Power transformers - Part 12: Loading guide for dry-type power transformers
1 Scope
This part of GB 1094 is applicable to dry-type transformers according to the scope of GB 1094.11. It provides the means to estimate ageing rate and consumption of life time of the transformer insulation as a function of the operating temperature, time and the loading of the transformer.
Note: For special applications such as wind turbine application transformers, furnace transformers, welding machine transformers, and others, the manufacturer shall be consulted regarding the particular loading curve.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB 1094.11 Power transformers - Part 11: Dry-type transformers (GB 1094.11-2007, IEC 60076-11:2004, MOD)
GB/T 11026.1 Electrical insulating materials - Properties of thermal endurance - Part 1: Ageing procedures and evaluation of test results (GB/T 11026.1-2003, IEC 60216-1:2001, IDT)
GB/T 18494.1-2001 Convertor transformers - Part 1: Transformers for industrial applications (IEC 61378-1:1997, IDT)
3 Terms and definitions
For the purpose of this document, the following terms and definitions apply.
3.1
long-time emergency loading
loading resulting from the prolonged outage of some system elements that will not be reconnected before the transformer reaches a new and higher steady state temperature
3.2
short-time emergency loading
unusually heavy loading of a transient nature (less than one time constant of the winding) due to the occurrence of one or more unlikely events which seriously disturb normal system loading
3.3
hot-spot
if not specifically defined, “hot-spot” means the hottest-spot of the winding
3.4
relative thermal ageing rate
for a given hot-spot temperature, the rate at which transformer insulation ageing is reduced or accelerated compared with the ageing rate at a reference hot-spot temperature
3.5
transformer insulation life time
the total time between the initial state for which the normal transformer insulation is considered new and the final state when due to thermal ageing, insulating field strength, short-circuit stress, or mechanical movement, which could occur in normal service and result in a high risk of electrical failure
3.6
AN cooling
cooling by natural air ventilation
3.7
AF cooling
method of cooling to increase the rated capacity of the transformer with fan cooling
4 Effect of loading beyond nameplate rating
4.1 General
Normal life expectancy is a conventional reference basis for continuous duty under design ambient temperature and rated operating conditions. The application of a load in excess of nameplate rating and/or an ambient temperature higher than specified ambient temperatures involves a degree of risk and accelerated ageing. It is the purpose of this part to identify such risks and to indicate how, within limitations, transformers may be loaded in excess of the nameplate rating.
4.2 General consequences
The consequences of loading a transformer beyond its nameplate rating are as follows:
——the temperatures of windings, terminals, leads, tap changer and insulation increase, and can reach unacceptable levels;
——products with enclosures are more sensitive to overload, leading to a more rapid increase in insulation temperature to unacceptable levels;
——as a consequence, there will be a risk of premature failure associated with the increased currents and temperatures. This risk may be of an immediate short-term character or may come from the cumulative effect of thermal ageing of the insulation in the transformer over many years.
Note: Another consequence of overload is an increased voltage drop in the transformer.
4.3 Effects and hazards of short-time emergency loading
Foreword i
Introduction iv
1 Scope
2 Normative references
3 Terms and definitions
4 Effect of loading beyond nameplate rating
4.1 General
4.2 General consequences
4.3 Effects and hazards of short-time emergency loading
4.4 Effects of long-time emergency loading
5 Ageing and transformer insulation life time
5.1 General
5.2 Life time
5.3 Relation between constant continuous load and temperature
5.4 Ageing rate
5.5 Life time consumption
5.6 Hot-spot temperature in steady state
5.7 Assumed hot-spot factor
5.8 Hot-spot temperature rises at varying ambient temperature and load conditions
5.9 Loading equations
5.10 Determination of winding time constant
5.11 Determination of winding time constant according to empirical constant
5.12 Calculation of loading capability
6 Limitations
6.1 Current and temperature limitations
6.2 Other limitations
Annex A (Informative) Ageing rate
A.1 Loading capability
A.2 Molecular structure
A.3 Reference documents
Annex B (Informative) Examples of life time consumptions for 3 load regimes
B.1 Example 1: Loading at constant temperature
B.2 Example 2: Load current Ii for a time t1 followed by load current Iu for a time t2
B.3 Example 3: Varying load current
Annex C (Informative) List of symbols
C.1 List of symbols
Bibliography
