Design of High-Gain DC-DC Converters for High-Power PV Applications

Renewable energy sources are penetrating the market in an ever increasing rate, especially in terms of Wind and Solar energies, with the latter being more suitable for the GCC region. Typically, Photovoltaic (PV) strings’ output voltage is limited to ~ 1500 V due to safety constraints, and thus requires boosting to higher DC levels (non-isolated step-up DC-DC transformer) suitable for High-Voltage DC (HVDC) and AC grid applications in order to provide the required DC-Link voltage level. Nevertheless, conventional non-isolated DC-DC converters provide a limited practical gain due to their parasitic elements. Other options include isolated DC-DC converters that utilize costly high-frequency transformers with limited power capability. Moreover, the isolation requirements of transformers in HVDC significantly increase the footprint of the converters. High-frequency transformers for high-power applications are hard to design and are usually associated with higher losses. Alternatively, connecting conventional DC-DC converters in different combinations can provide higher gains to the required levels, while maintaining the high efficiency requirements. This thesis proposes the cascade and/or series connection of DC-DC modules as a solution to the high-conversion ratio requirement, based on Cuk and Single-Ended Primary Inductor Converter (SEPIC) topologies, whose continuous input current is suitable for PV applications, and reduces the bulky capacitor filters at the input side. Detailed theoretical models of the proposed topologies are first derived, then their trends are practically verified by low power prototypes. Sensitivity analysis is also performed to assess the effect of small variations to the parasitic inductors’ resistances on the overall system gain, where the input inductor is found to have a considerable effect, especially at higher duty ratios (i.e. higher gains). High-power applications’ scenarios with their considerations are simulated to compare the different topologies and the results show a comparable efficiency of the proposed converters for a 1 –MW application with efficiencies higher than 90%

HVDC transmission : technology review, market trends and future outlook

HVDC systems are playing an increasingly significant role in energy transmission due to their technical and economic superiority over HVAC systems for long distance transmission. HVDC is preferable beyond 300–800 km for overhead point-to-point transmission projects and for the cable based interconnection or the grid integration of remote offshore wind farms beyond 50–100 km. Several HVDC review papers exist in literature but often focus on specific geographic locations or system components. In contrast, this paper presents a detailed, up-to-date, analysis and assessment of HVDC transmission systems on a global scale, targeting expert and general audience alike. The paper covers the following aspects: technical and economic comparison of HVAC and HVDC systems; investigation of international HVDC market size, conditions, geographic sparsity of the technology adoption, as well as the main suppliers landscape; and high-level comparisons and analysis of HVDC system components such as Voltage Source Converters (VSCs) and Line Commutated Converters (LCCs), etc. The presented analysis are supported by practical case studies from existing projects in an effort to reveal the complex technical and economic considerations, factors and rationale involved in the evaluation and selection of transmission system technology for a given project. The contemporary operational challenges such as the ownership of Multi-Terminal DC (MTDC) networks are also discussed. Subsequently, the required development factors, both technically and regulatory, for proper MTDC networks operation are highlighted, including a future outlook of different HVDC system components. Collectively, the role of HVDC transmission in achieving national renewable energy targets in light of the Paris agreement commitments is highlighted with relevant examples of potential HVDC corridors

Innovative energy management system for MVDC networks with black-start capabilities

Medium voltage DC (MVDC) networks are attracting more attention amid increased renewables penetration. The reliability of these DC systems is critical, especially following grid contingencies to maintain critical loads supply and provide ancillary services, such as black-start. This paper proposes an innovative energy management system (EMS) to maintain reliable MVDC network operation under prolonged AC grid contingencies. Similar EMS designs in literature tend to focus on limited operating modes and fall short of covering comprehensive elongated blackout considerations. The proposed EMS in this paper aims to preserve the distribution network functionality of the impacted MVDC system through maintaining a constant DC bus voltage, maximizing critical load supply duration, and maintaining the MVDC system black-start readiness. These objectives are achieved through controlling generation units between Maximum Power Point Tracking (MPPT) and Voltage Regulation (VR) modes, and implementing a smart load shedding and restoration algorithm based on network parameters feedback, such as storage State of Change (SoC) and available resources. Practical design considerations for MVDC network participation in AC network black start, and the following grid synchronization steps are presented and tested as part of the EMS. The proposed system is validated through simulations and scaled lab setup experimental scenarios

Performance evaluation of four grid-forming control techniques with soft black-start capabilities

