### Stationary UPS Sizing Calculations – Part Four

As we stated in the previous article “Stationary UPS Sizing Calculations -Part One” That Stationary UPS Sizing Calculations include:

1. The UPS sizing calculations,
2. Rectifier/Charger sizing calculations,
3. Inverter & Static Switch sizing calculations,
4. The Battery Sizing Calculations.

We explained the UPS sizing calculations in the above article and we explained in article “Stationary UPS Sizing Calculations -Part Two, the following calculations:

- Rectifier/Charger sizing calculations,

- Inverter & Static Switch sizing calculations,

- The Battery sizing calculations, which includes:

First: The Manufacturers’ methods, which include:

• Method#1: Watts per cell method,
• Method#2: Watts per bank method,
• Method#3: Ampere per cell method.

And in Article “Stationary UPS Sizing Calculations -Part Three”, we explained

The IEEE methods of Battery Sizing Calculations which includes:

• Method#1: The IEEE 485 method,
• Method#2: The IEEE 1184 method.

Today, we will explain the following:

• UPS Backup time calculation
• Selection and sizing of UPS protective devices (CBs or Fuses)

 UPS Backup time calculation (At Full Load)

 The Backup time sometimes is called Autonomy Time or discharge time or running time. There are two different methods to calculate the UPS/Inverter battery backup time as follows: Method#1: Using Battery Capacity and LoadMethod#2: Using Battery Discharge Rate

 Example#1: A 1500VA UPS with a 12V 100Ah battery, and the total wattage of your load is 800W, calculate the backup time?   Solution: The backup time can be calculated as follows: Backup Time (in hours) =Battery Capacity (in Ah) * Battery Voltage (in V) * Battery Efficiency (in %) / Connected Load (in W) Backup time = 100Ah x 12V x 0.7 ÷ 800W Backup time = 1.05 hours = 1.05 x 60 minutes = 63 minutes   Example#2: An inverter battery with a capacity of 150 Ah, an input voltage of 12 V and battery efficiency 0.9, and the loads are: Three tube lights 40 W each, Three fans 75 W each, and A Wi-Fi router 20 W Calculate the backup time of the inverter battery?   Solution: The total load will be: 3 tube lights = 40 x 3 = 120 Watts 3 fans = 75 x 3 = 225 Watts 1 Wi-Fi router = 1×20 Watts = 20 Watts So, the total load = 120 + 225 + 20 = 365 Watts. Backup Time (in hours) =Battery Capacity (in Ah) * Battery Voltage (in V) * Battery Efficiency (in %) / Connected Load (in W) Backup Time (in hours) = 150 Ah x 12 V x 0.9/ 365 = 4.44 hours The inverter battery will last around 4.44 hours to keep the loads running.

 Method#2: Using Battery Discharge Rate   The second method to calculate UPS/Inverter battery backup time is by using the battery discharge rate. The battery discharge rate is the rate at which the battery discharges during a power outage.   To calculate the backup time using this method, follow these steps: Step#1: Determine the discharge rate of the battery. This information can usually be found on the battery or in the product manual.Step#2: Determine the capacity of your battery. To determine the battery capacity in Ah, check the battery label or manual. For example, if the battery is labeled as 12V 100Ah, the battery capacity is 100Ah.Step#3: Calculate the backup time by dividing the capacity of the battery by the discharge rate. Backup time (in hours) = battery capacity (in Ah) ÷ discharge rate (in A)

 Example#3: If the discharge rate of a battery is 10A, and the capacity is 100Ah, calculate the backup time.   Solution: The backup time can be calculated as follows: Backup time (in hours) = battery capacity (in Ah) ÷ discharge rate (in A) Backup time = 100Ah ÷ 10A Backup time = 10 hours

 Important Note#1 If it is required to calculate the UPS Backup time calculation other % load other than full load use the following rule:   Backup Time (hours) at % load = Backup Time (hours) at full load / load%   For example#1, calculate the backup time at 50% load. It will be as follows: Backup Time (hours) at 50% load = Backup Time (hours) at full load / 0.5 = 1.05/0.5 = 2.1 hours.    Important Note#2 If you have many batteries (and of course, all have the same voltage, capacity, and charging/discharging characteristics) then when calculating the Backup time, multiply by the number of batteries and the rule will be as follows:  Backup Time (in hours) =Battery Capacity (in Ah) * Battery Voltage (in V) * the number of batteries * Battery Efficiency (in %) / Connected Load (in W/h)   For example#1: Calculate the backup time if we have 3 batteries of the same type. Solution: Backup Time (in hours) =Battery Capacity (in Ah) * Battery Voltage (in V) * the number of batteries * Battery Efficiency (in %) / Connected Load (in W/h) Backup time = 100Ah x 12V x 3 x 0.7 ÷ 800W Backup time = 3.15 hours = 3.15 x 60 minutes = 189 minutes

 Backup Time Common Scenarios   The typical range generally proposed is: Standard backup times of 10, 15 or 30 minutes Custom backup times   Some loads may only be required to have enough backup time to shut down safely, while some critical systems may need to operate for as long as possible. So, the optimal battery backup time must suit the particular business or application. The most common backup time solutions are as follows:   Solution#1: UPS with 10-15 minutes of runtime and no generator  This solution allows time to safely shut down connected equipment and save work-in-progress.   Solution#2: UPS with 10-15 minutes of runtime and a generator This solution will keep connected systems up and running until the generator powers on.   Solution#3: UPS with two or more hours of battery runtime In some cases, generators may not be practical and organizations that wish to remain up and running during an extended outage must rely solely on UPS batteries.

