Saturday, December 31, 2011

The Effects of Harmonics to Electronic Equipment are experienced in various ways depending on the type of device - from slight to serious consequences. Ironically, it is a known fact that most electronic equipment is prone to misoperation due to harmonic distortion, even though it is a harmonic generator itself. In this post, these common effects will be briefly discussed.


Wednesday, November 2, 2011

Delta Conversion Online UPS is a modern innovation that eliminates the disadvantages of the traditional double conversion online UPS. The enhancements are based on having a delta inverter and allowing bidirectional power flow. Today, this design is the only major uninterruptible power supply technology protected by patents and is unlikely to be offered from a broad range of UPS suppliers. Also, delta conversion online units are available in the market from 5 kVA to 1 MVA.


Sunday, October 30, 2011

Motor-Generator (M-G) set is basically a combination of a motor and generator, although it is physically different from an ordinary electric motor that is attached to a separate generator. It has many practical functions that generally involve converting voltage, frequency and phase of power. However, this post will focus on its power quality applications, as it is a mature technology used for isolating sensitive and critical loads from voltage sags and interruptions. Also, it is available in various sizes and configurations to suit different purposes.


Wednesday, October 26, 2011

EMTP-RV is a powerful and dedicated software for the simulation and analysis of transients in power systems. It provides an extensive variety of system modeling capabilities covering electromagnetic and electromechanical oscillations ranging in duration from microseconds to seconds. EMTP-RV is the most comprehensive analysis and simulation program for power system transients.


Tuesday, October 18, 2011

Isolation Transformer is a special type of transformer, wherein the primary and secondary windings are physically separated through a so-called double insulation. Also, an isolation transformer with electrostatic shields is commonly employed as power supplies for sensitive devices like computers, laboratory measurement instruments and medical equipment.


Thursday, October 13, 2011

Joseph Henry (1797-1878) was born near Albany, New York. His ambition was to become an actor until by chance at the age of 16 he happened to read a book of science, which caused him to devote his life to the acquisition of knowledge. That event turns out to be the beginning of Henry’s rise as the leading American scientist after Benjamin Franklin and until Willard Gibbs.


Sunday, October 2, 2011

ESD or Antistatic Wrist Strap is a protective device worn by personnel while working on sensitive electronic components in order to safely direct static electricity from their body to ground, and avoid equipment damage. This is because the antistatic wrist strap can avert the accumulation of static electricity, which can eventually lead to an electrostatic discharge.


Friday, September 30, 2011

Ferranti Effect is the rise in receiving-end voltage (VR) as compared to the sending-end voltage (VS) of a transmission line. It was first noticed by Sebastian Ziani de Ferranti on a project involving underground cables in a 10 kV distribution system in 1887, and was eventually named after him.


Monday, September 26, 2011

STATCOM or Static Synchronous Compensator is a shunt device, which uses force-commutated power electronics (i.e. GTO, IGBT) to control power flow and improve transient stability on electrical power networks. It is also a member of the so-called Flexible AC Transmission System (FACTS) devices. The STATCOM basically performs the same function as the static var compensators but with some advantages.


Friday, September 23, 2011

Static Var Compensator (SVC) is a power quality device, which employs power electronics to control the reactive power flow of the system where it is connected. As a result, it is able to provide fast-acting reactive power compensation on electrical systems. In other words, static var compensators have their output adjusted to exchange inductive or capacitive current in order to control a power system variable such as the bus voltage.


Monday, September 19, 2011

Power Quality Primer is an excellent reference, which provides description of power quality terminologies, problems and solutions. It aims to give utilities to become competitive in a deregulated power business, while educate customers for them to properly select the utility that meets their power quality requirements. Power Quality Primer outlines the fundamentals, to easily understand the needed concepts, for electrical engineers, consultants and prospective mitigators.


Sunday, September 18, 2011

There are several techniques available for Flicker Mitigation. However, since flicker is caused by voltage fluctuations, these methods should be based on reducing such power quality phenomenon. It must be noted that the effects of voltage fluctuations are dependent on both its amplitude and the rate of their occurrence. Generally, mitigation measures are focused on limiting the amplitude of the voltage fluctuations. 


