env2 - transformers environment

The Transformer EnvironmentEEnnvv22

E2 – Transformers Environnement - 04/2004 - 2 / 7 --

Introduction

Transformers, just like devices, are exposed to environments that

influence the way in which they age and operate. Such

environments have an aggressive effect on them and may even

destroy them should such effects be too long-lasting.

It is thus important to recall some rules and precautions to be

applied to protect transformers from aggressions of any kind,

whether or not the transformers are in operation.

Such is the purpose of this document.

Aggressive environmentsLike all transformers, the TRIHAL transformer is subject to physical

and chemical aggressions that depend on the quality of its

environment. Potential aggressions are:

- humidity,

- physical (dust, sand) and chemical (fumes) pollution,

- wind.

Such aggressions may occur during storage and operation of the

transformer.

To determine the type of environment of a site, reference can be

made to the set of IEC 60721 standards dealing with classification

and severity of environmental agents.

During the storage period :

During storage, the transformer is at ambient temperature. During

this period, its insulating materials may be attacked by ambient

humidity:

- humidity penetrating the centre of the material,

- surface condensation.

This may result in an arcing hazard when the transformer is

energised. Relative humidity must therefore be limited to less than

90% during storage, and absence of condensation must be

ensured prior to energisation.


During operation :

The transformer may be exposed to a variety of aggressive factors

when it is in operation:

High degree of ambient humidity

Despite the fact that windings operate at a temperature greater

than ambient temperature, a very high concentration of humidity

may lead to penetration of humidity in insulating materials

adversely affecting their properties.

Conductive dust

Dust accumulates on HV windings due to the effect of the electric

field and can result in reduction of creepage distances, thereby

encouraging arcing.

Hydrocarbon fumes (cutting oil fumes, etc.)

Due to the electric field, these hydrocarbons are concentrated

around the HV windings.

Once deposited on their surface, hydrocarbons may evolve

chemically as a result of coil temperature, particularly at the top,

and form run-outs or deposits that have an adverse effect on coil

surface electric fields, encouraging arcing. The presence of run-

outs or deposits may also favour accumulation of conductive dust.

Chemical pollution

Some chemical substances resulting from pollution may lead to

surface modification of insulators by chemical corrosion. Such

corrosion is affected by factors such as humidity and temperature.

Insulator surface modifications may result in deterioration of

electrical properties: modification of track resistance.


Dust, sand, salt spray with windThe effects of these natural environmental agents are closely

linked with wind and sometimes aggravated by wind. They can

affect transformers in various ways:

- penetration of dust in enclosures,

- deterioration of electrical properties, such as poor contact or

modification of track resistance,

- seizure or disturbance of fan movement,

- surface abrasion of insulators that can lead to modification of

humidity resistance,

- presence of conductive dust: this dust accumulates on HV coils

due to the electric field and can result in reduced creepage

distances thus encouraging arcing,

- obstruction of ventilation inlets.

Damp, hot atmospheres combined with chemically aggressive dust

cause corrosion, together with atmospheric salt spray.

Fine dust is hygroscopic, thus resulting in formation of a surface

conductive layer on HV coils that can lead to reduced creepage

distances, encouraging arcing.

Limits to be complied with :

To limit the effect of some of the above aggressive agents, their

severity must not exceed the following thresholds:

- relative humidity ≤ 90%

- sulphur dioxide ≤ 0.1 mg/m3

- nitrogen oxides ≤ 0.1 mg/m3

- dust and sand concentration ≤ 0.2 mg/m3

- marine salt concentration ≤ 0.3 g/m3

These conditions normally correspond to the following

environments for fixed applications protected against bad weather

conditions (see standard IEC 60721-3-3 ).

- areas located in urban regions with industrial activities or subject

to dense traffic conditions,

- places without specific precautions to minimise presence of dust

but that are not located near to dust sources.

Allowance for these environmental considerations is essential to

avoid reducing the lifetime of these transformers, which involve

serious investment and whose use spans a number of decades.


The thermal environment

To ensure optimum transformer cooling, we give below the

recommendations to be complied with for proper ventilation of

devices:

irrespective of room dimensions.

whether or not the transformers are equipped with a metal

protective enclosure.

