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Introduction
With the continuous increase of photovoltaic installed capacity, the compatibility requirements of the power grid for photovoltaic systems are becoming increasingly stringent – voltage fluctuations, harmonic distortion, three-phase imbalance and other issues have become the core bottlenecks restricting the smooth grid connection of photovoltaics. As a “signal bridge” between the photovoltaic system and the power grid, the stability and accuracy of the output signal of the current sensor directly determine the effectiveness of the grid connected control strategy. Under the dual demands of power grid upgrading and photovoltaic consumption, the “grid adaptation capability” of current sensors is shifting from an implicit requirement to a rigid indicator, becoming a key factor affecting the grid connection rate of photovoltaic projects.
1、 Three core challenges of grid compatibility for current sensors
In the scenario of photovoltaic grid connection, the dynamic changes on the grid side and the complex operating conditions on the system side pose targeted requirements for current sensors: firstly, the demand for wideband response. The harmonic interference in the power grid (mainly concentrated in the 2nd to 50th harmonics) is superimposed with the high-frequency switching signal of the inverter, requiring the sensor to accurately capture the wideband current signal of 20Hz-1kHz. Traditional sensors often focus on accuracy near the power frequency (50Hz), which can lead to signal attenuation in the harmonic frequency range, causing grid connected controllers to be unable to accurately identify harmonic components and affecting the execution of filtering strategies. Secondly, the ability to capture transient signals. Transient events such as sudden increases and decreases in grid voltage, lightning surges, etc. (lasting only milliseconds) can cause instantaneous fluctuations in the current of photovoltaic systems. If the response delay of the sensor exceeds 3 μ s, it will cause the grid protection device to trigger or refuse to operate, which may result in short-term disconnection or damage to the equipment. Thirdly, the ability to resist electromagnetic interference. The inverters, transformers, combiner boxes and other equipment in photovoltaic power plants are densely packed, forming a complex electromagnetic environment. If sensors lack effective electromagnetic shielding design, their output signals are easily interfered by clutter, leading to grid connected controllers receiving incorrect signals and exacerbating grid voltage distortion.
2、 Shortcomings in grid adaptation of traditional current sensors
The traditional current sensors that are still widely used in current photovoltaic projects are gradually exposing their shortcomings under the requirements of grid compatibility: on the one hand, the frequency response range is limited. The effective frequency response of most traditional Hall sensors is only 50Hz-2kHz. When dealing with grid harmonics, the signal attenuation rate for harmonics above the 20th order can reach more than 10%, which cannot meet the accuracy requirements for harmonic monitoring in GB/T 19964-2012 “Technical Regulations for Photovoltaic Power Station Access to Power System”. On the other hand, there is insufficient control over phase error. The phase synchronization between current and voltage directly affects the accuracy of power factor calculation. Traditional sensors can have a phase error of ± 3 ° over a wide load range, causing the grid connected power factor to deviate from the target value (usually requiring ≥ 0.9), triggering rectification requirements from the power grid dispatch department. In addition, in three-phase photovoltaic systems, the consistency deviation of traditional sensors (mostly above ± 1%) can exacerbate the imbalance of three-phase currents. When the imbalance exceeds 10%, it will be judged as grid connected failure by the power grid dispatch system.
3、 Optimization Path of Grid Adaptation Technology for Current Sensors
For the core requirements of grid connected scenarios, the technological optimization of current sensors needs to focus on three directions: firstly, expanding the frequency response bandwidth. By using new sensitive materials such as giant magnetoresistance (GMR) and tunnel magnetoresistance (TMR) to replace traditional Hall elements, the effective frequency response of the sensor is extended to 10Hz-10kHz, ensuring distortion free acquisition of harmonic signals and providing accurate data support for harmonic suppression algorithms in grid connected controllers. The second is to optimize the transient response and phase characteristics. By using digital signal processing technology, the sensor response time is compressed to within 1 μ s, and the phase error within a wide load range is controlled within ± 0.5 ° through phase compensation algorithm, ensuring the accuracy of power factor calculation and the reliability of transient protection. The third is to strengthen electromagnetic compatibility design. By adopting a double-layer shielding structure, low impedance grounding circuit, and differential signal output method, the sensor’s ability to suppress radiated electromagnetic fields and conducted interference is improved, ensuring that the signal-to-noise ratio of the output signal in complex electromagnetic environments is ≥ 60dB, meeting the requirements of grid connected equipment in GB/T 17626 “Electromagnetic Compatibility Testing and Measurement Techniques”.
Conclusion
In the industrial stage of photovoltaics moving from “self use” to “full grid connection”, grid compatibility has become one of the core competitiveness of photovoltaic projects. As the “signal source” of grid connected control, the technological iteration of current sensors has always revolved around the requirements of the power grid – from simply pursuing power frequency accuracy to considering wideband response, transient capture, and anti-interference capabilities. Essentially, it is a reflection of the deep adaptation of photovoltaic technology to the requirements of the power grid. In the future, with the improvement of the intelligence level of the power grid, current sensors will further develop towards “high precision, high reliability, and digitization”, becoming a key support for ensuring efficient grid connection of photovoltaics and promoting energy structure transformation.