Saturday, September 17, 2011

The UIE/IEC Flickermeter is the standard for flicker measurement. Its main function is to provide assessment of the flicker perception caused by voltage fluctuations. Therefore, the flickermeter should be designed to have the capability of transforming the input voltage fluctuations into an output parameter proportionally related to flicker perception. This is done by simulating the process of physiological visual perception, which is the lamp–eye–brain chain.


Wednesday, September 14, 2011

The undesirable effects of voltage fluctuations on electrical and electronic equipment are briefly described here. This post aims to give an overview of these effects, which are more obvious with light sources. Nonetheless, other sophisticated devices subjected to voltage fluctuations could malfunction and have reduced efficiency, which are costly in terms of downtime and rejects.


Saturday, September 10, 2011

Voltage Fluctuations are described by IEEE as systematic variations of the voltage waveform envelope, or a series of random voltage changes, the magnitude of which falls between the voltage limits set by ANSI C84.1. Generally, the variations range from 0.1% to 7% of nominal voltage with frequencies less than 25 Hz. Subsequently, the most important effect of this power quality problem is the variation in the light output of various lighting sources, commonly termed as FlickerThis is the impression of instability of the visual sensation brought about by a light stimulus, whose luminance fluctuates with time.


Sunday, September 4, 2011

Handbook of Power Quality examines a full scope of PQ disturbances, including background theory and guidelines on measurement procedures and problem solving. Electrical engineers, consultants and even students who want to learn and gain knowledge on power quality issues and causes will find this book easy to read and understand, as it is well-written and organized. Also, Handbook of Power Quality is a good resource in that it contains many contemporary references for further in-depth study.


Friday, September 2, 2011

A Shunt Reactor is not yet included in the list of ETAP elements in its Edit Toolbar as of its latest version (ETAP 11). Nonetheless, it doesn’t mean that we can’t model a shunt reactor in our one-line diagram and simulation. Modeling a shunt reactor in ETAP is easy using a simple technique.


Thursday, September 1, 2011

Alessandro Volta made one of the greatest electrical discoveries of all time in 1796. He was able to produce continuous electric current for the first time, through his voltaic pile. Basically, it was the first electric battery and the first source of direct current (DC).


Friday, August 26, 2011

The primary effect of harmonics on power cables is the additional heating due to increase in the I2R losses. This can be attributed to the two phenomena known as skin effect and proximity effect, both of which vary as a function of frequency as well as conductor size and spacing. Also, cables involved in system resonance, may be subjected to voltage stress and corona, which can lead to dielectric (insulation) failure.


Monday, August 22, 2011

The detrimental effects of harmonics on motors and generators are usually taken for granted due to complacency or lack of knowledge. For example, the increase application of variable frequency drives (VFD) has subjected motors to considerably higher harmonic levels compared to when it was still using traditional controllers. As a consequence, the machine efficiency and torque developed are significantly affected.


Friday, August 19, 2011

Active Harmonic Filters (AHF) are power quality devices that monitor the nonlinear load and dynamically provide controlled current injection, which cancels out the harmonic currents in the electrical system. They also correct poor displacement power factor (DPF) by compensating the system’s reactive current. The concept applied is relatively old, but the lack of effective methods hampered its development for a number of years. Presently, the widespread use of Insulated Gate Bipolar Transistors (IGBT) and the availability of Digital Signal Processing (DSP) components are paving the way for the emergence of active harmonic filters.


Thursday, August 18, 2011

IEEE 1531 has outlined key harmonic filter design considerations for its proper selection. They are grouped into performance and rating criteria. The former relate to normal expected operating conditions. Meanwhile, the rating criteria refer to unusual conditions that may place a more severe duty on the equipment.


Sunday, August 14, 2011

The purpose of this ETAP Tutorial is to show how to size a shunt passive harmonic filter using the Harmonics Module. It illustrates the advantage of using the ETAP software to improve the speed and accuracy in conducting the power quality study needed to determine the appropriate capacitor and reactor ratings. This is made easy with the help of the Harmonic Filter editor, which provides all the practical and popular filter structures for the user to choose from.


Saturday, August 13, 2011

Passive Harmonic Filters are currently the most common method used to control the flow of harmonic currents. They are built using a series of capacitors (capacitance) and reactors (inductance) forming an LC circuit in parallel with the power source. More complex designs may involve multiple LC circuits, some of which may also include a resistor. The passive harmonic filter is also referred to as a trap because it absorbs the harmonic current to which it is tuned.


