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Bushing CT & Circuit Breaker CT: Design, Selection & Integration Guide for GIS/AIS Substations (IEC 61869-2)
Meta Description: Comprehensive guide on bushing current transformers and circuit breaker integrated CTs. Covers design principles, insulation coordination, accuracy classes, GIS/AIS integration, transient performance, and selection per IEC 61869-2 and IEEE C57.13. Includes installation guidelines, testing procedures, and comparison with standalone CTs.
1. Introduction
Bushing Current Transformers (CTs) and Circuit Breaker (CB) integrated CTs are specialized instrument transformers designed to be mounted directly on high-voltage bushings or circuit breaker interrupter units. Unlike standalone window or wound-primary CTs, these integrated designs eliminate the need for separate CT housings, reducing substation footprint, cost, and insulation complexity.
These CTs are widely used in:
– Gas-Insulated Switchgear (GIS) — Compact, maintenance-free design
– Air-Insulated Switchgear (AIS) — Space-saving for high-voltage applications
– Circuit Breakers — Integrated protection and metering
– Power Transformers — Winding bushing CTs for differential protection
This guide systematically covers bushing CT and CB CT design principles, insulation coordination, accuracy characteristics, integration requirements, and selection methodology per IEC 61869-2:2012 and IEEE C57.13 standards.
2. Bushing CT Design and Construction
2.1 Basic Configuration
A bushing CT consists of:
High-Voltage Conductor (Bushing Center)
│
│ ┌─────────────────────────────┐
│ │ Insulation (Oil/Paper, │
│ │ Resin, or SF6) │
│ │ │
│ │ ┌───────────────────────┐ │
│ │ │ CT Core(s) │ │
│ │ │ (Toroidal, multiple) │ │
│ │ │ Mounted on bushing │ │
│ │ └───────────────────────┘ │
│ │ │
│ └─────────────────────────────┘
│
Secondary Terminals (at ground potential)
2.2 Multiple Core Configuration
Bushing CTs typically incorporate multiple independent cores for different applications:
| Core | Accuracy Class | Application | Location |
|---|---|---|---|
| Core 1 | 0.2S or 0.5 | Revenue metering | Top (near HV) |
| Core 2 | 0.5 or 1.0 | General metering/SCADA | Top-Middle |
| Core 3 | 5P20 or 5P30 | Overcurrent/Distance protection | Middle |
| Core 4 | TPY or 5P30 | Differential protection | Bottom (near ground) |
| Core 5 | 5P20 | Backup protection | Bottom |
2.3 Insulation System
The bushing CT shares the main bushing insulation system:
| Insulation Type | Application | Advantages | Disadvantages |
|---|---|---|---|
| Oil-Impregnated Paper (OIP) | 123-550 kV | Proven reliability, self-healing | Maintenance intensive, oil leaks |
| Resin-Impregnated Paper (RIP) | 123-245 kV | Maintenance-free, no oil | Limited to 245 kV |
| Resin-Impregnated Synthetic (RIS) | 72.5-170 kV | Light weight, compact | Higher cost |
| SF6 Gas | GIS applications | Compact, maintenance-free | SF6 handling, environmental concerns |
2.4 Core Mounting Methods
| Method | Description | Application |
|---|---|---|
| Slip-On | Core slips over bushing during assembly | Factory installation |
| Clamp-On | Split-core clamps around bushing | Retrofit, maintenance |
| Embedded | Core cast into resin bushing | GIS, compact design |
| Wound Primary | Secondary winding on separate core, primary is bushing conductor | Standard bushing CT |
3. Circuit Breaker Integrated CT
3.1 Configuration Types
3.1.1 Live Tank CT
┌─────────────────────────────────┐
│ Circuit Breaker Tank │
│ (at line potential) │
│ │
│ ┌─────────────────────────┐ │
│ │ Interrupter Unit │ │
│ │ (Arc chamber) │ │
│ └─────────────────────────┘ │
│ │
│ ┌─────────────────────────┐ │
│ │ CT Core(s) │ │
│ │ (at line potential) │ │
│ └─────────────────────────┘ │
│ │
└─────────────────────────────────┘
│
Insulating Support Column
│
┌─────────┴─────────┐
│ CT Terminal Box │
│ (at ground) │
│ via fiber optic │
│ or CT cable │
└───────────────────┘
3.1.2 Dead Tank CT
┌─────────────────────────────────┐
│ Circuit Breaker Tank │
│ (at ground potential) │
│ │
│ ┌─────────────────────────┐ │
