A Direct Replacement Solution for Legacy AIS Modernization

​Executive Summary:​​ Facing aging conventional Current Transformers (CTs) and Voltage Transformers (VTs) in Air-Insulated Substations (AIS)? Modernization can't mean rebuilding the substation. The CIT (Combined Instrument Transformer)​​ is engineered specifically as a robust, drop-in replacement for your legacy units. Preserving existing foundations and busbar connections, it delivers essential analog compatibility today while seamlessly enabling your phased digital future. Prioritizing ruggedness and field-proven reliability, it builds trust for replacing established technologies.

​The Challenge: Modernizing Mature AIS Infrastructure​
AIS assets form the backbone of global transmission and distribution networks. Thousands of sites rely on decades-old CTs and VTs nearing end-of-life. Replacing them presents unique challenges:

• ​High Cost of Structural Change:​​ Demolishing foundations, altering busbars, or expanding structures is prohibitively expensive and disruptive.
• ​Relay Compatibility Dependency:​​ Critical protection and metering schemes rely on established 5A/1A and 110V/100V analog inputs.
• ​Migration Strategy:​​ Immediate wholesale replacement with digital-only solutions is often impractical; a phased approach is essential.
• ​Trust Hurdle:​​ Legacy CTs/VTs have a proven track record. New technologies must demonstrate comparable reliability under harsh field conditions to gain acceptance.

​Direct Replacement, Dual Outputs, Proven Reliability​
This CIT is the answer, designed from the ground up as a direct, retrofittable upgrade for conventional CTs and VTs in AIS environments:

1. ​Direct "Drop-In" Retrofit Design (Core Enabler):​​
• ​Precision Footprint Matching:​​ Dimensions and mass are engineered to perfectly replicate the original CT/VT units being replaced.
• ​Identical Mounting & Busbar Interfaces:​​ Utilizes the existing foundation bolts and matches the existing busbar tap dimensions/configurations (e.g., clamp type, bolt holes). No cutting, welding, or busbar modifications required.
• ​Standard Terminal Box Placement:​​ Secondary connections terminate in locations familiar to technicians, positioned like the originals.
• ​Significant Installation Advantages:​​
• ​Radically Reduced Downtime:​​ Installation windows shrink from days to hours.
• ​Eliminated Civil Costs:​​ Avoids concrete work, structural modifications.
• ​Minimized Risk:​​ Simplified engineering, less complex lift plan, reduced potential for errors during busbar work.
2. ​Hybrid Output System: Supporting Today & Tomorrow:​​
• ​Legacy Analog Interface:​​
• ​Current Outputs:​​ Standard burden-compatible outputs: ​1A (5VA typical)​​ and ​5A (15VA or 30VA typical)​​ per protection/metering core.
• ​Voltage Outputs:​​ Standard ratio-compatible outputs: ​100V (Line-Neutral)​​ and ​110V (Line-Neutral)​, suitable for relays and meters. ​110V (Line-Line)​​ option available where required.
• ​Modern Digital Interface:​​
• Standards-Based: Digital output compliant with ​IEC 61850-9-2LE​ Sampled Values (SV) over Ethernet.
• Advanced Capabilities: Provides merged, synchronized, high-resolution sampled current and voltage data, enabling new possibilities for protection, control, and condition monitoring within digital substation architectures.
• ​Phased Migration Pathway:​​ Utilities can:
• ​Phase 1:​​ Connect existing analog protection/control systems to the CIT. Critical functions remain unchanged.
• ​Phase 2:​​ Route the digital SV stream to new Intelligent Electronic Devices (IEDs) or gateways for advanced applications or new bays.
• ​Phase 3:​​ Gradually decommission legacy IEDs as digital systems prove reliable, minimizing risk and spreading investment.
3. ​Ruggedized Design for Proven Field Reliability (Building Trust):​​
• ​Extreme Environment Ready:​​ Components selected and sealed to withstand temperature extremes (-40°C to +70°C operational), high humidity (IP67 ingress protection standard), salt fog (C5-M corrosion resistance), and severe pollution levels.
• ​Seismic Performance:​​ Designed to meet IEC 61869/IEEE C37 seismic requirements suitable for the installation zone.
• ​Advanced Insulation System:​​ Utilizes solid core insulation (e.g., SF6-free dry design with silicone shed composite housing, or SF6 gas) optimized for stability and long life under switching surges and temporary overvoltages (TOVs).
• ​Thermal & Overload Stability:​​ Generously rated primary conductors and secondary windings ensure performance under fault conditions and overload scenarios. Proven thermal stability test compliance.
• ​Reliability by Design:​​ Employs robust sensor technology (e.g., optimized Low-Power Coreless CTs/LPCTs, resistive/capacitive voltage dividers) with minimal active electronics. Focus on simplicity in critical paths.
• ​Validation Focus:​​ Extensive type testing (IEC 61869, IEEE C57.13) plus rigorous pre-deployment pilot testing in actual grid environments under varied operating conditions to build utility confidence and demonstrate operational equivalence to legacy technologies.
4. ​Technical Specifications Overview:​​

​Value Proposition: Reducing Risk & Cost for Grid Operators​

• ​Radically Lower Installation Cost & Time:​​ Eliminates structural modifications and complex busbar work. Commissioning faster.
• ​De-risked Modernization:​​ Maintains compatibility with existing, trusted protective relaying during transition. Proven analog reliability.
• ​Future-Proof Investment:​​ Built-in digital capabilities ensure readiness for digital substations without immediate obsolescence.
• ​Enhanced Resilience:​​ Rugged design offers longevity comparable to conventional CTs/VTs, minimizing future replacement cycles.
• ​Reduced Substation Footprint:​​ Replaces two conventional devices with one, improving buswork clarity and freeing space.
• ​Unified Data Source:​​ Single device provides synchronized current and voltage data, improving measurement correlation and enabling advanced analytics.