Lifetime Of DC-link Capacitors Research Articles

Enhanced DC-link voltage control of PMSG-based wind turbine generators by machine-side converter during asymmetrical grid faults

Under asymmetrical grid fault conditions, oscillating active power can occur on the grid-side converter (GSC) of permanent magnet synchronous generator (PMSG)-based wind turbine generators. The oscillating active power flows into the DC-link, causing voltage oscillations that can lead to a shortened lifetime of DC-link capacitors and additional harmonics injected into the grid. While conventional methods can mitigate the oscillating active power through negative sequence current injection by the GSC, this hinders the grid support of the GSC regarding negative sequence reactive current during grid fault. To address this issue, a novel control scheme is proposed that uses the machine-side converter (MSC) to transfer the oscillating power to the motor. Firstly, the active power capacity of the MSC is analyzed and compared with that of GSC, and the results show that MSC has enough capacity to accommodate the oscillating active power. On this basis, a novel MSC control scheme is proposed to transfer the oscillating active power on the DC-link to the motor, which has a much larger inertia to buffer the power oscillation. Compared with conventional methods, the proposed control scheme relieves the GSC from mitigating the power oscillation to support the grid. Simulation studies using DIgSILENT PowerFactory are presented to verify the effectiveness of the proposed control scheme for PMSG-based wind turbine generators.

Dual-Carrier-Based PWM Method for DC-Link Capacitor Lifetime Extension in Three-Level Hybrid ANPC Inverters

Power converters operate at varying low-, medium-, and high-voltage levels. Large electrolytic capacitors are usually used to sustain the associated dc-link current and stabilize the dc-link voltage. However, owing to different power converter applications, capacitors are affected by large ripples in the dc-link current, which increase heat exchange and reduce system reliability and lifespan. In this regard, this article proposes an innovative control technique for suppressing the ripple in the dc-link current and common-mode voltage (CMV) in hybrid active neutral point clamped inverters. The proposed control technique uses an improved dual-carrier-based pulsewidth modulation technique to create a new switching scheme by selecting the voltage vectors that contribute to the smallest ripples in the dc-link current according to each sector. Applying the proposed control technique significantly reduces the ripples in the dc-link current and CMV, which increases the reliability of the system and extends the lifetime of dc-link capacitors. The simulation and experimental results demonstrate the effectiveness of the proposed control strategy.

Common DC-Link Capacitor Harmonic Current Minimization for Cascaded Converters by Optimized Phase-Shift Modulation

This paper investigates the influence of a constant carrier phase shift on the DC-link capacitor harmonic current of cascaded converters used in fuel-cell and mild-hybrid electric vehicles. In these applications, a DC-DC converter can be adopted between the battery and the motor drive inverter in a cascaded structure, where the two converters share the same DC-link. Since the DC-link capacitor of such a system represents a critical component, the optimization of the converter operation to limit the current stress and extend the lifetime of the capacitor is an primary objective. This paper proposes the use of a carrier phase shift between the modulations of the two converters in order to minimize the harmonic current of the DC-link capacitor. By harmonic analysis, an optimal carrier phase shift can be derived depending on the converter configuration. Analytical results are presented and validated by hardware-in-the-loop experiments. The findings show that the pulse width modulation carrier phase shift between the interleaved boost converter and the voltage source motor drive inverter has a significant influence on the DC-link capacitor current and thus on its lifetime. A case study with two-cell and three-cell interleaved boost converters shows a possible DC-link capacitor lifetime extension of up to 390%.

Novel control approach for enhancing multi-performance of two-stage converter during power transfer from solar photo-voltaic array to AC grid

This paper presents a nonconventional control approach for augmenting various performances of two-stage power converter (DC-side boost converter and three-phase voltage source inverter (VSI)) during power transfer from solar photo-voltaic array to AC grid. In the application, intermediate DC link voltage is traditionally regulated to settle at a constant and high value. This fixed value is not optimum in respect of various performances of the converters under variable power transfer scenarios. In this respect, variable values of DC link voltage under different power transfer conditions are necessary, which is absent in conventional scheme. For achieving the control flexibility, two modifications of traditional control scheme are proposed as, omission of intermediate DC link voltage regulating loop and controlling of modulation index of current loop between 0.98 and 1 for VSI. In effect, significant multi-benefits are obtained as, (i) reduction of voltage stress on power switches to enhance their life span, (ii) reduction of size, cost and enhanced lifetime of DC link capacitor, (iii) improved efficiency of conversion system and (iv) reduction of total harmonic distortion (THD) during maximum power injection from solar photo-voltaic (PV) array to connected AC system. First three benefits are produced due to reduction of DC link voltage for achieving required power transfer and fourth benefit is achieved due to high valued modulation index (between 0.98 and 1). Both simulation and experimental studies verify superior performances of two-stage converter with the help of proposed logic under variable solar power conditions. Derived mathematical condition on interplay between DC link voltage value and power transfer amount at different power factors (unity and leading) provides insight of proposed control approach.

