Stationary UPS Sizing Calculations – Part Five

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.

 

Also, in Article “Stationary UPS Sizing Calculations -Part Four”, we explained:

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

 

Today, we will explain the following:

  • Selection and sizing of UPS Cables,
  • Sizing a generator set for UPS system

 

 

 

Selection and Sizing of UPS Cables

 

 

 

Selection of UPS Cables

 

The cross section of cables depends on two main factors:

  1. Permissible temperature rise,
  2. Permissible voltage drop.

For a given load, each of these parameters results in a minimum permissible cross section. The larger of the two must be used.

When routing cables, care must be taken to maintain the required distances between control circuits and power circuits, to avoid any EMI disturbances caused by HF currents.

 

  

 

1- Temperature Rise

 

Permissible temperature rise in cables is limited by the withstanding capacity of cable insulation.

Temperature rise in cables depends on:

  • Type of core (Cu or Al),
  • Installation method,
  • Number of touching cables type of cable, the maximum permissible current.

 

  

 

2- Voltage Drops

 

The maximum permissible voltage drops are:

For AC circuits (50 or 60 Hz):

If the voltage drop exceeds 3% (50-60 Hz), increase the cross section of conductors.

 

For DC circuit:

If the voltage drop exceeds 1%, increase the cross section of conductors.

 

To calculate the voltage drop for three-phase circuits, 50-60 Hz, 380 V / 400 V / 415 V, cos φ = 0.8, balanced system 3L+N, use the following table-1 for AC circuits & table-2 for DC Circuits:

 

Table-1 for AC circuits


Table-2 for DC Circuits

 

Important note:

  • Values in percent for a circuit of 100m (copper conductors), to calculate the voltage drop in a circuit with a length L, multiply the value in the table by L/100.

 

Example#1:

Consider a 70-meter 400 V three-phase circuit, with copper conductors and a rated current of 600 A.

Standard IEC 60364 indicates, depending on the installation method and the load, a minimum cross section.

We shall assume that the minimum cross section is 95 mm2. Calculate the voltage drop?

 

Solution:

It is first necessary to check that the voltage drop does not exceed 3%.

table-1 for three-phase circuits on the previous page indicates, for a 600A current flowing in a 300 mm2 cable, a voltage drop of 3% for 100 meters of cable, i.e. for 70 meters:

Voltage drop = 3×70/100=2.1%

Therefore less than 3% and it is acceptable

 

Example#2:

For a DC current of 1,000A in a ten-meter cable of 240 mm2. Calculate the voltage drop?

 

Solution:

From table-2, The voltage drop for 100 meters of 240 mm2 cable is 5.3%, i.e. for ten meters:

 

Voltage drop = 5.3×10/100=0.53%

Therefore less than 1% and it is acceptable

 

  


 

Sizing of UPS Cables

We will explain how to size different cables used for UPS system, which are:

  1. UPS Input Cables,
  2. UPS Output Cables,
  3. Neutral Conductors,
  4. UPS to Battery Cables.

 

 

 

 

1- UPS Input Cables

 

The cross section of cables required for the input of the UPS can be derived based on the input power in KVA which depend on the:

  • Connected Load,
  • Efficiency of the Inverter,
  • Battery charging Power,
  • Efficiency of Rectifier,
  • Input power factor of rectifier,
  • Minimum operating Voltage of Rectifier.

 

To size UPS Input Cables, follow the below (4) steps:

 

Step#1: Arrive at the input power of Inverter

Inverter Input Power= Capacity of UPS in KVA X Output Power Factor X 1000/Inverter Efficiency

 

Step#2: Calculate the battery charging power in W

Battery Charging Power = 2.2VX No of Cells X Charging Current

The charging current is typically 10% of AH Capacity

 

Step#3: Calculate the Input power of Rectifier in W

Rectifier Input Power = Inverter Input Power + Battery Charging Power / Efficiency of Rectifier

 

Step#4: Calculate the input current drawn

The rectifier input power calculated in step 3 needs to be converted to KVA by taking into consideration the input power factor:

 

Input Power in VA = Rectifier Input Power in W /Input Power Factor

Rated Current in (A) = VA / SQRT 3 X Vph-ph

where:

Vph-ph is the minimum operating Voltage of rectifier

 

As thumb rule, we can consider 2A/sq mm to arrive the cross section of the required cables.

