How to Select Unknown 36kV Voltage Transformer: Engineering Methodology
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How to Select Unknown 36kV Voltage Transformer: Engineering Methodology

April 11, 2026 Documents, High-Voltage Metering Box

Engineering Selection Guide: Unknown This comprehensive selection guide provides systematic methodology for specifying t...

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Engineering Selection Guide: Unknown

This comprehensive selection guide provides systematic methodology for specifying the Unknown voltage transformer in 35kV power systems. The selection process requires careful analysis of system parameters, load characteristics, accuracy requirements, and environmental conditions.

Step 1: Determine System Parameters

Before selecting the transformer, gather complete system data:

1.1 System Voltage

  • Nominal System Voltage: 35 kV
  • Maximum System Voltage: 36 kV (per IEC 60038)
  • Minimum System Voltage: 0.9 × 35 = 31.5 kV

1.2 System Frequency

  • Standard: 50 Hz (China, Europe, most of Asia/Africa)
  • Alternative: 60 Hz (Americas, parts of Asia)
  • Unknown Rating: 50/60 Hz compatible

1.3 System Configuration

  • Grounding: Solidly grounded / Resistance grounded / Ungrounded
  • Connection: Three-phase / Single-phase
  • Neutral: Effective / Non-effective

Step 2: Select Primary/Secondary Ratio

2.1 For Current Transformers

The CT ratio must be selected so that normal load current falls within 60-120% of rated primary current for optimal accuracy.

Selection Formula:

Iprimary_rated ≥ Iload_max / 0.6

Example Calculation:

Given:

  • Maximum load current: 380A
  • Minimum load current: 80A
  • System voltage: 35 kV

Calculation:

  1. Minimum CT primary: 380A / 0.6 = 633A
  2. Standard ratio selection: 750/5A or 800/5A
  3. Verify minimum load: 80A / 750A = 10.7% (acceptable for class 0.5)
  4. For class 0.2S, minimum load should be >1%: 80A / 750A = 10.7% ✓

2.2 Standard CT Ratios for 35kV Systems

Application Typical Ratios Accuracy Class
Feeder Circuits 100/5A to 600/5A 0.5S / 5P10
Transformer Feeders 400/5A to 1200/5A 0.5 / 5P20
Bus Tie / Section 1000/5A to 3000/5A 5P20 / 5P30
Generator Circuits 1500/5A to 5000/5A TPY / TPZ
Capacitor Banks 200/5A to 800/5A 5P20

Step 3: Determine Accuracy Class

3.1 Accuracy Classes for Metering

Class Ratio Error (%) Phase Displacement Application
0.2S ±0.2% ±10 minutes Revenue metering (high value)
0.5S ±0.5% ±20 minutes Revenue metering (standard)
0.5 ±0.5% ±30 minutes General metering
1.0 ±1.0% ±40 minutes Indicating instruments
3.0 ±3.0% Not specified General indication

3.2 Accuracy Classes for Protection

Class Composite Error Accuracy Limit Factor Application
5P10 5% 10 × rated current Overcurrent protection
5P20 5% 20 × rated current Differential protection
10P10 10% 10 × rated current Backup protection
TPY Transient performance Per IEEE C37.110 Generator/transformer differential

Step 4: Calculate Burden

Total secondary burden includes connected devices and lead resistance:

Stotal = Smeter + Srelay + Sleads

4.1 Lead Burden Calculation

Lead resistance:

Rlead = (ρ × L) / A

Where:

  • ρ = 0.0175 Ω·mm²/m (copper resistivity at 20°C)
  • L = lead length in meters
  • A = conductor cross-section in mm²

Lead burden (for 2-wire connection):

Slead = I² × Rlead × 2

4.2 Example Burden Calculation

Given Data:

  • Meter burden: 0.5 VA (electronic multifunction meter)
  • Relay burden: 1.0 VA (microprocessor protection relay)
  • Lead length: 35 meters
  • Conductor: 4 mm² copper
  • CT secondary current: 5A

Calculation:

  1. Rlead = (0.0175 × 35) / 4 = 0.153 Ω
  2. Slead = 5² × 0.153 × 2 = 7.65 VA
  3. Stotal = 0.5 + 1.0 + 7.65 = 9.15 VA
  4. Selected CT burden rating: 10 VA or 15 VA

Step 5: Verify Selection

5.1 Selection Checklist

Verify the following before finalizing selection:

  1. ☐ Rated voltage ≥ Maximum system voltage (36 kV)
  2. ☐ Primary current selected for 60-120% of normal load
  3. ☐ Accuracy class appropriate for application
  4. ☐ Total burden ≤ Rated burden
  5. ☐ Installation type matches location (Outdoor)
  6. ☐ Environmental conditions within limits
  7. ☐ Creepage distance adequate for pollution level
  8. ☐ Thermal and dynamic current ratings adequate

Complete Selection Example

Application: 35kV Feeder for Industrial Facility

System Data:

  • System voltage: 35 kV (Max: 36 kV)
  • Maximum load current: 450A
  • Minimum load current: 100A
  • Application: Revenue metering + overcurrent protection
  • Meter: Electronic multifunction meter (0.5 VA)
  • Relay: Microprocessor overcurrent relay (1.5 VA)
  • Lead length: 40 meters, 4 mm² copper
  • Installation: Outdoor
  • Ambient temperature: -10°C to +50°C

Selection:

  1. CT Ratio:
    • Minimum primary: 450A / 0.6 = 750A
    • Standard ratio: 800/5A
    • Check minimum load: 100A / 800A = 12.5% ✓
  2. Accuracy Class:
    • For revenue metering: 0.5S
    • For protection: 5P10
    • Selection: 2-core CT (0.5S + 5P10)
  3. Burden Calculation:
    • Rlead = (0.0175 × 40) / 4 = 0.175 Ω
    • Slead = 25 × 0.175 × 2 = 8.75 VA
    • Stotal = 0.5 + 1.5 + 8.75 = 10.75 VA
    • Selected: 15 VA (next standard rating)
  4. Final Specification:
    • Model: Unknown
    • Ratio: 800/5A
    • Core 1: 0.5S, 15 VA
    • Core 2: 5P10, 15 VA
    • Installation: Outdoor
    • Insulation: Oil-Immersed

This guide provides engineering methodology. For non-standard applications or special requirements, contact our technical team with complete system data.