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IEC 61869-4-2013 — Instrument Transformers — Combined Transformers
Key Insight: IEC 61869-4-2013 is the dedicated standard for combined instrument transformers, specifying design, testing, and application requirements for integrating current transformer (CT) and voltage transformer (VT) functions within a single insulation enclosure.
1. Scope and Overview of Combined Transformers
IEC 61869-4-2013 is Part 4 of the IEC 61869 instrument transformer series, specifically addressing combined instrument transformers. A combined transformer integrates CT and VT elements within the same insulation enclosure (typically SF₆ gas-filled or oil-immersed), sharing a common high-voltage conductor and insulation system to save installation space and reduce costs.
The standard applies to combined transformers with rated voltages of 72.5 kV and above, covering electrical performance, insulation requirements, accuracy classes, temperature rise limits, and electromagnetic compatibility. Corrigendum cor1-2014 primarily corrected partial discharge measurement procedures and nameplate marking clauses — care should be taken to reference the corrected version.
Technical Note: The core design challenge for combined transformers is electromagnetic coupling interference between CT and VT elements. Under high primary current, the strong magnetic field generated by the CT winding induces interference voltage in the VT winding, potentially compromising voltage measurement accuracy. The standard specifies dedicated shielding and layout requirements to mitigate this effect.
2. Accuracy Classes and Error Characteristics
A combined transformer must simultaneously satisfy the accuracy requirements of both CT and VT. The standard defines multiple accuracy classes ranging from metering grades (0.1, 0.2S) to protection grades (5P, 10P).
2.1 Current Transformer Errors
CT errors include current (ratio) error and phase displacement. In a combined structure, the presence of VT windings may alter the magnetic field distribution around the primary conductor, causing CT errors to deviate from expected values. Proper magnetic shielding (e.g., high-permeability alloy shields) is required during design to minimize this effect.
2.2 Voltage Transformer Errors
VT voltage error and phase displacement are primarily influenced by no-load current and leakage impedance. Combined transformers typically employ capacitive voltage divider (CVT) or inductive (IVT) designs. CVT designs are more common in combined transformers since the capacitor stack can simultaneously serve as insulation support.
| Accuracy Class | Application | Current Error Limit (±%) | Phase Displacement Limit (±minutes) | Combined Design Impact |
|---|---|---|---|---|
| 0.2S | Precision energy metering | 0.2 | 10 | Extra magnetic shielding |
| 0.5S | Boundary energy metering | 0.5 | 15 | Standard shielding |
| 1.0 | Distribution metering | 1.0 | 30 | Basic shielding |
| 5P | Overcurrent protection | 1.0 | 60 | Saturation characteristic |
| 10P | Earth fault protection | 3.0 | — | Remanence focus |
3. Engineering Design Practice and Insulation Coordination
Engineering Experience: When configuring combined transformers in GIS substations, SF₆ gas-insulated combined transformers are preferred. Compared to oil-immersed designs, SF₆ solutions offer advantages in partial discharge performance, maintenance convenience, and fire safety. However, SF₆ gas pressure variation compensation for insulation performance must be considered.
Insulation Design: The main insulation of a combined transformer must withstand the superimposed electric field stress of both CT and VT windings. The standard requires lightning impulse (LI), switching impulse (SI), and power-frequency voltage withstand tests. Altitude correction and pollution class considerations should follow IEC 60071-1.
Partial Discharge Measurement: Partial discharge (PD) level is a critical indicator of insulation quality. The standard specifies that at 1.2 Um/√3 voltage, PD levels must not exceed 10 pC (GIS type) or 20 pC (open-terminal type). Corrigendum cor1-2014 clarified test circuit connections and background noise subtraction methods.
Temperature Rise and Thermal Management: The combined structure causes CT and VT winding heat to accumulate. The standard specifies temperature rise limits (coil ≤ 65 K, core ≤ 50 K). For high-current applications (≥2000 A), forced air cooling or enlarged oil/SF₆ convection paths are recommended.
Common Design Deficiency: Open-circuiting the CT secondary winding in a combined transformer generates dangerously high voltage—a risk amplified by the confined space in combined designs. Overvoltage protection devices (bidirectional TVS diodes or varistors) must be installed on CT secondary circuits, and their operation must be verified during factory testing. Nameplates must clearly indicate both CT and VT ratios to prevent misconnection during maintenance.
4. Frequently Asked Questions
Q1: What advantages do combined transformers offer over separate CT+VT installations?
A: Key advantages include reduced installation space (approximately 40% footprint reduction), lower overall cost (shared insulation and enclosure), fewer sealing points reducing oil/gas leak risk, and simplified primary connections. Limitations include reduced flexibility after fixed CT/VT ratios are chosen and the need for complete replacement if either element fails.
Q2: Can a combined transformer simultaneously serve metering and protection needs?
A: Yes. Combined transformers typically feature multiple secondary windings: a high-accuracy winding (0.2S class) for metering and one or two protection-grade windings (5P or 10P class) for relaying. Each winding uses independent cores and shielding for electrical isolation and magnetic decoupling.
Q3: What were the main corrections in cor1-2014?
A: The corrigendum addressed: corrected detailed wiring diagrams for partial discharge test circuits, clarification of combined transformer nameplate marking format, and adjustment of ambient temperature correction factors for temperature rise tests. Users should reference the consolidated version incorporating the corrigendum.
Q4: What considerations apply to SF₆-insulated combined transformers in low-temperature environments?
A: SF₆ gas may liquefy at low temperatures (below -30°C), degrading insulation performance. In cold regions, low-temperature SF₆ mixtures (with N₂ or CF₄) or heating devices to maintain gas compartment temperature should be specified. The standard’s annex provides SF₆ gas pressure-temperature characteristic curves for design reference.
