The Need For Ups System Engineering Essay

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In a paper written by John D. Harnden in 1978, the description of the improved efficiency of solid state power conditioning was that of exhilaration

Power quality became a quite serious issue in the 1980, when failure of various loads was attributed to these loads drawing non-sinusoidal load from the supply, mainly due to their non- linear properties. These loads caused varying harmonic distortions in the AC supply line, which meaning that power was wasted, and the overall efficiency is reduced. Power conditioning equipments was brought in to remedy this situation, where this equipment was placed at the point of the incoming power supply where the harmonic distortion is most significant. These equipment have had a great impact in the heavy apparatus, and industrial areas for two decades earlier, so

Nowadays, market pressure pressures on improvement performance have driven the development of better solution for the

The need for UPS system for provision of uninterrupted power, and power conditioning of critical loads has seen a wider audience, since the onset of power electronics-based loads, which contain passive energy-storage-circuit elements such as inductors and capacitor, according to [1] and [2].

Uninterruptible Power Supply systems are described as systems that produce a steady and continuous alternating current to any applied load.

A Smart Uninterruptible Power Supply (UPS) system is been designed. The smart system uses the advances in power electronics to accomplish such as the buck and boost converters, which promises a better result as opposed to early technologies, to effectively supply a constant voltage to a critical load.

The online double conversion design strategy has been adopted for this project. This decision was based on the advantages of the online design strategy over others; these views are presented backed by clear literature included in this work.

The smart system contains a bridge rectifier, which takes the nominal AC supply for conversion into DC, which is feed to a Buck converter, whose purpose to step-down the bus voltage to one acceptable to the back-up battery. The back-up battery is feed energy from the output of the buck converter during normal mode of operation, i.e. the AC supply voltage is within the preset range. The load sees output from the back-up battery energy during back-up mode of operation, i.e. the AC supply voltage isn't within the preset range. This report showcases the design of the buck converter and steps taken to achieve its stepped down voltage.

A Boost converter follows the buck converter, which on receiving the stepped down voltage of the DC bus line, steps it up to a voltage imaginably greater than that need by the load. This extra voltage is a safety margin set to account for the inverter efficiency; hence the output voltage of the inverter is lower but right for the load.

The back end of the smart system sees the use of a full-bridge inverter, which achieves 90% efficiency, due to the employment of the sinusoidal pulse-width modulation (SPWM) switching control technique, programmed into the Amicus 18 development board that incorporates the Microchip PIC® microcontroller. The achievement of SPWM switching pattern from simulations into a program for the amicus 18 compiler is presented.

The inverter produces a sinusoidal output of harmonics content pushed to higher frequencies following the adoption of the SPWM switching pattern. These harmonic contents are presented and the design of appropriate small sized passive filter to attenuate the harmonics is discussed. The integration of the inverter with the passive filter, allows for the delivery a low total-harmonic distortion (THD) sinusoidal output to the load.

The ORCAD PSPICE simulation software is adopted to run various simulations of each of the power block, as well as the complete UPS system. The simulation results are explained and referenced to in the practical implementation.

The prototype UPS system was designed to power a 200 VA single-phase rated power load. The steady state performance of the UPS system under such a load is presented, the result explained and a tie in into the simulations and model design shown. A suitable conclusion to the project and report was reached, which ends this paper.

Scope

The scope of this encompasses theoretical analysis focused around the fundamentals of the UPS system, as well as the simulation and prototype building of the smart UPS system.

The fundamentals of the UPS system includes a definition of UPS systems, power quality and its issues, review of the existing UPS topologies, and then a look into the conventional UPS system and its main components, the developed smart UPS system and its main components, switching devices and electric energy storage.

This work presents the smart UPS system, which incorporates the single-phase online double conversion design strategy.

Aims

Objectives

The UPS System

Introduction

UPS System

Load

Mains AC supply

Figure 2.1: UPS System Installation [2]

An uninterruptible power supply (UPS) system can be looked at protection of an electrical load against various power quality issues that are seen in electrical systems. As seen in Figure 2.1 , the UPS system is always connected between the point of incoming mains AC supply and the electrical load needing protection against the supply fluctuations, as well as a total failure of the AC supply, as described in [3].

