Transformer industry
Fuses play a key role in transformer protection, providing short-circuit protection, overload protection, and secondary-side fault isolation. They are essential electrical components for ensuring safe transformer operation and preventing fault escalation. Because transformers (especially distribution transformers) are complex and expensive, and because faults can easily lead to serious consequences such as fires and oil spray, fuse configuration must be precisely tailored to the inductive load characteristics and fault modes.
1. Special Requirements for Transformers on Fuses
The electrical characteristics of transformers (such as magnetizing inrush current and short-circuit impedance) differ significantly from those of loads like capacitors and motors. Therefore, their corresponding fuses must meet the following core requirements:
Withstand magnetizing inrush current: When the transformer is switched on, the sudden change in core magnetic flux generates a magnetizing inrush current (typically 5-10 times the rated current, lasting 0.1-0.5 seconds). Fuses must withstand this instantaneous high current without melting, preventing false tripping.
Quickly interrupt short-circuit currents: When a short circuit occurs within a transformer (windings, core) or externally (outgoing lines, busbars), the short-circuit current can reach tens or even hundreds of times the rated current. Fuses must blow quickly, within milliseconds to tens of milliseconds, to prevent winding burnout, core damage, or tank explosion.
Adaptable overload characteristics: Long-term transformer overload (e.g., load exceeding rated capacity, poor heat dissipation) can cause winding temperatures to rise and accelerate insulation aging. Fuses must exhibit a "delayed blow" feature—maintaining resistance during minor overloads and opening within a set timeframe during sustained severe overloads, ensuring a balance between operational reliability and protection sensitivity.
Matching the breaking capacity: The rated breaking capacity of the fuse must be greater than the maximum short-circuit current at the installation point to ensure that the arc is safely extinguished upon opening, preventing arcing from causing equipment damage or personal injury.
II. The Core Protection Function of Fuses in Transformers
Depending on the transformer's voltage level (high voltage, low voltage) and installation location (primary, secondary), the fuse's protective function has different priorities and is primarily categorized into the following two types:
1. High-voltage Side Fuses: Primary Protection and Fault Isolation
High-voltage side fuses (mostly high-voltage current-limiting fuses) are the core protection components of distribution transformers (such as 10kV/0.4kV distribution transformers). Their primary functions include:
Internal short-circuit protection: When a transformer winding short circuits between turns, phases, or a core fault (causing local overheating and short circuiting) occurs, the high-voltage side current increases dramatically, causing the fuse to quickly open, disconnecting the high-voltage power supply and preventing the fault from spreading to the higher-level grid. This also protects the transformer from internal arcing, which could cause insulation burnout, oil spraying, or even explosion.
External short-circuit protection: When a short circuit occurs in the transformer's low-voltage output (e.g., a cable or load), the short-circuit current will be induced through the transformer to the high-voltage side. If the low-voltage side protection (e.g., circuit breaker) does not operate promptly, the high-voltage side fuse will act as a backup, shutting off the faulty circuit.
Overload protection: When the transformer is chronically overloaded (e.g., the load exceeds the rated capacity by more than 10% and continues to operate), the high-voltage side current will continuously exceed the rated value. The fuse will then delay opening based on its "ampere-second characteristic," preventing insulation aging and shortening of the winding life due to prolonged overheating.
2. Low-Voltage Side Fuses: Precision Secondary-Side Protection
Low-voltage fuses are typically installed on the low-voltage side of a transformer (e.g., the 0.4kV side) as "front-end protection" for the low-voltage circuit. Their primary functions include:
Low-voltage outgoing line short-circuit protection: When a short circuit occurs in the line (e.g., a cable) from the low-voltage distribution panel to the load, the low-voltage fuse quickly opens, isolating the faulty branch and preventing the fault from affecting the normal operation of the transformer's low-voltage busbar and other branches.
Load Overload Protection: When a single load (such as a motor or lighting circuit) is overloaded, the corresponding low-voltage fuse will open with a delayed action, protecting the load equipment and circuitry while maintaining the power supply to other loads.
Supplementary Protection: Serving as a "completion" to the low-voltage circuit breaker, the fuse can reliably operate when the short-circuit current is low (the circuit breaker does not meet the operating conditions) or when the circuit breaker fails to operate due to a fault, providing "double protection."
III. Common Application Scenarios and Configuration Methods
The configuration of fuses for transformer protection depends on the voltage level and protection requirements. Typical scenarios are as follows:
1. Distribution Transformer (10kV/0.4kV)
This is the most common fuse application scenario, typically employing a "high-voltage side fuse + low-voltage side fuse/circuit breaker" configuration:
On the high-voltage side: Connect a set of three high-voltage current-limiting fuses (such as the XRNT-10/6.3A, where the 6.3A value is calculated based on the transformer's rated current) in series between the transformer's high-voltage bushing and the high-voltage busbar as primary protection.
Low-voltage side: Inside the low-voltage distribution cabinet, each outgoing line is equipped with a low-voltage fuse (e.g., RT28-63/20A) to protect the outgoing line circuit and load. The low-voltage busbar master switch can be equipped with a fuse or circuit breaker for overall low-voltage side protection.
2. Small dry-type transformer (e.g., 380V/220V for workshop use)
A dedicated high-voltage fuse is installed on the high-voltage side (if the incoming line is 10kV); if the incoming line is low-voltage (e.g., 380V), a master low-voltage fuse is installed at the incoming line.
Each low-voltage outgoing line circuit is equipped with a separate fuse to provide "branch circuit protection."
3. Power Transformers (35kV and Above)
High-voltage side fuses are typically used as "backup protection" in conjunction with relay protection devices (such as differential protection and overcurrent protection). If the relay protection fails to operate due to a fault, the fuse will open, interrupting the fault.
Low-voltage side fuses or circuit breakers are installed to protect the low-voltage busbar and outgoing lines.
In summary, fuses are a low-cost, highly reliable first line of defense for transformer protection. Especially in distribution transformers, their rapid short-circuit interruption and ability to withstand magnetizing inrush currents are irreplaceable. Correct selection, proper configuration, and regular maintenance are key to ensuring fuse compatibility with transformers and achieving their full protective performance.