HV Stress Generator for Insulation Endurance Assessment

Failure of insulation systems due to power electronics

Despite these advantages, many developers and system engineers are not fully aware 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.

The first part of this article presented a current research topic on endurance tests of enamelled copper wires, to test for example the insulation system of motor windings.

A few questions may have arisen:

• Have you ever wondered how much your insulation materials are stressed by up to 200 kV/µs voltage slopes?
• Do you know how much displacement current will flow in your insulated hairpin wire under steep voltage pulses and how
  this affects the lifetime of the winding?
• Are you as a semiconductor manufacturer aware of the aging process of your packaging technology?

SAXOGY´s approach for a high voltage dv/dt pulse generator testbench

To develop the required test equipment, a circuit topology is needed that generates high voltage slopes and amplitudes 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 testbench.

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.

Due to the high required dv/dt and excellent adjustability, it quickly became apparent that SiC-MOSFETs should be used. However, as there was no single device that meets the requirements regarding breakdown voltage, it was necessary to connect multiple standard devices in series. Direct series connection, is complex, which led to 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 are involved in generating 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, resulting in increased displacement currents. This can pose a risk of 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. Similar to the principle of a “bucket chain”, energy is transferred from the power supply unit into the first cell and, in the next step, energy is passed on from the first 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 that the recharging of two cells works, one cell operates as a «charging cell» and cannot provide any voltage at the output for the duration of charging. All cell voltages in the system can be maintained by matching the charging frequency to the application.

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, but this results in additional losses and reduces the recharging efficiency.

Setting the correct stress level for your insulation.

Thanks to the modularity of the topology, 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.

SAXOGY^® and Hannover University of Applied Sciences have taken on the task of developing a test bench that realistically simulates the stresses of inverter operation and can be used for accelerated insulation endurance assessment. The corresponding project “ISODyn” was supported by the German ZIM research funding.

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.

Currently, there is no unified international standard for endurance testing under high-frequency voltage impulses for winding wires. Therefore, we refer to the existing chinese standard GB/T 4074.21-2018 and incorporate feedback from manufacturers to derive the following requirements according to figure 2 that the dv/dt pulse generator must meet.

As demonstrated by the research findings in the first part of this article, doubling the rise times results in roughly halving the lifetime of an insulation system. 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.

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 therefore very easy to operate.

Tailored solutions for your needs

The pulse generators are customized according to your specific 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 for you depends on your needs and the associated integration costs.

Meet us at PCIM 2024 – Hall 7 Stand 135 As every year, we will be exhibiting at PCIM 2024 in June. Perform your own live insulation tests and observe the generator in operation during a cup of coffee. Alternatively, we would be happy to talk to you sooner.

About

As experts in the field of power electronics, we have gathered comprehensive knowledge and experience that are reflected in this specialized article. With a solid background in measurement technology and a focus on safety aspects, Hannover University of Applied Sciences and SAXOGY^® provide a profound insight into the latest developments and challenges in this dynamic field.

This article appeared in Bodo’s Power Systems^® Magazine – April 2024. You can download the issue here: https://www.bodospower.com/current.aspx