Convex Optimization of PV-Battery System Sizing and Operation with Non-Linear Loss Models

In the literature, when optimizing the sizing and operation of a residential PV system in combination with a battery energy storage system, the efficiency of the battery and the converter is generally assumed constant, which corresponds to a linear loss model that can be readily integrated in an optimization model. However, this assumption does not always represent the impact of the losses accurately. For this reason, an approach is presented that includes non-linear converter and battery loss models by applying convex relaxations to the non-linear constraints. The relaxed convex formulation is equivalent to the original non-linear formulation and can be solved more efficiently. The difference between the optimization model with non-linear loss models and linear loss models is illustrated for a residential DC-coupled PV-battery system. The linear loss model is shown to result in an underestimation of the battery size and cost as well as a lower utilization of the battery. The proposed method is useful to accurately model the impact of losses on the optimal sizing and operation in exchange for a slightly higher computational time compared to linear loss models, though far below that of solving the non-relaxed non-linear problem.Comment: submitted to Applied Energ

Performance comparison between SiC and Si inverter modules in an electrical variable transmission application

This paper evaluates the performance of Silicon Carbide MOSFET and Silicon IGBT modules in a threephase inverter for Electrical Variable Transmission systems. For this purpose, two practical inverter setups were developed and compared. An increase of several percentage points is visible over the entire operating range for the Silicon Carbide prototype. The total energy efficiency increased by 3.7% for the rotor and by 11.2% for the stator, for the same test conditions

Modeling and Simulation of Power Electronic Circuits using Piecewise Smooth Differential Algebraic Equations

In the fields of power electronics and power systems, the modeling and simulation of switching converter circuits is often performed using idealized switch models instead of detailed semiconductor models. For such circuits, conventional circuit simulation methods are not directly applicable because the resulting differential algebraic equations (DAEs) are non-smooth and possibly discontinuous. Therefore, most existing power electronic circuit simulation tools reconfigure the circuit equations after each switch event so that the resulting DAE can be considered as smooth between switch events. This approach introduces several difficulties, necessitating advanced mathematical treatment and specialized numerical methods. This thesis introduces piecewise smooth differential algebraic equations as a new mathematical model class to facilitate the analysis of power electronic circuits with idealized switches. The new model class is used to formalize the required solution procedures and clarify the difficulties associated with switch events. Two other contributions of the thesis focus on improving the computational efficiency of existing methods. First, a new method for integration and interpolation is developed. With this method, fewer simulation steps are needed between switch events to achieve the same accuracy. The method is therefore useful for the simulation of circuits with a high switching frequency. Second, a circuit partitioning technique is introduced to reduce the computation time for matrix refactorization at switch events in simulation studies with a large number of switching devices.1. Introduction 2. Literature Review 3. Model Composition 4. Piecewise Smooth DAEs 5. Similarities with Complementarity Modeling 6. Solving Power Electronic Circuits 7. Numerical Integration and Interpolation 8. Circuit Partitioning 9. Conclusionsnrpages: 284status: publishe

On the Numerical Accuracy of Electromagnetic Transient Simulation with Power Electronics

© 2018 IEEE. This paper investigates the numerical accuracy of currently employed methods for electromagnetic transient simulation with power electronics. Existing tools often employ the second-order accurate trapezoidal method for numerical integration. However, solution techniques employed to facilitate ideal power-electronic switches do not always preserve this second-order accuracy at switch events. As a result, the step size required to achieve the desired level of accuracy is often smaller than expected, affecting simulation speed. Therefore, this paper aims to assess and improve the tradeoff between step size and accuracy. First, currently employed solution techniques are reviewed by deriving asymptotic estimates of the rates at which approximation errors decrease as the step size reduces. Afterward, a benchmark method is proposed that preserves second-order accuracy at switch events. The tools EMTP-RV and PSCAD are compared with this benchmark method for elementary circuit examples. The results demonstrate the potential to improve accuracy, by several orders of magnitude in some cases, without increasing the number of linear systems to be solved.status: publishe

Accurate second-order interpolation for power electronic circuit simulation

This paper presents a second-order accurate method for circuit simulation with idealized power electronic switches. The method combines the integration method TR-BDF2 with an improved quadratic interpolation technique for the localization of switch events. Next to preserving second-order accuracy after interpolation, the technique also preserves the capability to damp fast transients caused by small on-resistances and large off-resistances. Conventional interpolation techniques for integration methods do not have this damping property. This results in a loss of accuracy if a switch event occurs shortly after a transition to discontinuous conduction mode. Consequently, the step size required to achieve the desired level of accuracy is sometimes smaller than expected. With the improved interpolation technique, the second-order accuracy associated with TR-BDF2 is not affected at switch events. Numerical experiments confirm the improved accuracy of the proposed method. The method is compared with the simulation tools PSIM and PSCAD, and with the conventional interpolation polynomial of TR-BDF2 as used in Simulink.status: publishe

Multiobjective Battery Storage to Improve PV Integration in Residential Distribution Grids

This paper investigates the potential of using battery energy storage systems in the public low-voltage distribution grid, to defer upgrades needed to increase the penetration of photovoltaics (PV). A multiobjective optimization method is proposed to visualize the trade-offs between three objective functions: voltage regulation, peak power reduction, and annual cost. The method is applied to a near-future scenario, based on a real residential feeder. The results provide insight into the dimensioning and the required specifications of the battery and the inverter. It is found that an inverter without batteries already achieves part of the objectives. Therefore, the added value of batteries to an inverter is discussed. Furthermore, a comparison between lithium–ion and lead–acid battery technologies is presented.status: publishe

Automated Residential Demand Response Base on Dynamic Pricing

Against the background of more renewable energy, limited investment in power generation, ageing distribution and transmission infrastructure, and the electrification of energy, the demand side of the electricity system is gaining attention. Moreover, the increasing ability to automate household appliances allows to involve the demand side more easily. Therefore, this paper examines demand response of residential consumers possessing smart appliances. Starting from day ahead wholesale prices and renewable energy production, different cost reflective Real Time Pricing (RTP) schemes are developed. According to these schemes, white goods are scheduled to the lowest price period taking into account user preferences. These demand modifications bring about new insights in the impact of the introduction of RTP.status: publishe

Techno-Economical And Life Expectancy Modeling Of Battery Energy Storage Systems

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Power Flow Efficiency of a DC Distribution Grid

A model of a DC local distribution grid with penetration of distributed energy resources (DER) is studied that are expected to increase significantly in the future. Because the existence AC distribution networks are not optimally designed for bidirectional energy flows caused by the distributed energy resources (RES, storage energy systems, PHEVs), a replacement of delivering DC power in conductors for existing AC network is assumed. Therefore, a DC distribution grid with participation of renewable energy resources will be presented in different operational modes to evaluate the efficiency of power control solution using DC signal in terms of power flow and total losses. A comparison with AC power flow is the same grid is also introduced to clarify the viable alternatives of a low-voltage DC grid for bidirectional power flow in future.status: publishe