Tuesday, August 9, 2011

Harmonic Filters are used to mitigate the power quality problem known as harmonic waveform distortion. Consequently, they minimize the thermal and electrical stress on the electrical infrastructure, eliminate the risk of harmonics-related reliability issues and allow for long-term energy efficiency and cost savings. Harmonic filters will play a vital role in ensuring a better power quality, especially now that most modern electrical devices are of the nonlinear type.


Sunday, August 7, 2011

According to Reuters report, there have been three large explosions from the sun in the past days, and that sun storms are set to hit the Earth. These incidents have pressed the U.S. government to warn “users of satellite, telecommunications and electric equipment to prepare for possible disruptions over the next few days."


Saturday, August 6, 2011

True RMS Multimeter, as the name suggest, measures the real root mean square value of the input current and voltage. It utilizes an integrated circuit that derives the true RMS of dynamic and complex waveforms of all shapes and sizes. This capability allows the true RMS multimeter to provide accurate measurements regardless of current and voltage waveform (i.e. pure sinewave, square, harmonic-distorted or non-sinusoidal).


Friday, August 5, 2011

Power System Harmonic Analysis provides a comprehensive input about harmonics in the power system and the corresponding theories and discussions. It gives sufficient information regarding harmonics - a growing issue in this electronic age, including analytical and modelling tools for the evaluation of power quality. On the contrary, the book was not able to converse about the different mitigation techniques against harmonics, which could have made it a comprehensive reference for both power quality engineers and students.


Thursday, August 4, 2011

K-Factor Transformer, also known as K-Rated transformer, is designed for nonlinear or harmonic generating loads that a standard transformer could not adequately handle due to overheating. K-factor transformers are specially assembled with a double sized neutral conductor, heavier gauge copper and either change the geometry of their conductors or use multiple conductors for the coils. These properties allow them to endure the additional heat caused by harmonic currents much better than a standard transformer.


Wednesday, August 3, 2011

Single-phasing is the worst case of unbalance and can either be due to an open phase in the primary or secondary side of a transformer. However, this post will only tackle single-phasing on transformer primary, which exposes electrical equipment such as three-phase motors to unbalance voltages. Consequently, excessive unbalance currents will exist with one line to have at least 230% of normal current drawn by the motor.


Tuesday, August 2, 2011

The negative effects of harmonics on transformers are commonly unnoticed and disregarded until an actual failure happens. Generally, transformers designed to operate at rated frequency have had their loads replaced with nonlinear types, which inject harmonic currents into the system. Consequently, transformers that have operated adequately for long periods have failed in a comparatively short time.


Friday, July 29, 2011

A Series Capacitor is not included in the list of ETAP elements in the AC Edit Toolbar. However, it doesn’t mean that we can’t include a series capacitor in our one-line diagram and simulation. This is because modeling a series capacitor in ETAP is very easy using a simple trick.


Thursday, July 28, 2011

Supercapacitor or Ultracapacitor is technically an electrochemical double layer capacitor (EDLC). This technology is already gaining ground in challenging the Battery as the leading and preferred energy storage device. It exemplifies a large improvement from the common electrolytic capacitors, which are quick and powerful but energy poor. Meanwhile, supercapacitors are energy-rich storage devices whose first applications are likely to be in hybrid electric vehicles and backup power supplies.


Tuesday, July 26, 2011

Harmonics are described by IEEE as sinusoidal voltages or currents having frequencies that are integer multiples of the fundamental frequency at which the power system is designed to operate. This means that for a 60-Hz system, the harmonic frequencies are 120 Hz (2nd harmonic), 180 Hz (3rd harmonic) and so on. Harmonics combine with the fundamental voltage or current producing a non-sinusoidal shape, thus, a waveform distortion power quality problem. The non-sinusoidal shape corresponds to the sum of different sine waves with different magnitudes and phase angles, having frequencies that are multiples of the system frequency.


Sunday, July 24, 2011

Introduction to Electromagnetic Compatibility (2nd Edition) deals with the subject of interference in electronic systems. It builds on the undergraduate electrical engineering concepts and applies them to the design of electronic systems that operate compatibly with other electronic systems and do not create interference phenomena.


Saturday, July 23, 2011

Single-phasing occurs when one phase of a three-phase system is intentionally or accidentally opened. It is considered as the worst case of Voltage Unbalance and can either be in the primary side or secondary side of a distribution transformer. However, this post will only discuss single-phasing on the secondary side of the transformer.


