Health Estimation of Capacitor Bank

Aluminum electrolytic capacitors (AECs) are connected in a bank to meet the requirement of capacitance, voltage, and current rating for dc-link of power electronic converters. The performance of capacitor bank deteriorates with time due to the degradation of AECs in the bank. Further, each AEC in the bank degrades differently based on its core temperature, which in turn depends on the current and its equivalent series resistance (ESR). Therefore, health monitoring of a bank would require monitoring of individual AECs in the bank. However, extending the available literature on health monitoring of an AEC to the capacitor bank would require voltage/current sensors for individual AECs. To address the aforementioned issue, this work suggests a method to assess the health of individual AECs online in the bank without using sensors for each AECs. The proposed method involves estimation of current through each capacitor, which is used to determine the core temperature by using a thermal model of AEC. The core temperature is used for estimation of ESR and capacitance of each capacitor using physics-of-failure based degradation model. For the consistent matching of the degradation model to the actual degradation of the capacitor, the coefficients of the model are sequentially updated. For updating, an algorithm based on extended Kalman filter is used. The proposed scheme is experimentally validated on laboratory prototype of single-phase, grid-connected solar PV inverter under variable load conditions.
Details: https://ieeexplore.ieee.org/document/8536870

Health Monitoring of IGBT

A novel scheme is proposed for online condition monitoring of bond wires present in insulated gate bipolar transistor (IGBT) package. The proposed method detects bond wire degradation using on-state collector emitter voltage at the inflection point. Previously reported condition monitoring methods based on on-state collector-emitter voltage as a precursor of aging require an accurate knowledge of junction temperature which is difficult to measure online during an inverter operation. The key advantage of the proposed scheme is that it monitors the bond wire degradation irrespective of the junction temperature. Therefore, this technique is not affected by increase in junction temperature due to die attach degradation or change in ambient temperature. The proposed scheme is verified experimentally under realistic operating conditions.
Details: https://ieeexplore.ieee.org/document/7707420

Gan Based Compact DC-DC Converter

Gallium Nitride(GaN) power devices offer benefits like faster switching, lower on-resistance, lower device capacitances as compared to the current silicon(Si) devices. These benefits could be leveraged to reduce the size and weight of GaN based power electronic supplies. This work aims to evaluate the benefits and challenges of using GaN devices by experimentally comparing Si and GaN based Switch Mode Power Supply(SMPS) for satellite application. Two multi output flyback converter prototypes with Si and GaN switches are developed for the same specifications to observe the direct benefits and challenges of using GaN switches. It is observed that for same efficiency, GaN converter can have upto 10 times the switching frequency as compared to a Si converter. This results in significant reduction in capacitor and inductor size, resulting in a two fold improvement in overall power density. Further, this work also discusses the issues arising from high frequency switching in presence of circuit parasitics.
Details: https://ieeexplore.ieee.org/document/8707877

Transformer-less Solar Inverter

Owing to low cost, small size, and low weight, transformerless inverters became prominent in single-phase grid connected photovoltaic (PV) systems. Key issues pertaining to these inverters include suppression of common mode (CM) leakage current and improvement of conversion efficiency. Achieving higher efficiency in single-phase grid-connected photovoltaic systems depends on the number of stages involved in feeding power to the grid, predominantly, if the PV array voltage is less than the peak value of the grid voltage. In this work, an integrated dc-dc converter based grid-connected transformerless PV inverter is proposed which is aimed at maintaining high efficiency, even if the PV array voltage falls below the peak value of grid voltage (efficient operation at an extended input voltage range). A modulation strategy is discussed in order to minimize the flow of CM leakage current. Further, the efficiencies of certain transformerless inverter topologies are analyzed and compared with that of the proposed topology. Detailed simulation studies are carried out in MATLAB/Simulink environment to verify the analysis. Experimental results for a scaled down laboratory prototype are included as a proof-of-concept to validate the claims.
Details: https://ieeexplore.ieee.org/document/8125736

Secondary Controller for DC Microgrid

Sources in a dc microgrid are required to have a proportional current sharing and an improved voltage regulation. Conventional droop-based control techniques achieve the proportional current sharing, but the voltage regulation of an individual bus may be large due to a high value of droop gains. In the literature, secondary controllers are discussed to improve either the voltage regulation of one bus or the average voltage regulation of the entire system. Most of these techniques rely on the communicated values of both the terminal voltage and the output current of individual sources. This work puts forth a new approach to address the aforementioned issues. A figure of merit called quality index is introduced, which constitutes a weighted average of terms representing the voltage regulation and the power sharing at each source bus/node. An online algorithm to find the optimal droop coefficient based on the quality index is suggested. It does not require the knowledge of the system parameters. Furthermore, it reduces congestion in the communication network, as only output current information is exchanged among sources. Equations for the current sharing error and the voltage regulation in a microgrid system are derived based on droop gain values of the sources. Operation of the proposed algorithm is verified using a computer simulation. The method is validated on a scaled-down dc microgrid prototype.
Details: https://ieeexplore.ieee.org/document/8264690