│ │ Interrupter Unit │ │
│ └─────────────────────────┘ │
│ │
│ ┌─────────────────────────┐ │
│ │ CT Core(s) │ │
│ │ (directly accessible) │ │
│ └─────────────────────────┘ │
│ │
└─────────────────────────────────┘
│
CT Terminal Box (at ground)
3.2 Live Tank vs. Dead Tank CT
| Parameter | Live Tank CT | Dead Tank CT |
|---|---|---|
| CT Potential | Line potential | Ground potential |
| Insulation | Requires insulation to ground | Directly grounded |
| Signal Transmission | Fiber optic or CT cable | Direct cable |
| Maintenance | Complex (de-energize required) | Simple (accessible) |
| Cost | Higher | Lower |
| Application | AIS, GIS | GIS, Dead-tank AIS |
3.3 SF6 Circuit Breaker CT
Standard Configuration:
– Voltage Range: 72.5 kV to 550 kV
– Insulation: SF6 gas at 0.1-0.3 MPa
– CT Cores: 2-6 independent cores
– Secondary Current: 1A (standard for GIS)
– Accuracy Class: 0.2S to 5P30 per core
4. Technical Specifications
4.1 Standard Ratings
| Parameter | Bushing CT | CB Integrated CT |
|---|---|---|
| Voltage Range | 12 kV to 550 kV | 72.5 kV to 550 kV |
| Rated Primary Current | 200 A to 4000 A | 1200 A to 4000 A |
| Rated Secondary Current | 1A (GIS), 5A (AIS) | 1A (standard) |
| Rated Burden | 5 VA to 30 VA per core | 10 VA to 30 VA per core |
| Accuracy Class | 0.2S to 5P30 | 0.2S to TPY |
| Thermal Current (1s) | 40 kA to 100 kA | 40 kA to 80 kA |
| Dynamic Current | 100 kA to 250 kA | 100 kA to 200 kA |
4.2 Insulation Levels per IEC 60076-3
| System Voltage (Um) | Power Frequency Withstand (kV, 1min) | Lightning Impulse Withstand (kV peak) |
|---|---|---|
| 72.5 kV | 140 | 350 |
| 123 kV | 230 | 550 |
| 145 kV | 275 | 650 |
| 170 kV | 325 | 750 |
| 245 kV | 460 | 1050 |
| 362 kV | 630 | 1425 |
| 550 kV | 740 | 1800 |
4.3 CT Core Allocation (Typical)
| Application | Voltage Level | Core 1 | Core 2 | Core 3 | Core 4 | Core 5 |
|---|---|---|---|---|---|---|
| Transformer Protection | 123-245 kV | 5P30 (Diff) | 5P30 (Diff) | 5P20 (Backup) | 0.5 (Metering) | – |
| Line Protection | 123-245 kV | 5P20 (Distance) | 5P20 (Distance) | 5P20 (Backup) | 0.5 (Metering) | – |
| Busbar Protection | 123-245 kV | 5P30 (Diff) | 5P30 (Diff) | 5P20 (Backup) | 0.5 (Metering) | – |
| Generator Protection | 15-24 kV | TPY (Diff) | 5P30 (Backup) | 0.2S (Metering) | 0.5 (Excitation) | – |
5. Selection Methodology
5.1 Step-by-Step Selection Process
Step 1: Determine System Parameters
- System voltage (U_m)
- Rated primary current (I_n)
- Maximum fault current (I_sc)
- X/R ratio of fault circuit
- Protection scheme requirements
- Metering requirements
- Space constraints
Step 2: Select CT Configuration
| Application | Recommended Configuration |
|---|---|
| GIS Substation | Dead tank CT or embedded bushing CT |
| AIS Substation (Live Tank) | Live tank CB CT with fiber optic output |
| AIS Substation (Dead Tank) | Dead tank CB CT with direct cable |
| Transformer Bushing | Bushing CT (OIP or RIP) |
| Generator Terminal | Wound-primary CT or busbar CT |
Step 3: Determine Number of Cores
| Application | Minimum Cores | Typical Cores |
|---|---|---|
| Line Protection | 2 (Distance + Backup) | 3-4 |
| Transformer Protection | 2 (Diff + Backup) | 3-4 |
| Busbar Protection | 2 (Diff + Backup) | 3-4 |
| Metering Only | 1 | 1-2 |
| Generator Protection | 2 (Diff + Backup) | 3-4 |
Step 4: Verify Thermal and Dynamic Withstand
I_th_rated ≥ I_sc_max (maximum symmetrical fault current)
I_dyn_rated ≥ 2.5 × I_sc_max (peak dynamic current)
Step 5: Select Accuracy Class per Core
| Core Function | Recommended Class | Minimum ALF |
|---|---|---|
| Differential Protection | 5P30 or TPY | 30 |
| Distance Protection | 5P20 | 20 |
| Overcurrent Protection | 5P20 | 20 |
| Revenue Metering | 0.2S | – |
| General Metering | 0.5 | – |
| SCADA/Indication | 1.0 | – |
5.2 Bushing CT vs. Standalone CT Comparison
| Parameter | Bushing CT | Standalone CT |
|---|---|---|
| Space Required | Minimal (integrated) | Significant (separate) |
| Insulation | Shared with bushing | Independent |
| Cost | Lower at high voltage | Higher at high voltage |
| Flexibility | Fixed ratio, fixed cores | Changeable ratio |
