Env1 - Transformer's Thermal Strengh

8/11/2019 Env1 - Transformer's Thermal Strengh

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Env1 Transformers thermal stength 22/03/2004 - 2 / 6 -

Transformer heating

Unless otherwise specified by the customer, the transformer issized for nominal operating conditions, in which it will not heat upto the extent that equipment is likely to be damaged.

For "abnormal" operating conditions, however, it is important to

draw up precise design specifications, so that the overheatingcaused by abnormal situations does not shorten the transformersservice life.It is also possible to adjust the transformer to the load used duringperiods of normal operation.

These specific operating conditions, as well as the differentsolutions to overheating, are discussed in this document.

Ambient temperature

If the transformer is designed to withstand a "maximum ambienttemperature of 40C", the cooling air, therefore the ambienttemperature, must not, under any circumstances whatsoever,exceed :

- 30C (daily average),- 20C (annual average),- 40C (maximum acceptable temperature).

So, if the maximum acceptable temperature is 50 C, theapplicable daily and annual averages will be 40C and 30C

respectively.

Note : not only must the ambient temperature specificationsbe complied with, but the transformer room must also besuitably designed. Special attention should be paid to thevents in the walls, which are used to evacuate the heatgenerated by the transformer !

Standard CEI 905 defines acceptable load factors for differentambient temperatures, in continuous operation mode.So, even if a transformer is designed for a specific ambienttemperature, its load can be increased if the temperature drops,without affecting its service life. On the other hand, if the ambient

temperature rises, the load must be reduced.

The ambient temperature must not be below :- - 25C for outdoor transformers,- - 5C for indoor transformers.

load factors

0,8

0,85

0,9

0,95

1

1,05

1,1

1,15

1,2

-15 -5 5 15 25 35 45

annual ambient temperature in C

e.g.: for a transformer designed to withstand a maximum

ambient temperature of 40 C:

- If the ambient temperature exceeds the maximum

value by 5C, the standard de-rating rate for a

transformer in continuous operation mode is 0.97. This

will preserve the transformers service life. Therefore a

1000 kVA transformer should be de-rated to 970 kVA.

- If the ambient temperature is 10C less than

expected, a 1000 kVA transformer may operate at 1040

kVA.


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Operating overload

Current overloads

Start-up / blocking of electric motors :For a few seconds on start-up, the transformers electric motorsrequire a much stronger current than the rated current.

As the start-up process is often repeated several times, anundersized transformer will be subjected to overload.Similarly, a blocked motor requires a stronger current, and thetransformer undergoes overload for as long as the motor isblocked.

HarmonicsInductive loads, rectifiers and most converters generate harmonicsand cause relatively high distortion rates.This phenomenon is often observed in paper mills (choppedvoltage for paper cutting).This reduces the power of the transformer.

Short circuitTransformers are sized to withstand a 2-second short circuit (seestandard IEC 60076-5).If the short circuit lasts for longer than the standard 2 seconds, itcan cause significant overheating.

TrippingTransformers are also sized to withstand occasional tripping.More frequent tripping, however, can cause overheating.

Voltage overloads

Over-inductionThe transformers tapping point may not be properly adjusted tothe network voltage.This invariably leads to over-induction : the losses generated bythe magnetic core increase, causing overheating.

FrequencyOver-induction also occurs if the transformer is sized for afrequency above the network frequency. Again, the result isoverheating. However, this is not a common occurrence.

HarmonicsVoltage harmonics cause an increase in the losses generated bythe magnetic core. As a result, the magnetic core and the lowvoltage winding overheat.

Overheating due to the ambient temperature andinadequate ventilation

If the ambient temperature in the transformer room risesexcessively and / or the installation instructions (particularly thoseconcerning the design of the room) are not followed, the

transformer overheats.


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Standardised heating limits

Standard IEC 726 sets a limit to the average heating oftransformers in stabilised nominal operating conditions, whereasstandard IEC 905 specifies a maximum temperature for windinghot spots during periods of overload. The acceptable temperaturesaccording to these standards are:

For windings :- 145C in stabilised operation mode, with an average annual

temperature of 20C (which means a daily temperature of155C that must not, in any event, exceed 165C). Theaverage increase in winding temperature is therefore limited to100 K in assigned operation mode.

-190C maximum in periods of overload. This temperaturemust never be exceeded.

As far as the magnetic core, metal components and othermaterials adjacent to the windings are concerned, the standardstipulates, the temperature must not, in any circumstances,increase to the extent that it damages the magnetic core itself,or the components and materials adjacent to it.

Thermal protection

An optional thermal protection system is available, whichpermanently and automatically monitors the transformerstemperature. It is sized for continuous operation in nominaloperating conditions.It is based on sensors, which protect the transfo and measure thetemperature at the winding hot spot: as standards IEC 726 and 905are very specific regarding winding temperatures, these sensorsare situated at the very centre of the LV coil, in order to track thetemperature of the inner winding. Their thresholds are determinedaccordingly, and they are relayed by an electronic converter thattriggers the appropriate alarm whenever one of the temperaturethresholds is crossed.

Here is a theoretical temperature graph for a sensor in the LVchannel : it concerns a transformer that actually exists. Thesevalues are valid for an ambient temperature of 30C (for atransformer sized for 40C maximum), and for equipment fitted withan optimum cooling system.

temprature de sonde dans le canal BT

60

80

100

120

140

160

180

200220

240

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

ratio In

C

dmarrage des ventilos

puissance AN

puissance AF

Temprature de sonde PTC dans le canal d'enroulem

temprature de sonde dans le canal BT

60

80

100

120

140

160

180

200220

240

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

In ratio

C

Fan start-up

AN power

AF power

Temperature of the PTC sensor in the LV winding


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Conclusion

The thermal phenomena caused by overheating have a strongimpact on the operation and service life of the transformer,whatever the reasons for this overheating.They must be taken into account when designing the transformer,and the following related measures should be taken :

- The transformer room should be suitably designed (height,

breathing space around the transformer, size and position ofvents).

- A system should be installed for permanently supervising thetemperature of the windings. This system should trigger theappropriate alarms, according to the temperature reached.

- A ventilation system, specially developed by the transformermanufacturer, should be installed. This system should reducethe impact of accidental overloads, but allow for intentional,carefully controlled overloads.

It is therefore important that the future user meet with the

manufacturer to discuss the characteristics of the transformer, andhence prevent thermal incidents during the transformers servicelife, and the possibly disastrous consequences mentioned above.

Env1 - Transformer's Thermal Strengh

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