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Combined Transformer (CT+PT) Technical Guide: Selection, Testing & Application for MV Metering & Protection (IEC 61869)
Meta Description: Comprehensive guide on combined transformers (CT+PT) for medium-voltage metering and protection. Covers construction, accuracy classes, selection methodology, testing procedures, and compliance with IEC 61869. Includes application examples, wiring practices, and troubleshooting for JLS series combined transformers in power distribution networks.
1. Introduction
Combined Transformers (CT+PT) integrate current and voltage transformers in a single enclosure, providing a compact, cost-effective solution for metering and protection in medium-voltage power distribution systems. Unlike separate CT and PT installations, combined transformers offer:
– Space savings: Single enclosure reduces switchgear footprint
– Cost reduction: Lower installation and maintenance costs
– Simplified wiring: Integrated terminals, reduced cable requirements
– Improved accuracy: Matched CT/PT characteristics for energy metering
– Reliability: Fewer connections, reduced failure points
Combined transformers are widely used in:
– Metering boxes: JLS series (CT+PT for revenue metering)
– Switchgear: RMU, ring main units, compact substations
– Distribution networks: Overhead, underground, pad-mounted
– Industrial plants: Motor metering, feeder metering
This guide systematically covers combined transformer construction, accuracy requirements, selection methodology, testing procedures, and application per IEC 61869-2 (CT), IEC 61869-3 (PT), and relevant national standards.
2. Construction & Insulation Types
2.1 Standard Combined Transformer Configuration
┌─────────────────────────────────────┐
│ Combined Transformer │
│ │
│ ┌─────────────────────────────┐ │
│ │ PT Section │ │
│ │ (Primary, Core, Secondary) │ │
│ └─────────────────────────────┘ │
│ │ │
│ ┌─────────────────────────────┐ │
│ │ CT Section │ │
│ │ (Window, Core, Secondary) │ │
│ └─────────────────────────────┘ │
│ │ │
│ ┌─────────────────────────────┐ │
│ │ Terminal Box │ │
│ │ (CT S1/S2, PT a/n, Ground) │ │
│ └─────────────────────────────┘ │
└─────────────────────────────────────┘
2.2 Insulation Types
| Type | Insulation Material | Voltage Range | Characteristics | Application |
|---|---|---|---|---|
| Cast Resin | Epoxy Resin | 10-36 kV | Dry-type, maintenance-free, fire-safe | Indoor, switchgear, metering boxes |
| Oil-Immersed | Mineral Oil + Paper | 10-36 kV | Traditional, high reliability, requires maintenance | Outdoor, overhead, older networks |
| SF6 Gas | SF6 Gas | 10-36 kV | Compact, lightweight, sealed | GIS, hybrid switchgear |
| Polymer (Dry) | Silicone/EPDM + Resin | 10-36 kV | Lightweight, pollution-resistant | Outdoor, polluted environments |
2.3 Cast Resin Combined Transformers (Most Common)
Construction:
– PT Core: Laminated silicon steel, epoxy-encapsulated windings
– CT Core: Laminated silicon steel or amorphous alloy, window-type primary
– Encapsulation: Vacuum-cast epoxy resin, UV-resistant
– Terminals: Stainless steel, sealed terminal box
– Grounding: Dedicated ground terminal, single-point grounding
Advantages:
– Maintenance-free (no oil, no pressure)
– Fire-resistant (no flammable materials)
– Compact size, lightweight
– Suitable for indoor/outdoor installation
– High reliability, long service life
Limitations:
– Limited to 36 kV maximum (standard)
– Sensitive to mechanical impact during transport
– Higher cost than oil-immersed (initial)
3. Accuracy Requirements
3.1 CT Accuracy Classes
| Class | Ratio Error (%) | Phase Angle Error (minutes) | Application |
|---|---|---|---|
| 0.2S | ±0.2 (1-120% I_n) | ±10 | Revenue metering, high accuracy |
| 0.5S | ±0.5 (1-120% I_n) | ±30 | General metering, standard |
| 1 | ±1.0 (5-120% I_n) | ±60 | Indication, monitoring |
| 5P | ±1.0 (5P), ±3.0 (10P) | ±120 (5P), ±240 (10P) | Protection |
3.2 PT Accuracy Classes
| Class | Ratio Error (%) | Phase Angle Error (minutes) | Application |
|---|---|---|---|
| 0.2 | ±0.2 | ±10 | Revenue metering, high accuracy |
| 0.5 | ±0.5 | ±30 | General metering, standard |
| 1 | ±1.0 | ±60 | Indication, monitoring |
| 3P | ±3.0 | ±120 | Protection |
3.3 Combined Accuracy for Energy Metering
Total Error:
Total Error = √(CT_error² + PT_error² + Meter_error²)
Example:
CT: 0.5S (±0.5%)
PT: 0.5 (±0.5%)
Meter: 0.5S (±0.5%)
Total Error = √(0.5² + 0.5² + 0.5²) = ±0.87%
Standard Combinations:
| Application | CT Class | PT Class | Total Error | Standard |
|————|———|———|————|———-|
| Revenue Metering | 0.2S | 0.2 | ±0.35% | IEC 62053 Class 0.5S |
| General Metering | 0.5S | 0.5 | ±0.87% | IEC 62053 Class 1.0 |
| Protection | 5P | 3P | ±4.5% | IEC 60255 |
4. Selection Methodology
4.1 Selection Parameters
| Parameter | Description | Standard Reference |
|---|---|---|
| Rated Voltage (U_r) | System maximum voltage (Um) | IEC 61869-3 |
| Rated Primary Current (I_pn) | Load current range | IEC 61869-2 |
| Rated Secondary Current | 1A or 5A | IEC 61869-2 |
| Rated Secondary Voltage | 100V or 100/√3 V | IEC 61869-3 |
| Rated Burden (CT/PT) | VA per core | IEC 61869-2/3 |
| Accuracy Class (CT/PT) | Metering/Protection | IEC 61869-2/3 |
| Short-Time Current | Thermal withstand (1s/3s) | IEC 61869-2 |
| Dynamic Current | Mechanical withstand (peak) | IEC 61869-2 |
| Insulation Level | BIL, power-frequency withstand | IEC 60071 |
4.2 CT Ratio Selection
Rule: CT rated primary current should be 1.2-1.5× maximum load current.
