Different UPS power topologies affect the power supply system

The "high availability" of the UPS power supply system requires that the system must have four elements: reliability, functionality, maintainability and fault tolerance. The internal topology of the UPS plays a decisive role in the reliability and functionality of the system. The key elements of availability, especially maintainability and fault tolerance, depend largely on the UPS redundancy and distribution system selection. This article analyzes and evaluates different topologies within the UPS to explain how each structure affects the four elements of the "high availability" power supply system.

1. UPS topology: Offline UPS or backup power supply (SPS)

The offline topology is the simplest UPS configuration. Under normal operation, AC mains power flows directly through the UPS to the critical loads. The charger or "4-quadrant converter" converts the AC power to DC to charge the battery. The inverter is used to convert the DC power provided by the battery to AC to power the load when the mains fails. The inverter usually runs in standby mode to keep the battery charged. If the mains power exceeds the specified range, the inverter supplies power to the load from the battery. This topology is called "single conversion" because the power can only be converted once at any time (AC to DC or DC to AC). Under normal operation, a small amount of AC power is converted to DC to keep the battery powered. When the input mains power exceeds the specified range, the UPS detects the power failure, the switching relay is activated, and the output inverter starts the inverter to convert the DC power provided by the battery to AC to power the load.

Offline UPS are very inexpensive and are only suitable for home use or for powering a single computer workstation where only power outage protection is required. Offline products sometimes have built-in surge suppression and/or "buck and boost" circuitry to compensate for higher or lower input voltages, but do not attempt to provide any active input power conditioning.

Offline UPS scored high in design simplicity, but fell short in other measures. Offline UPS are generally used for single-phase non-critical loads. In particular, the lack of compatibility with generators makes them unsuitable for three-phase applications.

2. UPS topology: interactive UPS

Interactive UPS topology. This structure is similar to offline UPS products, except that a transformer or inductor is added in series between the mains power supply and the load. The inductor enables the UPS inverter to "interact" with the input power supply and adjust the output power connected to the load. In other words, the "buck and boost" circuit can make corresponding adjustments when the input voltage is high or low.

Four-quadrant converters are generally constant voltage devices. When the load or input voltage changes, the four-quadrant converter adjusts the output phase angle to make corresponding changes. Frequent changes in load will cause battery discharge, and frequent battery discharge will greatly shorten its service life.

Another disadvantage of interactive UPS products is that if battery power is not used, it will not be possible to completely isolate the important load from the input power. Frequency disturbances and poor power quality will directly affect the important load. If electrical isolation is not implemented, common mode noise will also directly affect the normal operation of the load.

Another disadvantage of interactive UPS products is that if battery power is not used, it will not be possible to completely isolate the important load from the input power. Frequency disturbances and poor power quality will directly affect the important load. If electrical isolation is not implemented, common mode noise will also directly affect the normal operation of the load.

Like offline UPS, interactive UPS products can be less expensive and more efficient because they only provide power to critical loads when input power is abnormal and only operate during battery life. Compared to offline UPS, interactive UPS devices lose some efficiency due to their series inductor and power conditioning functions. In addition, there is usually at least a small (but measurable) voltage drop when the UPS switches to battery power.

Generator compatibility

Offline UPS and interactive UPS products require the frequency and phase of the input power to be very stable. The power frequency must be stable because the inverter must track the power frequency to calibrate the voltage and current so that the system output frequency is the same as the input frequency, unless the UPS is running on batteries.

A typical operating problem is that when other loads on the generator are started, the generator output frequency will change significantly, causing the offline UPS or interactive UPS to discharge through the battery. This problem is particularly evident on natural gas generator sets. Repeated charging and discharging of the battery is likely to cause over-discharge of the battery, which will greatly shorten the battery life. Another potential problem is that when the UPS is loaded, the generator output will be unstable, that is, a sudden increase in the UPS load will cause the generator voltage and frequency to drop, causing the UPS to enter the battery discharge operating state. When the generator output stabilizes, the UPS returns to normal state. If the generator output voltage drops again, the UPS will enter the battery discharge state again.

The above problems do not occur with double conversion UPS. Double conversion UPS adjusts the input power and can tolerate large changes in power frequency while continuing to provide a stable output frequency without using battery power. In addition, major double conversion UPS manufacturers have developed technology to reduce output current distortion, greatly improving the compatibility of UPS and generators, making load specification selection more accurate. The ratio of double conversion UPS capacity to generator capacity can reach 1.25~1.5:1, and there will be no operating problems.

3. UPS topology: Online interactive structure with power factor correction function (including "Delta conversion" products)

In recent years, several companies have introduced online interactive three-phase UPS products with power factor correction. Compared with offline products and general interactive products, these products have improved the regulation performance of the power supply and improved the quality of the output power. However, there are also some disadvantages, such as active power regulation will reduce efficiency. In fact, when carrying non-linear loads (usually computer loads), its efficiency is generally lower than that of double conversion UPS products. And the complexity of this product structure is often even more complicated than that of double conversion UPS. What is even more misleading to users is that this type of online interactive product is even advertised as an "online" product because its inverter is always "running" to provide voltage regulation or input power factor correction.

