A f ul in u tive co rdination stu y, whic in olves the calc lation of harmonic c r ent f low in the s stem in order to determine the pro lematic tran mis ion l nes an the as es ment of t
General
The article discusses permissible distortion limits and performance measures for limiting telephone interference, specifically focusing on the telephone interference factor (TIF) and the product of RMS current and TIF (IT), as detailed in IEC TR 62001-1:2016, Clause 4 Strict adherence to these current-based criteria, particularly IT, is crucial for filter design, as they can significantly influence filter costs and necessitate larger filters, leading to additional requirements such as extra station space and shunt reactors for reactive power compensation Furthermore, several HVDC projects have faced challenges due to high levels of telephone interference during commissioning and early operation, highlighting the importance of understanding basic interference criteria and their implications for filter design.
Because these criteria, based on psophometric or C-message weighting of harmonics, are specific to evaluation of noise induced on telephone circuits electromagnetically coupled to
AC lines, they should only be specified where significant coupling between AC transmission _
This document offers guidance on identifying situations where telephone interference may occur, acknowledging that various factors influence this risk While definitive quantitative guidelines are unattainable, it is crucial to recognize the impact of harmonic currents propagated through the AC system Notably, significant harmonic currents generated by HVDC systems can affect lines far from the converter station, even after voltage transformations To accurately assess the potential for interference, a comprehensive inductive coordination study is essential, as it calculates harmonic current flow and evaluates the coupling between problematic transmission lines and adjacent telephone lines.
The specification of telephone interference should also take into account local particularities, as discussed in 3.2
A comprehensive paper by the Joint Task Force 02 of WG1 4.03/CC.02 provides an in-depth overview of the inductive coordination process and key factors influencing telephone interference The IT limits are derived from the Finnish telephone system, incorporating certain approximations of network characteristics This document emphasizes North American practices for IT limits, while the underlying principles and calculation methods are relevant globally, highlighting essential system parameters for technical specifications.
In systems susceptible to telephone interference, it is crucial to establish clear harmonic current and voltage performance criteria to safeguard the interests of the HVDC system owner Inadequate specifications can lead to severe consequences if telephone interference issues arise, resulting in costly and time-consuming resolutions after the HVDC system is operational The complexity of addressing these issues is heightened as they impact not only the HVDC owner but also the telephone system operator and its subscribers Legal or regulatory actions may force the HVDC system to halt operations until necessary modifications are made to the filtering system or mitigation measures are implemented Proper inductive coordination at the project's outset simplifies the resolution of potential problems.
When not carefully considered, requirements and their evaluation can result in overly complex and expensive designs Clause 3 aims to clarify various aspects of this topic by presenting theoretical insights, assessing practical experiences, and offering guidelines.
Determining the necessity for telephone interference limits
Voltage distortion control is crucial for electrical networks, but telephone interference varies by project This interference arises when harmonic currents in AC transmission lines run parallel to telephone lines, inducing disturbing voltages proportional to exposure length and mutual impedance Section 3.2 addresses harmonic limits related to telephone interference, including IT, TIF, I eq, and THFF, which aim to mitigate interference in cable wire telephone lines transmitting audible frequency signals, approximately between 200 Hz and 3000 Hz.
Quantitative guidance on the conditions for significant telephone interference in a project is challenging to provide, making it essential to consider qualitative guidelines If there are concerns about a project's vulnerability to telephone interference, conducting an inductive coordination study is recommended to inform the development of necessary specifications for buyer protection.
Conditions known to increase the susceptibility to AC-side telephone interference are the following:
• Long sections of exposure between AC lines carrying converter harmonic currents and telephone lines
• Close proximity of AC transmission lines and parallel telephone lines
• Even moderate separation distances and longitudinal exposures if combined with very high earth resistivity
• Open-wire telephone lines However, shielded twisted-pair telephone circuits provide only a partial reduction of coupling potential, and such circuits are by no means exempt from potential interference issues
• Radial transmission line(s) to the converter station, where all converter harmonics are forced into the one single-circuit or double-circuit line
• AC transmission systems having a hybrid overhead/underground design, with overhead lines interspersed with underground cable sections
AC transmission systems often feature numerous capacitor banks near converter stations, leading to various resonances within the AC network Analyzing these systems is complex due to the necessity of considering all possible combinations and permutations of capacitor bank statuses.
Harmonic currents generated by converter stations extend beyond the immediate AC lines, infiltrating multiple levels of the transmission network and even crossing transformers to different voltage levels This poses challenges, especially when lower-voltage lines are closely linked to telephone circuits While harmonic currents typically decrease with distance from the converter station, resonance conditions can lead to unexpectedly high levels of harmonic currents in second and higher-tier lines compared to those directly connected to the converter station.
The following conditions can be assumed to indicate non-existence of telephone interference issues at vocal frequency, and thus no need for psophometric or C-message weighted specifications:
• all exposed telephone circuits are fibre optic cable;
• multiplex systems (time or frequency multiplexing):
Global HVDC experience indicates that while some regions lack specified telephone interference limits, issues have not typically arisen Telephone systems share similarities across countries, but factors influencing interference can vary significantly In North America, telephone interference is a major concern due to the design of rural telephone and transmission systems, which increase exposure and proximity risks There are also strong legal protections for consumers and utilities, making excessive interference a serious economic risk for HVDC projects Conversely, in China, telephone lines are often located far from HV transmission lines Large HVDC projects may be prioritized for their national interest, allowing for rapid and cost-effective construction while addressing potential telephone noise concerns separately.
Many homes and small businesses in North America and other regions still rely on traditional twisted copper wires for phone connectivity, which have been in use for decades Despite challenges in deploying fiber optics, the existing copper network continues to offer high transmission speeds, suggesting that it will remain the industry standard for the foreseeable future, particularly in rural areas where the cost of installing fiber optic or coaxial cabling is prohibitive Meanwhile, in several countries, digital cellular technologies are rapidly replacing outdated analogue landline systems, with telecom operators' pricing structures encouraging a shift towards mobile phone usage.
Previous experience with utility and telephone companies regarding telephone interference from existing facilities serves as the most relevant reference, as it closely mirrors the specific conditions under which the new HVDC project will need to function.
