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In the research and field application of photovoltaic inverters and energy storage converter systems, the performance issues of current sensors are often attributed to device selection or parameter indicators. However, in a large number of practical projects, we have found that even with the use of current sensors with appropriate performance indicators, measurement results may still exhibit bias or instability. After further analysis, a factor that is often underestimated but has a significant impact gradually emerged – the layout of the busbar and the resulting stray magnetic field.
1、 Realistic scenario: Parameters are correct, but measurement results are ‘incorrect’
In laboratory environments, current sensors are typically tested under ideal conditions: a single conductor, a fixed location, and no strong magnetic interference in the surrounding area.
But in real photovoltaic systems, especially inside high-power density inverters, the situation is completely different. Multiple DC busbars are arranged in parallel, with alternating high currents distributed in a staggered manner on the AC side. Space limitations result in significant changes in the distance between sensors and conductors. After these factors are combined, the magnetic field perceived by the sensor no longer comes solely from the measured current itself.
In practical cases, common phenomena include: systematic deviation of current zero point between different machines; The same device experiences slow zero drift under different load conditions; The measurement fluctuation is significantly amplified under no-load or low current conditions. These problems are often not caused by the failure of the sensor itself, but by the continuous superposition of stray magnetic fields from the busbar on the measurement results.
2、 How stray magnetic fields affect current measurement
The essence of a current sensor is the perception and conversion of magnetic fields. Whether it is a flux gate or a Hall scheme, the presence of an external magnetic field may have an impact on the measurement results.
In photovoltaic systems with complex busbar layouts, stray magnetic fields mainly come from the following aspects:
a. Stacked magnetic field of adjacent mother bars
Multiple parallel busbars under different current directions and amplitudes will form an asymmetric magnetic field distribution, causing the sensor core to be in a non ideal magnetization state.
b. Time varying interference caused by high current on the AC side
The AC output current has periodic changes, and its magnetic field will affect the DC side sensor through spatial coupling, especially when the layout is compact.
c. Offset introduced by structural components and fixing methods
The bending of the busbar, fixing bolts, and metal brackets will all change the local magnetic field distribution, causing the theoretically symmetrical structure to be broken in practice.
For open-loop Hall current sensors, such external magnetic fields are easily superimposed directly on the measurement signal, resulting in zero point shift or increased noise in the low current region. For the closed-loop flux gate scheme, although it has stronger resistance to external magnetism, it may still impose additional burden on the compensation current under extreme layout conditions, affecting system consistency.
3、 Problems are more likely to be amplified under low current conditions
It is worth noting that the impact of stray magnetic fields in the busbar is particularly evident under low current or no-load conditions. This is because in the low current state, the magnetic field generated by the measured current itself is weak, and the proportion of external magnetic field increases instead.
In photovoltaic systems, morning, evening, low light, and partial load operation time account for a considerable proportion. If the current measurement is unstable under these operating conditions, it will not only affect the MPPT algorithm’s judgment, but may also lead to misjudgment of the system’s operating status.
4、 Response strategy: System level rather than device level approach
From practical engineering experience, it is often difficult to completely solve the problem of stray magnetic fields by simply changing the sensor model. A more effective strategy should start from the system level: optimize the direction and symmetry of the busbar, and minimize unnecessary magnetic field superposition; Reasonably plan the installation location of sensors, avoiding close contact with high current conductors or AC side busbars; Prioritize adopting technology routes with stronger resistance to external magnetism at key measurement points; Conduct validation testing under real layout during the prototype stage, rather than relying solely on device level indicators; By treating the current sensor as a part of the system’s magnetic environment rather than an isolated component, stable and reliable measurement results can be obtained in high-power density photovoltaic devices.
Conclusion
In photovoltaic systems, the effectiveness of current measurement is not solely determined by the sensors themselves. The seemingly “structural problem” of busbar layout and stray magnetic field often has a profound impact on measurement accuracy and stability during long-term operation. Only by fully recognizing and addressing this practical factor during the design phase can the value of current sensors in the system be truly realized.
