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BSI PD IEC TR 62001-5:2021

BSI PD IEC TR 62001-5:2021

$215.11

High-voltage direct current (HVDC) systems. Guidance to the specification and design evaluation of AC filters – AC side harmonics and appropriate harmonic limits for HVDC systems with voltage sourced converters (VSC)

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BSI 2021 136
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This part of IEC TR 62001, which is a Technical Report, provides guidance on the state-of-the art of VSC technology in relation to harmonics and predicted future developments, on the harmonic profile of present and predicted future VSC architectures and how they are characterised and modelled – as voltage sources, current sources, or otherwise. It also assesses the harmonic impedance of VSC and the possible impact on pre-existing background harmonics emanating from loads or generation units in the supply network and considers how VSC harmonics are assessed under current IEC standards and national regulations, and identify areas where improvements could be made, research can be needed, or other bodies consulted, for example when considering interharmonics. This document can be a reference source on the subject, which will also contain recommendations for use by those charged with modifying existing standards to adapt to VSC HVDC systems.

Issues relating to harmonics on the DC side of the converters, including DC grids, are deliberately not covered in this document, but are addressed by a separate CIGRE Technical Brochure [1]1.

PDF Catalog

PDF Pages PDF Title
2 undefined
4 CONTENTS
10 FOREWORD
12 INTRODUCTION
14 1 Scope
2 Normative references
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
15 3.2 Abbreviated terms
16 4 Basic aspects of VSC HVDC harmonics
4.1 General
17 4.2 Differences between VSC and LCC harmonic behaviour
18 4.3 Issues relating to VSC harmonics
19 4.4 Range of frequencies considered
Figures
Figure 1 – Frequency range of VSC waveform
20 4.5 Equivalent circuit of the converter for harmonic analysis
Figure 2 – Harmonic representation of a VSC station for harmonics analysis
21 4.6 Dual impact of a VSC converter on harmonic distortion at PCC
4.6.1 General
4.6.2 Converter generated harmonics
22 4.6.3 Pre-existing harmonics
Figure 3 – Harmonic contribution by the converter
Figure 4 – Amplification of the background harmonics
23 4.6.4 Combining the effects of converter-generated and pre-existing harmonics
Tables
Table 1 – Indicative summation exponents
24 5 Harmonic generation
5.1 General
25 5.2 Factors influencing harmonic generation
5.2.1 General
5.2.2 Converter topology
26 Figure 5 – Two-level converter
Figure 6 – Three-level converter
Figure 7 – Modular multi-level converter (MMC)
Figure 8 – Cascaded two-level converter (CTL)
27 5.2.3 Control
Figure 9 – HVDC VSC converter control structure
29 5.2.4 Power electronics hardware
30 Figure 10 – Interlocking example
31 5.3 Harmonic generation
5.3.1 General
5.3.2 Harmonic generation from VSC using switch type valves
Figure 11 – Semiconductor voltage drop
32 Figure 12 – References and carrier for a two level converterusing PWM with pulse number of 9
Figure 13 – Reference, carrier and the resulting phase voltage for one phase of a two level converter using PWM with pulse number of 9
33 Figure 14 – Harmonic spectrum, phase to ground, of a two level converterusing PWM with pulse number of 39
Figure 15 – Harmonic spectrum, phase to ground, of a two level converter using PWM with pulse number 39 after removal of the zero sequence orders
34 Figure 16 – Extended harmonic spectrum of a two level converter using PWM with pulse number 39 after removal of the zero sequence orders
35 Figure 17 – Fundamental and phase voltage for one phaseof a two-level converter using OPWM
Figure 18 – Harmonic spectrum, phase to ground, of a two-level converter using OPWM
36 Figure 19 – Harmonic spectrum, phase to ground, of a two level converterusing OPWM after removal of the zero sequence
Figure 20 – Extended harmonic spectrum, phase to ground, of a two-level converter using OPWM after removal of the zero sequence
37 Figure 21 – References and carriers for a three level converter with pulse number of 9
38 Figure 22 – Reference, carriers and the resulting phase voltagefor one phase of a three level converter with pulse number of 9
Figure 23 – Harmonic spectrum, phase-ground, of a three level converter,pulse number of 39
39 Figure 24 – Harmonic spectrum, phase to ground, of a three level converterwith pulse number of 39 after removal of the zero sequence
