Jan 26, 2021
Protection scheme of inverter failure in UPS
Abstract: The UPS inverter is directly connected to the load side, often because the user's load is complex and changeable and the user's wiring error causes the inverter to malfunction. The article focuses on the inverter output short-circuit protection method and the bridge-arm over-current protection method to improve the reliability of UPS.
UPS needs to realize long-term uninterrupted power supply for key loads, which puts forward extremely high requirements on the reliability of UPS. In the actual application environment, the user side may cause the UPS output short circuit or the inverter bridge arm to pass through due to operating errors or environmental factors. At this time, the inverter power tube will have a large current passing (this article mainly focuses on the IGBT. It can also be applied analogously to MOSFET).If such fault currents are not detected and effective protection actions are implemented, the collector or drain current of the IGBT will far exceed the safe working area, and the IGBT will be burned due to high power loss due to instantaneous high current. , There may also be overvoltage breakdown damage caused by overcurrent.
Output short-circuit protection: The UPS needs to be able to withstand the short-circuit impulse current repeatedly, and the inverter can be turned off after maintaining 200ms. This requires the current flowing through the IGBT during the short-circuit to be controlled at ICRM (repetitive peak current, generally ICRM=2IC nom) Inside.
Bridge arm through protection: For example, VT2 itself fails and is short-circuited by an external electrical connection. When VT1 is turned on, the bus is directly short-circuited through VT1. This through current rises very fast, generally within 10 μs that can rise to the IGBT rated current After the bridge arm through-through occurs, it is necessary to quickly detect this fault, and block and deadlock the IGBT drive until the system command is reset before allowing the drive signal to be turned on again. In the total life cycle, the general IGBT can not withstand this type of through current more than 100 times. This type of shoot-through protection needs to turn off the drive before the IGBT current does not exceed ISC (transient peak current, generally ISC=4IC nom), and turn off the inverter at the same time.
1 Output short circuit protection
The HALL current sensor is installed to detect the Lf inductor current. When an output short circuit occurs, if VT1 is turned on, the voltage UC1 is short-circuited through VT1 and the inductor Lf, and the inductor current rises rapidly. When the current is detected to a certain range (greater than the normal operating current, less than repeated Peak current ICRM), the VT1 and VT2 drives are blocked. At this time, the inductor current ILf begins to decrease. When the current drops to a certain level, the drive blockade signal is cancelled. If the output has been short-circuited during this process, when the next drive arrives, the inductor current starts to rise again. When the short-circuit protection point is reached, the IGBT drive is blocked again. After 200ms of repeating this, the software logic can judge that the output short-circuit has occurred at this time. Turn off the inverter.
2 Overcurrent protection for bridge arm through
First of all, in order to avoid the through failure caused by the driving signal of the upper tube VT1 and the lower tube VT2 due to the high level at the same time, on the one hand, it is necessary to consider adding a dead zone in the driver software, on the other hand, it is also necessary to adjust the upper and lower tubes on the hardware circuit. The drive waveforms of the two are interlocked by hardware. When the drive levels of the upper and lower tubes are at the same time, the drive voltage is automatically blocked.
In addition, the IGBT may also instantaneously break down due to overvoltage, or its own avalanche failure short-circuit, or short-circuit caused by electrical connections caused by external reasons. At this time, there will be large currents passing through the marks 2, 3, and 4. Protection measures can not completely avoid the possibility of bridge arm through [industrial electrical appliances network-cnelc] in the converter, so how to detect the through fault in time and protect the IGBT when the bridge arm through through occurs is particularly important. .
There are currently two types of bridge arm through protection circuits:
(1) Detect bus current
When the bridge arm bus current Ip suddenly increases to a certain multiple of the rated current, it is considered that a bridge arm through fault has occurred. At this time, all IGBT drives are blocked to eliminate the fault and avoid IGBT burning. This kind of detection circuit is suitable for single-phase small-capacity converters. For three-phase converters or large-capacity converters, due to the large bus rated current, when the single-phase bridge arm is directly connected, the Ip change is not obvious before the IGBT is damaged. Effective protection cannot be implemented.
(2) Use a driving optocoupler with overcurrent detection (for example, HCPL316J).
It can be seen from the characteristics of the IGBT that when the IGBT is turned on, the voltage across its C and E has a linear relationship with the current passing through it. The IGBT can withstand a peak current of 4 times its rated current within 10 μs. When a through-through occurs, the saturation of the UCE is detected. The voltage drop is used to judge the IGBT overcurrent, thereby blocking the drive signal. Its general circuit is shown as in Fig. 3.
When the HCPL316J is driven to a high level, the pin DESAT will charge the capacitor C with a typical charging current of 0.25mA. When the IGBT is turned on, overcurrent occurs, and the UCE rises sharply and exceeds the set UCE protection voltage. DESAT When the pin charging voltage is greater than 7V, HCPL316J will automatically block the drive and soft turn off the IGBT, thereby protecting the IGBT from passing large currents. Through the formula I×△T=C×△U, it can be calculated that the time required for the DESAT pin to charge voltage to 7V is 2.8μs, plus the HCPL action time of 2μs, to ensure that the total protection action time does not exceed the IGBT It can withstand the overcurrent limit time of 10μs, and it can also ensure that the IGBT is turned off before it burns out.
The function of the diode VD is to conduct forward current, which is used to detect the protection voltage drop UCE (DESAT) when the IGBT is turned on. When it is turned off, it blocks the high voltage of the main circuit. During the IGBT turn-off period, there will be a higher dUCE/dt between the C-E of the IGBT, which in turn causes the capacitor charging current between the C-E. In order to avoid false triggering caused by the charging current, it is best to use a fast recovery diode for this diode. The threshold voltage UCE, fault(th) of short circuit protection can be set by the reference voltage Uref (Uref=7V) set in the comparator of HCPL316J and the number of diodes with series connection. Actual protection voltage drop protection point voltage between C-E
where n is the number of diodes in series and UF is the forward voltage drop of the diode.
In order to improve the reliability of the UPS and ensure that it can protect itself in some fault situations, overcurrent protection measures must be taken into consideration for the power module of the inverter. First of all, the over-current fault should be detected in the shortest time, and it is clear whether it is an output short-circuit fault or a bridge arm through fault, and then an appropriate way is taken to protect the power module.