1. Introduction Microprocessor-based relay protection has been widely applied in electrified railways. Based on the concept of summarizing the past and developing the future, this paper raises some noteworthy issues regarding the current use of microprocessor-based relay protection in electrified railways and the development of new products, in order to draw the attention of designers and developers and provide improvements in practical work. 2. Brief Summary of Past Work Due to the special characteristics of the traction network and traction load, the feeder protection device of electrified railways has many characteristics compared with the line protection of power systems. The main requirements are: insensitivity to high-order harmonics in the traction load current; insensitivity to inrush current caused by the electric phase splitting of locomotives; adaptability to changes in operating mode; selectivity to the maximum load current and minimum short-circuit current; protection maloperation caused by the switching of parallel capacitor compensation devices on electric locomotives; and protection dead zone caused by series capacitor compensation. These special characteristics have been given sufficient attention in various microprocessor-based protection systems currently under development and have been addressed from the perspective of protection principles. For example, three-stage adaptive impedance protection and current increment protection are adopted with the comprehensive harmonic content as the control quantity. 2.2 Regarding Transformer Protection Considering the diverse types of traction transformers, such as single-phase transformers, single-phase transformer V/V connection, YN,d11 transformers, and impedance matching balance transformers, a widely applicable microprocessor-based protection device was developed. This device achieves applicability to different traction transformers by changing the control word or using the transformer type as a setting value. 2.3 Regarding Parallel Capacitor Compensation Protection A microprocessor-based protection device suitable for parallel capacitor compensation devices in electrified railways was developed. It is equipped with the seven protections required in reference [1], namely: instantaneous overcurrent protection, overcurrent protection, differential current protection, differential voltage protection, harmonic overcurrent protection, undervoltage protection, and overvoltage protection. [b]3 Several Issues [/b] 3.1 Microprocessor-based Feeder Protection 3.1.1 Protection Failure to Operate During Substation Out-of-Phase Short Circuit A schematic diagram of the electrical phase separation structure at the substation outlet is shown in Figure 1. When an electric locomotive passes through the electrical phase split at the substation outlet, if the pantograph is lowered as required by regulations, the probability of an out-of-phase short circuit at the substation outlet is much lower. If the pantograph is not lowered when passing through the electrical phase split, under the working voltage of the contact network, it is easy to cause discharge along the surface of the insulator, resulting in a short circuit between phases A and B. When an out-of-phase short circuit fault occurs in the substation, the fault state is an arc fault, the fault current is rich in harmonics and the value is small, the voltage drop between phases A and B to ground is very small, or even increases instead of decreasing. Existing feeder protection principles cannot protect well in this state. Reference [2] has a detailed analysis of this fault and has also proposed corresponding protection measures. Overall, the protection problem of this fault has not been completely solved and needs further research. [img=294,140]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/46-1.jpg[/img] 3.1.2 Protection fails to operate when AT traction network is broken and grounded AT power supply traction network has advantages in heavy-load and high-speed lines. my country's Beijing-Qinhuangdao, Datong-Qinhuangdao, and Zhengzhou-Wuhan lines all use this power supply method. A schematic diagram of the double-line traction network is shown in Figure 2. Since the positive feeder F is unsupported between the two towers, it is prone to breakage when the tension changes. When a breakage fault occurs, one side may be grounded and the other side may be suspended. If the power supply side is grounded and the non-power supply side is suspended (as shown in ① in Figure 2), the expression for the fault line impedance measured from the substation is shown in equation (1). [img=411,42]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/46-2.jpg[/img] [img=303,159]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/47-1.jpg[/img] If the power supply side is not grounded and the power supply side is floating (as shown in ② in Figure 2), the fault line impedance measured from the substation is as shown in equation (2). [img=383,156]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/47-2.jpg[/img][font=SimSun][/font]  In the formula, ZT, ZR, ZF, ZTR, ZFR, and ZTF are the unit self-impedance of the contact wire (T), rail (R), and positive feeder (F), respectively, and the unit mutual impedance between them; D represents the length of the AT section where the open-circuit ground fault occurs; x represents the distance from the endpoint to the AT closest to the power supply side; LA and LB represent the lengths of the up and down power supply arms.  According to the theoretical analysis in reference [3] and the field report in reference [4], when a positive feeder open-circuit ground fault occurs, the conventional principle feeder protection often fails to operate. At the same time, since the positive feeder F is erected outside the roadbed, the open-circuit ground fault is often a fault with high transition resistance, which further exacerbates the possibility of the conventional principle protection failing to operate. Although the current microprocessor-based feeder protection is equipped with adaptive current increment protection, which has solved the possibility of failure to operate during such faults to a certain extent, it still does not fundamentally solve the problem in principle. 3.2 Microprocessor-based transformer protection 3.2.1 Differential protection for balanced transformers Differential protection for Y/Δ connection impedance matching balanced transformers and Y/A connection balanced transformers can be implemented using either a two-relay or a three-relay method. Theoretical analysis shows that the three-relay method has the advantage of higher sensitivity compared to the two-relay method. When using the three-relay method, in order to overcome the maloperation of the differential protection for external faults on the high-voltage side when the neutral point of the high-voltage side of the transformer is grounded as shown in Figure 3, the current on the high-voltage side of the transformer should be the line current when constructing the differential protection, i.e., using equation (4). [img=383,99]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/47-6.jpg[/img][img=337,121]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/47-3.jpg[/img][img=337,166]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jdq/2002-2/47-5.jpg[/img] 3.2.2 Selection of Differential Action Characteristics In traction power supply systems, microprocessor-based transformer differential elements generally employ two-segment or three-segment broken-line operating characteristics. Three-segment broken-line characteristics offer higher sensitivity than two-segment broken-line characteristics, which is advantageous for protecting against inter-turn short circuits in the windings. Therefore, three-segment broken-line differential protection operating characteristics are recommended. 3.3 Parallel Capacitor Compensation Device Protection 3.3.1 Differential Current Protection with Multiple Branches Parallel Compensation The microprocessor-based parallel capacitor compensation device for electrified railways does not consider the impact of switching multiple branch parallel compensation devices (as shown in Figure 4) on protection performance. That is, currently, most settings are calculated based on the full operation of the 3rd, 5th, and 7th compensation branches. In reality, the operation of the 3rd, 5th, and 7th branches frequently changes. Therefore, a microprocessor-based protection device with adaptive automatic switching capabilities for multiple branch parallel capacitor compensation devices should be researched and developed. 3.3.2 Automatic Zero Adjustment of Voltage Transformers Since the two voltage transformers constituting the voltage difference have different characteristics, conventional voltage differential protection uses an auxiliary converter for zero adjustment. Microprocessor-based protection should utilize software for automatic zero adjustment. 4. Conclusion This paper first analyzes the successful experience in the research, development and application of microcomputer protection for electrified railways. Then, based on the actual situation of electrified railways, it points out some shortcomings in the current protection principle, hoping to attract the attention of protection design and research departments.