Article Content
5. Burden Calculation
The burden of a current transformer (CT) is the total impedance presented to its secondary winding, comprising connected devices (relays, meters, etc.) and wiring resistance. Accurate burden calculation ensures CTs operate within accuracy limits specified by standards such as IEC 61869 or IEEE C57.13.
Burden is typically expressed in volt-amperes (VA) at a standard secondary current (usually 1 A or 5 A). The total burden \( Z_b \) is calculated as:
\[
Z_b = Z_{\text{lead}} + Z_{\text{device}}
\]
Where:
- \( Z_{\text{lead}} = 2 \rho \frac{L}{A} \) (round-trip resistance of leads)
- \( \rho \) = resistivity of copper (~0.01724 Ω·mm²/m at 20°C)
- \( L \) = one-way lead length (m)
- \( A \) = cross-sectional area of conductor (mm²)
- \( Z_{\text{device}} \) = impedance of connected instruments
Example: For a 5 A CT with 20 m lead length (one way), 2.5 mm² copper wire, and a relay burden of 0.2 Ω:
\[
Z_{\text{lead}} = 2 \times 0.01724 \times \frac{20}{2.5} = 0.276\ \Omega
\]
\[
Z_b = 0.276 + 0.2 = 0.476\ \Omega
\]
\[
\text{Burden (VA)} = I_s^2 \times Z_b = 5^2 \times 0.476 = 11.9\ \text{VA}
\]
Thus, a CT rated for at least 15 VA would be selected to provide margin.
| Parameter | Symbol | Typical Value | Unit |
|---|---|---|---|
| Secondary Current | \( I_s \) | 1 or 5 | A |
| Lead Length (one way) | \( L \) | 10–100 | m |
| Conductor Area | \( A \) | 1.5, 2.5, 4 | mm² |
| Relay/Meter Burden | \( Z_{\text{device}} \) | 0.05–0.5 | Ω |
| Total Burden (VA) | \( S_b \) | 2.5–30 | VA |
Exceeding the rated burden causes CT saturation, leading to inaccurate measurements and potential relay misoperation. Always include a 20–25% safety margin in burden estimation.
6. Short-Circuit Considerations
Current transformers must withstand high short-circuit currents without mechanical damage or performance degradation. During faults, primary currents can reach 20–100 times nominal values, inducing large electromagnetic forces and thermal stress.
Two key parameters define short-circuit capability:
- Thermal Withstand Current (\( I_{th} \)): Maximum RMS current the CT can carry for 1 or 3 seconds without exceeding temperature limits.
- Dynamic Withstand Current (\( I_{dyn} \)): Peak current the CT can endure without mechanical failure due to electrodynamic forces.
Per IEC 61869-2, \( I_{dyn} \approx 2.5 \times I_{th} \) for 50 Hz systems. For example, a CT rated 20 kA/1s thermal withstand should handle ~50 kA peak dynamically.
Design implications include robust winding construction, secure core clamping, and adequate insulation. Inadequate short-circuit ratings may result in:
- Winding deformation or breakage
- Insulation puncture
- Core displacement affecting accuracy
Verification involves comparing system fault levels (from network studies) against CT ratings. If the prospective fault current exceeds \( I_{th} \) or \( I_{dyn} \), select a higher-rated CT or implement current-limiting devices.
| Short-Circuit Parameter | Description | Typical Rating | Standard Reference |
|---|---|---|---|
| Thermal Withstand (\( I_{th} \)) | RMS current for 1s or 3s | 10–63 kA | IEC 61869-2, IEEE C57.13 |
| Dynamic Withstand (\( I_{dyn} \)) | Peak fault current | 25–160 kA | IEC 61869-2 |
| Time Duration | Withstand time | 1 s or 3 s | System protection coordination |
| Fault Level Comparison | Must exceed system max | Yes | Mandatory |
Note: Indoor CTs often have lower short-circuit ratings than outdoor types. Always confirm ratings match the installation location and system configuration.
7. Environmental Conditions
Environmental factors significantly influence CT selection, longevity, and reliability. Key considerations include ambient temperature, humidity, altitude, pollution, and seismic activity.
Ambient Temperature: Standard CTs operate between –25°C to +40°C. Extreme temperatures affect insulation properties and core permeability. High temperatures accelerate aging; low temperatures may cause brittle fracture in resin-bonded units.
Altitude: Above 1000 m, air density decreases, reducing dielectric strength and cooling efficiency. Per IEC 60071, derating is required: for every 100 m above 1000 m, reduce voltage rating by 1% or increase creepage distance.
Pollution and Humidity: In coastal or industrial areas, salt or chemical deposits can cause tracking and flashover. CTs should have appropriate pollution degree ratings (e.g., PD III or IV per IEC 60664) and hydrophobic housings.
Seismic Requirements: In earthquake-prone zones, CTs must meet seismic withstand standards (e.g., IEEE 693 or IEC 60068-2-57). This includes dynamic testing for horizontal and vertical accelerations (typically 0.3g to 0.6g).
| Environmental Factor | Impact on CT | Mitigation Strategy |
|---|---|---|
| High Temperature (>40°C) | Insulation degradation, accuracy drift | Select extended temp range CTs; ensure ventilation |
| Low Temperature (<–25°C) | Brittle housing, oil solidification (oil-filled) | Use cold-climate designs; synthetic insulation |
| High Altitude (>1000 m) | Reduced dielectric strength, overheating | Derate voltage; increase clearances; sealed design |
| Humidity/Salt Spray | Corrosion, surface leakage | Stainless steel hardware; silicone rubber sheds |
| Seismic Activity | Mechanical failure, disconnection | Reinforced mounting; seismic-certified models |
Always specify environmental conditions during procurement. Outdoor CTs typically feature UV-resistant polymer housings, while indoor units may use epoxy resin.
8. CT Selection Checklist
Use this checklist during specification, procurement, and commissioning to ensure all critical aspects are addressed:
| # | Check Item | Verified? (✓/✗) | Remarks |
|---|---|---|---|
| 1 | Primary current matches system load and fault levels | ||
| 2 | Secondary current (1 A or 5 A) compatible with relays/meters | ||
| 3 | Accuracy class meets application (e.g., 0.5 for metering, 5P20 for protection) | ||
| 4 | Total burden ≤ CT rated burden (with 20% margin) | ||
| 5 | Thermal and dynamic short-circuit ratings exceed system fault levels | ||
| 6 | Environmental ratings suitable for location (temp, altitude, pollution, seismic) | ||
| 7 | Insulation level (BIL) matches system voltage | ||
| 8 | Physical dimensions fit available space and mounting arrangement | ||
| 9 | Compliance with relevant standards (IEC, IEEE, local regulations) | ||
| 10 | Secondary terminals accessible and labeled correctly |
This checklist should be completed jointly by protection engineers, designers, and site supervisors. Retain signed copies for project documentation and future audits.