Mitigation of DC Current Injection in Transformerless Grid-Connected Inverters By Weichi Zhang - School of Engineering Newcastle University United Kingdom

 Mitigation of DC Current Injection in Transformerless Grid-Connected Inverters Weichi Zhang B.Sc., M.Sc. A thesis submitted for the degree of Doctor of Philosophy April 2019 School of Engineering Newcastle University United Kingdom

Abstract With a large number of small-scale PV plants being connected to the utility grid, there is increasing interest in the use of transformerless systems for grid-connected inverter photovoltaic applications. Compared to transformer-coupled solutions, transformerless systems offer a typical efficiency increase of 1-2%, reduced system size and weight, and reductions in cost. However, the removal of the transformer has technical implications. In addition to the loss of galvanic isolation, DC current injection into the grid is a potential risk. Whilst desirable, the complete mitigation of DC current injection via conventional current control methods is known to be particularly challenging, and there are remaining implementation issues in previous studies. For this reason, this thesis aims to minimize DC current injection in grid-connected transformerless PV inverter systems. The first part of the thesis reviews the technical challenges and implementation issues in published DC measurement techniques and suppression methods. Given mathematical models, the performance of conventional current controllers in terms of DC and harmonics mitigation is analyzed and further confirmed in simulations and experiments under different operating conditions. As a result, the second part of the thesis introduces two DC suppression methods, a DC voltage mitigation approach and a DC link current sensing technique. The former method uses a combination of a passive attenuation circuit and a software filter stage to extract the DC voltage component, which allows for further digital control and DC component mitigation at the inverter output. It is proven to be a simple and highly effective solution, applicable for any grid-connected PV inverter systems. The DC link sensing study then investigates a control-based solution in which the dc injection is firstly accurately determined via extraction of the line frequency component from the DC link current and then mitigated with a closed loop. With an output current reconstruction process, this technique provides robust current control and effective DC suppression based on DC link current measurement, eliminating the need for the conventional output current sensor. Results from rated simulation models and a laboratory grid-connected inverter system are presented to demonstrate the accurate and robust performance of the proposed techniques. This thesis makes a positive contribution in the area of power quality control in grid-connected inverters, specifically mitigating the impact of DC injection into the grid which has influences on the network operating conditions and the design and manufacture of the PV power converter itself.

LINK: https://theses.ncl.ac.uk/jspui/bitstream/10443/4594/1/Zhang%20W%202019%20%28added%20to%20dspace%20on%20receipt%29.pdf

Analysis, Design, and Control of a Single-Phase Single-Stage Grid-Connected Transformerless Solar Inverter by Manisha Verma - Department of Electrical and Computer Engineering -Concordia University Montreal, Quebec, Canada.

 Analysis, Design, and Control of a Single-Phase Single-Stage Grid-Connected Transformerless Solar Inverter Manisha Verma A Thesis In the Department of Electrical and Computer Engineering Presented in Partial Fulfillment of the Requirements For the Degree of Master of Applied Science at Concordia University Montreal, Quebec, Canada. June 2019

