The core device and protection mechanism of photovoltaic inverter
Photovoltaic inverter is a very important equipment of photovoltaic system. Its main function is to convert the direct current generated by the photovoltaic module into alternating current. In addition, the inverter also undertakes the detection component, the power grid, the cable running state, and communicates with the outside world. , system security butler and other important functions. In the photovoltaic industry standard NB32004-2013, the inverter has more than 100 strict technical parameters, and each parameter can pass the certificate. The AQSIQ will also conduct random checks every year, the protection connection of photovoltaic grid-connected inverter products, contact current, power frequency withstand voltage of solid insulation, rated input and output, conversion efficiency, harmonic and waveform distortion, power factor. Nine items, such as DC component and AC output side over/under voltage protection, were tested. A new inverter, from development to mass production, takes more than two years to come out. In addition to over-voltage protection and other functions, the inverter has many little-known black technologies, such as leakage current control and heat dissipation. Design, electromagnetic compatibility, harmonic suppression, critical device protection, efficiency control, etc., require a lot of manpower and material resources to develop and test.
This paper mainly introduces the IGBT protection technology in the inverter.
The IGBT is a power device that plays the role of power conversion and energy transmission in the inverter and is the heart of the inverter. At the same time, IGBT is one of the most unreliable components in the inverter. It is very sensitive to the temperature, current and current of the device. It is slightly over-standard, and it can't be repaired. It can't be repaired. IGBT damage means inverter. The device needs to be replaced or overhauled. Therefore, IGBT is the key protection object of the inverter.
The above is the three modes of IGBT failure, the most common is the electrical fault, because the IGBT bears the current and voltage conversion, and the frequency is very high, the IGBT main circuit is too high, the driving voltage is too high, and the externally generated spike voltage may cause overvoltage. Damage; overload or short circuit of the inverter output may cause overcurrent. Secondly, the temperature is faulty. The IGBT generates a lot of heat during the operation. If the heat cannot be dissipated in time, it may be damaged by overheating. The mechanical failure is likely to be rare during production, processing and transportation. .
1, IGBT drive protection
The IGBT itself is a current switching device. When is it turned on, when is it turned off, how long the switch is controlled by the inverter's CPU, but the DSP output is a PWM signal, the speed is fast, but the power is not enough, the driver is the most The main function is to amplify the PWM signal.
The IGBT controls a large high-frequency and high-current, and generates an electromagnetic interference signal. The driver is closely spaced from the IGBT. Therefore, the driving circuit must have an isolation function. At present, the driving isolation scheme includes an optocoupler, an optical fiber, a pulse transformer, a magnetic coupling, and the like. Kind. The advantages and disadvantages of various methods are as follows:
2, IGBT over current / short circuit protection
When the IGBT is designed, the current generally has a margin of more than 10%. However, when the inverter is in operation, due to component and load short circuit, the load side fault causes overcurrent, and the load side has a particularly large inductive load. When starting and stopping, there is a large harmonic current. At this time, the inverter output current will be A sharp rise has caused the operating current of the IGBT to rise sharply. The IGBT short circuit is divided into two cases: the through-current occurs in the bridge arm of the converter, which is called a type of short circuit. The short-circuit point of the converter occurs on the load side, and the equivalent short-circuit impedance is large, which is called the second-type short circuit. The second type of short circuit is generally considered to be a serious overcurrent in the inverter. At the moment of the short circuit, if the relevant measures are not taken, the IGBT will quickly enter the desaturation, and the transient power consumption will exceed the limit and be damaged, because the IGBT is subjected to the overcurrent for only a few microseconds. Therefore, when a short circuit occurs, the IGBT should be turned off as soon as possible, and the turn-off speed should be gentle, to ensure that the rate of current change is within a certain range, to avoid the voltage being cut too fast and causing the voltage stress to exceed the limit and damage the IGBT, the active clamp Adding quick response measures in the solution enables the IGBT driver to operate as quickly as possible.
3, IGBT over temperature protection
When the ambient temperature of the inverter is too high and the inverter heats up poorly, continuous overheating will cause damage to the IGBT. If the device continues to short-circuit, the power generated by the large current will cause a temperature rise. If the chip temperature exceeds the silicon intrinsic temperature (about 250 ° C), the device will lose its blocking capability, and the gate control will not be protected, resulting in IGBT failure. In design, it mainly considers two aspects: First, strengthen and improve the heat dissipation conditions of IGBT tubes, including air duct design, heat sink design and production, and strengthen refrigeration; second, design overheat detection protection circuit, using IGBT module The built-in thermistor is very accurate to measure the IGBT heat dissipation temperature. When the temperature exceeds the set value, the IGBT is turned off to stop working.
4, IGBT mechanical fault protection
In order to facilitate heat dissipation, the IGBTs are connected by screws and mounted on the heat sink. The connection strength of this screw is very particular. It should be just right. If the power is too large, it will damage the IGBT. If the force is too light, during the transportation and installation process, the vibration will cause poor contact, the thermal resistance will increase, and the device will be over temperature damaged. When installing the IGBT, a special screwdriver is used. According to the IGBT model, the corresponding torque is used to ensure that the IGBT is firmly connected and will not be damaged.
in conclusion
IGBT is the most delicate, sensitive and most vulnerable device in the inverter; it is also the most expensive and most critical device in the inverter. The inverter must take many measures to protect her.
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