IEEE C57.91 2012

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IEEE Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators

Published By	Publication Date	Number of Pages
IEEE	2012	120

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Description

Revision Standard – Active. This guide provides recommendations for loading mineral-oil-immersed transformers and step-voltage regulators with insulation systems rated for a 65 C average winding temperature rise at rated load. This guide applies to transformers manufactured in accordance with IEEE Std C57.12.001 and tested in accordance with IEEE Std C57.12.90, and step-voltage regulators manufactured and tested in accordance with IEEE Std C57.15. Because a substantial population of transformers and step-voltage regulators with insulation systems rated for 55 C average winding temperature rise at rated load are still in service, recommendations that are specific to this equipment are also included.

PDF Catalog

PDF Pages	PDF Title
1	IEEE Std C57.91-2011 Front Cover
3	Title page
6	Notice to users Laws and regulations Copyrights Updating of IEEE documents Errata Interpretations
7	Patents
8	Participants
10	Introduction
12	CONTENTS
15	Important Notice 1. Overview 1.1 Scope 1.2 Purpose
16	2. Normative references 3. Definitions
17	4. Effect of loading beyond nameplate rating 4.1 General 4.2 Voltage and frequency considerations
18	4.3 Supplemental cooling of existing self-cooled transformers 4.4 Information for user calculations
19	5. Transformer insulation life 5.1 General
20	5.2 Aging equations
24	5.3 Percent loss of life
25	6. Ambient temperature and its influence on loading 6.1 General 6.2 Approximating ambient temperature for air-cooled transformers 6.3 Approximating ambient temperature for water-cooled transformers 6.4 Influence of ambient on loading for normal life expectancy
26	7. Calculation of temperatures 7.1 Load cycles 7.1.1 Load cycles, general
27	7.1.2 Method of converting actual to equivalent load cycle 7.1.3 Equivalent peak load 7.1.4 Equivalent continuous prior load
28	7.2 Calculation of temperatures 7.2.1 General 7.2.2 Components of temperature
29	7.2.3 Top-oil rise over ambient
30	7.2.4 Oil time constant
32	7.2.5 Winding hot-spot rise
34	7.2.6 Exponents for temperature rise equations 7.2.7 Adjustment of test data for different tap position
35	7.3 Computer calculation of loading capability
36	7.4 Bibliography for Clause 7
38	8. Loading of distribution transformers and step-voltage regulators 8.1 Life expectancy 8.1.1 General 8.1.2 Normal life expectancy 8.2 Limitations 8.2.1 General
39	8.2.2 Limitations for loading distribution transformers above nameplate rating 8.3 Types of loading 8.3.1 Loading for normal life expectancy under specific conditions 8.3.2 Loading by top-oil temperature
42	8.3.3 Continuous loading based on average winding test temperature rise 8.3.4 Short-time loading with moderate sacrifice of life expectancy (operation above 110 °C hottest- spot temperature) 8.4 Loading specific to voltage regulators 8.4.1 General
43	8.4.2 Restricted regulation 8.4.3 Loading with reduced voltages 8.4.4 Limitations for loading voltage regulators above nameplate rating 9. Loading of power transformers 9.1 Types of loading and their interrelationship
44	9.2 Limitations 9.2.1 Temperature or load limitations
45	9.2.2 Ancillary components 9.2.3 Risk considerations 9.3 Normal life expectancy loading 9.3.1 General
46	9.3.2 Influence of ambient temperature on normal life expectancy loading 9.3.3 Normal life expectancy loading by top-oil temperature 9.3.4 Normal life expectancy loading by average winding test temperature rise
48	9.4 Planned loading beyond nameplate rating 9.5 Long-time emergency loading 9.6 Short-time emergency loading
49	9.7 Loading information for specifications 9.8 Operation with part or all of the cooling out of service
50	Annex A (normative) Thermal evolution of gas from transformer insulation A.1 General
51	A.2 Experimental verification
52	A.3 Determination of equation parameters A.4 Example
53	A.5 Bibliography for Annex A
55	Annex B (normative) Effect of loading transformers above nameplate rating on bushings, tap changers, and auxiliary components B.1 Bushings B.1.1 General
56	B.1.2 Draw leads in bushings B.2 Tap-changers B.2.1 Tap-changers for de-energized operation (TCDO)
57	B.2.2 Load tap-changers
59	B.3 Bushing-type current transformers B.3.1 General B.4 Insulated lead conductors B.5 Bibliography for Annex B
60	Annex C (informative) Calculation methods for determining ratings and selecting transformer size C.1 General C.2 Calculation determining loading beyond nameplate rating of an existing transformer
65	C.3 Planned loading beyond nameplate (PLBN)
66	C.4 Long-time emergency loading (LTE)
67	C.5 Short-time emergency (STE) loading
69	Annex D (normative) Philosophy of guide applicable to transformers with 55 °C average winding rise (65 °C hottest-spot rise) insulation systems D.1 General
70	D.2 Aging equations
71	Annex E (normative) Unusual temperature and altitude conditions E.1 Unusual temperatures and altitude E.2 Effect of altitude on temperature rise E.3 Operation at rated kVA E.4 Operation at less than rated kVA
72	E.5 Bibliography for Annex E
73	Annex F (normative) Cold-load pickup (CLPU) F.1 General F.2 Duration of loads F.3 CLPU ratio
74	F.4 Other considerations
75	F.5 Bibliography for Annex F
76	Annex G (informative) Alternate temperature calculation method G.1 General G.2 List of symbols
80	G.3 Equations G.3.1 Introduction
81	G.3.2 Average winding temperature
83	G.3.3 Winding duct oil temperature rise over bottom oil
84	G.3.4 Winding hottest-spot temperature
86	G.3.5 Average oil temperature
88	G.3.6 Top and bottom oil temperatures
89	G.3.7 Stability requirements
90	G.3.8 Fluid viscosity and specific heats of materials
91	G.3.9 Summary of exponents G.3.10 Adjustment of rated test data for a different tap position G.3.10.1 Top- and bottom-oil rise over ambient
92	G.3.10.2 Average winding rise over ambient
93	G.3.10.3 Hottest-spot rise over ambient
94	G.3.11 Load cycles and ambient temperatures G.4 Discussion
95	G.5 Disclaimer statement G.6 Computer program Input data for computer program
105	G.7 Bibliography for Annex G
106	Annex H (normative) Operation with part or all of the cooling out of service H.1 General H.2 ONAN/ONAF transformers H.3 ONAN/ONAF/ONAF, ONAN/ONAF/OFAF, and ONAN/OFAF/OFAF transformers H.4 OFAF and OFWF transformers H.4.1 General
107	H.4.2 Calculations H.4.2.1 Equations
109	H.4.3 Caution
110	H.5 Forced-oil-cooled transformers with part of coolers in operation
111	Annex I (informative) Transformer insulation life I.1 Historical perspectives
112	I.2 Thermal aging principles
116	I.3 Example calculations
120	I.4 Bibliography for Annex I

Additional information

Standard Title	IEEE Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators
Published Code	IEEE
Publication Date	2012
Pages Count	120

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