Showing papers by "Marco Liserre published in 2016"

A Review of Passive Power Filters for Three-Phase Grid-Connected Voltage-Source Converters

Abstract: In order to reduce size and cost, high-order passive filters are generally preferred in power converters to cancel out high-frequency harmonics caused by pulsewidth modulation. However, the filter resonance peaks may require the use of passive dampers to stabilize the interactions between the load and source impedances. Furthermore, the stabilizing effect is more difficult to be guaranteed for cost-optimized filters, which are characterized by low-inductance and high-capacitance passive components. In this paper, several passive filter topologies used to interface voltage-source converters with the utility grid are reviewed and evaluated in terms of damping capability, stored energy in the passive components, and power loss in the damping circuit. In addition, the influences of different switching frequencies of power converters on the passive filter design are discussed in the range 1–15 kHz. Illustrative design examples of the passive filters and experimental data are also provided.

The Smart Transformer: Impact on the Electric Grid and Technology Challenges

Abstract: The increasing proliferation of renewable energy resources and new sizeable loads like electric vehicle (EV) charging stations has posed many technical and operational challenges to distribution grids [1]. Encouraged by attractive tax incentives and promotion policies, local grid end consumers are becoming not only consumers of electricity but, in many cases, also producers. The actual electric distribution system limits the use of renewable energy resources, offers poor EV infrastructure, and is based on a unidirectional information flow from sources to control centers.

Optimal Design of High-Order Passive-Damped Filters for Grid-Connected Applications

Abstract: Harmonic stability problems caused by the resonance of high-order filters in power electronic systems are ever increasing. The use of passive damping does provide a robust solution to address these issues, but at the price of reduced efficiency due to the presence of additional passive components. Hence, a new method is proposed in this paper to optimally design the passive damping circuit for the LCL filters and LCL with multituned LC traps. In short, the optimization problem reduces to the proper choice of the multisplit capacitors or inductors in the high-order filter. Compared to existing design procedures, the proposed method simplifies the iterative design of the overall filter while ensuring the minimum resonance peak with a lower damping capacitor and a lower rated resistor. It is shown that there is only one optimal value of the damping resistor or quality factor to achieve a minimum filter resonance. The passive filters are designed, built, and validated both analytically and experimentally for verification.

Frequency-Domain Thermal Modeling and Characterization of Power Semiconductor Devices

Abstract: The thermal behavior of power electronics devices has been a crucial design consideration, because it is closely related to the reliability and also the cost of the converter system. Unfortunately, the widely used thermal models based on lumps of thermal resistances and capacitances have their limits to correctly predict the device temperatures, especially when considering the thermal grease and heat sink attached to the power semiconductor devices. In this paper, frequency-domain approach is applied to the modeling of the thermal dynamics for power devices. The limits of the existing RC lump-based thermal networks are explained from a point of view of frequency domain. Based on the discovery, a more advanced thermal model developed in the frequency domain is proposed, which can be easily established by characterizing the slope variation from the bode diagram of the typically used Foster thermal network. The proposed model can be used to predict not only the internal temperature behaviors of the devices but also the behaviors of the heat flowing out of the devices. As a result, more correct estimation of device temperature can be achieved when considering the cooling conditions for the devices.

IGBT Junction Temperature Measurement via Peak Gate Current

Abstract: An electrical method for junction temperature measurement of MOS-gated power semiconductor devices is presented. The measurement method involves detecting the peak voltage over the external gate resistor of an insulated-gate bipolar transistor or mosfet during turn-on. This voltage is directly proportional to the peak gate current, and fluctuates with temperature due to the temperature-dependent resistance of the internal gate resistance. Primary advantages of the method include an immunity to load current variation, and a good linear relationship with temperature. A measurement circuit can be integrated into a gate driver with no disruption to operation and allows autonomous measurements controlled directly via the gate signal. Advantages and disadvantages of the method are discussed.