Note that these recommendations are not specific to the

transformer but apply to the electrotechnical sector in general.

They allow optimum use without deterioration of electrical

switchgear and avoid overheating situations causing deterioration

of insulators and premature ageing of equipment.

The stack effect principle

Hot air is less dense than cold air and naturally rises in ambient air.

Consequently, the hot air leaving the transformer in operation rises

towards the room ceiling. Efficient ventilation is based on its

capacity to expel hot air to the top of the room. For this purpose, a

cold air inlet must be placed as low as possible on the room wall,

and a hot air outlet as high as possible on the opposite wall.

The greater the clearance above the transformer and the greater

the amount of air rising and thus the better equipment cooling.

Placing cold air above the transformer prevents hot air from leaving

the device. The result is a hazardous rise in transformer

temperature.

This is the typical example of air conditioning placed above heating

apparatus. Although it is very cool in the transformer room, the

device temperature will rise until it trips the thermal protection

alarm, if any.

If unprotected, its insulators will undergo premature ageing.

The only solution available to the user is then to…stop the air

conditioning system.


Room design

Sizing rules

The aim of proper ventilation of the room is to dissipate all the heat

produced by heating apparatus (transformers, motors, heaters,

etc.) placed in the room.

In point of fact, in normal operating conditions, transformers and all

devices giving off heat generate losses P, expressed in kilowatts

(kW).

To discharge such losses, correct natural ventilation of the room

requires:

- cool incoming air for a net surface* S (m)² located at the bottom

of a room wall, close to the transformer base,

- warm outgoing air for a net surface* S’(m²) located at the top of

the opposite wall, if possible vertically to the transformer, and at a

height H (metre) with respect to the bottom inlet; the air outlet must

be larger than the inlet.

These net surfaces* are defined by the formulas:

The area above the transformer must remain free up to the ceiling

except for the connecting items.

These formulas apply for a substation:

- Installed at a maximum altitude of 1000 metres,

- Whose annual average ambient temperature is 20°C.

If these surfaces cannot be complied with, forced air circulation

must be organised by the installation of:

- a fan blowing cold air inside, via the bottom inlet, whose flow rate

Q (m3/ second) will also depend on the losses P (kW) with the

formula: Q = 0.10 x P- a fan discharging hot air outside, via the upper outlet whose flow

rate Q’ (m3/sec) will be: Q’ = Q x 1,1.

These two conditions can also be combined:

either by respecting inlet surface and outlet flow rate,

or by respecting inlet flow rate and outlet surface.

* Net opening surface: real surface of anopening, after deduction of all obstructing items(grid, bar, flap, etc.)

S = 0.18 x P and S’ = S x 1.10 √H

Natural cooling of the room


Application of these rules

- The degrees of protection

The cool air inlet and hot air outlet cross-sections used in the

above calculation are net surface areas*.

According to the IP degree for grids equipping wall openings, the

surfaces to be installed to ensure efficient air passage net cross-

section may be large: as an example 50% of the IP31 panel grids

of the Trihal transformer are pierced.

- Presence of other devices in the room

The presence of other devices, a source of heat in the room, must

be considered when sizing surfaces or air flow rate. The losses

that they give out at full power will be accounted for in P (KW).

- Fans installed under the transformer coils

The presence of these fans (“Forced air” option) does not change

in any way the rules mentioned above: they bring in cool air from

the air inlets and expel hot air outside the transformer or the metal

enclosure. Hot air must then be expelled from the room by the

outlets of appropriate dimensions or equipped with an air extractor.

Air quality

DustDust deposited in large amounts on the transformer parts acts as a

thermal insulator: the device temperature increases. Regular

cleaning must then be performed by suction (and not blowing!).

Cement works are particularly affected by this problem.

Ambient humidityHumidity is not an aggravating factor in overheating.

However, the presence of heating resistors in the room to remove

condensation must be taken into account in the sizing of rooms

and… the ventilation inlets.

With a room that is correctly designed and sufficiently ventilated,

the transformer can withstand the loads applied and even

overloads, provided that they are governed by balanced

management and conform to standards (see IEC 354 and IEC

905).

env2 - transformers environment

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