Cross Section of cables in sq mm = Rated Current in A(I) / 2

 

Step#5: adjust the calculated cross section from table-3 in below as per the rated current.

 

Table-3 for Minimum and maximum cross-sections of copper conductors suitable for connection of one copper cable per terminal. (Extract from IEC 60439-1)

  

 

 

 

2- UPS Output Cables

 

To size the cross section of the UPS Output cable, the output current needs to be calculated using the below formula:

Rated Current in A(I) = KVAX1000 / SQRT 3 X Vph-ph

Using the cable manufacturer’s datasheet and the conditions linked with routing and bunching of cables, the required cable can be selected.

 OR

 As thumb rule, we can consider 2A/sq mm to arrive the cross section of the required cables.

Cross Section of Cables in sq mm = Rated Current in A(I)/2

Then, adjust the calculated cross section from table-3 as per the rated current.

 

 

 

 

3- Neutral Conductors

 

In three-phase systems, the third-order harmonics (and their multiples) of single-phase loads add up in the neutral conductor (sum of the currents on the three phases).

For this reason, to size Neutral Conductors the following rule may be applied:

Neutral cross section = 2 x phase cross section in Sq mm

 

 

 

 

4- UPS to Battery Cables

 

The selection of UPS to battery bank cables has to be based on the current at minimum discharge voltage, which can be derived based on the below formula:

Battery Current = UPS Power Rating / UPS DC Voltage rating

Then, use Uninyvin cables tables-4&5 to get the proper size as per applied conditions.

Uninyvin cables are generally preferred for cables between UPS & battery due to high current carrying capacity and smaller cross sectional area.

 

Uninyvin cables tables-4

 

Uninyvin cables tables-5

 

 

 

 

Example#3:

Consider an installation that involves a Powerware 40kVA 9355(3-phase) UPS rated at 432VDC, 36 batteries, 6 cells/battery. Calculate the cable sizes required to install the UPS assuming the UPS will be operating at full load for a backup time of 12 hours.

Solution:

UPS rating; 40,000VA
UPS DC rating; 432V - 36 batteries

1- UPS input cable selection:
Step#1: Arrive at the input power of Inverter

Inverter Input Power= Capacity of UPS in KVA X Output Power Factor X 1000/Inverter Efficiency = 40*0.98*1000/0.95 = 41.263 KW

 

Step#2: Calculate the battery charging power in W

Battery Charging Power = 2.2VX No of Cells X Charging Current = 2.2*216*10=4.75 KW

The charging current is typically 10% of AH Capacity

 

Step#3: Calculate the Input power of Rectifier in W

Rectifier Input Power = Inverter Input Power + Battery Charging Power / Efficiency of Rectifier = 41.263 + 4.75 = 46 KW

 

Step#4: Calculate the input current drawn

The rectifier input power calculated in step 3 needs to be converted to KVA by taking into consideration the input power factor

 

Input Power in VA = Rectifier Input Power in W /Input Power Factor = 46000/0.96 =47.916 KVA

 

Rated Current in (A) = VA / SQRT 3 X Vph-ph = 47916/(1.73*400) = 69.24 A

where Vph-ph is the minimum operating Voltage of rectifier

 

Cross Section of cables in sq mm = Rated Current in A(I) / 2 = 69.24/2 = 34.62 mm2

 

Step#5: adjust the calculated cross section from table-3 as per the rated current.

 

minimum cable size required will be 34.62 mm2 (from table-3 for 80A, Cable 35 mm2 is the maximum standard)

2- UPS Output Cable selection:

 

Rated Current in A(I) = KVAX1000 / SQRT 3 X Vph-ph = 40*1000/(1.73*400) = 57.8 A

 

 

As thumb rule, we can consider 2A/sq mm to arrive the cross section of the required cables.

