IEC TR 62544: HVDC — Active Filters

IEC TR 62544, published in 2016 as a Technical Report by IEC Technical Committee 22 (Power Electronic Systems and Equipment), provides comprehensive guidance on active filters for high-voltage direct current (HVDC) converter stations. As HVDC systems proliferate worldwide and power quality requirements become increasingly stringent, active filtering technology has emerged as a critical solution for mitigating harmonic distortion on both the AC and DC sides of HVDC converters, particularly for line-commutated converter (LCC) HVDC systems where characteristic harmonics (11th, 13th, 23rd, 25th for 12-pulse converters) are inherent to the conversion process.

The report addresses the limitations of traditional passive filtering approaches, which require large physical footprints (often occupying 30-50% of the total converter station area), are sensitive to AC grid impedance variations and frequency deviations, and may create series or parallel resonances with the grid that amplify harmonics at specific frequencies. Active filters, by contrast, offer adaptive harmonic cancellation that can track changing system conditions, provide compensation for multiple harmonics simultaneously without filter bank switching, and occupy significantly less space. The report covers both AC-side and DC-side active filter configurations, providing engineers with the analytical tools needed to specify, design, and evaluate active filter performance in HVDC applications.

Active Filter Topologies and Configurations

The report classifies active filters for HVDC applications into several distinct topologies. Shunt active power filters (SAPFs) are the most common configuration, connected in parallel with the converter AC bus through a coupling transformer or interface inductor. SAPFs inject harmonic currents equal in magnitude but opposite in phase to the converter-generated harmonics, achieving cancellation at the point of common coupling. For LCC-HVDC systems, where the dominant harmonics are the 11th and 13th (for 12-pulse converters), a shunt active filter rated at 5-10% of the converter rated power can reduce the individual harmonic distortion (IHD) to below 0.5% at the AC bus, compared to 1-2% achievable with passive filters alone.

Series active filters are connected in series with the AC or DC bus and function as a harmonic isolator, presenting high impedance at harmonic frequencies while offering low impedance at the fundamental frequency. While less common than shunt topologies due to the need for full-rated series transformers and the challenge of handling fault currents, series active filters offer superior performance for specific applications such as mitigating harmonic interactions between multiple HVDC converters connected to a weak AC bus. The report describes hybrid configurations combining a small-rated active filter (typically 1-3% of converter power) with passive filter branches, where the active portion improves the passive filter’s tuning accuracy and dampens potential resonances. This hybrid approach offers an attractive cost-performance trade-off for upgrading existing LCC-HVDC stations where space for additional passive filters is constrained and the cost of fully-rated active filtering cannot be justified.

On the DC side, the report addresses DC active filters specifically designed to mitigate harmonic currents on HVDC transmission lines, which can cause interference with telephone lines and other metallic communication circuits running parallel to the DC transmission corridor. DC-side harmonics in LCC-HVDC systems are dominated by the 12th harmonic (600 Hz for 50 Hz systems, 720 Hz for 60 Hz systems) for 12-pulse converters, with additional non-characteristic harmonics under unbalanced AC conditions. DC active filters typically employ a shunt configuration connected between the DC bus and ground through a coupling capacitor, with the active inverter rated at 0.5-2% of the converter power for effective harmonic suppression. The report notes that in MMC-HVDC systems, DC-side harmonics are significantly lower due to the high number of voltage levels, making DC active filtering generally unnecessary for modern VSC-HVDC installations.

Control Strategies and Performance Evaluation

IEC TR 62544 describes the principal control strategies for HVDC active filters, which can be categorized as frequency-domain methods (based on Fourier analysis or synchronous reference frame transformation) and time-domain methods (based on instantaneous power theory or adaptive filtering). For LCC-HVDC applications, the most widely implemented approach is selective harmonic elimination using multiple synchronous reference frames (SRF), where each target harmonic is transformed to a rotating reference frame where it appears as a DC quantity and can be regulated using conventional PI controllers. A typical implementation requires six to eight SRF controllers operating in parallel to target the dominant harmonics (11th, 13th, 23rd, 25th, 35th, 37th), with the controller outputs summed to produce the active filter voltage reference.