What is a UPS?

A UPS is said to be a system which can provide a continuous, high-quality, and efficient power to electrical loads, according to [4]. An ideal UPS system, according to [4], would be expected to include the following functionalities:

Ability to suppress line transients and harmonic disturbances.

Regulated sinusoidal output voltage and current with a low total harmonics distortion (THD), irrespectively of load type.

No interruption even in switching time between various modes of operation.

Unity power factor.

Low cost, weight, size, and maintenance, but high efficiency.

Low electromagnetic interference and noise.

Why is a UPS necessary?

Ideally UPS systems are used to protect mainly power sensitive load known as 'critical loads', which are defined as loads that are quite susceptible to any type of variation seen during its normal operation to the incoming AC-input supply, as mentioned in [3].

As the range of microprocessor-based equipment varies and increases in the industrial and commercial sectors, so also the variances in the types of load that are termed to be critical. A number of electrical loads fall under the category of critical loads as mentioned in [3], some of which are:

computer systems, including those used in control system, data storage and processing

medical systems, such as life-support and monitoring systems

industrial processing

telecommunication equipment, like PABX

on-line transaction processes and management systems, such as internet banking and shopping.

Protection of these critical loads against the fluctuation of power or the failure cannot be over-exaggerated. Statistics show that the risk of not protecting the critical loads can range from loss to production (downtime) for few minutes to equipment damage bringing about inability to trade, and in few situations total short down in various business operations, as expressed by [3].

Power Problems

Interruption or in some cases, a total failure in the normal AC-input supply can occur for a number of reasons. All this reasons are summarized under power problem topic, which goes in depth to looking at various power problems.

In order to avoid lengthy discussion and confusion over power quality disturbances, the Institute of Electrical and Electrical Engineer's standardization body produced the IEEE Std 1159 - 2009 (latest version) [5], which clarifies the terminologies that describes the so-called electromagnetic disturbances.

Power Problem

Subcategories

Definition

Causes

Transients

Figure 2.3: Nine power quality problems [5]

How is a UPS used?

UPS Topologies

Introduction

In the 1970s, the market saw a massive boom for Uninterruptible Power Supplies (UPS), which was brought on by the need for power quality in large computer systems, as well as continuity of electrical power flow. The dramatic upsurge in digital technology saw the rise in the amount of applications that were sensitive to mains supply, which brought upon the need for more innovative and technologically advanced UPS systems.

In order to keep up with the advances in the UPS market, a diverse range of UPS systems, rating from a 100 VA to >1000 KVA, was conceived. The diversity in the UPS systems, lead to some confusion about the description used to identify the systems, where at the time, the commonly known are, 'on-line' UPS and 'off-line' UPS.

The 1970s saw the on-line UPS widely used, which was known to depict the load on AC-input mains power. This was to be rivalled in the 1980s by the development of the off-line UPS, which was known to represent the load not on the AC-input mains power. The standardisation body stepped in, after the development of a third topology in the 1990s, the 'line interactive', which was known for implementing reversible inverters.

The standard body formed by the International Electrotechnical Commission (IEC), later published the standard, IEC 62040-3. This standard splits the topologies into three types: passive standby, line interactive and double conversion, as illustrated in [6].

However, advanced studies in power electronics have driven these topologies even further and given more variety when it comes to choice of topology for any application. These studies have been concentrated on bettering the efficiency and reliability of the system, according to [7].

Whatever way a UPS topology is been constructed, a common feature to all is the inclusion of an energy storage element, such as a battery, which acts as the supply to load when the regular mains is interrupted or unavailable. UPS topologies are basically describing the design of the UPS itself, each with its own distinguishing performance characteristics.

There are a number of ways to classify the existing UPS systems but nowadays, these ways have been generalized into three categories, namely static, rotary and hybrid static or rotary, according to [7].

The static UPS system are used to describe the UPS as a solid-state equipment, as opposed to the rotary UPS systems that incorporate the use of moving part based around the motor or generator, according to [3]. The hybrid static or rotary UPS system on the other hand, brings together the common features of both static and rotary, as described by [7].

In this chapter, since this the static UPS sees its usage in majority of applications in recent years and the system developed by this work is based around this, the focus will only be on the description and make-up of the static UPS system.

