# Sequential-Fault Diagnosis Strategy for High-Speed Train Traction Systems Based on Unreliable Tests
## 1 Introduction  
High-speed trains have become the mainstream form of intercity transportation because of their comfort, convenience, safety, and punctuality. Due to the rapid increase in rail transit passenger flow, the requirements for train safety have become more stringent. Research on train safety issues [1, 2] is also emerging in an endless stream. A train information control system is the core of a high-speed train, and a traction system is the \"heart\" of the high-speed train information control system. Once a major failure occurs, it will bring great losses to society and the economy. Therefore, the real-time monitoring and testing of the traction systems are important technological means to ensure the safe operation of high-speed trains, so the evaluation of traction system test performance is essential. Testability is the evaluation of testing capability, which refers to the ability of a system to accurately and quickly determine its state, for example, whether it can work or whether its performance ability has decreased. Based on this, the system's internal structure is used to isolate faults. If a system has a good testability, it can efficiently and accurately detect and locate faults. Testability is widely used in various fields and can be applied to fault diagnosis [3, 4], as well as for analyzing power devices [5, 6]. Testability design comes into being with testing. Using design-for-testability (DFT) [7] can make a bus controller work more reliably and stably. One of the methods of DFT is scanning design [8, 9, 10, 11], which caneffectively improve the reliability of testing. Testability optimization can address the current shortcomings in the testability of a system by taking appropriate measures via certain means to meet the testability requirements of the system. For optimization purposes, testability can involve a flexible test system (FTS) with a phased mission model [12], a hybrid algorithm based on priority strategy and a genetic algorithm [13], the Non-dominated Sorting Genetic Algorithm (NSGA)II algorithm [14], and the pullout test optimization device based on the design of mechanical principles [15], etc. The way to provide a strong solution for train maintenance is to build an appropriate fault diagnosis strategy for a rail transit train traction system via the testability analysis and design of trains. Fault diagnosis [16; 17] is the key to ensuring safe railway operation. Diagnosis strategy construction is a key part of testability technology, and its goal is to determine the sequence of testing when fault detection and fault isolation are performed. The error-voltage-based, open-switch fault diagnosis strategy [18] can locate the faulty switch and improved slight fault diagnosis strategy for induction motor considering even and triple harmonics [19] can enhance the diagnosis accuracy for the induction motor under the slight fault situation. This reflects the importance of constructing diagnosis strategies. Because the traction system itself is relatively large and the internal electrical coupling relationship is very complex, there are inevitably some uncertain factors in the test process, which leads to the unreliability of our test results. However, a traditional testability analysis [20; 21] is based on accurate results. Methods such as testability analysis based on testability modeling [22; 23; 24; 25; 26] and testability analysis based on multi-signal modeling [27; 28; 29; 30; 31] cannot consider the uncertainty factors of these tests, and thus have many disadvantages regarding the construction of fault diagnosis strategy of high-speed train traction systems. Therefore, it is very necessary to study the construction method of a fault diagnosis strategy for high-speed train traction systems based on the unreliable tests. Therefore, in order to meet the needs of system fault diagnosis in the field of rail transit, this study attempts to apply testability technology to the testing capability analysis of rail transit train traction systems.