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Introduction
In the core architecture of photovoltaic systems, the electrical isolation between the high-voltage side (up to 1500Vdc) of the inverter and the low-voltage circuit of the control side is a key link to ensure equipment safety and signal reliability. As a “signal transmitter” that spans high and low voltage ranges, the magnetic isolation performance of current sensors directly determines the insulation safety, signal anti-interference ability, and long-term operational stability of the system. With the upgrading of photovoltaic system voltage levels from 1000Vdc to 1500Vdc and the high-frequency trend brought by SiC devices, the limitations of traditional magnetic isolation technology are gradually becoming prominent. New magnetic isolation solutions are becoming the core support for adapting to high-voltage and high-frequency photovoltaic scenarios through technological innovation.
1、Triple core demand for magnetic isolation in photovoltaic scenarios
The special working conditions of photovoltaic systems require targeted requirements for magnetic isolation technology of current sensors: firstly, high voltage insulation reliability. The insulation requirements of a 1500Vdc system are 50% higher than those of a 1000Vdc system, and must meet the mandatory requirements for creepage distance (≥ 8mm/kV) and electrical clearance in IEC 61730 “Safety Requirements for Photovoltaic Modules”. At the same time, it must withstand transient high voltage impacts such as lightning surges and grid overvoltages to avoid insulation breakdown and safety accidents. Secondly, high-frequency signals are isolated without distortion. The switching frequency of SiC inverters has exceeded 100kHz, and the high-frequency components in the current signal need to be transmitted through an isolation layer without attenuation. If there are parasitic capacitance and inductance in the isolation layer, it will cause signal phase shift and amplitude attenuation, affecting MPPT control accuracy and overcurrent protection response speed. Third, long-term stability and anti-aging ability. The temperature and humidity fluctuations in outdoor environments, as well as ultraviolet radiation, can accelerate the aging of isolation materials, leading to a decrease in insulation resistance and a decline in isolation performance. It is necessary to ensure that the isolation performance degradation does not exceed 10% of the initial value during the 25 year lifecycle, in order to avoid system failures caused by isolation failure.
2、The adaptation shortcomings of traditional magnetic isolation technology
The traditional magnetic isolation scheme, which is still widely used, has obvious limitations in high-voltage and high-frequency photovoltaic scenarios: on the one hand, the frequency bottleneck of power frequency magnetic core isolation. Traditional Hall sensors use a power frequency ferrite core as the isolation medium, which has a large parasitic inductance (usually>1 μ H). In high-frequency scenarios above 100kHz, it can cause a signal phase shift of more than 5 ° and an amplitude attenuation rate of 8% -12%, which cannot meet the precise measurement requirements of high-frequency currents. On the other hand, there are inherent deficiencies in the design of insulation structures. Some products meet high voltage requirements by simply increasing the thickness of the insulation layer, but do not optimize the creepage circuit design. In high humidity and salt spray environments, conductive paths are easily formed due to surface condensation, leading to a decrease in insulation resistance; At the same time, the anti-aging performance of traditional insulation materials (such as ordinary epoxy resin) is poor, and the insulation resistance may drop below 30% of the initial value after 5000 hours of damp heat aging test. In addition, the anti-interference short board of magnetic coupling isolation. Traditional magnetic isolation relies on the magnetic coupling of the magnetic core to transmit signals, which is susceptible to interference from high-frequency electromagnetic fields in power plants, resulting in signal crosstalk between the two ends of the isolation layer and a signal-to-noise ratio of less than 50dB, affecting the precise identification of current signals by the control circuit.
3、Three major evolutionary directions of magnetic isolation technology
In response to the core demands of photovoltaic scenarios, the magnetic isolation technology of current sensors is upgrading towards “high insulation, high frequency, and long lifespan”: firstly, the application of new isolation magnetic core materials. By using nanocrystalline alloys and amorphous magnetic cores instead of traditional ferrite cores, the parasitic inductance can be reduced to below 0.1 μ H. Within a wide frequency range of 10Hz-100kHz, the phase shift is controlled within ± 0.5 °, achieving distortion free isolation of high-frequency signals and adapting to the high-frequency conditions of SiC inverters. The second is the optimization of insulation structure and materials. The composite structure of “magnetic core isolation+double-layer insulation packaging” is adopted: the inner layer uses high voltage resistant and anti-aging polyimide (PI) film as the main insulation layer, and the outer layer uses modified epoxy resin for encapsulation; At the same time, optimize the design of the shell creepage path, increase the creepage distance to over 12mm/kV, and meet the insulation requirements of the 1500Vdc system. After 10000 hours of wet heat aging testing, the insulation resistance still remains above 10 ¹² Ω. The third is the integration of digital magnetic isolation solutions. By embedding an integrated digital isolation chip (such as a magnetic isolation ADC), the current signal is digitized on the high voltage side and transmitted to the low voltage side through a magnetic isolation channel, avoiding distortion and interference of analog signals during the isolation process. The signal-to-noise ratio of this scheme can reach over 70dB, and its ability to resist electromagnetic interference is improved by 40% compared to traditional schemes, while simplifying the complexity of system integration.
Conclusion
The upgrading trend of photovoltaic systems towards high voltage and high frequency is driving the transformation of magnetic isolation technology for current sensors from “basic insulation protection” to “high performance, long life, and high frequency adaptation”. The new magnetic isolation scheme, through material innovation, structural optimization, and digital integration, not only solves the frequency bottleneck and insulation aging problems of traditional technology, but also adapts to the application requirements of 1500Vdc systems and SiC devices. In the development stage of pursuing both safety and efficiency in the photovoltaic industry, magnetic isolation performance has become one of the core indicators for selecting current sensors. Only by breaking through the limitations of traditional technology and achieving high insulation, high-frequency, and long-life magnetic isolation solutions can we truly support the safe and stable operation of photovoltaic systems and provide reliable guarantees for the landing of high-voltage and high-frequency photovoltaic technology.