Static UPS

These systems are the most generally used wherever a UPS system is seen. This is so because of the ease of their use for a broad number of applications ranging from low-power telecommunication systems and personal computer (PC), to medium-power medical system to high-power utility system. Main advantages of the static UPS includes its high reliability, efficiency accompanied by low total harmonic distortion (THD), according to [7].

Static UPS modules all fall under two design types, known as off-line and on-line. These two types can be further sub-divided into five others; Standby and Ferro for the off-line UPS while the on-line divides into line-interactive, double conversion and delta conversion, see Figure 3.2 : Static UPS System Topology [8], where each refer to the UPS in terms of the power utility, the delivery system of the UPS upstream.

Figure 3.2: Static UPS System Topology [8]

A brief look into each one of the static UPS topology shown in Figure 3.2 : Static UPS System Topology [8] will be carried out next, focusing mainly on the five varying sub-divided topologies. This will be concluded with a brief discussion on the choice topology used for this work.

Off-Line (Passive Standby) UPS System

An Off-line UPS system in general terms, is one that is characterised by the power to the load directly being supplied by the AC-input supply, in normal mode of operation, meaning that the load is subjected to variations in input frequency and voltage within a preset limit.

The Passive Standby topology, as shown in Figure 3.3 : Off-line UPS with passive standby topology, has as its main blocks as, a AC/DC converter, which can serve as the battery charger, a battery bank, a DC/AC inverter, a static switch, which changes the power supply to load from the bypass path to the battery path and vice versa, then an output AC to load.

Static Switch

Bypass

AC Supply

AC/DC Rectifier (Charger)

DC/AC Inverter

Battery Bank

  AC Output

Normal Mode

Stored-Energy Mode

Static Switch

Figure 3.3: Off-line UPS with passive standby topology

The switching time of the static switch is wholly dependent on the DC/AC inverter start time, so it introduces a brief break in power supply, which is normally about 2 to 10ms; hence a fast inverter start is always called for, although most loads using this configuration would easily be fine with this short break. Since the inverter is normally off in this configuration, the UPS isn't correcting the power factor. The inverter is normally rated to 100% of the load demand and is connected in parallel to the load, stays off in the mode of operation, giving this configuration an improved overall efficiency.

There are two modes of operation for this type of configuration: normal and stored energy.

Normal Mode

During normal mode, the load is power directly from the mains AC supply through the bypass line, switched in by the static switch, the load will be susceptible to any disturbances in the mains supply, which fall within the preset tolerance level of the bypass voltage but most often this is reduced by the addition of a filter/condition (spike/ surge suppressor or radio frequency).

The AC/DC converter charges keeps the battery bank fully charged, so as to be able to provide power to the load during the stored energy mode of operation. The DC/AC inverter is conservatively off during the mode of operation and will only be turned on during the stored energy mode of operation.

Stored-Energy Mode

This mode will be called for only and only if the power falls outside the preset tolerance of the bypass line, where, the load will be supplied power through the battery bank via the inverter by the static switch, until the battery voltage reaches its discharge limit or the bypass line voltage back within the preset tolerances.

This topology comes with its advantages as well as disadvantages for its application in a UPS system.

Advantages

A simple design.

Small size.

Low capital and running cost.

Great overall efficiency.

These advantages owed mainly to the use of a low-grade inverter while the rectifier is very low rating, since its main purpose is a charger to the battery bank.

Disadvantages

The lack of regulation of the output voltage under normal mode of operation.

Longer switching time.

A poor performance with non-linear loads.

Due to these disadvantages, the application of the configuration is also limited to low-power ones such as personal computers typically < 2kVA.

Off-Line (Ferroresonant Standby) UPS System

This topology work in a similar fashion to that of the passive standby UPS system but in an improvement to that, it offers passive voltage regulation through its ferroresonant transformer, which is a non-linear transformer, see Figure 3.4 : A Ferroresonant Transformer [9] [9]. This transformer is been engineered to always provide nearly constant voltage at the secondary winding (output) despite any variation in the primary windings (input), by operating at a point of magnetic saturation, where the iron core is so strongly magnetized that no increase in magnetic flux is seen, though the winding current is increased, as described by [9].

