Analysis of UPS backend load damage cases
A 3K UPS with single-phase input and single-phase output, with back-end loads for office computers and LCD screens. Under normal mains power conditions, the UPS converts to battery inverter power supply, but the backend load is found to be damaged and unable to start.
Through the introduction of several cases, understand the possible situations that may cause damage to the UPS backend load.
Case 1
A certain customer uses a 3K UPS with single-phase input and single-phase output, with a backend load of office computers and LCD screens. Under normal mains power conditions, the UPS converts to battery inverter power supply, but the backend load is found to be damaged and unable to start.
On site inspection of the UPS mains power input first revealed that the front-end mains power was normal, while the zero live wire at the UPS input terminal had no voltage. Further inspection revealed that there is a 220VAC voltage between the UPS input terminal's neutral wire and the ground wire. Finally, it was found that the zero wire of the external input socket had poor contact.
The voltage from the input live wire to the input zero wire is the mains voltage (220VAC) (PE is connected to the input zero wire at the far end). The zero wire inside the machine is connected to the input live wire by an X capacitor and a sampling resistor (components connected at A and B), while the input zero wire is disconnected at C. Therefore, the potential at A and B inside the machine is basically the same, that is, the voltage between the output zero wire and PE is the mains voltage of 220VAC. After the input neutral line is disconnected, the UPS switches to battery inverter operation, and the inverter is in the local oscillator state without phase locking. The phase difference between the inverter output 220VAC between the live and zero lines and the mains supply 220VAC between the output zero and ground can vary from 0 to 360 degrees. Causing voltage fluctuations between the output live wire and ground wire (OUTL-PE) from tens of volts to three to four hundred volts. When the two are completely in phase, the maximum voltage between OUT-L and PE can reach over four hundred volts, which can cause damage to the input varistor (lightning protection circuit) of the backend load equipment.
Handling methods and suggestions：
After replacing the socket with poor contact with the external neutral wire, power on again, and the UPS is working normally. Various switching tests have also shown that the backend load is working properly. For single-phase UPS, measures should be taken to ensure that the input neutral line of the UPS is not disconnected separately.
Case 2
A certain customer uses a 6K UPS with single-phase input and single-phase output, with a backend load of office computers and LCD screens. After the power outage, the UPS switched to battery inverter power supply normally, but when calling again, it was found that the backend load had a sticky smell. The UPS has been supplying power normally.
Check that the output voltage of the UPS is normal. Check the input voltage of the UPS and find that there is a voltage of 220VAC between the neutral and ground wires. Finally, confirm that the live and neutral wires of the UPS input are reversed.
Because the input neutral wire is reversed, the voltage from the input neutral wire LIN to the input PE is the mains voltage (220VAC) (PE is connected to the far end of the input neutral wire), resulting in the output neutral wire OUTN and PE also having the same voltage.
If UPS operates on a bypass, the output OUT is the mains zero NIN. So the voltage between the output live wire OUTL and PE is very low, and the voltage between the output neutral wire OUTN and PE is 220VAC. It has no impact on the backend load.
If UPS operates in an inverter, the 220VAC output of the inverter will synchronously track the frequency and phase of the input voltage. The 220VAC inverter voltage between OUTL and OUTN cancels out the mains voltage between OUTN and PE, so the voltage between the output OUTL and PE is also very low. But if there is a power outage, the inverter will work at the local oscillator frequency and cannot synchronize the input. If you call again, the inverter will synchronize the input voltage, but there is a tracking synchronization stage. During this stage, the inverter is not synchronized with the input, resulting in an indefinite phase difference between the inverter voltage 220VAC between the output OUTL and OUTN, and the mains voltage 220VAC between OUTN and PE, which can vary from 0 degrees to 360 degrees. Causing voltage fluctuations between output (OUTL-PE) from tens of volts to three to four hundred volts. When the two are completely in phase, the maximum voltage between OUT-L and PE can reach over four hundred volts, which can cause damage to the input varistor (lightning protection circuit) of the user's equipment.
Handling methods and suggestions：
After rectifying the input wiring and re powering on, the UPS is working normally and the output voltage is also normal. Engineering construction should be carried out in accordance with regulations to ensure that the single-phase UPS input neutral wire is not reversed.
Case 3
Use two sets of 500KVA 1+1 UPS parallel systems as dual bus powered A/B inputs. One day, a customer reported that the main input switch of the B-way system's power distribution (ATS composed of ABB's 4P switch) was disconnected, and it was also found that some output column cabinets had voltage fluctuations displayed on the voltmeter, and some rear end loads were damaged.
After the engineer arrived at the site, the customer had already closed the power distribution main input switch and the system was supplying power normally. Upon checking the disconnected main input switch of the B system distribution, it was found that the ATS of the 4P was set to "manual" mode. On site inspection was conducted on the A/B UPS, and it was found that the power supply was normal. The internal history records of the A system machine were downloaded, and there were records of input interruptions. The on duty personnel in the computer room also reported that the lighting in the computer room was flashing at that time.
A comprehensive inspection was conducted on the output distribution cabinet and the rear column head cabinet, and it was found that the two input neutral wires of some column head cabinets were reversed.
Because the main input switch of the front-end system's power distribution is equipped with an undervoltage trip coil, system A is set to "automatic" and system B is set to "manual", after the mains flash, system A automatically recovers after the switch is disconnected. The back-end equipment has a record of short-term input interruptions, while system B keeps the switch disconnected. The switch has been set to "automatic".
Due to the disconnection of the 4P switch in System B, the zero line of the load in System B was suspended from the ground wire. In addition, the two power supply lines of the rear output column cabinet were wrongly connected, resulting in damage to the rear load of the wrongly connected part. The voltage fluctuation of the output column head cabinet reported on site, as well as the fact that the damaged load is connected to the wrong line column head cabinet, can also verify this point. According to the following figure, a detailed analysis can be conducted:
The output voltage of the two UPS inverters is stable at 220VAC, so the correctly connected load works normally. R1, R2 is the partial load where the neutral wire is wrongly connected. When the zero line of system B is connected to the ground, it can be known that x=0Vac in the figure, and the voltage applied to R1 and R2 is 220VAC, which can work normally. If the front zero line of system B is cut off from the ground, the zero line of system B will be suspended from the ground, and the voltage on R1 and R2 will be (x-220) and (x+220) Vac (vector sum). At this time, x will follow R1, and the size of R2 will fluctuate within the corresponding range. R1 and R2 may experience overvoltage/undervoltage.
Handling methods and suggestions：
The input main switch of system B has been set to "automatic", and the wrong zero wire in the backend output column cabinet has been rectified. These hidden dangers in the system have been eliminated and normal operation has been restored.
Dual power supply, it is necessary to ensure that there is no mixed connection between the two back-end load connections. On the contrary, if there is overvoltage damage to the load, it is mostly due to wiring problems.
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