Selection of uninterruptible power supply UPS

In order to choose a UPS device that is technologically advanced, economical, safe and reliable, you must not only be familiar with the performance characteristics of the UPS itself, but also understand the computer's requirements for the power supply system.

(1) Computer power supply system requirements The computer power supply should have the characteristics of voltage stabilization, voltage regulation, anti-interference, and uninterrupted power supply, as follows.

① Static change of voltage stability. In terms of computers, "surge voltage" refers to the change in the amplitude of the voltage sine wave; "pulse voltage" refers to the pulse value superimposed on the voltage sine wave (not changing periodically); those with periodic changes are called "oscillating voltage" (still superimposed on the sine wave voltage). Load change voltage drop requires that when the load changes between 0 and 100%, the power supply voltage drop is generally less than 10%.

② Frequency stability: The frequency variation range is required to be within a certain range.

③ Anti-interference. The power supply must be able to resist high-order harmonics, pulsating voltage and oscillating voltage.

④Uninterrupted power supply time.

Power outages may cause data loss, errors, etc. Different computers have different requirements for uninterrupted power supply time, some must not exceed 5ms, some must not exceed 10ms, and some must not exceed 20ms.

(2) Selection of capacity and number of phases When configuring a UPS for a computer system, the first issue to consider is how large a capacity should be selected so that it can achieve the best efficiency during use. The capacity of a UPS is expressed in kV·A, which depends on the size of the kV·A of the load. The kV·A of the load can be determined by actual measurement or estimation. The actual measurement method is to measure the load current when the power is on. If the load is single-phase, twice the product of the phase current and the phase voltage is used as the kV·A of the load; if it is a three-phase balanced load, three times the product of the maximum line current and the phase voltage is used as the kV·A of the load. The estimation method is to add the complex power of each load based on the data provided by the manufacturer to obtain the kV·A of the total load. A relatively simple method is to directly add the kV·A of each load to the kV·A of the total load, and then consider an appropriate safety factor (such as a safety factor of 30%) to determine the capacity of the UPS system to be selected.

In addition, when selecting the UPS system capacity, a certain margin should be left according to the load expansion capacity and the load surge kV·A (or surge current). If the capacity is too small, the equipment will often be in an overload state, seriously affecting its service life; if the capacity is too large, not only will the equipment fail to achieve its due efficiency, but it will also cause great economic waste.

After selecting the capacity, the next step is to determine the number of phases. Currently, there are several types of UPS, including three-phase input/three-phase output, three-phase input/single-phase output, and single-phase input/single-phase output. Generally speaking, large-capacity UPS are three-phase input/three-phase output, small-capacity UPS are mostly single-phase input/single-phase output, and medium and small-capacity UPS have both three-phase and single-phase, so you should choose carefully. Three-phase output power supply devices are not only complex in structure and expensive, but also more troublesome in maintenance. Therefore, single-phase output UPS should be selected as much as possible. In order to make the system work more stable and reliable, some uninterruptible power supplies now use three-phase input pulse width controlled rectifiers regardless of single-phase or three-phase output.

(3) Selection of battery power supply duration The UPS system is equipped with a maintenance-free sealed lead-acid battery pack, which acts as an energy storage device to power the inverter when the mains power is abnormal or outage, so that the load can be powered uninterruptedly. The choice of battery power supply duration is related to the number of local mains power outages and the length of each power outage. It can be determined by the user based on the local mains power outage record data and the power distribution situation. The general power supply duration is 10 to 30 minutes or longer. Due to different users' measures for voltage regulation, voltage stabilization, frequency stabilization, power supply mode (online or offline), the basic wiring method is different.

① No bypass, single operation mode. When the power grid is normal or faulty, it is powered by a UPS, which can ensure stable voltage and frequency, but has poor flexibility.

②Parallel operation mode (redundancy mode). With two UPS, when one fails, the other can continue to supply power and can be bypassed, which is highly reliable and flexible.

③ Single-set operation mode with mechanical switching to bypass. When the grid is normal, the inverter supplies power. When the inverter fails, the mechanical switch with contacts switches to bypass power supply. There is a momentary power outage during switching.

④ Single-set operation mode with static switch switched to bypass. The difference from single-set operation mode with mechanical switch to bypass is that this operation mode uses a contactless static switch (such as a bidirectional thyristor), and there is no instantaneous power failure during switching, but there may be waveform distortion. The UPS can be made into two operation modes: "online" and "offline".

⑤ Improved wiring method. Its wiring method is shown in the figure. The ferromagnetic voltage stabilizer has three coils, Wt is connected to the power grid, yVz is connected to the load, and W3 is connected to the converter. When the power grid is normal, the power grid is supplied to the load through the static switch WJ by the filter, and the battery is charged after rectification by the converter through W3. When the power grid fails, the control circuit automatically turns off the static switch, cuts off the power supply of Wt, and the battery is switched to supply power to W3 after inversion by the converter, and supplies power to the load after voltage stabilization and filtering. The converter can both stabilize the voltage and reverse. In this way, the battery has fewer charge and discharge times, the battery life is extended, and the output voltage frequency is more stable (the voltage stability can reach ±2%, and the frequency can be stabilized at ±1%~2%); but there are ferromagnetic components in the line, and the inertia is large.

Figure UPS improved wiring method