ABSTRACT

 Analysis, Design, and Control of a Single-Phase Single-Stage Grid-Connected Transformerless Solar Inverter Manisha Verma As energy utilization is increasing with the rise in the world’s power demand, the traditional energy sources are depleting at a high pace. It has led to attention drawn towards inexhaustible energy resources. There is a huge augmentation in the power generation from renewable energy sources (RES) like wind, solar, hydropower, biomass, etc. to reduce the stress on conventional energy sources like fossil fuels, oil, gas, etc. There has been a steep increase in interest for wind and solar energy systems. PV energy has been growing swiftly in the past two decades which made it most demanded power generation system based on RES. This worldwide requirement for solar energy has led to an immense amount of innovation and development in the Photovoltaic (PV) market. The Conventional grid-connected PV inverter was either with DC/DC converter or without DC/DC converter. These inverters were isolated using a transformer either on the grid (AC) side as a low-frequency transformer or as a highfrequency transformer on the DC side. Elimination of the transformer leads to a galvanic connection between the grid and PV module. This gives rise to the flow of leakage current which is disastrous for the system when it exceeds a specific value. Thus, minimization of this leakage current after the removal of the transformer has been an interesting topic explored by many researchers. Many topologies have been proposed targeting reduction in this leakage current either by 1.) Directly connecting the PV negative with neutral of utility grid or 2.) Disconnecting the PV panel side from AC side. This generally involved addition of more switches or diodes or supplementary branches to disconnect during the freewheeling period. Generally, the above-mentioned ways lead to a reduction in efficiency due to increased losses or complex circuitry. The motivation of this thesis is to design a transformerless inverter for single-phase PV grid-tied system with a smaller number of devices and still has minimum ground current. It discusses the prevailing inverter topologies in detail and then explains the modes of operation of the proposed inverter. A simple control strategy has been derived and passive elements of the inverter are designed. The simulation results presented have validated the theoretical claims. The experimental results which are similar to simulation results are evidence that the proposed topology is suitable for PV grid-tied systems. Also, the dynamic modeling of the inverter has been done to derive the plant transfer function. Then, the Proportional Resonant (PR) controller has been designed to ensure the flow of sinusoidal current into the grid with zero steady-state error and constant sinusoidal grid voltage irrespective of load change. The simulation and experimental results achieved high performance which makes this topology successful and promising for grid-tied PV systems.

LINK: https://spectrum.library.concordia.ca/985562/1/Verma_MASc_F2019.pdf

Control Design of a Single-Phase DC/AC Inverter for PV Applications by Haoyan Liu - University of Arkansas, Fayetteville

 Control Design of a Single-Phase DC/AC Inverter for PV Applications A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering by Haoyan Liu

Harbin University of Science and Technology Bachelor of Engineering in Automation, 2012 May 2016 University of Arkansas

Abstract 

This thesis presents controller designs of a 2 kVA single-phase inverter for photovoltaic (PV) applications. The demand for better controller designs is constantly rising as the renewable energy market continues to rapidly grow. Some background research has been done on solar energy, PV inverter configurations, inverter control design, and hardware component selection. Controllers are designed both for stand-alone and grid-connected modes of operation. For standalone inverter control, the outer control loop regulates the filter capacitor voltage. Combining the synchronous frame outer control loop with the capacitor current feedback inner control loop, the system can achieve both zero steady-state error and better step load performance. For grid-tied inverter control, proportional capacitor current feedback is used. This achieves the active damping needed to suppress the LCL filter resonance problem. The outer loop regulates the inverter output current flowing into the grid with a proportional resonant controller and harmonic compensators. With a revised grid synchronization unit, the active power and reactive power can be decoupled and controlled separately through a serial communication based user interface. To validate the designed controllers, a scaled down prototype is constructed and tested with a digital signal processor (DSP) TMS320F28335.

LINK:https://core.ac.uk/download/pdf/80559559.pdf

A new high-efficiency single-phase transformerless PV inverter topology Tamás Kerekes, Member, IEEE, Remus Teodorescu, Senior Member, IEEE, Pedro Rodríguez, Member, IEEE, Gerardo Vázquez, Student Member, IEEE, Emiliano Aldabas, Member, IEEE

A new high-efficiency single-phase transformerless PV inverter topology

Tamás Kerekes, Member, IEEE, Remus Teodorescu, Senior Member, IEEE, Pedro Rodríguez, Member, IEEE, Gerardo Vázquez, Student Member, IEEE, Emiliano Aldabas, Member, IEEE

ABSTRACT: There is a strong trend in the photovoltaic (PV) inverter technology to use transformerless topologies in order to acquire higher efficiencies combining with very low ground leakage current. In this paper a new topology, based on the H- Bridge with a new AC bypass circuit consisting in a diode rectifier and a switch with clamping to the DC midpoint is proposed. The topology is simulated and experimentally validated and a comparison with other existing topologies is performed. High conversion efficiency and low leakage current is demonstrated.