Power Routing in Modular Smart Transformers: Active Thermal Control Through Uneven Loading of Cells

Abstract: Increasing decentralized energy production challenges the distribution grid [1], [2], and, in many countries, power generation and consumption are spatially separated, meaning that energy must be transferred over a long distance [3]. This calls for novel ways to transfer power to the loads without overloading grid feeders and to connect new intelligent loads and storage [4], which typically form the actual electric grid hybrid (ac and dc) and couple with other energy networks (multimodal) [5]. In the current configuration, transformers are passive devices that do not enable dc systems to connect or interface the electric grid with other energy grids.

H8 Inverter for Common-Mode Voltage Reduction in Electric Drives

Abstract: This paper presents a modified two-level three-phase inverter for the reduction of the leakage current. With respect to a traditional two-level inverter, the proposed solution reduces the common-mode voltage (CMV), both in amplitude and frequency. Between the dc source and the traditional three-phase bridge, two active dc-decoupling devices and a voltage-clamping network have been added. A dedicated control strategy was developed adopting a modified space vector pulse-width modulation, oriented to the reduction of the CMV. Simulations showing the good performance of the solution are presented. A preliminary prototype was developed and experimental results are presented.

Thermal Stress Analysis and MPPT Optimization of Photovoltaic Systems

Abstract: In the last years, the optimization of the energy harvesting of photovoltaic systems during fast variable irradiance conditions has been an active area of research and of competition among the companies. The proposed fast maximum power point tracking (MPPT) algorithms can produce extremely variable loading of the power semiconductors resulting in a decrease of the system lifetime, which in consequence can nullify the economic advantage of higher energy harvesting. This work analyzes the problem with a deep theoretical and laboratory work. Then, a multiobjective MPPT, which limits the positive temperature gradient and the maximum junction temperature of the power semiconductors, is introduced and fully validated in the laboratory with a mission profile emulating variable irradiance conditions.

Analysis of the frequency-based control of a master/slave micro-grid

Abstract: Renewable energy penetration in the low-voltage grid faces several limitations due to the current grid topology. Master/slave micro-grids could help solving these issues, by offering additional services to the grid such as the power management of the distributed power energy sources. In some cases, the power produced by the distributed energy sources exceeds the local consumption of the low-voltage grid. The consequent reverse power flow can be either dangerous (for the medium voltage) or impossible (for a micro-grid with limited storage). The droop characteristic of commercial inverter can be exploited to avoid this behaviour. However, stability problems can arise due to a low phase-locked-loop (PLL) bandwidth. This study investigates the stability of this solution depending on the bandwidth of the PLL of the distributed power generation sources.

Reverse power flow control in a ST-fed distribution grid

Abstract: The increasing implementation of Distributed Generation (DG) in the distribution grids creates new challenges in controlling the voltage profile. If the DG production exceeds the load consumption, the power flow reverses through the MV/LV substations. The reverse power flow impacts mainly on the voltage profile, increasing further the voltage in LV and MV grids. At this regard the Smart Transformer offers a new possibility to avoid the reverse power flow in the MV grids. The ST can adapt the voltage waveform modifying the frequency in order to interact with the local DG: the DG PLL notices the frequency change and the generators, equipped with droop controllers, decrease their power output. This paper deals specifically with the stability issues of the DG PLL when a fast change in the frequency is applied for avoiding the reverse power flow in MV grid. If the PLL in the DG is tuned with a low bandwidth, it could result in oscillatory phenomena in the current controller of the DG. The evaluation of the stability analysis has been performed analytically and validated by means of Control-Hardware-In-Loop (CHIL).

Computational light junction temperature estimator for active thermal control

Abstract: The junction temperature of power semiconductors in power converters must not exceed its maximum limits and it is of major importance for several failure mechanisms. But still, the junction temperature is hard to access. Direct measurement is not practical for industrial applications, indirect measurements require substantial effort and available junction temperature models have high calculation effort. This work develops a simple junction temperature estimator, which is applied for a maximum junction temperature limitation and the capability to be applied for further algorithm relying on the junction temperature, referring to active thermal control. It is experimentally shown, that a second order estimator is sufficient to achieve high bandwidth estimation.