Defining telephone interference limits 1 1
General 1 1
IEC TR 62001-1:2016, Clause 4 provides general guidelines for establishing limits without extensive studies, particularly in situations with tight schedules, insufficient computational tools, or limited telephone system data, and where significant interference issues due to harmonic distortion are not anticipated It references IEEE Std 368-1977, which includes a table of suggested limits for high voltage (HV) and extra high voltage (EHV) transmission lines, emphasizing the necessity for a detailed case-by-case analysis of telephone interference The values in this table are illustrative, and the methodology for their derivation is not specified Subsequent standards, such as IEEE Std 519-1992, continue to address these considerations.
[4] and CAN/CSA-C22.3 No 3-98 [5]) have copied this table with no apparent verification of its validity On the other hand, experience shows that some HVDC schemes with a specified
IT emanating from a converter bus of between 25 000 A and 50 000 A function with no problems of telephone interference Applying these previous limits without any study is therefore not recommended
In cases where significant telephone interference risks are identified due to a specific HVDC project, a comprehensive study is essential to evaluate the performance specifications of AC filters Section 3.3 outlines the procedure for calculating the impact of a transmission line on nearby telephone lines, drawing on North American practices where interference issues are more prevalent While the focus is on telephone cable systems, the fundamental principles are applicable to other systems with varying susceptibility levels Additionally, illustrative tables of coupling values are included for reference.
Assessing the harmonic current flow in transmission lines near the HVDC project is essential to identify those that may impact telephone interference requirements and to evaluate potential interference levels Recommendations are provided regarding the necessary AC system information to be included in specifications for effective AC filter design.
Mechanisms of interference 1 1
Harmonic currents in transmission lines generate harmonic voltages and currents in adjacent installations The induced voltage, known as longitudinal voltage, can be measured between one end of a telephone conductor and the ground, with the remote end grounded The calculation of longitudinal voltage in any parallel conductor can be performed using specific formulas.
1 jn jn gn =∑ = × (1 ) where n is the harmonic number; j is the conductor number; k is the number of conductors on the transmission line;
Vg n is the longitudinal voltage at harmonic n;
I jn is the phasor current in conductor j at harmonic n;
Zm jn is the mutual impedance between conductor j and telephone line at harmonic n, including the screening effect of the ground wires and any other nearby grounded conductors
In Equation (1 ), the harmonic currents flowing in the transmission line are calculated by the HVDC converter contractor according to the method defined in the technical specifications
Mutual impedance is influenced by factors such as earth resistivity, the distance between transmission and telephone lines, line configuration, and frequency Inductive coordination studies necessitate calculating mutual impedances for numerous exposure scenarios between AC transmission lines and telephone lines This calculation often accounts for the impact of screening conductors, like shield wires or nearby conductive installations, and is typically performed using specialized software such as EMTP, CORRIDOR, MathCAD, and CDEGS For simpler cases, Dubanton equations can yield satisfactory results within a typical range of exposure values Additionally, calculating coupling for irregular exposures requires decomposing the exposures into parallel sections and summing them to determine the total coupling Some software, including the Crinoline toolbox in EMTP and CDEGS, can also compute mutual impedances for irregular exposures.
Modern telephone lines utilize shielded cables with twisted conductor pairs to transmit voice signals The shield effectively cancels out part of the induced voltage in the conductor pair by allowing current to flow through its grounded ends, particularly reducing interference at higher frequencies The interference voltage, known as metallic or transverse voltage, is the difference between the longitudinal voltages of both conductors and is what is ultimately detected across a telephone receiver.
NOTE The terms "common mode" for longitudinal and "differential mode" for transverse are also used
The balance of a circuit, defined as the ratio between metallic and longitudinal voltage, is influenced by frequency To account for this, the metallic voltage is adjusted to align with the frequency response of both the ear and the telephone system In North America, C-message weighting is utilized, whereas Europe employs psophometric weighting Various regions around the globe adopt either of these weighting methods.
The total effective noise is determined by calculating the square root of the sum of the squares of the weighted components for each harmonic The formula for the total weighted metallic noise voltage is provided as follows:
1 jn jn n (2) where m is the maximum order of harmonic to be considered;
C n is the C-message or psophometric weighting of harmonic n;
K n is the telephone circuit shielding factor at harmonic n;
B n is the telephone circuit balance at harmonic n.
Telephone companies typically provide the shielding and balance factors for telephone circuits In practice, shielding tends to improve with increasing frequency, whereas balance deteriorates as frequency rises Overall, the combined factor remains relatively constant across the frequency range of interest.
EMTP, CORRIDOR, MathCAD, and CDEGS are commercially available products that are mentioned for user convenience; however, this information does not imply any endorsement by IEC.
IEEE Std 1124-2003 offers extensive insights into calculating mutual impedances and understanding the parameters essential for inductive coordination studies The approach to inductive coordination for DC transmission lines mirrors that of AC lines Additionally, [10] provides valuable guidance on managing electromagnetic interference from power systems affecting telecommunication systems, while Annex A addresses the impact of voltage and current distortion on telephone interference levels.
Noise performance coordination levels 1 3
According to ITU-T EMC-1.6, applicable in Europe and beyond, the psophometric voltage measured across a 600 Ω resistance at one end of the line, with the remote end terminated in its characteristic impedance, must not exceed 0.5 mV.
North American standards recommend that the noise contribution on the customer loop should be limited to 20 dBrnC Telephone circuit noise is defined in relation to 1 pW in 600 Ω, corresponding to an applied voltage of 24.5 mV, and is expressed in decibels above this reference level.
N m is the metallic (transverse) noise expressed in dB above 24,5 mV
The corresponding metallic noise voltage is 0,245 mV, which is therefore stricter than the ITU counterpart (0,5 mV)
Since the influence of transmission lines on telephone interference is more predominant in North America, the following discussion will focus on the American practice
The basic quantities in the characterization of interference between HV transmission lines and telephone lines are:
N m (dBrnC) = N g (dBrnC) – B al (dB) (4) where
N g is the longitudinal noise to ground expressed in dB above 24,5 mV;
B al is the balance of the telephone circuit in dB (ratio of disturbing longitudinal voltage and the resulting metallic voltage)
Noise to ground arises from the influence of high-voltage (HV) transmission lines and their coupling with telephone lines This phenomenon is linked to the harmonic current levels in the transmission line, which can be managed by the network owner Additionally, the balance reflects the telephone system's susceptibility, making it the responsibility of the telephone company.
