Hamidreza Jafarian

Design and implementation of distributed control architecture of an AC-stacked PV inverter

A control scheme for a distributed Inverter Molecule™ architecture is analyzed and implemented in this paper. An Inverter Molecule is a building block for a novel distributed inverter architecture suitable for PV/Battery applications which exceeds the performance metrics obtained by Central/String- inverters with or without DC/DC optimizers or by Micro-Inverters. The proposed transformerless architecture utilizes low-voltage semiconductor devices with much higher yields than their high-voltage counterparts. A group of Inverter Molecules operate autonomously and cooperatively to maintain the grid interconnection requirements while providing maximum power control on each molecules PV panel. This paper overviews the state-of the-art control methods implemented mostly on cascaded H-bridge derived topologies and further establishes a distributed control scheme for Inverter Molecule architecture that does not need any high bandwidth communications except a heartbeat signal at line frequency. Experimental results are presented for a lab-scaled prototype.

Development of current measurement techniques for high frequency power converters

Current sensing plays dominant role in power converters, where current information can be used for controlling, monitoring and protection. One of the most challenging goals in modern power electronics converters is to increase switching frequency for the purpose of miniaturization, and improve the performance especially for end-users. Therefore, the need for accurate, lossless, and fast response current sensors is more highlighted. This paper presents a comprehensive review of different current sensing schemes for power electronics applications. The Challenges for implementation of conventional methods for high frequency (>1MHz) and high current converters will be addressed. More specifically, technical issue and hardware difficulties for developing Rogowski-based as well as Hall effect current sensors are discussed and finally, a new technique for improving the performance of Anisotropic Magneto-Resistive (AMR) at 1MHz and 30V with a new technique is shown.

Locking frequency band detection method for grid-tied PV inverter islanding protection

Islanding is an undesired event in which a portion of utility system containing both load and distributed resources remains energized while it is isolated from the remainder of the utility system. In case of islanding, successful detection is required to protect the equipment, and provide safety of the maintenance service workers. This paper shows an implementation of a novel algorithm named Locking Frequency Band Detection (LFBD) method on a distributed PV inverter architecture. Fast detection, easy implementation, no impact on the power quality and zero Non Detection Zone (NDZ) are among many benefits of the proposed algorithm.

Layout study of contactless magnetoresistor current sensor for high frequency converters

In this paper, we present a new technique to unify and intensify the magnetic fields that results in higher performance of Anisotropic Magneto-Resistive (AMR) current sensors and consequently develop a closed loop controller for 100W synchronous GaN buck converter at 1MHz. The closed loop operation of high switching frequency converters at high power has always been a big challenge due to lack of access to current information. The proposed method that also intensifies the magnetic fields through the sensor, significantly improves the bandwidth limits, and reduces electromagnetic interference (EMI) on AMR sensors, making them applicable for high switching frequency and high current power electronics converters. After verifying uniform distribution concept through simulation, we also implemented a prototype of AMR current sensors onto Printed Circuit Board (PCB) for verification of the concept at high frequency converter. We then present the design procedure and associated challenges of an integrated analogue peak current controller for creating the closed loop operation of a GaN buck converter at high switching frequency.

On reactive power injection control of distributed grid-tied AC-stacked PV inverter architecture

In this paper, two different Reactive Power Injection (RPI) methods for a fully distributed PV inverter architecture are investigated. RPI methods are parts of ancillary service requirements for modern PV systems to play a more active role in grid regulation and control in future high penetrated PV generation networks. The main objective of this paper is to demonstrate and test RPI methods on a panel-level AC-stacked PV-inverter string which is controlled with distributed control scheme with minimum communication requirements and propose a combined RPI method. Effectiveness and feasibility of distributed control architecture and RPI methods are verified by experimental results during normal operation and voltage disturbance conditions using a lab-scale PV inverter string setup.