To fight the effect of distortion to the sine wave due to the saturation, a 'tank circuit', shown as the resonant LC circuit in Figure 3.4 : A Ferroresonant Transformer [9], which is adjusted to match the power supply frequency, acts as a filter that discards any harmonics produced. This non-linear transformer varies from a normal linear transformer in terms of the output voltage, where in the case of the non-linear; the output voltage never strays outside a pre-set regulation band, typically 1% - 4%, irrespective of the variation to the input voltage while the linear transformer as a direct proportionality to its input, according to [8] and [9].

Figure 3.4: A Ferroresonant Transformer [9]

The inclusion of the ferroresonant transformer in the off-line UPS, offers a capability of riding through short losses in power due to its tank circuit, which is characterised by its ability to store energy for up to half cycle; hence when combined with the inverter and static transfer switch gives an uninterrupted transfer to an alternate source, according to [9]. The topology has in its configuration, as seen in Figure 3.4 : On-line UPS with ferroresonant standby topology, an AC supply, a battery charger, the battery bank, a DC/ AC inverter, the ferroresonant transformer and an output AC to load.

Ferroresonant Transformer

AC Supply

Battery Charger

DC/AC Inverter

Battery Bank

AC Output

Normal Mode

Stored-Energy Mode

Static Switch

Figure 3.4: On-line UPS with ferroresonant standby topology

There are two mode of operation of the ferroresonant standby topology: normal and stored-energy.

Normal Mode

During normal operating conditions, the load is supplied a constant voltage from the ferroresonant transformer through the static switch, regardless of variation in the input supply and provides good power conditioning for disturbance like that of electrical line noise. The input supply also charges the battery through the battery charger; hence the battery is susceptible to the input voltage variations.

Stored-Energy Mode

When there is a power outage of the input supply, this mode will be called upon via the static switch which sets the power to that from the inverter. During this short switch, the load won't see this interruption mainly due to the stored energy in the tank circuit of the ferroresonant transformer. Power is then supplied to the transformer from the battery through the inverter until the input power is restored but the UPS will shut down power to the load if either the battery voltage level reaches a preset lower limit (autonomy) or the inverter faults, if the input power is not restored.

The ferroresonant standby topology comes with its advantages as well as disadvantages for its application in a UPS system.

Advantages

Simplicity.

Preservation of energy in the resonant tank circuit, gives an ability to allow short interruption in power without any interruption to load.

Output voltage is always maintained constant despite substantial input voltage variation.

The non-linear transformer can tolerate excessive loading and even a momentary voltage surge.

Harmonic filtering between the input power and the load.

Disadvantages

Energy is squandered in the saturated iron core due to hysteresis, producing great heat in the process.

Frequency differences are not tolerated.

Voltages generated by the tank resonant circuit are quite high, so expensive capacitors are needed and any engineer working on this will be exposed to dangerous working voltages.

On-Line (Line-Interactive) UPS System

Line-interactive UPS, similar to the off-line UPS system, differing by the continuity of power to the load at all times via the inverter, which also operates as an AC/DC converter to charge the battery, allowing for mains AC supply to be conditioned at the input frequency. This configuration, as shown in Figure 3.5 : On-line UPS with line-interactive topology, consists of the AC-input supply, a static switch, a bidirectional converter, the battery bank and the output AC to load.

Static Switch

AC Supply

Bidirectional Converter

Battery Bank

AC Output

Battery Charger

DC/AC Inverter

Normal Mode

Stored-Energy Mode

Figure 3.5: On-line UPS with line-interactive topology

There are three modes of operation for this topology: normal, energy-stored and bypass.

Normal Mode

During this mode of operation, as long as the AC-input supply is within a pre-set range, the load is supplied power through static switch and the parallel connected bidirectional converter; hence the power to load is conditioned since the reactive power is always close to unity. The bidirectional converter also does the work of charging the battery when needed, while keeping the output voltage stable and sinusoidal. The current for the load is mainly taken from the AC line while the output frequency is dependent on the input AC-input supply.