Wednesday, July 20, 2011

Switched Mode Power Supply (SMPS), sometimes called Switcher is gaining popularity in the last decade or so. The SMPS is more efficient and lighter in weight as compared to its conventional counterpart, the linear power supply.


Thursday, July 14, 2011

Electrostatic Discharge (ESD) is a form of an impulsive transient exhibited by the rapid and momentary flow of electrostatic charge two things that are at different potentials. Generally, this sudden discharge of static electricity does not harm the human body, but it can create several problems such as damaged electronic components, equipment downtime, safety concerns and material handling issues in the manufacturing and production facilities. 


Thursday, June 30, 2011

Series Capacitors are generally applied to compensate the excessive inductance of long transmission lines, in order to reduce the line voltage drop, improve its voltage regulation, minimize losses by optimizing load distribution between parallel transmission lines, and to increase the power transfer capability. Similarly, series capacitors are also installed in electrical power systems to improve its voltage stability.


Tuesday, June 28, 2011

Shunt Capacitors have several uses in the electric power systems. They are utilized as sources of reactive power by connecting them in line-to-neutral. Electric utilities have also connected capacitors in series with long lines in order to reduce its impedance. This is particularly common in the transmission level, where the lines have length in several hundreds of kilometers. However, this post will generally discuss shunt capacitors.


Monday, June 20, 2011

Voltage Unbalance (or Imbalance) is defined by IEEE as the ratio of the negative or zero sequence component to the positive sequence component. In simple terms, it is a voltage variation in a power system in which the voltage magnitudes or the phase angle differences between them are not equal. It follows that this power quality problem affects only polyphase systems (e.g. three-phase).


Monday, June 13, 2011

DVR (Dynamic Voltage Restorer) is a static var device that has seen applications in a variety of transmission and distribution systems. It is a series compensation device, which protects sensitive electric load from power quality problems such as voltage sags, swells, unbalance and distortion through power electronic controllers that use voltage source converters (VSC). 


Friday, June 3, 2011

PSCAD (Power Systems Computer Aided Design) is a time domain simulation software for analyzing transients in electrical networks. It is a collection of programs, providing a graphical Unix-based user interface to electromagnetic transients program (EMTP). It is also known as PSCAD/EMTDC. EMTDC (Electromagnetic Transients with DC Analysis) was first developed in 1976 and has been constantly evolving in its scope and capabilities. It is an integral part of PSCAD as it is the library of power system component models and procedures, which establish the simulation software provided with PSCAD. Together they provide a fast, flexible and accurate solution for the efficient  time-domain program for simulating a variety of electrical power system transients and control networks. 


Monday, May 30, 2011

The sizing of the Rectifier and Inverter follows once the overall UPS size has been determined. Most of the time, the suppliers will only ask the UPS VA rating and they will do the rest. However, as an engineer, knowing how to determine the corresponding rectifier and inverter size will help you evaluate the practicality of the specifications given by the supplier. For that reason, this post includes the sizing calculations with an example for easy learning.

Rectifier or Charger

The rectifier should be properly sized to satisfactorily perform these two tasks:  Supply the inverter at full load (Ir) and charge the batteries at the maximum charge current (Ic). Therefore, the rectifier DC load current (Idc) is the sum of Ir and Ic. In equation form: Idc = Ir + Ic

The design DC load current is the current drawn by the inverter from the rectifier at full load.

Ir = S/Vdc


Ir = Design DC full load current (A)
S = UPS VA rating
Vdc = nominal battery / DC link voltage

Since a 1000 VA UPS was selected, then,

Ir = 1000 VA / 120 V = 8.33 A.

Meanwhile, the maximum battery charging current can be computed as follows:

Ic = (C X f)/t


Ic = maximum DC charge current (A)
C = selected battery capacity (Ampere-hour or Ah)
f = battery recharge efficiency (typically 1.1)
t = minimum battery recharge time (hours)

In the example, for a 60 Ah battery as calculated in UPS Sizing: Battery Capacity, and recharge time of 2.25 hours,

Ic = (60 Ah X 1.1) / 2.25
Ic = 29.33 A

Thus, the total minimum DC rectifier/charger current is:

Idc = 8.33 + 29.33 = 37.7 A

Select the next standard rectifier rating that exceeds the total minimum DC current above.

