Foreword: As a component, the inverter has many application fields and is also an important unit of the new energy system. The performance of the inverter directly affects the battery utilization rate and the service life of the equipment. Voltage and current sensors, as important "sensing organs" of the inverter, monitor and feedback key parameters in real time, and collect and calculate by the CPU, and finally make precise control according to the preset conditions. Current and voltage sensors play an important role in fault protection and equipment protection. This paper systematically describes the application scenarios of voltage and current sensors in various types of inverters and the selection strategies of some devices. CHIPSENSE voltage and current sensor play an important role in it.
First. An overview of the inverter system and the core role of sensors
The inverter converts the low-voltage direct current/or the direct current of the energy storage battery into a low-voltage alternating current of a specific frequency through the high-speed switching action of the power electronics. For example, in the home storage inverter, the 48V 46800 battery pack is boosted to 220V Ac @50Hz through the inverter to provide various power devices at the back end. Its typical topology includes DC input, DC/DC boost, DC/AC inverter, and waveform shaping AC output. Voltage and current sensors can play an important role in each link, depending on the goal pursued by the designer.
The overall block diagram is roughly as follows:
Voltage and current sensors play a central role in these aspects:
Real-time monitoring:
Real-time, accurate acquisition of various parts of the current, voltage parameters, passed to the CPU, CPU high-speed A/D, the analog signal into a digital signal for the program control algorithm to provide data basis;
Closed-loop control, process:
Through the monitored voltage and current data, the program performs data operation and logic analysis and judgment, and then realizes the precise control of power devices (SiC /MOSFET/IGBT) to achieve the purpose of closed-loop control;
Fault protection and device protection:
The CPU detects the signals output by the voltage and current sensor devices, and makes corresponding judgments on abnormal states such as overcurrent, short circuit, and leakage of the equipment, and finally triggers the protection mechanism.
Second. The practical application of voltage and current sensors in various aspects of the inverter
1. DC input link:
The battery pack itself usually has a complete BMS system, including battery charging protection and low-voltage protection for low discharge, so it generally does not need to be tested. From the perspective of safety regulations, it is usually only necessary to consider the leakage current monitoring of photovoltaic cells/energy storage batteries. In terms of hardware circuit reliability, it can be considered that the leakage current sensor is arranged here. Therefore, it is a low-voltage area of the power supply, which has less interference and is easy to handle. It will be an ideal location;
2. DC/DC boost:
The DC/DC boost link usually relies on a complete circuit to increase from a low voltage such as 48V to about 300V. Therefore, it is generally not necessary to detect the voltage after boosting; EMC/EMI considerations can be considered to add lightning protection and surge protection here to prevent damage to the back-end components and cause equipment abnormalities. The following lightning protection part of the circuit can meet the test of anti-2000V surge/EFT, etc. See the circuit below:
3. DC/AC inverter link: IGBT protection, bridge arm control and fault protection
Compared with MOS transistors, IGBTs are much more expensive, in order to protect the power transistor from being damaged as much as possible; or, in order to protect the upper and lower bridge arms from burning or even electrical fires due to damage to the power transistor, it is necessary to place a high-precision, fast response time closed-loop Hall current sensor on the + 300V bus to detect such faults. For example, the CN2A series current sensor of CHIPSENSE has a response time of < 0.5us and an accuracy of about 0.2%. Most CHIPSENSE current sensors have a good performance.
The response time of CHIPSENSE CN2A current sensor is less than 0.5 us.
This high-precision closed-loop Hall sensor from CHIPSENSE is placed here to serve two main functions: 1) detect the total output current for calculating and displaying the total output power; 2) protect like a fuse; once the MOS/IGBT of a bridge arm is damaged, it will inevitably cause + 300V to pass through to the ground, resulting in a short circuit. After placing this device, such faults will be quickly detected;
Although the response time of the closed-loop current sensor is less than 0.5us, considering that the waveform data output by the inverter is generated by the calculation of the internal program of the CPU, which leads to a large amount of running, the CPU may not be able to quickly match the response time of 0.5us. Increasing the working frequency of the CPU or upgrading the CPU is certainly a way, but it is best to add a maximum value detection circuit: the analog quantity is then converted into a high and low level digital signal through a window comparator, and finally sent to the interrupt port of the CPU to achieve a fast response, quickly cut off the output of the IGBT, and play a protective role.
Considering that the response speed of the hardware circuit will be much higher than that of the program, then this protection signal can also be introduced into the enable terminal of the driver IC of the MOS/IGBT, which can be turned off faster than the program.
If it is a price consideration, cheaper open-loop Hall devices can also be used, such as CHIPSENSE AN3V/AN1V/AS1V series current sensor, which have a response time of about 3-5 US. So CHIPSENSE current sensor is the best choice for customers when looking for alternative products.
4.AC output link: voltage quality, less harmonics
The load of the inverter is usually used to supply power directly to electrical equipment, power supplies, etc. Therefore, it is necessary to add an inductor to reduce the PWM to an approximate sine wave to improve the power quality of the power supply and reduce harmonics.
In this process, sensors are usually not required, and voltage monitoring is not required.
Application challenges and coping strategies in special environments
Try to choose high-precision, low-temperature drift closed-loop Hall devices. The temperature drift of the device is less than 50ppm/℃, which can effectively avoid some false protection caused by temperature drift. A more reliable approach is to introduce the silicone rubber heating film/PTC heater commonly used in BMS systems. The program is used as the main control to control the start and stop of the heating film, and the mechanical thermostat is used as the secondary protection, which can effectively improve the operating reliability of the entire inverter device under the whole temperature environment.
Packaging: Devices are usually designed to meet IP67 requirements to better meet insulation voltage requirements
High current application scenario
The copper thickness of the PCB is usually 0.5oz, 1oz, 2oz, 5oz, etc. Obviously, in order to flow through a large current, a larger copper cross-sectional area is required, so either widen the wiring or thicken the copper layer thickness; or at the same time widen the line width, thicken the copper layer, or something else. Even so, not only does it increase the cost significantly, but it is also difficult to meet the reliability requirements by relying on the PCB's wiring to go up to 100A: the heat generated by the copper layer cannot be dissipated under the wrapping of the solder mask.
In these high current situations, you can consider a Hall current sensor with a threading method. Up to 10mm2 of copper wire/copper bar can easily flow through 100A. The PCB can choose FR4 with a thickness of 0.5 oz copper layer, and open corresponding pad vias on the PCB. This is still a low-cost solution. Inside, CHIPSENSE can also make customized current sensors.
Conclusion
Voltage and current sensors in photovoltaic inverters have gradually evolved from basic measurement components to core guarantees for system safety and efficiency, and are constantly being recognized by engineers. With the gradual introduction of silicon carbide devices (SiC) devices into the inverter industry, the continuous improvement of switching frequency is bound to push the sensor industry to a new level.
CHIPSENSE is a national high-tech enterprise that focuses on the research and development, production, and application of high-end current and voltage sensors, as well as forward research on sensor chips and cutting-edge sensor technologies. CHIPSENSE is committed to providing customers with independently developed sensors, as well as diversified customized products and solutions.
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