Anti-Interference Design Strategies for DC Operational Power-Supply Systems
1 Introduction
DC operational power-supply systems (DCOPS) are the unsung guardians of power plants, substations, data centres and petrochemical complexes. While the primary function is to deliver uninterrupted 110 V or 220 V dc to protective relays, trip coils, SCADA and emergency lighting, the electromagnetic environment in which they work is becoming increasingly hostile. High-voltage switchyards generate 2 kV–20 kV common-mode transients, variable-frequency drives inject differential-mode noise, and 5G/LTE antennas couple radio-frequency (RF) energy into battery leads. A single 1 µs spike that exceeds the 1 500 V insulation rating of a relay can cause a false trip, transformer outage and million-dollar losses. This paper reviews the interference mechanisms specific to DCOPS and presents a coherent design methodology—grounding, shielding, filtering, surge suppression, layout and software—that has been validated in 50 substations and 5 000 UPS installations worldwide.
DC operational power-supply systems (DCOPS) are the unsung guardians of power plants, substations, data centres and petrochemical complexes. While the primary function is to deliver uninterrupted 110 V or 220 V dc to protective relays, trip coils, SCADA and emergency lighting, the electromagnetic environment in which they work is becoming increasingly hostile. High-voltage switchyards generate 2 kV–20 kV common-mode transients, variable-frequency drives inject differential-mode noise, and 5G/LTE antennas couple radio-frequency (RF) energy into battery leads. A single 1 µs spike that exceeds the 1 500 V insulation rating of a relay can cause a false trip, transformer outage and million-dollar losses. This paper reviews the interference mechanisms specific to DCOPS and presents a coherent design methodology—grounding, shielding, filtering, surge suppression, layout and software—that has been validated in 50 substations and 5 000 UPS installations worldwide.
2 Interference taxonomy in DCOPS
2.1 Conducted low-frequency (10 kHz–150 kHz)
Six-pulse rectifiers create 300 Hz ripple and sidebands that modulate the dc bus. Although amplitude is small (≤2 %), the long cable runs (up to 300 m) behave as antennas, re-radiating into adjacent control cables.
2.1 Conducted low-frequency (10 kHz–150 kHz)
Six-pulse rectifiers create 300 Hz ripple and sidebands that modulate the dc bus. Although amplitude is small (≤2 %), the long cable runs (up to 300 m) behave as antennas, re-radiating into adjacent control cables.
2.2 Conducted high-frequency (150 kHz–30 MHz)
IGBT rectifiers switching at 16 kHz–50 kHz produce dv/dt 5 kV µs⁻¹. Stray capacitance between battery cells and steel racks drives common-mode (CM) currents through the protective earth (PE) network.
IGBT rectifiers switching at 16 kHz–50 kHz produce dv/dt 5 kV µs⁻¹. Stray capacitance between battery cells and steel racks drives common-mode (CM) currents through the protective earth (PE) network.
2.3 Transient over-voltages
Circuit-breaker restrikes, lightning and capacitor-bank switching create 1.2/50 µs 4 kV differential and 8 kV CM surges. Because the DCOPS battery presents a low impedance, the surge is converted into a 1 000 A current spike that shares paths with sensitive electronics.
Circuit-breaker restrikes, lightning and capacitor-bank switching create 1.2/50 µs 4 kV differential and 8 kV CM surges. Because the DCOPS battery presents a low impedance, the surge is converted into a 1 000 A current spike that shares paths with sensitive electronics.
2.4 Radiated RF (30 MHz–1 GHz)
Hand-held radios (5 W, 400 MHz) 1 m from an open rack induce 15 V in 10 cm of unshielded wire—enough to flip a latch in a 3.3 V microcontroller.
Hand-held radios (5 W, 400 MHz) 1 m from an open rack induce 15 V in 10 cm of unshielded wire—enough to flip a latch in a 3.3 V microcontroller.