Active thermal management for a single-phase H-Bridge inverter employing switching frequency control

Abstract: A thermal controller can be employed to reduce the thermal swing consequent to power cycling. This is demonstrated in this work for a full bridge inverter without a priori knowledge of the mission profile to which the converter is a subsystem, with the objective to reduce the thermal cycling without measurement of the junction temperature. The performance of the thermal controller is experimentally demonstrated by comparing a system, which operates with constant switching frequency with a system equipped with differently tuned thermal controllers. The impact of active thermal control on lifetime is investigated. 1) Introduction Power electronic converters are widely used for electric vehicles, aircrafts and grid connected applications, such as renewable energies. The reliability of these systems is essential, because failures cause downtimes and thus costs [1]. The most important failure mechanism is based on thermal cycling and is addressed in [2],[3]. Thermal cycling is affected by ambient temperature variations and power cycling, leading to mechanical stress within the module, which causes aging and finally failures. Semiconductor manufacturers optimize hardware to increase the reliability. Beside improved hardware, active thermal control enables to decrease the effects of power cycling [4]. However, in many systems, the main target is to maximize the efficiency, which is especially true in photovoltaic applications. Another way to improve the cost efficiency is to increase the lifetime of the system by a reduction of thermal cycling. In [5] active thermal control has been introduced to drive the system in safe operation during a fault of a subsystem, but not to increase lifetime during normal operation. This work proposes to modify the switching frequency to control the semiconductor losses, leading to reduced thermal cycles and increased lifetime under fast changing load conditions. The thermal control relies on electrical measurements and does not require additional sensors to reduce the thermal cycling. It is shown that the appropriate tuning is mandatory to find the optimal system behavior. The active thermal control is first described in theory and then tested on an experimental setup consisting of an H-bridge inverter connected to a passive load. 2) Active thermal control The junction temperature Tj of the power semiconductors depends on the losses of the device Ploss, the thermal resistance between junction to ambient Rth,ja and the ambient temperature Ta. Tj = Ta + Ploss ⋅ Rth,ja (1) Since the ambient temperature cannot be influenced and the thermal resistance is dependent on the hardware, the losses are controlled to reduce the thermal cycling. In high power applications, usually the switching losses Psw and the conduction losses Pcond are dominant, while driving or blocking losses are neglected: Ploss = Psw + Pcond (2) The conduction losses can be approximated with a second order polynomial, where iload pk is the peak alternating current, m is the modulation factor and cos (φ) is the power factor: Pcond = ( 1 2π + m⋅cos(φ) 8 ) ⋅ uce,0 ⋅ iload pk + ( 1 8 + m⋅cos(φ) 3π ) ⋅ rce(iload pk ) 2 (3) The switching losses can be expressed with the turn on losses Eon, the turn off losses Eoff, the switching frequency fsw, the dc-link voltage Udc, the load current and an empirical factor c, suggested to be chosen to 1.3 in [7] Psw = fsw ⋅ (Eon + Eoff) ⋅ i load pk Iref ( Udc Udc,ref ) c (4) Based on the bondwire liftoff failure mechanism, which is characterized by the Coffin-Manson equation, the power semiconductors can undergo an specific number of thermal cycles Nf, which is exponentially dependent on the magnitude of the thermal cycles ΔTJ and average temperature Tj . A = 302500, α = −5.039 and Ea = 9.89E − 20 are the Scheuermann fitting parameters, found for average power modules while k is the Boltzmann constant. As can be seen from the α value, the importance of the amplitude of the thermal for lifetime is critical. The impact of Tj is less relevant, but an increased temperatures reduces the expected lifetime as well. Nf = A(ΔTJ) e ( Ea kTj ) (5) The idea proposed in this paper is to reduce the thermal cycling by regulating determined profiles of losses via switching frequency variations. Some hypothesis will be considered: The full-bridge (shown in Figure 1) operates in hard-switching conditions at variable modulation ratio on a RL load. The H-bridge is driven by a unipolar modulation (with freewheeling in the upper and lower sides of the H-bridge). The modulation index is changed according to a mission profile. Figure 2 shows the proposed control method. The common operation resides in the losses model, shown in equations (3) and (4), that allows estimating the average losses over a fundamental output period. The difference between the actual losses and a low-pass filtered version constitutes the power difference that the active thermal control aims at reducing. A look-up table that links the power difference to the frequency increase is adopted, so that a higher switching frequency is selected when the output power is reducing (with gain ∆fmax ∆Pmax ). Instead, when the output power is increasing, the switching frequency is locked to the minimum. This non-linear control aims at preventing the cooling of the power module when the output power is reduced, but does not work when the modules is heating up. This behavior