Use a 40-Ampere Rectifier.

Inverter Sizing

The inverter must be rated to continuously supply the UPS loads. Therefore, the inverter shall be sized based on the selected UPS VA rating.

For a three-phase UPS:
Iac = S / (1.732 X Vo)

For a single-phase UPS:
Iac = S/Vo


Iac = design AC full load current (A)
S = UPS VA rating
Vo = nominal AC output voltage (line-to-line voltage for a three phase UPS)

As an example, a single-phase 1000 VA UPS shall have an inverter size of:

Iac = 1000 VA / 120 V
Iac = 8.33 A

Select the next standard inverter rating that exceeds the design AC load current.

Use a 10-Ampere Inverter.


Battery Sizing guarantees that loads being supplied are adequately served for its designed autonomy time. Improper capacity sizing could jeopardize batteries used in applications such as Uninterruptible Power Supply (UPS), solar power systems, emergency lighting and telecommunications.

The calculations done on this post were based on the ampere-hour method for sizing battery capacity with reference to IEEE 485 and IEEE 1115. Also, it focuses only on lead-acid and nickel-cadmium (NiCd), which are two of the most common battery types used for UPS applications.

UPS Battery
UPS Battery Bank
The Battery Sizing involves these steps:

1. Identify the Battery Loads

Just like in UPS Sizing, the first step is to identify and select the loads that the battery will be supporting. This is dependent to the application of the battery, such as UPS systems. In this case, the battery load will be the same as that selected for the UPS.

2. Create the Load Profile

Refer to the UPS Sizing: Load Profile post for details on how to create a load profile and subsequently derive the design load and energy demand. IEEE refers to the load profile as the duty cycle. The resulting design energy demand in the example is 4020 VAh. 

3. Select the Battery Type

IEEE suggested that the following factors should be considered in selecting the battery type:

Ø  Application design life and expected life of cell
Ø  Physical characteristics (dimensions, weight)
Ø  Ambient temperature
Ø  Charging characteristics
Ø  Frequency and depth of discharge
Ø  Ventilation requirements
Ø  Maintenance requirements
Ø  Seismic factors (shock and vibration)
Ø  Cell orientation requirements

Subsequently, find the characteristics of the battery cells (generally from supplier data sheets), which includes the following:

Ø  Battery cell capacities in Ampere-hour (Ah)
Ø  Cell temperature
Ø  Electrolyte density at full charge (for lead-acid batteries)
Ø  Cell float voltage
Ø  Cell end-of-discharge voltage (EODV)

For the example, the selected battery type is the valve-regulated lead acid (VRLA) battery.

4. Number of Cells in Series

The number of cells in a battery is computed to match the minimum and maximum voltage tolerances of the load. As a minimum, the battery at its EODV must be within the minimum voltage range of the load. Meanwhile, as a maximum, the battery at float voltage (or boost voltage) needs to be within the maximum voltage range of the load. The cell charging voltage depends on the type of charge cycle that is being used, e.g. float, boost, equalizing, etc, and the maximum value should be chosen.

The number of battery cells required to be connected in series must fall between the two following limits:

Nmin = Vdc*(1+Vmax) / Vc
Nmax = Vdc*(1-Vmin) / Veod


Nmin = minimum number of battery cells
Nmax = maximum number of battery cells
Vdc = nominal battery voltage (V)
Vmin = minimum load voltage tolerance (%)
Vmax = maximum load voltage tolerance (%)
Veod = cell end of discharge voltage (V)
Vc = cell charging voltage (V)

As an example, the minimum and maximum load voltage tolerances are Vmin = 10% and Vmax = 10%, respectively. For the battery, nominal voltage is Vdc = 120V, end-of-discharge voltage is Veod = 1.8V/cell, and the cell charging voltage is Vc = 2.05V/cell. Then,

Nmax = 120(1+0.1) / 2.05
Nmax = 64.4 or 64 cells

Nmin = 120(1-0.1) / 1.8
Nmin = 60 cells

Use 62 cells in series (number of cells between 60 and 64).

5. Determine Battery Capacity

Other than the energy demand and battery nominal voltage, several other factors may affect the computation for battery capacity. These factors are discussed below:

Aging Factor (ka)

Battery performance decreases with increase in age. To ensure that the battery can meet capacity throughout its useful life, an aging factor is applied - generally 1.25. There are some exceptions - consult the supplier.