2.5 Electrostatic discharge (ESD)
Dry-air indoor substations (<20 % RH) generate 15 kV ESD from maintenance personnel. Although energy is low, the 1 ns rise-time couples through front-panel apertures into the gate-driver power supply.
Dry-air indoor substations (<20 % RH) generate 15 kV ESD from maintenance personnel. Although energy is low, the 1 ns rise-time couples through front-panel apertures into the gate-driver power supply.
3 Grounding and bonding—foundation of immunity
3.1 Single-point vs mesh
IEC 60364-5-54 recommends a meshed equipotential network for HV substations. A 50 mm × 5 mm copper ring main encircles the battery room; every rectifier chassis, battery rack and cable tray is bonded with 16 mm² Cu. Measured resistance <0.1 Ω between any two points limits CM voltage developed by 1 kA surge to 100 V.
3.1 Single-point vs mesh
IEC 60364-5-54 recommends a meshed equipotential network for HV substations. A 50 mm × 5 mm copper ring main encircles the battery room; every rectifier chassis, battery rack and cable tray is bonded with 16 mm² Cu. Measured resistance <0.1 Ω between any two points limits CM voltage developed by 1 kA surge to 100 V.
3.2 Segmented earth reference
To prevent surge currents from traversing the signal earth, DCOPS uses a “three-earth” hierarchy:
To prevent surge currents from traversing the signal earth, DCOPS uses a “three-earth” hierarchy:
- PE: safety earth, carries fault and surge currents
- FE: functional earth for shields and filters
- SE: sensitive earth for micro-electronics
The three are bonded at one single point (star) next to the battery negative terminal. High-frequency impedance is further reduced by 0.1 µF RF capacitors between FE and PE at 1 m intervals.
3.3 Battery negative treatment
IEEE 946 allows either floating or earthed negative. Floating offers better surge immunity (no return path) but risks electrostatic build-up. A 1 kΩ, 100 W resistor plus 63 A varistor clamps the floating bus to PE, bleeding static while blocking power-frequency current.
IEEE 946 allows either floating or earthed negative. Floating offers better surge immunity (no return path) but risks electrostatic build-up. A 1 kΩ, 100 W resistor plus 63 A varistor clamps the floating bus to PE, bleeding static while blocking power-frequency current.
4 Cable routing and shielding
4.1 Segregation classes
Following IEC 61892-6, cables are grouped:
4.1 Segregation classes
Following IEC 61892-6, cables are grouped:
- Class A: power (>110 V, battery leads)
- Class B: control (24–110 V)
- Class C: data (RS-485, CAN)
Minimum spacing 300 mm horizontal, 1 m vertical. Where crossing is unavoidable, a 90° angle is used and an earthed metallic divider is inserted.
4.2 Steel conduit vs shielded cable
A 2 mm steel conduit provides 60 dB attenuation at 1 MHz, but is bulky. For new Li-ion racks, double-shielded 0.75 mm² CYKYY cable (braid + foil, 85 % coverage) gives 55 dB and is 70 % lighter. Both ends of the shield are bonded to FE; the inner shield is floated at the field device to avoid ground-loop current.
A 2 mm steel conduit provides 60 dB attenuation at 1 MHz, but is bulky. For new Li-ion racks, double-shielded 0.75 mm² CYKYY cable (braid + foil, 85 % coverage) gives 55 dB and is 70 % lighter. Both ends of the shield are bonded to FE; the inner shield is floated at the field device to avoid ground-loop current.
4.3 Optical fibre for signalling
Current-differential protection and BMS communication use multimode fibre with ST connectors. Fibre removes the last CM path; measured surge current drops from 80 A to <1 A during 8 kV injection.
Current-differential protection and BMS communication use multimode fibre with ST connectors. Fibre removes the last CM path; measured surge current drops from 80 A to <1 A during 8 kV injection.
5 Input-side filtering
5.1 Rectifier reflection
A 12-pulse rectifier already attenuates 5th and 7th harmonics by 80 %. A small 2 % commutation inductor (0.1 pu) reduces notch depth from 350 V to 120 V, cutting CM current 6 dB.
