In recent years, the International Maritime Organization (IMO), Europe, and the United States and other countries have set up different emission control areas (ECA) for ship exhaust pollutants to enforce more stringent pollutant emission regulations. In order to meet the current IMO Tier III emission regulations, an after-treatment device must be installed in the exhaust system of the ship power plant to reduce the ship NOx emissions. At present, selective catalytic reduction technology (SCR) is one of the main technical routes to resolve excess NOx emissions of marine diesel engines, and is the only NOx emission reduction technology recognized by the IMO that can be used for various ship engines. Compared with the conventional low-pressure SCR system, the high-pressure SCR system can be applied to low-speed marine diesel engines that burn inferior fuels, but its working conditions are relatively harsh, and it can be susceptible to operational problems such as sulfuric acid corrosion, salt blockage, and switching delay during the actual ship tests and ship applications. Therefore, it is necessary to improve the design method and matching strategy of the high-pressure SCR system to achieve a more efficient and reliable operation. This article summarizes the technical characteristics and application problems of marine diesel engine SCR systems in detail, tracks the development trend of the catalytic reaction mechanism, engine tuning, and control strategy under high sulfur exhaust gas conditions. Results showed that low temperature is an important reason for the formation of ammonium nitrate, ammonium sulfate, and other deposits. Additionally, the formed deposits will directly affect the working performance of the SCR systems. The development of SCR technology for marine low-speed engines should be the compromise solution under the requirements of high sulfur fuel, high thermal efficiency, and low pollution emissions. Under the dual restrictions of high sulfur fuel and low exhaust temperature, the low-speed diesel engine SCR systems will inevitably sacrifice part of the engine economy to obtain higher denitrification efficiency and operational reliability.
The high-pressure SCR system is suitable for low-speed marine engines that burn high-sulfur fuel, but it will cause the increase of the exhaust back pressure of main engine, which will affect the main engine’s performance and fuel consumption. At the same time, due to the large weight and size of the SCR reactor, the overall heat storage performance of the SCR reactor is large, which makes the temperature difference between the front and back of the turbine in the start-up and shutdown of the main engine and transient conditions, affecting the transient response performance of the turbine. In this paper, 6S46ME marine low-speed diesel engine and its high-pressure SCR system are taken as the research object, the co-simulation model is constructed by using GT-Suite coupled with a programming software, and the accuracy of the test model is verified. The influence of high-pressure SCR system on the performance of main engine is analyzed, and it finds that the fuel consumption of the main engine at low load increases significantly greater than that at high load, and the exhaust temperature of the main engine changes differently at different loads. On this basis, the co-simulation model was used to analyze the matching performance of high-pressure SCR system under the transient conditions such as the main engine Tier II/Tier III mode switching process, fast-loading, normal-loading and fast-unloading. The results show that the slow cut-in/cut-out of SCR reactor is beneficial to the stable operation of the main engine system, and the cut-in/cut-out time of high-pressure SCR system greatly affects the transient fuel consumption of the main engine, and the maximum increase of fuel consumption is about 3.6 g/kW·h; In addition, the thermal inertia of the reactor has a greater impact on the performance of the main engine at low load, but not at high load.
Submodule (SM) open-circuit failures severely affect the reliable operation of modular multilevel converter (MMCs). A comprehensive SM open-circuit failure diagnosis process includes three steps with failure detection, localization, and classification. However, the existing diagnosis methods perform these steps separately, which makes the diagnosis processcomplicated. Furthermore, the efficiency is relatively low with separate diagnosis steps. To address this issue, a diagnosis method with variance measurement is proposed in this paper to combine failure detection, localization, and classification. The proposed method makes use of two fault characteristics of SM open-circuit failures: 1) SM capacitor voltages have distinct distributions upon different faulty IGBTs; 2) Numerous SMs in the MMC make the failure diagnosis equivalent to statistical outlier analysis. On account of the fault characteristics, a variance-based index called VAR is proposed to evaluate the distributions of SM capacitor voltages. The VAR values are then analyzed with quartile analysis, which is a statistical outlier analysis method, to complete the diagnosis process. The proposed failure diagnosis method features high reliability, high compatibility, and high cost-effectiveness. Finally, the feasibility of the proposed method is verified through both simulations and experiments under various failure scenarios. INDEX TERMS Open-circuit failure, failure diagnosis, modular multilevel converters, statistical analysis.
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