Sandeep Anand

Distributed Control to Ensure Proportional Load Sharing and Improve Voltage Regulation in Low-Voltage DC Microgrids

DC microgrids are gaining popularity due to high efficiency, high reliability, and easy interconnection of renewable sources as compared to the ac system. Control objectives of dc microgrid are: 1) to ensure equal load sharing (in per unit) among sources; and 2) to maintain low-voltage regulation of the system. Conventional droop controllers are not effective in achieving both the aforementioned objectives simultaneously. Reasons for this are identified to be the error in nominal voltages and load distribution. Though centralized controller achieves these objectives, it requires high-speed communication and offers less reliability due to single point of failure. To address these limitations, this paper proposes a new decentralized controller for dc microgrid. Key advantages are high reliability, low-voltage regulation, and equal load sharing, utilizing low-bandwidth communication. To evaluate the dynamic performance, mathematical model of the scheme is derived. Stability of the system is evaluated by eigenvalue analysis. The effectiveness of the scheme is verified through a detailed simulation study. To confirm the viability of the scheme, experimental studies are carried out on a laboratory prototype developed for this purpose. Controller area network protocol is utilized to achieve communication between the sources.

Reduced-Order Model and Stability Analysis of Low-Voltage DC Microgrid

Depleting fossil fuels, increasing energy demand, and need for high-reliability power supply motivate the use of dc microgrids. This paper analyzes the stability of low-voltage dc microgrid systems. Sources are controlled using a droop-based decentralized controller. Various components of the system have been modeled. A linearized system model is derived using small-signal approximation. The stability of the system is analyzed by identifying the eigenvalues of the system matrix. The sufficiency condition for stable operation of the system is derived. It provides upper bound on droop constants and is useful during planning and designing of dc microgrids. Furthermore, the sensitivity of system poles to variation in cable resistance and inductance is identified. It is proved that the poles move further inside the negative real plane with a decrease in inductance or an increase in resistance. The method proposed in this paper is applicable to any interconnecting structure of sources and loads. The results obtained by analysis are verified by detailed simulation study. Root locus plots are included to confirm the movement of system poles. The viability of the model is confirmed by experimental results from a scaled-down laboratory prototype of a dc microgrid developed for the purpose.

Optimal voltage level for DC microgrids

Depleting fossil fuels, increasing energy demand and concern over climate change due to CO2 emission motivate the use of renewable sources. However, supplying electronics, variable speed drives and LED loads from renewable sources requires multiple ac-dc and dc-ac conversions. This causes substantial energy wastage before end use. To address this limitation, dc system is suggested, which offers high efficiency and reliability with low system cost. In this paper, suitable power electronic interfaces for sources/loads/storage elements are discussed along with their performance requirements. Further, efficiency, cost and safety issues of the dc system with various voltage levels are compared. With suitable case studies, optimal dc voltage level is determined for residential and commercial application. The proposed dc system with this optimum voltage level offers 10–22% improvement in energy efficiency over the conventional ac system.

Transformer-Less Grid Feeding Current Source Inverter for Solar Photovoltaic System

High efficiency and operating life of grid feeding solar photovoltaic (PV) inverters are demanded. Due to reduced dc-link capacitor requirement, current source inverter (CSI) offers higher reliability than the voltage source based solar inverter. However, conventional three-phase pulse width modulated (PWM) current source based solar inverter injects high earth leakage current into the grid. In order to suppress this current, an isolation transformer can be used. Use of this transformer increases the cost and size, and decreases overall efficiency. In order to address the aforementioned limitations, a modified CSI is proposed in this paper. The proposed inverter suppresses the earth leakage current without using an isolation transformer, thereby increasing the efficiency and reducing cost as compared to conventional current source based solar inverters. A mathematical model of the system is derived based on which controller for the operation of the inverter is designed. The effectiveness of the scheme is verified through detailed simulation study. To confirm the viability of the scheme, experimental studies are carried out on a scaled-down laboratory prototype.

DC Voltage Controller for Asymmetric-Twin-Converter-Topology-Based High-Power STATCOM

A four-level static compensator integrating two 2-level converters, supplying/absorbing reactive power to/from the grid, is reported in our earlier paper. Reduced component count, simpler layout for switches, and smaller dc-link capacitor values are the attractive features of the proposed topology over the diode clamped and cascaded multilevel converters. This paper suggests further improvements in this topology. Suitable selection of the dc-link voltage values reduces distortion in the current fed to the grid. In addition, circuit topology is modified to avoid the split-capacitor dc links. This reduces the number of independent dc capacitor voltages to be controlled and eliminates the flow of third-harmonic current through the transformer. In order to improve the performance, a phase-shifted carrier-based pulsewidth modulation technique is used. A mathematical model of the system is derived, based on which a controller for the scheme is designed. The effectiveness of the scheme is verified through detailed simulation study. To confirm the viability of the scheme, experimental studies are carried out on a scaled-down laboratory prototype developed for the purpose.

