Converter transformers - Part 3: Application guide
1 Scope
This document provides information to users about specific topics related to industrial and HVDC converter transformers with design, construction, testing and operating conditions differing from conventional transformers used in power systems. In addition, it is the aim of this document to provide manufacturers with the technical background of GB/T 18494.1-2014 and GB/T 18494.2-2022.
It is intended that this document is used to supplement and not replace or supersede GB/T 13499-2002, since many of the general principles contained within it are equally applicable to converter transformers.
2 Normative references
The following documents contain requirements which, through reference in this text, constitute provisions 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/T 1094.1-2013 Power transformers - Part 1: General (IEC 60076-1: 2011, MOD)
Note: There is no technical difference between the referenced content from GB/T 1094.1-2013 and that from IEC 60076-1: 2011.
GB/T 1094.5 Power transformers - Part 5: Ability to withstand short circuit (GB/T 1094.5-2008, IEC 60076-5: 2006, MOD)
GB/T 1094.14 Power transformer - Part 14: Liquid-immersed power transformer using high-temperature insulation materials (GB/T 1094.14-2022, IEC 60076-14: 2013, MOD)
GB/T 18494.1-2014 Converter transformers - Part 1: Transformers for industrial applications (IEC 61378-1: 2011, MOD)
Note: There is no technical difference between the referenced content from GB/T 18494.1-2014 and that from IEC 61378-1: 2011.
GB/T 18494.2-2022 Converter transformers - Part 2: Transformers for HVDC applications (IEC/IEEE 60076-57-19: 2017, MOD)
Note: There is no technical difference between the referenced content from GB/T 18494.2-2022 and that from IEC/IEEE 60076-57-19: 2017, MOD.
3 Terms and definitions
For the purposes of this document, the terms and definitions given in GB/T 18494.1-2014 and GB/T 18494.2-2022 apply.
4 Symbols
For the purposes of this document, the following symbols apply:
FSE——enhancement factor for stray losses in structural parts;
FWE——enhancement factor for winding eddy losses;
fh——frequency at harmonic order number h, Hz;
fx——frequency used to determine the distribution of eddy losses (≥150Hz, HVDC transformer applications only);
f1——rated frequency and also the fundamental frequency (50Hz or 60Hz);
h——harmonic order number;
Ieq——r.m.s. value of the equivalent sinusoidal test current giving the winding in-service load loss, A;
Ih——harmonic current having order number h, A;
ILN——r.m.s. value of the non-sinusoidal in-service load current in the winding under consideration at rated converter load for transformers for industrial applications, A;
For HVDC transformers, it is the square root of the sum of squares from fundamental current to the 49th harmonic current under specific load conditions and related harmonic current spectrum ( , 49 is the highest harmonic calculated), A;
Ir——rated current, the r.m.s. value of the nominal operating current of the winding, including harmonics, calculated in the same way as ILN above, used to define the rated impedance (HVDC transformer applications only), A;
IX——r.m.s. value of the load loss test current at frequency fx, A;
I1——r.m.s. value of the fundamental component of the rated line side current, A;
——ohmic losses at current I1, W;
——ohmic losses at rated current (HVDC transformer applications only), W;
Kh——ratio of the current Ih to the rated current I1;
kh——ratio of the current Ih to the rated current Ir (HVDC transformer applications only);
N——number of six-pulse bridges in series from the neutral of the d.c. line to the rectifier bridge connected to the converter transformer;
PN——load loss in service with current ILN, W;
PSE1——stray losses in structural parts (excluding windings) with current I1, W;
PWE1——eddy losses in windings with current I1, W;
Px——load loss measured at current Ix, W;
P1——load loss measured at current I1, W;
R——d.c. resistance of windings including internal leads, Ω;
Sr——rated power (VA);
Uac——a.c. separate source test voltage for the valve side windings (r.m.s. Value.), V;
Udc——d.c. separate source test voltage for the valve side windings, V;
Udm——highest d.c. voltage per valve bridge, V;
Um——highest system voltage of the line side winding, V;
Upr——polarity reversal test voltage for the valve side windings (d.c. Voltage), V;
Uvm——maximum phase to phase a.c. operating voltage of the valve side windings, V;
U1——r.m.s. value of the fundamental component of the line-to-line rated voltage, V.
Note: “Valve side” and “line side” define the external connections of the converter transformers. Line side refers to the winding connected to the a.c. network and valve side to the winding connected to the converter.
5 Rating data
Foreword i
Introduction v
1 Scope
2 Normative references
3 Terms and definitions
4 Symbols
5 Rating data
6 Winding configurations
7 Tappings and impedances – HVDC applications
8 Insulation aspects and dielectric tests
9 Losses
10 Core and sound aspects
11 Transformer specifications
12 Short circuit considerations
13 Components
14 Maintenance
15 Monitoring
16 Design review of converter transformers
Bibliography
Figure 1 Bridge connection for six-pulse arrangement
Figure 2 Bridge connection for twelve-pulse arrangement
Figure 3 Transformer connections star-delta and star-star or alternatively, delta-delta and delta-star to give a 30 electrical degree separation between the valve-side voltages
Figure 4 15° phase shifting is preferably realized by extended delta or zigzag connection
Figure 5 Double-inverse-star connection
Figure 6 Connection of transductor
Figure 7 Autotransformer concept for open phases with coarse and fine regulation steps
Figure 8 Conventional autotransformer concept for closed phases with coarse and fine regulation steps
Figure 9 Conventional autotransformer concept for closed phases with multi coarse regulation
Figure 10 Booster autotransformer concept for closed phases with coarse and fine regulation steps
Figure 11 Autotransformer concept for open phases with coarse and fine regulation steps
Figure 12 Transformer concept for closed phases with coarse and fine regulation steps
Figure 13 Two basic arrangements of the individual windings of a two-winding transformer
Figure 14 Mutual impedance
Figure 15 Typical impedance pattern
Figure 16 Components of a typical industrial converter transformer insulating system
Figure 17 Insulation system, equivalent R-C circuit
Figure 18 Voltage distribution before and immediately after polarity reversal
Figure 19 AC/DC conversion - Simplified sketch
Figure 20 Leakage fields for a three-winding transformer with closely coupled valve side windings
Figure 21 Leakage fields for a three-winding transformer with decoupled valve side windings
Figure 22 Leakage fields for a three-winding transformer with loosely coupled double concentric valve side windings
Figure 23 Leakage fields for a three-winding transformer with loosely coupled double-tier valve side windings
Figure 24 Short-circuit fault conditions in rectifier bridge
Figure 25 Arrangement of valve bushings
Figure 26 Examples of a.c., d.c. and combined electric field dispositions adjacent to HVDC bushings and associated electric insulation systems
Table 1 Arrangements of two windings
Table 2 Data for calculation of load losses with harmonic currents
Table 3 Monitoring types