I INTRODUCTION Photovoltaic inverters become more and more widespread within both private and commercial circles. These grid connected inverters convert the available direct current supplied by the photovoltaic (PV) panels and feed it into the utility grid. According to the latest report on installed PV power, during 2007 there has been a total of 2.25GW of installed PV systems, out of which the majority (90%) has been installed in Germany, Spain, USA and Japan. At the end of 2007 the total installed PV capacity has reached 7.9 GW of which around 92% is grid connected [1].

There are two main topology groups used in case of grid connected PV systems and they are: with and without galvanic isolation [2]. Galvanic isolation can be on the DC side, in the form of a high frequency DC-DC transformer or on the grid side in the form of a big-bulky AC transformer. Both of these solutions offer the safety and advantage of galvanic isolation, but the efficiency of the whole system is decreased, due to power losses in these extra components. In case the transformer is omitted the efficiency of the whole PV system can be increased with an extra 1-2%. The most important advantages of transformerless PV systems can be observed in Fig. 1, like: higher efficiency, smaller size and weight compared to the PV systems that have galvanic isolation (either on the DC or AC side). 1 Fig. 1 has been made from the database of more than 400 commercially available PV inverters, presented in a commercial magazine about PV systems [3]. Transformerless inverters are represented by the dots (Transformerless), while the triangles represent the inverters that have a low-frequency transformer on the grid side (LF-transformer) and last the stars represent the topologies including a high-frequency DC-DC transformer (HF-transformer), adding a galvanic isolation between the PV and grid. The conclusion drawn from these graphs is that transformerless inverters have higher efficiency, smaller weight and size than their counterparts with galvanic separation. Transformerless PV inverters use different solutions to minimize the leakage ground current and improve the efficiency of the whole system, an issue that has previously been treated in many papers [4]-[11].

LINK: http://seer.upc.edu/material/ficheros_publicaciones/26552ZRV_PV_Inverter.pdf

A Digital Control System for UPS Systems with Smart Grid Capability-Author Mardani Boroujeni, Fatemeh-Faculty Schulich School of Engineering- Institution University of Calgary

 

A Digital Control System for UPS Systems with Smart Grid Capability by Fatemeh Mardani Boroujeni

 

UNIVERSITY OF CALGARY

A THESIS SUBMITTED TO THE FACULTY OF GRADUATE STUDIES IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE GRADUATE PROGRAM IN ELECTRICAL ENGINEERING CALGARY, 

ALBERTA AUGUST, 2019.

  Abstract 

Smart grids have recently become the center of attention for modernizing the grid system. In future smart-grids, energy storage systems are one of the key components, which can complement intermittent renewable energy sources and in turn increase reliability and eciency of the grid system. Modern Uninterruptible Power Supply (UPS) systems can provide storage capacity for future smart grids since they usually include batteries. UPS systems can also provide instant electrical power to sensitive equipment and grid during various events such as brownout, power failures, spikes, voltage surges, EMI/RF noise, and frequency variations.

Future UPS systems require to have much better dynamics in order to deal with transients. The control system of UPS systems mainly determines their dynamical performance and transient response. The existing state-of-the-art UPS control systems are based on linear PI controllers for the most part. Thus, current UPS systems usually show a sluggish transient response and they are not suited for future smart grid applications where instant power is required to maintain the system.

In this thesis, new UPS systems with improved transient response are proposed. The proposed UPS system utilities a new controller that is able to improve the dynamic performance and allows for various smart grid functionalities. The proposed control system is based on the adaptive control theory, which adaptively changes the controller's parameters based on the UPS operating conditions. Furthermore, the proposed control system isolates the double-frequency ripple from the battery in the normal/charging mode as well as in the backup/discharging mode. Therefore, the new UPS system is well-suited for single-phase systems utilizing lithium-ion battery as storage. Mathematical analysis, simulation, and experimental results are presented to verify the performance of the proposed control system and demonstrate its superior performance.

LINK:https://prism.ucalgary.ca/bitstream/handle/1880/110715/ucalgary_2019_mardaniboroujeni_fatemeh.pdf?sequence=2&isAllowed=y