Unique Power Electronics and Drives Experimental Bench (PEDEB) to Facilitate Learning and Research

Experimentation is important for learning and research in the field of power electronics and drives. However, a great deal of equipment is required to study the various topologies, controllers, and functionalities. Thus, the cost of establishing good laboratories and research centers is high. To address this problem, the authors have developed a “Power Electronics and Drives Experimental Bench” (PEDEB), whose details are given in this paper. This unique kit includes reconfigurable hardware modules, which can be interconnected to achieve more than 14 different circuit topologies. Moreover, the software (controller) is accessible to users, thereby facilitating quick verification and testing of new ideas. A 2-kVA prototype of the PEDEB was developed and tested for various possible modes of operation. The kit is being used for a first-semester post-graduate laboratory course on “Power Electronics and Drives.” This paper includes observations and learning from experiments on dc-dc buck converter, an induction motor drive, and a grid feeding inverter conducted using the PEDEB.

Online Monitoring Technique for Aluminum Electrolytic Capacitor in Solar PV-Based DC System

To interconnect low-voltage solar photovoltaics (PV) with dc system, a dc-dc boost converter is required. To minimize the switching frequency oscillations in solar PV voltage, aluminum electrolytic capacitors (AECs) are connected between solar PV and the converter. Operational life of AECs depends on electrical and environmental parameters and is less than that of solar PV panels. Equivalent series resistance (ESR) of AEC increases with life, thereby increasing the voltage ripple across PV terminals. This results in less power extraction from solar PV. This paper proposes a method for health monitoring of AEC using the parameters measured for maximum power point tracking (MPPT) (PV voltage and current). The proposed technique is applicable for both continuous conduction mode and discontinuous conduction mode of operation of the converter. Effect of temperature on ESR is incorporated for accurate determination of the health of capacitor. Key advantage is that no additional current or voltage sensor is required to implement this technique, thereby offering a low-cost solution. Further, the proposed technique does not require significant processing capabilities and is implemented in the same digital controller used for MPPT. Detailed simulation studies are carried out and results are included in this paper. A laboratory prototype of dc-dc boost converter is developed for experimentation. Results are in agreement with that obtained from simulation studies.

Evaluation of Vce at Inflection Point for Monitoring Bond Wire Degradation in Discrete Packaged IGBTs

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.

Lifetime Monitoring of Electrolytic Capacitor to Maximize Earnings From Grid-Feeding PV System

Electrolytic capacitors are popularly used in single-phase grid-feeding solar photovoltaic (PV) inverters to suppress the second harmonic and switching frequency voltage ripples. With aging equivalent series resistance (ESR) of capacitor increases and its capacitance value decreases, which lead to increase in dc-link voltage ripple. Oscillations in PV operating point around its maximum power point (MPP), results in reduction of average output power and revenue generated. To address this, frequent replacement of capacitors are required, which may lead to increased cost. Therefore, capacitors must be replaced at an optimal period to ensure maximum earnings. To realize this, a technique for monitoring of power extraction efficiency (PEE) is proposed in this paper. Further, criteria for replacement of capacitor based on the measured values of PEE is suggested. Mathematical model relating the capacitance and ESR values to the PEE is derived. Effect of variation in temperature and solar radiation on PEE is discussed. Detailed simulation studies are carried out using MATLAB-Simulink. A scaled down laboratory prototype of inverter is developed. The proposed technique is implemented in the existing digital processor/controller used for MPP tracking, thereby avoiding additional circuits/sensors. PEE estimated by simulation and experimentation are found to be within 1% of each other.

Comparative analysis of power density in Si MOSFET and GaN HEMT based flyback converters

Gallium Nitride (GaN) based power devices have the potential to achieve higher efficiency and higher switching frequency than those possible with Silicon (Si) power devices. In literature, GaN based converters are claimed to offer higher power density. However, a detailed comparative analysis on the power density of GaN and Si based low power dc-dc flyback converter is not reported. In this paper, comparison of a 100 W, dc-dc flyback converter based on GaN and Si is presented. Both the converters are designed to ensure an efficiency of 80%. Based on this, the switching frequency for both the converters are determined. The analysis shows that the GaN based converter can be operated at approximately ten times the switching frequency of Si-based converter. This leads to a reduction in the area product of the flyback transformer required in GaN based converter. It is found that the volume of the flyback transformer can be reduced by a factor of six for a GaN based converter as compared to a Si based converter. Further, it is observed that the value of output capacitance used in the GaN based converter reduces by a factor of ten as compared to the Si based converter, implying a reduction in the size of the output capacitors. Therefore, a significant improvement in the power density of the GaN based converter as compared to the Si based converter is seen.