Application Analysis of Current Sensor in Photovoltaic Grid-connected Inverter
Published time: 2020-03-18 11:30:00
The photovoltaic market is becoming saturated. Under the situation of fierce competition, reducing production costs and improving product reliability has become the main way for manufacturers to seize the market; the current sensor is the core detection element in photovoltaic grid-connected inverters. At the same time that product stability is required, high-precision power metering must also be considered.
If classified from the design principles of current sensors, commonly used technologies are open-loop, closed-loop, and fluxgate. Generally, current sensors with different principles are selected to achieve corresponding functions according to different applications.
The open-loop Hall current sensor is based on the direct-measure Hall principle. When the magnetic flux generated by the primary current on the primary side is concentrated in the magnetic circuit by a high-quality magnetic core, the Hall element is fixed in a small magnetic circuit opening air gap space. Here, the change of the magnetic flux is linearly detected. After the Hall voltage output by the Hall device is processed by a special circuit, the secondary side outputs a follow voltage consistent with the waveform of the primary side. This voltage can accurately feedback the change of the current of the primary side.
The closed-loop Hall current sensor is based on the magnetically-balanced Hall principle, that is, the closed-loop principle (also called magnetically-balanced Hall). When the magnetic flux generated by the primary current on the primary side is concentrated in the magnetic circuit through a high-quality magnetic core, the Hall element It is fixed in the air gap to detect the magnetic flux. The multi-turn coil wound around the magnetic core outputs a reverse compensation current to offset the magnetic flux generated by the primary current, so that the magnetic flux in the magnetic circuit is always kept at zero. With the processing of special circuit, the output end of the sensor can output a signal (current output or voltage output) that accurately feedbacks the current change on the primary side.
As we all know, the topology of a typical distributed photovoltaic inverter (as shown in the figure below) includes a DC input link (string input bus), a DC boost link (Boost MPPT line), and a DC inverter AC link (DC / AC line ), And AC output (leakage current detection), current detection is essential in every link.
DC link open loop current sensor
At present, most manufacturers choose open-loop current sensors on the DC side (string current detection or DC / DC Boost line input current detection), because the DC side current detection is only for measurement and does not participate in protection, so the accuracy requirements It is not very high, and usually the accuracy of 1% -2% can meet the requirements. As for the lack of temperature characteristics, you can use software algorithms to correct and compensate for hard parameter indicators such as zero temperature drift and accuracy. Contribute to the consistent use of the current sensor. And the cost of the open-loop current sensor is lower than that of the closed-loop current sensor, so the advantages of the open-loop sensor on the DC side are obvious.
AC Link Closed Loop Current Sensor
At present, most domestic manufacturers use closed-loop sensors on the AC side, because the output of the AC-side current sensor is generally used for software control. If the accuracy is too low, it will affect the detection and control of some key quantities. For example, the detection and extraction of DC components. Although the acceptance value of DC components is different in each country, it needs to be controlled at 0.5% or even 0.25% of the nominal output current. Therefore, only closed-loop sensors can meet the requirements of high accuracy.
At present, with the high integration of photovoltaic modules, the improvement of new device processes, and advances in technology developed by inverter manufacturers, the power of individual modules of photovoltaic inverters has become larger and larger, and the power density has become higher and higher. For current sensors, The choice also raises higher requirements. In addition to having conventional electrical performance, it also requires:
a) Small size, high insulation and voltage resistance, high integration, easy to automate production
When the layout space for current measurement on a printed circuit board is relatively small, a chip-type current measurement scheme is ideally used. Integrate the primary conductor and directly surface mount the device to the printed circuit board, thereby reducing manufacturing costs and avoiding confusion about various soldering processes. LEM's latest GO-SMS (bottom left) / HMSR-SMS (bottom right) series current sensors are chip-type current sensors in SMD packages.
In addition to meeting the small size, the original secondary pin design of GO-SMS also achieved 7.5mm creepage and clearance distance, respectively. The original secondary pin design of HMSR-SMS reached 8.0mm creepage and Clearance distance. The package uses 600 CTI material to accumulate, making it have high isolation performance (test isolation voltage: 4.3kVrms / 50 Hz / 1min), among which HMSR chip type current sensor is specially used for solar system with 1500Vdc DC input.
b) 10kA anti-surge capability
At present, the design of inverter manufacturers generally has the risk of lightning surges on the lines directly connected to photovoltaic modules (PV panels) or the power grid. In order to help manufacturers simplify the lightning protection design at network ports, HMSR-SMS In consideration of the anti-surge capability of the primary side, a specially optimized primary conductor is designed. When the primary side passes the lightning surge current of 10kA 8 / 20us, the chip can still work normally without any failure.
c) Built-in overcurrent protection alarm function
GO-SMS / HMSR-SMS chip-type current sensor can be used for peak current detection. It is used to compare the true value with the set point (protection point). The protection point can be set using the built-in (factory default) or external ( (The user modifies the value), and outputs a valid low-level alarm message through the dedicated OCD pin to notify the controller (DSP) of the over-current signal generation so that the DSP can respond quickly to protect the IGBT and other devices in the circuit.
In addition to the above features, LEM's chip-type current sensors can measure nominal AC, DC, pulse, and hybrid isolated currents, with a wide measurement range of ± 3 x Ipn and a bandwidth of 100kHz.
GO-SMS / HMSR-SMS chip-type current sensor design is based on the unique LEM open-loop Hall-effect ASIC technology of the HG2 ORION platform. It is marked with CE mark and conforms to EN50178 standard. Compared with traditional discrete technologies, it has a wider operating temperature range (-40 to + 85 ° C), better offset and gain drift, and linearity.
GO-SMS / HMSR-SMS chip-type current sensor operates through a + 5V power supply and provides a configurable reference voltage (2.5V). The gain and offset are fixed and set. The output corresponding to the rated measurement current The voltage is equal to the input or output reference voltage ± 0.625 V. It can provide a rated current detection capability of 10-30A, which is very suitable for string current detection on the DC side and input current detection of the DC / DC Boost circuit.Tag: Current Sensor