Gate driver for the active thermal control of a DC/DC GaN-based converter

Abstract: Wide-Band-Gap power semiconductors based on SiC and GaN offer some significant advantages compared to Si-devices, in particular higher switching speed and higher operating temperature. These features offer potentially increased power density, which makes the temperature management critical especially for the PCB and components to which the GaN is connected. In this paper, an active gate driver with active thermal control is implemented and can be used to alter the losses of a DC/DC buck converter based on GaN transistors, with the aim of reducing the thermal cycling thus improving the converter's lifetime.

Design of External Inductor for Improving Performance of Voltage-Controlled DSTATCOM

Abstract: A distribution static compensator (DSTATCOM) is used for load voltage regulation, and its performance mainly depends upon the feeder impedance and its nature (resistive, inductive, stiff, nonstiff). However, a study for analyzing voltage regulation performance of DSTATCOM depending upon network parameters is not well defined. This paper aims to provide a comprehensive study of design, operation, and flexible control of a DSTATCOM operating in voltage control mode. A detailed analysis of the voltage regulation capability of DSTATCOM under various feeder impedances is presented. Then, a benchmark design procedure to compute the value of external inductor is presented. A dynamic reference load voltage generation scheme is also developed, which allows DSTATCOM to compensate load reactive power during normal operation, in addition to providing voltage support during disturbances. Simulation and experimental results validate the effectiveness of the proposed scheme.

Smart Transformer reliability and efficiency through modularity

Abstract: The application of the Smart Transformer (ST) in the electrical distribution system follows the trend to install more intelligent devices in the grid. In competition with the traditional transformer, the winning argument of the ST can be the grid services, while the system needs to be designed for the targets of high efficiency, high reliability and high availability. This work proposes a modular ST design, which uses power semiconductors rated for lower current and voltage for high efficiency, while the reliability and the availability are targeted by directly routing the power within the modular system. An overview is given on promising modular ST architectures and the concept of power routing for improved reliability is presented.

Quadruple Active Bridge DC-DC converter as the basic cell of a modular Smart Transformer

Abstract: One of the main challenges of a Solid-State transformer (SST) lies in the dc-dc conversion stage. In this work, a Quadruple-Active-Bridge (QAB) dc-dc converter is investigated to be used as the basic module for the whole dc-dc stage. Besides the feature of high power density and soft-switching operation, the QAB converter provides a solution with a reduced number of high frequency transformers, since more bridges are connected to the same multi-winding transformer. To ensure soft-switching in the full operation range of the converter, two modulation strategies are investigated: the phase-shift modulation and the triangular current modulation. The theoretical analysis is developed for both modulation strategies and a comparison between them is carried out. In order to validate the theoretical analysis, a 20 kW prototype was built and tested.

