) is proposed. To check the effectiveness of the proposed BFCL, its performance is compared with that of the series dynamic braking resistor (SDBR). A harmonic performance improvement by the proposed method is also analyzed. A three-phase-to-ground (3LG) fault was applied to one of the double circuit transmission lines connected to the wind generator system. Simulation was carried out by using MATLAB/Simulink software. Simulation results show that the proposed BFCL is a very effective device to achieve the FRT and suppress fault current that eliminates the need for circuit breaker replacement. Also, the BFCL improves the harmonic performance and helps follow harmonic grid code. Moreover, it was found that the BFCL works better than the SDBR and has some distinct advantages over the SDBR. [ABSTRACT FROM PUBLISHER]

Stand-alone microhydro systems based on the self-exited induction generator (SEIG) are popular in developing countries. However, an electronic based controller is required to maintain the quality (voltage and frequency) of the generated power within an acceptable range. Conventionally, this controller is designed using a phase-controlled thyristor circuit (or more recently using a dc chopper circuit) and a fixed resistance dump load to receive the excess generated power. Generally, in small communities the total dissipated power in the dump load is greater than the consumed power by all the households. This problem of inefficient power usage can be solved by distributing the controller among households. The objective of this paper is to improve the distributed electronic load controller (DELC). As with the original DELC, the proposed improved DELC is used for two purposes: First, consuming the excess generated power individually by each household; and second, operating as a complementary device for voltage and frequency regulation of the SEIG. The detailed design and experimental verifications of the proposed improved DELC including the design of an appropriate input filter to minimize the network current distortion are presented here. Additionally, MATLAB simulations are used to study the interaction between the proposed improved DELC and the powerhouse controller (the ELC). [ABSTRACT FROM AUTHOR]

Wind turbine power converter system (WTPCS) is a crucial device in a wind energy conversion system. This paper presents the new failure rate models and a reliability evaluation technique for the (WTPCS) considering effects of wind speeds, which can be named by the multistate probability analysis method. The case studies on the WTPCS of a 2 MW wind turbine with a permanent magnet synchronous generator are conducted using wind speed data at Lauwersoog and Valkenburg wind sites in Holland. The results indicate that reliability of the WTPCS is affected significantly by variations of wind speed and the rated wind speed of wind turbine. The effectiveness of the proposed method is demonstrated by examples. [ABSTRACT FROM AUTHOR]

Doubly fed induction generator (DFIG) technology is the dominant technology in the growing global market for wind power generation, due to the combination of variable-speed operation and a cost-effective partially rated power converter. However, the DFIG is sensitive to dips in supply voltage and without specific protection to “ride-through” grid faults, a DFIG risks damage to its power converter due to overcurrent and/or overvoltage. Conventional converter protection via a sustained period of rotor-crowbar closed circuit leads to poor power output and sustained suppression of the stator voltages. A new minimum-threshold rotor-crowbar method is presented in this paper, improving fault response by reducing crowbar application periods to 11–16 ms, successfully diverting transient overcurrents, and restoring good power control within 45 ms of both fault initiation and clearance, thus enabling the DFIG to meet grid-code fault-ride-through requirements. The new method is experimentally verified and evaluated using a 7.5-kW test facility. [ABSTRACT FROM PUBLISHER]

In conventional parallel hybrid energy storage systems (HESSs), there are usually two cascaded converters active when transferring energy between its energy storage systems (ESSs) that result in the overall efficiency drop. Moreover, the dc-link capacitor has to be oversized to limit the voltage ripple during simultaneous energy exchange between the ESSs and load feeding. The integrated reconfigurable configuration for HESSs proposed in this paper allows direct energy transfer between the ESSs by using only a single converter and bypassing the dc link. This reduces the stress on the dc link, reduces the dc-link capacitor size, and improves the efficiency. The proposed configuration can be reconfigured to operate in different operating modes: “feeding a load” mode, “regenerative” mode, “energy exchange” mode, “energy exchange and feeding a load” mode, “balancing” mode, “charging” mode, and “backup” mode. To control the proposed HESS, a battery peak power limiting method is used to allocate the power components between the ESSs. The operating principle of the proposed configuration and its different modes are explained and experimentally verified. [ABSTRACT FROM PUBLISHER]

This paper presents the performance of a self-excited induction generator (SEIG) with a voltage and frequency controller (VFC) in a standalone microhydro power generating system. The VFC consists of a distribution static compensator (DSTATCOM) and a direct torque controlled variable frequency induction motor drive (VFIMD) operating as an electronic load controller (ELC). The DSTATCOM is an insulated-gate bipolar transistor-based three-leg voltage source inverter with a self-sustaining dc bus. The main objective of DSTATCOM is to regulate the system voltage through reactive power compensation. In addition to voltage regulation, the DSTATCOM compensates the harmonics generated by nonlinear loads as well as ELC. The ELC controls the system frequency through the active power balance. The VFIMD of the ELC drives a pump and it consumes the power in excess of consumer load power to maintain constant power generation at the SEIG terminals, which in turn regulates the system frequency. A prototype-proposed VFC is developed and tested with a variety of loads. Experimental results demonstrate that the proposed VFC can effectively control the system voltage and frequency. [ABSTRACT FROM PUBLISHER]

