HVDC Systems
High-voltage direct current (HVDC) transmission systems present unique EMC challenges that differ fundamentally from those of conventional AC power systems. The power electronic converters at the heart of HVDC technology generate harmonic currents and voltages across a wide frequency spectrum, creating interference that affects both the power system itself and nearby communication and electronic equipment. Understanding and managing these EMC aspects is essential for successful HVDC project development and operation.
The growing deployment of HVDC for long-distance transmission, submarine cables, and asynchronous grid interconnections makes HVDC EMC increasingly important. Modern voltage source converter (VSC) technology has expanded HVDC applications to include offshore wind farm connections and urban cable systems, bringing HVDC equipment closer to populated areas and sensitive electronic installations. This evolution requires continuous advancement in EMC analysis methods and mitigation techniques.
Converter Station EMC
Converter stations are the primary sources of electromagnetic interference in HVDC systems. These facilities house the power electronic equipment that converts between AC and DC, along with associated transformers, filters, and control systems. The EMC challenges in converter stations include both emissions from the conversion process and immunity of the sophisticated control equipment to those emissions.
Line-Commutated Converter Characteristics
Traditional line-commutated converters (LCC) using thyristor valves generate characteristic harmonics determined by the pulse number of the converter configuration. A twelve-pulse converter, standard for most HVDC installations, produces AC-side harmonics at orders 12n plus or minus 1 (11th, 13th, 23rd, 25th, etc.) and DC-side harmonics at orders 12n. The magnitudes of these harmonics depend on the converter operating point, particularly the firing angle and overlap angle.
The commutation process in LCC converters creates additional EMC effects. During commutation, two valves conduct simultaneously, creating a temporary short circuit between phases that draws current from the AC system. The abrupt current changes at commutation produce broadband interference extending to frequencies well above the characteristic harmonics. This commutation noise can couple to control cables and communication systems within the converter station.
Voltage Source Converter Characteristics
Voltage source converters (VSC) using insulated gate bipolar transistors (IGBTs) switch at much higher frequencies than LCC converters, typically several hundred hertz to several kilohertz. This high switching frequency moves the dominant harmonic content to higher frequencies where filtering is more effective, but it also creates emissions in frequency ranges that may interfere with radio communication systems.
Modular multilevel converters (MMC), the dominant VSC topology for HVDC, distribute the voltage across hundreds of submodules, each containing a capacitor and switching devices. The resulting voltage waveform closely approximates a sinusoid, dramatically reducing harmonic content compared to two-level or three-level converters. However, the large number of switching events still produces measurable emissions, and the distributed architecture creates new EMC challenges related to internal circulating currents and submodule voltage balancing.
Valve Hall Electromagnetic Environment
The valve hall housing the converter valves experiences intense electromagnetic fields during operation. Electric field gradients around valve components can approach corona inception levels, requiring careful attention to electrode shapes and clearances. The rapid voltage changes during switching create displacement currents that couple to surrounding structures and can induce voltages in control and monitoring circuits.
Shielding of the valve hall is essential for controlling external emissions. Metal-clad buildings with proper attention to door seals and penetrations provide significant attenuation. Cable entries, ventilation openings, and viewing windows all require treatment to maintain shielding effectiveness. Internal partitioning may be used to isolate particularly intense emission sources from sensitive control areas.
DC Line Effects
HVDC transmission lines exhibit EMC characteristics distinct from those of AC lines. The absence of power frequency alternating fields eliminates some interference mechanisms while the DC operating mode introduces others. Corona on DC lines behaves differently from AC corona, and the presence of space charge creates effects not found on AC systems.
DC Corona Phenomena
Corona discharge on DC conductors is fundamentally different from AC corona because the polarity remains constant. Positive conductors produce positive corona with space charge that moves away from the conductor, while negative conductors produce negative corona with space charge that tends to remain near the conductor surface. This asymmetry affects both the electrical characteristics and the EMC behavior of bipolar DC lines.
