.

CHAPTER-1

INTRODUCTION

1.1 INTRODUCTION:

Power Electronics has three faces in power distribution, first one that introduces valuable industrial and domestic equipment, a second one that creates problems and finally a third one that helps to solve those problems. Modern semiconductor switching devices are being utilized more and more in a wide range of applications in distribution networks, particularly in domestic and industrial loads. Examples of such applications widely used are Adjustable Speed motor Drives (ASD’s), Diode and Thyristor rectifiers, Uninterruptible Power Supplies (UPS), computers and their peripherals, consumer electronics appliances (TV sets for example) and arc furnaces. These semiconductor devices present nonlinear operational characteristics, which introduce contamination to voltage and current waveforms at the Point of Common Coupling (PCC) of industrial loads. Unexplained computer network failures, premature motor burnouts, humming in telecommunication lines and transformer overheating are only a few of the damages that quality problems may bring into home and industrial installations. Complications related to the use of non-linear loads for these systems have been a major issue for a long time for both power providers and users alike.

Power problems are partially solved with the help of LC passive filters. However, this kind of filter cannot solve random variations in the load current waveform. They also can produce series and parallel resonance with source impedance. To solve these problems, shunt active power filters have been developed, which are widely investigated today. These filters work as current sources, connected in parallel with the nonlinear load, generating the harmonic currents the load requires.

However, the cost of these active filters is high, and they are difficult to implement in large scale. Additionally, they also present lower efficiency than shunt passive filters. For these reasons, different solutions are being proposed to improve the practical utilization of

active filters. One of them is the use of a combined system of shunt passive filters and series active filters. This solution allows one to design the active filter for only a fraction of the total load power, reducing costs and increasing overall system efficiency.

Now a days voltage Source Converter (VSC) based custom power device are increasingly being used in custom power applications for improving the Power Quality (PQ) of power distribution systems. Devices such as Distribution Static Compensator (DSTATCOM) and Dynamic Voltage Restorer (DVR) have already been in use. A DSTATCOM can compensate for distortion and unbalance in a load such that a balanced sinusoidal current flows through the feeder, it can also regulate the voltage of a distribution bus. A DVR

can compensate for voltage sag/swell and distortion in the supply side voltage such that the voltage across a sensitive/critical load terminal is perfectly regulated. A Unified Power Quality Conditioner (UPQC) can perform the functions of both DSTATCOM and DVR. The UPQC consists of two Voltage-Source Converters (VSCs) that are connected to a common DC bus. One of the VSCs is connected in series with a distribution feeder, while the other one is connected in shunt with the same feeder. The DC links of both VSCs are supplied through a common DC capacitor.

1.2 AIM OF THE PROJECT:

Power Quality (PQ) survey reports have highlighted voltage sag as the prime reason for which particularly production industries suffer huge loss. Most of these voltage sensitive critical loads are non-linear in nature due to application of fast acting semiconductor switches and their specific control strategy. Undoubtedly they have revolutionized the state of the art technology in almost every field, but their large-scale presence in a system pose some major concerns as they affect the distribution utility in some highly undesirable way.

Primarily, this kind of load currents is rich in harmonics and they may require some reactive Volt Amperes (VA) as well. The harmonic currents flowing through the finite source impedance of the utility supply cause the voltage distortion at the Point of Common Coupling (PCC) to the other loads. It results in malfunction of control signals, protection and metering of other loads and system metering devices.

The aim of the IUPQC is two-fold:

• To protect the sensitive load L-2 from the disturbances occurring in the system by regulating the voltage.

• To regulate the bus B-1 voltage against sag/swell and or disturbances in the system.

In order to attain these aims, the shunt VSC-1 is operated as a voltage controller while the series VSC-2 regulates the voltage across the sensitive load.