Electrical coordination standards ([5], [1 3], [1 9]) define performance thresholds for metallic noise, longitudinal noise and balance on normal business or residential lines which are cable lines as described in Tables 1 to 3
Table 1 – Performance thresholds for metallic noise
M etall ic noise threshol ds dBrnC
N oi se level performance category
Table 2 – Performance thresholds for longitudinal noise
Longitudinal noi se thresholds dBrnC
Table 3 – Performance thresholds for balance
The "recommended" noise levels serve as essential design and maintenance benchmarks, while the "acceptable" category is only a temporary measure until improvements are made or customer complaints arise Although "not recommended" noise levels may be tolerated if they minimally affect metallic noise performance, levels exceeding 20 dBrnC are unlikely to trigger complaints, whereas those above 30 dBrnC likely will Consequently, many communication companies aim for no more than 5% of network lines to exceed 20 dBrnC, with none surpassing 30 dBrnC In the context of a new HVDC project, the power company must manage longitudinal noise from transmission line harmonics to adhere to the recommended limit of 80 dBrnC.
Influence of power transmission lines 1 4
The impact of power transmission lines is determined by the current of each conductor at various harmonics, typically ranging from 1 to 49, as outlined in Equation (2) To facilitate the establishment of a power influence limit by the transmission system owner and to assess the performance of AC harmonic filters by the HVDC contractor, it is essential to consolidate these distinct phase and harmonic components into a single value that reflects their overall effect on telephone interference levels The subsequent section elaborates on the definition of the power influence limit.
Converting transmission line harmonic currents from phase quantities to symmetrical components is essential, as the harmonic currents produced by a HVDC converter are classified into positive or negative sequences based on harmonic order Although the converter station theoretically does not generate zero sequence (residual) harmonic currents, the interaction of positive and negative sequence current components in a non-symmetrical electrical component, like a transmission line, can lead to the creation of residual harmonic currents These residual currents are significant due to their strong coupling with telephone lines Consequently, harmonic currents in a transmission line are typically represented as balanced mode currents (positive or negative sequence) and residual mode currents, which are processed independently to simplify specifications, as one mode usually predominates.
Equation (2) can be split in two components of metallic voltages:
V mbal is the weighted metallic noise voltage in balanced mode (V);
Z mbn is the mutual impedance in balanced mode at harmonic n (Ω);
I bn is the balanced mode current at harmonic n (A);
V mr is the weighted metallic noise voltage in residual mode (V);
Z mrn is the mutual impedance in residual mode at harmonic n (Ω);
I rn is the residual mode current at harmonic n (A)
The equations can be simplified using the concept of "equivalent disturbing current," which refers to a theoretical single frequency reference current in a hypothetical conductor positioned between line conductors This current generates the same weighted noise in an adjacent communication circuit, enabling the calculation of metallic noise voltage as shown in Equation (7).
I eq is the C-message weighted equivalent disturbing current (A);
The mutual coupling impedance, denoted as Z m1, represents the interaction between the notional conductor and the communication circuit at a reference frequency of 1 kHz This impedance also accounts for the screening effect of shield wires on transmission line towers and other grounded conductors, measured in ohms (Ω).
K 1 is the telephone circuit shielding factor at the reference frequency;
B 1 is the communication circuit unbalance factor at the reference frequency;
The equivalent disturbing current I eq (A) combines the effect of each individual harmonic current with Equation (8):
I n is the the single frequency RMS current at harmonic n (A); m is the the maximum harmonic number to be considered;
The weighting factor \( H_n \) is used to represent the frequency-dependent coupling between telephone cables and AC transmission lines, normalized to 1 kHz It incorporates the frequency-dependent characteristics of \( Z_{mn} \cdot K_n \cdot B_n \) across the harmonic range from \( n = 1 \) to \( n = m \).
The H n factor for a project is determined by evaluating the frequency dependencies of mutual impedances, shielding factors, and balance for each exposure Since each exposure theoretically necessitates a unique H n factor, a representative value must be established for all exposures within the project A method proposed by [9] outlines how to develop an appropriate H n factor Notably, the combined effects of shielding and balance remain relatively constant across frequencies, indicating that this factor effectively represents the characteristics of mutual impedance as a function of frequency.
The "IT product," commonly used in North America, serves as an alternative to the "equivalent disturbing current" (A) and is a key measure of transmission line influence.
I n is the single frequency RMS current at harmonic n; m is the maximum harmonic number to be considered;
W n is the single frequency TIF weighting at harmonic n (=C n 5 n f o )
Equation (10) consolidates various harmonic contributions into a single value, reflecting the cumulative effect of all harmonic currents in a line The IT can be viewed as a specific instance of I eq, which presumes a linear frequency dependence of the coupling The IT product is applicable to both the balanced and residual components of harmonic currents While the balanced IT significantly impacts telephone lines near the transmission line, its effect diminishes rapidly with increased separation, particularly in areas with low earth resistivity Conversely, the residual IT has a broader influence and is more sensitive to variations in earth resistivity.
The residual current, denoted as I eq, or the balanced current, can be defined based on the specific component of the AC transmission line currents being analyzed The I eq criterion offers the benefit of establishing an H n factor that aligns more closely with the frequency-dependent coupling characteristics of a given exposure However, the IT method remains effective in most scenarios, provided that the data accuracy is taken into account.
In various scenarios, either balanced mode coupling or residual mode coupling may dominate; however, it is essential to consider both modes in most cases A combination of these coupling modes can be achieved by applying a correction factor \( K_b \), which is calculated based on the most critical exposures.
Z mb1 is the balanced mode coupling at reference frequency;
Z mr1 is the residual mode coupling at reference frequency
The K b factor gives the appropriate weighting to the balanced mode harmonic currents I n (A): bn 2
I rn is the the total residual mode current at harmonic n;
I bn is the the balanced mode current at harmonic n;
K b is the the ratio of balanced mode coupling to the residual mode coupling at reference frequency
Equation (1 2) employs RSS summation as a compromise due to the complexity of calculating the phase angle difference between the residual and balanced modes in each telephone line The factor \( K_b \) in Equation (1 2) introduces inaccuracies in evaluating the impact of the balanced mode, as it relies on the most critical exposures In cases where the inductive coordination study indicates that the balanced mode is dominant, the equation can be rearranged to apply the correction factor to the residual mode This adjustment allows for the calculation of the equivalent disturbing current or \( IT \) as shown in Equations (8).