Active thermal balancing for modular multilevel converters in HVDC applications

Abstract: The modular multilevel converter (MMC) has become a very attractive solution for interfacing high voltages hybrid networks. The MMC enables scalability to different power levels, full controllability provided by IGBTs and can achieve very high efficiencies by using a low switching frequency method as the nearest level modulation (NLM). However, in order to limit failures of the power modules, the thermal stress of the submodules (SMs) should be properly studied. For NLM a capacitor voltage balancing algorithm is required and this algorithm, as demonstrated in this paper, offers already good thermal balance among the cells of the MMC. However, at low power factor, operation which could occur in case of low-voltage ride through and of reactive power injection, the mentioned algorithm is not effective anymore. This paper proposes an active thermal balancing algorithm which is embedded in the previously mentioned capacitor voltage balancing algorithm. The purpose of the active balancing is to achieve an equal heat distribution among the submodules to enhance the lifetime. The junction temperatures with and without active thermal balancing are studied in simulation for an HVDC application. The paper proves that thermal balance of MMC can be significantly improved.

Enhanced Control of DFIG Wind Turbine Based on Stator Flux Decay Compensation

Abstract: For the doubly fed induction generator (DFIG)-based wind energy conversion system, the decaying flux and negative flux are the main reasons to cause the DFIG rotor overcurrent, during grid faults. The stator decaying flux characteristics versus the depth and instant of the stator voltage variation are analyzed first. On the basis of the stator flux performances, the enhanced control strategy is proposed to limit the decaying flux approximately into zero in half fundamental period by changing the stator voltage drop and recovery mode, and consequently the rotor transient current pulse is significantly reduced during grid faults. The experimental results based on the 7.5 kW DFIG setup are carried to validate the correctness and feasibility of the proposed strategy. Dynamic voltage restorer (DVR) can be one of the applications. With the proposed strategy, the DVR only works in a half fundamental period and its output voltage amplitude is half of the stator voltage variation, during the grid voltage drop and recovery, respectively. As a consequence, the DVR can be rated for lower power saving cost. The simulation results based on MATLAB/Simulink using a 2 MW DFIG and the experimental results based on the 7.5 kW DFIG validate the effectiveness and feasibility of the DVR-based low-voltage ride through strategy.

Smart transformer-based hybrid grid loads support in partial disconnection of MV/HV power system

Abstract: Double circuit lines are common for transmitting the electrical power in high voltage (HV) and/or medium voltage (MV) power system. During the faults in one of the lines or transformers of the double circuit lines, one line is disconnected from the system and the healthy line is utilized for supplying the entire load. In that case, the transformer supplying the entire load could be overloaded. For the safe operation of the transformer, it is needed to disconnect some of the loads. This partial disconnection of MV/HV power system can severally effect the performance of critical loads. Recently, power electronic based transformer equipped with effective control and communication called smart transformer (ST) has been proposed for installation in the distribution system in place of conventional transformer. One of the most important feature of ST is to allow for connection of ac and dc grid forming hybrid grid which allow easy integration of renewable energy sources and storage. Considering these feature of ST, this paper proposes a new functionality of the ST where it provides continuous power to a section of the loads during the partial disconnection of MV/HV power system and improves the performance of power system. This new feature of ST has been proved through power system computer aided design (PSCAD) software based simulation results. Power hardware in loop (PHIL) and control hardware in loop (CHIL) is under development to test the idea.

Bidirectional series-resonant DC-DC converter with fault-tolerance capability for smart transformer

Abstract: The Series-Resonant dc-dc converter (SRC) is widely used in several application and it became very popular in Smart Transformer application. In this application, fault tolerance is a highly desired feature and it is obtained through redundancy. This paper proposes a reconfiguration scheme for the SRC for the case of failure in one semiconductor, which could drastically reduce the need of redundancy. Using the proposed scheme, the full-bridge based SRC can be reconfigured in a half-bridge topology, in order to keep the converter operational even with the failure (open circuit or short circuit) of one switch. The theoretical analysis is carried out for the unidirectional SRC and then extended to the bidirectional topology, since bidirectionality is required in smart transformer application. To verify the feasibility of the proposed scheme, the converter is tested experimentally in a 700 V to 600 V prototype with 10 kW of output power. A IGBT short-circuit fault is tested and the results confirms the effectiveness of the proposed approach.

