Basic technical guidance for UPS selection

In important data centers, enterprise information centers, and other important data gathering places, UPS plays a vital role. It can not only convert voltage and frequency, provide backup time after power outages, but also isolate the power grid from electrical appliances, providing high-quality clean power for electrical appliances.

Common UPS types on the market now include backup UPS, online interactive UPS and online UPS. Many brands and manufacturers tout the selling points of their products, making users lose their direction in choosing UPS products. This article summarizes the quality and selection issues of UPS for user reference.

UPS technology has gone through four generations of development. The combination of IGBT rectification and IGBT inverter has become the mainstream technology of UPS products today. It adopts high-frequency pulse width modulation IGBT rectifier technology and has strong adaptability to the power grid. The input power factor can be as high as 0.99 without a filter. Its advantages are as follows:

1. Input voltage range: <±25%

2. Input power factor: 0.99

3. Harmonic component <3% green power

4. No energy loss will be caused by adding filters

5. The machine is light in weight - easy to install and transport

shortcoming:

1. The rectifier capacity is limited by existing technical conditions (<200KVA)

2. Not as stable as silicon controlled rectifier

The main purpose of using IGBT rectification is to improve the power factor of the input end to ≈1 - to make the input current as close to a sine wave as possible while reducing DC pulsation and harmonic interference to expand the UPS input voltage range.

1. Diesel engine/UPS capacity matching

Note: The capacity of the front-end generator of the UPS parallel system is 1.8 times that of a single unit. If the user's investment conditions allow, the generator set should also have a margin of 1.25 times.

2. UPS capacity and model selection

1. User load; UPS output redundancy (70-80%)

2. The load peak factor (3:1) cannot exceed the inverter overload capacity

3. The load apparent power (KVA) cannot exceed the UPS rated power * power factor conversion coefficient

4. Three-phase load imbalance <30%

3. UPS energy consumption indicators

UPS working loss, ventilation volume, air conditioning configuration:

Full load loss (KW) = kVA × Cosφ × (6-7.5%) air conditioning cooling capacity

Output power conversion - Altitude, the capacity is reduced by 1% for every 100 meters increase in altitude (typical UPS operating altitude: 1000 meters)

4. Capacity and model selection

1. UPS output power conversion for loads with different power factors (typical UPS output PF-0.8 type)

2. Cable entry mode: upper cable entry/lower cable entry

3. Choose superior models to increase price competitiveness

PW technical power 40KVA 60KVA 120KVA 200KVA

PW commercial power 20, 30, 4050, 6080, 100, 120160, 200

5. Battery calculation and configuration

1. Battery calculation: constant current calculation method (discharge rate)

2. Approximate calculation method: suitable for low power and long time configuration

Calculation formula:

AH = (kVA × Cosφ)/h (inverter efficiency) × number of batteries × U level (single battery voltage) × discharge rate

3. Accurate calculation: constant power calculation method

(1) Determination of cut-off voltage:

1.67V/cell: discharge for 30 minutes; 1.75V/cell: discharge for 30~60 minutes;

1.83V/cell》Discharge for 60 minutes

(2) Calculate the constant power data of each cell:

W/cell=（kVA ´Cosj）/［h（inverter efficiency） ´ number of battery cells］

(3) Select the battery specifications that meet the calculation results according to the manufacturer's constant power discharge data table

6. UPS Configuration

1. Power distribution: cables and switches

(1) Input switch capacity and cable specifications:

Three-phase electrical speed calculation method: Input current (A) = 1.8XKVA, switching coefficient X1.2

Single-phase electrical speed calculation method: Input current (A) = 5XKVA, switching coefficient X1.2

(2) Output switch capacity and cable specifications:

Three-phase electrical speed calculation method: Output current (A) = 1.5XKVA, switching coefficient X1.2

Single-phase electrical speed calculation method: Output current (A) = 4XKVA, switching coefficient X1.2

(3) Battery switch capacity and cable specifications: Discharge current (A) = kVA ´Cosj/U battery voltage

Switching coefficient (X1.2)

(4) Cable length and voltage drop: For example, 70mm cable resistance is 0.26Ω/km

(5) Neutral and ground wire specifications: Neutral wire = 1 to 1.5 times the phase wire, ground wire = phase wire

(6) Simple calculation method for cable rated current

(7) Circuit breaker specifications: R10, R16, R20, R25, R32, R40, R50, R63, R80, R100, R125, R160, R200, R250, R320, R400, R630, R800, R1250

2. Isolation transformer

(1) High frequency machine equipped with 380V/380V output isolation transformer: capacity KVA = UPS KVA

Use △/Y0 type isolation transformer, output neutral point grounding, Y/Y type transformer bypass backfeed

This may cause a dangerous over-voltage condition.

(2) UPS equipped with 380V/220V output isolation transformer:

Output capacity loss 20-30%

There is interference feedback to the inverter, so choose a transformer with high efficiency and low interference.

(3) Bypass isolation transformer: realize neutral line electrical isolation