The photovoltaic inverter is a very important device in the photovoltaic system. Its main function is to convert the direct current from photovoltaic modules into alternating current. In addition, the inverter is also responsible for detecting the operating status of components, power grids, and cables, and communicating with the outside world. , system security housekeeper and other important functions. It takes more than two years to produce a brand-new inverter, and a lot of manpower and material resources need to be invested in R&D and testing in the early stage.
As a power device, IGBT 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 and current of the device. If it exceeds the standard, it will explode and cannot be repaired. Therefore, IGBT is the key protection object of the inverter.
The core application of IGBT protection technology in photovoltaic inverters is reflected in four aspects: drive protection, overcurrent/short circuit protection, overtemperature protection, and mechanical fault protection, as follows.
1. Drive protection
The IGBT itself is a current switching device, and the switching time is controlled by the CPU of the inverter, but the DSP output is a PWM signal, which is very fast, but the power is not enough. The main function of the driver is to amplify the PWM signal.
The IGBT controls a large high frequency and large current, which will generate electromagnetic interference signals. The driver is very close to the IGBT, so the driver circuit must have an isolation function. At present, the driver isolation solutions include optocouplers, optical fibers, pulse transformers, and magnetic couplers. kind.
2. Over current/short circuit protection
When designing IGBTs, the current generally has a margin of more than 10%. However, when the inverter is working, due to the short circuit of components and loads, the fault on the load side causes overcurrent, the load side has a particularly large inductive load, and there is a large harmonic current when starting and stopping. At this time, the output current of the inverter will A sharp rise will result in a corresponding sharp rise in the operating current of the IGBT.
IGBT short-circuit is divided into two situations: a straight-through occurs in the bridge arm of the converter, which is called a short-circuit of the first type, and the short-circuit point of the converter occurs on the load side, and the equivalent short-circuit impedance is large, which is called a short-circuit of the second type. The second type of short circuit can generally be regarded as a serious overcurrent of the inverter. At the moment of the short circuit, if relevant measures are not taken, the IGBT will quickly enter desaturation, and the transient power consumption will exceed the limit and be damaged, because the IGBT is only subjected to overcurrent for 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 current change rate is within a certain range, so as to avoid the voltage stress exceeding the limit and damage to the IGBT due to the excessively fast turn-off. Quick response measures are added to the scheme, so that the IGBT driver can act as soon as possible.
3. Over temperature protection
When the ambient temperature of the inverter is too high, the heat dissipation of the inverter is poor, and the continuous overheating will lead to the damage of the IGBT. If the device is continuously short-circuited, the power consumption generated by the high current will cause a temperature rise. If the chip temperature exceeds the intrinsic silicon temperature (about 250°C), the device will lose its blocking ability, and the gate control will not be able to protect, resulting in the failure of the IGBT.
In the design, two aspects are mainly considered: first, strengthen and improve the heat dissipation conditions of IGBT tubes, including air duct design, design and manufacture of radiators, and strengthen refrigeration; second, design an overheat detection and protection circuit, using IGBT modules on The built-in thermistor is very accurate to measure the heat dissipation temperature of the IGBT. When the temperature exceeds the set value, the IGBT will be turned off to stop working.
4. Mechanical fault protection
In order to facilitate heat dissipation, IGBTs are connected by screws and installed on the radiator. The connection strength of this screw should be just right, neither too strong nor too light. If the force is too large, it will damage the IGBT; if the force is too light, during transportation and installation, due to vibration, it will cause poor contact, increased thermal resistance, and over-temperature damage to the device. When installing the IGBT, a special screwdriver will be used. According to the IGBT model, the corresponding torque is used to ensure that the IGBT connection is firm and will not be damaged.
The IGBT is the most easily damaged device in the inverter, and it is also the most expensive and critical device in the inverter. Therefore, in order to protect the IGBT better, the inverter must take more corresponding protection measures.
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