Stored-Energy Mode

In this operating mode, the bidirectional converter acts as an inverter and supplies power to the load from the battery bank, when the AC-input supply voltage falls outside the preset tolerance. The AC-input supply is prevented from a back flow of voltage from the inverter, since the static switch is disconnected from it. The UPS system will return to normal mode of operation, when the AC-input supply is within the preset tolerances again or the battery autonomy (end-of-discharge) is reached.

Bypass Mode

The line-interactive topology may include this mode for a maintenance bypass. This mode allows for load to be supplied power via an external bypass line in the occasion of internal malfunction of the UPS, by this work come be done with satisfaction of continual uninterruptible power to load.

The line-interactive topology offers some advantages and throws back some disadvantages to its use for a UPS system.

Advantages

Lower costs in comparison to the double conversion of equal power rating.

Good harmonic suppression for the input current.

Due to a single conversion in this topology, its efficiency is higher than that of the double conversion.

Disadvantages

Absence of isolation of the load from the AC line.

With non-linear loads, it has poor efficiency, poor guard against spikes and over voltages.

Due to the inverter in parallel and not in series with the load, output voltage conditioning is limited.

In addition to its disadvantages, the fact that frequency regulation is not likely, the line-interactive topology cannot be used for delicate load of medium to high power rating; it is therefore advised to be used in application requiring low power.

On-line (Double Conversion) UPS System

In this configuration (Figure 3.6 : On-line UPS with a double conversion topology), the DC/AC inverter is in series between the AC supply and the load, where power will flow through it continuously, irrespectively of the mode of operation. The topology consists of the rectifier/charger, the battery bank, the inverter, the static switch, with an optional bypass AC-input supply and a manual maintenance bypass option as shown by Figure 3.6 : On-line UPS with a double conversion topology, according to [6].

Manual Maintenance Bypass

AC Supply

AC/DC Rectifier /Charger

DC/AC Inverter

Battery Bank

AC Output

Normal Mode

Stored-Energy Mode

Static Switch

Bypass AC Supply

Bypass Mode

Only connected if normal AC supply exists

Figure 3.6: On-line UPS with a double conversion topology

There are three modes of operation for this topology: normal, energy-stored and bypass.

Normal Mode

Power is continuously supplied to the load, through the rectifier/ charger and inverter combination, during this mode of operation; hence a double conversion happens, that is, AC-DC-AC, where this topology derives its name from, according to [6].

Stored-Energy Mode

The stored-energy mode of operation comes into play when the AC-input supply voltage is seen to have fallen outside the preset tolerances, at which point the battery bank will continuously supply powers the load through the inverter.

The UPS system will only exit this mode, when the AC-input supply is restored or returns back to within the preset tolerances or when the preset battery autonomy (end-of-discharge) is reached. [6]

The load voltage will move in phase with the input voltage through a phase-locked loop (PLL) system, when the AC-input supply is returned. The double conversion allows for excellent line conditioning, where the battery bank is charged by the AC/ DC converter and supplies power to the load through the inverter. This puts the AC/ DC converter as the highest cost of the double conversion topology with the highest power rating too [7].

Bypass Mode

The bypass mode is called upon by the static switch or static bypass, if there is any internal malfunction, overcurrent (in-rush or fault clearing), or when the battery is at its end-of-discharge, where the inverter shuts down. Bypass however, implies the output and input frequency must be identical, this is in order to ensure successful power transfer. [7], [6]

Synchronisation between the bypass AC supply and the AC-input supply is needed, which will ensure the power transfer is done instantaneously [6]. As an extra option, the maintenance bypass is added to this topology, constructed to allow for maintenance to the UPS system be carried out without interruption to the flow of power to load. The maintenance bypass is manually operated by a switch.

As with other topologies, the double conversion topology offers its own advantages and disadvantages, these are listed next.

Advantages

Double conversion topology, offers greatest degree of critical supply integrity, since the load, in most cases, always receives processed power through the inverter.

Isolation of the load from the upstream fluctuations such as over voltages, surge or spikes.

Precise output voltage regulation and a very wide tolerance of the AC-input voltage variations.

In the event of AC-input failure, power transfer to battery bank is instantaneous.

A high level performance under transient or steady state conditions.

Output frequency can be precisely regulated or can be changed by simply disabling the static switch.

An added option for system maintenance manually.

Disadvantages

The rectifier in the double conversion topology makes it achieves a lower power factor and high total harmonic distortion (THD) at the input.

