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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Capacitor Voltage Ripple Estimation and Optimal Sizing of Modular Multi-Level Converters for Variable-Speed Drives

This article deals with the sizing of submodule capacitors for modular multilevel converters (MMCs) used for variable speed drives for medium voltage and high voltage electric machines. Variable speed drives are required to run over a wide frequency range from zero to a few hundred Hz or even over 1 kHz. Under low frequency operation (<30 Hz), a large current ripple flowing through the MMC submodule capacitors will cause significant capacitor voltage fluctuation. This article proposes an optimal submodule capacitor selection method based on the common-mode voltage injection. The proposed sizing model includes two functions: 1) accurately estimating the capacitor voltage ripple based on the drive system parameters; and 2) identifying the minimum capacitance of the submodule capacitors based on the allowed maximum capacitor voltage fluctuations. Simulation and experimental results show that the error between the actual and estimated values of the voltage fluctuation is within 3% using the proposed model.

42 ENGINEERING↗
DOE OSTI2022

Fast and fault-tolerant model predictive control of MMCs under selective harmonic elimination

Selective harmonic elimination (SHE) is a suitable pulse-width modulation (PWM) tech-nique for some applications of modular multilevel converters (MMC) with lower numberof semiconductor submodules (SM). However, SHE can make it difficult to control circu-lating currents in the MMC. Moreover, the MMC is vulnerable to SM switch failures thatreduce its output power magnitude and quality. This paper proposes a fault-tolerant modelpredictive control strategy for MMCs operating with SHE (FT SHE-MPC). The effects ofswitch failure on phase voltage waveforms are studied first. Then, a method is proposedto design the FT SHE waveform to preserve its harmonic performance. Next, the pro-posed FT SHE-MPC is designed to increase the capacitor voltages during an SM failure tomaintain the fundamental component of the faulty phase voltage and control the circulat-ing currents, maintaining balance among line voltages and the output power of the MMC.Additionally, the proposed method offers as much as 60% faster FT transition time and asmuch as 36% lower capacitor voltage ripple. The advantages of the proposed method areverified through offline and real-time simulation studies on a three-phase nine-level MMCin PSCAD/EMTDC software and RTDS.

42 ENGINEERING↗
DOE OSTI2022

Aluminum Electrolytic Capacitor Vulnerability Evaluation in DC Power Supplies at the Spallation Neutron Source

The service life of electrolytic capacitors is a concern for long-term reliable operation of power supplies. The Power Conversion Group at the Spallation Neutron Source (SNS) facility of Oak Ridge National Laboratory is responsible for more than 500 power supplies with over 1000 electrolytic capacitors installed in these power supplies. Most of the electrolytic capacitors have been in operation since 2007 and are either well over or close to the manufacturer provided lifetime. This paper addresses this vulnerability to ensure continued reliable operation of the SNS. The power supply circuit is simulated in a MATLAB/Simulink environment to quantify the capacitor ripple current and operating voltage. Combined with manufacturer data, this information is used to estimate capacitor lifetime. Utilizing the simulation results and lifetime projections of the capacitors, capacitor replacements for the power supplies is prioritized. To assess end-of-life, the ambient temperature and ripple current of the capacitor is used. Based on the manufacturer's data and scientific literature review, capacitor end-of-life is established. The results of the simulations and analysis is presented. Data on the highest priority supplies will also be presented, and statistics on the capacitor performance, reliability and remaining lifetime will be shown.

Harave, Sudarshan↗
NASA NTRS1975

Evaluation of wet tantalum capacitors after exposure to extended periods of ripple current, volume 2

The application of tantalum capacitors in the Viking Lander includes dc voltage and ripple current electrical stress, high temperature during nonoperating times (sterilization), and high vibration and shock loads. The capacitors must survive these severe environments without any degradation if reliable performance is to be achieved. A test program was established to evaluate both wet-slug tantalum and wet-foil capacitors under conditions accurately duplicating actual Viking applications. Test results of the electrical performance characteristics during extended periods of ripple current, the characteristics of the internal silver migration as a function of extended periods of ripple current, and the existence of any memory characteristics are presented.