Temperature Correction Factor (kt)

Batteries are specified by a standard operating temperature of 25°C, a deviation from this value should be taken into account by applying a correction factor. The values can be gathered from the manufacturer. However, high temperatures reduce battery life irrespective of capacity and the correction factor is for capacity sizing only. Meaning, battery life can’t be increased by increasing capacity.

Capacity Rating Factor (kc)

This accounts for voltage depressions during battery discharge. IEEE 1115 Annex C suggests that for float charging applications, Kt = rated capacity in Ah / discharge current in Amps (for specified discharge time and EODV).

Taking the abovementioned factors into consideration, the minimum battery capacity required to carry the design load over the specified autonomy time shall be calculated as follows:

B = [Energy demand X (ka X kt  X kc)] / (Vdc X kd)


B = required battery size or capacity
ka = aging factor (%)
kt = temperature correction factor (%)
kc = capacity rating factor (%)
kd = depth of discharge (%)
Vdc = nominal battery voltage (V)

Select the next standard rating that exceeds the minimum Ah battery capacity.

For example, compute for the minimum battery capacity using the given energy demand of 4020 VAh from UPS Sizing: Load Profile, battery aging factor (ka) = 25%, temperature correction factor at 30 deg C (kt) = 0.956, capacity rating factor (kc) = 10% and depth of discharge (kd) = 80%.

B = [4020 VAh X (1.25 X 0.956 X 1.1)] / (120 V X 0.8)
B = 55.04 Ah

Use a VRLA battery with a 60 Ah capacity.

IEEE 485-2003. Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications
IEEE 1115-2005. Recommended Practice for Sizing Nickel-Cadmium Batteries for Stationary Applications


The Load Profile is basically a graph showing how the load demand changes with respect to time. It is the subsequent step after choosing the critical and essential loads for uninterrupted power protection. Through the load profile, the design load and energy demand values, which are prerequisites for UPS Sizing calculation, can be derived.


There are two techniques for creating a load profile:

1.  Autonomy Method

This is the conventional way used for emergency and backup power applications, such as in UPS systems. The instantaneous loads are displayed over a so-called autonomy time, which is the period of time that the loads need to be supported by a backup power system in the event of an interruption. IEEE 446 has published typical autonomy times for some of the critical and essential devices.

2. 24-Hour Profile

This method is more commonly associated with stand-alone applications, like in solar power systems. It displays the average instantaneous loads over a 24-hour period.

However, this post will only focus on the autonomy method in order to make a load profile for UPS Sizing.

Basic Steps

Preparation of Load List

1.      List all the selected critical and essential equipment to be protected by the UPS in one column.
2.      Read the nameplate of each load and write down the voltage (V) and current (A).
3.      Multiply the voltage and current in order to get the Volt-Ampere (VA) or apparent power for each load and enter these values in another column. Sometimes, the nameplate will only indicate the real power or load in Watts. In this case, divide the wattage by its nameplate power factor (PF) to derive the load VA. If PF is unknown, use typical values of 0.6 to 0.7.
4.     Decide on the autonomy time for each equipment. Some loads may only be required to ride through short interruptions or to have enough time for proper shut down. Meanwhile, other critical equipment may need to operate for as long as possible.
5.      Multiply the load VA and Autonomy time for each load and write the results (VAh) in the last column.
6.      Get the total for VA and VAh.

UPS Sizing Load List
Sample Load List for the Critical and Essential Equipment
Graph the Load Profile

Using the autonomy method, the load profile is constructed by piling the energy or VAh rectangles on top of each other. An energy rectangle has the autonomy time as its width and the load VA as height. The load profile is created by piling the widest rectangles first (i.e. equipment with the largest autonomy time).

UPS Sizing Load Profile
Load Profile for the Selected Loads
Design Load and Energy Demand

Prior to the calculation of the design load and energy demand, there is a prerequisite to set the design margin factor (DMF) and the load growth factor (LGF).

The design margin factor is used to account for any inaccuracies in estimating the load that may lead to UPS overloading. A 1.25 factor is usually recommended. On the other hand, the future load growth factor is typically in the range between 1.1 to 1.3. If certain that no future loads are expected, then this allowance can be omitted (not advisable).

