WZ
W. Zhao
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11 records found
1
PCB transformers are emerging as a promising alternative to medium-frequency wire-wound transformers due to their numerous advantages. However, research on FR-4, the primary PCB insulation material, remains limited. This study investigates the dielectric performance of PCB electrodes through interlayer breakdown tests at 50 Hz, yielding an aging curve for this condition. Additionally, layer-to-layer breakdown tests were conducted at 50 Hz and 1 kHz, revealing a reduction in time to breakdown with increasing frequency. Notably, interlayer breakdown is significantly more likely, exhibiting a breakdown strength eight times lower than that of layer-to-layer breakdown.
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PCB transformers are emerging as a promising alternative to medium-frequency wire-wound transformers due to their numerous advantages. However, research on FR-4, the primary PCB insulation material, remains limited. This study investigates the dielectric performance of PCB electrodes through interlayer breakdown tests at 50 Hz, yielding an aging curve for this condition. Additionally, layer-to-layer breakdown tests were conducted at 50 Hz and 1 kHz, revealing a reduction in time to breakdown with increasing frequency. Notably, interlayer breakdown is significantly more likely, exhibiting a breakdown strength eight times lower than that of layer-to-layer breakdown.
The future power grid must accommodate large-scale integration of variable renewable energy sources. Power electronic (PE)-based components, e.g. onverters, will play an essential role in the operation of the power system. However, the harmonics and transients generated by these PE-based components can significantly affect the lifetime and reliability of various power system components. These disturbances, particularly harmonics and transients, are challenging to eliminate. Thus, a more effective strategy is to assess the effects and enhance the durability and performance of the system components. To ensure the reliability of critical system components, these should meet stringent specifications and undergo rigorous testing prior to installation. Using a modular cascaded H-bridge (CHB) based high-voltage arbitrary waveform generator (HV-AWG), capable of replicating the dielectric stresses induced by PE components, enables a more accurate assessment of component resilience under operational conditions.
The CHB-based HV-AWG can be divided into several submodules; each module consists of three key components: a driver, a medium-frequency transformer, and an H-bridge equipped with HV rectifiers. Although various types of HV-AWGs exist, the modular CHB-based HV-AWG excels due to its superior high-voltage capability, broad operating frequency bandwidth, simple topology, compact size and low manufacturing costs. To successfully realize the CHB-based HV-AWG design, several technical challenges must be addressed, including the development of the insulation system for the medium-frequency transformer and the design of the highvoltage switch within the H-bridge.... ...
The CHB-based HV-AWG can be divided into several submodules; each module consists of three key components: a driver, a medium-frequency transformer, and an H-bridge equipped with HV rectifiers. Although various types of HV-AWGs exist, the modular CHB-based HV-AWG excels due to its superior high-voltage capability, broad operating frequency bandwidth, simple topology, compact size and low manufacturing costs. To successfully realize the CHB-based HV-AWG design, several technical challenges must be addressed, including the development of the insulation system for the medium-frequency transformer and the design of the highvoltage switch within the H-bridge.... ...
The future power grid must accommodate large-scale integration of variable renewable energy sources. Power electronic (PE)-based components, e.g. onverters, will play an essential role in the operation of the power system. However, the harmonics and transients generated by these PE-based components can significantly affect the lifetime and reliability of various power system components. These disturbances, particularly harmonics and transients, are challenging to eliminate. Thus, a more effective strategy is to assess the effects and enhance the durability and performance of the system components. To ensure the reliability of critical system components, these should meet stringent specifications and undergo rigorous testing prior to installation. Using a modular cascaded H-bridge (CHB) based high-voltage arbitrary waveform generator (HV-AWG), capable of replicating the dielectric stresses induced by PE components, enables a more accurate assessment of component resilience under operational conditions.
The CHB-based HV-AWG can be divided into several submodules; each module consists of three key components: a driver, a medium-frequency transformer, and an H-bridge equipped with HV rectifiers. Although various types of HV-AWGs exist, the modular CHB-based HV-AWG excels due to its superior high-voltage capability, broad operating frequency bandwidth, simple topology, compact size and low manufacturing costs. To successfully realize the CHB-based HV-AWG design, several technical challenges must be addressed, including the development of the insulation system for the medium-frequency transformer and the design of the highvoltage switch within the H-bridge....
