I. Reference Basis

GB 51348-2019 Code for Electrical Design of Civil Buildings

DL/T 5491-2014 Technical Code for Design of AC Uninterruptible Power Supply System in Electric Power Engineering

12SDX101-2 Civil Building Electrical Design Calculation and Examples

Ii. Terminology

Uninterruptible power supply equipment (UPS)

A power supply device composed of a converter, a switch, and an energy storage device (battery), etc., which can maintain the continuity of power supply to the load when the input power fails or disappears.

2. Online UPS

A power supply device composed of a converter, a static transfer switch and an energy storage device (battery), etc. This power supply device can provide the load with power that meets the requirements regardless of whether the AC input power is interrupted or not, and whether the voltage or waveform meets the requirements or not.

3. Bypass

A power supply circuit set up to ensure power supply to the load during UPS equipment failure, temporary overload or shutdown for maintenance. It is generally composed of an isolation transformer, a voltage regulator, a static transfer switch and a manual maintenance switch, etc.

4. Parallel connection

A UPS connection method, consisting of two or more UPS hosts, with their AC output terminals connected together for parallel operation, and the AC input terminals can be connected in parallel or not.

5. Parallel redundancy

The load is shared by two or more parallel UPS hosts. When one or several UPS hosts fail, the remaining UPS can handle the entire load.

6. Dual redundancy

To meet the requirements of redundant configuration control systems or dual power supply loads, two independent UPS systems are set up. Each system consists of a UPS host, bypass, and distribution cabinet, etc., and can provide two mutually backup power supplies for the load.

7. Static transfer switch

An electronic fast transfer switch in UPS that realizes the conversion from inverter to bypass or bypass to inverter.

8. Backstop diode

A device that prevents current from flowing from the UPS rectifier to the battery but allows current to flow from the battery to the UPS inverter, used when the UPS shares the battery with other systems.

9. Current peak coefficient

When the output current of the UPS is a periodic non-sinusoidal current, the ratio of the peak value of the non-sinusoidal current to its effective value. The magnitude of the peak coefficient reflects the UPS's ability to supply power to nonlinear loads.

10. Switching time

The time required to switch from inverter power supply to bypass power supply or from bypass power supply to inverter power supply.

11. Power correction coefficient

When the load power factor deviates from the rated power factor of the UPS, the ratio of the actual load capacity that the UPS can carry to its rated capacity.

12. Capacity Reduction Coefficient (Kd)

When the installation site of the equipment is above an altitude of 1000m, the ratio of the actual load capacity that the UPS can carry to its rated capacity.

Iii. Design Principles

The main AC power input of the UPS is preferably 380V three-phase three-wire input. For UPS with a capacity less than 10kVA, 220V single-phase input can be used for AC input.

2. The DC input circuit should be equipped with a backstop diode, and the reverse breakdown voltage of the backstop diode should not be less than twice the rated input DC voltage.

When an independent battery pack is set up for the UPS, the nominal voltage of the battery pack should not exceed 500V.

4. Bypass isolation transformers and self-ignition voltage regulators should adopt dry natural air-cooled structures.

5. When the UPS has a single-phase output, the bypass is recommended to use a 380V two-phase two-wire input, or a 220V single-phase input can also be adopted. When the UPS output is three-phase, its bypass should adopt 380V three-phase input.

6. The bypass static switch is preferably a hybrid type of electronic and mechanical transfer switch, and its switching time should not exceed 5ms (1/4 cycle).

7. The manual maintenance bypass switch should have a synchronous interlocking function.

8. UPS batteries are recommended to use valve-regulated sealed lead-acid batteries or acid-proof lead-acid batteries.

9. When the UPS uses single-phase output, two-pole circuit breakers should be used in the power supply and feeder circuits. When the UPS uses three-phase output, four-pole circuit breakers should be used in the power supply and three-phase load circuits, and two-pole circuit breakers are recommended for the single-phase load circuit.

10. Copper core fire-resistant cables are recommended for UPS power distribution circuits.

11. When the UPS has a single-phase output, a three-core cable is recommended for the feeder circuit. When the UPS has a three-phase output, five-core cables are recommended for the three-phase load feeder circuits, while three-core cables are preferred for the single-phase load feeder circuits.

12. Overload capacity: It should not be lower than 125% per 10 minutes, 150% per 1 minute, or 200% per 5 seconds.

13. Overall efficiency: It should not be less than 90%.

14. Noise: It should not exceed 65dB (A).

15. Protection grade: It should not be lower than IP20.

16. Mean Time Between Failures (MTBF) : It should not be less than 25,000 hours.

Iv. Selection of Uninterruptible Power Supply (UPS

When the power load does not allow power interruption or in important places where the allowed power interruption time is in the millisecond range, an emergency backup power supply UPS should be installed.

2. UPS is suitable for capacitive and resistive loads.

When supplying power to information network systems, the rated output power of the UPS should be greater than 1.5 times the total rated power of the information network equipment. When supplying power to other electrical equipment, its rated output power should be 1.3 times the maximum calculated load.

When two UPS units are selected for parallel power supply, the rated output power of each UPS unit should be greater than 1.2 times the total rated power of the information network equipment.

The maximum inrush current of the load should not exceed 1.5 times the rated current of the UPS device.

6. The capacity of the UPS battery pack should be selected by the user based on the allowable power interruption time requirements of the specific project.

7. The operating system of the UPS should be considered as a continuous operating system.

8. When the UPS capacity is large, it is advisable to take measures to control high-order harmonics on the power supply side.

9. The AC input terminal of the UPS can be equipped with an input filter and shall comply with the following provisions:

When fully loaded, the input current distortion rate (THDi) should be less than 5%, and the input power factor should be greater than 0.93.

2) When operating at half load, the input current distortion rate (THDi) should be less than 7%, and the input power factor should be greater than 0.90.

10. The output voltage waveform of the UPS shall be a continuous sine wave and shall comply with the following provisions:

When under full linear load, the voltage distortion rate (THDu) should be less than or equal to 2%.

2) When fully loaded with nonlinear loads, the voltage distortion rate (THDu) should be less than or equal to 4%.

11. When the isolation transformer at the output end of the UPS is in TN-S or TT grounding form, the neutral point should be grounded.

12. Large-capacity UPS should have standard communication interfaces and be open to third-party software.

13. Large-capacity UPS should have the function of monitoring each battery and be able to display it on the monitor screen.

14. UPS should be set up relatively centrally in different regions.

15. When the input power of the UPS is directly provided by a self-provided diesel generator set, the ratio of its capacity to that of the diesel generator should not be less than 1:1.2. The power supply time for the initial installed capacity of the battery should not be less than 15 minutes.

16. Emergency power supply time for UPS devices

To ensure that electrical equipment is shut down in accordance with the operation sequence, the rated discharge time of its battery can be determined based on the longest time required for shutdown, generally ranging from 8 to 15 minutes.

2) When there is a backup power supply, to ensure the continuous power supply to the electrical equipment, the rated discharge time of the battery should be considered as waiting for the backup power supply to be put into operation, and it is generally advisable to take 10 to 30 minutes. When an emergency generator is set up, the emergency power supply time of the UPS can be shorter.