Application of fuses in transformers
The core application of fuses in instrument transformers is as a low-cost, highly reliable short-circuit protection element. They protect the instrument transformer itself, connected secondary equipment (such as instruments and relays), and the secondary circuit, preventing equipment burnout or even larger accidents caused by short-circuit faults. The secondary-side characteristics of instrument transformers (especially voltage and current transformers) determine the specific application scenarios and selection requirements of fuses.
I. Core Application Scenario: Secondary Side Protection of Voltage Transformers (VT/PT)
The core function of a voltage transformer is to step down high voltage to low voltage proportionally (e.g., the standard 100V) for measurement by secondary equipment or use in relay protection. Short-circuiting the secondary side is strictly prohibited — due to the extremely small internal resistance of the voltage transformer, a short circuit will generate an enormous short-circuit current, which can instantly burn out the transformer windings or secondary circuit equipment. Therefore, fuses are the most critical protection components for the secondary side of voltage transformers.
Specific Functions
Rapid Interruption of Short-Circuit FaultsWhen a winding short circuit, wiring error, or internal short circuit of secondary equipment occurs on the secondary side of the voltage transformer, the short-circuit current will quickly trigger the fuse to blow, cutting off the faulty circuit within tens of milliseconds. This prevents overheating and burning of the transformer windings, insulation breakdown, and simultaneously protects precision equipment such as connected electricity meters and relay protection devices.
Isolation of Faulty CircuitsAfter the fuse blows, it can isolate the faulty part from the normal secondary circuit, preventing the fault from spreading to secondary equipment of other voltage levels and ensuring the normal operation of non-faulty circuits.
Prevention of Protection Maloperation/Refusal to OperateWithout protection on the secondary side of the voltage transformer, a short-circuit fault may cause the secondary voltage to disappear, triggering maloperation of relay protection devices that rely on voltage signals (e.g., undervoltage protection), or causing devices such as overvoltage protection to lose judgment criteria and refuse to operate. The rapid interruption by the fuse can reduce the interference of faults on the protection system.
II. Limited Application Scenario: Secondary Side Protection of Current Transformers (CT)
The function of a current transformer is to convert large current into small current proportionally (e.g., the standard 5A or 1A). Opening the secondary side is strictly prohibited (an open circuit will generate extremely high induced voltage, endangering personal and equipment safety). However, the risk of short circuits is lower than that of voltage transformers, so fuses are rarely used and only applied in specific scenarios.
Specific Functions (Limited to Specific Cases)
Branch Protection for Multiple Secondary WindingsLarge current transformers often have multiple independent secondary windings (for measurement, metering, and protection respectively). If a short circuit occurs in one winding branch, a fuse can be connected in series in that branch to prevent the short-circuit current from affecting the normal output of other windings.
Simplified Protection for Low-Voltage Small-Capacity CTsFor small-capacity current transformers in low-voltage systems (e.g., 380V), the secondary side short-circuit current is small. Fuses can be used instead of more complex protection devices to achieve low-cost short-circuit protection.
Key Precautions
It is strictly prohibited to connect fuses in series in the main circuit (the common circuit for all windings) on the secondary side of current transformers — if the fuse blows, it is equivalent to an open circuit on the secondary side, which will generate induced voltage of several thousand volts or even higher, potentially breaking down insulation and burning personnel.
III. Core Requirements for Fuse Selection for Transformers
The secondary circuit characteristics of transformers (low voltage, small current, and instantaneous short-circuit current) determine that their fuses must meet the following special requirements, which are significantly different from ordinary power fuses:
IV. Typical Problems and Solutions in Application
Frequent False Blowing of Fuses
Causes: Undersized rated current of the selected fuse, hidden grounding in the secondary circuit (resulting in excessive leakage current), ferroresonance of the voltage transformer (generating overcurrent).
Solutions: Re-calculate the secondary circuit current and replace the fuse with an appropriate rated current; inspect the insulation of the secondary circuit and eliminate grounding points; install a resonance elimination device on the primary side of the voltage transformer.
Fault Expansion After Fuse Blowing
Causes: Insufficient breaking capacity of the fuse, resulting in continuous arcing when blowing, which ignites the insulation of surrounding cables.
Solutions: Replace with dedicated fuses with higher breaking capacity; optimize the wiring of the secondary circuit to avoid flammable materials near the fuse.
Open-Circuit Risk of Fuses on CT Secondary Side
Causes: Incorrect series connection of fuses in the main secondary circuit of CT, or improper selection of branch fuses leading to blowing.
Solutions: Strictly prohibit installing fuses in the main secondary circuit of CT; branch fuses must match the CT capacity and be combined with overcurrent relays to achieve dual protection.
Summary
The application of fuses in transformers is centered on the secondary side protection of voltage transformers, serving as the "last line of defense" to ensure the safety of transformers and secondary systems. In current transformers, however, their application scenarios must be strictly restricted to avoid causing open-circuit risks. The selection and installation of fuses must be combined with the type of transformer, secondary circuit parameters, and fault characteristics to achieve reliable protection effects.