The CHB-based HV-AWG can be divided into several submodules; each module consists of three key components: a driver, a medium-frequency transformer, and an H-bridge equipped with HV rectifiers. Although various types of HV-AWGs exist, the modular CHB-based HV-AWG excels due to its superior high-voltage capability, broad operating frequency bandwidth, simple topology, compact size and low manufacturing costs. To successfully realize the CHB-based HV-AWG design, several technical challenges must be addressed, including the development of the insulation system for the medium-frequency transformer and the design of the highvoltage switch within the H-bridge....
Journal article
(2025)
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Zhaoxin Wang, Xing Wei, Claus Leth Bak, Filipe Faria da Silva, Tianming Luo, Weichuan Zhao, Peter Vaessen, Henrik Sorensen, Mohamad Ghaffarian Niasar
As the voltage levels of solid-state transformers (SSTs) increase using medium-voltage switches, high-frequency transformers (HFTs) used inside SSTs are subjected to increased electrical stress. This stress, characterized by high voltage, high-frequency pulsewidth modulation (PWM) voltage, can cause insulation partial discharge (PD) and potentially lead to failure of the HFT insulation system. While PD behavior under power-frequency sinusoidal voltage has been extensively studied, the behavior of HFT insulation under PWM square pulse conditions is less well understood. To address this gap, a high voltage high-frequency PWM voltage PD test platform is developed and high-frequency current transformer (HFCT) and ultra-high-frequency (UHF) antenna are used for PD signal detection. PD tests are performed under a variety of PWM conditions including PWM frequency, rise time, voltage amplitude, and different insulation layers to thoroughly investigate the HFT insulation behavior. The PD characteristics of repetitive PD inception voltage and phase-resolved PD patterns at different PWM conditions are recorded and analyzed under PWM conditions. In addition, this article explores the underlying PD mechanisms of the HFT insulation under high-frequency PWM stress, providing insights to explain the observed test results. The findings from this research provide essential references and lay a solid foundation for future advances in optimal design, health monitoring, reliability analysis, and lifetime prediction for HFTs in power electronics applications.
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As the voltage levels of solid-state transformers (SSTs) increase using medium-voltage switches, high-frequency transformers (HFTs) used inside SSTs are subjected to increased electrical stress. This stress, characterized by high voltage, high-frequency pulsewidth modulation (PWM) voltage, can cause insulation partial discharge (PD) and potentially lead to failure of the HFT insulation system. While PD behavior under power-frequency sinusoidal voltage has been extensively studied, the behavior of HFT insulation under PWM square pulse conditions is less well understood. To address this gap, a high voltage high-frequency PWM voltage PD test platform is developed and high-frequency current transformer (HFCT) and ultra-high-frequency (UHF) antenna are used for PD signal detection. PD tests are performed under a variety of PWM conditions including PWM frequency, rise time, voltage amplitude, and different insulation layers to thoroughly investigate the HFT insulation behavior. The PD characteristics of repetitive PD inception voltage and phase-resolved PD patterns at different PWM conditions are recorded and analyzed under PWM conditions. In addition, this article explores the underlying PD mechanisms of the HFT insulation under high-frequency PWM stress, providing insights to explain the observed test results. The findings from this research provide essential references and lay a solid foundation for future advances in optimal design, health monitoring, reliability analysis, and lifetime prediction for HFTs in power electronics applications.
Journal article
(2024)
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W. Zhao, S. Ghafoor, Gijs Willem Lagerweij, Gert Rietveld, P.T.M. Vaessen, M. Ghaffarian Niasar
This paper comprehensively reviews several techniques that address the static and dynamic voltage balancing of series-connected MOSFETs. The effectiveness of these techniques was validated through simulations and experiments. Dynamic voltage-balancing techniques include gate signal delay adjustment methods, passive snubbers, passive clamping circuits, and hybrid solutions. Based on the experimental results, the advantages and disadvantages of each technique are investigated. Combining the gate-balancing core method with an RC snubber, which has proven both technically and commercially attractive, provides a robust solution. If the components are sorted and binned, voltage-balancing techniques may not be necessary, further enhancing the commercial viability of series-connected MOSFETs. An investigation of gate driver topologies yields one crucial conclusion: magnetically isolated gate drivers offer a simple and cost-effective solution for high-frequency (HF) applications (2.5–50 kHz) above 8 kV with an increased number of series devices. Below 8 kV, it is advantageous to move the isolation barrier from the gate drive IC to an optocoupler and isolated supply, allowing for a simple design with commercially available components.