An IT value can be converted to its equivalent 1 000 Hz current to calculate, with Equation
The assessment of telephone interference risk can be derived from longitudinal noise levels, specifically at 80 dBrnC, alongside a balance of 60 dB and a typical shielding factor of 10 dB While the shielding factor can fluctuate based on cable type, exposure length, frequency, and grounding, 10 dB serves as a standard value for significant exposure lengths Tables 4 and 5 illustrate the maximum lengths of parallel exposures corresponding to balanced and residual IT levels, influenced by earth resistivity and separation.
Table 4 – Illustrative maximum telephone line length to achieve the
N orth American recommended longitudinal Ng level, as a function of balanced IT level, earth resistivity and separation distance
M axi mu m exposu re length km
NOTE Balanced IT level is 1 0 000 A
Tables 4 and 5 illustrate the characteristics of a typical 230 kV transmission line featuring a horizontal configuration and two steel overhead shield wires In balanced mode, coupling increases with the distance between phases, which is proportional to the transmission line's nominal voltage Conversely, coupling in residual mode exhibits minimal dependency on this distance Additionally, transmission line configurations that reduce the separation between phases and the telephone line, such as vertical configurations, result in significantly lower coupling to balanced IT A typical ratio of 10 to 1 between balanced IT and residual IT basic levels is established for high-voltage transmission lines.
The figures presented are based on the assumption of negligible grounding resistance at both ends of the exposure; however, in reality, grounding resistance can influence the induced voltage by diminishing the shielding effect of shield wires Additionally, these figures presuppose a constant current along the transmission line, which holds true for a perfectly balanced line terminated by its characteristic impedance, but this is typically not the case in practical scenarios.
Annex B provides an example that demonstrates the various steps needed to determine the maximum length of a transmission line, considering the specific exposure characteristics, key features of the telephone system, and the suggested values.
N g Using the same procedure, a user can recalculate such tables for his own situation, for example with a different line, or different assumptions about acceptable limits, shielding
The maximum exposure length is inversely proportional to the IT level, and so the results of these tables can be proportionally adjusted to apply to alternative IT limits
The results show that for this particular transmission line configuration
• short exposures and low IT are required when the separation distance is low,
• when the earth resistivity is high, the separation distance should be much larger, for the same exposure length, and
• for line configuration with low balanced coupling, residual IT becomes prevalent even at low earth resistivity
Table 5 – Illustrative maximum telephone line length to achieve the N orth American recommended longitudinal Ng level as a function of residual IT level, earth resistivity and separation distance
M axi mu m exposu re l ength km
NOTE Residual IT level is 1 000 A
Determination of IT limits for a specific project 1 9
3.3.5 will focus on the detailed studies to be performed in order to produce the technical specification of AC filters for a specific HVDC project There is very little reference material available on this subject from previous experience, so 3.3.5 will concentrate on the description of the main parameters affecting the penetration of harmonics into the HV network, and the preparation of technical specifications
In the past, calculating harmonic currents in AC transmission line conductors for telephone interference control was deemed too complex due to the vastness of the transmission system and the numerous network configurations However, advancements in computational tools and improved transmission element models have made these studies more manageable and accurate An example in section 3.3.5.3 demonstrates the capability to replicate severe harmonic amplification in a transmission system using a detailed simulation model Ultimately, a calculated telephone interference limit based on the actual characteristics of the network is preferable to an arbitrary limit.
To ensure effective management of harmonic currents in a planned HVDC project, it is crucial to first identify the transmission lines that significantly amplify these currents These lines will influence the telephone interference requirements of the project An inductive coordination study must then be conducted to establish the telephone interference limit profile along these lines Finally, all pertinent data should be incorporated into the technical specifications, enabling the HVDC contractor to accurately replicate the harmonic current flow and optimize the filter design using a straightforward calculation method.
Pre-specification studies are essential for identifying harmonic current issues in large AC systems, particularly when multiple telephone lines are involved The proximity of a high-voltage direct current (HVDC) system necessitates extra effort to assess its impact on telephone influence requirements Therefore, initiating these studies early in the project is crucial to avoid delays in the project schedule.
3.3.5.2 Identification of the decisive transmission lines
Harmonic currents can flow in transmission lines connected to a converter station as well as in remote lines influenced by network characteristics, such as the presence of cables To identify which transmission lines are affected, a harmonic penetration study is conducted This study utilizes the same digital tools employed for the impedance locus analysis, focusing on the harmonic currents in transmission lines when a unitary current is injected at the converter's connection point.
Transferred currents are determined as a function of frequency, up to about 3 000 Hz, at least for the main harmonic orders generated by HVDC converters
The converter produces balanced sequence harmonics that distribute among the various transmission lines linked to the converter bus, influenced by their harmonic impedances, which are determined by their electrical properties and end impedances These harmonic currents may intensify along the lines or at distant points within the network Therefore, it is essential to monitor the harmonic currents at multiple locations throughout their entire length, considering all potential network configurations and operating conditions.
Transmission lines can convert harmonic currents, whether positive or negative, injected by the converter into ground mode and vice versa, unless the conductor configuration is perfectly symmetrical, which is rarely the case Even when conductors are transposed for fundamental frequency, the coupling between telephone lines and transmission lines is more significant in ground mode, making it essential to consider this harmonic current Additionally, the magnitude of the opposite sequence (negative versus positive) can exceed that of the current source at distant points in the network.
To achieve realistic results, transmission lines must be modeled using an adequate three-phase representation, incorporating accurate earth resistivity assessments It's essential to consider mutual impedance for transmission lines within the same right of way, as adjacent lines significantly influence zero sequence impedance Ideally, the impedance at the connection point should match that of the future converter installation to facilitate the flow of induced balanced and zero sequence harmonic currents Although this information may not be available during the pre-specification study stage, a low impedance should be assumed for characteristic harmonics likely to be addressed by tuned AC filter branches Additionally, converter transformers will provide a grounding path for zero sequence currents through their grounded wye-delta windings At the load end, various network conditions are taken into account, with network elements modeled in balanced and zero sequences, including transformers and shunt capacitors, ensuring proper grounding connections.