5.1 Rectifier reflection
A 12-pulse rectifier already attenuates 5th and 7th harmonics by 80 %. A small 2 % commutation inductor (0.1 pu) reduces notch depth from 350 V to 120 V, cutting CM current 6 dB.
5.2 EMI filter topology
A two-stage LC filter (L1 = 300 µH, C1 = 1 µF X2; L2 = 100 µH, C2 = 0.47 µF) is inserted between rectifier and battery. Resonance at 9 kHz is damped by 2 Ω/2 W in parallel with C2. Insertion loss: 45 dB at 150 kHz, 60 dB at 500 kHz—sufficient to meet CISPR 22 Class B with 6 dB margin.
A two-stage LC filter (L1 = 300 µH, C1 = 1 µF X2; L2 = 100 µH, C2 = 0.47 µF) is inserted between rectifier and battery. Resonance at 9 kHz is damped by 2 Ω/2 W in parallel with C2. Insertion loss: 45 dB at 150 kHz, 60 dB at 500 kHz—sufficient to meet CISPR 22 Class B with 6 dB margin.
5.3 Y-capacitor safety
Y-capacitors from dc (+) and (–) to PE must not exceed 1 µF total to keep earth-leakage <30 mA under normal operation. A 0.47 µF/1 kV ceramic is chosen; its 50 Hz leakage is 7 mA, leaving headroom for ageing.
Y-capacitors from dc (+) and (–) to PE must not exceed 1 µF total to keep earth-leakage <30 mA under normal operation. A 0.47 µF/1 kV ceramic is chosen; its 50 Hz leakage is 7 mA, leaving headroom for ageing.
6 Surge protection coordination
6.1 Zone concept
IEC 62305 divides the installation into lightning-protection zones (LPZ). DCOPS battery room is LPZ 1; rectifier cubicle LPZ 2; control PCB LPZ 3. Surge-arresters are cascaded:
6.1 Zone concept
IEC 62305 divides the installation into lightning-protection zones (LPZ). DCOPS battery room is LPZ 1; rectifier cubicle LPZ 2; control PCB LPZ 3. Surge-arresters are cascaded:
- LPZ 0→1: 25 kA, 1 000 V spark-gap at incomer
- LPZ 1→2: MOV 40 kA, 600 V clamp
- LPZ 2→3: 15 V TVS diode array on PCB
6.2 DC-specific metal-oxide varistor (MOV)
Standard 275 V ac MOVs are unsuitable for 220 V dc because thermal run-away occurs at 1.3×Vdc. A 420 V dc-rated MOV with 150 J cm⁻³ energy density and thermal fuse is used. Coordination study (ETAP) shows 1 500 A, 8/20 µs current splits: 70 % to spark-gap, 20 % to MOV, 10 % to cable resistance—keeping clamp voltage ≤650 V.
Standard 275 V ac MOVs are unsuitable for 220 V dc because thermal run-away occurs at 1.3×Vdc. A 420 V dc-rated MOV with 150 J cm⁻³ energy density and thermal fuse is used. Coordination study (ETAP) shows 1 500 A, 8/20 µs current splits: 70 % to spark-gap, 20 % to MOV, 10 % to cable resistance—keeping clamp voltage ≤650 V.
6.3 Isolated spark-gap for battery strings
To prevent dc arc persistence inside a 250 V battery, a hermetically sealed spark-gap with 350 V breakdown is placed across each 50-cell section. Arc extinction is guaranteed by the 1.5 V/cell counter-voltage when the surge collapses.
To prevent dc arc persistence inside a 250 V battery, a hermetically sealed spark-gap with 350 V breakdown is placed across each 50-cell section. Arc extinction is guaranteed by the 1.5 V/cell counter-voltage when the surge collapses.
7 PCB-level measures
7.1 Stack-up
A 4-layer board (signal-ground-power-ground) with 0.2 mm prepreg gives 250 pF cm⁻² inter-plane capacitance, providing a low-impedance return path up to 200 MHz. Tracks carrying 24 V logic are flanked by ground vias every 10 mm, reducing loop area to 0.3 cm²—equivalent induced voltage 0.5 V at 10 MHz, 1 A µs⁻¹.
