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PD IEC TR 62001-5:2021 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), 2021
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- CONTENTS
- FOREWORD
- INTRODUCTION
- 1 Scope
- 2 Normative references
- 3 Terms, definitions and abbreviated terms [Go to Page]
- 3.1 Terms and definitions
- 3.2 Abbreviated terms
- 4 Basic aspects of VSC HVDC harmonics [Go to Page]
- 4.1 General
- 4.2 Differences between VSC and LCC harmonic behaviour
- 4.3 Issues relating to VSC harmonics
- 4.4 Range of frequencies considered
- Figures [Go to Page]
- Figure 1 – Frequency range of VSC waveform
- 4.5 Equivalent circuit of the converter for harmonic analysis
- Figure 2 – Harmonic representation of a VSC station for harmonics analysis
- 4.6 Dual impact of a VSC converter on harmonic distortion at PCC [Go to Page]
- 4.6.1 General
- 4.6.2 Converter generated harmonics
- 4.6.3 Pre-existing harmonics
- Figure 3 – Harmonic contribution by the converter
- Figure 4 – Amplification of the background harmonics [Go to Page]
- 4.6.4 Combining the effects of converter-generated and pre-existing harmonics
- Tables [Go to Page]
- Table 1 – Indicative summation exponents
- 5 Harmonic generation [Go to Page]
- 5.1 General
- 5.2 Factors influencing harmonic generation [Go to Page]
- 5.2.1 General
- 5.2.2 Converter topology
- Figure 5 – Two-level converter
- Figure 6 – Three-level converter
- Figure 7 – Modular multi-level converter (MMC)
- Figure 8 – Cascaded two-level converter (CTL) [Go to Page]
- 5.2.3 Control
- Figure 9 – HVDC VSC converter control structure [Go to Page]
- 5.2.4 Power electronics hardware
- Figure 10 – Interlocking example
- 5.3 Harmonic generation [Go to Page]
- 5.3.1 General
- 5.3.2 Harmonic generation from VSC using switch type valves
- Figure 11 – Semiconductor voltage drop
- 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
- 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
- Figure 16 – Extended harmonic spectrum of a two level converter using PWM with pulse number 39 after removal of the zero sequence orders
- 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
- 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
- Figure 21 – References and carriers for a three level converter with pulse number of 9
- 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
- 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 [Go to Page]
- 5.3.3 Harmonic generation from VSC using controllable voltage source type valves
- Figure 26 – Voltage source representation of the MMC
- Figure 27 – Valve voltage generation
- Figure 28 – Harmonic spectrum for one arm of the MMC converter
- Figure 29 – Harmonic spectrum for one arm of the MMC converter(extended frequency range)
- Figure 30 – Reference and carriers for three adjacent cells
- Figure 31 – Zoomed – reference and carriers for three adjacentcells and resulting voltage
- Figure 32 – Reference and voltage for one arm
- 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
- Figure 35 – Voltage synthesization with optimum time stepof the valve control operation
- Figure 36 – Voltage synthesization with an alternative time stepof the valve control operation
- Figure 37 – Illustrative impact of sorting and selection algorithmson interharmonic generation
- 5.5 Impact of non-ideal conditions on harmonic generation
- 6 VSC HVDC as a harmonic impedance [Go to Page]
- 6.1 General
- 6.2 Passive impedance
- 6.3 Active impedance [Go to Page]
- 6.3.1 General
- 6.3.2 Ideal VSC behaviour
- Figure 38 – Active and passive impedance elements [Go to Page]
- 6.3.3 Impact of practical control system features
- Figure 39 – Control of AC voltage or current [Go to Page]
- 6.3.4 Example of impact of control
- 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
- 7 Adverse effects of VSC HVDC harmonics [Go to Page]
- 7.1 General
- 7.2 Telephone interference [Go to Page]
- 7.2.1 General
- 7.2.2 Extended higher frequency range of VSC harmonics
- 7.2.3 Interharmonics
- 7.2.4 AC cable connecting HVDC station to the PCC
- 7.3 PLC, metering and ripple control [Go to Page]
- 7.3.1 General
- 7.3.2 Extended higher frequency range of VSC harmonics
- 7.3.3 Interharmonics
- 7.4 Railway signal interference
- 7.5 Digital telecommunications systems
- 8 Harmonic limits [Go to Page]
- 8.1 General
- 8.2 Deleterious effects of excessively low limits
- 8.3 Standards and practice
- 8.4 Perception of VSC in setting limits
- 8.5 Emission and amplification limits
- 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
- 8.8 Higher frequency harmonics [Go to Page]
- 8.8.1 General
- 8.8.2 IEEE Std 519-2014 [7]
- Table 3 – Current limits for system rated > 161 kV [Go to Page]
- 8.8.3 Shortcomings in the context of VSC
- 8.9 Even order harmonic limits
- 8.10 Interharmonics [Go to Page]
- 8.10.1 General
- 8.10.2 Treatment of interharmonics in existing standards
- Table 4 – Summary of IEC TR 61000-3-6 [5] recommended voltage planning levels [Go to Page]
- 8.10.3 Discussion and recommendations
