Technical Article

Reliable Testing for Insulation Endurance Assessment

May 15, 2024 by Konrad Domes

Engineers may be unaware of the drawbacks of faster switching semiconductors. It is important to consider these potential risks as they affect the insulation system and develop reliable test equipment.

This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.

 

The development of power semiconductors, particularly wide bandgap (WBG), has progressed significantly, resulting in higher blocking voltages, reduced switching losses due to faster switching times, and the ability to create more compact systems by reducing passive components and optimizing space in power electronic systems.

Despite these advantages, many developers and system engineers are unaware of the potential drawbacks of faster switching semiconductors. The development of fast power semiconductors bears new risks and challenges for established electrical systems, particularly the insulation system. It is important to consider these potential threats and develop reliable test equipment.

 

High-Voltage dv/dt Pulse Generator Test Bench

A circuit topology must generate high-voltage slopes and amplitudes to develop the required test equipment while ensuring permanent, non-destructive operation. Saxogy has developed an innovative circuit concept over the past two years that meets the new load requirements for a scalable high-voltage insulation test system for various applications. Figure 1 presents the hardware components of the pulse generator, and Figure 2 shows a prototype of a complete test bench.

Traditional solutions are reaching their limits due to fast-switching components. To avoid overloading our system and risking its destruction, it has been designed as a multi-level system and developed with a strict insulation approach. While not exceeding the standard usage level of the components, we attain voltage slopes that are often multiple times greater than usual.

 

Figure 1. SAXOGY’s Adjustable Slope HVGenerator. Image used courtesy of Bodo’s Power Systems [PDF]

 

Due to the high required dv/dt and excellent adjustability, it quickly became apparent that SiC-MOSFETs should be used. However, as no single device met the requirements regarding breakdown voltage, multiple standard devices had to be connected in series. Direct series connection is complex, requiring a cell-based cascaded H-bridge topology.

Simultaneously switching individual cells can significantly increase the output voltage slope, depending on the number of cells used. For example, if a single cell switches with a slope of 20 kV/μs, the slope can triple to 60 kV/μs if three cells generate the output voltage.

Traditionally, the cascaded H-bridge topology requires individual transformers in each cell for the power supply. However, these transformers exhibit high coupling capacitances and increasing displacement currents. This can risk damaging the insulation and ultimately lead to transformer failure over time. Hence, Saxogy has invented an innovative topology that operates with only a single power supply and, at the same time, can be extended for higher voltage levels.

The inverter topology is extended with an additional charging path. Like the principle of a “bucket chain,” energy is transferred from the power supply unit into the first cell to the second cell. This process continues until the top cell in the system has also been recharged, and the charging sequence starts all over again. To ensure the recharging of two cells works, one cell operates as a charging cell and cannot provide any voltage at the output for the charging duration. Cell voltage can be maintained by matching the charging frequency to the application.

 

Figure 2. Example of an early prototype testbench. Image used courtesy of Bodo’s Power Systems [PDF]

 

Figure 3 displays the topology of the Saxogy dv/dt pulse generator using a three-stage system with a supply voltage of 750 volts. The switching configuration for positive output voltage and charging of cell two is shown. Cell one functions as a charging cell and does not output any voltage but is connected in parallel to cell 2 via the switch T5. The current flow (red arrow) between the DC link capacitances is limited by a current rise-limiting inductance and a diode in the charging path, which also prevents oscillations of the resonant circuit. Alternatively, a current-limiting resistor can be used instead of the inductance, resulting in additional losses and reducing recharging efficiency.

 

Figure 3. SAXOGY’s advanced transformerless multi-level topology. Image used courtesy of Bodo’s Power Systems [PDF]

 

Setting the Correct Stress Level

Thanks to the topology’s modularity, the output voltage can be adjusted as required by the application. To cover current and future insulation tests, the generator can provide bipolar voltages from 0.4 kVpp to 12 kVpp.

The rectangular voltage waveform can be set in a wide range from 2 kHz to 20 kHz to apply additional stress to the test specimen and shorten the test duration.

Doubling the rise times roughly halves an insulation system’s lifetime. To vary the rise time, we used a high level of expertise to dynamically adjust the voltage dv/dt slope in real time using a self-developed gate driver. This ensures an almost linear voltage slope, resulting in a constant displacement current over the entire voltage rise.

 

Figure 4. Adjustable rise times and optional overshoot via passive network. Image used courtesy of Bodo’s Power Systems [PDF]

 

The gate driver is designed to enable a switching behavior that keeps the generator voltage overshoot below 2%. Figure 4 demonstrates the adjustability of the voltage gradient at 1200 V and the almost linear voltage slope by displaying three out of sixteen voltage gradient settings. To expand the range of applications for the high-voltage dv/dt generator, an additional overshoot can be generated by using external passive RL networks (dashed lines). This enables testing of motor windings in worst-case scenarios.

The generator is addressed via Modbus TCP and is easy to operate.

The pulse generators are customized according to your requirements to create an optimal test bench. They are available in different versions, all housed in a 19” rack-mountable enclosure. The most suitable variant depends on your needs and the associated integration costs.

 

Figure 5. Available options for the HV dv/dt Pulse generator. Image used courtesy of Bodo’s Power Systems [PDF]

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine and is co-authored by Konrad Domes, Philipp Berkemeier, and Felix Schönlebe of Saxogy Power Electronics GmbH and Benjamin Sahan, Christian Staubach, Kevin Kaczmarek, and Stefan Reddig of Hannover University of Applied Sciences.