1.2.1 Voltage sag:

Electronic devices function properly as long as the voltage (or the driving force) of the electricity feeding the device stays within a consistent range. There are several types of voltage fluctuations that can cause problems, including surges and spikes, sags, harmonic distortions, and momentary disruptions. Voltage sag is not a complete interruption of power; it is a temporary drop below 90 percent of the nominal voltage, and they normally last from 3 to 10 cycles (or 50 to 170 milliseconds). Voltage sags are probably the most significant Power Quality (PQ) problem facing industrial customers today and they can be a significant problem for large commercial customers as well.

There are two sources of voltage sags: external (on the utility’s lines upto consumer’s facility) and internal (within consumer’s facility).

Common causes of external voltage sags are lightning, storms, start-up of large loads at neighboring facilities. Internal causes of voltage sags can include starting major loads and grounding of wiring problems.

Whether or not voltage sag causes a problems will depend on the magnitude and duration of the sag and on the sensitivity of consumer’s equipment. Many types of electronic equipment are sensitive to voltage sags, including variable speed drive controls, motor starter contactors, robotics, programmable logic controllers, controller power supplies, and control relays. Much of these equipments are used in applications that are critical to an overall process, which can lead to very expensive downtime when voltage sags occur.

1.2.2 Current harmonics:

Harmonics can be put as high frequency current and voltage distortions within a power system whose frequencies are integral multiples of the fundamental system frequency (i.e., 50Hz). These power harmonics have become much more prevalent with the development of high efficiency electronic equipment which form nonlinear load. When talking about harmonics in power installations, it is the current harmonics that are of most concern because the harmonics originate as currents and most of the ill effects are due to these currents.

Harmonic current components lead to:

Increase in power system losses.

Cause excessive heating in rotating machinery.

Creates significant interference with communication circuits that share common right-of-ways with AC power circuits.

Generate noise on regulating and control circuits causing erroneous operation of such equipment.

These current harmonics further manifest themselves into voltage harmonics when passing through system impedance. Hence it is very important to mitigate the effect of current harmonics at the point of common coupling itself.

1.3 POWER QUALITY:

Power quality is simply the interaction of electrical power with electrical equipment. If electrical equipment operates correctly and reliably without being damaged or stressed, it can be considered that

the electrical power is of good quality. On the other hand, if the electrical equipment malfunctions, is unreliable, or is damaged during normal usage, we would suspect that the power quality is poor.

At the generating station utilities create a perfect sine wave with their generators, free of transients, harmonic distortion and high frequency noise. It is then transmitted at various voltage levels to the end user. If the generator were at the customer site, that would be the ideal solution. Since distribution systems are exposed to lightning, accidents, component failures, power factor switching transients, grid switching transients and customers who create harmonics and send them back onto the utility system distorting voltage and adding to reliability problems.

As a general statement, any deviation from normal of a voltage source (either DC or AC) can be classified as a power quality issue. Power quality issues can be very high-speed events such as voltage impulses / transients, high frequency noise, wave shape faults, voltage swells and sags, unbalance in voltage and current and total power loss. Power quality issues will affect each type of electrical equipment differently. They may cause equipment heating, measurement faults and scores of other similar problems.

1.4 POWER QUALITY DISTURBANCES:

Harmonic distortion: Continuous or sporadic distortions of the 50 hertz (HZ) voltage sine waveform, usually caused by microprocessor-based loads in the building such as computer power supplies, lighting ballasts, and electronic adjustable speed drives. Harmonics can also be transmitted from an energy user down the block. These can cause telecommunications or computer interference, overheating in motors, transformers, or neutral conductors, decreased motor performance, deterioration of power factor-correction capacitors or erratic operation of breakers, fuses, and relays.

Harmonic distortion

Interruption, momentary: A very short loss of utility power that lasts up to 2 seconds, usually caused by the utility switching operations to isolate a nearby electrical problem.

Momentary interruption

Interruption, temporary: A loss of utility power lasting more than 2 minutes caused by a nearby short circuit due to something like animals, wet insulators, or accidents. Corrected by; automated utility switching.