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This paper comprehensively reviews several techniques that address the static and dynamic voltage balancing of series-connected MOSFETs. The effectiveness of these techniques was validated through simulations and experiments. Dynamic voltage-balancing techniques include gate signal delay adjustment methods, passive snubbers, passive clamping circuits, and hybrid solutions. Based on the experimental results, the advantages and disadvantages of each technique are investigated. Combining the gate-balancing core method with an RC snubber, which has proven both technically and commercially attractive, provides a robust solution. If the components are sorted and binned, voltage-balancing techniques may not be necessary, further enhancing the commercial viability of series-connected MOSFETs. An investigation of gate driver topologies yields one crucial conclusion: magnetically isolated gate drivers offer a simple and cost-effective solution for high-frequency (HF) applications (2.5–50 kHz) above 8 kV with an increased number of series devices. Below 8 kV, it is advantageous to move the isolation barrier from the gate drive IC to an optocoupler and isolated supply, allowing for a simple design with commercially available components.
As the integration of renewable energy sources into the grid increases, the insulation systems of grid components such as transformers and switchgear encounter significant challenges due to the transients and harmonics generated by power-electronic-based converters. A test generator capable of replicating these component stresses is essential to accurately evaluate these insulation systems under real-grid conditions. This paper proposes a modular cascaded H-bridge-based high-voltage arbitrary waveform generator, prototyped with three stages to generate customized waveforms (triangular, sawtooth, pulse, and complex) up to 8 kV. The H-bridge modules are designed using Si MOSFETs with a maximum blocking voltage of 4.5 kV. The input to the HV H-bridge module is provided by a 10 kV medium-frequency transformer, whose design is described with a focus on the insulation system and winding configuration. This transformer is driven by a zero-voltage switching driver. This arbitrary waveform generator excels in several aspects, including a straightforward design procedure, compact size, high voltage capability, ease of integration, and cost.
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As the integration of renewable energy sources into the grid increases, the insulation systems of grid components such as transformers and switchgear encounter significant challenges due to the transients and harmonics generated by power-electronic-based converters. A test generator capable of replicating these component stresses is essential to accurately evaluate these insulation systems under real-grid conditions. This paper proposes a modular cascaded H-bridge-based high-voltage arbitrary waveform generator, prototyped with three stages to generate customized waveforms (triangular, sawtooth, pulse, and complex) up to 8 kV. The H-bridge modules are designed using Si MOSFETs with a maximum blocking voltage of 4.5 kV. The input to the HV H-bridge module is provided by a 10 kV medium-frequency transformer, whose design is described with a focus on the insulation system and winding configuration. This transformer is driven by a zero-voltage switching driver. This arbitrary waveform generator excels in several aspects, including a straightforward design procedure, compact size, high voltage capability, ease of integration, and cost.
In the future, series-connected SiC MOSFETs will become widely used in power-electronic-based medium and high voltage (HV) applications such as modular multilevel converters (MMCs). Balanced voltage sharing among the SiC MOSFET string is hard to be achieved due to possible deviation on MOSFET switching delay. In this paper, two types of gate signal delay adjustment methods (gate balancing core and improved RC snubber method) are presented to solve the unbalanced voltage sharing caused by MOSFET turn-off delay differences. The methods are validated with laboratory experiments performed on low-voltage prototypes and a performance comparison of these two approaches is given. Both methods significantly improve the drain-source voltage sharing, but the gate balancing core method is most suitable for HV and high-frequency applications.
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In the future, series-connected SiC MOSFETs will become widely used in power-electronic-based medium and high voltage (HV) applications such as modular multilevel converters (MMCs). Balanced voltage sharing among the SiC MOSFET string is hard to be achieved due to possible deviation on MOSFET switching delay. In this paper, two types of gate signal delay adjustment methods (gate balancing core and improved RC snubber method) are presented to solve the unbalanced voltage sharing caused by MOSFET turn-off delay differences. The methods are validated with laboratory experiments performed on low-voltage prototypes and a performance comparison of these two approaches is given. Both methods significantly improve the drain-source voltage sharing, but the gate balancing core method is most suitable for HV and high-frequency applications.
In this study, the immediate time to breakdown for pure epoxy resin is measured by applying a ramp sinusoidal voltage signal at certain frequency (50Hz, 500Hz and 5000Hz) until breakdown occurs. The HV testing procedures, epoxy resin sample and experimental set-up preparation and Weibull analysis of obtained results are elaborated. The experiments were carried out on epoxy samples with the thickness between 0.1mm and 0.2mm and the thickness was divided into three groups for more accurate statistical analysis.