Grid-Forming Converters (GFC) can be controlled as independent, self-starting, voltage sources. This feature is essential for power converters to achieve successful black-start sequence initiation. Conventional grid-following converters are not capable of self-starting an islanded network. GFC control thus exploits wider grid support and network restart potential. This study analyzes and compares four GFC controllers to assess their generic and soft black-start (ramping voltage) capabilities. The compared techniques are: Droop Control, Power Synchronizing Control (PSC), Virtual Synchronous Machine (VSM), and Matching control. These techniques are selected based on their direct voltage reference control flexibility. Various simulations are performed with common parameters to assess the response of each technique under similar conditions against load, DC voltage and active power reference disturbances, in addition to their soft-start readiness. The results demonstrate the high-level compatibility of these four controllers with soft black-start through successful and timely ramping voltage reference tracking. Moreover, the four considered control techniques achieve satisfactory performance, with VSM demonstrating more flexibility due to its tunable virtual inertia parameter (J)

Modified grid-forming converter control for black-start and grid-synchronization applications

The increasing grid integration of converter- based generation and distributed resources is necessitating a review to the classical synchronous generators dominant network models, and to equip converters with the necessary control capabilities such as inertia emulation and black-start to carry on the required ancillary services provision. This paper proposes a modified grid-forming converter control technique based on virtual synchronous machine (VSM). The modified controller uses a ramping voltage reference to achieve soft transformer energization and eliminate high-magnitude inrush current that can traditionally exceed converters rating. Grid-synchronizing functionality is added to the VSM controller to achieve smooth transition from black-start to grid-connected mode while maintaining grid-forming operation. This is achieved by gradually adjusting the power loop reference through a dedicated synchronizing controller, which is disconnected once grid-connection is achieved. A MATLAB/ Simulink case study is presented to illustrate the controller performance with a 40 MVA simulated converter. The results demonstrate successful black-start sequence execution, starting by soft transformer energization, followed by a block load pickup and grid-synchronization. The sequence is achieved with minimal transformer inrush current, and with seamless transition to the grid-connected operating mode

Black-start provision from grid-forming converters : a new network restoration paradigm

Black-start service has been long associated with conventional power plants such as large synchronous generators. Given the rising penetration of converter-based resources such as solar, wind and battery storage, this thesis addresses black-start provision from the power electronic-based grid-forming converters (GFCs). Key challenges for converters black-start utilization can be attributed to their input resources intermittency, limited overcurrent capabilities against high network energization inrush currents, in addition to identifying suitable controllers for reliable GFC operation and black-start compatibility. To tackle these points, the investigations in this thesis begin by proposing an innovative energy management system (EMS) that aims to maintain the converter input DC side reliable operation for prolonged periods, especially when a GFC is interfaced to a DC network consisting of multiple resources. The EMS is validated through simulations and a scaled lab setup. Then, the high transformer energization inrush currents are addressed through analyzing techniques that require direct GFC control manipulation such as soft energization (SE), against classical methods such as controlled switching (CS). Detailed theoretical transformer models are derived to quantify inrush current influencing factors. A new SE voltage ramp-rate estimation framework is then introduced, given the arbitrary ramp-rates definition in the literature. These techniques are tested in a detailed case study, where a GFC is used to energize a large network consisting of multiple transformers, and under various sensitivity scenarios. Four GFC controllers are benchmarked to assess their SE and black-start compatibility, namely: droop, power synchronizing control (PSC), virtual synchronous machine (VSM), and matching control. VSM grid-forming control is selected based on the comparison, and modifications to its classical form are proposed to improve its black-start compatibility such as voltage support and grid-synchronization. The modified controller is validated in complete black-start scenarios, through simulation and novel power hardware in the loop (PHiL) experiments that enable testing hardware converters to energize and synchronize to simulated networks in digital real-time simulation (DRTS) platforms. Overall, the presented in-depth analysis and investigations aim to provide thorough insights to researchers and industrial engineers on black-start feasibility from GFCs.Black-start service has been long associated with conventional power plants such as large synchronous generators. Given the rising penetration of converter-based resources such as solar, wind and battery storage, this thesis addresses black-start provision from the power electronic-based grid-forming converters (GFCs). Key challenges for converters black-start utilization can be attributed to their input resources intermittency, limited overcurrent capabilities against high network energization inrush currents, in addition to identifying suitable controllers for reliable GFC operation and black-start compatibility. To tackle these points, the investigations in this thesis begin by proposing an innovative energy management system (EMS) that aims to maintain the converter input DC side reliable operation for prolonged periods, especially when a GFC is interfaced to a DC network consisting of multiple resources. The EMS is validated through simulations and a scaled lab setup. Then, the high transformer energization inrush currents are addressed through analyzing techniques that require direct GFC control manipulation such as soft energization (SE), against classical methods such as controlled switching (CS). Detailed theoretical transformer models are derived to quantify inrush current influencing factors. A new SE voltage ramp-rate estimation framework is then introduced, given the arbitrary ramp-rates definition in the literature. These techniques are tested in a detailed case study, where a GFC is used to energize a large network consisting of multiple transformers, and under various sensitivity scenarios. Four GFC controllers are benchmarked to assess their SE and black-start compatibility, namely: droop, power synchronizing control (PSC), virtual synchronous machine (VSM), and matching control. VSM grid-forming control is selected based on the comparison, and modifications to its classical form are proposed to improve its black-start compatibility such as voltage support and grid-synchronization. The modified controller is validated in complete black-start scenarios, through simulation and novel power hardware in the loop (PHiL) experiments that enable testing hardware converters to energize and synchronize to simulated networks in digital real-time simulation (DRTS) platforms. Overall, the presented in-depth analysis and investigations aim to provide thorough insights to researchers and industrial engineers on black-start feasibility from GFCs

