What is a photovoltaic string?
In a photovoltaic power generation system, several photovoltaic modules are connected in series to form a circuit unit with a certain DC output, which is called a module string or group string.
String current sensing has several typical characteristics:
1. The number of current detection channels is usually large, typically 8 or 16 (combiner box), while the inverter varies depending on its MPPT design.
2. A certain level of accuracy is required for its current detection, but no metering or calculation is needed. Its greater significance lies in real-time monitoring of the component's power generation status.
Applications of string current sensing:
junction box
A combiner box is a device that allows users to connect a certain number of identical photovoltaic cells in series to form photovoltaic strings, and then connect several photovoltaic strings in parallel to a photovoltaic combiner box for current collection and monitoring. The combiner box is a typical application of string current collection.
Inverter
With the rise of distributed power stations and commercial and residential photovoltaic power generation, especially in China, string current sensors play an indispensable role as the first line of monitoring for photovoltaic panels.
Iterative upgrades of current sensing components
Principles
Because real-time monitoring of the current emitted by photovoltaic panels is required, the focus is typically on a DC current of 7A to 10A (the current for bifacial modules will be even higher). There are many solutions for detecting this level of DC current, such as resistors and optocoupler Hall effect sensors. Here, we can discuss the updates and iterations of current detection solutions in photovoltaic combiner boxes.
Resistor scheme:
It exhibits high accuracy and fast response speed in low-frequency, small-amplitude current measurements. In industrial applications, where electrical isolation between the measurement circuit and the measured current is not required, the shunt is the preferred low-cost solution for converting current signals into voltage signals.
Single-chip Hall effect solutions (SO8 package, etc.):
Taking combiner boxes as an example, the earliest DC combiner boxes did not have current detection functions. They were mainly used to connect photovoltaic arrays (strings) and inverters, providing lightning protection and overcurrent protection. However, for large-scale photovoltaic power plant projects, it is necessary to add intelligent data acquisition devices specifically for monitoring the operating parameters of the battery strings in the photovoltaic array to ensure real-time monitoring of the equipment.
Early combiner box manufacturers preferred to use resistors (Shunt) to detect current. Resistors have advantages such as simplicity and low cost. However, their disadvantages are also obvious: their large temperature drift and non-isolation characteristics make them unsuitable for use in photovoltaic systems. If a special resistor isolation solution is chosen, better operational amplifiers and linear optocouplers are required, and the overall cost is not low.
Furthermore, in the actual operation of photovoltaic power plants, a certain percentage of the enclosures have caught fire and burned. Investigations revealed that a significant portion of the cause was that the resistors were constantly energized during prolonged use, leading to loosening at the mounting points and causing abnormal heat generation and even arcing. Ultimately, the root cause was that the resistance detection solution was non-isolated, which is clearly unsuitable for industrial-grade systems like photovoltaics.
This issue is frequently discussed at various photovoltaic forums and annual meetings. The market generally believes that resistor-based current sensing solutions in photovoltaic systems pose potential risks and are generally not recommended. Gradually, mainstream power plant operators have begun specifying in their tender documents the use of isolated current sensors (Hall effect) to replace resistors.
There are also differences in the selection of Hall current sensors. Based on different working principles, they are generally divided into open-loop and closed-loop systems.
During the period when Hall effect sensors were required to replace resistors, the available current sensor solutions were actually quite limited. Only a few foreign sensor manufacturers had mature solutions, and the open-loop solutions at the time were not yet mature. Under these circumstances, everyone had no choice but to use expensive closed-loop solutions. Subsequently, domestic manufacturers seized the opportunity to launch localized, low-cost closed-loop solutions.
While closed-loop sensors offer a relative advantage in accuracy, several issues arise in practical applications. For instance, the numerous coil turns in closed-loop sensors make them prone to breakage after thermal expansion and contraction following potting. Furthermore, the large number of coil turns means that the high energy coupling during thunderstorms can easily damage the sensor's internal chip. However, the most fundamental problem remains: even inexpensive closed-loop sensors are still prohibitively expensive compared to string current sensing.
During the same period, open-loop solutions gradually matured, from LEM's HO series to the proliferation of domestic open-loop solutions. In terms of cost and reliability, open-loop current sensors truly achieved a cost-effective and reliable string current detection solution.
Note: Single-chip Hall effect sensors (such as ACS712) are positioned in an awkward position between resistors and Hall current sensors (magnetic ring type). Due to their poor voltage withstand and current overshoot capability, they are not recognized by most photovoltaic users.
In open-loop current sensor solutions, Magtron, a company whose intelligent current sensors are driving the increasing popularity of its customers, is primarily due to its mature self-developed SOC chips and keen market insight, which enable it to find suitable solutions for customers in a short time and provide corresponding reliable components.
--Structure
Once open-loop Hall effect sensors became mainstream, they were the preferred choice for string-type DC current sensing. After finding a suitable principle, people began to consider structural issues.
Mainstream through-core Hall effect sensors achieve complete isolation and offer a high level of safety. Almost all sensors are designed with a vertical mounting configuration. This design continues the traditional vertical approach for sensors.
There is no absolute good or bad among different solutions, but when applied to suitable projects, they can greatly reduce your labor costs and overall costs.
Let's take combiner boxes as an example again. Comparing the SNEC 2014 and 2016 photovoltaic exhibitions, it's easy to see that mainstream combiner box solutions are moving towards power density integration. This is what we mentioned earlier: in today's world where there isn't much differentiation in the underlying principles, structural innovation can bring entirely new design ideas to the entire solution.
From the flattened monitoring module to the stacked positive and negative fuses, to the use of copper busbars instead of AWG wires, and even the design of dual fuse bases, a series of structural upgrades have made the entire combiner box more compact, continuously improving production efficiency. The overall chassis volume can be reduced by more than 1/4, and the advantages are self-evident.
When discussing production efficiency, string current sensing in distributed inverters must be mentioned. It serves a similar function to the combiner box. As part of the inverter, its design is more compact. Traditional vertical installations result in a large PCB board footprint and inconvenient wiring. String cables must first pass through the sensor's hole before being inserted into the MC4 terminal block inside the inverter housing. This double wiring process creates production efficiency issues for workers.
Horizontal current sensors are perfectly suited for this type of application. Their through-holes fit snugly against empty spaces on the PCB, while the PCB holes are flush with the MC4 wiring holes. This allows for easy wiring in one step, while taking up only a small portion of the inverter's internal space. Further exploration of this horizontal sensor design reveals that it is particularly well-suited for current detection in a wide range of applications.
Of course, Magtron offers different solutions for different locations, giving users more options.
Magtron is a global leader in AllProgramamble PGA Sensors and Magnetoelectric Sensing SoCs, dedicated to enabling next-generation, smarter, higher power density, and differentiated magnetoelectric sensing system solutions. Driven by the industry-wide trend towards Industrial IoT and intelligent sensing, Magtron's innovative technologies such as Quadcore, RCMU, and iShunt enable highly integrated and user-friendly magnetoelectric sensing, particularly current sensor applications, while achieving high power density and, for the first time, high-speed software-defined sensors.
