High-Voltage Load Switch - Fuse Combination Electrical Appliance Solution: Safety Application Guide Based on Transfer Current

I. Core Issue and Objective​
This solution aims to address the safety risks arising from the mismatch between the core parameter "transfer current" of the "load switch-fuse combination electrical appliance" and the actual system short-circuit current when protecting power transformers. The goal is to provide a clear set of guidelines for selection, verification, and application, ensuring that the combination electrical appliance operates correctly and reliably during transformer faults. This prevents the load switch from being damaged due to interrupting currents beyond its capability and safeguards the entire distribution system.

​II. Key Concept: Transfer Current​

1. ​Definition and Mechanism​
   The transfer current is the critical current value that determines whether a fault current is interrupted by the fuse or the load switch. Its occurrence is closely related to the working mechanism of the combination electrical appliance:
   • ​Small fault current: The fuse of one phase (the first-to-clear phase) melts first, and its striker triggers the load switch mechanism, causing all three poles of the load switch to open simultaneously and interrupt the remaining two-phase current.
   • ​Large fault current: All three fuses melt almost simultaneously and rapidly, interrupting the fault current before the load switch opens.
   • The transfer current is precisely the boundary between these two operating modes.
2. ​Official Determination Method​
   According to IEC standards, the transfer current (Itr) is determined based on:
   • The total break time of the load switch (T0): The time from the activation of the fuse striker to the complete separation of the load switch contacts.
   • The time-current characteristic curve of the fuse: On the characteristic curve with a manufacturing deviation of -6.5%, the current value corresponding to an operating time of 0.9 × T0 is the transfer current.
3. ​Classification and Influencing Factors​
   • ​Rated transfer current: The standard value provided by the manufacturer, based on the maximum fuse element rating.
   • ​Actual transfer current (Ic,zy)​: The value that must be verified in engineering applications, derived from the characteristic curve based on the actual selected fuse element rating and T0.
   • ​Main influencing factors: The break time T0 of the load switch is the primary factor. A smaller T0 results in a larger transfer current. The characteristics of the fuse itself are also a factor.

​III. Core Application Principles and Verification Process​

1. ​Golden Rule​
   To ensure safety, the following condition must be met:
   The value of the three-phase short-circuit current on the low-voltage side busbar of the transformer, converted to the high-voltage side (Isc) > Actual transfer current of the combination electrical appliance (Ic,zy)
   • ​When met: The three-phase short-circuit current is interrupted by the fuse, protecting the load switch.
   • ​When not met: The load switch is forced to interrupt the current (approximately the two-phase short-circuit current) and withstands harsh Transient Recovery Voltage (TRV), making interruption failure highly likely and leading to accidents.
2. ​Selection and Verification Steps​
   To correctly apply the combination electrical appliance, the following steps must be followed:
3. ​Collect system parameters: Obtain system short-circuit capacity, transformer capacity, and impedance voltage.
4. ​Preliminary selection: Based on the transformer's rated current, preliminarily select appropriate fuse specifications and load switch type.
5. ​Calculate key currents:
   o Calculate the three-phase short-circuit current on the low-voltage side of the transformer and convert it to the high-voltage side (Isc).
   o Based on the selected fuse specifications and the T0 time of the load switch, refer to the manufacturer's provided curve to obtain the actual transfer current (Ic,zy).
6. ​Perform core verification: Compare Isc and Ic,zy.
   o If Isc > Ic,zy, the verification passes, and the solution is essentially safe.
   o If Isc < Ic,zy, the solution carries risks, and optimization measures must be taken (see Part IV).
7. ​Final capability verification: Confirm whether the rated transfer current interruption capability of the selected load switch is greater than the calculated Ic,zy. This serves as the final safety barrier.

​IV. Guidance for Different Scenarios​

1. ​Transformer Capacity ≤ 630kVA​
   • ​Solution: Using a combination electrical appliance is generally safe and economical.
   • ​Explanation: As shown in the table, for 500kVA and 630kVA transformers (with 4% impedance), the condition Isc > Ic,zy is easily met when the system short-circuit capacity is sufficient.
   • ​Recommendation: Ordinary pneumatic load switch combination electrical appliances can be selected.
2. ​Transformer Capacity 800 ~ 1250kVA​
   • ​Solution: High-risk range, strict verification is mandatory.
   • ​Analysis: As shown in the table, even with a transformer impedance of 6%, it is difficult to meet the condition Isc > Ic,zy for transformers with a capacity of 800kVA and above. If vacuum or SF6 load switches with smaller T0 are selected, their transfer current is larger, making the condition even harder to meet.
   • ​Optimization measures:
   o Prioritize the use of pneumatic load switches with a longer break time (T0) to reduce the transfer current and make it easier to meet the condition.
   o Communicate actively with manufacturers to inquire whether vacuum or SF6 load switches can be adjusted (by increasing T0) to achieve a smaller transfer current value.
   o If the condition cannot be met after calculation and verification, the combination electrical appliance solution should be abandoned.
   • ​Final recommendation: For 1000kVA and 1250kVA transformers, especially dry-type transformers, it is strongly recommended to directly use circuit breakers.
3. ​Transformer Capacity > 1250kVA​
   • ​Solution: Circuit breakers must be used for protection and control.
   • ​Explanation: The short-circuit current level at this capacity exceeds the reliable protection range of combination electrical appliances. Circuit breakers are the only safe choice.

​V. Summary and Special Notes​

1. ​Verification is mandatory: Never rely solely on experience or simply apply combination electrical appliances based on transformer capacity. The calculation and comparison of Isc and Ic,zy must be performed.
2. ​Consider the impact of load switch type: Do not blindly assume that vacuum or SF6 load switches with stronger interruption capabilities are superior. Their smaller T0 results in a larger transfer current, which may make it harder to meet the core verification condition and instead introduce risks.
3. ​Importance of system short-circuit capacity: The system short-circuit capacity directly affects the value of Isc. In systems with smaller short-circuit capacities, such as industrial parks or grid endpoints, the above issues become more pronounced, and extra caution is required during selection.