Ward, C. M.↗
NASA NTRS1974

Evaluation of wet tantalum capacitors after exposure to extended periods of ripple current, volume 1

The application of tantalum capacitors in the Viking Lander includes both dc voltage and ripple current electrical stress, high temperature during nonoperating times (sterilization), and high vibration and shock loads. The capacitors must survive these severe environments without any degradation if reliable performance is to be achieved. A test program was established to evaluate both wet-slug tantalum and wet-foil capacitors under conditions accurately duplicating actual Viking applications. Test results of the electrical performance characteristics during extended periods of ripple current, the characteristics of the internal silver migration as a function for extended periods of ripple current, and the existence of any memory characteristics are presented.

Watson, G. W.↗
DOE OSTI2020

DC Link Capacitor Reduction with Feedforward Control in Series-Series Compensated Wireless Power Transfer Systems

This paper presents the use of feedforward control to reduce the input side DC link capacitance of series-series compensated wireless power transfer (WPT) systems. Compared to conventional control schemes for WPT systems, the proposed feedforward-based approach achieves significant reduction in the DC link capacitor without any complicated voltage or current sensing requirements from the secondary side. This results in more compact hardware architecture. The proposed method shows minimal increase in the turn-on switching loss of the inverter. The switching loss is analyzed, and detailed results are presented relating the switching loss to the DC link capacitance and voltage ripple for proper tradeoff between losses and capacitor size. Simulation and experimental results presented validate the proposed scheme.

Mukherjee, Subho↗
NASA NTRS2017

The Impact of Harness Impedance on Hall Thruster Discharge Oscillations

Hall thrusters exhibit characteristic discharge voltage and current oscillations during steady-state operation. The lower frequency breathing-mode current oscillations are inherent to each thruster and could impact thruster operation and PPU design. The design of the discharge output filter, in particular, the output capacitor is important because it supplies the high peak current oscillations that the thruster demands. However, space-rated, high-voltage capacitors are not readily available and can have significant mass and volume. So, it is important for a PPU designer to know what is the minimum amount of capacitance required to operate a thruster. Through SPICE modeling and electrical measurements on the Hall Effect Rocket with Magnetic Shielding (HERMeS) thruster, it was shown that the harness impedance between the power supply and the thruster is the main contributor towards generating voltage ripple at the thruster. Also, increasing the size of the discharge filter capacitor, as previously implemented during thruster tests, does not reduce the voltage oscillations. The electrical characteristics of the electrical harness between the discharge supply and the thruster is crucial to system performance and could have a negative impact on performance, life and operation.

Pinero, Luis R.↗
NASA NTRS2017

The Impact of Harness Impedance on Hall Thruster Discharge Oscillations

Hall thrusters exhibit characteristic discharge voltage and current oscillations during steady-state operation. The lower frequency breathing-mode current oscillations are inherent to each thruster and could impact thruster operation and power processing unit (PPU) design. The design of the discharge output filter, in particular, the output capacitor is important because it supplies the high peak current oscillations that the thruster demands. However, space-rated, high-voltage capacitors are not readily available and can have significant mass and volume. So, it is important for a PPU designer to know what is the minimum amount of capacitance required to operate a thruster. Through Simulation Program with Integrated Circuit Emphasis modeling and electrical measurements on the Hall Effect Rocket with Magnetic Shielding thruster, it was shown that the harness impedance between the power supply and the thruster is the main contributor towards generating voltage ripple at the thruster. Also, increasing the size of the discharge filter capacitor, as previously implemented during thruster tests, does not reduce the voltage oscillations. The electrical characteristics of the electrical harness between the discharge supply and the thruster is crucial to system performance and could have a negative impact on performance, life and operation.