A lower efficiency than its other counterparts, which is mainly due to the presence of its double conversion.

There is a high price to pay for this topology.

In retrospective of the drawbacks of the double conversion, it is the most preferred topology, when it comes to power conditioning and regulation, load protection or overall performance, mainly due to its numerous advantages.

On-line (Delta-Conversion) UPS System

This topology was developed in order to compensate for the deficiencies of the typical line interactive and double conversion UPS systems. The delta conversion UPS system (Figure 3.7 ) consists of two bidirectional converters; both connected the battery bank, the static switch and a series delta transformer. [7], [10]

Static Switch

AC Supply

Series Bidirectional Converter (Charger)

Parallel Bidirectional Converter

Battery Bank

AC Output to Load

Normal Mode

Stored-Energy Mode

Series Transformer

Figure 3.7:On-line UPS with a delta-conversion topology

The series bidirectional converter is rated at 20% of the output power and connected to the AC-input supply through the series transformer. It serves as a current regulator, ensuring the input power factor is at unity and compensating for any differences between the input and output voltages.

The parallel bidirectional converter, serves as the normal inverter is connected in parallel to the load, and fully rated at 100% of the output power. Using pulse-width modulation (PWM) control, the parallel converter stabilizes the load voltage and regulates the battery charging, when the AC-input supply is within the preset tolerances. [7], [8] and [10]

A description of the two modes of operation of the delta-conversion topology: normal and stored-energy, would follow.

Normal Mode

During this mode of operation, if the load is at 100%, battery fully charged and no difference between the load and AC-input supply, the load is supplied all power directly from this AC-input through the series transformer with no power through the converters.

When a voltage sag of the AC-input supply is seen the converters are called into action, where they compensate for the sag by taking from the AC-input supply, firstly through the parallel converter, then the series converter and finally through the series transformer, in that reverse order, adding to the reduced voltage to ensure 100% rated power is feed to load, see Figure 3.7 .

Static Switch

AC Supply

Series Bidirectional Converter (Charger)

Parallel Bidirectional Converter

Battery Bank

AC Output to Load

Normal Mode

Extra power taken from AC supply

Series Transformer

Figure 3.7: Delta-conversion UPS power flow during voltage sag

For a voltage swell, the series converter takes in the extra voltage from the AC-input supply through the series transformer and feeds it forward to the load, adding to the lower voltage from the AC-input supply through the parallel inverter, see Figure 3.7 .

Static Switch

AC Supply

Series Bidirectional Converter (Charger)

Parallel Bidirectional Converter

Battery Bank

AC Output to Load

Normal Mode

Extra power taken to AC supply

Series Transformer

Figure 3.7: Delta-conversion UPS power flow during voltage swell

To charge the batteries, extra power is taken from the mains and passed backwards through the parallel converter; hence the battery charging is done solely by the parallel converter. [10]

Stored-Energy Mode

When AC-input supply falls outside the preset tolerances, power is delivered to the load the from battery bank through the parallel inverter, in this mode of operation. The parallel inverter is synchronised with the AC-input supply, therefore it controls the output voltage and frequency through an internal frequency reference set by the pulse-width modulation control scheme. [10]

Advantages

High efficiency due to absence of conversion of the major percentage of power, around 85%, flowing from the AC-input supply to the load.

Can be used in high power rated applications, which specific efficiency as a key priority.

Disadvantages

It comes with a complicated control scheme, posing a hindrance to its usage in certain applications.

An evidently lack of electrical isolation between the load and the incoming upstream AC-input supply.

Choice of Topology

In order to conclude on any type of topology to use for a UPS system, the following factors are ultimately the deciding factors.

Rated power of the load to be supported by the UPS system.

The available cost

Main Components of a UPS System

Introduction

The main components of any UPS systems are undoubtedly the inverter, rectifier/ charger and a storage bank such as the battery, see figure xx. These three components are not necessarily used in all systems but typically when an uninterruptible system is described, they are the dominant components, according to [3]. In this chapter, the composition of these blocks will be looked into, but first the interaction along the UPS system in terms of voltage levels will be introduced.

