Forced ventilation cyclesin transformers

TT1100

T10 - Forced ventilation cycles 13/04/2005 - 2 / 4 -

Introduction

Transformers, when in operation, are subject to no-load losses andload losses.This energy causes a rise in the temperature of the transformersactive part, i.e. the magnetic core and the windings.It is therefore normal for the transformer to be hot while it isrunning. This heating is taken into account when the equipment isdesigned and is, moreover, standardised.

At the customers request, fans can be installed beneath thetransformer windings to alleviate excessive heating caused byabnormal operating and/or installation conditions.This request must be made when the order is placed, as it is notalways possible to act once the transformer has been installed onsite.

Why install fans?

If fans are installed, the transformer can be overloaded withoutoverheating. This overloading can be caused by:

- The ambient temperature around the transformer

In a poorly-ventilated room, where the ambient temperatureexceeds normative values over a short period of time, fans can beused to punctually cool down the transformer.The ambient temperature problem will not be solved, but thepunctual operation of the fans will stop the heat protection systemfrom starting up and bringing the transformer to a halt.

- The through-current or the voltage applied to thetransformer

It is important to understand that load losses are proportional to thesquare of the through-current.It is sometimes necessary to increase the load of the transformerfor short periods, without having to oversize the transformer whenit is designed, because it will not have to withstand this overloadpermanently.Fans keep the temperature of the transformer below standardisedlimits, so that the power of the transformer can be increasedwithout shortening its service life.

Sizing the transformer

The transformer is therefore sized to permanently maintain- its nominal output- at nominal ambient temperature,in a naturally ventilated room, i.e. without fans.

The flow rate of the fans, which has a direct influence on their size,is determined according to the overload specified by the customer.On site, as soon as the load or the ambient temperature becomesexcessive, the fans start up to stop the temperature from climbingas a result of the overload.

T10 - Forced ventilation cycles 13/04/2005 - 3 / 4 -

Heat protection

This is based on the following:

- temperature sensors, which are placed at the hottest points oneach column of the active part; they are serially connected andtheir resistance increases significantly if any of them is exposed tothe maximum temperature authorised;- an electronic converter, which permanently monitors theresistance of these sensors ; if it detects an excessive level ofresistance, it switches a contact to give an order;- 2 tangential fan ramps, which are fixed to either side of eachcolumn and start up on receiving an order from the converter viathe relays. They blow ambient air into the windings.

This protection system is sized for permanently-runningtransformers in nominal operating conditions. Therefore, the fansare not continuously working, and stop as soon as the temperaturehas dropped well below the alarm threshold.

At nominal output, the alarm threshold is activated by the dailyambient temperature (30C if the order specifies a maximumambient temperature of 40C) and the tripping threshold isactivated by the maximum authorised ambient temperature (40Cif the order specifies a maximum ambient temperature of 40C).

At nominal output, the fans start up when the ambient temperaturematches the annual ambient temperature (if the order specifies amaximum ambient temperature of 40C) so that the alarmthreshold is not activated before the fans come into operation.

The fans continue to operate for 20 minutes after the temperaturehas dropped by 20C. In practice, the temperature falls by morethan 20C (up to 50C).

Transformer and fan operation

Whatever the degree of overload, the fans always operate at fullpower (in relation to the maximum overload specified by thecustomer).

temprature de la sonde dans le canal BT

0

20

40

60

80

100

120

140

160

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6

fraction In

C

dmarrage des ventilateurs

fonctionnement AN

fonctionnement AF

Air Natural (AN) running

Fans starting

Air Forced (AF) running

Temperature collected by the sensor in the LV channel

Portion I rated

T10 - Forced ventilation cycles 13/04/2005 - 4 / 4 -

The fans operate according to the duration and extent of theoverload.

Punctual overload:If the overload is punctual, the fans operate for around half anhour.

Lasting overloadIf the overload continues, the fans do not stop, as the temperaturedoes not go down to the fan stop threshold.On the other hand, the following might happen if the fans are verypowerful:

- the power of the fans is such that the temperature drops to thestop level,- when the fans stop, the temperature climbs again, as thetransformer is still overloaded,- as soon as the tripping threshold is reached again, the fansstart up again and the cycle continues indefinitely if the operatingconditions are not changed.

In this case, the transformer is subjected to very significanttemperature cycles.

This thermal shock frequency is not recommended for resin-moulded HV coils, as it can cause cracks in the solid insulatingmaterial.

To avoid this problem, the fan tripping threshold should bereduced, and the time-lag lengthened.

Bibliography - Standards

Standard IEC 60076-11, 2004

70

80

90

100

110

120

130

55 56 57 58 59 60heures

C

surface HT int col B canBTa canBTb canBTcHV interior surface column V LV channel u LV channel v LV channel w

hours