Abstract: This paper discusses the multi-level coordination issues of protection components in the newly issued DC design code, proposes suggestions for the selection of DC system fuses and DC circuit breakers, and presents the field test results of typical differential configurations in DC systems. Keywords: DC system, differential coordination, protection test Introduction With the continuous progress of China's power industry, power systems are developing towards ultra-high voltage and large capacity. Providing control, protection, signaling, and operating power for these large-capacity power equipment necessitates raising the safe, reliable, and economical operation of DC systems to a new level. During normal operation, the DC system provides closing power for circuit breakers and DC power for relay protection, automatic devices, and communications. During faults, especially when AC power is interrupted, the DC system provides a safe and reliable DC power supply for relay protection and automatic devices, circuit breaker closing and tripping, and emergency lighting, which is the fundamental guarantee for the correct operation of power system relay protection, automatic devices, and circuit breakers. In DC circuits, fuses and circuit breakers are the main protective components for overcurrent and short-circuit faults in each outgoing line of the DC system. They are used for disconnecting and isolating the power supply network of feeder circuits. The appropriateness of their selection and operating value settings, as well as the selective coordination between upstream and downstream protection levels, directly affects whether system faults can be limited to a minimum. This is crucial for preventing system damage, accident escalation, and serious damage to main equipment. Therefore, improving the accuracy of fuse and circuit breaker selection and configuration is of great significance for improving the safety and reliability of power system operation. 1. Main Problems of Differential Coordination Because substation DC systems supply a wide range of power and have a wide circuit distribution, many branches in a DC network often require circuit breakers or fuses for protection, often divided into three or four levels in series. This raises the issue of correctly selecting protection schemes and coordinating protection between upstream and downstream protection levels. 1.1 Mixed Use of AC/DC Circuit Breakers Due to the different arc ignition and extinguishing processes of AC and DC circuit breakers, AC/DC circuit breakers with the same rated value do not have completely identical DC power interruption capabilities. Using AC circuit breakers instead of DC circuit breakers or mixing AC and DC circuit breakers is one of the main causes of protection malfunctions exceeding the rated current. The instantaneous operation of a circuit breaker uses the magnetic tripping principle, with the criterion being the peak current. The rated value of the circuit breaker is the effective value, while the peak value of AC current is higher than the effective value. Under the same set value, the actual rated value of the AC circuit breaker is higher than that of the DC circuit breaker in a DC circuit. Furthermore, because the arc extinguishing principles of AC and DC circuit breakers are different, AC circuit breakers used in DC circuits cannot effectively and reliably extinguish DC arcs, easily causing malfunctions exceeding the rated current. 1.2 Fuse Quality and Parameter Issues The technical data for fuses provided by various manufacturers are obtained during product type testing, and the breaking capacity of the fuses is verified under the effective value of the periodic component of the AC power supply. The fuse element operation selection and matching characteristic curve is also an AC ampere-second characteristic curve. This differs somewhat from the actual situation when a short circuit fault occurs in a substation's DC system. The differential coordination provided by various fuse manufacturers and design manuals determines the upper and lower differentials based on the same model and fuse element material to ensure selectivity. When different types of fuses are present in the circuit, the differential coordination between fuses should be given even greater attention. Furthermore, due to the large number of low-voltage electrical appliance manufacturers, product quality cannot be fully guaranteed; therefore, even fuses from the same manufacturer and of the same model may have varying parameters. 1.3 Inappropriate Selection of Rated Differentials Between Upper and Lower Levels: Fuses operate on a thermal effect principle, while circuit breakers combine magnetic and thermal effects, resulting in different ampere-second characteristic curves and thus different coordination differentials. The differential coordination between circuit breakers and between circuit breakers and fuses should not be based solely on the coordination specifications between fuses. 2. Field Test of Differential Configuration of Fuse and DC Circuit Breaker To comply with the relevant provisions of the newly issued DL/T5044-2003 "Technical Specification for DC Design of Power Engineering" (hereinafter referred to as the Design Specification), and to verify whether several typical differential configuration schemes of circuit breakers and fuses in the DC system of substations meet the requirements of selective protection, this study explored the differential coordination between DC circuit breakers, the coordination between DC circuit breakers and fuses, and the selection and configuration between their upper and lower levels. Field tests were conducted on typical differential protection coordination schemes of some DC circuit breakers and fuses in the DC system of substations under the jurisdiction of Shijiazhuang Power Supply Company. Tests were also conducted on three-stage DC circuit breakers with time delay function to confirm the coordination conditions for achieving selective protection. 2.1 Selection of Short-Circuit Current Based on the maximum short-circuit current that may occur at the installation site of the DC circuit breaker and fuse, the test elements were connected in series for short-circuit tests. To ensure that the test current is higher than the maximum short-circuit current that may occur at the site, an appropriate margin was added. The short-circuit current selection result is as follows: a. For 300Ah and 200Ah battery banks, the short-circuit current should be 2000A for short-circuit faults in the closing feeder cable. b. For 100Ah battery banks, the short-circuit current should be 1000A for short-circuit faults in the closing feeder cable. c. For short circuits at the outlet of fuses or DC circuit breakers at the end of control and protection circuits, the short-circuit current should be 200A. 2.2 Test Scheme and Results Based on an investigation of the protective electrical appliances installed and used in the DC system of the substation, and in accordance with the relevant provisions of the design code, 12 test schemes in 4 categories were determined: fuse-fuse, circuit breaker-fuse, fuse-circuit breaker, and circuit breaker-circuit breaker. 