Temporary interruption

Sag: A short-term decrease in voltage lasting anywhere from milliseconds up to a few seconds. Sags starve a machine of the electricity it needs to function, causing computer crashes or equipment lock-ups. Usually caused by equipment start-up such as elevators, heating and air-conditioning equipment, compressors, and copy machines or nearby short circuits on the utility system.

Voltage sag

Swell: A short term increase in voltage lasting anywhere from milliseconds up to a few seconds. They are usually caused due to start/stop of heavy loads and poorly regulated transformers. Voltage swells may lead to damage of sensitive equipment.

Voltage swell

1.5 SOLUTIONS TO POWER QUALITY PROBLEMS:

There are three ways to solve the problems of power quality and provide quality power customized to meet user’s requirement:

System improvement.

Use mitigation equipment based on power electronics.

Improvement of equipment immunity.

Of these, the best way to handle power quality problems is to mitigate the effects of distorted voltage or current at the Point of Common Coupling. This would ensure that the harmonics are restricted from entering the distribution system and contaminating the system power as a whole. Thereby, the other loads connected to the system are provided with clean power.

Conventionally, passive filters have been used to mitigate the effect of power supply discrepancies, such as line current and voltage harmonics, and increase the load power factor. However, in applications these passive second order filters present the following disadvantages:

The source impedance strongly affects filtering characteristics.

As both the harmonic and fundamental current components flow into the filter, the filter must be rated by taking into account both currents.

When the harmonic current components increase, the filter can be overloaded.

Parallel resonance between the power system and the passive filter causes amplification of harmonic currents on the source side at a specific frequency.

The passive filter may fall into series resonance with the power system so that voltage distortion produces excessive harmonic currents flowing into the passive filter.

The increased severity of harmonic pollution in power networks has attracted the attention of power electronic and power system engineers to develop dynamic and adjustable solutions to the power quality problems. One such solution lies in the use of FACTS controllers, which have acquired significance in recent times.

FACTS is one aspect of the power electronics revolution that is taking place in all areas of electrical energy. A variety of powerful semiconductor devices not only offer the advantage of high speed and reliability of switching but, more importantly, the opportunity offered by a variety of innovative circuit concepts based on these power devices enhance the value of electrical energy.

FACTS controller: FACTS controller is a power electronic-based system and other static equipment that provide control of one or more AC transmission system parameters.

1.6 BASIC TYPES OF FACTS CONTROLLERS:

Series Controllers:

The series controller could be variable impedance such as capacitor, reactor, etc., or a power electronics based variable source of main frequency, sub-synchronous and harmonic frequencies (or a combination) to serve the desired need. In principle, all series controllers inject voltage in series with the line. Even variable impedance multiplied by the current flow through it, represents an injected series voltage in the line. As long as the voltage is in phase quadrature with the line current, the series controller only supplies or consumes variable reactive power. Any other phase relationship involves handling real power as well.

Shunt Controllers:

The shunt controller could also be variable impedance, variable source or a combination of these. In principle, all shunt controllers inject current into the system at the point of connection. Even variable shunt impedance connected to the line voltage causes variable current flow and hence represents

injection of current into the line. As long as the injected current is in phase quadrature with the line voltage, the shunt controller only supplies or consumes variable reactive power. Any other phase relationship involves handling real power as well.

Combined series-shunt controllers:

Combined series-shunt controllers could be a combination of separate shunt and series controllers, which are controlled in a coordinated manner, or a Unified Power Quality Controller with series and shunt elements. In principle, combined shunt and series controllers inject current into the system with the shunt part of the controller and voltage in series with the line using series part of the controller. However, when the shunt and series controllers are unified, there can be a real power exchange between the series and shunt controllers via the power link.

Using the concept of basic facts controllers and understanding the requirement of the given power quality problem, it is possible to provide the consumer with power, that is tailored to suit the load requirement.

Custom power: Custom power is the concept of employing power electronic (static) controllers in 1kv through 38kv distribution systems for supplying a compatible level of power quality necessary for adequate performance of selected facilities and processes.

