Dry Type Transformer Burn Causes and Countermeasures

Transformer Burn1

Dry type transformer due to the limitations of the operating environment, very easy to appear too high temperature phenomenon, core, winding overheating seriously affect the insulation of equipment, poor ventilation and heat dissipation, moisture, etc., will intensify the internal heating of equipment. transformer burn should not be ignored, should be based on the basic principles of heat and common causes of proper treatment, so as to avoid accidents, seriously affect the safe operation of equipment.

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Table of Contents

What are the main causes of dry-type transformer burn?

Transformer burning2

Transformer insulation resistance drop

(1) Due to moisture caused by the insulation decline. At present, all the transformers in the tube have been replaced by resin-cast dry-type transformers, and the windings are sealed by resin, so most of the drop in insulation resistance is caused by the destruction of the resin on the surface of the windings or the moisture of some insulation materials. When the dry-type transformer in the use of the process of moisture, due to corrosive substances in the water, so that the enameled wire mould deterioration, insulation paint off or insulation degree decline, resulting in between the turns, between the layers of the discharge phenomenon, thus causing a short circuit between the turns or layers of short circuit. When the short circuit generally first heat, and then appear odor, smoke, so when the dry-type transformer odor, should immediately stop power to check the cause.

(2)Due to environmental pollution caused by the insulation decline. At present, the location of the transformer and distribution substation and partition in the tube is close to coal mines and highways, soot pollution is serious, and there is also the phenomenon of willow and poplar wadding entering the interval in spring and summer, and the equipment cleaning cycle is longer than the pollution cycle, so there is a hidden danger of the insulation of the equipment dropping due to pollutants and causing discharge along the surface.

Learn more  about The Dry-type Transformer

Heating of the iron core

(1) core quality and internal insulation problems. Dry-type transformer core made of silicon steel sheet stacked, when in the production process due to the quality of silicon steel sheet, there will be core magnetic density is not enough, easy to saturate, the operation of the core heating, thus affecting the transformer load capacity, silicon steel sheet in the gluing due to construction problems appear gaps, will cause internal partial discharge, overheating.

(2) There are multiple points of grounding in the core. Core multi-point grounding is mainly the core from the metal shell insulation distance is not enough, fixed bolt installation direction error caused and core insulation distance is not enough, the insulation of the core is damp or damage, other foreign body lap caused by the core grounding. When the core is grounded at multiple points, there is a ring current inside the core, which is equivalent to a small short circuit, and there is resistance in the short circuit part, so when there is a ring current it will cause the core to heat up. When the multi-point grounding and a long time is not dealt with, transformer continuous operation, core abnormal heating will affect the core internal insulation, and then cause more serious core overheating, core will burn, serious even make the transformer accident.

Winding heating

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The reasons for winding heating are: ① in the manufacturing process of copper wire cross-sectional area selection is unreasonable, resulting in operation when there is a shock load or surge heat; ② design and operating environment does not match, running load, but the space is small, poor heat dissipation caused by overheating; ③ annual cyclical maintenance test process due to personnel quality problems, in the maintenance equipment left short wiring, fuses, metal tools caused by winding short circuit; ④ The temperature detection element installed outside the winding is shorted due to poor contact, causing the winding to heat up.

Poor ventilation and heat dissipation system

Dry-type transformers are exposed to the air, through the air circulation and distribution room industrial air conditioning to regulate equipment heat, there is no direct cabinet cooling device, the cabinet only closed observation window, can not carry out a good temperature transfer, transformer operation will produce a certain copper loss and iron loss, transformer winding and iron core heat, heat generation is too large when the need for continuous heat dissipation, and the current tube distribution cabinet only automatic heating system. Effective heat dissipation is not possible. When the transformer winding temperature rises to a certain level, it will inevitably damage the insulation properties of the system and cause the insulation parts to deteriorate. Therefore, the operating temperature of the transformer windings needs to be reduced as much as possible.