 Selection and sizing of UPS protective devices (CBs or Fuses)

The right selection and sizing of UPS protective devices has a very important role since the best sizing of UPS and the best choice of configuration can be compromised by a wrong choice of only one circuit-breaker.

We have 3 important circuit-breakers in the protection scheme of a UPS:

1. The UPS input breaker (CB1)
2. The static bypass switch input breaker (CB2)

Usually, these breakers are given by the manufacturers. However, the below table and Figure show how to select the circuit-breakers in the protection scheme of a UPS

Figure-1 shows how to select the circuit-breakers in the protection scheme of a UPS

Fig-1

 Example#4: A UPS feed two production lines each with Normal power load 125 KW and other loads are: Compressor 11 KW Water Treatment Plant 41 KW And from the manufacturer data sheet: UPS input current distortion factor =0.9 UPS efficiency =0.9 UPS power factor =0.9   Size the UPS input breaker?   Solution:   P = Ʃ KW of UPS Loads = 2*125 + 11 + 41  = 302 KW and S = (P/(Eff*W) + 0.25P/W) / PF S = (302/(0.9*0.9) + 0.25*302/0.9) / 0.9 = 507.5 KVA Then: Ib = S /(√3*V) = 507.5*1000/(√3*400) = 732.5 A   Select the nominal current of the UPS input breaker (In) to the nearest higher standard Value 800 A and which verify the following conditions: Ib ≤ In ≤ Iz ---------------   732.5 < 800 < Iz Iz ≥ K2*In/1.45 ------------ Iz ≥ 1.45 * 800/1.45     Example#5: From example# , Size the UPS Downstream Load breaker?   Solution: S = P / PF S = 302/ 0.9 = 335.56 KVA Then: Ib = S /(√3*V) = 335.56*1000/(√3*400) = 484.9 A   Select the nominal current of the UPS Downstream Load breaker to the nearest higher standard Value 630 A and which verify the following conditions: Ib ≤ In ≤ Iz ---------------   484.9 < 630 < Iz Iz ≥ K2*In/1.45 ------------ Iz ≥ 1.45 * 630/1.45

 DC breaker selection A DC breaker, which is fundamental to protect the battery cabinet in case of a fault in between the cabinet and the inverter inside the UPS. The Energy storage connected to a UPS consists of battery strings in parallel. Each string has its own disconnection/protection method, such as switch disconnector, MCB, MCCB or a fuse, depending on its rating. The parallel strings are then connected to the UPS either through a switch disconnector (if the protection was already provided by a fuse or circuit breaker on a string level), or a DC circuit breaker to provide DC protection in the paralleling switchgear at the point of UPS connection.   To Size the battery circuit breaker, follow the below (4) steps: Step#1: The battery charging current after a long period power outage=full charger output (N+1 rectifiers) = Rectifiers quantity * Rectifier ampere Step#2: The maximum charging current of each string = the battery charging current/ string quantity Step#3: The battery circuit breaker sizing current = 1.25 x the maximum charging current of each string Step#4: Select the rating of DC circuit breaker to be next higher standard value.  Location and Number of battery overcurrent devices: Notes for sizing DC breaker: DC side isolated from ground Maximum breaking capacity to be selected according to the prospective short circuit current for different installation Circuit breaker size has been selected considering maximum voltage and maximum discharge current Probability of fault occurring between the batteries and DC circuit breaker is not considered, and the circuit breaker shall be installed as close as possible to the batteries. Ambient temperature up to +40°C Maximum discharge current refers to 1.7V/cell as battery cut off voltage Always refer to UPS technical data sheets for details on number of blocks vs autonomy and temperature

Selectivity between UPS protective devices

• Selectivity is defined in IEC 60947-2 “Low voltage Equipment – part 2: Circuit breakers”, and we can accordingly explain it as the selectivity during a fault between two protection devices (i.e., circuit breakers) connected in a series, where the protection device closer to the fault would trip without tripping the upstream protection devices.
• This is mainly achieved to isolate the fault and maintain the supply for other circuits that are not directly connected to the fault, and if selectivity is not achieved between circuit breakers, the purpose of installing an expensive UPS system is defeated.
• It is required that you trip the CB3 circuit breaker and isolate the faulty section before tripping CB2 and dropping the entire load supplied by the UPS. It is also important to isolate the fault as quickly as possible to restore the output voltage for the load bus supplied by UPS.
• Selectivity tables are provided by all manufacturers to define the selectivity between its different products, as shown in the. Here, for example, if the load downstream was protected by ABB MCB S803 B 63, the upstream breaker could be Tmax XT2 100 A, which provides partial selectivity up to 4.5 kA only, or 160A, which provides total selectivity.