The standard emphasizes the critical importance of the phase-locked loop (PLL) for active filter performance. Under weak AC grid conditions (SCR below 3), the PLL must extract the fundamental frequency phase angle with high accuracy despite significant voltage distortion. Conventional synchronous reference frame PLLs (SRF-PLL) may exhibit phase errors of several degrees under highly distorted conditions, degrading active filter performance. The report describes advanced PLL architectures suitable for active filter control, including the decoupled double synchronous reference frame PLL (DDSRF-PLL), the second-order generalized integrator PLL (SOGI-PLL), and the moving average filter PLL (MAF-PLL). Each offers different trade-offs between dynamic response speed (important for fault ride-through) and harmonic rejection capability (important for steady-state filtering performance).

Performance evaluation criteria for HVDC active filters are defined in terms of individual harmonic distortion (IHD), total harmonic distortion (THD), telephone influence factor (TIF), and IT product (I*T). For modern HVDC installations, typical performance requirements specified by grid operators include IHD below 0.5% for all individual harmonics up to the 50th order, THD below 1.0% at the AC bus, and TIF below 50 for the DC line. The report provides test procedures for verifying active filter performance under various operating conditions, including rated power operation, low power operation where harmonics are proportionally more significant, and transient conditions during power step changes and AC fault events. The standard also addresses the interaction between active filters and nearby passive filters or capacitor banks, recommending impedance scan analysis (both frequency-domain and time-domain) to identify potential resonance conditions before installation.

Engineering Design Insights for HVDC Active Filters

From an engineering implementation perspective, several critical design considerations emerge from IEC TR 62544. First, the coupling transformer or interface inductor for shunt active filters must be designed to handle the combined stress of fundamental frequency voltage and harmonic currents. The transformer rating is typically 1.5-2 times the active filter rated power to account for the harmonic copper losses and increased core losses at elevated frequencies. For medium-voltage direct connection (10-35 kV bus), an air-core interface inductor may be preferred over a transformer to avoid core saturation from DC components and to provide a more linear inductance characteristic over the operating frequency range. The design must ensure that the interface impedance does not create a parallel resonance with the AC grid impedance at a frequency where significant harmonic current exists.

Second, the switching frequency selection for the active filter inverter involves fundamental trade-offs. Higher switching frequencies (3-5 kHz for IGBT-based active filters) provide better harmonic cancellation bandwidth and lower filter size but increase switching losses. For HVDC active filters, which operate continuously at rated power, the additional 0.3-0.5% in losses from higher switching frequency must be capitalized into the life-cycle cost analysis. Advanced modulation techniques such as selective harmonic elimination PWM (SHE-PWM) or optimized pulse patterns (OPP) can achieve the required harmonic cancellation with significantly lower switching frequencies (800-1200 Hz), reducing losses by 30-40% compared to conventional PWM at the expense of dynamic response. For applications requiring both high steady-state performance and fast transient response, hybrid modulation schemes that switch between SHE-PWM (steady-state) and conventional PWM (during transients) offer a promising approach.

Third, the report highlights the importance of redundancy and fault tolerance in HVDC active filter design. The active filter power stage should be designed with redundant inverter modules (N+1 configuration) such that the failure of a single module does not degrade filtering performance. The control system should include independent overcurrent protection, DC bus overvoltage protection, and grid synchronization monitoring. In the event of active filter tripping, the HVDC converter must be able to continue operation with reduced harmonic performance (relying on passive filters only) without exceeding equipment thermal limits. This requires coordination between the active filter protection system and the HVDC converter control, typically implemented through a station-level harmonic management system that dynamically adjusts the converter operating point or power level when filtering capacity is degraded.