1. Introduction

2. Role of Outdoor VTs in DC Grids and HVDC Systems

• Control and Regulation: HVDC converters (line-commutated converters [LCC] or voltage-source converters [VSC]) require real-time DC voltage feedback to maintain stable operation. VTs provide precise measurements to adjust converter firing angles (LCC) or modulation indices (VSC), preventing voltage instability or converter tripping.

• Protection: DC faults (e.g., pole-to-pole, pole-to-ground) propagate rapidly, with fault currents rising to thousands of amperes in milliseconds. VTs detect overvoltage/undervoltage transients, triggering protective actions (e.g., converter blocking, DC circuit breaker operation) to limit damage.

• Monitoring and Metering: VTs enable energy accounting (e.g., between wind farms and the grid), track insulation health (via partial discharge monitoring), and support condition-based maintenance of HVDC links. In DC distribution grids (e.g., 380V DC for data centers), they ensure load balance and safety.

3. Technical Challenges of DC Voltage Measurement

3.1 Insulation Stress in DC Systems

• Steady-State Stress: Unlike AC, where voltage alternates, DC voltage maintains a constant polarity, causing continuous electric field stress on insulation. This leads to space charge accumulation—charges trapped in insulation materials (e.g., epoxy, oil) that distort electric fields, increasing the risk of partial discharge (PD) and insulation breakdown over time.

• Polarity Effects: Outdoor VTs exposed to humidity or pollution may develop surface leakage currents with polarity-dependent behavior. For example, positive DC voltage accelerates ion migration in water droplets, increasing surface conduction and PD risk.

• Transient Overvoltages: Switching operations (e.g., converter start-up) or faults generate DC transients (e.g., voltage spikes up to 2× rated voltage) that test insulation integrity. Outdoor VTs must withstand these without arcing or degradation.

3.2 Measurement Accuracy in DC

• No Zero-Crossing: AC VTs leverage zero-crossings to reset magnetic cores, but DC VTs lack this feature, requiring non-magnetic measurement principles (e.g., resistive or capacitive 分压 [voltage division]).

• Temperature Sensitivity: DC measurement components (e.g., high-voltage resistors) are prone to drift with temperature, a critical issue in outdoor environments where temperatures may swing from -40°C to +60°C.

• Electromagnetic Interference (EMI): HVDC converter stations generate high-frequency noise (from switching devices like IGBTs), which can couple into VT secondary circuits, distorting DC measurements. Outdoor installations, with longer cable runs, are particularly vulnerable.

3.3 Environmental Stressors in Outdoor DC Applications

• Offshore HVDC: Salt spray and high humidity accelerate corrosion of metal components and degrade insulator surfaces, increasing leakage currents.

• Desert/Arctic DC Grids: Extreme temperatures cause thermal expansion/contraction, stressing seals and insulation. UV radiation in deserts degrades polymeric materials (e.g., silicone insulators).

• Urban DC Distribution: Pollution (e.g., dust, industrial emissions) deposits on insulators, creating conductive paths that bias DC measurements.

4. Types of Outdoor VTs for DC Grids and HVDC

4.1 Resistive Voltage Dividers (RVDs)

• Principle: A chain of high-voltage resistors (e.g., ceramic, metal-oxide) divides the primary DC voltage proportionally, with a low-voltage secondary signal extracted from the bottom resistor.

• Design Features:

• Outdoor Adaptations: Resistors are housed in weatherproof enclosures (stainless steel or FRP) with silicone rubber bushings to prevent moisture ingress.

• Temperature Compensation: Resistors with low temperature coefficients (e.g., ±50ppm/°C) or active cooling (heat sinks) minimize drift in extreme temperatures.

• EMI Shielding: Metal shields around the resistor chain and coaxial secondary cables reduce noise coupling.

• Advantages: Simple design, fast response (suitable for transient detection), low cost for medium voltages (≤±320kV).

• Limitations: High power dissipation (I²R losses) at high voltages, requiring heat management; vulnerable to pollution-induced leakage currents.