hollow cathodes↗
NASA NTRS2017

The Impact of Harness Impedance on Hall Thruster Discharge Oscillations

Hall thrusters exhibit characteristic discharge voltage and current oscillations during steady-state operation. The lower frequency breathing-mode current oscillations are inherent to each thruster and could impact thruster operation and power processing unit (PPU) design. The design of the discharge output filter, in particular, the output capacitor is important because it supplies the high peak current oscillations that the thruster demands. However, space-rated, high-voltage capacitors are not readily available and can have significant mass and volume. So, it is important for a PPU designer to know what is the minimum amount of capacitance required to operate a thruster. Through Simulation Program with Integrated Circuit Emphasis modeling and electrical measurements on the Hall Effect Rocket with Magnetic Shielding thruster, it was shown that the harness impedance between the power supply and the thruster is the main contributor towards generating voltage ripple at the thruster. Also, increasing the size of the discharge filter capacitor, as previously implemented during thruster tests, does not reduce the voltage oscillations. The electrical characteristics of the electrical harness between the discharge supply and the thruster is crucial to system performance and could have a negative impact on performance, life and operation.

ion engines↗
NASA NTRS2007

Reliability Effects of Surge Current Testing of Solid Tantalum Capacitors

Solid tantalum capacitors are widely used in space applications to filter low-frequency ripple currents in power supply circuits and stabilize DC voltages in the system. Tantalum capacitors manufactured per military specifications (MIL-PRF-55365) are established reliability components and have less than 0.001% of failures per 1000 hours (the failure rate is less than 10 FIT) for grades D or S, thus positioning these parts among electronic components with the highest reliability characteristics. Still, failures of tantalum capacitors do happen and when it occurs it might have catastrophic consequences for the system. This is due to a short-circuit failure mode, which might be damaging to a power supply, and also to the capability of tantalum capacitors with manganese cathodes to self-ignite when a failure occurs in low-impedance applications. During such a failure, a substantial amount of energy is released by exothermic reaction of the tantalum pellet with oxygen generated by the overheated manganese oxide cathode, resulting not only in destruction of the part, but also in damage of the board and surrounding components. A specific feature of tantalum capacitors, compared to ceramic parts, is a relatively large value of capacitance, which in contemporary low-size chip capacitors reaches dozens and hundreds of microfarads. This might result in so-called surge current or turn-on failures in the parts when the board is first powered up. Such a failure, which is considered as the most prevalent type of failures in tantalum capacitors [I], is due to fast changes of the voltage in the circuit, dV/dt, producing high surge current spikes, I(sub sp) = Cx(dV/dt), when current in the circuit is unrestricted. These spikes can reach hundreds of amperes and cause catastrophic failures in the system. The mechanism of surge current failures has not been understood completely yet, and different hypotheses were discussed in relevant literature. These include a sustained scintillation breakdown model [1-3]; electrical oscillations in circuits with a relatively high inductance [4-6]; local overheating of the cathode [5,7, 8]; mechanical damage to tantalum pentoxide dielectric caused by the impact of MnO2 crystals [2,9, 10]; or stress-induced-generation of electron traps caused by electromagnetic forces developed during current spikes [11]. A commonly accepted explanation of the surge current failures is that at unlimited current supply during surge current conditions, the self-healing mechanism in tantalum capacitors does not work, and what would be a minor scintillation spike if the current were limited, becomes a catastrophic failure of the part [l, 12]. However, our data show that the scintillation breakdown voltages are significantly greater that the surge current breakdown voltages, so it is still not clear why the part, which has no scintillations, would fail at the same voltage during surge current testing (SCT).

Teverovsky, Alexander↗
DOE OSTI2020

A High-Speed High-Power-Density Non-Heavy Rare-Earth Permanent Magnet Traction Motor

There has been a global push for higher torque density and lower cost traction motors. The use of non-heavy rare-earth (non-HRE) permanent magnet materials in traction motors are being investigated for the sustainability, profitability and affordability of electric vehicles and to broaden their adoption. A 20,000-rpm permanent magnet traction motor using non-HRE magnet materials is proposed. A dual three-phase winding configuration driven by a segmented dual three-phase drive is proposed to reduce the current ripple, and as a result, the DC Link capacitor, as well as to ease the voltage constraint at high speed and eliminate any risk of uncontrolled regeneration. Furthermore, the dual winding configuration enables a fault tolerant design which is useful for reliability. The paper presents comprehensive electromagnetic, thermal and mechanical designs and analyses using an integrated approach. The results have shown that the design is robust against demagnetization and confirmed its thermal and mechanical viability.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Predictive Direct DC-Link Control for 7.2 kV Three-Port Low-Inertia Solid-State Transformer With Active Power Decoupling