Transformer load three-phase unbalance

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Transformer load three-phase unbalance

Dry-type transformers have copper loss and iron loss two kinds of loss, iron loss is generally unchanged, copper loss will change according to the changes in the load. When three phase load unbalance operation, will cause phase current is too large, thus causing winding overheating, accelerate dry transformer aging, insulation performance decline, reduce the service life of dry transformer, and even burned winding.

At the same time three phase unbalanced conditions running dry-type transformer will produce zero sequence flux, zero sequence flux flow through the circuit composed of metal parts, and steel materials do not have the function of magnetic conductivity, so there will be hysteresis phenomenon and eddy current loss, so that the steel parts heat up, resulting in transformer temperature rise too fast.

Increased line losses

Transformer three-phase load imbalance should not be greater than 15%, only with a small amount of single-phase load three-phase transformer neutral line current should not exceed 25% of the rated current. Three phase load unbalance will produce zero sequence flux, zero sequence flux in the neutral line will become neutral line current, not only the phase line will have loss, while the neutral line will also produce loss, thus increasing the line loss.

At present, most of the load within the three-phase four-wire system, in the three-phase load balance line power loss is minimal, when the three-phase load imbalance in the state of operation, the line in the imbalance of the current generated by the additional loss will make the line in the power increase, resulting in line heating, increase line loss, but also increase the impact on the line.

Shorten the life of the load

When there is a load imbalance, the larger load will impact with a transformer dry transformer burning causes and countermeasures electrical equipment, accelerating the frequency of replacement of components, but also to increase the cost of equipment maintenance, for the transformer and distribution equipment, in the three-phase load imbalance conditions of operation, the output current of each phase is unequal will lead to its internal three-phase voltage drop is not equal, resulting in When the voltage is unbalanced, it is easy to cause the user transformer burn in one phase with high voltage, while the user power equipment in one phase with low voltage may not be available. Therefore, when the three-phase load is unbalanced, it will seriously affect the quality of the load operation and operational safety.

Read more: Dry-type Distribution Transformers Solution for Public Building

Situation Analysis of Transformer Burn

Accident overview

Two accidents occurred at an interval of 9 months, transformer burn model with the same SCB10-80/10 epoxy cast dry-type transformer with a capacity of 80 kV-A. 1#SB had a single-phase ground fault, causing winding burn, 2#SB had a high operating temperature and winding burn.

Incident investigation

Associated events

The investigation found that the institute was a construction and renovation institute, and due to poor control during the excavation of the outdoor site, rainwater backed up into the cable trench of the distribution room during the thunderstorm season, and was subsequently only drained without effective drying.

The office was far from the station area and the heating equipment could not meet the living heating, so the logistics added graphene wall-mounted heating devices to protect the staff heating, but there was no communication with the professional person in charge of the production department during the construction process, resulting in the load being connected to one phase. At the time of exiting all wall-hung devices the three-phase load was A: 7 A, B: 6 A, C: 9 A, with an unbalance of 23%. As the load was not large, the infrared temperature measurement showed that the transformer core was running at 37°C. When the wall-hung devices were put into operation, the load was A: 7 A, B: 37 A, C: 9.2 A, with an unbalance of 117% and a serious unbalance, and the core was running at 67°C.


(1) Water in the cable trench was damp at the same time as the ventilation and heat exchange did not meet the requirements, causing the insulation of the equipment to drop and the windings to heat up at high loads.

(2) Serious imbalance of the three-phase load, resulting in excessive zero sequence current and heating of the neutral line. When F-class insulation material is used, the limit working temperature is 155℃, the maximum temperature rise should be less than 100℃, the highest operating temperature is not enough to make the transformer burn, but due to the internal moisture of the equipment, the insulation performance decreases, and the possibility of equipment burn exists under the influence of comprehensive factors.

(3)equipment quality problems, equipment running for 3 years, the load is relatively small, there may be internal insulation layer in the manufacture of the subsequent processing without drying, resulting in internal slight discharge gradually expand for equipment burn.

Read on:Ultimate Transformer for Guide

How to Prevent Transformer Burn?