Modelling of DC-DC converters with continuous input current for high power PV applications

Renewable Energy sources integration to the grid is becoming more vital due to the market share increase. The integration of PV farms, for instance, usually requires a DC-DC conversion stage to boost up the source voltage. In addition, DC-DC converters with continuous input current reduce the bulky input capacitor requirements. This saves the cost, and enhances the reliability of the system since these capacitors exhibit short life time. This paper presents a complete mathematical modeling of two appropriate converters for such application, namely the Cuk and SEPIC DC-DC converters. The parasitic non-idealities are included to account for gain deterioration at high duty cycles, which is a vital point in high power applications especially when high gains are required. The performance of both converters is then compared and their small signal models are derived and presented in terms of both the input voltage and duty cycle, which provides an additional path for performance assessment. The theoretical models are verified practically. 2016 IEEE.Scopu

Quartz Crystal Microbalance Electronic Interfacing Systems: A Review

Quartz Crystal Microbalance (QCM) sensors are actively being implemented in various fields due to their compatibility with different operating conditions in gaseous/liquid mediums for a wide range of measurements. This trend has been matched by the parallel advancement in tailored electronic interfacing systems for QCM sensors. That is, selecting the appropriate electronic circuit is vital for accurate sensor measurements. Many techniques were developed over time to cover the expanding measurement requirements (e.g., accommodating highly-damping environments). This paper presents a comprehensive review of the various existing QCM electronic interfacing systems. Namely, impedance-based analysis, oscillators (conventional and lock-in based techniques), exponential decay methods and the emerging phase-mass based characterization. The aforementioned methods are discussed in detail and qualitatively compared in terms of their performance for various applications. In addition, some theoretical improvements and recommendations are introduced for adequate systems implementation. Finally, specific design considerations of high-temperature microbalance systems (e.g., GaPO4 crystals (GCM) and Langasite crystals (LCM)) are introduced, while assessing their overall system performance, stability and quality compared to conventional low-temperature applications

Transformer inrush current mitigation techniques for grid-forming inverters dominated grids

The use of inverters-based resources (IBRs) is rising rapidly in power networks due to increased renewable energy penetration. This requires revisiting of classical network operation standards. For instance, high transformer energization inrush current has been studied extensively under the classical network paradigm. Whereas this paper investigates transformers' energization techniques in the context of inverters dominated grids, where inverters with limited short-circuit current are expected to utilize their grid-forming capabilities for black-start. Common transformer energization techniques such as controlled switching and soft energization are first analyzed with a new perspective aiming to assess their feasibility when used with grid-forming inverters and existing network assets. Parameters influencing soft energization voltage ramp-up time (Tramp) are investigated, and a new Tramp estimation framework for transformer energization from IBRs is introduced. Due to the variability of available point-on-wave circuit breakers (CBs) in distribution networks, controlled energization using single-pole and three-pole CBs is investigated for various configurations and their application limits are identified. A comprehensive case study is then presented using a test network with multiple transformers to benchmark the performance and requirements of each technique when the network is energized from an IBR, followed by a set of practical recommendations

New Fast Arctangent Approximation Algorithm for Generic Real-Time Embedded Applications

Fast and accurate arctangent approximations are used in several contemporary applications, including embedded systems, signal processing, radar, and power systems. Three main approximation techniques are well-established in the literature, varying in their accuracy and resource utilization levels. Those are the iterative coordinate rotational digital computer (CORDIC), the lookup tables (LUTs)-based, and the rational formulae techniques. This paper presents a novel technique that combines the advantages of both rational formulae and LUT approximation methods. The new algorithm exploits the pseudo-linear region around the tangent function zero point to estimate a reduced input arctangent through a modified rational approximation before referring this estimate to its original value using miniature LUTs. A new 2nd order rational approximation formula is introduced for the first time in this work and benchmarked against existing alternatives as it improves the new algorithm performance. The eZDSP-F28335 platform has been used for practical implementation and results validation of the proposed technique. The contributions of this work are summarized as follows: (1) introducing a new approximation algorithm with high precision and application-based flexibility; (2) introducing a new rational approximation formula that outperforms literature alternatives with the algorithm at higher accuracy requirement; and (3) presenting a practical evaluation index for rational approximations in the literature