4.2 Capacitive Voltage Dividers (CVDs)

• Principle: Two capacitors (high-voltage [C1] and low-voltage [C2]) form a voltage divider, with secondary voltage $V_2=V_1\times (C1/(C1+C2))$. For DC, CVDs often include parallel resistors to discharge capacitors during faults.

• Design Features:

• Outdoor Adaptations: C1 uses ceramic or polymer capacitors with high dielectric strength; the divider is enclosed in a gas-tight tank (SF₆ or N₂) to prevent moisture.

• Pollution Resistance: Composite insulators with hydrophobic surfaces (silicone rubber) reduce leakage current effects.

• Advantages: Low power loss (no steady-state current), robust to temperature variations, suitable for high voltages (±500kV to ±1100kV).

• Limitations: Slow response to transients (due to capacitor charging); requires careful matching of C1 and C2 to maintain accuracy.

4.3 Optical Voltage Transformers (OVTs)

• Principle: Utilize the Pockels effect—electro-optic crystals (e.g., lithium niobate) change refractive index under DC voltage, altering the polarization of light transmitted through them. A photodetector converts this change into a voltage signal.

• Design Features:

• Outdoor Adaptations: Crystal sensors are sealed in hermetic enclosures; fiber-optic cables (immune to EMI) transmit light between sensor and electronics.

• UV and Temperature Resistance: Crystals are mounted on thermally stable substrates (e.g., invar) to minimize mechanical stress from temperature swings.

• Advantages: No electrical connection between high-voltage and low-voltage sides (enhanced safety), wide bandwidth (DC to MHz), immunity to EMI—ideal for HVDC converter stations.

• Limitations: High cost; crystal aging (over years) can degrade accuracy; sensitive to vibration (challenging for offshore applications).

4.4 Hybrid Voltage Transformers

• Principle: Combine resistive/capacitive dividers with optical sensing for redundancy. For example, a CVD provides steady-state measurement, while an RVD captures transients, with data fused to ensure reliability.

• Design Features: Modular enclosures allow separate maintenance of divider and optical components; outdoor-rated for -40°C to +70°C operation.

• Advantages: Balances cost, speed, and accuracy; suitable for critical HVDC applications (e.g., ±800kV links).

5. Key Design Features for Outdoor DC VTs

5.1 Insulation Systems

• Solid Insulation: Epoxy resin with alumina fillers (for RVDs) or silicone rubber (for bushings) provides high dielectric strength (≥20kV/mm) and resistance to UV and pollution.

• Gas Insulation: SF₆ (for CVDs) or dry air (for OVTs) offers excellent insulation in compact enclosures, with pressure monitoring to detect leaks.

• Liquid Insulation: Synthetic esters (in hybrid VTs) resist moisture absorption and maintain dielectric strength at low temperatures (-40°C), suitable for arctic HVDC.

5.2 Environmental Protection

• Corrosion Resistance: Enclosures use 316L stainless steel (offshore) or powder-coated aluminum (onshore) to withstand salt spray and H₂S (in coastal DC grids).

• Pollution Mitigation: Composite insulators with "self-cleaning" hydrophobic surfaces (silicone rubber) shed water and prevent dust accumulation, reducing leakage currents.

• Thermal Management:

• Heaters (thermostatically controlled) prevent condensation in cold climates.

• Heat pipes or finned enclosures dissipate heat in high-temperature regions (e.g., desert DC microgrids).

5.3 EMI Immunity

• Shielding: Conductive enclosures (e.g., copper-clad steel) block radiated EMI from converters; twisted-pair or fiber-optic secondary cables minimize conducted noise.

• Filtering: Low-pass filters (cutoff ≤1kHz) in secondary circuits remove high-frequency switching noise without distorting DC signals.

5.4 Safety and Reliability

• Overvoltage Protection: Metal-oxide varistors (MOVs) across secondary circuits clamp transient voltages from lightning or faults.

• Redundancy: Dual-sensor designs (e.g., two RVDs in parallel) ensure measurement continuity if one sensor fails—critical for HVDC links with no backup.

6. Selection Criteria for Outdoor DC VTs

6.1 Voltage Rating

• Continuous Operating Voltage: Must match the DC system voltage (e.g., ±320kV for offshore wind HVDC, ±1100kV for ultra-high-voltage links).