| Maintenance | Complex (bushing replacement) | Simple |
| Replacement | Requires bushing replacement | CT only |
| Application | 123 kV and above | All voltages |
6. Integration in GIS/AIS Substations
6.1 GIS Integration
Design Features:
– CT cores embedded in SF6 tank or busbar
– Secondary terminals in grounded compartment
– Fiber optic or direct cable connection to relay panel
– Standardized core allocation per IEC 61869-2
Advantages:
– Compact footprint (50-70% smaller than AIS)
– Maintenance-free operation (20-30 years)
– Excellent transient performance
– Weather-independent
Standard Configurations:
| GIS Type | CT Configuration | Voltage Range |
|---|---|---|
| Compact GIS | Embedded CT in busbar | 72.5-170 kV |
| Standard GIS | Dead tank CT in CB module | 123-550 kV |
| Hybrid GIS | Bushing CT on outgoing | 123-245 kV |
6.2 AIS Integration
Design Features:
– CT mounted on CB tank (live tank) or in grounded tank (dead tank)
– Secondary signal transmitted via fiber optic (live tank) or direct cable (dead tank)
– Standardized core allocation per utility specification
Advantages:
– Proven technology
– Easier maintenance access
– Lower initial cost at lower voltages
– Flexible configuration
6.3 Signal Transmission Methods
| Method | Application | Advantages | Disadvantages |
|---|---|---|---|
| CT Cable (Cu) | Dead tank, short distance | Simple, reliable | Burden, voltage drop |
| Fiber Optic | Live tank, long distance | No burden, EMI immune | Complex, expensive |
| Rogowski Coil + Electronic Unit | Digital substation (IEC 61850-9-2) | Wide bandwidth, digital output | Requires merging unit |
7. Testing Procedures
7.1 Factory Tests per IEC 61869-2
| Test | Purpose | Acceptance Criteria |
|---|---|---|
| Turns Ratio Test | Verify ratio for each core | Within ±0.5% of nameplate |
| Polarity Test | Verify terminal markings | Correct polarity |
| Insulation Test | Verify insulation integrity | Withstand voltage, no breakdown |
| Excitation Test | Verify knee-point voltage (protection cores) | V_k ≥ specified |
| Composite Error Test | Verify accuracy class | Error ≤ class limit |
| Partial Discharge Test | Verify insulation quality | < 5 pC at 1.1Um/√3 |
| Lightning Impulse Test | Verify impulse withstand | No breakdown |
| Thermal Test | Verify thermal withstand | No damage, ratio unchanged |
7.2 Field Commissioning Tests
| Test | Method | Acceptance Criteria |
|---|---|---|
| Visual Inspection | Check for shipping damage | No damage, proper mounting |
| Insulation Resistance | Megger test | > 1000 MΩ |
| Ratio Test | Primary/secondary injection | Within ±1% of factory |
| Polarity Test | Battery and voltmeter | Correct polarity |
| Excitation Test | Secondary voltage injection | V_k ≥ 90% of factory |
| Burden Measurement | Measure actual burden | ≤ rated burden |
| Fiber Optic Test (if applicable) | Optical power measurement | Within specified range |
| Secondary Injection | Verify relay operation | Relay operates correctly |
7.3 Periodic Maintenance Tests
| Test | Interval | Acceptance Criteria |
|---|---|---|
| SF6 Gas Pressure (GIS) | Annual | Within specified range |
| Insulation Resistance | 3-6 years | > 1000 MΩ |
| Ratio Test | 6-10 years | Within ±1% of factory |
| Excitation Test | 10 years or after fault | V_k ≥ 80% of factory |
| DGA (Oil CTs) | Annual or after fault | Per IEC 60599 |
| Partial Discharge | 6-10 years | < 10 pC |
8. Common Issues and Troubleshooting
8.1 Bushing CT Issues
| Issue | Symptoms | Cause | Solution |
|---|---|---|---|
| Partial Discharge | PD activity, noise | Insulation defect, contamination | Replace bushing |
| Moisture Ingress | Increasing tan δ, low IR | Seal failure, condensation | Dry out, replace seal |
| Core Saturation | Relay maloperation, waveform distortion | Excessive burden, low ALF | Reduce burden, increase ALF |
| Open Secondary | High voltage, arcing | Loose connection, broken wire | Tighten, replace wire |
| Ratio Change | Metering error, relay under-reach | Shorted turns, tap issue | Replace CT core |
8.2 CB Integrated CT Issues