When analyzing converter connections, it is crucial to avoid unrealistic scenarios such as parallel resonance, which can lead to high currents at distant locations while only a low current is present at the injection point These parallel resonance conditions create significant harmonic impedance at the connection point, causing the majority of harmonic currents to be diverted into AC filters, particularly at specific harmonic frequencies where tuned filters are typically installed Utilizing the TIF criterion in technical specifications helps mitigate the risk of severe current amplifications For a theoretical illustration, refer to Annex C.
To effectively manage harmonic voltages at the connection point, it is essential to ensure they remain below a specified TIF level The requirement for harmonic voltage distortion will consequently regulate the TIF level For example, a HVDC project adhering to IEC TR 61 000-3-6 standards will restrict individual TIF levels to around 50 for odd harmonics that are not multiples of 3, and approximately 20 for other harmonics Since the harmonic emissions from the converter installation must be below these planning levels, it is possible to consider even lower TIF values However, despite TIF limitations, conditions that lead to amplification at distant locations may be counterbalanced by the effectiveness of filtering at specific harmonics, allowing for the consideration of potentially unrealistic scenarios.
Figure 2 illustrates the ratio of positive sequence current at the receiving end to that at the sending end of a 230 kV, 124 km long transmission line, fully transposed at 60 Hz, along with its positive sequence impedance calculated using EMTP A shunt element with negligible zero sequence impedance and 800 Ω resistive positive and negative sequence impedance is connected to the sending end, while the receiving end is grounded with an earth resistivity of 1,000 Ω-m High transfer factor ratios occur near the line's resonance frequency, whereas values below 1 are observed at low line impedance Characteristic harmonics require tuned filters, which are most effective near line resonance and less so at low impedance It is concluded that limiting IT at the sending end is not suitable if the telephone line is positioned at the middle or receiving end of the transmission line.
Figure 2 illustrates the ratio of ground mode current at the receiving end compared to the positive sequence current at the sending end The residual harmonic currents exhibit similar magnitudes and frequency dependencies both in the middle of the line and at the receiving end Additionally, Figure 2 reveals a relationship between the transfer ratio of the residual mode and line impedance, akin to the findings presented in Figure 1.
Figure 1 – Conversion factor from positive sequence current at the sending end to positive sequence current at the receiving end, and input impedance of a 230 kV line,
Figure 2 – Conversion factor from positive sequence current to residual current, and input impedance of a 230 kV line, 1 24 km long, 1 000 Ω-m
Li ne im pe da nc e (k Ω )
Li ne im pe da nc e (k Ω )
I pos_receiving/lpos_sendingZline_pos
Digital simulations must consider the accuracy of network component data, the limitations of impedance models in the frequency domain, and the variability of component impedance under different ambient and system conditions This is typically achieved by analyzing a range of frequencies around the harmonic of interest, as outlined in IEC TR 62001-3:2016 The frequency range is selected based on the estimated accuracy of the network model; for example, when analyzing the 11th harmonic, the worst-case scenario would account for injected current frequencies varying from 627 Hz to 693 Hz.
1 1 ± 5 %) or 522,5 Hz to 577,5 Hz (50 Hz times 1 1 ± 5 %)
The objective of a harmonic penetration study is to identify key transmission lines that contribute to telephone interference by calculating the harmonic currents transferred to nearby lines Telephone interference is directly related to the balanced and residual harmonic currents that run parallel to telephone lines To accurately determine the induced voltage in these lines, it is essential to derive the three sequence components in the transmission line, each characterized by its magnitude and phase angle Furthermore, the definition of the interference transfer (IT) assumes a linear relationship between the mutual impedance of the transmission and telephone lines with frequency, which may not fully align with actual coupling characteristics.
A more effective method for assessing the relative telephone influence of various transmission lines is to utilize test lines placed in parallel with the lines under examination The voltages induced in these test lines provide a precise indication of telephone influence, independent of shielding and balance effects, as long as the separation between the test lines mirrors that of the actual telephone lines Multiple test lines can be employed to represent different separations relevant to the telephone lines in question The comparison of each transmission line's influence will be based on a C-message weighted voltage that corresponds to the longitudinal induced voltage previously discussed.
Pre-existing harmonics and future growth
When determining the telephone interference limit for a project, it is essential to account for existing harmonic sources in the network and anticipate future increases in harmonic levels The installation of a large AC filter can elevate the current of interharmonics on transmission lines, as these shunt filters can absorb pre-existing harmonics Consequently, introducing a new shunt filter may lead to an increase in harmonic current flow, resulting in potential telephone interference.
The impact of pre-existing harmonics on AC filter design is thoroughly examined in clause 3, focusing on harmonic voltage distortion within the network Similar principles are applicable to the telephone influence of transmission lines, where controlling harmonic current in specific lines is crucial Figure 3 illustrates a straightforward example of utilizing pre-existing harmonics to address telephone interference.
I cn injected harmonic currents from the converter
Z sn AC network harmonic impedance
I sn AC network harmonic current
U fn filter (or optionally pcc) busbar harmonic voltage
U on specified pre-existing harmonic voltage source
Figure 3 – Simple circuit for calculation of harmonic performance taking into account pre-existing harmonics
The design of new converter AC filters is significantly influenced by pre-existing large harmonic sources at the same voltage level or nearby To achieve a realistic assessment of telephone influence on critical transmission lines, an aggregate criterion should be utilized, as outlined in IEC 62001-3:2016, Clause 5 When such harmonic sources are identified, the technical specifications must include all necessary data to evaluate their harmonic generation.
Incorporating pre-existing harmonics from various voltage levels necessitates a reliable model that includes consistent harmonic voltages and impedances However, creating such models is challenging due to the limited availability of harmonic sources, uncertainty in harmonic levels within the network, and the complexity of network modeling Additionally, studies on telephone interference in extensive networks may yield inaccurate results due to the limitations of network models at higher frequencies, which are only precise up to the 20th harmonic as per IEC TR 61 000-3-6 This issue is exacerbated in scenarios with higher residual mode coupling, where assessing earth resistivity and surrounding screening circuits is difficult A model that combines worst-case harmonic impedances with planning levels may lead to overly conservative IT levels, potentially overlooking optimal AC filter solutions Ideally, measuring the pre-existing harmonic levels at a network bus is preferred, but this requires consistent source voltage and network impedance sets to achieve realistic outcomes, with measurements encompassing most expected operating conditions and contingencies.