Voltage Interaction

As this report focuses on the transformerless UPS system, an example of voltage conversion through this will be used. Considering a system, fed with a 240Vac single-phase mains supply with an intended output of 240Vac, the voltage levels will be

Inverter

According to the IEEE-SA standards dictionary, an inverter, also known as a power inverter, is described as a machine, system or device that produces an alternating current (AC) by changing from a direct current (DC) [11]. However, an inverter is sometimes used to describe a variable-frequency drive; a device used in an AC electric motor, such as an induction motor, to controls its operating speed that converts incoming fixed AC from one frequency into another [12].

An inverter can be a single-phase or three-phase based topology; since this paper brings to life the single-phase UPS, only the single-phase inverter will be discussed. This discussion will start with the classification of inverters, a brief description of each,

Classification of Inverters

In UPS systems, the main purpose of the inverter is to convert the DC link voltage, i.e. energy from the battery bank, into an AC output suitable for the connected load; hence the focus in this paper will be on the DC - AC Inverter topology, which takes the typical block diagram as seen in figure xx. There are two types related to this topology: current source inverter (VSI) and voltage source inverter (CSI)

Current Source Inverter

VDC

IAC

Output Current

IDC

L

Figure 4.3: Block Diagram of a Current Source Inverter (CSI)

Voltage Source Inverter

The most widely used class of inverter is the voltage source inverter, as this has seen a great deal of attention

IDC

VDC

IAC

Output Voltage

C

Figure 4.3: Block Diagram of a Voltage Source Inverter

Principle of Operation

There are two main types used to configure a typical single-phase inverter: half-bridge inverter and full-bridge (H-bridge) inverter [7]. The main difference between is the use of four switches in the full-bridge inverter to the two switches used in the half-bridge inverter.

Commonality between these configurations is the use of transistor switches. The switches are driven by signals, which turn them on and off as desired. The drive switching signal to the switches must always be in anti-phase, i.e. 180° off of phase, in order to achieve the desired output, otherwise, a short circuit is created and damage to equipment is imminent. Also, an extra measure is needed to ensure that there is always a dead band between the time one switch goes off and the other turns on, otherwise a shoot-through fault will be experienced.

This section of the paper will elaborate on these two types, particularly on the full-bridge inverter, as its application is more commonly favoured, also a note that from here onwards, when an inverter is referred to only the VSI inverter topology is shown.

Half-Bridge Inverter

The half-bridge inverter has its two switches connected in series across the DC supply, as in Figure 4.3 : Half-Bridge Inverter Configuration, the top S1 switch connected to the positive terminal while the bottom switch S2 is connected to the negative terminal. The point of connection between the two switches provides the output, either negative or positive.

Figure 4.3: Half-Bridge Inverter Configuration

A simple truth table summarizes the switching pattern and mode of operation of the half-bridge inverter topology (Table xx), corresponding to figure xx.

Table 4.3.: Half-Bridge Inverter Truth Table

S1

S2

Output Condition

Off

Off

No output

Off

On

Negative

On

Off

Positive

On

On

Short Circuit (Shoot-Through)

Advantages

Simplicity of using a lower number of switches

Cheaper cost and simple control

Disadvantages

Lower efficiency

Only bipolar (single-pulse) PWM can be used to drive its switches

Output is a square wave rich in odd harmonics

Requires a large filter to bring it to the desired sinusoid waveform

These setbacks ensure that this topology is only used for low-power applications; medium- and high-power application will need to be supplied using the full-bridge topology.

Full-Bridge Inverter

The full-bridge inverter, see Figure 4.3 : Full-Bridge Inverter Configuration, has its four switches S1, S2, S3 and S4, connected in series two-by-two in two legs. The connection points in the middle of the switches provide the output, which can be either negative, positive or zero.

Figure 4.3: Full-Bridge Inverter Configuration

A simple truth table summarizes the switching pattern and the operation of the full-bridge inverter topology (Table 4.3.: Full-Bridge Inverter Truth Table), corresponding to Figure 4.3 : Full-Bridge Inverter Configuration.

