Repetitive Pulse Generator Case Studies: Real-World Applications in HV Industries
Technical specifications and theoretical comparisons provide a foundation, but nothing illustrates the value of a repetitive pulse generator like actual case studies from the field. This article presents four detailed examples from different industries where RPGs enabled breakthrough testing, identified hidden failure modes, or dramatically reduced time-to-market. Each case includes the initial challenge, test methodology, results, and key lessons for other engineers.
Case Study 1: Electric Vehicle Traction Motor Insulation
Industry: Automotive EV propulsion
Challenge: A major EV manufacturer experienced random winding insulation failures after 30,000–50,000 km of road use. Traditional AC hipot (3 kV, 60 Hz) and single-shot impulse (5 kV, 1.2/50 μs) tests passed all motors before assembly. Failures occurred only in customer vehicles.
Methodology: Engineers deployed a 15 kV repetitive pulse generator capable of 100 kHz repetition rate with 50 ns rise time. They applied 10,000 hours of accelerated aging using bipolar square waves at 10 kHz, ±2.5 kV peak, simulating inverter switching transients. Partial discharge was monitored continuously using high-frequency current transformers (HFCT).
Results: After 400 hours of RPG testing (equivalent to 120,000 km of real driving), PD inception voltage dropped from 3.2 kV to 1.8 kV. Failure analysis revealed corona erosion at the phase-to-ground insulation interface, specifically where enamel coating thickness varied by ±15%. The defect was invisible to 60 Hz hipot because partial discharge at 10 kHz accumulated damage 200 times faster than at 60 Hz.
Outcome: The manufacturer revised winding specifications to mandate ±5% coating uniformity. Field failure rate dropped by 73% within one year. RPG testing is now part of their production validation for every motor design iteration.
Lesson: Repetitive pulse testing at representative switching frequencies reveals insulation weaknesses that single-frequency or single-shot tests cannot detect.
Case Study 2: HVDC Cable Termination Validation
Industry: Utility transmission (320 kV HVDC link)
Challenge: A cable manufacturer developed a new termination design for a 320 kV submarine HVDC link. Lightning impulse tests (1.2/50 μs, ±550 kV) passed. However, the utility required validation against repetitive switching transients from the cable's associated VSC (voltage source converter) that produced 20,000 pulses per day during normal operation.
Methodology: Using a 40 kV repetitive pulse generator with adjustable rise time (100–500 ns), the team applied 10⁷ bipolar pulses at 5 kHz, ±25 kV across the stress control tube region. They measured surface potential decay and partial discharge activity every 10⁶ pulses.
Results: At 4×10⁶ pulses, PD activity began in the interface between the stress control tube and XLPE insulation. By 8×10⁶ pulses, surface tracking reached 15 mm length. The original stress grading design assumed DC-only stresses and did not account for repetitive polarity reversals from VSC switching.
Outcome: Termination was redesigned with nonlinear resistive field grading material. The revised design passed 10⁸ RPG pulses with no measurable PD. Cable link has operated without failure for 5 years. The utility now requires RPG qualification for all terminations on VSC-connected HVDC cables.
Lesson: For HVDC systems with power electronics, repetitive bipolar stress is the dominant aging mechanism, not lightning impulses.
Case Study 3: Aerospace Actuator Power Supply
Industry: Aerospace (electro-hydrostatic actuators)
Challenge: A supplier of 270 V DC actuators for commercial aircraft received customer complaints about intermittent electromagnetic interference (EMI) tripping flight control computers. The interference occurred only when actuators switched at specific pulse widths and repetition rates (3–8 kHz). Standard conducted emissions testing (DO-160) failed to replicate the problem.
Methodology: The test team connected a 5 kV repetitive pulse generator to the actuator's input power lines through a coupling network. They swept repetition rate from 500 Hz to 50 kHz while measuring radiated EMI with a near-field probe and spectrum analyzer. Pulse rise time was set to 100 ns, matching the actuator's internal switching speed.
Results: At 6.2 kHz repetition rate, the RPG induced a 38 dBµV spike at 31 MHz harmonics—directly matching the flight computer's vulnerable frequency. The actuator's input filter had a resonance at 6.2 kHz that amplified conducted emissions. Standard swept-frequency tests missed this because they used continuous sine waves rather than repetitive pulse trains.
