Selection and requirements of AC uninterruptible power supply

The AC uninterruptible power supply system, usually referred to as UPS, is mainly composed of a rectifier, an inverter, a static switch, a bypass system, a battery pack, and a control and display unit. Its basic wiring is as follows:

In normal working state of UPS, the AC working power supply is rectified and inverted to provide the load with AC 220V constant frequency and constant voltage power supply; when the AC working power supply disappears or the rectifier fails, the DC battery pack supplies power to the load through the locking diode and inverter. When the inverter fails and the output voltage is abnormal or overloaded, the static switch switches to the bypass backup power supply to supply power to the load.

1. UPS capacity calculation

The rated capacity of a UPS usually refers to the apparent power (kVA) of the inverter AC. Therefore, when selecting the rated capacity of a UPS, in addition to considering the static load of a certain power factor, it is also necessary to consider the dynamic load as well as factors such as temperature changes, battery performance changes, and design redundancy requirements.

1. Calculation formula:

Sn=KP∑/cosφ

Sn-------UPS calculated capacity (kVA)

P∑-------Calculated power of all loads (kW)

K----Reliability coefficient, generally 1.4

cosφ-------Load power factor, generally 0.7

Then Sn=2P∑(kVA)

[page]2. Notes:

Capacity calculation requires accurate load statistics. In actual projects, a certain margin is often taken when calculating load statistics, and the reliability coefficient of 1.4 is used during calculation. The rated capacity of the selected UPS is too large, which is a waste for users.

2. Cable cross-section selection

UPS devices are powered by two types of power: AC and DC. The reliability of the power supply is directly related to the reliability of the UPS device power supply.

1. Cable cross-section calculation formula:

The selection of cable cross-section should mainly meet the requirements of continuous allowable working current and allowable voltage drop. The cables for power supply and feeder loops must be selected through calculation.

(a) Select according to the continuous allowable working current: I≦KIz---------------------------------- (1)

(b) Check according to voltage loss:

DC line: S=200L ρ I/△U% U -----------------------------------------(2)

Single-phase AC: S=200L ρ I/△U% U -----------------------------------------(3)

Three-phase AC: S=173L ρ I/△U% U -----------------------------------------(4)

Where I -------Calculated working current of all loads (A)

Iz-------Rated current carrying capacity of the cable under standard laying conditions (A)

K-------Comprehensive correction coefficient under different laying conditions

S-------Cable cross section calculated according to voltage drop requirement (mm²)

L-------The length of the power supply line (m)

ρ -------Resistivity of cable material

△U%-------The voltage drop allowed by the equipment

U-------Line operating voltage, three-phase is line voltage, single-phase is phase voltage, DC is battery terminal voltage (V)

2. Notes:

(a) In general, the power supply is not far from the installation location of the UPS device, and the rated current carrying capacity Iz of the cable under standard laying conditions determines the cross-section of the power supply cable; however, for feeder circuits with larger loads and longer distances, the voltage drop often determines the cross-section of the selected cable. Do not make estimates based on so-called experience to avoid accidents.

(b) According to the importance of the power supply load of the UPS device, its feeder cable should use copper core power flame retardant cable and should meet various fire protection requirements.

[page]3. Reliable power supply

1. AC power supply

When the UPS device is equipped with a dedicated bypass, the main circuit and bypass power supplies should be provided separately as far as possible and supplied from different AC bus sections to ensure the reliability of the power supply. The main and bypass power supplies are considered as one operation and one backup, and the capacity is not double-counted; when conditions are limited, the main circuit and bypass power supplies can also be combined into one power supply.

2. DC power supply

There are two common schemes for UPS device DC power supply, one is to configure an independent self-use battery pack, and the other is to draw from the controlled and protected DC battery pack (power plant, substation, etc.). No matter which scheme is adopted, attention should be paid to the routine monitoring and maintenance of the battery pack to ensure that the DC battery pack can play a role when needed.

IV. Others

1. The UPS device should adopt single-phase output mode, without considering the load balance problem, and can fully utilize the rated capacity of the UPS .

2. The UPS device should have a synchronization control device to synchronize the inverter output with the bypass power supply to ensure disturbance-free switching between circuits.

3. UPS devices generate a lot of heat. Generally, the heat generation capacity accounts for about 8% of the total capacity. Electronic equipment is sensitive to ambient temperature. When designing, full attention should be paid to heat dissipation to ensure the performance and life of the UPS.

V. Conclusion

This article mainly analyzes some problems existing in UPS selection and provides methods for the correct selection of UPS. The ultimate goal is to provide users with products with good cost performance and ensure the safe and reliable operation of equipment.

References:

1. "Electrical Design Manual for Power Engineering" ------ Water Resources and Electric Power Press

2. "Power System DC System Design Manual" ------ China Electric Power Press