When it comes to renewable energy devices, maximum efficiency and high-power density play a key role in increasing their market potential. The adoption of such energy, such as solar power, requires efficient and reliable power converters to regulate voltage and current, achieve maximum power point tracking, and ensure optimal system performance. Wide bandgap devices, like silicon carbide (SiC) and gallium nitride (GaN), offer improved efficiency and reliability compared to traditional converters. This development has enabled the creation of competent and reliable PV converters, which are in high demand as the use of solar energy continues to grow.

Wide bandgap (WBG) devices present challenges when used in renewable energy systems (RES). Fast switching speeds can lead to increased electromagnetic interference (EMI) and degraded system performance while minimizing stray capacitance and inductance is crucial to avoid overvoltage. High market prices hinder widespread adoption, but paralleling Si and SiC devices allows for quasi-soft switching and improved efficiency at light to medium loads in PV and wind power converters.

WBG Devices in PV Converters and Wind Turbine Converters

PV inverters are characterized based on their power output, and they can be classified into four main categories, namely residential (<10kW), small commercial (10-100kW), large commercial (100-250kW), and utility applications (250kW1MW). Figure 1 depicts a typical commercial-scale PV inverter system, where the efficiency and power density of the system are primarily influenced by the dc/ac inverter and the output passive filter.

WBG devices have the potential to increase the switching frequency and operating temperature of the inverter system, resulting in a more compact and energy-efficient design. By utilizing WBG devices, the system can achieve higher power density and improved outputs, making it an attractive option for various applications.

SiC devices are rated at higher voltage levels, typically at 1.2 kV or 1.7 kV, while GaN devices are rated at lower voltage levels, typically no more than 600V. The higher voltage rating of SiC devices allows for their use in medium and high-power applications such as power converters, motor drives, and grid-tied inverters. On the other hand, the lower voltage rating of GaN devices makes them suitable for low-power applications such as microinverters and small string inverters for residential and commercial solar installations.

Due to this, such devices have also emerged as an attractive solution for power conversion in wind turbine systems. Most present-day wind turbines utilize variable-speed generators, which require control by power converters. WBG devices are ideal for solid-state power conversion in wind turbines, offering high efficiency and power density, which is important given that over 75% of wind turbines sold require power converters for control. This includes power converters, passive filters, and cooling systems.

Furthermore, the high-temperature stability of WBG devices makes them more reliable and enables better thermal management in the power conversion unit. The use of WBG devices in wind turbine systems has the potential to improve system performance, reduce maintenance costs, and increase energy yield.

Improved Efficiency Wind Turbine Power Converters Using “SIC+SI” Hybrid Devices

Using three-phase transformer less PV inverters in commercial-scale solar systems poses a challenge due to the large power ratings involved. One major impediment to implementing SiC devices in such inverters is their high cost. However, this challenge can be overcome by utilizing hybrid devices, which combine SiC and Si devices in parallel connection to achieve higher efficiency at light or medium load conditions, thereby reducing the required SiC ratings and decreasing device cost.

The 400A/1.2kV hybrid module's basic structure comprises one SiC MOSFET (100A/1.2kV) and three Si IGBTs (100A/1.2kV). The parallel connection of SiC and Si devices allows for efficient operation at various load conditions, with the Si devices taking up the load at low or medium load conditions, while the SiC device takes up the high load.

Fig 1: “SiC+Si” 400A/1.2kVhybrid device.

Fig 2: “SiC MOSFET + Si IGBTs” switching pattern.

The switching pattern depicted in Figure 2 for the hybrid SiC MOSFET-IGBT devices is designed to achieve quasi-zero voltage switching (ZVS) during the turn-on and turn-off of the devices. This is done by turning on the SiC MOSFET earlier than the IGBTs during turn-on and turning off the SiC MOSFETs later than the IGBTs during turn-off. The turn-on and turn-off delays are carefully selected to be shorter than the dead time between complementary switches and longer than the turn-on/turn-off time specified in the manufacturer's datasheet for each device.

However, it should be noted that during the turn-on and turn-off delays, the entire load current flows through the SiC MOSFET, which may lead to device reliability issues under heavy load conditions. Therefore, this switching pattern is recommended only for light or medium load conditions. Overall, the careful selection of turn-on and turn-off delays is critical for achieving quasi-ZVS and improving the efficiency of hybrid SiC MOSFET-IGBT devices. Nonetheless, the impact of load conditions on device reliability should also be taken into account when designing power electronics systems using these devices.

Fig 3a: Efficiency comparison of DFIG power converters using hybrid devices and Si IGBTs. Fig 3b: DFIG power converter efficiency compared between "SiC+Si" hybrid devices and Si-IGBTs at varying wind speeds.

The simulation results presented in Fig. 3a & 3b demonstrate the potential of using hybrid devices based on major Si IGBTs and minor SiC MOSFET/JFET to improve the efficiency of wind turbine power converters. It is noteworthy that the efficiency gain of using hybrid devices becomes more significant as the switching frequency increases. This is because the switching losses of SiC devices increase proportionally with the switching frequency, while the conduction losses of Si IGBTs decrease as the switching frequency increases. Therefore, a higher switching frequency can fully exploit the benefits of using hybrid devices.

Moreover, the efficiency comparison results in Fig. 3b indicate that the DFIG converters using hybrid devices have higher efficiency than the Si-based counterparts. This is due to the lower conduction losses and faster switching speed of SiC devices, which can reduce the power losses and improve the dynamic response of the converter. Overall, these simulation results suggest that using hybrid devices can be a cost-effective approach to improve the efficiency and performance of wind turbine power converters, and can contribute to the wider adoption of wind power generation.

Challenges of Wide Bandgap Devices in Renewable Energy Power Converters

The utilization of WBG devices in power converters for renewable energy conversion systems can greatly enhance system performance. Nevertheless, this application of WBG devices also brings practical challenges that need to be addressed.

To address the challenge of increased EMI, several recommended solutions have been proposed, including shielding, filtering, and layout optimization. To attenuate EMI signals in power converters, shielding can enclose the noise source, filtering can insert passive filters, and layout optimization can reduce radiation by shortening distances between high-frequency circuits and using ground planes.

WBG devices face reliability challenges due to high dv/dt and di/dt, but these can be addressed with snubber circuits, increased gate resistance, and optimized driving schemes. Snubbers suppress overvoltage and overcurrent, while increased gate resistance slows down switching speed to reduce stress. Optimized driving schemes improve switching performance and reduce dv/dt and di/dt stress.

The cost of WBG devices impedes their adoption in RES power converters, but a hybrid approach using Si and SiC devices can enhance efficiency without raising costs significantly. Hybrid paralleling and co-pack modules reduce switching losses by up to 40%, allowing for a higher operating frequency. Replacing Si-based three-level NPC converters with WBG-based two-level topologies achieves higher efficiency and reduced complexity and cost, making it a viable option for RES power converters.

Conclusion

The experiment analyzed the performance benefits of utilizing SiC and GaN devices in power converters for PV and wind power systems. A low-cost, high-efficiency hybrid device was proposed and simulated for large-rated power converters used in RES. The simulation results verified the efficiency improvements of this hybrid device, making it a cost-effective solution for large PV and wind systems, as to traditional systems. The tests also discussed challenges associated with using WBG devices in RES, including increased EMI and high system cost, and reviewed potential solutions. These findings provide valuable insights for future industrial users and researchers in related fields.