7.1 Stack-up
A 4-layer board (signal-ground-power-ground) with 0.2 mm prepreg gives 250 pF cm⁻² inter-plane capacitance, providing a low-impedance return path up to 200 MHz. Tracks carrying 24 V logic are flanked by ground vias every 10 mm, reducing loop area to 0.3 cm²—equivalent induced voltage 0.5 V at 10 MHz, 1 A µs⁻¹.
7.2 Galvanic isolation
Digital isolators (ADuM5401) with 2.5 kV rms and 100 kV µs⁻¹ CMTI separate microcontroller from gate drivers. An on-chip iso-power dc/dc delivers 150 mW, eliminating opto-coupler ageing.
Digital isolators (ADuM5401) with 2.5 kV rms and 100 kV µs⁻¹ CMTI separate microcontroller from gate drivers. An on-chip iso-power dc/dc delivers 150 mW, eliminating opto-coupler ageing.
7.3 Watch-dog with hysteresis
A window-watchdog disables PWM if supply deviates ±5 %. Hysteresis (50 mV) prevents chatter during 100 Hz ripple. Mean-time-to-dangerous-failure (MTTFD) improves from 8 000 h to 120 000 h.
A window-watchdog disables PWM if supply deviates ±5 %. Hysteresis (50 mV) prevents chatter during 100 Hz ripple. Mean-time-to-dangerous-failure (MTTFD) improves from 8 000 h to 120 000 h.
8 Software filtering and redundancy
8.1 Median filter
Voltage and current samples are sorted; the median of five consecutive values is used. Rejection ratio for 1-sample spike: 100 %. Execution time: 3 µs on Cortex-M4 at 120 MHz.
8.1 Median filter
Voltage and current samples are sorted; the median of five consecutive values is used. Rejection ratio for 1-sample spike: 100 %. Execution time: 3 µs on Cortex-M4 at 120 MHz.
8.2 Majority-vote trip logic
Three independent microcontrollers (MCU-A, B, C) sample the same resistive divider through separate ADCs. A trip is executed only if at least two agree. Probability of nuisance trip due to EMI: 10⁻⁹ per hour—meeting SIL-3.
Three independent microcontrollers (MCU-A, B, C) sample the same resistive divider through separate ADCs. A trip is executed only if at least two agree. Probability of nuisance trip due to EMI: 10⁻⁹ per hour—meeting SIL-3.
8.3 Frequency dithering
The IGBT switching frequency is dithered ±4 % around 18 kHz with 200 Hz pseudo-random sequence. Peak emission at 18 kHz drops 6 dB, easing filter size 20 %.
The IGBT switching frequency is dithered ±4 % around 18 kHz with 200 Hz pseudo-random sequence. Peak emission at 18 kHz drops 6 dB, easing filter size 20 %.
9 Special topics for Li-ion batteries
9.1 BMS immunity
Each BMS slave uses local 5 V dc/dc with 1 kV isolation. Cell-voltage measurement traces are guarded by ground pour on adjacent layer; CMRR >90 dB at 100 kHz. A 1 nF Y-cap from each cell to local ground shunts RF current, preventing 10 V offset that would otherwise unbalance the stack.
9.1 BMS immunity
Each BMS slave uses local 5 V dc/dc with 1 kV isolation. Cell-voltage measurement traces are guarded by ground pour on adjacent layer; CMRR >90 dB at 100 kHz. A 1 nF Y-cap from each cell to local ground shunts RF current, preventing 10 V offset that would otherwise unbalance the stack.
9.2 Arc-fault detection
Series arcs in battery strings produce 100 kHz–1 MHz bursts. A Rogowski coil with 10 MHz bandwidth feeds an FFT. Energy in 200–500 kHz band >50 mV² Hz⁻¹ for >1 ms is classified as arc; the main contactor opens within 5 ms. False positives due to rectifier switching are avoided by blanking the 16 kHz fundamental and harmonics.
