Trends In The Standards For Dry Type Transformers

With the increasing use of Dry type transformers, it is important for the utilities and the regulators to have norms for losses for Dry type transformers – as we have norms for oil immersed transformers in form of star labelling programme... - Sailesh Purohit, Thomas Lim, Radoslaw Szewczyk

Today the urban metropolitan cities, industries, power plants, high population density areas with sky scrapers, malls, andairports, metro rails have become a necessity. The spiral growth in the civic, business and industrial development has forced the system designers to look for safe and reliable solutions.

The public utilities have a big challenge to cater to the exponentially growing demand – and simultaneously maintaining the distribution system healthy and reduce the outage of the transformers.

The utilities with high population densities like CESC Kolkata; BEST Mumbai and Reliance Mumbai, Orissa and Assam have converted their distribution transformers from oil to dry – depending on the safety and maintenance needs, as these utilities have identified that working with the traditional oil immersed distribution transformers may not help them solve the concerns around safety and reliability. Considering the changing trends and the growth in the Dry type transformer markets, the need of the hour is to establish the right kind of standards that define the safety and reliability of these transformers.

What is the present state of the utilities?

The utilities in the country face difficult challenges on safety of oil immersed transformers, almost all the cities in the country are old and congested – where we can find oil filled transformers in very close vicinity to the houses buildings etc. The examples are the narrow by-lanes of cities like Delhi, Jaipur, Gwalior and Indore.

With the increase in population leading to the increase in load, the transformers in these locations inside the crowded cities have become susceptible to fire hazards (with oil immersed transformers).

India is a vast country and have a variety of climatic conditions – ranging from extreme hot nearing 50 Dec C in the hot deserts of Thar to a vast coast line with a high humid environment – and also high mountain ranges having sub-zero temperatures in places like Kargil. We need transformers that can work in all environments, pollution levels, and provide utmost safety to the establishments and people around them.

The most common technologies prevalent in the dry type transformer space are Open ventilated Dry type and Cast Resin technologies, while driving the specifications for Dry type transformers – all concerns are related to the fire safety, environmental and climatic conditions.

If we see the newspaper headlines related to the oil immersed distribution transformers, we often find pictures of the recent fire on 25th August 2014 in the narrow by lanes of Chandani Chawk in Delhi. Shown below:

The impact of the incident was: 25 shops were destroyed and put to flames although no human causality was reported. Some incidents turn very severe, and cause incidents like UPHAR CINEMA – where 59 people lost their lives as an oil immersed transformers caught fire in the basement.

The Indian Electricity Rules 1956 were amended on 25th Nov 2000, and mandated the use of Dry type transformers in all indoor applications. However, the rule says ‘Dry type’ – but it is now the responsibility of the standards to define ‘HOW FIRE SAFE?’

The IS 11171:1985 is the present standard for the Dry type transformers in India. This standard is derived from the IEC 726 1982, subsequently the ICE was called the IEC 60076-11 the 60076 series.

The second most important aspect for the use of Dry type transformers is the energy efficiency. With the increasing use of Dry type transformers it is equally important for the utilities and the regulators to have norms for losses for Dry type transformers –as we have norms for oil immersed transformers in form of star labelling program.

Standards bodies provide an important platform for the cumulative experience of the users and manufacturers, and leading in the education and introduction of practices for the improvement on the quality, reliability and safety of products and services.

The standards document developed are thus ‘living’ documents and are constantly evolving as the society demands change.

To address the changing needs of the country and increasing demand for the Dry type transformers, BIS (Bureau of Indian Standards) is looking to revise the IS 11171:1985 as per IEC standards 60076-11.

How do the standards stand today?

IS11171:1985 titled ‘Dry-type transformer’ is the primary reference and application standard for Dry type transformers in India.

IS11171:1985 is primarily based on IEC 726 (1982) ‘Dry type power transformers’ issued by the International Electro-technical Commission.

IEC 726 (1982) has been replaced by IEC60076-11 (2004-05) with the same title, and now forms a part of the IEC60076 series of standards on power transformers.

Based on the experiences and issues of users and manufacturers, there are some major additions in the IEC60076-11 standard focusing on improving the dry type transformer reliability and safety.

What are the major differences in the IEC 60076-11 and the present Dry type standards IS 11171:1985? Learning from the cumulative experience of the users and manufacturers, the new draft document for IS 11171 (subject to revision) includes some additional tests to define the fire safety standard of the transformer, the climatic conditions standard for the transformer and the environmental standard for a dry type transformer. We will now have a closer look at the new draft standards – and what it would mean to the transformer manufacturers users and the specifies.

Additions proposed in the IS 11171 draft

Three distinct additions in the IS 11171 standards draft are:

  • Environmental classification
  • Fire behaviour classification
  • Climatic classification.

These three classifications are added because of concerns expressed by users including:

How does the Dry type transformer product perform under high humidity or pollution?
How safe is Dry type transformer under fire?
The classifications intend to offer more clarity on the performance of the Dry type transformers under different operation conditions.

Environmental classification

Defining the environmental conditions in which the Dry type transformers can operate reliably in terms of humidity, condensation, pollution and ambient temperature.

Three operation conditions for considerations

  • No condensation and pollution (class E0)
  • Occasional condensation and limited pollution (class E1)
  • Frequent condensation and heavy pollution (class E2)

The insulation system resistance level to moisture and pollution varies with the insulation material used and processes applied. The classifications are based on the different pollution conditions impacting the transformer, the worst condition are described E 2 as under.