A rational approach to determining limits for a specific HVDC project is to focus solely on harmonic generation from the HVDC converters, including both the new project and nearby converters Experience from a major utility with numerous high voltage capacitor banks indicates that telephone interference has not been an issue during operation Theoretically, any capacitor bank, along with its series reactor, acts as a sink for high order harmonics within an HV network Most background harmonic sources are likely found at the distribution level or in remote locations, where they interact with HVDC filters through transmission line and transformer impedances, which effectively dampen potential amplification due to loads and transformers.
Interference in telephone lines can arise from various sources, including the distribution network that parallels telephone lines To mitigate this, a conservative strategy suggests capping the contribution from each source to 17 dBrnC, taking into account the RSS summation of these contributions It is crucial to ensure that the combined impact of the new project and nearby converter installations does not exceed the established limits for metallic noise.
The maximum contribution from a distribution line to the total metallic noise is 20 dBrnC, which does not elevate it to an unacceptable threshold level For example, when considering the contributions from both sources, the overall impact remains within acceptable limits.
20 dBrnC, the total noise will be 23 dBrnC, well below 30 dBrnC For one source at 25 dBrnC and the other at 20 dBrnC, the total noise will be 26 dBrnC
To determine the optimal influence level, it may be necessary to implement mitigation measures across several telephone lines Given the conservative nature of the calculation method and the ability to tolerate higher noise levels temporarily, it is advisable to wait for measurements from the most affected lines during acceptance tests to validate the need for these measures Early-stage measurements are recommended to address any telephone interference issues, ensuring that the susceptibility levels of the telephone system and distribution influence are acceptable.
When planning for future growth in harmonic generation, particularly with upcoming HVDC converter installations, it is crucial to account for potential increases, although predicting these installations beyond the near term is often challenging Background sources that progressively increase can be managed through complaints to telephone companies, as acknowledged by industry standards A proper allowance for future growth must take into account the network's maturity, the anticipated evolution of the telephone system, and the implications of surpassing recommended metallic noise levels.
Recommendations for technical specifications
To ensure an accurate assessment of interference influence (IT or I eq) by the HVDC contractor, the technical specifications must clearly define the network and line characteristics once the limit along the critical transmission lines is established This information should be presented conveniently in the specifications, and it is the responsibility of the utility or its consultant to provide a well-defined calculation method for optimal filter design.
Most technical specifications define telephone interference limits at the converter's network connection point, using balanced weighted current or voltage based on an impedance locus that reflects various network conditions However, this approach fails to manage amplification at remote locations and does not consider the distribution of interference current across different transmission lines in a meshed network connected to a converter bus Additionally, it overlooks the generation of ground mode harmonics and the residual current from transmission lines, which often significantly contributes to telephone interference.
The technical specification must clearly outline the electrical characteristics of the transmission line to minimize telephone interference, allowing for some tolerance in line length In a straightforward scenario involving a radial transmission line linking the converter to the network, a line model with an impedance locus at the opposite end can be defined This approach has been successfully implemented in a significant project.
Extending the concept to multiple radial lines significantly increases the number of potential network impedances to consider, as all combinations of impedances in each locus must be evaluated This raises concerns about the consistency of operating conditions across different loci Furthermore, the networks at each line end may be interconnected through other network components, necessitating a mutual impedance to accurately model their interactions For specifications that require limiting residual harmonic currents, additional information on the zero sequence harmonic impedance at the load end is essential.
The complete network data can be included in the technical specification, detailing the operating conditions and contingencies to consider While this approach utilizes the most accurate model for representing harmonic current flow in the network, it demands extensive data and may prolong the study during the bidding period.
Selecting the right method for defining telephone interference limits hinges on factors like network complexity, the number of telephone lines, and project timelines During the bidding phase, a straightforward approach is essential to streamline bidder evaluations and ensure uniformity in technical proposals However, bidders must receive sufficient information to accurately estimate filter costs In contrast, the final project design requires a more detailed method to facilitate optimal filter design.
In cases where a network's complexity exceeds the capability of a suitable equivalent circuit for technical specifications, or when a simplified approach is needed for bidding, a specific method can be employed if certain conditions are met Observations indicate that the amplification of IT along a transmission line correlates with the magnitude of the sending end impedance Additionally, a relationship can be established between IT at transmission line locations adjacent to telephone lines and the network impedance at the converter's connection point during penetration studies If high levels of IT transfer along critical transmission lines correspond to elevated network impedance values, this relationship can be incorporated into technical specifications for key harmonics This method can also enhance the accuracy of the induced voltage ratio in a test line relative to the injected harmonic current at the converter, as detailed in section 3.3.5 Ultimately, this approach facilitates a more optimized design compared to the traditional method of using a single IT level at the converter exit alongside an impedance locus, with a relevant case study provided in Annex D.
Once the specification model is set, the technical specification of an HVDC project requiring telephone interference limitation should include
– I eq or IT limits along each transmission line with telephone line exposures,
– the definition of the performance indices, either I eq or IT (Equations (8) or (1 0)) with the values of H n if required,
– the values of K b for each transmission line when residual and balanced mode coupling is considered (Equation (1 1 )),
– normal operating conditions to be considered for the calculation of telephone influence limit with the possibility of defining relaxed limits for degraded conditions,
Consequences for filter design
This subclause considers the possible consequences for AC filter design of imposing a low IT limit
When the specified IT limit is excessively low, filter designers may discover that simply providing tuned shunt filters for the primary harmonic characteristics is insufficient In such cases, it becomes essential to implement sharply tuned branches to address higher-order characteristics, as opposed to relying solely on the conventional high-pass filter types.
The introduction of numerous finely tuned branches leads to high-impedance anti-resonances at intermediate frequencies, causing previously negligible non-characteristic harmonics to be amplified and significantly impact the IT To address this, tuned filters may be necessary for these harmonics, or a broad-band damped filter could be added alongside the sharply tuned arms to provide damping at the anti-resonance points This can result in a proliferation of small filter branches, which can be expensive to construct and maintain, while also posing challenges for reactive power balance.