- 8.11 Interharmonics discretization and grouping methodologies [Go to Page]
- 8.11.1 Suggested method
- Figure 41 – Proposed grouping methodology
- Figure 42 – Comparison with grouping methodology of IEC 61000-4-7 [3]
- Figure 43 – Centred harmonic subgroup [Go to Page]
- 8.11.2 Power quality indices for interharmonic grouping
- Figure 44 – Harmonic group [Go to Page]
- 8.11.3 Network impedance loci for interharmonic grouping
- Figure 45 – Harmonic impedance frequency ranges for LCC
- 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
- 8.13 Assessment of THD, TIF, THFF, IT
- 8.14 Measurement and verification of harmonic compliance
- 8.15 Recommendations
- 9 Harmonic mitigation techniques [Go to Page]
- 9.1 General
- 9.2 Passive filtering
- 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
- 9.3 Active damping and active filtering by converter control
- 9.4 Optimization between passive and active mitigation
- 9.5 Specific mitigation issues and techniques [Go to Page]
- 9.5.1 Unbalanced phase reactances or voltages
- Figure 50 – Example of a converter station scheme with asymmetrical phase reactances
- Figure 51 – Example of converter plant and control scheme
- Figure 52 – Current control scheme
- 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 [Go to Page]
- 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
- 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 [Go to Page]
- 9.5.3 Harmonic cross-modulation between AC and DC sides
- 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 [Go to Page]
- 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
- 10 Modelling [Go to Page]
- 10.1 Provision of models
- 10.2 Time and frequency domain
- 10.3 Modelling of the converter control for harmonic and resonance studies
- 10.4 Converter linearization by analytical approach [Go to Page]
- 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 [Go to Page]
- 10.4.4 Advantages of analytical method
- 10.4.5 Drawbacks of analytical method
- 10.5 Deriving the converter impedance by numerical approach [Go to Page]
- 10.5.1 Methodology
- Figure 61 – Model evolution in decreasing complexity
- Figure 62 – Switching function model of MMC arm
- Figure 63 – Time domain to frequency domain stratagem [Go to Page]
- 10.5.2 Advantages of numerical method
- Figure 64 – Example of a circuit to linearize a network and a VSC including controllers [Go to Page]
- 10.5.3 Drawbacks of numerical method
- 10.6 Choice between analytical and numerical methods
- 10.7 Model validation
- 10.8 Network impedance modelling
- 11 Harmonic stability [Go to Page]
- 11.1 General
- 11.2 Literature review
- Figure 65 – Dynamic interactions between components and study framework
- 11.3 Definitions
- 11.4 Theory [Go to Page]
- 11.4.1 General
- 11.4.2 Passive harmonic resonance
- Figure 66 – RLC circuit and time-domain response to a step disturbance
- Figure 67 – Connection of the converter station to a passive network
- Figure 68 – Bode plot of the converter, network and equivalent impedances [Go to Page]
- 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 [Go to Page]
- 11.4.5 Harmonic instability
- Figure 70 – Bode plot of the converter passive and active impedance
- 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
- Figure 73 – Bode plot of the VSC and network impedance,including active converter effects
- 11.5 Analysis methods [Go to Page]
- 11.5.1 General
- 11.5.2 Network impedance scans
- Figure 74 – Results of EMT simulation study of the investigated system [Go to Page]
- 11.5.3 Passivity analysis
- Figure 75 – Example output of passivity analysis [Go to Page]
- 11.5.4 Impedance-based stability analysis
- Figure 76 – Comparison of passivity analysis of converter systemwithout (blue line) and with (red line) harmonic damper
- Figure 77 – Simple network, consisting of source and load
- Figure 78 – Loop gain of the simple network
- Figure 79 – Bode diagram of the frequency dependent impedanceof a converter and the grid
- Table 5 – Phase margins at intersections
- Figure 80 – Small-signal representation of two interconnected AC systems [Go to Page]
- 11.5.5 Modal analysis in rotating reference frame
- Figure 81 – Sample impedance stability results [Go to Page]
- 11.5.6 Electro-magnetic-transient simulation
- Figure 82 – Sample modal analysis results [Go to Page]
- 11.5.7 Recommendations
- 11.6 System-wide studies
- 11.7 Real experiences of harmonic stability in the context of HVDC systems [Go to Page]
- 11.7.1 General
- 11.7.2 Case A: High power rating VSC HVDC system
- Figure 83 – Circuit configuration of the negative resistance test case
- Figure 84 – Frequency response of Network 1 and the converter station [Go to Page]
- 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
- Figure 87 – Schematic view of the main componentsof the case B grid connection system [Go to Page]
- 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
- 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
- 12 Conclusion
- Bibliography [Go to Page]