Table 4.3.: Full-Bridge Inverter Truth Table

S1

S2

S3

S4

Output Condition

Off

Off

Off

Off

No output

Off

Off

On

On

Zero

Off

On

Off

On

Negative

On

Off

On

Off

Positive

On

On

Off

Off

Zero

On

On

On

On

Short Circuit (Shoot-Through)

Advantages

Better efficiency

Disadvantages

Complex control technique

Output AC Voltage

As seen from the two inverter configuration, half-bridge and full-bridge, the output voltage though alternating, is a square waveform, whose amplitude is determined by the DC voltage input and frequency by the control drive signals frequency.

For usage on critical loads, which this report deals with, a square waveform output is simply not sufficient but rather a 50Hz sine waveform is required. The widely used method of obtaining an output that is of a sinusoidal form is the Pulse-Width Modulation (PWM), this method and its control techniques are discussed next.

Pulse-Width Modulation (PWM)

PWM is a technique of varying the pulse width of a control signal in order to control the output waveform. The pulse width is described as the duration between the 50% amplitude points on the leading edge and the trailing edge of the pulse, as shown by [13]Figure 4.3 : Square waveform describing pulse width [13], unless otherwise specified [14].

Figure 4.3: Square waveform describing pulse width [13]

The output voltage is directly proportional to the duty cycle and the amplitude of the DC input voltage, where the duty cycle is calculated using equation 1.

where, d is the duty cycle, pw is the pulse width in seconds and T is the period in seconds (s).

Since the amplitude of the DC input voltage is usually fixed, the duty cycle of the control signal into the switches, is varied to shape the output voltage. The effect of the variation shown in Figure 4.3 : The effect of varying the duty cycle [13], is what is known as PWM, the duty cycle calculated by equation 1.

Figure 4.3: The effect of varying the duty cycle [13]

A Smart UPS System

Traditional UPS

Power Electronics Solution

For this report, the

Choice of Topology - Online UPS System

The decision to implement an online UPS system was predominating down to the effectiveness of this topology. Other consideration was down to factors such as the size of UPS, load requirement, and load process requirements of which there are

Sizing the UPS

Main Components

The implementation of the transformerless UPS architecture began by taking a brief review of the system's main components, described in stages shown as blocks in figure xx. Each of the blocks will be broken into and the internal composition will be presented.

Inverter

As the main and most challenging component block is the inverter. It plays the most important role of producing the accurate power output at the rated frequency to the critical load, so a detailed attention to the inverter is a must. As already elaborated in previous chapter, the choice of an H-bridge topology is most advantageous for the online double conversation; hence this is used for the implementation. The next decision then lied in the type is switch to be used to for the H-bridge, the process of which will be presented.

Choice of Switch - Power MOSFET

Deciding on the most adequate switch to use lied mainly between the transistor types; IGBT and power MOSFET, this is because for the purpose of the use in the UPS system, fast switching is needed, that is the switch must be capable of operating at high switching frequency. The comparison between these two shown in table xx , gives a clear indication that the MOSFET is most adequate for the requirement.

Speed

The speed of the MOSFET can be generally represented as the total time it takes to change state from on to off or vice versa, knowing that a considerable amount of power is dissipated when a MOSFET not in either of its operating states. The speed is therefore calculated by adding the turn-on delay time, rise time, turn-off delay time, and fall time, resulting in a speed of 115ns for the IRF740, extracted from the datasheet (figure xx); hence a maximum frequency of approximately 8.7MHz.

Figure 5.3: Switching Speeds of IRF740 from the Datasheet

Heat Effects

Free-wheel diode

MOSFET Driver

In order to efficiently power on the MOSFET switch, the voltage supplied to the gate terminal, gate-source voltage must be greater than the MOSFET's threshold voltage (VGS > VTH) and the supply voltage to the drain VDS must be greater than VGS - VTH; hence the need for an appropriate driver circuit to ensure this..

The ICL7667 dual power MOSFET driver used is one that is designed to convert TTL level signals to high current outputs

Figure 5.3: Schematic Diagram for the MOSFET Driver Circuit

Circuit

Combining the Power MOSFET and the driver circuit, the finished inverter circuit diagram can be seen in figure xx, where

The power MOSFET used is the IRF740, with rating of 400V, RDS(ON) of less than 55mΩ and ID of 10A.

Opto-isolator

Also known as optocoupler, this device is a very

Microcontroller - Myamicus

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