Causes and Solutions for High Failure Rate of Distribution Transformers

1. Causes of Failure in Agricultural Distribution Transformers

(1) Insulation Damage

Rural power supply typically utilizes 380/220V mixed supply systems. Due to the high proportion of single-phase loads, distribution transformers often operate under significant three-phase load imbalance. In many cases, the imbalance exceeds the permissible range specified in standards, causing premature aging, deterioration, and failure of transformer winding insulation, ultimately leading to burnout.

When distribution transformers experience prolonged overload conditions, low-voltage side line faults, or sudden significant load increases, damage can occur due to inadequate protection. The absence of protection devices on the low-voltage side, combined with high-voltage side drop-out fuses failing to operate in a timely manner (or not operating at all), allows transformers to carry currents far exceeding their rated values (sometimes several times the rated current). This causes dramatic temperature increases, accelerating insulation aging and leading to winding burnout.

After extended operation, sealing components such as rubber beads and gaskets deteriorate, crack, and fail. If not promptly detected and replaced, this leads to oil leakage and declining oil levels. Moisture from the air then enters the insulating oil in large quantities, drastically reducing its dielectric strength. Severe oil deficiency may expose the tap changer to air, causing moisture absorption, discharge, short circuits, and transformer burnout.

Manufacturing defects also contribute to failures. Inadequate production processes, incomplete varnish impregnation of winding layers (or poor-quality insulation varnish), insufficient drying, and unreliable winding joint welding can create insulation vulnerabilities. Additionally, adding substandard insulating oil during maintenance or allowing moisture and impurities to enter during repairs degrades oil quality and insulation strength, eventually causing insulation breakdown and transformer failure.

(2) Overvoltage

Lightning protection often fails due to grounding resistance values that don't meet requirements. Even when initially compliant, grounding systems can deteriorate over time due to steel corrosion, oxidation, breakage, or poor welds, causing dramatic increases in grounding resistance and rendering transformers vulnerable to lightning damage.

Improper lightning protection configuration is common. Many agricultural distribution transformers have surge arresters installed only on the high-voltage side. Since rural power supply predominantly uses Yyn0 connected transformers, lightning strikes can generate both positive and reverse transformation overvoltages. Without low-voltage side surge protection, transformers become significantly more susceptible to damage from these overvoltages.

Rural 10kV power systems frequently experience ferromagnetic resonance. During resonance overvoltage events, the primary current of distribution transformers can increase dramatically, burning windings or causing bushing flashover and even explosion.

(3) Poor Operating Conditions

During summer high-temperature periods or when transformers operate continuously under overload conditions, oil temperatures rise excessively. This severely impacts heat dissipation and accelerates insulation aging, deterioration, and failure, significantly shortening transformer service life.

(4) Improper Tap Changer Operation or Poor Quality

Rural electricity consumption features dispersed loads, strong seasonal patterns, large peak-valley differences, long low-voltage lines, and significant voltage drops, resulting in substantial voltage fluctuations. This leads to rural electricians frequently adjusting transformer tap changers themselves. Most of these adjustments don't follow proper procedures, and DC resistance values aren't measured and compared after operation before returning to service. Consequently, many transformers fail due to improper tap changer positioning, poor contact, increased contact resistance, and burned tap changers.

Poor-quality tap changers with inadequate static and dynamic contact, or mismatched position indicators (where external markings don't correspond to actual internal positions), can cause discharge and short-circuit accidents after commissioning, resulting in damaged tap changers or entire windings.

(5) Transformer Core Grounding Issues

Quality problems in distribution transformers can cause the insulation varnish between silicon steel sheets to age prematurely during long-term operation, resulting in multi-point core grounding and transformer failure.

(6) Long-Term Overload Operation

With rural economic development, electricity demand has increased dramatically. However, insufficient addition of new transformers or replacement with higher-capacity units has forced existing transformers to operate continuously under overload conditions. Additionally, the high proportion of single-phase loads in rural areas makes three-phase load balancing difficult to achieve, causing certain phases to experience severe long-term overloading while neutral line currents significantly exceed permissible values. These conditions ultimately lead to transformer burnout.

2. Countermeasures

According to relevant standards, every distribution transformer must have three fundamental protections: lightning protection, short-circuit protection, and overload protection.

For lightning protection, surge arresters must be installed on both high and low-voltage sides of the transformer, with zinc oxide arresters being the preferred option.

Short-circuit and overload protections should be considered separately. High-voltage side drop-out fuses should primarily protect against internal short-circuit faults within the transformer, while overload conditions and low-voltage line short circuits should be handled by low-voltage circuit breakers or fuses installed on the low-voltage side.

During operation, phase load currents should be regularly monitored using clamp meters to verify that three-phase load imbalance remains within permissible limits. When excessive imbalance is detected, immediate load redistribution is necessary to bring the imbalance within standard limits.

Regular inspection and testing of distribution transformers according to prescribed schedules is essential. Inspectors should check oil color, level, and temperature for normality, and look for oil leakage. Bushing surfaces should be examined for flashover or discharge marks. Any abnormalities must be addressed immediately. Periodic cleaning of transformer exteriors to remove dirt and contaminants from bushings and other surfaces is also recommended.

Before each annual thunderstorm season, a thorough inspection of both high and low-voltage surge arresters and grounding leads must be conducted. Non-compliant arresters must be replaced. Grounding leads must be free of broken strands, poor welds, or breaks. Aluminum conductors should never be used for grounding leads; instead, 10-12mm diameter steel rods or 30×3mm flat steel strips are recommended.

Grounding resistance should be tested annually during winter under favorable weather conditions (at least one week of continuous clear weather). Non-compliant grounding systems must be remediated.

Connections between transformer terminals and overhead line conductors on both high and low-voltage sides should use copper-aluminum transition components or copper-aluminum equipment clamps. Before connection, surfaces of these components should be polished with fine-grade sandpaper and coated with an appropriate amount of conductive grease.

When operating transformer tap changers, procedures must be strictly followed. After adjustment, the transformer should not be immediately returned to service. Instead, DC resistance values of each phase should be measured with a bridge before and after operation and compared. If no significant changes are observed, the post-operation phase-to-phase and line-to-line DC resistance values should be compared, with phase differences not exceeding 4% and line differences less than 2%. If these criteria aren't met, the cause must be identified and corrected. Only after meeting these requirements should the transformer be returned to service.