How to ensure the selectivity between UPS protective devices?

The table below indicates how to determine the Ir (overload; thermal or longtime) and Im (short-circuit; magnetic or short time) thresholds to ensure selectivity, depending on the upstream and downstream trip units.

 Type of downstream circuit Ir upstream / Ir downstream ratio Im upstream / Im downstream ratio Im upstream / Im downstream ratio Downstream trip unit All types Magnetic Electronic Distribution >1.6 >2 >1.5 Asynchronous motor >3 >2 >1.5

Notes:

• Time selectivity must be implemented by qualified personnel because time delays before tripping increase the thermal stress (I2t) downstream (cables, semi-conductors, etc.). Caution is required if tripping of CB2 is delayed using the Im threshold time delay
• Energy selectivity does not depend on the trip unit, only on the circuit-breaker

In the next Article, we will explain the following:

• Selection and sizing of UPS Cables
• Sizing a generator set for UPS system
• UPS room ventilation calculation

Subject Of Pervious Article

Article

Applicable Standards for UPS Systems

• What is a UPS?
• Why do we need a UPS?
• UPS Rating
• Classification of UPS:

1-Voltage range,

2-No. of phases,

3- Mobility,

4- Technological design,

5- Physical Size/capacity,

6- Form factor/ configurations:

6.1- “N” System Configuration

6.2- “N+1” System Configuration, which includes:

• Isolated Redundant Configuration (N +1)
• Parallel Redundant Configuration (1+1)
• Parallel Redundant Configuration (N +1)
• Parallel Redundant Configuration (N +2) and so on

6.3- Parallel Redundant with Dual Bus Configuration (N+1 or 1+1)

6.4- Parallel Redundant with STS Configuration

• Parallel Redundant Configuration (1+1) + STS
• Parallel Redundant Configuration (N+1) + STS

6.5- System plus System 2(N+1), 2N+2, [(N+1) + (N+1)], and 2N

7- According to UPS Topology

7.1 Off-line or Standby UPS,

7.2 Line Interactive UPS,

7.3 Standby-Ferro UPS,

7.4 Online Double Conversion UPS,

7.5 The Delta Conversion On-Line UPS.

8- According to UPS Distribution Architecture

• 8.1 Centralized UPS Configuration,
• 8.2 Distributed (Decentralized) UPS Configuration,
• 8.2.1 Distributed UPS-Zonewise Configuration
• 8.3 Hybrid UPS Configuration.

Conventional (Monolithic) Vs Modular UPS System:

• Deploy UPSs in parallel,
• Deploy UPSs in Series,
• Use modular UPS products.

Three Basic Configurations Of Mains And Bypass For A UPS System:

• Single mains,
• Single mains without bypass,
• Dual mains.

9-According to Use of transformers with the UPS

• Transformer based,
• Transformer less UPS,
• Transformer less UPS with external input/ output transformer.

Transformer Arrangements in Practical UPS Systems:

1-Transformer options for the “single mains” configuration

2-Transformer Options for the “Dual Mains” Configuration

3- Transformer options for “single mains without bypass”

Classification and Types of UPS – Part Nine

Components of Online Double Conversion UPS:

• 1- Rectifier,
• 2- Inverter,
• 3- Energy Storage system:
• 3.1 Battery

### Components of Online Double Conversion UPS– Part One

3.1.1 Battery Configurations

• Serial Strings,
• Parallel Strings.

3.1.2 Battery Size and Location

3.1.3 Battery Transition Boxes

3.1.4 Battery Monitoring

3.2 Energy Storage System – Flywheel

3.3 Energy Storage system – Super Capacitors

3.4 Hydrogen Fuel Cells

4- Static switch

Earthing Principles of UPS Systems

Evaluation Criteria for Selecting an UPS

### Evaluation Criteria for Selecting an UPS-Part One

Example: Selecting an Uninterruptible Power Supply (UPS)

UPS System Ratings and Service Conditions

• First: from IEC 60146-4
• Second: according to American standards

The UPS sizing calculations steps

### Stationary UPS Sizing Calculations – Part One

2- Rectifier/Charger Sizing Calculations

3- Inverter sizing calculations & Static Switch Sizing

4- The Battery sizing calculations

First: The Manufacturers’ methods, which include:

• Method#1:Watts per cell method
• Method#2:Watts per bank method
• Method#3:Ampere per cell method

Second: The IEEE methods of Battery Sizing Calculations which includes:

• Method#1: The IEEE 485 method,
• Method#2: The IEEE 1184 method.