 

Cross Section of Cables in sq mm = Rated Current in A(I)/2 = 57.8/2 = 28.9 A


That is, minimum cable size required will be 28.9 mm2 (from table-3 for 63 A, cable 25 mm2 is the maximum standard)

3- Battery cable Selection:

Battery Current = UPS Power Rating / UPS DC Voltage rating

Battery Current = 40000VA / 432V = 92.59A

 

Then, use Uninyvin cables tables-5 to get the proper size as per applied conditions.


For single cable, the minimum cable size that can be used for this battery current should be 16 mm2.

For 3 bunched cables, the minimum cable size that can be used for this battery current should be 25mm2.


Hence, cables required are:

Input cable – 35 mm2
Output cable – 25 mm2.
Battery cable - for single cable 16 mm2 & for 3 bunched cables 25 mm2.

 

 

 

Sizing a generator set for UPS system

 

  

 

An uninterruptible power supply (UPS) is used in data centers to prevent loss of service, data loss and damage to sensitive IT and communications equipment. This is done during short-term power disturbances and blackouts, but is not designed to deliver backup power indefinitely, so for extended blackout, a standby generator will be needed as a backup source for the UPS.


How Generators Interface with UPS Systems?

The two systems (standby diesel generator and UPS) are connected by an automatic mains failure (AMF) panel (see Fig-1). This is installed upstream of the UPS system.

 

- When the AMF panel senses a mains power failure:

 

The UPS system feed the critical load from its battery,

A signal contact initiates the generator to start after an extended mains failure time; delays of 2 s to 10 s are typical

Once the generator is up to full cycle speed and capable of supplying power, the UPS system input synchronizes to the output supply to the generator.

When this is achieved the UPS system draws on the supply from the generator and its batteries go into recharge.

 

 

- When the mains power supply returns:

The generator will continue to operate for several cycles (may be 2 minutes) to ensure the mains power supply is stable.

The UPS system then synchronizes back to the mains power supply and

The generator is issued a signal contact from the AMF panel to power off.

The UPS uses the mains power to recharge its batteries again.

 

 

 

Fig.1: Typical protected power installation with Mains Failure Detection and Change Over

 

A typical continuous power sequence for a power outage and UPS ride-through to generator set power is shown above for a UPS is shown in Fig-2.

 

Fig-2: A typical continuous power sequence

 

 

 

 

When sizing UPS systems and generators it is important to classify the loads to be protected into critical, essential and non-essential loads.

So, When sizing the generator system the load normally consists of the:

Critical loads + Essential Loads

 

The critical load is generally supported by the UPS system and so the sizing becomes:

UPS system + Essential Loads

 

 

 

 

1- Ups system

 

  • Critical loads are generally IT servers and networks.
  • It is important with the UPS system to consider the full load applied to the generator by the uninterruptible power supply. 
  • UPS systems are not 100% efficient and so a 100kVA UPS at full load will draw more than 100kVA in Apparent Power.
  • In addition the battery may require charging adding more load to the generating set by the UPS rectifier circuit.
  • The input voltage waveform and harmonics may also have to be considered to ensure compatibility between the UPS system and its generating set.

 

 

 

 

2- Essential Loads

 

  • Essential loads include air conditioning, lighting and security systems, alarm and communications systems whose operation must be maintained during a mains power supply failure.
  • Non-essential services may include printing which may not be required during a power outage.
  • For each of the essential loads to be supported it is important to note their start-up and running currents and AC voltage and phase requirements. These may be available on system rating plates, on datasheets or within installation and operation manuals or manufacturer’s website pages. Where they are not available the most accurate way to size a generator is through a site electrical survey performed by a qualified electrician or electrical contractor by measuring using a current clamp.