Shunt AC filters effectively reduce harmonic voltage at their connection point However, if the network's harmonic impedance is low, substantial harmonic currents may still flow into the network Consequently, shunt filters may not always be the optimal solution for limiting interharmonics (IT).
The addition of series filters in outgoing circuits can enhance the relative impedance of the network compared to shunt filters While sharply-tuned shunt filters target specific troublesome frequencies, a low q-factor, broad-band series filter can effectively increase impedance across a wider frequency range, particularly between the 20th and 30th harmonics Series filters can be constructed using reactors similar to those for PLC filtering, supplemented with small capacitors and resistors on insulated platforms, ensuring minimal losses and maintaining reactive power balance To improve reliability and availability, installing isolators and a bypass switch for the series filter is advisable, although simple bus links can also serve this purpose.
Active filters, which can cancel low magnitudes of outgoing harmonic current over a wide frequency range, can also be an efficient way to limit IT (see IEC TR 62544 [1 9])
Customers must recognize that setting a low IT limit in filter design can lead to significant and costly consequences It is essential to only specify such limits when absolutely necessary and well-justified.
Telephone infrastructure mitigation options
This document does not delve into specific mitigation measures for the telephone system; however, it provides a basic list of options for power system engineers These options may be considered for particular phone lines or areas after conducting a detailed study to identify vulnerable circuits.
To enhance the performance of existing telephone equipment that may have deteriorated over time, several mitigation measures can be implemented These measures can restore or even improve the balance and shielding of telephone lines to their original values.
For effective shielding against power line induction, it is essential to ground both ends of the cable shield and bond all shield openings Additionally, reducing the shield grounding impedance along the exposure can enhance protection.
2) Improvement of customer loop balance
The balance of the customer loop can be influenced by the switching centre equipment, telephone cables, and customer equipment To enhance circuit balance, several measures can be implemented, including the use of modern cables that offer significantly improved balance.
Several devices can be installed on the telephone circuit to mitigate induced noise a) Bridged ringers and ringer isolators
Ground-connected ringers can create an imbalance in telephone pair wires To address this issue, bridge ringers and ringer isolators are utilized Additionally, longitudinal chokes play a role in mitigating these effects.
Such chokes add high longitudinal impedance in the circuit avoiding longitudinal to metallic noise conversion c) Negative impedance converters
The negative impedance converter serves as an active transmission gain element, effectively sensing and amplifying shield or spare-pair induced currents to enhance the shielding effect Additionally, induction neutralizing transformers play a crucial role in this process.
Telephone cables are grounded beyond power exposure at each side of the transformer, utilizing one or more pairs as the primary exciting winding The secondary voltage connects to the remaining pairs, applying a 180° out-of-phase voltage that partially cancels the induced voltage This device is typically most effective in its application.
50 Hz or 60 Hz and low order harmonics
Another option to mitigate telephone interference problems is to use a different transmission system The following transmission systems use a signal immune to voice frequency interference e) Carrier systems
This system operates with a carrier frequency ranging from 40 kHz to 100 kHz, which is modulated by audio-frequency telephone signals This modulation allows for multiplexing, thereby accommodating a greater number of subscribers on a single telephone line However, it may not be cost-effective for a limited number of subscribers Additionally, fiber optic cables are also mentioned as a relevant technology.
This system uses optical fibres for telephone communication While this system is forecast to increase in the future, it can not be yet economical for rural areas g) Cell phones
This system uses a radio signal and a network of antennas to transmit the information Cell phone service can not be available everywhere in some countries h) Digital systems
Various digital technologies have now achieved wide penetration and are immune from audio-frequency interference
Mitigating noise on telephone circuits is insufficient if unweighted induced voltages pose safety risks or exceed the capabilities of standard mitigation equipment For further details on noise mitigation measures for telephone systems, refer to sources [9] and [13] Additionally, the rapid advancements in digital technology are likely to enhance the immunity of telephone systems against audio frequency interference.
Experience and examples
General
Publicly available information on telephone interference levels for HVDC schemes in operation is limited This scarcity is largely due to performance measurements being conducted during system tests, which often occur under more favorable conditions than those for which the filter is intended A specific instance of interference and its resolution is detailed in section 3.6.4 below.
This article provides a concise review of design requirements implemented to limit telephone interference, highlighting that inadequate criteria have evolved or been replaced over time The findings indicate that, with few exceptions, the application of current-based criteria is predominantly confined to the American continent Additionally, the article discusses experiences from actual HVDC projects and includes a simple design example in Annex E.
Review of design requirements
This article reviews the performance criteria for HVDC schemes from the 1970s to the present A summary of performance limits is provided in Table 6, which compiles data from published references ([20] to [22]) and additional CIGRÉ session papers related to the respective projects.
• 1 2 out of 48 schemes had current based criteria;
• most schemes had TIF (or THFF) requirements;
• schemes with IT (I p ) requirements are almost all located in North or South America;
• only a few schemes did not have criteria related to telephone interference
Expanding the table to include current schemes would reveal a consistent trend, with IT (I p) requirements predominantly limited to the American continent While most schemes specify TIF (THFF) requirements, some of the more recent ones do not provide either TIF or IT requirements However, practical limits on individual distortion generally suggest calculated TIF levels around 30.
Regional variations in IT usage are influenced by differing national guidelines and recommendations, which stem from diverse experiences These experiences are shaped by the distinct structures of power transmission and telecom subscriber systems that vary across different regions globally.
Table 6 – Some HVDC schemes – Specified telephone interference criteria
N ame of H VDC System Location Year TIF,
- TH FF, % IT, kA Ip, A Remar k
Konti-Skan 1 And 2 Denmark-Sweden 1 965/88/2005 50
New Zealand Hybrid New Zealand 1 965/92 1 26 EDV, EDI
N ame of H VDC System Location Year TIF,
- TH FF, % IT, kA Ip, A Remar k
TI F of 40 corresponds to a THFF of about 1 %.
Measured current levels of schemes in service
Table 6 outlines the design requirements for HVDC schemes, but actual measurements during converter operation often reveal performance that falls short of these calculated values This discrepancy arises because measurements are usually taken under more favorable conditions than those considered in design calculations Despite this difference, the measured performance remains a topic of interest, and two illustrative examples are presented below.
