PV string current sensor application and iteration

What is a PV string:

In a photovoltaic power generation system , a plurality of photovoltaic modules are connected in series to form a circuit unit having a certain DC output, which is referred to as a component string or a string.

String current sensing has several typical features:

1. The number of circuits for detecting current is generally large, and there are typically 8 channels and 16 channels (combiner boxes), and the inverters are different according to their MPPT designs.

2. There is a certain requirement for the accuracy of the detected current, but no measurement or calculation requirements are required. The greater significance is to monitor the state of the component's power generation in real time.

Application of string current detection:

Confluence box

It means that the user can connect a certain number and the same size of photovoltaic cells in series to form a photovoltaic series, and then connect several photovoltaic series in parallel to the photovoltaic combiner box to form a device for collecting and monitoring. The combiner box is representative of a typical application for string currents.

Inverter

With the rise of distributed power stations and commercial home photovoltaic power generation . Especially in China, string current sensors play an indispensable role as the first monitoring of photovoltaic panels.

Iterative upgrade of current detection originals

Principle

Because the current sent by the photovoltaic panel needs to be monitored in real time, the current is generally about 7A ~ 10A DC current (the current of the double-sided component will be larger). There are many DC solutions to detect this gear. There are detection schemes such as resistors and optocoupler Halls. Here we can talk about the new iteration of the current detection scheme in the PV combiner box.

Resistance scheme:

In low frequency small amplitude current measurement, it shows high precision and fast response speed. In the industrial field, where it is not involved in the electrical isolation between the measuring loop and the current being measured, the shunt is the preferred low-cost solution for converting the current signal into a voltage signal.

Single-chip Hall scheme (SO8 package, etc.):

Take the example of the combiner box, the DC combiner box has the first function without current detection. It is mainly used to connect photovoltaic arrays (strings) and inverters to provide lightning protection and overcurrent protection. However, for large-scale PV power plant projects, intelligent acquisition devices must be added to monitor the operating parameters of the battery strings in the PV array to ensure real-time monitoring of the equipment.

In the early combiner boxes, manufacturers liked to use the Shunt scheme to detect current. The resistor has the advantages of simple use and low cost. However, its shortcomings are also obvious, its large temperature drift and non-isolated characteristics make it not very suitable for use in photovoltaic systems . If you choose a special resistor isolation scheme, you need to choose a better op amp and linear optocoupler, and the overall cost is not low.

In the actual operation of the photovoltaic power station, a certain proportion of the box body was burnt and burned. A large part of the reason for the investigation is that the long-term use of the resistor is always in the energized state, and the looseness at the fixed ends of the two ends causes the contact resistance to be abnormally heated or even arced. The root cause is that the resistance detection scheme is a non-isolation scheme, which is obviously not suitable in industrial grade systems such as photovoltaics.

On this issue, the major PV forums and annual conferences are also often talked about. The market generally believes that the resistance scheme as a current detection scheme in photovoltaic systems has potential risks and is not recommended in principle. Gradually, the mainstream power station began to specify the use of an isolated current sensor (Hall) to replace the resistor in the tender.

There are also differences in the choice of Hall current sensors. The rings and closed loops are generally separated according to the working principle.

In the period when the Hall was used to replace the resistor, there were not many alternative current sensor solutions. Only a few foreign sensor manufacturers had mature solutions, and the open-loop solution at that time was not mature. Under such circumstances, everyone can only use the expensive closed-loop solution on the scalp. Immediately after the domestic manufacturers took advantage of the opportunity to launch a localized cheap closed-loop solution.

The closed loop has a relative advantage in accuracy, but there are several problems in actual use. For example, the number of turns of the closed-loop sensor is large. After the glue is applied, the coil is prone to breakage after being subjected to thermal expansion and contraction. Adding more coil turns couples a larger amount of energy when encountering thunder weather, which can easily damage the sensor's internal chip. But the most fundamental problem is that even the cheap version of the closed-loop sensor, its cost price is not affordable for string current detection.

In the same period, the ring-opening scheme gradually matured, from the HO series of LEM to the ones of the domestic open-loop scheme. In terms of cost and reliability, the open-loop current sensor truly realizes a cost-effective and reliable string current detection scheme.

Note: The single-chip Hall (such as ACS712) between the resistor and the Hall current sensor (magnet ring) is positioned slightly, and is not recognized by most PV users due to its low voltage and current overshoot capability. .

In the open-loop current sensor solution, the current sensor intelligently promotes the company Magtron is favored by more and more customers. The fundamental reason is that its mature self-developed SOC chip and keen market insight can serve customers in a short time. Find the right solution and provide the right pieces.

--Structural articles

When the Open Ring Hall became mainstream. This scheme is preferred for DC current detection of string types. After finding the appropriate principle solution, everyone began to consider the structural problems.

The mainstream core-through Hall is completely isolated and has a high level of safety. Almost all sensors are designed in a vertical format. This design continues the previous vertical solution of the sensor.

Different solutions are not absolutely good or bad, but for the application in the appropriate project, you will greatly reduce your labor costs and overall costs.

Here is an example of a combiner box. Comparing SNEC's 14-year PV show with the 16-year PV show, it's not hard to find that the mainstream combiner box solution is moving in the direction of power density integration. This is also what we have mentioned. In today's principle that the principle scheme is not much different, the structural innovation can bring new design ideas to the whole scheme.

From the flattened monitoring module to the top and bottom stack of positive and negative fuses, to the use of copper bars instead of AWG lines, and even the design of double fuse bases. The structural upgrades of some columns make the space of the entire combiner box more compact and the production efficiency continues to increase. The overall chassis size can be reduced by more than 1/4, and the advantages are self-evident.

When it comes to productivity, you have to mention the string current detection of distributed inverters . It plays a similar role as the combiner box. As part of the inverter, it will be designed more closely. The traditional vertical installation encounters a situation where the PCB board occupies a large area and the threading is inconvenient. The string of wires must first pass through the hole of the sensor and then plug into the MC4 terminal block of the inner casing of the inverter. Two threading will cause problems in productivity for workers.

The current sensor in the form of a bed is just right for this location. The perforation is close to the vacancy of the PCB board, and the PCB hole is close to the MC4 wiring hole. Allowing the threading to be in one step, while occupying only a small portion of the interior of the inverter. If you dig deeper into this type of sensor, you will find that a large part of the field is more suitable for detecting current in this way.

Of course, depending on the location, Magtron offers different options that give users more choice.

Magtron is a global leader in All Programamble PGA Sensors and Magneto-Sensor SoCs, delivering a new generation of smarter, higher power density and differentiated magnetoelectric sensing system solutions. Driven by the industry's general trend of industrial IoT and sensor intelligence, Magtron's innovative technologies such as Quadcore, RCMU, and iShunt enable magnetoelectric sensing, especially current sensor applications, to be highly integrated and easy to use while achieving high performance. Power density is also the first time to achieve high-speed software for sensors.