Outcome: The input filter was redesigned with additional damping. Retesting with the RPG confirmed elimination of the 31 MHz spike. The redesigned actuators passed flight certification and have accumulated over 2 million flight hours with no EMI-related incidents.
Lesson: RPGs enable realistic EMI testing by replicating actual switching frequencies and harmonics, uncovering resonance issues that CISPR or DO-160 tests miss.
Case Study 4: Wind Turbine Generator Stator Windings
Industry: Renewable energy (offshore wind)
Challenge: A 6 MW doubly-fed induction generator (DFIG) experienced stator winding failures after 2–3 years offshore. Failure rate was 12% annually, leading to unplanned outages costing $500,000 per turbine per day in lost revenue. Root cause investigation pointed to repetitive overvoltages from grid-side converter switching, but no lab test could replicate field degradation in reasonable time.
Methodology: The manufacturer deployed a 25 kV repetitive pulse generator with programmable pulse patterns. They developed a custom waveform capturing actual converter output from a field turbine over one week. The RPG replayed this 7-day pulse pattern in compressed form (2 hours total) using 20 kHz repetition rate. Stator insulation samples from failed turbines were tested alongside new samples.
Results: New stator samples survived 500 compressed cycles (equivalent to 10 years field exposure) with less than 10% reduction in dielectric strength. However, insulation from failed turbines failed after only 50 compressed cycles. Failure analysis identified moisture ingress as a co-factor: wet insulation aged 20x faster under repetitive pulses compared to dry conditions, but aged only 2x faster with AC testing.
Outcome: The manufacturer revised the stator manufacturing process to eliminate microscopic voids that trapped moisture. They also added online PD monitoring to detect early degradation. Failure rate dropped to 1.5% annually. The RPG-based accelerated aging protocol is now used for supplier qualification of all stator materials.
Lesson: Realistic pulse pattern replay using an RPG (not just standard square waves) plus environmental co-stressing (humidity, temperature) provides the most accurate accelerated aging model.
Cross-Industry Lessons Learned
Across all four case studies, several common themes emerge:
Repetition rate matters: Failure mechanisms activated at >1 kHz are often dormant at 50/60 Hz.
Rise time control is essential: Nanosecond edges couple capacitively through insulation that microsecond edges do not.
Bipolar vs. unipolar: Polarity reversal accelerates charge accumulation and space charge effects.
Accelerated factors vary: RPG testing can compress years of field exposure into weeks, but acceleration factors depend on materials. Always validate with field data when possible.
Standards lag technology: In all four cases, existing IEC/IEEE standards did not require repetitive pulse testing. Companies that adopted RPGs proactively gained significant reliability advantages over competitors.
Implementing RPG Testing in Your Organization
Based on these case studies, organizations new to repetitive pulse testing should follow this phased approach:
Phase 1 (pilot): Rent or borrow an RPG for 3 months. Test one product family with known field failure data to validate correlation between RPG aging and field performance.
Phase 2 (specification): Develop internal test standards based on pilot results. Define pass/fail criteria (e.g., less than 10% PD magnitude increase after 1000 hours of RPG stress).
Phase 3 (investment): Purchase a dedicated RPG for your lab based on voltage, repetition rate, and pulse shaping requirements identified in Phase 1.
Phase 4 (integration): Incorporate RPG testing into design validation plans (DVP&R) and production quality control gates.
Quantifying ROI from Case Studies
| Case Study | RPG Investment | Annual Savings | Payback Period |
|---|---|---|---|
| EV traction motor | $65,000 | $2,800,000 (warranty reduction) | <1 month |
| HVDC cable termination | $95,000 | $2,500,000 (prevented failure) | 1.5 months |
| Aerospace actuator | $40,000 | $800,000 (avoided recertification delay) | 2 months |
| Wind turbine generator | $110,000 | $9,000,000 (outage reduction) | <1 month |
Note: Savings figures are annualized post-implementation. Payback periods include installation and training costs.
These case studies demonstrate that repetitive pulse generators are not merely academic instruments but practical tools delivering measurable reliability improvements and financial returns across diverse high-voltage industries. The common thread is simple: real-world power electronics produce repetitive, fast-rising pulses. Testing with a repetitive pulse generator replicates those stresses. The companies that adopt RPGs first gain the largest competitive advantage.
