1 General
1.1 Scope and object
This Standard applies to power transformers complying with the series of GB 1094 and GB 6450, etc.
It is intended to provide information to users about:
— Certain fundamental service characteristics of different transformer connections and magnetic circuit designs, with particular reference to zero-sequence phenomena;
— System fault currents in transformers with YNynd and similar connections;
— Parallel operation of transformers, calculation of voltage drop or rise under load, and calculation of load loss for three-winding load combinations;
— Selection of rated quantities and tapping quantities at the time of purchase, based on prospective loading cases;
— Application of transformers of conventional design to convertor loading;
— Measuring technique and accuracy in loss measurement.
Part of the information is of a general nature and applicable to all sizes of power transformers.
Several chapters, however, deal with aspects and problems which are of the interest only for the specification and utilization of large high-voltage units.
The recommendations are not mandatory and do not in themselves constitute specification requirements.
Information concerning loadability of power transformers is given in GB/T 15164, for oil-immersed transformers, and GB/T 17211, for dry-type transformers.
Guidance for impulse testing of power transformers is given in GB/T 7449.
1.2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of this Standard. At the time of publication, the editions indicated were valid. All normative documents are subject to revision, and parties to agreements based on this Standard are encouraged to investigate the possibility of applying the most recent edition of the normative documents indicated below.
Contents
1 General
1.1 Scope and object
1.2 Normative references
2 Characteristic Properties of Different Three-phase Winding Combinations and Magnetic Circuit Designs
2.1 Y-, D-, and Z-connected windings
2.2 Characteristic properties of combinations of winding connections
2.3 Different magnetic circuit designs
3 Characteristic Properties and Application of Auto-connected Transformers
3.1 By definition, an auto-connected transformer is a transformer in which at least two windings have a common part (see 3.1.2 of GB 1094.1-1996).
3.2 The reduction factor or auto-factor, α
3.3 Short-circuit Impedance and Leakage Flux Effects
3.4 System restrictions, insulation co-ordination
3.5 Voltage regulation in system-interconnection autotransformers
4 Zero-sequence Properties — Neutral Load Current and Earth Fault Conditions, Magnetic Saturation and Inrush Current
4.1 Introduction of the symmetrical components of a three-phase system
4.2 Impedance parameters for symmetrical components
4.3 Single-line equivalent diagram of the transformer for zero-sequence phenomena
4.4 Magnetizing impedance under asymmetrical conditions — zero-sequence voltage and magnetic circuit geometry
4.5 Zero-sequence and delta windings
4.6 Zero-sequence and zigzag windings
4.7 Zero-sequence impedance properties of different transformer connections
4.8 Continuous zero-sequence loading (neutral point current)
4.9 Magnetic circuit reluctance and magnetizing impedance, steady-state saturation under abnormally high power frequency voltage
4.10 Transient saturation, inrush current
4.11 Geomagnetically induced current and parasitic currents from d.c. systems
5 Calculation of Short-circuit Currents in Three-winding, Three-phase Transformers (Separate Winding Transformers and Auto-connected Transformers) with Earthed Neutrals
5.1 General
5.2 Notations of systems and windings
5.3 Transformer parameters
5.4 Impedances of systems I and II
5.5 Summary of cases studied in this subclause
6 Parallel Operation of Transformers in Three-phase Systems
6.1 Matching three-phase connections and phase-angle relations
6.2 Difference in ratio, circulating current
6.3 Unequal short-circuit impedances
6.4 Variation of short-circuit impedance across the tapping range, influence of winding arrangement
7 Calculation of Voltage Drop for a Specified Load, Three-winding Transformer Load Loss
7.1 Introduction: the need for voltage drop calculation
7.2 The short-circuit impedance and equivalent diagram of a two-winding transformer
7.3 Description of the load
7.4 The voltage drop equations
7.5 Voltage drop calculation in per cent notation
7.6 Equivalent diagram for multi-winding transformers, T-equivalent impedance elements for a three-winding transformer
7.7 Allocation of load losses to individual windings in three-winding transformers
7.8 Example of calculation of voltage drop and load loss for a three-winding transformer
7.9 Example of calculation of combined load loss and allocation of losses to individual windings in an auto-connected three-winding transformer
8 Specification of Rated Quantities and Tapping Quantities
8.1 Introduction
8.2 Standardized specifications of ratings and tappings, effect of the width of the tapping range
8.3 Procedure for the determination of rated and tapping quantities
8.4 Details of the procedure, step by step
9 Convertor Applications with Standard Transformers
9.1 Influence of distorted voltage
9.2 Influence of distorted current, general
9.3 Overall eddy losses in windings
9.4 Stray losses in mechanical parts
9.5 Combined additional loss, possible de-rating
9.6 Local hotspots
10 Guide to the Measurement of Losses in Power Transformers
10.1 Test results, guarantees, tolerances, uncertainty limits
10.2 Traceability, quality aspects on measuring technique
10.3 Fundamental sources of error in power transformer load loss measurement
10.4 Phase angle error of a conventional loss measuring system, possibility of correction
10.5 Advanced measuring systems
10.6 Measurement of no-load loss
Appendix A (Informative) Basic Relations for Single-phase and Two-phase Earth Faults