Thermal-based finite control set model predictive control for IGBT power electronic converters

Abstract: Thermal cycling is one of the main sources of aging and failure in power electronics. A possibility to reduce the thermal stress of semiconductors is to control the losses occurring in the semiconductor devices with the target to reduce the thermal cycles. This approach is known as active thermal control. The hardest limit of the existing active thermal control approaches is that they do not offer a general framework where the optimal switching sequence is selected in order to fulfill the applications demand and reduce the thermal stress in specific semiconductors. The goal is to achieve the minimum thermal stress for the best possible overall performance. For this purpose finite control-set model predictive control (FCS-MPC) seems the optimal approach because it allows including of non-linear thermal and lifetime related models into the control law. A precise control of the thermal stress in the semiconductors can be achieved as the optimal switching vector is directly applied to the physical system. This allows to avoid overrating of the used module or to increase its lifetime. In the paper the approach is proven using simulation and experimental results.

Frequency adaptive control of a smart transformer-fed distribution grid

Abstract: An advanced service provided by the Smart Transformer (ST) is the decoupling between the Medium Voltage (MV) and Low Voltage (LV) grids. In the LV side, the ST can modify the waveform's frequency in order to interact with the droop controllers of the local generators to control the power demand among the sources without affecting the MV grid. However, most of the existing controllers for power converters cannot guarantee good performances under variable frequency conditions. To address this issue, a frequency adaptive control scheme based on the Fractional-Order Repetitive Control (FORC) as well as Frequency Locked Loop (FLL) is proposed in this paper. This proposed scheme provides fast online parameter tuning capability in order to be highly adaptive to variable frequencies, and it can be implemented either in a ST converter or in distributed generators. In this work simulation and experimental results are provided to demonstrate the effectiveness and advantages of the proposed scheme.

Circulating current control for modular multilevel converter based on selective harmonic elimination with ultra-low switching frequency

Abstract: Multilevel converters (MCs) are utilized in medium voltage (MV) high power applications due to its higher efficiency than two level converters. On the other hand, modular multilevel converters (MMCs) provide several advantages with regard to other MCs, such as higher scalability, reliability and no requirement of a common DC capacitor. Particularly, low switching frequency modulations, such as (2N+1) selective harmonic elimination (SHE) — pulse width modulation (PWM), may improve the efficiency of MMCs when they are utilized in MV and high power applications, where the number of sub-modules is not high. This work presents a new circulating current control for MMC when (2N+1) SHE-PWM is utilized. Therefore, it is possible to operate the converter simultaneously with low switching frequency and low capacitor voltage ripple at every sub-module besides a correct energy balance between arms. In addition, a new method to implement (2N+1) SHE-PWM for MMCs, which is also valid to implement standard SHE-PWM for any MC, is provided. Using this method, different equation systems are not required for every switching pattern. In this way, this technique provides simultaneously both the switching patterns and the firing angles which solve the SHE problem, simplifying the searching task. Simulation results which have been obtained from a MMC with 5 sub-modules at every arm, have validated the novel proposed circulating current control. Furthermore, the spectrum of the simulated line to line voltage waveform has proved the correct performance of the proposed (2N+1) SHE-PWM implementation method. Several sets of angles have been provided throughout the m a range, where 17 harmonics have been controlled.

