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Model-driven development of solar power conversion systems

July 31, 2019

bijeshpoyil-July 31, 2019

The increasing depth of penetration of distributed energy resources such as grid tied solar inverters has resulted in the power conversion community to look for better, more efficient, and cost-effective solutions for these markets. There are many algorithms and topologies in the literature to improve the output quality and efficiency of the power conversion process. Silicon vendors are coming up with new control processors with features and hardware support to implement these algorithms efficiently. It is very expensive to build the hardware prototype of a full inverter and to experiment with its performance under various conditions. Moreover, any malfunctioning of the algorithm during experimentation could damage the entire system. There are also associated safety standards to be met for the products in these markets. So the power conversion industry has always been slow in adopting these innovations into the final product.

Model driven development has been adopted as a solution to this problem. In model driven development, a full model of the system is built and simulated before a hardware prototype is generated. This verifies the algorithm functionality and reduces the risk significantly. Moreover, current modeling tools support code generation directly from the model that simplifies and relaxes safety certification criteria. However, the industry has not embraced full model driven development mainly because 1) the performance of the end product and the model varies significantly, and 2) the generated code is not very efficient for the target control processor and requires manual modification before taking it to the product.

In this article, we discuss techniques and approaches that can make the model performance very close to the final product performance to minimize the risk of hardware changes and delays. We also discuss how the code can be generated efficiently from these models to get the product faster to market.

Model development guidelines

Consider a simplified diagram of a grid-tied residential solar inverter (Figure 1). The solar radiation on the solar panel generates dc proportional to the intensity of the radiation. The converter converts this dc to ac, which can be used by home appliances and also can be fed to the grid. Current and voltages from various points in the signal chain are sensed by appropriate sensors and will be fed to the control processor in the inverter. The algorithm running on the control processor analyzes these signals and controls the power modules such that the generated current and voltage are of required frequency, magnitude, and phase with the grid. In this case, the solar panel acts as the power source, and the grid and the home appliances act as the sink. In a different power conversion system, the sources and sink would be different, but most of them will fall into the structure shown in Figure 2.


Figure 1. Solar inverter system

The primary aim of a power conversion system/algorithm designer is to arrive at the right components and algorithms for the block’s control processor and converter hardware (Figure 2) and meet the desired performance for all source and load variations. So it is important to clearly know the environment the system is going to operate in while designing the system. For example, while designing a solar inverter for a system (Figure 1), the designer should know the places the inverter is expected to install, variations in intensity of solar radiations, the efficiencies of the solar panel, grid conditions, etc. In a model driven development, the designer first creates the model of the converter, simulates the expected variation, and verifies that the model works as expected. Most often the modeling tools will provide models and library blocks for modeling sources and sinks. For example, Simscape Power Systems from Mathworks has models for grids, photovoltaic (PV) panels, and various loads. These can be used to simulate and verify various use cases of the system.


Figure 2. Power conversion components

 

The system performance depends on all the components of the system. In some cases, the designer has the freedom to start the design from scratch and decide on all the components of the system to meet constraints on source and load. In some other cases, part of the system may already be fixed due to reasons outside the control of the designers, and their degree of freedom is limited to few components. In this article, we assume the main aim of the designer is to choose and implement the right control algorithm for an existing topology—but most of the guidelines explained can be applied to a generic case as well.

>> Continue reading "Structuring the Model" on page two of this article originally published on our sister site, EDN: "Fast prototyping and deployment of control algorithms for power conversion applications."

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