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In the on-site operation of photovoltaic and energy storage systems, a recurring phenomenon is that the equipment runs smoothly under rated or high load conditions, and various parameters appear normal. However, once it enters the early morning, evening, or partial load operation phase, various measurement abnormalities, control jitter, or inconsistent data problems begin to appear. Low load conditions often become the most concentrated “amplifier” for on-site problems.
This phenomenon is not accidental, but is caused by the differences between current measurement characteristics, system design assumptions, and actual operating conditions.
1、 The signal amplitude decreases, and the proportion of interference increases rapidly
Under low load conditions, the current amplitude significantly decreases and the measured signal itself weakens. At the same time, the inherent noise sources in the system, including zero offset, thermal noise, common mode interference, and stray magnetic fields, do not decrease proportionally with the current.
When the effective signal and interference are at the same level, errors that could have been ignored under high loads quickly become apparent, manifested as current fluctuations, zero drift amplification, or channel inconsistency. This is also why many systems perform well in rated power testing, but have frequent problems in low light conditions.
2、 Zero stability is truly tested under low loads
The zero point performance of current sensors is often masked during high current operation; Under low load conditions, zero stability directly determines the reliability of the measurement.
During long-term operation, temperature changes, device aging, and mechanical stress can all affect the zero point. If the zero point control is insufficient, even a small deviation in the low current range may be misjudged by the system as a valid signal, thereby affecting the control algorithm’s judgment. This issue is particularly prominent in photovoltaic systems, as weak light and partial load operation time account for a considerable proportion.
3、 System design assumptions often lean towards rated operating conditions
During the system design and validation phase, testing resources and focus are often concentrated on rated power and extreme operating conditions. This approach itself is not a problem, but it also easily forms an implicit assumption: as long as the system performs stably under rated conditions, the overall system has reliability.
However, low load conditions are not simply “scaled down rated conditions”. Control strategies, measurement links, and interference environments exhibit different characteristics at low signal levels. If these operating conditions are not fully validated during the design phase, on-site issues will be concentrated in the early stages of operation or specific time periods.
4、 The control strategy is more sensitive in the low current region
Under low load conditions, MPPT、 The current loop and power regulation algorithm are more sensitive to changes in the measurement signal. Minor measurement fluctuations may trigger frequent adjustment actions, manifested as power oscillations or frequent switching of operating states.
This type of problem is not due to the failure of the algorithm itself, but rather the insufficient measurement signal quality to support the precise judgment of the control strategy. Low load conditions often become concentrated exposure points with insufficient matching between measurement and control.
5、 The difference in on-site environment is amplified under low load
The differences between different installation environments, such as busbar layout, grounding methods, and electromagnetic environment, are often masked by strong signals under high load conditions. Under low load conditions, the impact of these differences on measurement results is amplified, resulting in inconsistent performance between different devices.
This is also why some problems are difficult to reproduce in the laboratory, but repeatedly appear on multiple devices on site.
Conclusion
Low load conditions are not abnormal states, but rather one of the normal operating conditions of photovoltaic systems. The frequent occurrence of on-site problems in this working condition essentially reflects the insufficient adaptability of the system to low signal environments. Only by fully understanding and verifying the measurement and control characteristics under low load operating conditions during the design phase can we avoid problems from being exposed on site and build a truly stable and reliable photovoltaic system.
