I. Possible Causes of Transformer Burnout
Absence of Fuses on High/Low Voltage Sides: Some distribution transformers lack fuses on both the high and low voltage sides. Even when dropout fuses or "horn" fuses are installed, aluminum or copper wire is often used as a substitute for the proper fuse element. This prevents the fuse from melting correctly during low-voltage short circuits or overloads, leading to transformer burnout.
Improper Configuration of High/Low Voltage Fuse Elements: Fuse elements on transformers are often oversized. When severe overloads occur, the incorrectly sized fuse fails to blow, resulting in the transformer burning out.
Unbalanced Phase Loads: Due to the prevalence of single-phase power supply for rural lighting, combined with arbitrary wiring practices during construction and inadequate management, distribution transformers frequently experience unbalanced phase loads. Over time, this causes the insulation of the heavily loaded phase winding to age and fail, burning out the transformer.
Tap Changer Issues:
Unauthorized adjustment of the tap changer can result in improper contact or misalignment, leading to overheating and burnout.
Poor quality tap changers can cause incomplete contact of the star point contacts, leading to short circuits or ground discharges.
Oil Leakage and Insulation Degradation: Oil leakage is a common external abnormality. Over years of operation, rubber gaskets and seals at connection points can age and crack, causing leaks. This reduces the oil level and allows moisture to enter, degrading insulation performance and potentially leading to discharge short circuits and transformer burnout.
Lightning Strikes: High and low voltage lines are mostly overhead. Failure to install or properly maintain 10kV arrestors, or delays in their deployment, means the transformer lacks protection against lightning strikes, which can cause burnout.
Multiple Grounding Points in the Core: Multiple ground points in the iron core create circulating currents, generating localized heat that can damage the transformer.
Effects of Low-Voltage Short Circuits: When a ground fault or phase-to-phase short circuit occurs on the low-voltage side, a short-circuit current 20 to 30 times the rated current can be generated. This immense current induces significant mechanical stress in the high-voltage windings, causing them to compress. Repeated stress from such events can loosen insulation components and core clamps, leading to distortion or rupture of the high-voltage coils. The accompanying extreme heat can also destroy the transformer in a very short time.
Human Factors and External Damage:
Galvanic Corrosion: Transformer bushings typically use copper studs, while overhead lines often use aluminum cables. The connection between copper and aluminum is prone to galvanic corrosion, increasing resistance and heat.
Bushing Flashover: Flashover and discharge on bushings are also common abnormalities, often caused by dirt, moisture, or damage.
II. Countermeasures and Solutions
Install and Maintain Fuses: During new installations, ensure high and low voltage fuses are fitted. During operation, promptly replace any blown or stolen fuses.
Correct Fuse Element Sizing:
For transformers with a capacity above 100kVA, configure high-voltage fuse elements at 1.5 to 2.0 times the rated current.
For transformers with a capacity below 100kVA, configure high-voltage fuse elements at 2.0 to 3.0 times the rated current.
Low-voltage side fuse elements should be rated slightly higher than the rated current of the transformer.
Monitor and Balance Loads: Strengthen load monitoring, especially during peak periods. Use a clamp meter to measure the load on each distribution transformer and adjust loads accordingly to prevent unbalanced three-phase operation.
Regulate Tap Changer Properly: Generally, tap changer adjustment is not recommended if the low-voltage side voltage is within the ±7% range. Any necessary adjustments must be performed by qualified technical personnel.
Regularly Check Balance: Periodically check if the three-phase currents are balanced or exceed rated values. If severe imbalance is found, take immediate corrective action.
Maintain Lightning Arrestors: Before the annual thunderstorm season, send all transformer lightning arrestors to a testing department for inspection. Reinstall them only after they pass the tests.
Pre-Commissioning Tests: Before putting a transformer into service, perform the following tests:
Switch the on-load tap-changer three times – ensure no maloperation.
Use the test button three times – ensure correct operation.
Use a test resistor for ground fault testing three times – ensure correct protection operation.
Regular Cleaning and Grounding Checks: Periodically clean dirt from transformer bushing surfaces. Inspect bushings for signs of flashover. Check the grounding system for good contact, and ensure the grounding conductor is free from broken strands, weld cracks, or breaks. Use a megohmmeter to verify that the grounding resistance does not exceed 4Ω.
In conclusion, the operational reliability of distribution transformers is closely linked to the quality of management. With diligent and meticulous work from management personnel, incidents of transformer burnout can be effectively prevented.
III. Daily Operation and Management of Transformers
Strengthen Routine Inspections, Maintenance, and Periodic Testing:
Conduct routine maintenance, promptly cleaning oil stains and dust from the transformer and its high/low voltage bushings. This prevents flashover discharges during humid or rainy weather, which can cause phase-to-phase shorts on bushings and blow high-voltage fuses, disrupting normal operation.
Monitor the transformer's oil level and color. Regularly check the oil temperature, especially during periods of significant load variation, large temperature swings, or inclement weather. For oil-immersed transformers, the top oil temperature during operation must not exceed 95°C, and the temperature rise should not exceed 55°C. To prevent accelerated aging of windings and oil, the top oil temperature rise should not frequently exceed 45°C.
Measure the transformer's insulation resistance. Check all leads for secure connections, paying special attention to the low-voltage output connections for good contact and abnormal temperature.
Enhance load monitoring. During peak electricity usage periods, increase the frequency of load measurements for each transformer. Promptly adjust distribution transformers with unbalanced three-phase currents to prevent excessive neutral current from burning out lines, which could damage user equipment and the transformer itself. For transformers with the Yyn0 connection group, three-phase loads should be balanced as much as possible; using only one or two phases for supply is prohibited. The neutral current should not exceed 25% of the low-voltage side rated current. Strive to ensure the transformer operates without overload or significant phase imbalance.
Prevent External Damage:
Choose a suitable installation location. The site should meet user voltage requirements but avoid remote, desolate areas prone to lightning strikes or theft. Locations that are too remote also hinder regular maintenance and management by operational staff.
Avoid installing low-voltage metering boxes directly on the transformer. Over time, the box's glass may break, or the transformer's low-voltage terminals may be damaged without timely repair. This can lead to water ingress, short-circuiting the meter and subsequently damaging the transformer.
Do not allow unauthorized adjustment of the tap changer to prevent phase-to-phase shorts caused by improper contact, which can burn out the transformer.
Install insulating covers on the high and low voltage terminals of the transformer. This protects against natural elements and foreign object intrusion, which is particularly important in narrow residential areas or forests with high animal activity, preventing short circuits caused by objects falling onto the terminals.
Regularly patrol the lines and clear vegetation from the right-of-way to prevent tree branches from contacting the conductors, which could cause low-voltage short circuits and damage the transformer.