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In this study, the immediate time to breakdown for pure epoxy resin is measured by applying a ramp sinusoidal voltage signal at certain frequency (50Hz, 500Hz and 5000Hz) until breakdown occurs. The HV testing procedures, epoxy resin sample and experimental set-up preparation and Weibull analysis of obtained results are elaborated. The experiments were carried out on epoxy samples with the thickness between 0.1mm and 0.2mm and the thickness was divided into three groups for more accurate statistical analysis.
With widely use of power electronics in medium voltage level, there is increasing concern about behaviour of insulation material under arbitrary voltage stress. Solid-gas insulation is a general choice in medium voltage level. Therefore, it’s unavoidable to encounter surface discharge problem. In this paper, the surface discharge on epoxy-air interface is investigated, which is a commonly used insulation material. Point to plate electrode was used to test the surface flashover voltage and partial discharge (PD) behaviour at 50Hz, 500Hz, 1kHz, 2kHz and 5kHz. As the result, with increasing frequency, PDIV keeps roughly constant and flashover voltage first increases and then decrease, and the frequency obtaining highest flashover voltage is related to gap length.
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With widely use of power electronics in medium voltage level, there is increasing concern about behaviour of insulation material under arbitrary voltage stress. Solid-gas insulation is a general choice in medium voltage level. Therefore, it’s unavoidable to encounter surface discharge problem. In this paper, the surface discharge on epoxy-air interface is investigated, which is a commonly used insulation material. Point to plate electrode was used to test the surface flashover voltage and partial discharge (PD) behaviour at 50Hz, 500Hz, 1kHz, 2kHz and 5kHz. As the result, with increasing frequency, PDIV keeps roughly constant and flashover voltage first increases and then decrease, and the frequency obtaining highest flashover voltage is related to gap length.
In this work, breakdown strength and lifetime curve of oil-impregnated paper (OIP) are compared at 50Hz and 1500 Hz. For the ramp breakdown tests, sinusoidal voltage ramp of 1 kVp/s is used. The ramp breakdown experiments is conducted on OIP samples made of single and double sheets of OIP. Weibull plots and interpretation of the results are presented. In order to obtain lifetime curves, a step sinusoidal voltage waveform whose peak value is close to the minimum OIP ramp breakdown voltage is applied on single sheet of OIP samples. Time to breakdown is measured, and each experiment is repeated between 20-30 times to obtain good statistical results. The experiments were focused on four different voltage levels and the lifetime curves are plotted for those two target frequencies. Parameters of the lifetime curves are extracted and the interpretation is given.
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In this work, breakdown strength and lifetime curve of oil-impregnated paper (OIP) are compared at 50Hz and 1500 Hz. For the ramp breakdown tests, sinusoidal voltage ramp of 1 kVp/s is used. The ramp breakdown experiments is conducted on OIP samples made of single and double sheets of OIP. Weibull plots and interpretation of the results are presented. In order to obtain lifetime curves, a step sinusoidal voltage waveform whose peak value is close to the minimum OIP ramp breakdown voltage is applied on single sheet of OIP samples. Time to breakdown is measured, and each experiment is repeated between 20-30 times to obtain good statistical results. The experiments were focused on four different voltage levels and the lifetime curves are plotted for those two target frequencies. Parameters of the lifetime curves are extracted and the interpretation is given.
Impulse voltage distribution on disk winding
Calculation of disk series capacitance using analytical method
Nonlinear voltage distribution along the transformer winding during a lightning overvoltage can cause severe electric stress specially to the area of the winding close to the high voltage terminals. For high voltage transformers different methods such as interleaved winding, electrostatic shield wires, and wound-in shield wires are implemented on the first few disks to improve such nonlinear voltage distribution. Through application of those methods, the series capacitance of the first few disks is increased which results in reduction of electric stress on the highest stress region close to the high voltage terminal. In this paper disk series capacitances are calculated using the energy summation method and a simplified analytical formula is derived for different configuration of disk winding construction.
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Nonlinear voltage distribution along the transformer winding during a lightning overvoltage can cause severe electric stress specially to the area of the winding close to the high voltage terminals. For high voltage transformers different methods such as interleaved winding, electrostatic shield wires, and wound-in shield wires are implemented on the first few disks to improve such nonlinear voltage distribution. Through application of those methods, the series capacitance of the first few disks is increased which results in reduction of electric stress on the highest stress region close to the high voltage terminal. In this paper disk series capacitances are calculated using the energy summation method and a simplified analytical formula is derived for different configuration of disk winding construction.