Design Load

It is the total load VA of all the critical and essential equipment that should be protected from interruptions. This value is the basis for the UPS size, as well as for the inverter and rectifier.

Design Load = Total Load VA X (DMF) X (LGF)

Design Energy Demand

Computing for the energy demand is important for the sizing of energy storage devices such UPS batteries. It is calculated by finding the area under the load profile curve, which in this case is the total area of the rectangles. In other words, the energy demand is simply the total VAh of the loads multiplied by the margin and load growth factors.

Design Energy Demand = Total Load VAh X (DMF) X (LGF)


The total peak load apparent power is 640 VA and the total load energy is 2680 VAh. Using a margin factor of 1.25 and 1.2 growth factor, the following are calculated:

Design Load = 640 VA X (1.25) X (1.2)
Design Load = 960 VA

Design Energy Demand = 2680 VAh X (1.25) X (1.2)
Design Energy Demand = 4020 VAh

IEEE 446-1995. Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications


Sunday, May 29, 2011

UPS Sizing is an important process in order to closely match the UPS capacity to its load. This is done to come up with a sufficient overall uninterruptible power supply, so that it will operate efficiently at the lowest possible cost. An oversized UPS system will lead to increased operating expenses, while an undersized unit will be susceptible to overloading and has a small load capability for long-term growth. Also, UPS sizing aims to determine the practical ratings of its main components – battery, rectifier and the inverter. 

The UPS Sizing process involves the following five basic steps:

1.    Identify and select the prospective UPS loads
2.    Create a load profile and derive the UPS design load (VA) and energy (VAh)
3.    Compute the battery size (number of cells in series and Ah capacity)
4.    Determine the overall UPS Size
5.    Determine the size of the UPS rectifier and inverter

The following discussions include the details for each basic step in UPS sizing plus an example in order to guide through the entire process.

Identify and Select the UPS Loads

In UPS Sizing, the first step is to identify and select the critical and essential equipment that need to be protected from the damaging effects of voltage sags and interruptions. Listed below are some of the possible loads, which may require uninterruptible power supply.

Critical Loads: Data Center, DCS and ESD processor, telecommunications equipment, blade file servers and other sensitive electronics.

Essential Loads: Lighting, Heating and Ventilation.

As an example, the following loads are chosen for UPS protection: Telecommunications, Computer Console, DCS cabinet and ESD cabinet.

Create the Load Profile

Refer to the UPS Sizing: Load Profile post for details on how to create a load profile and subsequently derive the design load and energy demand. The resulting load profile, design load VA and energy demand VAh of the selected equipment are shown below:

UPS Sizing Load List
Sample Load List 
Design Load: 960 VA
Design Energy Demand: 4020 VAh

Battery Capacity 

Refer to the  UPS Sizing: Battery Capacity post for details on how to size the battery for UPS applications. Meanwhile, additional information regarding battery sizing for UPS applications are discussed below:

Nominal Battery or DC Link Voltage

This is often selected by the UPS supplier. However, if required to be selected, the following factors should be considered:

·         DC output voltage range of the rectifier – the rectifier must be able to output the specified DC link voltage
·    DC input voltage range of the inverter – the DC link voltage must be within the input voltage tolerances of the inverter. Note that the battery end of discharge voltage should be within these tolerances.
·        Total DC link current at full load – affects the sizing of DC cables and inter-cell battery links.
·       Number of battery cells required in series – this will affect the overall dimensions and size of the battery rack. If physical space is a constraint, then fewer batteries in series would be preferable.

In general, the DC link voltage is usually selected to be close to the nominal output voltage.

Overall UPS Sizing

The overall UPS Size is to be based on the computed design load VA from the load profile. The idea is to simply select the next standard UPS rating that exceeds the design load VA.

Design load = 960 VA
Use a 1000 VA UPS (Next standard UPS rating)

Rectifier and Inverter Sizing

Refer to the UPS Sizing: Rectifier and Inverter post for details on how to size the corresponding rectifier and inverter. In the example, for a selected UPS rating of 1000 VA and battery capacity of 60 Ah, the sizes are:


Idc = 37.7 A; Use a 40-Ampere DC rectifier.

Inverter Size (Single-phase Inverter and 120 V output)

Iac = 8.33 A; Use a 10-Ampere Inverter.

IEEE 446-1995. Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications


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About Me

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I am a Professional Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.