Develop a management system for the operation of dry-type transformers

Incorporate the used variable load monitoring into the daily inspection, and conduct infrared temperature measurement on the operating transformer once a day, and at the same time strictly prevent the distribution room from entering water and getting damp. When the dry-type transformer is damp, it should be dried in time and the fire door should be opened every week for ventilation and heat exchange to ensure that the distribution room is dry. The two AC power supplies are hot standby for each other, and are run in rotation on a monthly basis, with the switchover taking place on the first day of each month, with 1# power supply running in a single month and 2# power supply running in a double month.

Strengthen standardized operation management

Strictly implement standardized operation in the process of inspection and repair work, do a good job of rechecking the equipment after repair, controlled by the team leader, work leader, work team manager and substation duty officer in multiple links, eliminate the internal remains of the equipment and prevent multi-point grounding of the iron core. At the same time, standard parts are used to prevent the phenomenon of insufficient insulation distance caused by over-long bolts. When insufficient insulation is found during the test process step-by-step insulation resistance is measured to ground for individual windings, clamping pieces, piercing screws, insulation layers, etc. to confirm the parts with insufficient insulation.

Strengthening control of winding temperature

Monitor the transformer winding operating temperature, use infrared temperature measurement for accurate recording, find overheating in time to count the operating load at that time, and switch the AC power system, when it is determined that the load is due to the load in time to adjust the electrical load. At the same time, a temperature and humidity meter is installed in the distribution room, and the indoor temperature is paid attention to in real time in summer, and industrial air conditioning is put into the distribution room in time, and the door of the distribution room is opened regularly for ventilation to reduce the ambient temperature in order to facilitate heat dissipation from the transformer. Thoroughly clean the transformer winding, core, insulation layer on the accumulation of ash during the inspection and maintenance, in order to facilitate transformer heat dissipation.

Use of scientific and technological means to prevent transformer overheating

(1) Install arc light device. Arc light device with automatic fault location function, installed in the cabinet can occur in the equipment fault, accurate positioning fault point, confirm the specific location of the arc light, while the system for self-test, issue an alarm signal, timely notification of the duty officer to exit the system.

(2) retrofitted temperature monitoring unit. At present, the temperature and humidity meter is installed in the upper cabinet, through the inductor can start the heater when the temperature decreases, in the back of the cabinet to add temperature monitoring unit can be monitored through the sensing probe, fluorescent optical fiber, temperature measuring device, etc., while the probe diameter is less than 3 mm, can monitor -20 ~ 150 ℃, the measurement accuracy of ± 1 ℃, can operate in the electromagnetic environment, can be connected to the existing integrated self-system, automatic monitoring equipment operation.

(3) Innovative modification of the rear cabinet observation window. Design and make visualisation of the rear cabinet exhaust device, using industrial exhaust fans, taking power from the AC power supply of the front cabinet, using the existing temperature and humidity controller node, able to turn on the heater when the temperature is low, able to conduct the exhaust circuit when the temperature is too high, while both observing the internal operation and the function of ventilation and heat exchange.

(4) Increase partial discharge tests on a cyclical basis. Increase the monitoring of partial discharge test of air cabinets, and through partial discharge test, discover the hidden danger points which are not easily found by routine maintenance test, and eliminate them in time to prevent the expansion of equipment damage due to partial discharge.

Strengthen load management

(1) Improve the monitoring of the three-phase load of power transformers and incorporate it into the daily management regulations and assessment system of transformer and distribution substations.

(2) Strengthen the access management system for domestic electricity consumption in substations and distribution stations to ensure the reasonableness and balance of three-phase load distribution.

(4)Focus on zero sequence currents, use electricity meters to focus on zero sequence currents during peak load hours and make adjustments when unbalance is found to exceed 25%, and choose a reasonable neutral line cross-section during installation.

Read my article on Selection of Grounding Cables for Dry-Type Transformer Shells


Through the above means can be more scientific planning distribution of low-voltage load distribution, standardize the collection and collation of load base information, while through the analysis of accidents to find out the possible causes, point by point, improve the operation and management methods of substation, more effective protection of dry-type transformer operation safety.

Keep on reading: What Is The Best Way To Dry Out a Wet Transformer?

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