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

Wone Store

07/22/2025

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.

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

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.

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.

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.

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

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.

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.