Introduction: Distribution transformers are critical equipment within the entire power supply system. They primarily convert high-voltage, low-current alternating current (AC) power into low-voltage, high-current power of the same frequency based on electromagnetic inductance principles, supplying electricity for daily user needs. Therefore, the proper operation of distribution transformers directly impacts users' daily lives and production activities, making it of paramount importance. Currently, with the substantial increase in electrical appliances, the required electrical load and power density are also rapidly escalating. Distribution transformers are being used more extensively, and their numbers are growing rapidly. However, some power authorities lack routine inspection and maintenance of distribution transformers, leading to their persistent operation at low efficiency. In severe cases, this causes transformer damage, resulting in direct economic losses.
I. Operational Inspection of Distribution Transformers
1. Inspection of the current in distribution transformers.
After distribution transformers are put into operation, special attention must be paid to monitoring current levels during overload conditions. Current meters are typically installed in distribution panels for direct monitoring. Where no current meter is present, a clamp-on ammeter may be used. The primary focus is on verifying three-phase current balance and ensuring no phase exceeds its rated current. The three-phase load current imbalance rate can be calculated using the following formula: K% = 0-line current ÷ (Phase A current + Phase B current + Phase C current) × 100%. The three-phase load current imbalance rate at the output of distribution transformers should be less than 10%. For the starting point of low-voltage busbars, the three-phase load current imbalance rate should be less than 20%. Prolonged overloading of transformers directly impacts their service life and significantly increases operational losses.
2. Inspection of noise from distribution transformers.
Under normal operating conditions, a distribution transformer typically emits a steady humming sound. If a fault occurs, the transformer's sound pattern will change. To identify the issue, place one end of an insulated rod against the transformer casing and hold the other end close to your ear for careful listening. If the hum is interspersed with crackling or popping sounds, this indicates internal insulation damage, leading to core breakdown. If the noise becomes low, muffled, and heavy, it typically indicates transformer overload or a short circuit. A noticeable increase in sharpness suggests excessively high line voltage. A sudden rise in noise level indicates loose internal components within the distribution transformer.
3. Inspection of the oil level in distribution transformers.
Distribution transformer oil primarily serves as an insulator and coolant. The normal oil level in a transformer should typically be at one-third of the oil gauge on the oil conservator. Both excessively high and low oil levels indicate abnormal conditions. Transformer overload can cause excessive oil temperature, thereby raising the oil level. An abnormally low oil level, especially if it drops below the tank cover, accelerates winding aging, promotes moisture ingress, and reduces the transformer's insulation integrity. When the oil level drops below the windings, it increases the risk of phase-to-phase or phase-to-ground breakdowns. If the oil level falls below the top of the cooling tubes, oil circulation ceases. The transformer then loses its ability to dissipate heat, causing temperatures to rise sharply and potentially leading to the destruction of the distribution transformer.
4. Inspection of High-Voltage and Low-Voltage Fuses for Distribution Transformers
Low-voltage overcurrent in the circuit will cause the low-voltage fuse of the distribution transformer to blow. Primary causes of low-voltage overcurrent include: low-voltage line short circuits, transformer overload, damaged insulation in electrical equipment, and insufficient fuse cross-sectional area selection. Key reasons for high-voltage fuse blowouts in transformers are: transformer insulation breakdown, high-voltage fuse failure, improper fuse cross-sectional selection or installation, and low-voltage network short circuits. Upon detecting a blown fuse, first identify the fault-especially when two or more phases are affected. Only proceed with fuse replacement after confirming the fault. Primary fuse selection generally follows multiples of the transformer's rated current: 1-3 times for 10-100kVA transformers, and 1.5-2 times for 100kVA and above.
II. Operation and Maintenance of Distribution Transformers
1. Conduct insulation resistance measurements and establish measurement standards.
To ensure the normal operation of distribution transformers, it is necessary to measure their insulation resistance. During insulation testing, separate measurements must be taken for each coil's insulation resistance to ground and between coils. Since most distribution transformers used at the author's workplace employ star-connected wiring, both pressing four terminals and pressing three terminals form a circuit path. Therefore, insulation resistance measurements must be conducted separately for each configuration: to ground and between low-voltage terminals. A 2500V megohmmeter must be used for insulation resistance measurement, with readings taken after the pointer stabilizes-typically after 1 minute. It is crucial to note that after completing the insulation resistance test, the measured equipment must undergo discharge treatment. Key factors affecting insulation resistance values include ambient temperature, the applied test voltage, and the duration of voltage application.
2. Daily Operation and Maintenance Management Measures for Transformers
(1) In addition to regularly checking the transformer oil level, monitoring oil temperature is also critical, especially in environments with significant load fluctuations, large temperature differentials, or harsh climates, where inspection frequency should be increased. For oil-immersed distribution transformers, the top oil temperature during operation should remain below 95°C, with a temperature rise not exceeding 55°C. Ideally, the top oil temperature rise should be less than 45°C to prevent accelerated degradation of windings and oil.
(2) Measure the transformer's insulation resistance and inspect the tightness of all leads. For low-voltage output connections, verify both the secure fastening and normal temperature conditions.
(3) Regularly clean and wipe oil stains from the distribution transformer's surface and dust from high/low-voltage bushings to prevent pollution flashovers during rainy weather. Such flashovers can cause phase-to-phase short circuits in bushings, leading to transformer malfunction.