Promising for applications including renewable energy and electric vehicle fast charging, a medium-voltage (MV) solid-state transformer (SST) typically has multiple modules series stacked, which requires the modules to be single phase. One critical issue of the single-phase SST is the double-line-frequency power ripple. Traditionally, large passives are used to buffer the ripple, resulting in significantly increased volume and cost. This article for the first time proposes active power decoupling (APD) for the SST and a predictive direct DC-link control method, using a current-source single-stage SST as an example. An electrolytic capacitor-less buffer port is used to absorb the ripple, which tolerates up to a 30% voltage ripple for small capacitance. The APD SST control is challenging. First, the current-source SST realizes isolation, DC link, and multiport in a single stage. Second, different from conventional SSTs or APD converters, the low-inertia DC link in this SST has a 40% switching ripple, which is difficult to stabilize. To address the control challenge, a direct DC-link control architecture and a predictive control method are proposed. Simulation and experimental results verify the proposed control method on an MV SiC modular soft-switching solid-state transformer (M-S4T) prototype. Here, the proposed APD reduces the SST volume by 53.8%. Importantly, the proposed concept is not limited to the M-S4T but is generic to other SSTs or APD converters.

14 SOLAR ENERGY↗
NASA NTRS2012

Physics Based Modeling and Prognostics of Electrolytic Capacitors

This paper proposes first principles based modeling and prognostics approach for electrolytic capacitors. Electrolytic capacitors have become critical components in electronics systems in aeronautics and other domains. Degradations and faults in DC-DC converter unit propagates to the GPS and navigation subsystems and affects the overall solution. Capacitors and MOSFETs are the two major components, which cause degradations and failures in DC-DC converters. This type of capacitors are known for its low reliability and frequent breakdown on critical systems like power supplies of avionics equipment and electrical drivers of electromechanical actuators of control surfaces. Some of the more prevalent fault effects, such as a ripple voltage surge at the power supply output can cause glitches in the GPS position and velocity output, and this, in turn, if not corrected will propagate and distort the navigation solution. In this work, we study the effects of accelerated aging due to thermal stress on different sets of capacitors under different conditions. Our focus is on deriving first principles degradation models for thermal stress conditions. Data collected from simultaneous experiments are used to validate the desired models. Our overall goal is to derive accurate models of capacitor degradation, and use them to predict performance changes in DC-DC converters.

Kulkarni, Chetan↗
NASA NTRS1974

High performance dc-dc conversion with voltage multipliers

An experimental 100W 1000V dc-dc converter using a capacitor diode voltage multipler (CDVM) with a nominal frequency of 100 kHz is studied. A component weight of about 1 kg/kW was obtained. Design equations for current, output -ripple and -power, efficiency and output voltage are derived. Agreement between experimental results and calculations is fairly good except for ripple.

Harrigill, W. T., Jr.↗
DOE OSTI2025

DC-Link Capacitor Design for a Neutral-Point-Less Three-Level Dual-Phase Inverter for Traction Application

Conventional multilevel inverters, such as neutral￾point-clamped and T-type inverters, have gained popularity in electric vehicle applications due to their advantages, including high voltage range, high power capability, low switching losses, low total harmonic distortion, and low electromagnetic interference. However, these traditional multilevel inverters require a neutral point connection to generate a zero-voltage vector. The neutral point current oscillates at three times the fundamental frequency, leading to voltage imbalance and overvoltage stress on the power modules. Additionally, the use of two stacked DC-link capacitors increases the volume required for the same overall capacitance and complicates packaging due to ripple current and heat dissipation from separate components. This is a significant concern for traction drive units, where space is limited. In this paper, a neutral-point-less multilevel dual three￾phase inverter topology is investigated for traction inverter applications. Simulation results demonstrate that the proposed topology effectively retains the benefits of multilevel operation while utilizing a single DC-link capacitor. The inverter model was simulated in conjunction with an industry-standard battery model to evaluate the potential for capacitor size reduction compared to a conventional three-level inverter.

