In a photovoltaic power generation system, several photovoltaic modules are connected in series to form a circuit unit having a certain DC output, referred to as a component string or a string.
String current detection has several typical features:
1. The number of detection currents is generally more, and there are typically 8 16-way (converge boxes). The inverters are different according to their MPPT design.
2. There is a certain requirement for the accuracy of its current detection, but no measurement or calculation requirements are required. The greater significance lies in real-time monitoring of the state of power generation of components.
String Current Detection Application:Confluence box
It means that users can connect a certain number of photovoltaic cells with the same specifications to form a photovoltaic series, and then connect several photovoltaic series in parallel to the PV combiner to achieve the convergence and monitoring devices. The combiner box is a typical application representative of the string current.
Inverter
With the rise of distributed power stations and commercial home photovoltaic power generation. Especially in China, the string current sensor plays an indispensable role as the first monitoring of photovoltaic panels.
Current iteration of current detection
PrincipleBecause of the need for real-time monitoring of the current emitted by the photovoltaic panel, it is concerned that the current is generally 7A ~ 10A DC current (double-sided components of the current will be greater). There are many DC schemes to detect this stall. There are such detection schemes as resistance and optocoupler Hall. Here you can talk about the new iteration of the current detection scheme in the PV combiner box.
Resistance scheme:
In the low-frequency low-amplitude current measurement, high accuracy and fast response speed are exhibited. In the industrial field, a shunt is the preferred low-cost solution for converting a current signal into a voltage signal without involving electrical isolation between the measurement loop and the measured current.
Single-chip Hall solution (SO8 package, etc.):
Hall current detection scheme (magnetic ring type):
Take the confluence box as an example, the DC combiner box had no current detection function at the earliest. It is mainly used to connect photovoltaic arrays (arrays) and inverters to provide lightning protection and over-current protection. However, for large-scale photovoltaic power plant projects, intelligent acquisition devices must be added to monitor the operating parameters of the battery strings in the photovoltaic array to ensure real-time monitoring of the equipment.
In the early convergence box, manufacturers like to use the Shunt scheme to detect the current. Resistors have the advantages of simple use and low cost. However, its disadvantages are also obvious. Its large temperature drift and non-isolated properties make it not very suitable for use in photovoltaic systems. If you choose a special resistor isolation scheme, you need to choose a good op amp and linear optocoupler, and its overall cost is not low.
And in the actual operation of photovoltaic power plants, a certain percentage of the cabinets have been burned. A large part of the reason for the investigation is that the resistors used for a long period of time are always in an energized state, and the looseness at the two ends of the terminals causes abnormal heat due to contact resistance and even arcing. The fundamental reason is that the resistance detection scheme is a non-isolated scheme and it is obviously not suitable for industrial-grade systems such as photovoltaics.
On this issue, the major photovoltaic forums and annual conferences also talked about from time to time. The market generally believes that the resistance scheme has potential risks as a current detection scheme in the photovoltaic system and is not recommended in principle. Gradually, the mainstream power station began to specify in the bidding documents that an isolated current sensor (Hall) be used instead of the resistor.
There are also differences in the selection of Hall current sensors. According to different working principles, the ring and the closed loop are generally separated.
In the period when it was required to use Hall instead of resistance, there were not many current sensor options available. Only several foreign sensor manufacturers had mature solutions, and the open-loop solution at that time was not mature. In such circumstances, everyone can only use the expensive closed-loop solution. Followed by the domestic manufacturers to take advantage of the introduction of the localization of the cheap closed-loop program.
Closed-loop has a relative advantage in accuracy, but there are several problems in practical use. For example, the number of turns of the closed-loop sensor is larger. After the glue is filled, the coil is susceptible to breakage after being subjected to thermal expansion and contraction. In addition, more coil turns are coupled with greater energy when they encounter thunderstorms, which easily damages the sensor's internal chip. But the most fundamental problem is that even the inexpensive version of the closed-loop sensor has a cost price that string current detection cannot tolerate.
At the same time, the open-loop plan gradually matured, from the HO series of LEM to the domestic blooming plan. In terms of cost and reliability, the open-loop current sensor truly enables a cost-effective and reliable string current detection solution.
Note: The single-chip Hall (such as the ACS712) located between the resistor and the Hall current sensor (magnetic ring type) is positioned slightly, and is not approved by most photovoltaic users due to its poor withstand voltage and current overshoot capability. .
In the open-loop current sensor solution, Magtron is favored by more and more customers. The fundamental reason is that its mature self-developed SOC chip and a keen market insight can find the right solution for customers in a short time. Provide reliable parts.
-- StructureWhen Open Loop Hall became mainstream. Any type of DC current detection, will prefer this program. After finding a suitable rationale, everyone began to consider structural issues.
The main core of the Hall is completely isolated and has a high safety level. Almost all sensors are designed to be erected. This design continues the previous vertical solution of the sensor.
Different schemes are not absolutely good or bad, but applying them to suitable projects will greatly reduce your labor costs and overall costs.
Here is still an example of a confluence box. Comparing SNEC's 14-year PV exhibition with the 16-year PV exhibition, it is not difficult to find that mainstream convergence box solutions are moving in the direction of power density integration. This is also what we have mentioned. Today, when the principle scheme is not much differentiating, structural innovation can bring new design ideas to the entire scheme.
From the flat monitoring module to the stacking of positive and negative fuses, to the use of copper wire instead of AWG wire, and even double fuse wire base design. Some of the column's structural upgrades have made the entire confluence box more compact and the production efficiency has been continuously improved. The overall chassis size can be reduced by more than 1/4, and the advantages it brings are self-evident.
When it comes to production efficiency, we have to mention the string current detection of distributed inverters. It plays a similar role as the confluence box. As part of the inverter, it will be designed more closely. The traditional vertical installation encounters a large PCB board area and inconvenient threading. The string must first pass through the hole of the sensor and be inserted into the MC4 terminal lug of the inner shell of the inverter. Two times of threading will cause workers problems in production efficiency.
The horizontal current sensor is just suitable for this kind of place. The perforation closely fits the empty space of the PCB board, and the PCB hole is in close contact with the MC4 wiring hole. Let threading go in one step while occupying only a small part of the space inside the inverter. Dig deeper into this type of horizontal sensor you will also find that a large part of the field is more suitable for current detection.
Of course, depending on the location, Magtron offers different options to give users more options.
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