• the top-liquid and average liquid temperature rises in a steady-state condition with dissipation of total losses; • the average winding temperature rise at rated current for the averag
Identification symbols
Transformers shall be identified according to the cooling method employed For liquid-immersed transformers, this identification is expressed by a four-letter code as described below
First letter: Internal cooling medium:
• O: mineral oil or synthetic insulating liquid with fire point ≤ 300 °C;
• K: insulating liquid with fire point > 300 °C;
• L: insulating liquid with no measurable fire point
Second letter: Circulation mechanism for internal cooling medium:
• N: natural thermosiphon flow through cooling equipment and in windings;
• F: forced circulation through cooling equipment, thermosiphon flow in windings;
• D: forced circulation through cooling equipment, directed from the cooling equipment into at least the main windings
Third letter: External cooling medium:
Fourth letter: Circulation mechanism for external cooling medium:
NOTE 1 In this standard, the use of insulating liquids K and L is considered only for safety and environmental reasons
In transformers with forced directed insulating liquid circulation (indicated by the second code letter D), the liquid flow rate through the main windings is primarily controlled by pumps rather than the load A small portion of the liquid flow may be redirected as a controlled bypass to cool the core and other components outside the main windings Additionally, regulating windings and other low-power windings may experience non-directed circulation of bypass liquid.
In transformers utilizing forced, non-directed cooling (indicated by the second code letter F), the liquid flow rates through the windings fluctuate with the load, and are not directly correlated to the flow pumped through the cooling system.
Transformers with alternative cooling methods
Transformers can be designed with various cooling methods, and the specification along with the rating plate must indicate the power values at which the transformer meets the temperature rise limits for these alternatives.
The rated power of the transformer, or an individual winding in a multi-winding transformer as defined by IEC 60076-1, determines the power value for alternative cooling methods with the highest cooling capacity These alternative cooling methods are typically organized in ascending order based on their cooling capacity.
The ONAN/ONAF transformer features a set of fans that can be activated as needed during high loading conditions In both scenarios, the circulation of the insulating liquid relies solely on the thermosiphon effect.
The transformer is equipped with cooling systems that include pumps and fans, but it is designed to operate at a lower rated power when relying on natural cooling, such as during auxiliary power failures or reductions.
Air-cooled transformers
Normal ambient temperature limits for power transformers are given in IEC 60076-1
With regard to normal temperature rise requirements, the temperatures at the intended installation site should not exceed:
+ 30 °C monthly average, of the hottest month;
NOTE The average temperatures are to be derived from meteorological data as follows (see IEC 60076-1)
– half the sum of the average of the daily maxima and the average of the daily minima during a particular month, over many years;
– one-twelfth of the sum of the monthly average temperatures.
Water-cooled transformers
Normal cooling condition for water cooled transformers is a temperature of cooling water at the inlet not exceeding 25 °C at any time or a 20 °C yearly average
If the operating water temperature is higher than this, then a lower temperature rise should be specified (see IEC 60076-1)
General
Temperature rise requirements are specified according to different options:
• a set of requirements which refer to continuous rated power (see 6.2)
• an additional set of explicitly specified requirements, that relate to a prescribed loading cycle (see 6.4)
The additional requirements primarily apply to large system transformers, where emergency loading conditions require special consideration These guidelines are not intended for regular use with small and medium-sized standardized transformers.
It is assumed throughout this part that the service temperatures of different parts of a transformer can each be described as the sum of the external cooling medium temperature
(ambient air or cooling water) and the temperature rise of the transformer part
Normal temperature rise limits apply unless other service conditions are specified In such cases, the limits of temperature rise shall be modified as indicated in 6.3
No plus tolerance is permitted on temperature rise limits.
Temperature rise limits at rated power
For transformers rated up to 2,500 kVA (833 kVA for single-phase) with a tapping range of ± 5%, the temperature rise limits must adhere to the principal tapping that corresponds to the rated voltage, as specified in IEC 60076-1.
For transformers with a rated power exceeding 2,500 kVA or a tapping range greater than ±5%, temperature rise limits must be adhered to for each tapping, considering the specific tapping power, voltage, and current.