Ion flow from corona creates a space charge region around DC lines that affects the electric field distribution at ground level. Unlike the instantaneous field of AC lines, the DC field has a static component from the conductor voltage plus a contribution from the space charge that varies with corona activity. The total field at ground level can exceed the field from the conductor potential alone, particularly under the positive pole of a bipolar line.
Radio Interference from DC Lines
Radio noise from DC corona differs from AC corona noise in several important ways. The statistical characteristics of noise pulses depend on polarity, with positive corona generally producing higher noise levels than negative corona under comparable conditions. The frequency spectrum and the variation with weather conditions also differ from AC patterns.
Measurement and prediction of DC line radio interference have historically received less attention than AC line interference, resulting in less refined prediction methods. Recent research has improved understanding of DC corona noise mechanisms and developed empirical prediction formulas applicable to modern HVDC projects. Standardized measurement procedures analogous to those for AC lines help ensure consistent characterization.
Audible Noise from DC Lines
DC lines typically produce less audible noise than AC lines at comparable voltage levels because the absence of the power frequency component eliminates the characteristic hum heard near AC lines. The broadband noise from corona remains, producing a hissing or crackling sound similar to the foul-weather component of AC line noise.
The response of DC line noise to weather differs from AC lines. Fair-weather noise levels on DC lines may be relatively high due to persistent corona from surface contamination. Rain can actually reduce noise levels by washing contaminants from the conductor surface, opposite to the effect on AC lines. This different weather response affects the selection of appropriate statistical descriptors for noise characterization.
Harmonic Generation
HVDC converters are significant sources of harmonic currents and voltages that can propagate throughout connected AC and DC networks. Managing these harmonics is essential for meeting power quality requirements and preventing interference with other network equipment. The complexity of harmonic analysis in HVDC systems requires sophisticated modeling techniques.
AC-Side Harmonics
Harmonic currents injected into the AC system by HVDC converters flow through the network impedance, creating harmonic voltage distortion. The magnitude and distribution of these effects depend on the converter operating point, the network impedance at each harmonic frequency, and the presence of other harmonic sources in the network. System resonances can amplify certain harmonics, sometimes at frequencies not directly generated by the converter.
Non-characteristic harmonics arise from imbalances in the converter or the AC system. Unequal transformer impedances, asymmetric firing angles, or unbalanced AC voltages can produce harmonics at frequencies other than the theoretical values. These non-characteristic harmonics are often lower in magnitude than characteristic harmonics but may fall at frequencies where network impedance is high, causing disproportionate voltage distortion.
DC-Side Harmonics
Harmonics on the DC side of HVDC converters create ripple voltage on the DC transmission line. This ripple can couple to parallel communication circuits through magnetic and electric field coupling, causing interference that increases with harmonic frequency due to more efficient coupling at higher frequencies. For cable systems, the voltage ripple can stress the cable insulation and terminations, potentially affecting long-term reliability.
The DC smoothing reactor reduces harmonic current flowing into the DC line but cannot eliminate harmonics from the voltage waveform at the converter terminals. The transmission line itself acts as a distributed filter, with standing wave effects creating complex patterns of current and voltage harmonics along the line. Terminal impedances at both ends of the line influence these patterns and must be considered in harmonic analysis.
Telephone Interference
Harmonic currents in the audio frequency range can interfere with telephone circuits running parallel to power lines. The Telephone Influence Factor (TIF) or Psophometric Weighting quantifies this interference potential by applying frequency-dependent weighting that reflects human hearing sensitivity and telephone receiver characteristics. HVDC harmonics, with their concentration at specific frequencies, create distinctive interference patterns different from the continuous spectrum of AC power frequency harmonics.
Interference with digital communication systems follows different patterns than traditional telephone interference. Digital systems may be sensitive to specific frequencies or to broadband noise levels, depending on the modulation and coding techniques employed. The transition from analog to digital telephony has generally reduced interference problems, but new sensitivity issues arise as communication technology continues to evolve.