| Load Current (A) | Recommended CT Ratio | Notes |
|---|---|---|
| 10-30 | 30/5, 50/5 | Small load |
| 30-60 | 50/5, 75/5 | Medium load |
| 60-100 | 100/5, 150/5 | Standard feeder |
| 100-200 | 150/5, 200/5 | Large feeder |
| 200-400 | 300/5, 400/5 | Main feeder |
| 400-600 | 500/5, 600/5 | Substation main |
4.3 PT Ratio Selection
Rule: PT rated primary voltage must match system maximum voltage (Um).
| System Voltage (kV) | Um (kV) | PT Ratio | Secondary Voltage |
|---|---|---|---|
| 10 | 12 | 10000/√3 : 100/√3 | 100/√3 V |
| 20 | 24 | 20000/√3 : 100/√3 | 100/√3 V |
| 35 | 40.5 | 35000/√3 : 100/√3 | 100/√3 V |
4.4 Burden Selection
Rule: Combined transformer rated burden must exceed total connected burden.
Typical Burdens:
| Device | Burden (VA) | Power Factor |
|——–|————|————-|
| Electronic Meter | 1-2 VA | 0.8-1.0 |
| Digital Relay | 0.5-1 VA | 0.8-1.0 |
| Indication Meter | 2-5 VA | 0.8 |
| Total Connected | 5-10 VA | 0.8 |
Selection: Rated burden ≥ Total connected burden × 1.25
4.5 Selection Decision Tree
Determine application:
│
├── Revenue Metering
│ ├── CT: 0.2S, PT: 0.2
│ ├── Ratio: CT (1.2-1.5× load), PT (matches Um)
│ └── Burden: ≥ 5 VA
│
├── General Metering
│ ├── CT: 0.5S, PT: 0.5
│ ├── Ratio: CT (1.2-1.5× load), PT (matches Um)
│ └── Burden: ≥ 5 VA
│
└── Protection
├── CT: 5P, PT: 3P
├── Ratio: CT (matches fault current), PT (matches Um)
└── Burden: ≥ 10 VA
5. Wiring & Installation Practices
5.1 Standard Wiring Diagram
HV Bus
│
├── PT Primary (A, N)
│
├── CT Primary (Window, cable passes through)
│
├── Terminal Box
│ ├── CT Secondary: S1, S2 → Meter/Relay
│ ├── PT Secondary: a, n → Meter/Relay
│ └── Ground: Dedicated terminal
│
└── Meter/Relay
├── Current Input: I_a (from CT S1/S2)
├── Voltage Input: U_a (from PT a/n)
└── Ground: Single-point grounding
5.2 Installation Requirements
| Requirement | Description | Standard Reference |
|---|---|---|
| Mounting | Vertical, secure bracket | Manufacturer spec |
| Clearance | Air insulation distance per Um | IEC 60071 |
| Cable Routing | CT/PT secondary cables separate from power cables | IEC 61869 |
| Grounding | Single-point grounding at meter/relay panel | IEC 61869 |
| Torque | Terminal connections per manufacturer spec | Manufacturer spec |
| Sealing | Terminal box IP54 or better | IEC 60529 |
5.3 Common Installation Errors
| Error | Consequence | Correction |
|---|---|---|
| CT secondary open circuit | High voltage, insulation damage, safety hazard | Never open CT secondary, short before disconnecting |
| PT secondary short circuit | High current, overheating, damage | Install fuse/MCB on PT secondary |
| Multiple grounding points | Ground loop, measurement error | Ground at meter/relay panel only |
| Incorrect polarity | Reverse power, metering error | Verify polarity during installation |
| Exceeding rated burden | Accuracy degradation, overheating | Verify total burden ≤ rated burden |
6. Testing & Commissioning
6.1 Factory Acceptance Tests (FAT)
| Test | Purpose | Standard Reference |
|---|---|---|
| CT Ratio Test | Verify ratio for each core | IEC 61869-2 |
| PT Ratio Test | Verify ratio | IEC 61869-3 |
| CT Polarity Test | Verify polarity | IEC 61869-2 |
| PT Polarity Test | Verify polarity | IEC 61869-3 |
| Insulation Test | Verify withstand voltage | IEC 61869-2/3 |
| Accuracy Test | Verify accuracy class | IEC 61869-2/3 |
| Burden Test | Verify rated burden | IEC 61869-2/3 |
6.2 Site Acceptance Tests (SAT)
| Test | Method | Acceptance Criteria |
|---|---|---|
| Visual Inspection | Check for shipping damage | No damage, proper installation |
| Insulation Resistance | Megger test | > 1000 MΩ |
| CT/PT Ratio Test | Primary/secondary injection | Within ±1% of factory |
| Polarity Test | DC method or relay tester | Correct polarity |
| Secondary Injection | Verify meter/relay operation | Accurate measurement |