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2020

Soft-Switching Solid-State Transformer With Reduced Conduction Loss

Solid-state transformers (SSTs) are a promising solution photovoltaic (PV), wind, traction, data center, battery energy storage system (BESS), and fast charging electric vehicle (EV) applications. The traditional SSTs are typically three-stage, i.e., hard-switching cascaded multilevel rectifiers and inverters with dual active bridge (DAB) converters, which leads to bulky passives, low efficiency, and high electromagnetic interference (EMI). This article proposes a new soft-switching solid-state transformer (S4T). The S4T has full-range zero-voltage switching (ZVS), electrolytic capacitor-less dc link, and controlled dv/dt, which reduces EMI. The S4T comprises two reverse-blocking current-source inverter (CSI) bridges, auxiliary branches for ZVS, and transformer magnetizing inductor as a reduced dc link with 60% ripple. Compared with the prior S4T, an effective change on the leakage inductance diode is made to reduce the number of the devices on the main power path by 20% for significant conduction loss saving and retain the same functionality of damping the resonance between the leakage and resonant capacitors and recycling trapped leakage energy. The conduction loss saving is crucial, being the dominating loss mechanism in SSTs. Importantly, the proposed single-stage SST not only holds the potential for high power density and high efficiency but also has full functionality, e.g., multiport dc loads integration, voltage regulation, and reactive power compensation, unlike the traditional single-stage matrix SST. The S4T can achieve single-stage isolated bidirectional dc–dc, ac–dc, dc–ac, or ac–ac conversion. It can also be configured input-series output-parallel (ISOP) in a modular way for medium-voltage (MV) grids. Hence, the S4T is a promising candidate for the SST. The full functionality, e.g., voltage buck–boost, multiport, etc., and the universality of the S4T for the dc–dc, dc–ac, and ac–ac conversion are verified through the simulations and experiments of two-port and three-port MV prototypes based on 3.3 kV SiC MOSFETs in dc–dc, dc–ac, and ac–ac modes at 2 kV.

14 SOLAR ENERGY↗
DOE OSTI2021

Robust Predictive Control for Modular Solid-State Transformer With Reduced DC Link and Parameter Mismatch

This paper presents the analysis and implementation of a predictive control method for dc-link regulation and voltage balance in a cascaded modular reduced dc-link solid-state transformer (SST). Passive components like bulky dc links limit the power density of power converters, especially medium-voltage (MV) SST. Reduced dc-link or low-inertia converters can dramatically reduce the size, cost, and weight by tolerating larger dc-link ripples and improve the reliability with electrolytic capacitor-less dc link. However, a small dc link leads to tight coupling between the input and the output stages, which is a challenge for control design. In stacked low-inertia converters (SLIC), the low-inertia converter modules are stacked for MV applications, resulting in coupling between the modules and making the control more challenging. A new model predictive control method which can achieve deadbeat regulation on the dc link without weighting factors has been proposed to address this novel problem. This paper focuses on analyzing the condition of the low-inertia dc link up to 80% ripple, the robustness of the control under parameter mismatches, high-order terms, and important implementation issues such as model-based sampling and computation delay compensation. Significantly, the high-order terms are introduced because of the large dc-link ripple. These high-order terms are unique to the SLIC and negligible in conventional high-inertia converters. A discrete-time large-signal model is built to capture the dc-link’s nonlinear dynamics, and the eigenvalues of a small-signal Jacobian matrix are analyzed with Floquet theory to evaluate stability, using the modular soft-switching solid-state transformer (M-S4T) as an example of the SLIC. Simulation and experimental results of an MVDC M-S4T verify the analysis and the predictive control method. Finally, the general application of the predictive control to low-inertia converters is compared against a conventional PI controller using a reduced dc-link active-front-end (AFE) rectifier as an example.

14 SOLAR ENERGY↗