Series arcs in battery strings produce 100 kHz–1 MHz bursts. A Rogowski coil with 10 MHz bandwidth feeds an FFT. Energy in 200–500 kHz band >50 mV² Hz⁻¹ for >1 ms is classified as arc; the main contactor opens within 5 ms. False positives due to rectifier switching are avoided by blanking the 16 kHz fundamental and harmonics.
10 Measurement techniques
10.1 CM/DM separation
A 50 Ω, 0°/180° power combiner splits the signal so that DM and CM can be measured separately during surge injection (IEC 61000-4-16). Resolution: 40 dB CMRR up to 30 MHz.
10.1 CM/DM separation
A 50 Ω, 0°/180° power combiner splits the signal so that DM and CM can be measured separately during surge injection (IEC 61000-4-16). Resolution: 40 dB CMRR up to 30 MHz.
10.2 1 GHz transient recorder
A 4-channel, 12-bit, 1 GS s⁻¹ oscilloscope with fibre-optic isolated probes (60 dB CMRR, 200 MHz) captures sub-nanosecond events without earth-loop corruption.
A 4-channel, 12-bit, 1 GS s⁻¹ oscilloscope with fibre-optic isolated probes (60 dB CMRR, 200 MHz) captures sub-nanosecond events without earth-loop corruption.
11 Case study – 500 kV GIS substation, Inner Mongolia
Environment: 40 thunderstorm days/year, radio transmitter 500 m, VHF 150 MHz, 50 W.
Configuration: 220 V DCOPS, 200 Ah Li-ion, 2 × 100 A rectifiers, 25 m cable run.
Interference tests:
Environment: 40 thunderstorm days/year, radio transmitter 500 m, VHF 150 MHz, 50 W.
Configuration: 220 V DCOPS, 200 Ah Li-ion, 2 × 100 A rectifiers, 25 m cable run.
Interference tests:
- 8 kV contact ESD on battery rack – no trip
- 4 kV 1.2/50 µs surge injected at rectifier – 620 V clamp, system operational
- 10 V m⁻¹ RF field (80 MHz–1 GHz) – measured bus deviation 0.3 V
- 1 kA lightning strike on gantry 30 m away – dc bus dipped 18 V for 2 ms, relays remained energised.
After one year of operation, no nuisance trips were logged; BMS error frames <0.01 %. The design is now the utility’s corporate standard.
12 Economic impact
Cost adder for full anti-interference package (filters, fibre, MOV, PCB redesign): USD 0.02 per watt of DCOPS capacity. For a 10 kW system this is USD 200—less than the cost of one unplanned outage. Mean-time-between-spurious-trip improved from 2.5 years to >50 years, translating into USD 4 000 NPV saving per substation.
Cost adder for full anti-interference package (filters, fibre, MOV, PCB redesign): USD 0.02 per watt of DCOPS capacity. For a 10 kW system this is USD 200—less than the cost of one unplanned outage. Mean-time-between-spurious-trip improved from 2.5 years to >50 years, translating into USD 4 000 NPV saving per substation.
13 Conclusions
- A systematic, standards-based approach—zone grounding, layered shielding, coordinated surge protection and software voting—raises DCOPS immunity by 20–40 dB across the 0 Hz–1 GHz spectrum.
- Fibre-optic communication and isolated gate-power remove the last galvanic paths, making even 8 kV surge injection benign.
- Li-ion chemistry introduces new HF coupling paths; local RF shunting and arc-fault discrimination are now mandatory.
- The incremental cost is <2 % of system CAPEX while reliability improves two orders of magnitude, proving that “immunity is cheaper than immunity testing after the fault.”
As substations become digital and renewables proliferate, the DCOPS must evolve from a passive battery room into an EMI-hardened, cyber-secure power hub. The design strategies presented here provide a future-proof template for mission-critical installations worldwide.
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