How to Choose a Lightning Impulse Test System

Lightning impulse testing is a critical method to verify the insulation performance of high-voltage equipment, such as transformers, cables, switchgear, and surge arresters, against transient overvoltage caused by lightning strikes. Selecting the appropriate lightning impulse test system depends on the type of equipment being tested, its rated voltage, and the testing standards.

1. Key Considerations for Selecting a Lightning Impulse Test System

(1) Test Voltage

• The lightning impulse voltage is determined based on the rated voltage of the equipment and applicable standards like IEC 60060, IEC 60076, or IEEE C57.
• Standard Voltage Multipliers:
  • The impulse test voltage is typically 2.5× to 3.0× the rated voltage of the equipment.

(2) Waveform Requirements

• The system must generate the standard 1.2/50µs lightning impulse waveform:
  • 1.2µs rise time.
  • 50µs fall time to half-peak voltage.
• Ensure the system meets the requirements for front time, time to half-value, and waveform tolerance.

(3) Energy and Power Requirements

• The energy (kJ) required depends on the capacitance of the equipment under test and the peak test voltage.
• Energy is stored in the impulse test system’s capacitors, which discharge to produce the lightning impulse.

(4) Test Equipment Type

• Identify the type of equipment being tested:
  • Transformers.
  • GIS (Gas-Insulated Switchgear).
  • Cables.
  • Surge arresters.

(5) Compliance with Standards

• Ensure the system complies with relevant standards:
  • IEC 60060-1: General high-voltage testing techniques.
  • IEC 60076-3: Transformer insulation tests.
  • IEEE C57.98: Transformer impulse testing.

2. Voltage Selection for Lightning Impulse Testing

Standard Test Voltages Based on Rated Voltage

Voltage Selection Guidelines

1. For Transformers:
   • Test voltage depends on insulation level and rated voltage.
   • Example: A 220kV transformer typically requires a 1,050kVp impulse voltage.
2. For Cables:
   • Lightning impulse test voltage = 2.5× the rated line-to-ground voltage.
   • Example: A 110kV cable requires a test voltage of approximately 275kVp.
3. For GIS:
   • Impulse test voltage is generally 1.5× to 2.0× the rated voltage.

3. Energy (kJ) and Power Selection

Formula for Energy Calculation:

W=1/2CVV

Where:

• W: Energy in joules (J) or kilojoules (kJ).
• C: Capacitance of the test object (in farads).
• V: Peak test voltage (in volts).

Step-by-Step Energy Calculation:

1. Determine Capacitance:
   • Typical values:
     • Transformers: 100pF to 500pF.
     • Cables: 200pF to 1,000pF per km.
     • GIS: 50pF to 200pF per meter.
2. Calculate Energy:
   • Use the formula above to determine the required energy storage.

Examples:

Case 1: Transformer (220kV)

• Capacitance: C=200pF=200×10−12F
• Test Voltage: V=1,050kVpV = 1,050kVp.

W=12(200×10−12)(1,050,000)2=110.25 J=0.11 kJ

Case 2: Cable (110kV, 5km)

• Capacitance: C=500pF/km×5km=2,500pF=2.5×10−9F
• Test Voltage: V=275kVpV = 275kVp.

W=12(2.5×10−9)(275,000)2=94.53 kJ

Case 3: GIS (500kV, 20m)

• Capacitance: C=100pF/m×20m=2,000pF=2.0×10−9F
• Test Voltage: V=1,800kVpV = 1,800kVp.

W=12(2.0×10−9)(1,800,000)2=3,240 J=3.24 kJ

4. Choosing the Test System Based on Voltage and Energy

5. Additional Considerations

(1) Portability

• For on-site testing, portable or modular systems are ideal.
• Fixed systems are better for laboratory or factory testing.

(2) Expandability

• Choose a modular impulse test system for scalability in voltage and energy.

(3) Waveform Quality

• Ensure the system generates accurate 1.2/50µs impulse waveforms, with minimal distortion.

(4) Safety Features

• Overvoltage protection.
• Automatic shutdown in case of failure.
• Grounding and discharge mechanisms for operator safety.

(5) Compliance with Standards

• The system must meet IEC 60060 for high-voltage testing techniques and specific equipment standards.

6. Practical Example Scenarios

Scenario 1: Testing a 35kV Transformer

• Rated Voltage: 35kV.
• Test Voltage: 2.85×35kV=100kVp2.85 × 35kV = 100kVp.
• Capacitance: 300pF.
• Energy Calculation: W=12(300×10−12)(100,000)2=1.5 kJ
• Recommended System:
  • Voltage: ≥100kVp
  • Energy: 2kJ

Scenario 2: Testing a 220kV Cable (10km)

• Rated Voltage: 220kV.
• Test Voltage: 1.5×220kV=330kVp1.5 × 220kV = 330kVp.
• Capacitance: 500pF/km×10km=5,000pF500pF/km × 10km = 5,000pF.
• Energy Calculation: W=12(5,000×10−12)(330,000)2=2.72 kJ
• Recommended System:
  • Voltage: ≥350kVp
  • Energy: 5kJ

Scenario 3: Testing a 500kV GIS

• Rated Voltage: 500kV.
• Test Voltage: 1.8×500kV=900kVp1.8 × 500kV = 900kVp.
• Capacitance: 2,000pF2,000pF.
• Energy Calculation: W=12(2,000×10−12)(900,000)2=8.1kJ
• Recommended System:
  • Voltage: ≥1,000kVp
  • Energy: 10kJ

7. Summary

Voltage Selection

• Use a multiplier of 2.5×2.5× to 3.0×3.0× the rated voltage for impulse testing.
• Add a safety margin of 10–20% for reliability.

Energy Selection

• Calculate energy using W=1/2CVV
• Ensure the system has sufficient energy to generate the required waveform.

System Recommendations

• For portable needs, use compact systems.
• For large-scale or future needs, select modular systems with expandable voltage and energy capacity.

By following these guidelines, you can select a lightning impulse test system that meets your testing requirements safely and effectively.