Madhav Manjrekar

Dynamic analysis and controller design for a center-point-clamped ac-ac converter

In this paper, a novel topology based on center-point-clamped circuit configuration for ac-ac power conversion has been presented. This converter introduces a unique methodology to clamp the voltage to the mid-point of the input transformer thus reducing voltage blocking requirements on the bi-directional switches employed in such direct ac-ac converter. Operating principles as well as dynamic analysis for the proposed converter have been presented in the paper. A feedback controller has been designed for closed loop control of the proposed center-point-clamped ac-ac converter. Simulation results presented in the paper verify that the proposed converter offers high power transfer efficiency compared to conventional ac-dc-ac converters, and smaller output voltage ripple along with reduced voltage stress on the bi-directional switches when compared to other direct ac-ac converters.

State space analysis and duty cycle control of a switched reactance based center-point-clamped reactive power compensator

A new approach for static reactive power compensation has been presented in this paper. Conventional thyristor controlled Static VAr Compensators (SVC) have inherent disadvantages like slow response times and poor harmonic performance as these FACTS devices are based on slow switching power electronics devices. Alternately, Pulse Width Modulated (PWM) dc-ac inverters and direct ac-ac converter structures offer higher bandwidth and push the spectral content to higher switching frequencies that are easier to filter. However, the application space of this approach is limited by the low voltage blocking capability of power devices employed in these converters. A center-point-clamped ac-ac direct power converter has been reported recently in literature which operates on the principle of neutral-point-clamped dc-ac inverter. By clamping the grid voltage to its mid-point, the center-point-clamped converter structure reduces voltage stress on the bi-directional switches by 50%. Compared to the conventional two-level and multilevel dc-ac inverters, the proposed compensator based on direct ac-ac conversion has a simpler structure and control. The operating principle as well as dynamic analysis for the proposed VAr compensation approach has been presented in the paper. A feedback controller has been designed for closed loop control. Simulation results presented in the paper verify that proposed converter offers better control of reactive power, retrofit capability, and reduced voltage stress on the bi-directional switches. Furthermore, it has been shown that leading and lagging reactive compensation can be accomplished with a smooth control of the reactance through duty cycle modulation.

Control strategies for solar panel companion inverters

This paper investigates a unique methodology that converts the conventional dc voltage output of a photovoltaic solar panel to switched quasi-square wave voltages with variable pulse width, which when aggregated realize a superior quality multilevel waveform that can be directly interfaced with the power grid. Termed Solar Panel Companion Inverters, this alternative approach offers advantages of micro-inverters in realizing panel-level maximum power point transfer and realization of system-level cost benefits of a central inverter approach. Various control strategies such as Unsorted Pulse Width Modulation, Sorted Stair Case Modulation and Sorted Pulse Width Modulation for these Solar Panel Companion Inverters are presented and analyzed in the paper. These control schemes are evaluated against a variety of solar irradiance operating scenarios with varying co-efficient of variation and varying mean irradiance levels. The resultant energy yields are documented for each control scheme. It is shown that the proposed structure of Solar Panel Companion Inverters is able to collectively perform as a conventional central inverter when irradiance conditions are uniform. However, under non-uniform irradiance conditions, proposed structure surpasses performance of central inverter in terms of power throughout and can be a commercially competitive solution when compared to microinverters for interfacing solar panels with the power grid.

Center-point-clamped AC-AC buck-boost harmonic compensator based power quality device

A novel approach for mitigation of power quality issues have been presented in the paper. This paper introduces a Center-Point-Clamped AC-AC Buck-Boost Converter based power quality device for industrial and residential customers at the leaf end of the distribution network. This converter utilizes the principle of center-point-clamping to reduce the voltage stress on the power semiconductor switches. It can buck, boost and eliminate unwanted line frequency harmonics from the input voltage by controlling the duty ratio of the switches, thereby providing desired voltage at the customer side with minimal THD. A closed loop strategy has also been presented in the paper. This feedback regulator improves the steady state and transient performance of the converter. Simulation results displayed in the paper verify the feasibility and effectiveness of the proposed power quality device in mitigation of power quality issues like voltage sag, voltage swell and harmonics at multiple order of line frequency, have been displayed in the paper. A small scale experimental prototype rated at 120V, 1-kW has been constructed in the laboratory. Experimental results further corroborate the theoretical and simulated analysis, and demonstrate the merits of the proposed approach.

Design of a center-point-clamped AC-AC converter based power-line conditioner

A new approach for power line conditioning has been presented in the paper. State of the art line-conditioning devices like Static Voltage Restorer (SVR), Dynamic Voltage Restorer (DVR) and AC-AC power line conditioner offer higher bandwidth and low filter size requirement due to their high switching frequencies. However, commercial utilization of such devices is limited by the voltage blocking capability of power semiconductor devices employed in these converters. Recent literature reports a center-point-clamped converter topology in which the grid voltage is clamped to its midpoint to reduce the voltage stress on the bi-directional switches in the converter by 50%. This paper introduces a line conditioner based on the center-point-clamped ac-ac converter topology. A duty cycle control scheme has also been implemented for feedback control of the line conditioner. Simulation results verify that proposed line conditioner offers superior spectral content, high power transfer efficiency and reduced voltage stress on the bi-directional switches. Furthermore, the control scheme has been shown to provide up to 10% load voltage regulation under dynamic line voltage and loading conditions. Detailed experimental results from a scaled down laboratory prototype rated 500 W, verifying the operation and closed loop control of the proposed power line conditioner have been presented in this paper.

