## 1 Introduction
The following advantages of direct torque control can be highlighted from an analysis of papers comparing direct torque control systems and vector control systems:  
1. High dynamic torque response [33; 34; 35];
2. Structural simplicity [36; 37; 38];
3. High resistance to stochastic disturbances of traction motor parameters [39; 40; 41];
4. Implementation of simplified circuit algorithms for the compensation of higher harmonic components of the traction current, which lead to pulsation of the torque even in the absence of asymmetric emergency asymmetric modes [42; 43; 44]. Taking into account the above factors, the authors assume that the use of direct torque control is a promising method for constructing a traction drive system for electric rolling stock compared to a vector control system. Therefore, the traction drive of electric rolling stock with direct torque control was chosen as the subject of research. During traction drive operation, asynchronous motor windings may be damaged, resulting in asymmetry of the stator phase current system. This contributes to the occurrence of torque pulsation on the motor shaft [45; 46; 47]. The same consequences are also caused by asymmetric modes in the induction motor supply system, i.e., by the asymmetry of the autonomous voltage converter arms [48; 49; 47]. The conducted analysis suggests that the asymmetric modes of operation of the traction drive system are caused both by the emergency modes of operation of the traction drive system and the changes in the operating modes of the electric rolling stock. In the case of emergency modes of operation, the motor or autonomous voltage inverter with the asymmetric modes of the windings should be cut off in order to prevent further development of the motor defect. In normal modes, the traction drive system should resume operation without cutting off any of the units. In view of the above, the task is to identify the emergency mode of the traction drive under real operating conditions. The existing operational diagnostic systems used on an electric rolling stock perform comparison of the phase current values of phase currents of the stator that are used to detect asymmetric modes in the traction drive windings [50; 51]. Analysis has suggested that the operational factors cause complexities in the diagnostic task of diagnosing as they promote asymmetry in the traction drive system in the absence of the emergency modes. Maximum current protection is used in the protection system of both the inverter [52; 53; 54] and the traction motor [55; 56]. In the case of application of the highest current protection, the cut-off of the damaged element takes place where the value of phase current exceeds a certain preset value after a certain time period has passed after the diagnosis. As a result, the damage to the traction motor is aggravated, and the repair of the motor will be more capital-intensive, while operation of the autonomous voltage inverter implies certain risks of damage in these modes. Therefore, the authors consider that it would be rational and reasonable to diagnose emergency asymmetric modes in the traction drive system in the early stages of occurrence of damage. Moreover, when designing the system of operational diagnostics that enables the detection of emergency asymmetric modes in the traction drive system, it would be reasonable to differentiate between the asymmetric modes resulting from transient processes and the asymmetric modes resulting from the damage of the traction drive elements. To implement this kind of diagnostic system, it is necessary to use the diagnostic parameter that would enable the detection of emergency asymmetric emergency modes in the traction drive system, taking into account the operational factors causing the asymmetric modes even during normal operation of the traction drive. It is preferable that the design of a diagnostic system involves a very limited number of hardware modifications in the traction drive system.