| Grounding Test | Verify single-point grounding | No ground loop |
6.3 Periodic Maintenance
| Test | Interval | Acceptance Criteria |
|---|---|---|
| Visual Inspection | Annual | No damage, clean |
| Insulation Resistance | 3-6 years | > 1000 MΩ |
| CT/PT Ratio Test | 6-10 years | Within ±1% of baseline |
| Accuracy Test | 6-10 years | Within accuracy class |
| Terminal Torque Check | Annual | Per manufacturer spec |
7. Standards & References
7.1 IEC Standards
| Standard | Title | Relevant Sections |
|---|---|---|
| IEC 61869-2 | Current Transformers | §5 (Performance), §6 (Tests) |
| IEC 61869-3 | Voltage Transformers | §5 (Performance), §6 (Tests) |
| IEC 62053 | Electricity Metering | §21 (Class 0.2S, 0.5S) |
| IEC 60071 | Insulation Coordination | §1 (Definitions) |
7.2 National Standards (China)
| Standard | Title |
|---|---|
| GB 20840.7 | Combined Instrument Transformers |
| DL/T 725 | Power System CT/PT Technical Specifications |
| JB/T 10441 | Combined Transformers for MV |
8. Engineering FAQ
Q1: Can I use a combined transformer for protection?
A: Yes, but ensure CT/PT accuracy classes match protection requirements (typically 5P/3P). Combined transformers for protection must withstand short-time currents and have adequate burden capacity. Verify transient performance (CT) and TVR (PT) for high-speed protection.
Q2: How do I verify combined transformer accuracy in the field?
A:
– Use a CT/PT tester to measure ratio and phase angle
– Compare with factory/base values
– Verify within accuracy class limits (0.5S: ±0.5%, 0.2S: ±0.2%)
– Test at multiple burden points (25%, 50%, 100% rated burden)
Q3: What happens if the CT secondary is open-circuited?
A: An open-circuited CT secondary causes:
– High voltage (hundreds to thousands of volts) across secondary terminals
– Core saturation, overheating
– Insulation breakdown, fire hazard
– Safety hazard (electric shock)
Solution: Never open CT secondary. Short secondary terminals before disconnecting.
Q4: How do I select the correct CT ratio for a combined transformer?
A:
– Calculate maximum load current
– Select CT ratio: I_pn = 1.2-1.5 × I_load_max
– Example: Load = 150A → Select 200/5A CT
– Verify accuracy at minimum load (≥ 5% I_pn for 0.5S, ≥ 1% I_pn for 0.2S)
Q5: Can I replace separate CT and PT with a combined transformer?
A: Yes, combined transformers can replace separate CT and PT installations if:
– Voltage and current ratings match
– Accuracy classes meet metering/protection requirements
– Space and mounting dimensions match
– Burden capacity is sufficient
Benefits: Space savings, cost reduction, simplified wiring.
9. Conclusion
Combined transformers (CT+PT) provide a compact, cost-effective solution for metering and protection in medium-voltage power distribution systems. Proper selection, installation, and testing are critical to ensure accuracy, reliability, and safety.
Key selection principles:
– Accuracy: 0.2S/0.2 for revenue metering, 0.5S/0.5 for general metering, 5P/3P for protection
– Ratio: CT (1.2-1.5× load), PT (matches Um)
– Burden: Rated burden ≥ Total connected burden × 1.25
– Installation: Single-point grounding, correct polarity, torque verification
– Testing: Verify ratio, accuracy, insulation, grounding during commissioning
Design checklist:
☐ Application determined (metering, protection)
☐ CT/PT accuracy classes selected (0.2S/0.2, 0.5S/0.5, 5P/3P)
☐ CT/PT ratios selected (matches load, Um)
☐ Rated burden verified (≥ total connected burden × 1.25)
☐ Insulation level verified (matches Um, BIL)
☐ Installation practices specified (grounding, polarity, torque)
☐ Testing procedures defined (ratio, accuracy, insulation)
☐ Maintenance schedule planned
Technical Reference: IEC 61869-2/3, IEC 62053, GB 20840.7, DL/T 725
Product Reference: Duomatech JLS series (cast-resin combined transformers, 10-36 kV) — optimized for MV metering and protection applications