(4) During peak consumption periods, intensify load monitoring. Conduct meticulous load measurements for each distribution transformer, increase measurement frequency, and promptly adjust transformers exhibiting three-phase current imbalance. This prevents lead burnout caused by excessive neutral line current, thereby averting equipment damage.
3. Preventing the Impact of External Forces on Transformers
(1) Installing insulating covers at both the high-voltage and low-voltage terminals of distribution transformers can effectively prevent damage from external objects. In forested areas with frequent animal activity, adding high- and low-voltage insulating covers can effectively prevent low-voltage short circuits caused by falling objects on the transformer's terminal posts, thereby avoiding transformer burnout.
(2) Select installation locations judiciously. Distribution transformers must meet user voltage requirements while avoiding excessively high elevations to prevent lightning strikes. Optimal placement should facilitate maintenance personnel operations.
III. Analysis and Handling of Common Faults in Distribution Transformers
1. Lightning Strikes Causing Damage to Distribution Transformers
Typically, the high and low voltage lines of distribution transformers are introduced and discharged via overhead lines. During a lightning strike, ultra-high voltages dozens of times higher than the rated voltage are generated across the windings. Without surge arresters and low-voltage surge arresters installed, the transformer windings are subjected to short-circuit current surges, damaging the inter-turn insulation. According to extensive survey data, lightning-induced failures account for over 30% of all distribution transformer incidents.
Lightning Protection Measures for Distribution Transformers
Install surge arresters as overvoltage protection to prevent internal insulation breakdown caused by high-voltage lightning waves introduced through high/low-voltage lines. Regular testing of grounding resistance prevents excessive values caused by issues like solder joint failure. If grounding resistance exceeds standards, the high current from a lightning strike on the distribution transformer cannot be effectively diverted underground. Instead, it may reverse-voltage the lightning surge through the grounding wire, elevating it to a high voltage that acts on the transformer, significantly increasing the risk of transformer burnout. Surge arresters must be installed at optimal locations. High-voltage arresters should be positioned near the high-voltage bushings closest to the distribution transformer, minimizing direct lightning ingress. Low-voltage arresters should be installed near the low-voltage bushings closest to the transformer, ensuring timely activation before lightning waves reach the equipment.
2. Short-Circuit Faults Causing Damage to Distribution Transformers
When a single-phase ground fault or phase-to-phase short circuit occurs on the low-voltage side of a distribution transformer-especially in the case of a nearby short-circuit fault-a powerful short-circuit current exceeding 20 times the rated current is generated. This current acts on the transformer's high-voltage winding, causing a rapid rise in internal temperature and generating significant magnetic impact forces. This leads to winding compression, with the stress dissipating once the short-circuit fault is resolved. Repeated stress impacts on the transformer can easily cause loosening or detachment of insulating resin beads and pads, loosening of core clamp bolts, and deformation of the high-voltage winding. Consequently, the distribution transformer may burn out within an extremely short time.
Three primary causes contribute to transformer short-circuit faults: First, external forces-such as falling branches during high winds, broken branches snapping power lines, or vehicles colliding with utility poles-can trigger short circuits. Second, improper installation or maintenance of low-voltage circuit breakers-including the absence of residual current devices (RCDs) or failure to activate them promptly during faults-prevents timely tripping. Third, improper installation or inadequate maintenance of low-voltage meter boxes can cause close-proximity short circuits.
Short-circuit protection for distribution transformers involves installing overcurrent protection devices. Typically, this includes fuses on the high-voltage side and a main residual current device (RCD) on the low-voltage side to safeguard against short circuits or overloads. To enhance transformer protection, careful selection of fuse elements and low-voltage overcurrent protection settings is essential. When selecting high-voltage fuses, ensure they blow promptly during short circuits at internal or external bushing points. For low-voltage RCD overcurrent settings, values are typically set at approximately 1.3 times the transformer's low-voltage rated current. For overcurrent protection settings on low-voltage branch circuits, values should generally be lower than the main protector's overcurrent trip setting and also below the transformer's low-voltage rated current. Overcurrent values are typically selected based on the maximum current-carrying capacity of the conductors. This ensures the distribution transformer trips promptly during overloads, facilitating fault line troubleshooting and achieving genuine protection for the distribution transformer.
3. Distribution Transformer Fire Failures
The primary causes of distribution transformer fires include oil-related ignition and internal transformer faults. Fire failures severely impact transformers, necessitating immediate fire suppression measures.
When a distribution transformer catches fire, immediately disconnect the power supply, stop the cooler operation, and initiate firefighting measures while promptly determining the cause of the fire. For fires caused by oil spilling onto the transformer's top cover, immediately open the oil drain valve to restore the oil level to normal and extinguish surface flames. If the fire originates from an internal distribution transformer fault, avoid draining oil under any circumstances. Draining oil at this stage risks triggering a transformer explosion with catastrophic consequences. The author recommends employing a nitrogen agitation fire suppression system for extinguishing. This system offers a simple structure, high operational reliability, minimal environmental impact, significant fire suppression effectiveness, and convenient maintenance.
IV. Conclusion
With the continuous advancement of national power equipment, electricity consumption has been increasing year by year. Consequently, the maintenance and repair of distribution transformers have become increasingly critical. Understanding and mastering the common causes of distribution transformer failures and their corresponding corrective measures enables prompt troubleshooting, thereby minimizing losses to the greatest extent possible.