Take double-conversion online UPS as an example. According to the definition of UPS, UPS has two functions: one is that it can still provide uninterrupted power to the load when the mains fails; the other is to reduce voltage surges, high-voltage spikes, Transient over-voltage, voltage sag, line noise, frequency deviation, continuous low voltage and low-quality mains power supply interruption are transformed into high-quality electric energy with constant voltage, constant frequency, no noise interference, and no power supply interruptions. For this reason, the composition of UPS power circuit should include the following parts.
① Input filter: used to filter out radio frequency interference (RFI), electromagnetic interference (EMI) and other high-frequency interference in the mains power supply.
②AC/DC rectifier or power factor corrector: It is used to convert the low-quality AC power input from the mains power into DC power, so that it can be inverted into high-quality power with constant voltage and constant frequency, and charge the energy storage battery.
③DC/AC inverter: It is used to convert the DC power output by the AC/DC rectifier and the DC power stored in the battery when the mains power is cut into constant voltage and constant frequency high-quality power to supply uninterrupted power to the load.
④ Output AC filter: used to filter out the harmonics in the output voltage obtained by the inverter and improve the sinusoidal quality of the UPS output voltage.
⑤Charger: Charge the battery after rectifying the electric energy of the mains power supply, which is used to extend the time of supplying power to the load when the mains power fails.
⑥DC/DC converter: used to convert the DC voltage of the battery into the 350~400V DC voltage required by the inverter to meet the needs of the inverter to output 230V AC voltage.
⑦Bypass: When the UPS is overloaded or the inverter fails, it is used to immediately switch to the mains power supply to ensure uninterrupted power supply to the load.
⑧Input transfer switch: When the mains power fails, it is used to switch the load to power supplied by the battery through the inverter to prevent the electric energy of the UPS from being poured back into the mains power supply.
⑨ Output transfer switch: used to switch between bypass and UPS power supply.
⑩Battery: Energy storage device in UPS. When the mains power is normal, the mains power is charged to the battery through rectification to store energy; when the mains power fails, the load is converted to the power supplied by the storage battery to the load through the inverter’s inverting action. It is a key component of UPS to realize uninterrupted power supply function.
The composition of the current high-power UPS has changed, eliminating the need for chargers and DC/DC converters, and increasing the battery voltage to 400V and then directly connecting it between the rectifier and the inverter, making the UPS circuit and its The composition has been significantly simplified, and the reliability of the UPS has also been improved.
The following describes the working principle of UPS uninterruptible power supply.
The key to UPS’s uninterrupted power supply is the introduction of energy storage batteries. When the mains power is normal, the poor quality AC power of the mains power is rectified into DC power through the rectifier. On the one hand, this DC power is transformed into constant voltage and constant frequency high-quality AC power through the inverter to supply power to the load. On the other hand, it also charges the battery to store energy: when the mains fails or is interrupted, the control circuit immediately The mains power supply is converted to the high-quality AC power with constant voltage and constant frequency from the storage battery through the inverter to continue supplying power to the load to ensure that the load receives uninterrupted high-quality AC power.
In addition, the key for UPS to achieve constant voltage and constant frequency high-quality AC power output is to have an input rectifier and an output inverter: the input rectifier can remove voltage surges, high voltage spikes, transient overvoltages, and transient overvoltages in the mains power supply through rectification. Voltage sag, frequency deviation, continuous low voltage and other inferior factors are eliminated; the output inverter can make the output voltage of the UPS achieve constant voltage and constant frequency, and the harmonic components in the output voltage can be filtered by the output AC filter. So that the AC output power of UPS becomes high-quality AC power.
There are two types of rectifiers suitable for UPS applications: one is the SCR multi-phase phase-controlled rectifier with input transformer boost, which can make the input power factor of the mains power reach 0.90; the other is the IGBT Boost switch that boosts itself. Rectifier, which can increase the input power factor of the mains to above 0.99.
There are also two kinds of inverters suitable for UPS applications: one is SPWM inverter, which can reduce the distortion rate of the output voltage waveform to less than 10%; the other is SPWM cascade superimposed inverter, which can reduce the distortion rate of the output voltage waveform to less than 10%. The distortion of the output voltage waveform can be reduced to less than 5%.
In addition, large and medium UPSs are different from medium and small UPSs. Large and medium UPSs have the following characteristics.
①Large and medium UPS are generally three-phase input. For high-power UPS, if single-phase input is used, it will cause the three-phase unbalance of the mains power load. Generally speaking, when the UPS capacity is less than 10kV. A, single-phase input is used; when the UPS capacity is greater than 10kV. A, three-phase input is used.