| Issue | Symptoms | Cause | Solution |
|---|---|---|---|
| Fiber Optic Failure | No signal, communication error | Broken fiber, dirty connector | Clean/replace fiber |
| SF6 Leak | Low pressure alarm, insulation failure | Seal degradation, damage | Repair leak, refill SF6 |
| Core Heating | Abnormal temperature rise | Excessive burden, eddy currents | Reduce burden, check wiring |
| Vibration | Noise, mechanical stress | Loose mounting, resonance | Tighten, add damping |
9. Standards and References
9.1 IEC Standards
| Standard | Title | Relevant Sections |
|---|---|---|
| IEC 61869-2 | Current Transformers | §5 (Performance), §6 (Tests) |
| IEC 62271-1 | High-Voltage Switchgear | §5.103 (CT requirements) |
| IEC 62271-200 | AC Metal-Enclosed Switchgear | §6 (CT integration) |
| IEC 60076-3 | Power Transformers – Insulation | §3 (Insulation levels) |
9.2 IEEE Standards
| Standard | Title | Relevant Sections |
|---|---|---|
| IEEE C57.13 | Instrument Transformers | §3 (Requirements) |
| IEEE C37.09 | CT Selection for Protective Relaying | Full document |
| IEEE C57.13.1 | Requirements for CTs | §4 (Bushing CTs) |
10. Engineering FAQ
Q1: Can bushing CTs be replaced without replacing the bushing?
A: Generally no. Bushing CTs are integral to the bushing insulation system. Replacement requires:
– De-energizing the equipment
– Removing the bushing
– Replacing the bushing (with new CT cores)
– Reinstalling and testing
For retrofit applications, clamp-on or window CTs can be installed externally, but with reduced accuracy and sensitivity.
Q2: Why do GIS substations typically use 1A secondary current?
A: 1A secondary current is preferred in GIS because:
– Reduced burden: I²R losses are 25× lower than 5A
– Longer cable runs: Secondary cables from GIS to relay panel can be 100-200m
– Compact design: Smaller CT cores fit in confined GIS spaces
– Digital integration: Modern IEDs accept 1A inputs
Q3: How do I verify CT polarity in a live tank CB?
A: For live tank CBs:
– Use battery and voltmeter method at secondary terminals
– Verify polarity matches relay requirements (subtractive)
– Use primary injection test with relay to verify direction
– Check relay vector display under load
Q4: What is the typical lifespan of a bushing CT?
A:
– OIP bushing CT: 25-40 years (with oil maintenance)
– RIP/RIS bushing CT: 30-40 years (maintenance-free)
– GIS embedded CT: 30-40 years (SF6 monitored)
– Key aging indicators: Increasing PD, tan δ, oil DGA
Q5: Can I use bushing CTs for differential protection?
A: Yes, but ensure:
– CT characteristics match on all windings (same ratio, class, ALF)
– CTs are positioned at transformer bushings (line side and neutral side)
– Knee-point voltages are matched
– Transient performance is compatible (TPY for EHV systems)
11. Conclusion
Bushing CTs and Circuit Breaker integrated CTs are essential components in modern high-voltage substations, offering compact design, reduced footprint, and integrated protection and metering capabilities. Their design shares the main insulation system of the bushing or CB, providing excellent insulation performance and reliability.
Key selection principles:
– Match configuration to substation type: GIS (dead tank), AIS (live/dead tank)
– Allocate cores per application: Protection, metering, SCADA
– Verify thermal and dynamic withstand: Match system fault levels
– Consider signal transmission: CT cable (dead tank) vs. fiber optic (live tank)
– Plan for maintenance: Access, testing, and replacement requirements
Design checklist:
☐ System voltage and fault level verified
☐ CT configuration selected (bushing, live tank, dead tank)
☐ Number and type of cores allocated
☐ Accuracy classes specified per core
☐ Thermal and dynamic withstand verified
☐ Signal transmission method selected
☐ Integration with GIS/AIS verified
☐ Testing and maintenance procedures defined
Technical Reference: IEC 61869-2:2012, IEC 62271-1/200, IEEE C57.13-2016, IEEE C37.09-2007
Product Reference: Duomatech LZZBJ9 series (cast-resin standalone CTs), LJWD series (oil-immersed CTs) — for applications requiring standalone CTs