2.3 Result Analysis 2.3.1 Fuse-Fuse Design Code Requirements: If the type of fuse and the material of each fuse element are the same, to ensure selectivity, the rated current difference between adjacent fuse elements in the circuit must be at least two levels. The premise of this test is that products from the same factory, model, and batch are selected. Because they have the same ampere-second characteristics, the coordination between the two levels is good. When the short-circuit current reaches 10 to 30 times that of the upper level, they all operate correctly, showing good coordination characteristics. Shijiazhuang Power Supply Company has uniformly replaced the fuses in the DC circuit of the substation, which meets the site conditions for this test. 2.3.2 Circuit breaker-fuse design code requires that a fuse can be installed as a protective device at the next level of the circuit breaker. The rated current of the circuit breaker should be 4 times or more than the rated current of the fuse. The test results are consistent with the design code. However, the premise of this test is that the short-circuit current is 8 to 9 times the rated current of the upper level, just entering the instantaneous trip zone of the upper level circuit breaker. If the short-circuit current increases to a certain value, according to the test results in reference [2], the upper and lower levels will operate simultaneously, causing over-tripping. Therefore, in engineering applications, in addition to the circuit breaker's rated current being 4 times or more greater than the fuse's rated current, the maximum short-circuit current should also be verified to be no more than 10 times the rated current of the upstream circuit breaker. 2.3.3 Fuse-Circuit Breaker This test condition is stricter than the design specifications, and the fuse's rated current is 1.6 times greater than the circuit breaker's rated current. The test results are consistent with the design specifications. However, the short-circuit current in this test is 12 to 13 times the rated current of the upstream fuse. Therefore: a. The battery outlet fuse is selected according to the battery's 1-hour discharge capacity and is one level higher. Its maximum short-circuit current is within this range and can be matched with the next-level circuit breaker. It is not necessary to verify the maximum short-circuit current. b. If used in the downstream circuit, because the fuse's melting speed increases with the increase of the short-circuit current, while the circuit breaker's tripping speed remains basically unchanged, when the short-circuit current is large enough, the two will act close together, causing over-current. The test results in reference [2] also confirm this point. Therefore, in engineering applications, in addition to the circuit breaker's rated current being at least four times greater than the fuse's rated current, the maximum short-circuit current should also be verified to be no more than 10 to 12 times the rated current of the upstream circuit breaker. 2.3.4 DC Circuit Breakers – DC Circuit Breakers 2.3.4.1 Two-Stage DC Circuit Breakers Two-stage DC circuit breakers operate correctly and coordinate well when the short-circuit current is 8 to 10 times the rated current of the upstream switch and under 4 to 5 level differential coordination. However, if the short-circuit current reaches or exceeds 10 times the rated current of the upstream circuit breaker, both upstream and downstream circuit breakers enter the fast-acting zone and operate simultaneously, causing cascading tripping. Furthermore, because the inherent operating time of molded case DC circuit breakers is longer than that of miniature circuit breakers, the selective short-circuit current value of a coordination where the upstream circuit breaker is paired with a molded case circuit breaker and the downstream circuit breaker with a miniature circuit breaker is higher than that of a similar coordination. According to relevant domestic tests, its selective limit current extends to approximately 20 times the upstream rated current. 2.3.4.2 Three-stage DC Circuit Breaker When a three-stage DC circuit breaker is used, with a three-stage upper stage and a two-stage or three-stage lower stage, the stage difference is 2 levels. It operates correctly within a short-circuit current range of 25 to 40 times the rated current of the upper stage circuit breaker. The design code does not specify stage coordination for three-stage DC circuit breakers. Test results show that because the short-delay time of the upper stage circuit breaker is greater than the full breaking time of the lower stage circuit breaker, the upper stage short-delay can return, thus achieving small stage coordination. Furthermore, the influence of short-circuit current does not need to be considered, which can meet the requirements of the design code regarding multi-stage coordination in DC distribution panel design schemes. 3. Recommendations for DC System Protection Scheme Selection 3.1 Fuses – Fuses should be selected from reputable manufacturers using the same factory and model, facilitating the unified replacement of existing fuses in the substation. However, for newly built stations, since the fuses are all integrated into the equipment, it is difficult to guarantee that they are from the same manufacturer and of the same model, especially to guarantee the requirement of "identical materials for all fuse components" as required by the design specifications. Therefore, the coordination of fuses from different manufacturers and of different models should have a larger level difference. 3.2 The coordination between fuses and circuit breakers (two-stage) and between two-stage circuit breakers of the same type should not only comply with the design specifications, but also verify that the maximum short-circuit current should not exceed 8 to 10 times the rated current of the upstream component. 3.3 The coordination of circuit breakers of different models should consider the inherent operating time of the circuit breakers, and it must be ensured that the inherent operating time of the upstream circuit breaker is not less than that of the downstream one. It is recommended to use a coordination of upstream molded case circuit breakers and downstream miniature circuit breakers. 3.4 Three-stage circuit breakers can achieve small level difference coordination and do not need to consider the influence of short-circuit current, which can meet the requirements of the design specifications for multi-level coordination in the design scheme of DC distribution panels. References: Fan Ruifeng, Shao Weixiang. A Brief Study on the Differential Coordination Test of Air Circuit Breakers and Fuses in DC Systems [J]. DC Power Supply, 2004, (2).