Figure 25 – Extended harmonic spectrum, phase to ground, of a three level converter with pulse number of 39 after removal of the zero sequence
40 5.3.3 Harmonic generation from VSC using controllable voltage source type valves
Figure 26 – Voltage source representation of the MMC
42 Figure 27 – Valve voltage generation
Figure 28 – Harmonic spectrum for one arm of the MMC converter
43 Figure 29 – Harmonic spectrum for one arm of the MMC converter(extended frequency range)
44 Figure 30 – Reference and carriers for three adjacent cells
45 Figure 31 – Zoomed – reference and carriers for three adjacentcells and resulting voltage
Figure 32 – Reference and voltage for one arm
46 5.4 Interharmonics
Figure 33 – Harmonic spectrum for one arm of a CTL converter
Figure 34 – Harmonic spectrum for one arm of a CTL converter –extended frequency range
47 Figure 35 – Voltage synthesization with optimum time stepof the valve control operation
48 Figure 36 – Voltage synthesization with an alternative time stepof the valve control operation
Figure 37 – Illustrative impact of sorting and selection algorithmson interharmonic generation
49 5.5 Impact of non-ideal conditions on harmonic generation
50 6 VSC HVDC as a harmonic impedance
6.1 General
51 6.2 Passive impedance
6.3 Active impedance
6.3.1 General
6.3.2 Ideal VSC behaviour
Figure 38 – Active and passive impedance elements
52 6.3.3 Impact of practical control system features
Figure 39 – Control of AC voltage or current
53 6.3.4 Example of impact of control
54 6.4 Impact on amplification of pre-existing harmonics
Figure 40 – Illustrative impact of the I-control inner control loop time response(to 5 % relative error) on the positive sequence converter impedance
55 7 Adverse effects of VSC HVDC harmonics
7.1 General
56 7.2 Telephone interference
7.2.1 General
7.2.2 Extended higher frequency range of VSC harmonics
7.2.3 Interharmonics
57 7.2.4 AC cable connecting HVDC station to the PCC
7.3 PLC, metering and ripple control
7.3.1 General
58 7.3.2 Extended higher frequency range of VSC harmonics
7.3.3 Interharmonics
59 7.4 Railway signal interference
7.5 Digital telecommunications systems
60 8 Harmonic limits
8.1 General
8.2 Deleterious effects of excessively low limits
61 8.3 Standards and practice
62 8.4 Perception of VSC in setting limits
8.5 Emission and amplification limits
63 8.6 Relevance of standards for VSC
8.7 Existing standards
Table 2 – Indicative planning levels for harmonic voltages (in percent of the fundamental voltage) in MV, HV and EHV power systems
64 8.8 Higher frequency harmonics
8.8.1 General
65 8.8.2 IEEE Std 519-2014 [7]
Table 3 – Current limits for system rated > 161 kV
66 8.8.3 Shortcomings in the context of VSC
8.9 Even order harmonic limits
8.10 Interharmonics
8.10.1 General
67 8.10.2 Treatment of interharmonics in existing standards
Table 4 – Summary of IEC TR 61000-3-6 [5] recommended voltage planning levels
68 8.10.3 Discussion and recommendations
69 8.11 Interharmonics discretization and grouping methodologies
8.11.1 Suggested method
70 Figure 41 – Proposed grouping methodology
Figure 42 – Comparison with grouping methodology of IEC 61000-4-7 [3]
71 Figure 43 – Centred harmonic subgroup
72 8.11.2 Power quality indices for interharmonic grouping
Figure 44 – Harmonic group
73 8.11.3 Network impedance loci for interharmonic grouping
Figure 45 – Harmonic impedance frequency ranges for LCC
74 8.12 Assessment as a harmonic voltage or current source
Figure 46 – Harmonic impedance frequency ranges for VSC with proposed methodology
Figure 47 – Harmonic impedance frequency ranges for VSCwith IEC 61000-4-7 grouping methodology
75 8.13 Assessment of THD, TIF, THFF, IT
76 8.14 Measurement and verification of harmonic compliance
77 8.15 Recommendations
78 9 Harmonic mitigation techniques
9.1 General
9.2 Passive filtering
79 Figure 48 – AC filter located at primary (network) side of converter transformer
Figure 49 – AC filter located at the secondary (converter) side of converter transformer
80 9.3 Active damping and active filtering by converter control
81 9.4 Optimization between passive and active mitigation
9.5 Specific mitigation issues and techniques
9.5.1 Unbalanced phase reactances or voltages
82 Figure 50 – Example of a converter station scheme with asymmetrical phase reactances
Figure 51 – Example of converter plant and control scheme
83 Figure 52 – Current control scheme
84 Figure 53 – Time-domain response of positive and negative sequence voltages and currents and active power when the converter does not compensate for effect of phase reactance unbalances
85 9.5.2 Power oscillations due to AC supply voltage unbalance