Both examples indicate a 95% probability value, meaning that this threshold is not exceeded during the one-week measurement period The analysis does not account for variations in load, different filter configurations, or distinctions between converter and pre-existing harmonics Notably, in the last example, the dominant harmonic current is the 5th harmonic, which flows between the EHV and HV lines and is not associated with the HVDC converter station.
A 600 MW scheme was implemented with a Total Harmonic Distortion Factor (THFF) of 1% Comprehensive measurements of harmonic voltages and currents were conducted both with and without the converter in operation The substation facilitated the connection of five incoming lines, including one Extra High Voltage (EHV) line and four High Voltage (HV) lines at 400 kV.
The results summarized in Table 7 indicate that, following detailed modeling of both the 3-phase AC system and telephone cables, induced noise levels were calculated based on measured data from locations near and far from the converter station Notably, there was no significant increase in telephone interference in eight out of nine areas studied, with the exception of one area that experienced elevated noise levels due to its proximity and extended exposure to the 400 kV AC line.
Table 7 – Measured 95 % values of THFF and I pe of a 600 MW scheme (3 phases)
The second example involves measurements from a 300 MW HVDC scheme designed for a TIF of 50 at the converter bus, with data collected only while the converter was operational The findings, which represent 95% values of psophometric weighted voltages and currents over a week, are summarized in Table 8.
The two stations are located in contrasting environments, with one in a rural area and the other in a densely populated region Notably, there were no reported instances of telephone interference along the AC lines connecting these stations.
Table 8 – Measured 95 % values of THFF and I pe of a 300 MW scheme (3 phases)
Example of actual telephone interference problems
3.1 highlights the need for the customer to base his performance requirements on inductive coordination studies, where it is perceived that there is a risk of telephone interference and also not to specify performance indices based simply on typical values and/or past practice
To amplify this point, following the commissioning of a recent HVDC project, telephone
Many users near the converter station experienced interference, which was consistent with the station's operation Several noteworthy points emerged regarding this issue.
The performance criteria for the converter station AC busbar stipulate that the harmonic voltage distortion must not exceed 1% for individual harmonics or 1.5% for the total (from harmonics 2 to 49) Additionally, the telephone interference factor (TIF) should not increase by more than 50 Overall, these performance standards are considered reasonable and not excessively demanding.
2) The network harmonic impedance characteristics (as seen from the converter station AC busbar) were defined by the simplified approach given in IEC TR 62001 -1 :201 6, 7.3.1
3) No inductive co-ordination studies were performed prior to the compilation of the enquiry document, neither were there discussions with the relevant telephone operators to determine the likelihood of potential interference Furthermore, no detailed studies were performed to identify harmonic transfer impedances between the AC harmonic filter busbar and remote busbars within the network to determine potential high levels of amplification of harmonic voltage and/or current from the converter station
4) On commissioning and during initial operation, there were many complaints (approximately 200) regarding audible noise on customer telephone circuits, and it was apparent that the noise being experienced was due to the presence of an 1 1 50 Hz component on the circuits with levels of –60 dBm 3 being measured with the HVDC link in service; such levels were deemed unacceptable on communication circuits The affected customers were connected to exchanges in the vicinity (approximately 5 km) of a substation, some 35 km distant from the converter station AC busbar In this area, there are several overhead lines all at the same network voltage level as the converter bus with relatively high mutual coupling to adjacent open wire telephone circuits
5) Measurements of harmonic voltage distortion at the converter station AC busbar with the interconnector in service indicated that the RSS total distortion was less than 1 % with
The 23rd harmonic (1 1 50 Hz) was measured at approximately 0.1%, and the calculated Total Harmonic Distortion (TIF) was significantly below 50 This indicates that the performance criteria outlined for the AC harmonic filter design have been successfully met.
6) However, at the remote substation referred to in IEC 62001 -3:201 6, Clause 4 [1 8], whilst the level of 23 rd harmonic voltage distortion showed an attenuation (to approximately 0,02 %, i.e barely measurable) compared with the converter station busbar, there was a significant amplification of 23 rd harmonic current from the converter station to this remote busbar Calculated values of IT (from harmonic current measurements) for overhead line feeders into this substation were in the order of 30 000 to 40 000 with a significant 23 rd harmonic contribution Such levels of IT would be consistent with potential telephone customer complaints
7) The above phenomena at the remote substation were indicative of a significant series resonance (transfer impedance) between the converter station busbar and the particular remote substation, which was later confirmed by detailed modelling of the network characteristics
8) To reduce the telephone interference to acceptable levels, one of the AC harmonic filters was temporarily reconfigured as a 22 nd harmonic single frequency tuned filter As a permanent solution, a 23 rd harmonic series "blocking" filter circuit was installed in one of the outgoing circuits from the converter station (no further permanent shunt connected AC filters could be connected because of reactive power exchange limits)
In conclusion, the example emphasizes the importance of assessing the potential for significant mutual coupling between overhead transmission lines and telephone open wire circuits, even in areas distant from the converter station Additionally, it is crucial to conduct studies to evaluate the self-impedance and, more importantly, the harmonic transfer impedance characteristics related to these busbars.
The article discusses the difference between two units of measurement: dBm, which is referenced to 1 mW, and dBrnC, which is referenced to 1 pW It highlights that there is a 90 dB difference between these units, with -60 dBm equating to +30 dBrnC when the C weighting term in dBrnC is disregarded.
Experience in China, showing no interference problems
In China, long-distance communication primarily relies on optical fiber cables, even for connections between urban communication hub stations The last 1 to 2 kilometers to customers in urban areas typically utilize standard cables or overhead wires, while rural areas may have longer stretches of these ordinary communication wires High Voltage (HV) and Extra High Voltage (EHV) transmission lines above 110 kV are supported by specialized towers that do not carry communication cables However, in the 35 kV utility system, distribution towers may also support some communication cables, allowing for parallel routing or crossings with power distribution lines.
500 kV transmission lines typically traverse agricultural fields and mountainous regions to minimize environmental impact In these remote areas, there are fewer communication cables running alongside the transmission lines, resulting in a reduced risk of interference While developing regions may have some communication cables parallel to the 500 kV lines, no reports of interference from communication companies have been recorded.