②Large and medium UPS generally have three-phase output. When the load is a single, single-phase device, the UPS is required to have a single-phase output; when the load is multiple devices, the UPS is required to be a three-phase output.
For UPS with three-phase output, the balance of the three-phase load must be considered. Generally, UPSs with better performance basically have the function of 100% unbalanced three-phase load, which can allow one phase load to be zero and the other phase load to be 100% to work stably for a long time.
③ Large and medium-sized UPS are still using controllable bridge rectification. This is because the rectifier filter capacitors of large and medium-sized UPSs are large, and the transient inrush current when the rectifier starts is large. Therefore, a controlled rectification method can be used to achieve soft start, so that the establishment of rectified voltage becomes a relatively gentle process and avoids The starting impulse current is too large.
In addition, the rectified voltage is stable, which can improve the input dynamic characteristics of the UPS. The rectifier can be directly used for charging, and the charging method can be changed from the original simple constant voltage and current limiting method to the programmable or monitorable method, thereby eliminating the need for charging Circuit.
The controllable rectifier can be used to protect against the mains lack of phase, phase error, serious voltage or frequency exceeding the standard, DC bus overload or short circuit, etc.; it can limit the rectified charging current; it can treat the city that exceeds the input range. The electric voltage is stopped for rectification.
It can also be equipped with a 12-pulse rectifier, which can reduce the DC pulsation on the output DC bus; second, it can reduce the harmonic interference to the mains power supply.
④Static bypass switch. Because there is a circulating current in the conversion process between inverter and bypass, the requirements for the conversion conditions are higher for large and medium UPSs.
The input range of the static switch and the range of the rectified mains input should be separated (generally speaking, the input range of the static switch is smaller).
If the mains power exceeds the input range of the static switch, the static switch is not allowed to switch from bypass to inverter. Therefore, when the mains power exceeds the static switch input range and has not yet exceeded the rectified input mains range, the inverter should be turned on by the DC start mode first, and then the rectifier input switch should be closed to provide electric energy from the mains.
⑤Switch setting. For large and medium-sized UPS, the corresponding protection switch should be set, so that on the one hand, each part can be distinguished, and on the other hand, clear operation steps can be established.
a The rectifier input switch can be used to control the on/off of the rectifier input.
b. The static bypass switch can be used to control the on/off of the static bypass input.
The C battery switch can be used to reduce the self-discharge of the UPS battery to control the on/off of the UPS battery pack.
d The inverter switch, like a small UPS, is used to start or shut down the inverter.
e Manual bypass switch: For large and medium-sized UPS, due to the large input and output currents, thick wires, and strict fixing methods, it may be necessary to stop or change the wires during maintenance, so manual bypass switches should generally be set.
When the inverter fails (or shuts down) and the UPS is in the bypass state, first close the manual bypass switch, and then disconnect the output switch, rectifier input switch, and static bypass switch respectively. At this time, the load can be switched off. Intermittently, the utility power is directly supplied by the manual bypass, and the UPS is completely isolated from the utility power and the load for thorough maintenance. When the maintenance is completed, first close the static bypass switch and output switch, and then disconnect the manual bypass switch, so that the load is transferred from manual bypass to static bypass without interruption, and then close the rectifier input switch to start the inverter , To restore the inverter’s output power supply.
⑥ Shut down. Because the load of large and medium UPSs is relatively important, in order to improve the reliability of the UPS power supply system, parallel operation is often considered, and sometimes the parallel mode is considered to expand the capacity of the UPS power supply system.
In various parallel schemes, the following differences should be noted.
a Whether it is a backup parallel or a true output redundant parallel, the reliability of the system can be improved.
h. If the capacity expansion system is not in redundant use, as long as one of the dual machines fails, the system cannot meet the requirements, so its reliability system should be a series system.
C. Although the output reliability of the system using backup parallel and true output redundant parallel is the same, this is based on the premise that the reliability of a single UPS remains unchanged. In fact, when the output redundancy parallel mode is adopted, it will often form a situation where two UPSs are connected in parallel with each other, which will affect the reliability of a single UPS.
d. There are some opportunities to share the battery line, which is conducive to the full use of the battery, but the sharing of the battery will inevitably form the correlation between the two UPSs, thereby affecting the reliability.
⑦ Power distribution requirements. For the installation of large and medium-sized UPS, the power distribution in the computer room should meet the capacity requirements, and the wire diameter should be selected according to the capacity, and the corresponding zero and ground voltage requirements for the power distribution should be made according to the load’s requirements for the voltage between the zero and ground wires ; For three-phase input UPSs, the input three-phase wires must be arranged in order.