Voltage stability analysis using a complete model of grid-connected voltage-source converters

Abstract: Due to the increasing popularity of renewable energies, a significant share of the power generation in future microgrids is expected to originate from converter-interfaced Distributed Energy Resources (DERs). Traditionally, idealized device models are used to conduct grid stability studies. For instance, a DER interfaced via a Voltage-Source Converter (VSC) would be modeled as an ideal current or power source (depending on the control schemes), ignoring non-ideal behavior like the response of the converter synchronization. However, such a simplification may lead to misjudging the stability, in particular for weak microgrids. To address this issue, ZIP models of grid-interfaced VSCs, which take into account both the control scheme and the synchronization, are developed in this paper. The influence of the synchronization response on the stability of a weak microgrid system is demonstrated using a benchmark system simulated in MATLAB/Simulink. It is shown that the idealized models normally used for static stability analysis do underestimate the voltage stability issue in the investigated microgrid system.

Loss balancing of three-level inverters in electric vehicles for low speed operation

Abstract: The most stressful operation point for the inverter of an electric vehicle drive is at low speed. In this operation point, one power semiconductor is mainly stressed because of a high loss concentration. The possibilities of distributing the losses among the power semiconductors through a new modulation is studied for the three-level NPC and the T-Type inverter. The effect of the proposed modulation on the sizing of the power semiconductors is shown by application of the power semiconductor rating. For both inverter topologies, the presented modulation strategy is verified experimentally.

Control and communication in the Smart Transformer-fed grid

Abstract: The Smart Transformer (ST) is a solid-state transformer performing not only a voltage step-down function but also energy and information management. With the help of control and communication technology, the ST can increase hosting capacity of renewable energies and provide ancillary services to utility and customers. To better exploit the potential of a ST-fed grid, a proper design of architecture, control scenarios and its corresponding communication network has to be done. This paper aims at giving a better understanding of the functionality of the ST-fed grid and of the associated communication challenges.

Wide frequency range medium-voltage grid impedance analysis by current injection of a multi-MW power converter

Abstract: More and more energy is fed especially by decentralized converters from renewable energy sources into the grid Therefore, it is important to know the grid behavior at a wide frequency range from the fundamental frequency up to the kHz-range This paper presents an advanced impedance medium-voltage measurement system with a high frequency range for an electrical medium-voltage grid A sinusoidal current of a predefined frequency is injected into the grid With this, the grid impedance is calculated from the measured magnitude and phase of the injected current and the resulting voltage The measurements will be executed at different frequencies to produce approximate continuous impedance versus frequency characteristic of the grid at the measured coupling point Results of a ready to use, low-voltage impedance measurement system are discussed, and the design considerations the basic concept and first simulation results of the developed medium-voltage system are presented in this paper

Multi-frequency power routing for cascaded H-bridge inverters in smart transformer application

Abstract: The smart transformer is a solid state transformer with advanced control functionalities that can efficiently manage a low-voltage micro-grid by also supporting the medium-voltage grid. Cascaded H-bridge (CHB) converters proved to be a suitable option to realize the MV stage of the smart transformer due to their modularity and multi-level output. Normally the power is equally split among the CHB cells, however, in order to delay failures of the system, certain cells can be unloaded if premature deterioration is detected. In this work, multi-frequency power transfer is used to control the power processed by the dc/dc converters that supply the dc link of the CHB. The potential is analyzed analytically and validated experimentally.

Topology and control strategy for accelerated lifetime test setup of DC-link capacitor of wind turbine converter

Abstract: Back-to-back converters for wind turbine systems (WTS) feature capacitors in the DC-link to maintain a stable DC-link voltage and to decouple the generator from the grid. Electrolytic and film capacitors can be chosen to this purpose. Long-term field experience and recorded failure data reveal that capacitor failures are one of the main reasons for the downtime of WTS. The ripple current accelerates the wear-out of the capacitors, which is strongly dependent on the mission profile of the system. Therefore, it becomes important to estimate the useful lifetime of a capacitor as a function of the ripple current. In this paper, aging characteristics of electrolytic capacitors are described, and a power converter topology and its control strategy are designed to perform accelerated lifetime tests. The voltage and ripple current of the capacitor under test (CUT) can be controlled with low THD to correlate wear-out with ripple.