 

 

 

 

The sizing of a diesel generator to feed an uninterruptible power supply maybe calculated from one of the following methods:

  1. The sizing formula,
  2. The Rule of thumb.

 

 

 

 

First: the sizing formula

 

KVA gen = (KVA UPS + KVA battery charge)/ (0.8 x ∩) + KVA Essential loads

 

Where:

∩ is the efficiency of the UPS

0.8 is the deration factor

 

KVA battery charge generally ranges from 0 to 25 percent of KVA UPS (15 percent is typical). If it’s unknown, use 25 percent of KVA UPS for an approximation

If UPS efficiency is unknown, we recommend the following guidelines:

  • Use 0.85 if UPS is less than 100 kW
  • Use 0.875 if UPS is greater than or equal to 100 kW and less than 500 kW
  • Use 0.90 if UPS is greater than or equal to 500 kW

 

 

 

 

Second: the Rule of thumb

Combining a UPS system with a gen set presents some special considerations to ensure compatibility. We use the following four-step procedure to size Cat gen sets that have static UPS systems as part of all of their load:

Step#1: Find the UPS input kW

Step#2: Set minimum-size gen set

In this step, establish the gen set size needed to contain waveform distortion as follows:

  • If your UPS system has a six-pulse rectifier, minimum standby get set equals UPS input kW x 1.6.
  • For a 12-pulse rectifier, the formula is UPS input kW x 1.4.

Step#3: Consider other loads

Be sure to size the gen set to accommodate other loads in the application. Establish the kW of the other loads, then add it to the UPS input kW x 1.15 for battery charge.

Step#4: Gen set sizing

Round up to the nearest larger size standby gen set.


Example#4:

The UPS system is rated 200 kVA/180 kW- six-pulse rectifier; other loads connected to the gen set total 100 kW. Size the proper generator to feed these loads?

Solution:

Step#1:

From supplier data, the UPS input is 255 kW, including battery recharge.

Step#2:

255 kW x 1.6 (six-pulse rectifier) = 408 kW minimum standby rated gen set.

Step#3:

(255 kW x 1.15) + 100 kW = 393 kW minimum standby rated gen set with other loads.

Step#4:

408 kW standard is larger than 393 kW, therefore a standby gen set of at least 408 kW is recommended.

 

 

 

 

Important Notes

 

  • The UPS and generator manufacturers should be consulted for more accurate sizing advice before finally committing to purchase.
  • Most generators can operate in prime or standby power mode. In a UPS server room or data center application, the generating set will typically operate in a standby power mode as a supplementary power source to the UPS system.
  • A Prime mode of operation is a continuous operation. This mode may be chosen where the mains supply is continuously unstable or not present. An example would be a remote application.
  • However, when used for power protection, backup generators can be started up at any time and for any duration. Because of this unpredictable demand, choosing the more conservative ‘Continuous’ rating is recommended.
  • When listing the loads it is important to note whether they require single or three phase loads. A three-phase generator can support single phase loads but the individual loading per phase must be balanced across the three phases. This means that you should have 33% of the total load for each phase of a three-phase output generating set.
  • The frequency parameters laid down by UPS manufacturers usually requires that the diesel generator be fitted with an electronic governor so as to maintain the frequency within plus or minus one per cent under all conditions.

 

 

In the next Article, we will explain the following:

  • Battery room ventilation calculation,
  • Installation and testing of UPS.

So, please keep following.

 

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,

 

Classification and Types of UPS – Part One

5- Physical Size/capacity,

6- Form factor/ configurations:

6.1- “N” System Configuration

Classification and Types of UPS – Part Two

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)

Classification and Types of UPS – Part Three

 

 

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

Classification and Types of UPS – Part Four

 

 

 

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.

Classification and Types of UPS – Part Five

 

 

 

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.

Classification and Types of UPS – Part Six

 

 

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.

Classification and Types of UPS – Part Seven

 

 

 

Transformer Arrangements in Practical UPS Systems:

1-Transformer options for the “single mains” configuration

2-Transformer Options for the “Dual Mains” Configuration

Classification and Types of UPS – Part Eight

 

 

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

Components of Online Double Conversion UPS – Part Two

 

 

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

Evaluation Criteria for Selecting an UPS-Part Two

 

 

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

Stationary UPS Sizing Calculations – Part Two

 

 

 

 

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

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

Stationary UPS Sizing Calculations -Part Three

 

 

- UPS Backup time calculation

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

 

Stationary UPS Sizing Calculations – Part Four

 


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