Filter Design
Harmonic filters are essential components of HVDC converter stations, serving to limit harmonic injection into connected AC systems and to provide reactive power for converter operation. Filter design balances multiple objectives including harmonic reduction, reactive power compensation, loss minimization, and cost control. EMC considerations influence filter requirements and configurations.
AC Filter Types
Tuned filters provide high attenuation at specific frequencies, typically the dominant characteristic harmonics. Single-tuned filters target individual harmonics, while double-tuned and triple-tuned designs address multiple harmonics with fewer components. The sharp frequency response of tuned filters makes them sensitive to component tolerances and temperature variations, requiring careful quality factor selection.
High-pass filters provide broadband attenuation above a cutoff frequency. Second-order and third-order high-pass designs offer different tradeoffs between low-frequency harmonic attenuation and high-frequency performance. C-type filters combine tuned and high-pass characteristics, providing both targeted low-frequency attenuation and broad high-frequency coverage. Filter bank design typically combines multiple filter types to achieve the required performance across the harmonic spectrum.
DC Filter Requirements
DC-side filters reduce the harmonic content of voltage and current on the DC transmission line. Unlike AC filters, DC filters must block the DC component while attenuating AC harmonics. The design must consider the voltage and current ratings at DC plus the superimposed AC components at each harmonic frequency.
Smoothing reactors are the primary DC-side filtering elements, providing impedance that limits the rate of change of DC current during transients and reduces harmonic ripple. Additional filter branches may be added to target specific harmonics, particularly for cable systems where harmonic voltage limits are more stringent than for overhead lines.
Active Filtering
Active filters using power electronics can provide adaptive harmonic compensation that responds to changing operating conditions. For HVDC applications, active filtering may be integrated into the converter control or implemented as separate compensators. The flexibility of active filters makes them attractive for situations where passive filter performance is inadequate or where the harmonic spectrum varies significantly with operating point.
Hybrid solutions combining passive and active elements leverage the strengths of both approaches. Passive filters handle the bulk of the harmonic current at relatively low cost, while active elements provide fine tuning and adaptation. This approach can be particularly effective for addressing non-characteristic harmonics that vary with system conditions.
Electrode Effects
HVDC systems using ground or sea return operate with electrodes that inject DC current into the earth or sea. These electrodes create local effects including chemical reactions, heating, and electromagnetic fields that require careful management. Even systems designed for bipolar metallic return typically include electrodes for use during single-pole operation or emergencies.
Electrode Current Distribution
Current from ground electrodes spreads through the earth following complex three-dimensional paths determined by soil resistivity structure. Near the electrode, current density is highest and heating is most intense. At greater distances, the current disperses and eventual return to the remote electrode may involve paths through conductive geological formations far from the direct line between electrodes.
The DC magnetic field from electrode currents extends over large areas and can affect compass readings for navigation. Pipeline and cable crossings may experience DC voltages that accelerate corrosion or interfere with cathodic protection systems. Coordination with operators of these facilities is necessary to identify potential impacts and implement mitigation measures.
Electrode Site Selection
Electrode sites are selected based on soil resistivity, accessibility, environmental considerations, and separation from structures that might be affected by electrode currents. Low-resistivity sites reduce electrode resistance and operating voltage, limiting heating and chemical effects. Coastal sites with access to seawater may provide low resistance with less local impact than land electrodes.
Environmental assessment of electrode sites considers effects on groundwater, vegetation, and marine life for sea electrodes. Chlorine generation at sea electrodes can affect water chemistry in the immediate vicinity. Heating of soil around land electrodes can dry out the soil and damage roots of nearby vegetation. Electrode design and operating limitations address these effects.
Return Path Configurations
The choice of return path configuration profoundly affects HVDC system EMC characteristics. Metallic return, ground return, and bipolar configurations each have distinct advantages and EMC implications. Understanding these differences guides system design decisions.
