Combined Instrument Transformer (CIT) Solution: Engineering Design & Integration Perspective

​1. Core Solution Concept: Modular Platform with Shared Insulation​

• ​Design:​​ Develop a unified, modular platform housing both current and voltage sensing functions within a single, optimized structure.
• ​Insulation:​​ Utilize a shared insulating envelope. Two options are engineered:
• ​SF6 Gas:​​ Proven high-dielectric strength and excellent arc-quenching properties for higher voltage classes (e.g., 72.5 kV and above). Design incorporates gas density monitoring and proven sealing technology.
• ​Composite Housing (Solid Insulation):​​ Environmentally sustainable solution using high-grade polymer materials with silicone sheds. Ideal for lower to medium voltages or where SF6 avoidance is mandated. Optimized for creepage distance and pollution performance.
• ​Modularity:​​ Design internal components and interfaces to allow for:
• Scalability across different voltage classes (e.g., through insulator length adjustment).
• Adaptation to specific bushing interface requirements.
• Potential for future sensor technology upgrades.

​2. Integrated Sensing Technology Implementation​

• ​Current Measurement:​​
• ​Sensor:​​ High-accuracy, temperature-compensated Rogowski coils. Selected for:
• ​Wide Dynamic Range:​​ Excellent linearity from small fractions of nominal current up to high fault currents (e.g., >40 kA).
• ​No Saturation:​​ Fundamental advantage over iron-core CTs, eliminating saturation risk during faults.
• ​Lightweight:​​ Significantly reduces mechanical stress on the overall structure.
• ​Integration:​​ Coils strategically placed within the insulator envelope, concentric with the primary conductor. Secure mechanical mounting resistant to vibration.
• ​Voltage Measurement:​​
• ​Sensor:​​ High-stability capacitive voltage dividers (CVDs) as standard. Resistive dividers (RVDs) considered for specific DC or wide-bandwidth applications requiring fast transient response.
• ​Integration:​​ CVD sensing electrodes (low-impedance) integrated directly into the insulator structure. Precision grading electrodes ensure uniform field distribution and thermal/pollution stability. Critical shielding prevents external field interference.

​3. Advanced Electromagnetic Field Modeling & Isolation (Critical Engineering Challenge)​​

• ​Modeling:​​ Mandatory, high-fidelity 3D Finite Element Method (FEM) modeling of the entire platform:
• Precisely characterizes internal electromagnetic fields under all operational conditions (sinusoidal, transient, distorted waveforms).
• Evaluates proximity effects from conductors, enclosure, and adjacent phases.
• ​Minimizing Crosstalk:​​
• ​Physical Separation:​​ Optimal geometric arrangement of sensing elements (coils, CVD electrodes) based on modeling results. Maximize distance within constraints.
• ​Active Shielding:​​ Implementation of grounded electrostatic shields strategically placed between sensor elements based on field simulation data.
• ​Guard Rings:​​ Utilize conductive guard rings around Rogowski coil outputs to drain displacement currents.
• ​Precise Measurement Isolation:​​
• ​Dedicated Signal Paths:​​ Routing signals from individual sensors using shielded, twisted-pair cabling within the enclosure immediately upon capture.
• ​Compensated Circuit Design:​​ Electronic conditioning circuits designed with crosstalk cancellation techniques informed by FEM models.
• ​Validation:​​ Rigorous factory testing (including harmonic injection tests) to characterize and verify isolation margins and crosstalk levels (< 0.1% specified).

​4. Integrated Digital Processing & Standardized Interfaces​

• ​Onboard Signal Processing:​​
• Dedicated, low-power ASICs or high-reliability microcontrollers directly integrated onto the sensor platform or adjacent sealed module.
• Functions include: Rogowski coil integrator, scaling, ADC conversion, harmonic computation (if applicable), linearization, temperature compensation, and timestamping.
• ​Standardized Digital Output:​​
• ​Embedded Interfaces:​​ Incorporate IEC 61869 compliant digital output circuitry directly within the CIT unit.
• ​Protocols:​​ Standardized support for:
• ​IEC 61850-9-2:​​ Sampled Values (SV) stream over Ethernet (typically multicast).
• ​IEC 61850-9-3LE:​​ Lightning Edition SV profile for guaranteed low-latency determinism.
• ​Additional Options:​​ Provision for legacy outputs (analog, IEC 60044-8 FT3) where required via optional modules.
• ​Data Quality:​​ Integrated Merging Unit (MU) functionality meeting relevant IEC 61869 accuracy (TPE/TPM class) and timing (PLL synchronization) standards.

​5. Engineering Design & Integration Considerations​

• ​Thermal Management:​​ Models include thermal performance analysis. Power dissipation from electronics actively managed using low-power components, potential localized heatsinks, and optimized convection paths within the insulator.
• ​EMC/EMI Robustness:​​ Conformal coating, shielded enclosures, ferrites, and optimized grounding strategies applied to internal electronics. Surge protection compliant with relevant standards (IEC 61000-4-5).
• ​Mechanical Integrity:​​ Structural analysis performed for seismic loads, wind loading, ice loading, and dynamic forces during faults. Optimized use of materials (composite/porcelain/SF6) contributes to lower seismic mass.
• ​Factory Calibration & Testing:​​ Comprehensive calibration against reference standards (optical/VTBI methods). Includes verification of EM isolation effectiveness, timing accuracy, protocol compliance, and full-power dielectric testing.
• ​Lifecycle & Serviceability:​​ Designed for minimal maintenance (especially SF6 or solid insulation). Modular electronics potentially accessible/testable without major disassembly. End-of-life disposal pathways considered (SF6 recovery/recycling).

​Benefits Realized through this Design & Integration Approach:​​

• ​Footprint Reduction:​​ Up to 40-50% space savings vs. separate CTs/VTs – crucial for retrofits and compact GIS/AIS designs.
• ​Enhanced Accuracy & Safety:​​ Eliminates traditional CT saturation risks, improves transient response (Rogowski/CVD), reduces external connections/risks.
• ​Simplified Installation:​​ Single unit mounting and commissioning significantly reduce field labor and cabling complexity.
• ​Lower Lifecycle Costs:​​ Reduced installation, cabling, civil work, maintenance overhead.
• ​Digital Substation Readiness:​​ Direct IEC 61850-9-2/3LE output enables seamless integration into modern protection, control, and monitoring systems (SAS).
• ​Future-Proof Platform:​​ Modular design accommodates evolving sensor technologies and communication standards.
• ​Reduced Environmental Impact (Solid Insulation Option):​​ Eliminates SF6 usage and associated risks.