NOTE 1 The load losses are different for different tappings and sometimes also the no-load loss when variable flux voltage variation is specified
NOTE 2 In a separate winding transformer, the tapping with the highest load loss is normally the tapping with the maximum current
NOTE 3 In an auto-transformer with tapping, the tapping with the highest load loss depends on how the tappings are arranged
In a multi-winding transformer, if the rated power of one winding matches the total rated power of the other windings, the temperature rise requirements apply to all windings operating at rated power simultaneously However, if this condition is not met, specific loading combinations must be identified and defined to establish the temperature rise limits.
For transformers with multiple stacked winding sections, the temperature limit must be based on the average measurements of these sections, provided they are of equal size and rating.
The temperature rise limits specified in Table 1 apply to transformers with solid insulation classified as class 105 °C per IEC 60085, which are immersed in mineral oil or synthetic liquids with a fire point not exceeding 300 °C (indicated by the first code letter: O).
The limits refer to steady state conditions under continuous rated power, and 20 °C average yearly temperature of the external cooling medium
If not otherwise agreed between manufacturer and purchaser, the temperature rise limits given in Table 1 are valid for both Kraft and upgraded paper (see also IEC 60076-7)
Requirements for Temperature rise limits
Average winding (by winding resistance variation):
– ON and OF cooling systems
There are no specific numerical limits for the temperature rise of magnetic cores, electrical connections, shields, and structural components within the tank However, it is essential that these components do not reach temperatures that could damage adjacent parts or accelerate the aging of the insulating liquid If deemed necessary, a temperature rise limit for the magnetic core surface can be established through agreement between the manufacturer and the purchaser.
NOTE 4 For some designs, the hot-spot winding temperature rise limit may imply lower top-liquid and/or average winding temperature rises than those indicated in the table
NOTE 5 The rules for determining the hot-spot winding temperature rise are given in 7.10
For large power transformers filled with mineral oil, conducting in-oil dissolved gas analysis (DGA) during temperature rise tests can effectively identify potential overheating issues.
NOTE 7 For large power transformers, the temperature rise of tank and cover surfaces can be checked by means of a thermographic infrared camera
In cases of low resistance windings with multiple bolted connections, such as those found in low voltage furnace transformers, accurately determining the average winding temperature rise through resistance variation can be challenging and prone to significant uncertainty As an alternative, and upon mutual agreement between the manufacturer and purchaser, the winding temperature rise requirements may be restricted to the hot-spot winding temperature rise, which should be assessed through direct measurement.
Temperature rise limits for transformers having higher temperature resistant insulation systems and immersed in a less flammable liquid (code letter K or L) are subject to agreement.
Modified requirements for special cooling conditions
General
If the installation site does not meet the normal cooling conditions specified in Clause 5, the temperature rise limits for the transformer must be adjusted according to the guidelines provided below.
Air-cooled transformers
If the external cooling medium temperature at the site surpasses the normal values specified in section 5.1, all temperature rise limits in Table 1 must be adjusted by the same excess amount The resulting values should be rounded to the nearest whole number in degrees Kelvin.
Recommended ambient temperature reference values and relevant temperature rise limit corrections are given in Table 2
Table 2 – Recommended values of temperature rise corrections in case of special service conditions
K a Yearly average Monthly average Maximum
35 45 55 –15 a Referred to the values given in Table 1
NOTE 1 No rules are given for ambient temperatures lower than the normal ones The temperature rise limits given in Table 1 are applied unless otherwise specified by the purchaser
NOTE 2 The values given in the Table 2 may be interpolated
For installations located at altitudes exceeding 1,000 meters above sea level, while the factory itself is at a lower elevation, the permissible temperature increases during testing at the factory must be adjusted downward accordingly.
For naturally cooled transformers ( AN), the maximum allowable increases in top-liquid, average, and hot-spot winding temperatures must be decreased by 1 K for every 400 m that the installation's altitude exceeds 1,000 m.
• for a forced-cooled transformer (… AF), the reduction shall be 1 K for every 250 m exceeding 1 000 m
A corresponding reverse correction may be applied in cases where altitude of the factory is above 1 000 m and the altitude of the installation site is below 1 000 m
Any altitude correction shall be rounded to the nearest whole number of degrees kelvin
When the allowable temperature rise limits of a transformer are lowered due to elevated cooling medium temperatures or high-altitude installations, this must be clearly marked on the rating plate, in accordance with IEC 60076-1 standards.
NOTE 3 When standardized transformers are to be used at high altitudes, a reduced value of power may be calculated, which from the point of view of cooling and temperature rise corresponds to service with rated power under normal ambient conditions.