• Withstand Capabilities:

• DC withstand voltage: ≥1.2× rated voltage for 1 hour (to test insulation under steady stress).

• Impulse withstand voltage: ≥2.5× rated voltage (1.2/50μs impulse) to survive lightning strikes.

6.2 Accuracy and Bandwidth

• Steady-State Accuracy: Class 0.2 (±0.2% error) for metering and control; Class 1 (±1% error) for protection.

• Transient Response: Rise time ≤10μs to capture fault transients (critical for VSC-HVDC, where faults propagate in <5ms).

• Frequency Bandwidth: DC to ≥10kHz to measure ripple (from converters) and switching harmonics.

6.3 Environmental Ratings

• Temperature Range: -40°C to +70°C for most outdoor applications; -55°C to +85°C for arctic or desert use.

• Ingress Protection: IP65 or higher to resist dust and water jets (offshore VTs may require IP66).

• Corrosion Resistance: Compliance with ISO 9227 (salt spray testing) for offshore use (≥1000 hours without rust).

6.4 Compliance with Standards

• IEC 61869-11: Specifies requirements for DC voltage transformers, including accuracy classes and test methods.

• IEC 62501: Applies to HVDC converter stations, mandating VT performance under EMI and transient conditions.

• CIGRE TB 854: Guidelines for outdoor equipment in HVDC systems, covering insulation and environmental adaptation.

7. Installation and Maintenance Best Practices

7.1 Installation Considerations

• Mounting: Use vibration-dampened brackets (for offshore platforms) to protect OVT crystals from mechanical stress. Elevate VTs ≥500mm above ground to avoid standing water.

• Cabling: Secondary cables (fiber-optic for OVTs, shielded copper for RVDs/CVDs) must be routed in UV-resistant conduits. Avoid parallel runs with high-voltage DC cables to reduce EMI coupling.

• Grounding: A dedicated low-impedance ground (≤1Ω) for the VT enclosure prevents voltage buildup; separate grounds for high-voltage and low-voltage sides avoid ground loops.

7.2 Maintenance Protocols

• Visual Inspections: Quarterly checks for insulator tracking, corrosion, or seal damage. Clean insulators with deionized water (offshore) or dry air (deserts) to remove salt/pollution.

• Electrical Testing:

• Insulation Resistance: Annual tests (≥10,000MΩ at 5kV) to detect moisture in RVD/CVD insulation.

• Accuracy Calibration: Every 2–3 years using DC high-voltage sources (e.g., 100kV DC calibrators) to correct for drift.

• PD Monitoring: For HVDC VTs, continuous PD monitoring (via embedded sensors) detects early insulation degradation.

• Optical System Checks (for OVTs): Annual verification of light source stability and fiber attenuation to ensure signal integrity.

8. Case Study: Outdoor VTs in a ±800kV HVDC Link

• Salt spray and 100% humidity (offshore).

• DC voltage up to ±816kV (102% of rated).

• EMI from 50kV IGBT converters.

• Type: Hybrid (CVD + OVT) for redundancy.

• CVD: SF₆-insulated, 316L stainless steel enclosure, composite insulators with 35mm/kV creepage distance (pollution class IV).

• OVT: Pockels crystal sensor, fiber-optic link, temperature-compensated electronics.

• Accuracy: Class 0.2 (steady-state), 10μs transient response.

9. Future Trends

• Digitalization: Embedded IoT sensors (e.g., temperature, humidity, PD) enable real-time condition monitoring via 5G/LoRa, reducing maintenance costs in remote locations.

• Wide Bandgap (WBG) Integration: VTs optimized for WBG-based converters (SiC/GaN) with higher switching frequencies (up to 100kHz) require wider bandwidth (DC to 1MHz).

• Sustainability: SF₆-free designs (using dry air or eco-friendly gases like g³) align with global decarbonization goals.

• AI-Enhanced Calibration: Machine learning algorithms correct for temperature/pollution-induced drift, improving long-term accuracy without manual intervention.

10. Conclusion