A novel comparative robustness index for evaluating the performance of grid-tied PV inverters

Increasing the integration of PV systems requires more reliable and more robust PV inverter architectures. This paper presents a novel index called Smart Inverter Robustness Index (SIRI) to evaluate the robustness of grid-tied PV inverters. Proposed index considering four major operational characteristics such as efficiency, Total Harmonic Distortion (THD), Maximum Power Point Tracking (MPPT) effectiveness and Power Factor (PF) compliance provides a comparative evaluation of PV inverters. Robustness of a single-phase grid-tied PV inverter under varying manufacturing inaccuracies is analyzed by using SIRI index and a boundary for robust operation is determined. Statistical analysis and Random Latin Hypercube Sampling (RLHS) algorithm are used to model the impact of physical variations of passive components on PV inverter operation with minimum sampling points. Effectiveness and feasibility of proposed index is verified by simulation and experimental results of a single-phase grid-tied PV inverter.

Hybrid Current-/Voltage-Mode Control Scheme for Distributed AC-Stacked PV Inverter With Low-Bandwidth Communication Requirements

This paper shows the feasibility of a novel decentralized control scheme for the grid-tied ac-stacked photovoltaic (PV) inverter architecture. The proposed decentralized control scheme with low-bandwidth communications requirements enables a fully distributed PV inverter architecture. Detailed modeling and theoretical analyses will be provided to support the existence of such decentralized control scheme. Simulation and experimental results on a representative laboratory-scale prototype will be presented under symmetrical and asymmetrical conditions as well as 10% grid voltage sag to show the effectiveness of the proposed control scheme in different operating conditions.

Controller robustness analysis of grid-tied AC-stacked PV inverter system considering manufacturing inaccuracies

This paper analyzes the impact of current sensing inaccuracies due to aging and ambient condition variations on robust operation of PV inverter systems. A novel index for analyzing the robustness of grid-tied AC-stacked PV inverter architectures has been used, which provides an opportunity for comprehensive robustness evaluation by considering four major operational characteristics of grid-tied PV inverters as efficiency, Total Harmonic Distortion (THD), Power Factor (PF) compliance, and Maximum Power Point Tracking (MPPT) effectiveness. In addition, a comparative analysis between a single inverter and an AC-stacked inverter configuration is provided to study the impact of AC-stacking on robust operation of grid-tied PV inverters. This paper also proposes a possible state estimation solution for improving the robustness of the system using Kalman filter approach.

Decentralized Active and Reactive Power Control for an AC-Stacked PV Inverter With Single Member Phase Compensation

This paper proposes a decentralized control scheme for controlling active and reactive power of grid-tied ac-stacked photovoltaic (PV) inverter architecture using single member phase compensation. Reactive power control is required for the next generation of grid-tied smart PV inverter systems in power networks with high PV penetration. The decentralized control scheme proposed in this paper allows for a fully distributed architecture, both in terms of active and reactive power control and physical implementation of a PV system. This will result in higher reliability and potentially lower cost with minimum communications requirements. A decentralized controller enables higher switching frequencies that can shrink passive components. Therefore, string voltage variations due to voltage drop across passive components will be negligible and the system will be controlled with minimum communications requirement. The relative gain array approach has been used to study the feasibility of the decentralized control scheme. Detailed modeling and analysis are provided to show the effectiveness of the proposed decentralized approach and the effect of this approach on control implementation. Finally, effectiveness of the proposed decentralized approach is verified using mathematical modeling, simulation, and a lab-scale experimental setup in different operation conditions.

Analysis of smart inverter functions of decentralized grid-connected AC-stacked PV inverter architecture

The main objective of this paper is to analyze the feasibility of decentralized control approach in providing smart inverter functions for grid-tied AC-stacked PV inverter architecture. Decentralized controller allows fully distributed architecture both in terms of control and physical implementation of a PV system, resulting in higher reliability and potentially lower cost. In this paper, Relative Gain Array concept has been used to determine the best locally measured input-output pairing sets for designing the proposed decentralized controller. Proposed approach by sending supervisory commands to only one inverter minimize the required communications for controlling reactive power and mitigating harmonics. Detailed modeling and analysis are provided to show the feasibility of the proposed decentralized approach. Proposed decentralized control scheme is verified by offline simulation and real-time hardware-in-the-loop setup.