Brief test description (most severe condition)

  • Transformer is placed in test chamber with
  1. Humidity greater than 93%
    B. Atomised water conductivity 0.5-1.5 S/m period of 6 hrs
  • Within 5 min after removing from the test chamber, the transformer is injected with 1.1 times rated voltage for 15 min

PIC showing the environmental test E2 on a Dry transformer in the test chamber…

Fire behaviour classification

Defining the risks of fire on the dry type transformers. Expected safety performance of the dry type transformer in terms of flammability, opaque smoke and toxic substances emitted.

Two operation conditions for consideration:

  • No fire risk to consider(class F0)
  • Fire hazard (class F1)

Though the transformer may not be the cause of fire, but under an external fire, there should be minimal contribution of thermal energy, emission of opaque smoke and toxic gases.

Test measurements

  • Temperature variation over time (heat contribution by object)
  • Optical transmission of light (smoke opacity)
  • Gases emitted (toxicity)

The test conducted for the fire behaviour is a destructive one. The coil is placed in the chamber and put to fire to see the behaviour of the coil under fire.

PIC showing the fire behaviour F1 class: the transformer coil is placed in the test setup…

Brief test description 

Test facilities

  • Dimensions and construction of fire chamber
  • Monitoring devices (temperature, optical, gas detection)

Ignition source
Ethyl alcohol placed directly below 40mm away from the test object – and fire to last for approximately 20 min

Heat source
Heating panel 800mm (h) by 500mm (w) with expected temperature of 750degC for 40 min

Climatic classification

Defining the temperature in which the dry type transformers can operate reliably in. Two operation conditions for consideration

  • Ambient Temperature of -5 deg C (Class C1)
  • Ambient Temperature of -25 deg C(Class C2).

The difference in the thermal expansion between the solid insulation and conductor may induce mechanical stresses (thermal shock) causing the insulation system to fail.

Test description (most severe condition)
Transformer is placed in test chamber controlled at -25degC for 12 hrs. Current at two times the rated current is injected into the transformer until rated temperature of the transformer is reached.

PIC showing the coil of an OVDT transformer after the test…

Transformer returned to normal ambient for 12 hrs and subjected to separate source and induced voltage test.

A typical setup for the behavior test as described in the IEC 60076-11…

What makes Dry type transformers safer and reliable?

Nomex paper
For the new trends on the standards, Nomex paper will provide superior performances. It provides a unique combination of properties found in no other insulating materials.
Long term stability for continuous exposure to 220 Deg C
Will not melt, flow or support combustion below 250 Dec C
Strong resistance to acids and alkalis
Compatible with all varnishes, resins, adhesives, and fluorocarbons
Stable properties over a wide range of temperature and moisture conditions
Inherently flame retardant; self-extinguishing

Nomex is a class C insulation material, which performs at 220 Deg C. The insulation used on the conductors for Relitran co-branded transformers is 2Mil Nomex paper and all other solid insulation material used for manufacturing of these transformers is class H.

When the fire behaviour test is performed on dry type transformers (IEC 60076-11) using Nomex has a better Optical Transmission Factor (OTF) during burning. The graph below indicates OTF (Visibility) of about 60% to 75% for a time period ranging from 5 minutes to 45 minutes (Max standard temperature range) of burning. The graph below translates into better visibility as the smoke is not dense or black. In case of fire, people do not die of fire but the damage is due to the suffocation in toxic smoke. The injuries to people trapped in the fire are due to stampede because of poor visibility.

The graph in the fig. below compares the flame resistance behaviour of Nomex with other materials used in the dry type transformers, It is found that the Limiting Oxygen Index of Nomex is very high, which means that it requires more oxygen to burn – and when this insulation is burnt, it does not support combustion so it does not spread fire.

Environmental and climatic conditions performance of Nomex based OVDT transformers

VDT Transformers are well protected from invasion of water high quality vacuum impregnation technology using the most modern equipment. Nomex is not adversely affected by water, even if it does become exposed to 95% RH

  • It retains 80% of dielectric strength
  • It retains low dielectric constant and dissipation factor
  • It retains superior volume resistivity.

The VPI (Vacuum Pressure Impregnated) Varnishes used with UL approval for usage at 180 deg°C and higher for continuous operation in the hottest areas. Nomex insulation is fully coated with varnish makes the encapsulation of the transformer.

Temperature limits for various insulations classes

The insulation system of transformers represent the highest temperature the electrical winding can accept while operating at the highest load condition (Hot Spot) and should have insulating materials which at least match this rating.

Highest Temperature = Average Winding Rise (K) + Max. Ambient + Hot Spot Allowance


Every B class city in India has a part called the old city where the population density is very high. The government needs to think of redefining the need for installation of dry type transformers in the distribution utilities not only in the indoor application – but also in outdoors where the population density is very high, especially in these old city areas.

The OVDT Nomex based designs are safer, reliable and help in reducing the environmental foot print. With the increasing load demand and the load density of individual household, these transformers with inherent overload capacities are the best option for the builders, utilities and the industry.

The OVDT transformers made out of Nomex insulation will tend to resist to the fire – as the material does not support fire, and in unfortunate case of fire these transformers will help save the human lives and reduce the loss due to fire. The inherent properties of the insulating material used and the VPI technology make the transformers resistant to environmental effects. The higher temperature endurance of the Nomex helps the OVDT transformers bear short-term overload without damage to the insulation.


Leave a Reply