Emre Kantar

Control strategies for grid connected PWM-VSI systems

This paper evaluates the current control methods for three-phase voltage-source-inverters (VSIs) connected to the grid via LCL-filters. Current feedback methods from both line and converter sides are compared and theoretical analysis is stated. The analysis is based on the existence of a critical resonant frequency dividing the controlling frequency interval into two regions; namely low and high resonant frequency regions. The feedback types and damping methods are evaluated for both low and high resonant frequency regions with regard to resonant and low frequency poles of the system. In the low resonant frequency region, damping of LCL-filter resonance is necessary for line current feedback; while the current controller provides inherent damping when converter current feedback is utilized. Moreover, total harmonic distortion (THD) and power factor performance of the designed system employing either line or converter current feedback is assessed under varying load conditions. Besides, thorough theoretical approaches supported with simulation results are presented.

Design and performance analysis of a grid connected PWM-VSI system

Pulse-width modulation (PWM) voltage source inverters (VSIs) are favorable interface devices to the power grid for renewable energy systems. This paper deals with the design of the LCL-filter and inverter for the grid-connected VSI. A complete design procedure for both reactive and passive components of LCL-filter is demonstrated with a new iterative approach in selection of the filter inductors. The design process for a two-level VSI is clearly illustrated through an example and the dynamic response is investigated under insufficient and sufficient damping cases. In this study, comprehensive analyses have been conducted to maximize the performance and efficiency. For this purpose, the line current total harmonic distortion (THD) and power factor performance of the designed system is assessed under various load conditions. Besides, the effects of the utilization of different PWM patterns on efficiency are compared and contrasted under different loads with altering switching frequencies. Simulation results validate theoretical findings throughout the design phase.

Optimal Design of Grid-Connected Voltage-Source Converters Considering Cost and Operating Factors

This paper assesses the feasibility of three-phase pulse-width modulated (PWM) three-level voltage-source converters (VSC), namely neutral-point-clamped (3L-NPC) and T-type (3L-T) as alternatives to two-level converter (2L-VSC) for low-voltage multimegawatt renewable energy grid-connected converter applications. For this purpose, a novel design algorithm that takes switching frequency, capacity factor, modulation index, PWM scheme(s), and converter topology(ies) as input is unveiled to provide the design steps and performance benchmark for the specified topologies and modulation methods. Design of the LCL filter is also addressed, and equivalent output ripple performance criteria is used to benchmark the converter performances for a set of selected filter parameters. Main contribution of this algorithm is that it covers cost and operating factors of the design during the hardware design phase by introducing/modifying the terms of total cost of ownership (TCO) and return on investment (ROI) time considering the major energy markets. The design is detailed for 1-MVA system, and the results are summarized for 0.5 and 2-MVA systems. The study shows that 3L topologies perform more efficiently than 2L-VSC (T-type being the most prominent-with the shortest ROI), whereas the difference becomes much more prominent at the multimegawatt range, provided that all three yield equivalent outputs fulfilling the stringent grid codes.

Factors influencing the tangential AC breakdown strength of solid-solid interfaces

The combination of two solid dielectrics (interface) increases the risk of formation of microscopic cavities reducing the breakdown strength (BDS) of the interface considerably, particularly when the electric field has a tangential component. The main purpose of this paper is to investigate the impact of the applied contact pressure and composite elastic modulus on the tangential ac BDS of the solid-solid interfaces experimentally. In the experiments, three different contact pressures were applied using different mechanical loads with two different materials having different elastic moduli, i.e. cross-linked polyethylene (XLPE) and silicon rubber (SiR). Two rectangular prism shaped samples were placed between two vertical Rogowski shaped electrodes either in air or oil. The type of the interface (air/oil) is highlighted duly upon showing the results. Increase in contact pressure caused relatively higher increase in the tangential BDS of dry SiR-SiR (assembled in air) than that of XLPE-XLPE, revealing that elastic modulus facilitated significantly to reduce the mean void size in SiR that in turn improved the tangential BDS. Likewise, the tangential BDS of hybrid interfaces formed by XLPE-SiR specimens increased by 43% compared to that of XLPE-XLPE interface at the same pressure. Additionally, the same set of experiments assembled in oil reveals that the presence of oil enhanced the tangential BDSs around 2-3 times for all three-interface cases. Moreover, with the increase of applied pressure the tangential BDS of air-filled and oil-filled cavities tended to get significantly higher.

LCL-filter design for low-voltage high-power grid-tied voltage-source converter considering various damping methods

The limited switching frequency, size and weight concerns, and the stringent limits for the injected grid current harmonics challenge the implementation of LCL grid filters for low-voltage multi-megawatt (multi-MW) renewable energy converters. Traditional design procedures of such filters employed in low power and high switching frequency converters may not hold for a multi-MW filter interfacing a low-voltage converter switching at low frequency to the electric grid. This paper proposes an LCL-filter design algorithm spanning a wide power range of multi-MW converters where impacts of favored current control method, damping method and pulse-width modulation (PWM) technique are reflected within the design phase. Filter design algorithm was tested under 0.5, 1, and 2-MVA applications. The impacts of designated damping and current control methods were elaborated in terms of dynamic and steady-state performance using 1-MVA converter design case.