Figure 54 – Time-domain response of positive and negative sequence voltages and currents and the active power when the converter controls phase currents to be balanced
86 Figure 55 – Power oscillations between AC and DC sides due to unbalanced AC conditions when the converter does not control the fluctuations of energy between arms and the grid currents
87 9.5.3 Harmonic cross-modulation between AC and DC sides
88 Figure 56 – Influence of distortions at the AC and DC side voltagesand the propagation through the control
Figure 57 – 6th harmonic content in DC side voltage of MMC
89 9.5.4 Cross-modulation of DC side fundamental frequency current
Figure 58 – Resulting AC side voltage with modification of control at t = 4 s
90 10 Modelling
10.1 Provision of models
10.2 Time and frequency domain
91 10.3 Modelling of the converter control for harmonic and resonance studies
92 10.4 Converter linearization by analytical approach
10.4.1 General
10.4.2 VSC-MMC linearized model
10.4.3 Input impedance
Figure 59 – VSC HVDC transmission system
Figure 60 – VSC station model using the small-signal approach
93 10.4.4 Advantages of analytical method
10.4.5 Drawbacks of analytical method
10.5 Deriving the converter impedance by numerical approach
10.5.1 Methodology
94 Figure 61 – Model evolution in decreasing complexity
Figure 62 – Switching function model of MMC arm
Figure 63 – Time domain to frequency domain stratagem
95 10.5.2 Advantages of numerical method
Figure 64 – Example of a circuit to linearize a network and a VSC including controllers
96 10.5.3 Drawbacks of numerical method
10.6 Choice between analytical and numerical methods
10.7 Model validation
97 10.8 Network impedance modelling
99 11 Harmonic stability
11.1 General
100 11.2 Literature review
Figure 65 – Dynamic interactions between components and study framework
101 11.3 Definitions
102 11.4 Theory
11.4.1 General
11.4.2 Passive harmonic resonance
Figure 66 – RLC circuit and time-domain response to a step disturbance
103 Figure 67 – Connection of the converter station to a passive network
Figure 68 – Bode plot of the converter, network and equivalent impedances
104 11.4.3 Active behaviour of converters
11.4.4 Active impedance of a VSC with a generic current control
Figure 69 – Dynamic scheme of the current controller and phase reactor
105 11.4.5 Harmonic instability
Figure 70 – Bode plot of the converter passive and active impedance
106 Figure 71 – Example of a network composed of a VSC and a frequency-dependent AC system for the study of control interactions
Figure 72 – Dynamic interaction between the active VSC impedanceand the network passive impedance
107 Figure 73 – Bode plot of the VSC and network impedance,including active converter effects
108 11.5 Analysis methods
11.5.1 General
11.5.2 Network impedance scans
Figure 74 – Results of EMT simulation study of the investigated system
109 11.5.3 Passivity analysis
111 Figure 75 – Example output of passivity analysis
112 11.5.4 Impedance-based stability analysis
Figure 76 – Comparison of passivity analysis of converter systemwithout (blue line) and with (red line) harmonic damper
113 Figure 77 – Simple network, consisting of source and load
Figure 78 – Loop gain of the simple network
114 Figure 79 – Bode diagram of the frequency dependent impedanceof a converter and the grid
Table 5 – Phase margins at intersections
115 Figure 80 – Small-signal representation of two interconnected AC systems
116 11.5.5 Modal analysis in rotating reference frame
Figure 81 – Sample impedance stability results
118 11.5.6 Electro-magnetic-transient simulation
Figure 82 – Sample modal analysis results
119 11.5.7 Recommendations
11.6 System-wide studies
120 11.7 Real experiences of harmonic stability in the context of HVDC systems
11.7.1 General
11.7.2 Case A: High power rating VSC HVDC system
121 Figure 83 – Circuit configuration of the negative resistance test case
Figure 84 – Frequency response of Network 1 and the converter station
122 11.7.3 Case B: Offshore wind farm
Figure 85 – Phase angle from Figure 84 zoomed in the y axis
Figure 86 – AC voltage at PCC1 and zoomed extract
123 Figure 87 – Schematic view of the main componentsof the case B grid connection system
124 11.7.4 Case C: Back-to-back converter in a 500 kV network
Figure 88 – Example of frequency scan at the offshore substation in case B
125 Figure 89 – Illustrations of the system in case C
Figure 90 – Bode diagram of converter and grid impedances in case C and time-domain simulation with the control implemented in the EMT tool
126 12 Conclusion
128 Bibliography

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BSI PD IEC TR 62001-5:2021
$215.11