Improving DC-Link Capacitor Lifetime for Three-Level Photovoltaic Hybrid Active NPC Inverters in Full Modulation Index Range

This article presents a dc-link capacitor lifetime improvement method for three-level photovoltaic hybrid active neutral-point clamped inverters in full modulation index range. The capacitors are one of the components with vulnerable reliability in a power conversion system. The dc-link capacitor current ripples lead to temperature rise and wear-out failure of the capacitors. The accumulated thermo-mechanical damages from the temperature rise reduce the capacitor lifetime and reliability of the system. The proposed modulation method suppresses effectively the dc-link capacitor current ripple to improve the capacitor lifetime. In this article, the proposed method is discussed via a comparison with conventional space-vector pulsewidth modulation in full modulation index range. The modulation index range is divided into two regions, namely medium-vector modulation and hybrid modulation region. The capacitor lifetime is evaluated by the capacitor lifetime model according to the modulation methods and modulation regions. The effectiveness of the proposed modulation method is verified through simulation and experimental results.

Lifetime Prediction of DC-Link Capacitors in Multiple Drives System Based on Simplified Analytical Modeling

Lifetime prediction of dc-link capacitors in a single drive has been discussed before, which indicates that the capacitor in a standard drive meets serious reliability challenges and in a slim drive does not. However, in most of the applications, drives are connected in parallel with the power grid. The large amount of harmonic distortion produced by nonlinearity drives may transmit and couple between grid and drives, which changes the stresses of devices as well as the dc-link filters. Therefore, the estimated results in a single drive cannot be extended to multiple drives any more. This article investigates the lifetime of dc-link capacitors in multiple drives system. First, by decoupling the interactions among grid-connected drives, a simplified equivalent circuit model and its analytical model to obtain the dc-link continuous current in multiple drives is proposed, which releases the designers from configuring the large simulation for multiple drives. Then, applying the lifetime prediction method, the lifetime of dc-link capacitors in multiple drives is investigated, in terms of types of drives, numbers of drives, and grid conditions. The results show that the lifetime of the standard drives extends in the multidrive systems and the lifetime of the slim drives decreases in the multidrive systems, which break the previous mind. Finally, based on the proposed analytical model and lifetime estimation method, the capacitor sizing from reliability aspect for multiple slim drives is given. The outcomes of the lifetime investigation could be a guideline for the design of the capacitive dc link in multidrive systems.

Fault Ride-Through Strategy for Two-Stage Grid-Connected Photovoltaic System Enabling Load Compensation Capabilities

This paper proposes an extended Kalman filter (EKF) based control strategy for fault ride-through operation in two-stage grid-connected photovoltaic (GPV) system. Unlike the conventional controllers for ride-through operation, the proposed strategy does not compromise with power quality improvement features in the system while enabling ride-through operation. The controller accounts for nonlinear loads in the system, grid harmonic-currents elimination, and grid-currents balancing even during the harmonic/distorted grid voltages. The IEEE Standard-1547.4 compels the distributed resource to ride-through during voltage disturbances caused by faults. For the ride-through operation, a limit is imposed on PV active power injection to prevent inverter overcurrents and dc-link energy aggregation, which reduces the lifetime of dc-link capacitor. The reactive power is fed to the grid, as per the depth in voltage-sag. The derated PV array power is supplied in cases where the inverter cannot handle the utmost PV-power. The power quality improvement is ensured using EKF state estimator, which precisely estimates the fundamental load currents. In distribution network with modern nonlinear loads, especially at far radial ends, the grid voltages are prone to huge diversions and the proposed controller provides a possible solution to maintain active/reactive power support and maintain power quality in the network. The effectiveness of the strategy is demonstrated through simulations and experiments. Under all disturbances, the harmonic content in grid currents is observed within limits, in accordance with the IEEE Standard-519.

Benchmarking of capacitor power loss calculation methods for wear-out failure prediction in PV inverters

Aluminium electrolytic capacitors (e-caps) are among the components most prone to failure in power electronic systems. Thermal stress is a critical factor which affects the lifetime of dc-link capacitors. Therefore, the estimation of power loss and temperature are important steps in lifetime prediction of these components. This paper aims to carry out a qualitative benchmarking of different capacitor loss estimation methods, focusing on the component level reliability. Four different equivalent series resistance (ESR) models are benchmarked for 2 different mission profiles based on complexity, estimated losses, thermal stresses, and B1 lifetime: 1) constant ESR, 2) temperature dependent ESR, 3) frequency dependent ESR, and 4) frequency/temperature dependent ESR. Capacitors in the dc-link of a PV inverter are evaluated using an offline look-up table (LUT) approach, considering two different mission profiles (MP), and following a normal distribution parametric variation. For the lifetime model and parameters employed, the results for this case study indicated a maximum difference of 7.1% in the B1 lifetime estimated by approaches 1 and 4.

Impact of Input Voltage Sag and Unbalance on DC-Link Inductor and Capacitor Stress in Adjustable-Speed Drives

This paper investigates the impact of input voltage unbalance and sags on stresses in the dc-bus choke inductor and dc-bus electrolytic capacitors of adjustable-speed drives (ASDs). These stresses are primarily attributable to the rectifier's transition into single-phase operation, giving rise to low-order harmonic voltages (120 Hz, 240 Hz, etc.) that are applied to the dc-link filter components. These harmonics elevate the ac-flux densities in the dc choke core material significantly above values experienced during normal balanced-excitation conditions, causing additional core losses and potential magnetic saturation of the core. It is shown that the effects of voltage unbalances and sags on the dc-link capacitor lifetime will be the same when either line inductors or a dc-link choke inductor are used if the dc choke-inductance value is twice the value of the line inductance. Simulations and experimental tests are used to verify the accuracy of predictions provided by closed-form analysis and simulation for a 5-hp or 3730-W ASD system.