Water-cooled transformers
If the maximum or annual cooling water temperature at the site surpasses the limits specified in section 5.2, all designated temperature rise limits must be decreased by the same amount as the excess temperature The resulting values should be rounded to the nearest whole degree.
NOTE The rule given above does not apply for water temperatures lower than the normal one In that case, an agreement between manufacturer and purchaser is necessary
The influence of differing ambient temperature or altitude on the air cooling of the tank shall be disregarded.
Temperature rise during a specified load cycle
By agreement between manufacturer and purchaser, temperature rise limits can be guaranteed and/or a special test regarding load cycle operation specified (see IEC 60076-7)
General
This article outlines the procedures for determining temperature and temperature rise values during factory testing, as well as the methods for replacing service loading conditions with equivalent test procedures.
During the temperature rise test, the transformer shall be equipped with its protective devices
(for example, Buchholz relay) Any indication from these devices during the test shall be noted and the case investigated
For transformers with multiple rated power values, such as those utilizing different cooling methods, a temperature rise test is typically required for each rating However, the manufacturer and purchaser may agree to reduce the number of tests conducted.
Temperature of the cooling media
Ambient temperature
For the temperature rise test, the cooling air temperature should be in the range between
10 °C and the maximum ambient temperature for which the transformer is designed
The interpretation of the test results shall be subject to agreement if the external cooling medium temperature during the test is outside the limits indicated
At least four sensors shall be provided and the average of their readings shall be used to determine the ambient temperature for the evaluation of the test results
For testing large power transformers, it is essential to increase the number of sensors to six This enhancement helps minimize uncertainty and improves the accuracy of the average readings.
Readings should be taken at regular intervals (e.g., every ten minutes), or automatic continuous recording may be used
Around an ONAN transformer, the ambient sensors shall be placed at a level about halfway up the cooling surfaces
The sensors shall be distributed around the tank, about 2 m away from the perimeter of tank and cooling surfaces, and protected from direct heat radiation
For a forced-air-cooled transformer, the sensors shall be placed in the air at about 0,5 m from the intake of the coolers
When using separate cooling equipment located at least 3 meters away from the transformer tank, it is essential to measure the ambient temperature around the cooling equipment following the same guidelines previously mentioned.
To ensure optimal performance, it is crucial to prevent the recirculation of hot air around the transformer Proper placement of the transformer should minimize airflow obstructions and maintain stable ambient conditions.
To ensure accurate measurements during the final phase of testing, it is crucial to minimize fluctuations in cooling-air temperature Implementing heat sinks with a thermal time constant comparable to that of the transformer can help prevent rapid variations in readings caused by turbulence.
Water temperature
For the temperature rise test, the cooling water temperature should be in the range between
5 °C and the maximum water temperature for which the transformer is designed
The interpretation of the test results shall be subject to agreement if the water temperature is outside the limits indicated above
Temperature measurements should be taken at the cooling equipment's intake, with readings of temperature and water flow rate recorded at regular intervals, such as every ten minutes, or through automatic continuous recording.
Precautions shall be taken to minimize the variations of water cooling flow and temperature during the test period.
Test methods for temperature rise determination
General
The standard method for the determination of the steady-state temperature rises on the test floor is the equivalent test in short-circuit connection according to 7.3.2 below
In certain situations, with mutual agreement, testing can be conducted using rated voltage and current by connecting to an appropriate load, primarily for transformers with low rated power.
A back-to-back method can be utilized, where two transformers are connected in parallel, with one being the transformer under test This setup allows for the transformer under test to be excited at its rated voltage, enabling the flow of rated current through its windings by using different voltage ratios or an injected voltage.
Test by short-circuit method for two winding transformers
In this test, the transformer is evaluated without applying rated voltage and rated current at the same time Instead, it is subjected to the calculated total losses, which are derived from two separate measurements: the load loss at a reference temperature and the no-load loss.
The purpose of the test is to establish:
• the top-liquid and average liquid temperature rises in a steady-state condition with dissipation of total losses;
• the average winding temperature rise at rated current for the average liquid temperature rise as determined above;
• the hot-spot winding temperature rise at rated current and for the top-liquid temperature rise as mentioned above
This is achieved in two testing steps: a) Total loss injection
The top-liquid and average liquid temperature rises are established when the transformer is subjected to a test current corresponding to the total losses of the transformer (see
According to IEC 60076-1, the test current must exceed the rated current sufficiently to generate an additional loss equivalent to the no-load loss at rated voltage, resulting in a corresponding increase in winding temperature rise.