Modeling longitudinal breakdown strength of solid-solid interfaces using contact theory

The main purpose of this paper is to explore the impact of applied contact pressure, surface roughness and elastic modulus on the AC tangential breakdown strength (BDS) of dry and wet-mate solid-solid interfaces both experimentally and theoretically. Experiments were performed using samples made of cross-linked polyethylene (XLPE) and silicone rubber (SiR) at three different contact pressures. The average diameters of interface voids were estimated using results from surface roughness measurements combined with contact theory. The expected breakdown values were assumed to be determined by the gas discharges and to be proportional to the real contact area in case of dry and wet assembly, respectively. Both theory and experiments showed increasing tangential breakdown values with increasing contact pressure in case of both dry and wet-mate interfaces. The effect of reducing the elasticity modulus is to shrink the void size and increase the real contact area of the interface. The breakdown strength of interfaces between soft SiR materials was found to be 3-4 times higher than that of the harder XLPE materials.

Interfacial breakdown between dielectric surfaces determined by gas discharge

This paper examines the influence of the elastic modulus and applied contact pressure on the tangential AC breakdown strength (BDS) of polymer solid-solid interfaces theoretically and experimentally. In the experiments, three different materials with different elastic moduli, namely cross-linked polyethylene (XLPE), cured end product of epoxy resin (EPOXY), and polyether ether ketone (PEEK) were employed under various contact pressures. The BDS of each interface increased as the contact pressure was augmented. As the contact pressure became threefold, the interfacial BDS rose by a factor of 2.4, 1.7, and 1.8 in the case of the PEEK, EPOXY, and XLPE interface in a sequence following the decrease of the elastic modulus. Under the same contact pressure, it was observed that the lower the elastic modulus, the higher the BDS. The proposed theoretical approach tested two different mechanisms in determining the gas pressure inside the cavities. Both mechanisms suggested decreasing BDS values as the elastic modulus was augmented; however, the estimated results deviated widely from the experimental data as the pressure was significantly increased in the case of first proposed mechanism whereas the second mechanism correlated with the experimental data much better.

Longitudinal AC breakdown voltage of XLPE-XLPE interfaces considering surface roughness and pressure

The interfacial breakdown between two dielectric surfaces has been reported to represent one of the principal causes of failure for power cable joints and connectors; thus, a better understanding of interfacial breakdown mechanisms is vital. The primary purpose of this paper is to investigate the influence of the surface roughness and interfacial pressure on the tangential AC breakdown strength (BDS) of solid-solid interfaces experimentally. The three-dimensional surface texture parameters are utilized to characterize the morphology of the surfaces. Experiments were performed using samples made of cross-linked polyethylene (XLPE) at three different contact pressures. The surface roughness was varied by polishing the surfaces using four different sandpapers of different roughness. Each surface topography was then assessed using a 3-D optical profilometer. Next, the samples were assembled under ambient laboratory conditions. The experimental results showed a good correlation between the tangential BDS and the surface roughness. The results suggested that reducing the surface roughness resulted in decreased mean height of the surface asperities by nearly 97% and increased the real contact area of the interface considerably. As a result, the tangential BDS rose by a factor of 1.85–2.15 with increasing pressure. Likewise, the increased contact pressure yielded augmented tangential BDS values by a factor of 1.4–1.7 following the decrease of the roughness.

On the tangential AC breakdown strength of polymer interfaces considering elastic modulus

The interfacial breakdown between two dielectric surfaces was reported to represent one of the leading causes of failure for power cable joints and connectors, in which elastic modulus of the dielectric material plays a key role. The primary motivation of this paper is to study the influence of the elastic modulus of the polymer insulation on the tangential AC breakdown strength (BDS) of polymer interfaces experimentally. In the experiments, four different materials with different elastic moduli were employed under various contact pressures: polyether ether ketone (PEEK), cured end product of epoxy resin (EPOXY), cross-linked polyethylene (XLPE), and silicone rubber (SiR). The BDS of each interface increased as the contact pressure was augmented. As the contact pressure became threefold, the interfacial BDS rose by a factor of 2.4, 1.7, 1.8, and 1.4 in the case of the PEEK, EPOXY, XLPE and SiR interface, in a sequence following the decrease of the elastic modulus. Under the same contact pressure, it was observed that the lower the elastic modulus, the higher the BDS.

Effect of pressure and elastic modulus on tangential breakdown strength of solid-solid interfaces

Breakdown strength (BDS) of the interface between the two solid dielectrics is much lower than the BDS of the bulk materials due to formation of microscopic cavities at the interface. The main motivation of this paper is to explore the impact of the applied contact pressure and composite elastic modulus on the AC tangential BDS of dry-mate, wet-mate and oil-mate solid-solid interfaces experimentally. In the experiments, two different contact pressures were applied using different mechanical loads with two different materials having different elastic moduli, i.e. cross-linked polyethylene (XLPE) and silicon rubber (SiR). Two rectangular prism shaped samples were placed between two vertical Rogowski shaped electrodes in air, water or oil. Field simulations were also performed on the test objects to assess the electric field distribution and they show a good agreement with the experiments. The experiments show that the elastic modulus, contact pressure and void-filling medium (air/water/oil) have a significant impact on the tangential BDS.