The doubly fed induction generator (DFIG) equipped with self-commutated insulated gate bipolar transistor (IGBT) voltage source converter (VSC) is one of the most popular topologies used in wind power systems. It has the ability to control active and reactive power independently. The reactive power capability is subject to several limitations which change with the operating point. Around synchronous operating point, a special attention is needed since the limitation of maximum junction temperature of the IGBTs cause a reduction on maximum permissible output current at the rotor side. This paper investigates the thermal behavior of the converter using semiconductor losses and thermal model based on the IGBT manufacturer datasheet. Different pulse-width modulation (PWM) types, including continuous and discontinuous types are applied and the results of reactive power capability are compared. Simulation results show that appropriate selection of PWM type is necessary at around synchronous speed to increase the maximum permissible rotor current as well as reactive power capability. [ABSTRACT FROM AUTHOR]

This article proposes an improved DC-link series IGBT chopping strategy for the brushless DC motor (BLDCM) drive equipped with a small DC-link capacitor fed by single-phase AC source. In this strategy, different vector sets are established according to the switching state of the DC-link IGBT and the operation mode of BLDCM. By analyzing the effect of each vector on phase current change rate, a vector selection-based current control scheme is presented. In particular, when the rectifier output voltage is less than the average line voltage of BLDCM, the combined action of vectors with only DC-link IGBT chopping is designed, in order to decrease the converter switching times and reduce the DC-link capacitance at the same time. In addition, the constraint of the DC-link capacitor voltage is analyzed to satisfy the reliable regulation of the motor phase currents, and the calculation method of the DC-link capacitance is given furthermore. The proposed strategy can achieve the smooth operation of the motor with small DC-link capacitance. Meanwhile, the total switching times of the converter are reduced, thereby improving the converter efficiency. The contrastive experimental results verify the correctness of the theories and the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

Thermal coupling between adjacent insulated gate bipolar transistor (IGBT) or diode chips is the result of nonuniform temperature distribution in a multichip IGBT module. This affects the junction temperatures and hence the total power loss predicted for the module. The study first investigates the impact of thermal coupling effect on the junction temperatures through a finite element method, and then develops a thermal coupling impedance model to represent such effects. The effect is shown to reduce with the distance exponentially. The model result agrees well with the test. The validated model is then used to predict the junction temperature swings during operational power cycling in a doubly fed induction generator wind turbine, showing the difference between the rotor and grid side converters. The model presented and the results obtained may be important for reliability evaluation and condition monitoring in the wind turbine power converters as well as in other multichip-paralleled power electronic systems. [ABSTRACT FROM AUTHOR]

In this research, six hybrid machine topologies, utilizing both a field winding and permanent magnets, are investigated for fixed-speed dc generation. The hybrids are compared with the wound-rotor and permanent magnet synchronous machines. To this end, analytical system models are utilized to predict machine-rectifier behavior. A general approach used to develop the machine models is discussed; the rectifier and filter models are also presented. The system models are used in a multi-objective design process to establish the tradeoff between overall mass and loss. Detailed losses, geometries, and rectifier waveforms obtained from the design studies are presented for four of the machine types. [ABSTRACT FROM AUTHOR]

Transient stability is a major concern for doubly fed induction machine (DFIM). A DFIM-based wind generator is readily affected by faults at the grid side as its stator windings are interfaced to grid. However, the wind generators need to remain connected and continue operation during faults at the grid side according to the grid code requirements. Therefore, it is important to enhance the transient stability of the DFIM-based wind generators. To achieve enhanced transient stability of the DFIM, a bridge-type fault current limiter (BFCL) is proposed in this study. Symmetrical as well as unsymmetrical faults were applied to the test system to check the efficacy of the BFCL in transient stability enhancement. Simulations were carried out in Matlab/Simulink environment. To demonstrate the effectiveness of the proposed BFCL, its performance is compared with that of the series dynamic braking resistor (SDBR). Simulation results show that the BFCL is a very effective device to attain better stabilization of the DFIM and outperforms the SDBR in all aspects. [ABSTRACT FROM AUTHOR]

Analysis of an experimentally measured daily load profile reveals that the residential power demand has a high percentage of low power duration over a day. In a stand-alone residential fuel cell power system, the power inverter designed for the peak power requirement will be operating at light loads, where its efficiency is lower, most of the time. Thus, improving the light-load efficiency will provide considerable hydrogen (energy) savings for the stand-alone residential power system. A solution to improve the power inverter efficiency at light loads is proposed. Both simulation and experimental results are given. Results show that the light-load efficiency can be improved by employing and uniquely controlling a parallel IGBT–MOSFET switch combination in a half-bridge inverter topology. It is also shown that substantial savings on hydrogen usage can be realized through the use of new inverter design in stand-alone operations. [ABSTRACT FROM PUBLISHER]