A grid connected PV micro-inverter with optimized battery storage utilization

Photovoltaic (PV) installations on the residential scale have been increasing tremendously and projected to continue to do so. Battery energy storage at the residential level has also become critical due to the increased adoption of residential scale PV. This paper proposes a new micro-inverter topology with integrated energy storage for PV applications. The proposed topology has a structure similar to that of a flying capacitor multilevel inverter, with reduced switch count. This provides advantages such as design simplicity while being more cost effective than existing topologies. A unique control scheme is also proposed in this paper, which mitigates PV intermittencies while also optimizing battery utilization. The proposed topology and control technique are verified by simulation for a residential scale micro-inverter rated 230 W. The simulation results validate the micro-inverter operation and the control methodology.

Modeling and analysis of an ePFC (enhanced power flow controller) with conduction angle control

Series Flexible AC Transmission Systems (FACTS) devices have been employed to increase power transfer capability of transmission networks and to provide direct control of power flow over designated transmission routes. However, high costs and reliability concerns associated with implementing one large FACTS device capable of altering the power flow in a wide transmission network have limited widespread deployment of FACTS solutions. Recently, concept of Distributed FACTS (D-FACTS) was proposed as an alternative approach to realize cost-effective power flow control through multiple, small, fixed series impedance injections. This paper extends the functionality of D-FACTS concept by introducing variability in impedance injection of D-FACTS devices, thereby improving their controllability. Furthermore, this paper also presents a more detailed analytical treatment of such a topology termed enhanced Power Flow Controller (ePFC). It is shown that employing 1st order (assume s sinusoidal voltage across compensation capacitor) and 2nd order (assumes sinusoidal current in the transmission line) fundamental impedance model are inaccurate methods to analyze effective impedance inserted by ePFC. Instead, a new mathematical model that is based on sinusoidal voltage difference between two end buses is proposed. The efficacy of this approach and its advantages as compared to existing models are presented.

Maximum power point tracking for solar panel companion inverters

Solar Panel Companion Inverter (SPCI) has been proposed as an H-bridge inverter embedded with a PV module. Each inverter converts the DC voltage output of the module, to a quasi-square wave voltage with variable pulse width. When these voltages are aggregated across multiple panels connected in a series string, they synthesize an AC waveform that can be interfaced with the power grid. The inverter functionality is distributed along the string, with no microinverter, no central inverter, and no voltage boosting required. In this paper, a new Maximum Power Point Tracking (MPPT) algorithm for Solar Panel Companion Inverters (SPCI) is proposed. This MPPT algorithm implements maximum power extraction at a system level. Also, closed loop current control scheme for a grid connected SPCI is implemented. Simulation results demonstrating the working of the closed loop current control and maximum power point tracking algorithm, for various operating scenarios are presented.

Synthetic inertia for BESS integrated on the DC-link of grid-tied PV inverters

The significant mechanical inertia of the rotor in a synchronous generator is crucial for facilitating the cooperative grid forming capability of multiple such generators on the transmission and distribution network. This paper demonstrates that the DC link capacitance in a PV inverter is the analogous form of inertia for such a system, albeit having much smaller magnitude. Furthermore it proposes using storage integrated on the DC link to synthesize extra inertia by programming the storage power electronics controller to achieve an emulated capacitance. Inertia added to distributed energy resources (DERs) in this manner is the first step towards mitigating the power quality issues arising from increased renewable penetration on the network. Demonstration of the acquired inertia dynamics and verification of the required controller design is shown through a detailed simulation model.

A novel system architecture to extract maximum energy from photo voltaic modules

Photovoltaic energy is one of the key renewable energy resources of present day. Optimal usage of maximum power available from photovoltaic panels still remains an engineering challenge. Focusing on this issue, a novel photovoltaic power generation system based on a low voltage DC generation with panel level MPPT based on look up table, medium frequency square wave AC link, medium frequency transformer, cyclo-converter based medium frequency to line frequency conversion and three phase grid integration is proposed in this paper. Each PV panel is connected to a boost converter to achieve MPPT and these systems are connected to the same DC bus followed by a medium frequency (400–500 Hz) single phase inverter. The output of the inverter is fed to the medium frequency transformer which provides isolation and also the high voltage square wave on the secondary side. A 3-phase cyclo-converter controlled by a dedicated switching pattern is connected to the secondary of the transformer which unfolds the medium frequency into line frequency and the output of cyclo-converter is connected with grid via line inductor. The design, analysis and simulation via MATLAB/Simulink presenting the efficacy of the proposed topology are the topics of discussion of this article.