Impulse Voltage Testing of Power Cables Using HV Lightning Impulse Voltage Generator Systems
Power cables form the backbone of urban distribution networks, submarine transmission links, and industrial power systems. Their insulation must withstand transient overvoltages generated by lightning strikes and switching operations. Lightning impulse voltage testing using HV lightning impulse voltage generator systems is a key type test for cable systems, particularly for extra-high-voltage (EHV) and submarine cables. This article discusses the standards, circuit requirements, and practical considerations for impulse testing of power cables.
Purpose of Impulse Testing on Power Cables
Impulse testing verifies that cable insulation can withstand transient overvoltages without breakdown. Specific objectives include:
Confirming the dielectric design of the cable insulation system.
Detecting manufacturing defects such as voids, contaminants, or incomplete impregnation.
Verifying accessory performance, including joints and terminations.
Validating the cable system after installation, particularly for submarine cables.
Providing quality assurance for high-value cable projects.
Applicable Standards
Cable impulse testing is governed by several international standards:
IEC 60230: Defines impulse tests on cables and their accessories.
IEC 60840: Specifies test methods for power cables with extruded insulation for rated voltages above 30 kV up to 150 kV.
IEC 62067: Covers power cables with extruded insulation for rated voltages above 150 kV up to 500 kV.
IEC 60060-1: Defines standard lightning impulse waveforms and tolerances.
IEEE 48: Provides test procedures for cable terminations.
These standards specify the test voltage levels, waveform parameters, and acceptance criteria.
Characteristics of Cables as Test Objects
Cables present unique challenges for impulse testing due to their electrical characteristics:
High Capacitance: Cable capacitance per unit length is substantial, and long cable lengths can reach several microfarads, requiring high generator energy.
Distributed Parameters: Cables behave as distributed networks, causing waveform distortion along the length.
Low Surge Impedance: The surge impedance of cables is typically much lower than that of overhead lines, affecting the impulse response.
Attenuation and Dispersion: High-frequency components of the impulse attenuate and disperse as they travel along the cable.
Accessory Effects: Joints and terminations introduce additional capacitance and may affect waveform shape.
Circuit Configuration for Cable Testing
Impulse testing of cables requires a carefully designed circuit to achieve the standard waveform:
Generator Energy: The impulse generator must have sufficient energy to charge the cable capacitance to the test voltage.
Front and Tail Resistors: Resistor values must be selected based on the cable capacitance to achieve the standard 1.2/50 microsecond waveform.
Short Connections: Minimize lead length between the generator and cable terminal to reduce inductance.
Divider Placement: Connect the impulse voltage divider close to the cable terminal, accounting for the effect of cable length.
Far-End Termination: The far end of the cable should be terminated according to the test plan, either open-circuited or grounded.
Test Procedure
A typical cable impulse test follows these steps:
Preparation: Inspect cable terminations, clean surfaces, and verify the cable is at ambient temperature.
Circuit Verification: Apply a reduced-level impulse to verify waveform and measurement.
Full-Level Impulses: Apply the specified test voltage, typically ten impulses of each polarity for type tests.
Waveform Recording: Record all waveforms and analyze for consistency.
Post-Test Examination: Inspect the cable and accessories for visible damage, and perform subsequent dielectric tests as required.
Waveform Control Challenges
Achieving standard impulse waveforms with cables requires attention to several factors:
Front Time Control: High cable capacitance tends to lengthen the front time, requiring low inductance connections and appropriate front resistance.
Tail Time Control: Tail resistance must be selected to achieve the standard decay time despite the high capacitance.
Overshoot: Damped oscillations at the peak must be limited to acceptable levels.
Voltage Drop: The impulse generator must maintain sufficient voltage at the cable terminal despite the load.
Reflections: Wave reflections from the far end can distort the waveform at the near end, requiring careful termination.
Fault Detection During Impulse Testing
Impulse testing can reveal defects through waveform analysis:
Sudden Voltage Collapse: Indicates breakdown of the cable insulation or accessories.
Changes in Waveform Shape: Subtle alterations across repeated impulses suggest developing defects.
Partial Discharge Activity: High-frequency oscillations may indicate discharge within the insulation.
Inconsistent Results: Variations between impulses or polarities warrant further investigation.
Digital recording systems with high sampling rates are essential for capturing these diagnostic indicators.
Special Considerations for Submarine Cables
Submarine cables require impulse testing after installation as part of acceptance testing. Challenges include:
Testing long cable lengths with high total capacitance.
Accessing cable terminals at sea or on remote shores.
Coordinating testing with installation schedules.
Ensuring safety of personnel and equipment in marine environments.
Compact impulse generators and portable test systems are often used for these applications.
Conclusion
Impulse voltage testing of power cables using HV lightning impulse voltage generator systems is essential for verifying insulation integrity and ensuring reliable service. The high capacitance and distributed nature of cables demand careful circuit design, adequate generator energy, and rigorous waveform analysis. By following international standards and applying sound engineering practices, manufacturers and utilities can ensure that cable systems will withstand transient overvoltages throughout their operational life.