The top-liquid temperature and cooling medium temperature are monitored, and the test is continued until steady-state liquid temperature rises are established
The initial phase of the test can conclude when the top-liquid temperature rise rate drops below 1 K/h and stays at that level for 3 hours For tests with discrete readings, the mean of the last hour's readings is used as the test result In cases of continuous automatic recording, the average value from the last hour is considered Additionally, rated current injection is applied.
Once the top-liquid temperature rise is confirmed, the test should proceed without interruption, reducing the test current to the rated current for the connected winding combination This condition must be sustained for one hour, during which continuous temperature records of the top-liquid, winding hot-spot (if applicable), and external cooling medium should be documented at intervals of no more than five minutes.
At the conclusion of the hour, the resistances of the windings are assessed either by quickly disconnecting the supply and performing short circuits or, alternatively, by employing the superposition method This method involves injecting a low-value direct current into the windings while measuring the load current without turning off the supply.
The average temperatures of the two windings are calculated based on resistance variations, considering the decrease in liquid temperature when the current is reduced to its rated value, along with changes in the temperature of the external cooling medium.
To accurately determine the temperature rise of the hot-spot winding, the highest temperature reading before disconnection should be taken into account This measurement must then be corrected as specified in section 7.10.3, considering that the liquid temperature decreases when the current is reduced to its rated value.
The two steps of the test can be merged into a single power application, positioned between load loss and total loss For liquid-immersed transformers, the temperature rise values for the top liquid, average liquid, and windings must be calculated using the correction rules specified in section 7.13 Additionally, the power applied during the test must be a minimum level.
80 % of the total loss value.
Test modification for particular transformers
Two-winding transformer with a tapping range larger than ± 5 %, or having a rated power exceeding 2 500 kVA
The temperature rise test for transformers is performed with the transformer connected to the maximum current tapping, as outlined in IEC 60076-1 During the latter part of the test, the tapping current corresponding to this maximum tapping is utilized.
During the initial phase of the test, the total losses must match the highest total loss recorded at any tapping, which typically corresponds to the maximum current tapping This phase is crucial for assessing the maximum rise in top-liquid temperature To evaluate the winding temperature rise at the maximum current tapping, the liquid temperature rise used should reflect the total losses associated with that tapping If the initial test results are based on different data, the values will be recalculated accordingly.
When the rated power of one winding matches the total rated powers of the other windings, the initial test will generate a total loss equivalent to the rated power (or tapping power) across all windings.
In cases where the standard does not apply, specific loading scenarios with various combinations of individual winding loads are defined The scenario resulting in the highest total loss will dictate the test power used to assess the liquid temperature rise.
The temperature rise value for an individual winding above liquid shall be obtained with rated current in the winding
In assessing the winding temperature rise above the cooling medium, the liquid temperature increase for the specific loading scenario will be recalculated based on the total loss injection test, as outlined in section 7.13 Additionally, the winding temperature rise above the liquid will be determined for each applicable winding.
Guidance for recalculation of losses in multi-winding transformers is given in IEC 60076-8
The injection of total loss for the determination of liquid temperature rise may be made:
To accurately simulate the loading case, inject the current that corresponds to the total losses in one winding while simultaneously short-circuiting or connecting the other windings to an impedance.
In certain cases, it may be permissible to leave specific windings of a transformer open, particularly if they have a low rated power and minimal impact on the overall loss By increasing the current in the other windings, the desired total loss can still be achieved without short-circuiting or closing the less significant windings.
In cases where the aforementioned methods cannot be fully implemented due to testing facility limitations, it may be permissible to conduct the test with a reduced loss, allowing for a deviation of up to 80% from the correct value Subsequently, the measured temperature value must be adjusted in accordance with section 7.13.
The details of the temperature rise test for a multi-winding transformer should, as a rule, be presented and agreed already at the tender stage
For multi-core transformers lacking external winding connections, it is essential to establish the temperature rise test method during the tender phase Additionally, the incorporation of extra test bushings may be beneficial.
Determination of liquid temperatures
Top-liquid temperature
The top-liquid temperature( )θ o is conventionally determined by one or more sensors immersed in the insulating liquid at the top of the tank or, in pockets in the cover
The recommended number of pockets is the following:
− rated power from 20 MVA to