Metallic Return
Metallic return systems use a conductor for the return path, eliminating the need for ground electrodes and their associated effects. This configuration prevents DC current injection into the earth and the resulting interference with pipelines and other grounded structures. The return conductor may be a dedicated low-voltage cable, a conductor integrated into the cable design, or one pole of a bipolar system used as return during single-pole operation.
The metallic return conductor carries the full DC current and must be rated accordingly. For overhead lines, the return conductor may be of smaller cross-section than the main poles since it operates at low voltage. Cable systems may use the cable sheath or armor as the return path, with appropriate design for current-carrying capacity and corrosion protection.
Ground Return
Ground return systems use the earth as the current return path, offering significant cost savings for long submarine crossings where a metallic return conductor would be expensive. The electrodes at each terminal inject and collect the return current. This configuration is common for monopolar submarine links connecting island systems or crossing long water bodies.
The EMC implications of ground return operation are significant. DC current flow through the earth creates potential gradients that can corrode pipelines and affect telecommunications cables. The magnetic field from electrode currents can influence compass readings over a wide area. Careful coordination with potentially affected parties and continuous monitoring during operation help manage these effects.
Bipolar Operation
Bipolar HVDC systems transmit power on two poles of opposite polarity. Under normal balanced operation, the neutral current is small and ground electrodes carry only imbalance current. This configuration largely eliminates the EMC concerns associated with ground return while providing the reliability benefit of continued operation if one pole is lost.
The imbalance current during bipolar operation depends on differences between the two poles in converter performance, line parameters, and fault conditions. Control systems work to minimize this imbalance, but some neutral current typically flows through the ground electrodes. The electrodes must be designed for continuous operation at the expected imbalance current plus short-term overload during single-pole operation.
Control System EMC
The sophisticated control systems required for HVDC operation must function reliably in an electromagnetic environment dominated by the converter switching operations. Control system EMC involves both protecting the control equipment from interference and ensuring that control actions do not create excessive emissions or instability.
Control Hardware Requirements
HVDC control equipment must meet stringent immunity requirements to operate reliably in the converter station environment. Valve firing commands must be transmitted and received correctly despite the intense electromagnetic fields near the converter valves. Protection systems must detect faults and initiate converter blocking without interference from normal switching transients. Equipment specifications typically require immunity levels well above those for general industrial applications.
Fiber optic communication between the control room and the valve hall provides immunity to electromagnetic interference on critical signal paths. Light signals are inherently immune to electric and magnetic field coupling, though the electronic interfaces at each end remain susceptible and must be properly designed and installed. Redundant optical links provide reliability for critical functions such as valve firing and protection.
Control System Interactions
HVDC control systems interact with the AC networks at both terminals and with any other HVDC links sharing the same AC systems. These interactions can create stability problems or amplify harmonics at frequencies related to control system dynamics. Multi-infeed situations where several HVDC links terminate in close proximity in the same AC network are particularly challenging from both stability and EMC perspectives.
Controller tuning must consider the electromagnetic interactions between the HVDC system and the AC network. Modes of oscillation involving the DC link and AC network components can be excited by control actions if not properly damped. Electromagnetic transient simulations help identify potential interaction problems and verify that control parameters provide adequate stability margins across the range of operating conditions.
Summary
HVDC systems present a distinctive set of EMC challenges centered on the power electronic converters that enable DC transmission. Converter stations generate harmonics across a wide frequency range, requiring extensive filtering to meet power quality and interference limits. DC transmission lines exhibit corona behavior and interference characteristics different from AC lines, requiring adapted prediction and mitigation techniques. Ground electrodes for current return create local electromagnetic effects and must be carefully sited and designed. The choice of return path configuration involves tradeoffs between economics, EMC performance, and environmental impact. Control systems must operate reliably in the intense electromagnetic environment while avoiding interactions that could cause instability or excessive emissions. As HVDC technology continues to evolve and deployment expands, EMC expertise remains essential for successful project development and operation.