Traditionally, these products are referred to as interactive UPS, where the series transformer and output inverter interact with the input mains to adjust the output voltage. Some products have a small input inverter/charger (sometimes called a "delta" inverter) to adjust the input voltage. The small inverter is generally connected to the DC bus, which provides a channel for power conversion for the output (main) inverter. The output inverter corrects the input power factor and inverts the battery energy output to provide load power protection when the power is off.

In normal mode (standard input power waveform, UPS load is linear load), the input isolation switch, AC input static switch and output isolation switch are closed, and the AC power directly supplies power to the load output. The input inverter is used as a charger to float charge the battery system. In the above ideal (unrealistic) state, the main inverter is not working. When the UPS input voltage is abnormal, the delta converter generates a corresponding voltage, which is connected to the input voltage through the buck/boost transformer.

The input voltage is superimposed (added or subtracted) to adjust the output voltage and ensure the stability of its output. This regulation principle is similar to some electronic voltage regulators currently on the market. When the input voltage exceeds the adjustable range, the main inverter starts to work, inverts the DC power provided by the battery and outputs it to provide full-scale output power. At this time, the mains input static switch must be disconnected to prevent the power from backflowing to the input side. Its function is similar to that of an offline UPS.

Line-interactive UPSs that use rectified mains input static switches are extremely susceptible to failure when the input power fails because they cannot shut down quickly enough, causing the inverter to reverse power to the failed input power terminal and then shut down.

Online interactive UPS (or Delta conversion) can also provide load harmonic current and input power factor correction. The main inverter generates the required compensation current, including harmonic current and fundamental current. As long as the inverter is running, whether it is used for voltage correction inverter/charger (Delta converter) or main inverter for harmonic current and power factor correction, additional losses will occur, greatly reducing the efficiency of the equipment, so its actual efficiency is far lower than its advertised efficiency.

4. UPS topology: Double conversion online UPS

True online or double conversion UPS products are extremely popular UPS topologies. The rectifier converts the input AC power to DC power, which supplies power to the internal DC bus of the UPS. The output inverter converts DC power to AC power, outputs AC power with a stable voltage and frequency, and supplies power to important loads. During normal operation, the battery connected to the DC bus is charged by floating charge. When the input power exceeds the specified range, the battery supplies power to the inverter and important loads.

The advantages of double conversion online UPS configuration are as follows:

(1) Important loads are completely isolated from the input AC power.

(2) The critical load is always powered by the output inverter, which is always powered by the internal DC bus. When the input power fails, the output voltage will not switch over because the inverter is always running from the DC input. The voltage and frequency of the input mains may change, but the double conversion UPS will not be affected because the rectifier will only

Provides DC power to the DC bus. For example, the UL33 series UPS can run for a long time, even when the battery input voltage is 15% lower than its nominal voltage, and can recharge the battery. It can continue to operate at a voltage 20% lower than the nominal voltage without discharging the battery. Similarly, if the input frequency is below or exceeds the specified range, the rectifier will continue to provide DC power, and the output inverter will continue to output 50Hz power without using battery power.

(3) The output inverter is usually equipped with an isolation transformer that can provide an additional neutral line. The UPS can thus achieve electrical isolation and provide common-mode noise protection for the load.

(4) Double conversion UPS has built-in dual input wiring structure, that is, it has rectifier input and bypass circuit input wiring terminals. Users may choose single input UPS for ease of installation, but dual input UPS products have higher fault tolerance. (5) True double conversion UPS can be used in dual bus power supply system. In all operating modes, the double conversion UPS in the system is synchronized with the specified reference source: AC power, battery or backup generator operation mode.

(6) If the input power line fails, the UPS will enter battery power mode. At this time, the UPS rectifier will prevent current from flowing back from the DC bus to the input terminal of the forward stage.

V. Conclusion

All UPS topologies can provide power outage protection and play a role in the current power protection industry:

(1) Offline UPS is a simple and low-cost power outage protection solution . It is only suitable for single-phase computer applications.

(2) The performance of interactive UPS is much better than that of offline UPS, but it still cannot completely eliminate all faults and interference caused by AC power and oil generator. It can provide certain power regulation for small network applications. It can be applied to single-phase computers and non-critical server applications.

(3) Online UPS with power factor correction provides better power regulation for small network applications. Suitable for single-phase workstations and non-critical server applications. Not suitable for three-phase applications in factories.

(4) Double conversion UPS has the best performance and provides protection against all power failures. It is recommended for all important single-phase and three-phase applications, especially high availability and 24/7 uninterrupted operation applications.