Custom power controller: An active power electronic device has the ability to perform current interruption and/or voltage regulation in the distribution system to improve power quality.

Two classes of custom power devices are:

Reactive power injection/harmonic compensation devices: Protect the source (Utility) from the load.

Voltage sag/Interruption mitigation devices: Protect the load from the source (Utility).

CHAPTER-2

LITERATURE SURVEY

CHAPTER-2

LITERATURE SURVEY

2.1 INTRODUCTION:

A literature survey forms the basis on which a project can be built or developed. It forms the core to which ideas can be added and developed in to a comprehensive system, which will be able to cover the deficiencies of some of the existing system.

This chapter deals with the data and information accumulated after referring to many books, articles and technical papers written by well known authors. Some of them have been discussed in brief.

2.2 LITERATURE:

Fujita and H. Akagi, “The unified power quality conditioner: the integration of series- and shunt-active filters,” IEEE Trans. Power Electron., vol. 13, no. 2, pp. 315–322, Mar. 1998.

2. A. Ghosh, A. K. Jindal, and A. Joshi, “Design of a capacitor-supported Dynamic Voltage Restorer (DVR) for unbalanced and distorted loads,” IEEE Trans. Power Del., vol. 19, no. 1, pp. 405–413, Jan. 2004.

3. A. Ghosh and G. Ledwich, “A unified power quality conditioner (UPQC) for simultaneous voltage and current compensation,” Elect. Power Syst. Res., vol. 59, no. 1, pp. 55–63, 2001.

4. M. Hojo and T. Funabashi “Unified Power Quality Conditioner for Dynamic Voltage Restoration And Fault Current Limitation,” IEEE Trans. Power Del., vol. 45, no. 1, pp. 205–213, Jan. 2005.

2.2.1 H. Fujita and H. Akagi, “The unified power quality conditioner: the integration of series- and shunt-active filters,” IEEE Trans. Power Electron., vol. 13, no. 2, pp. 315–322, Mar. 1998.

This paper deals with unified power quality conditioners (UPQC’s), which aim at the integration of series-active and shunt-active filters, which is not only to compensate for current harmonics produced by nonlinear loads, but also to eliminate voltage flicker/imbalance appearing at the receiving terminal from the load terminal. Theoretical comparison among three types of control methods for the series-active filter has clarified that the combination of current and voltage-detecting methods is suitable for voltage flicker/imbalance elimination and harmonic compensation. The flow of instantaneous active and reactive powers has shown that installation of the shunt-active filter is effective in performing dc-voltage regulation. Although the specific UPQC dealt with in this paper provides no power factor correction in order to minimize the required rating of the shunt-active filter, the general UPQC is capable of improving “power quality” as well as improving power factor.

2.2.2 A. Ghosh, A. K. Jindal, and A. Joshi, “Design of a capacitor-supported Dynamic Voltage Restorer (DVR) for unbalanced and distorted loads,” IEEE Trans. Power Del., vol. 19, no. 1, pp. 405–413, Jan. 2004.

In this paper the operating principles, structure, and control of a DVR that is supplied by a dc capacitor are discussed. It has been shown that when the load is nonlinear, even an ideal DVR may not be able to suppress voltage spikes unless a low impedance path from the PCC to ground is provided through a shunt capacitor. Three different configurations are analyzed. It is shown that the DVR with a capacitor connected in shunt with it gives the good performance. It has been shown that the success of the DVR depends on the choice of these two capacitors. Their values must be chosen judiciously as discussed in the paper. Furthermore, the value of the dc capacitor also plays an important role. This value is chosen such that the DVR can ride through and provide voltage support during transients.

2.2.3 A. Ghosh and G. Ledwich, “A unified power quality conditioner (UPQC) for simultaneous voltage and current compensation,” Elect. Power Syst. Res., vol. 59, no. 1, pp. 55–63, 2001.

In this paper the topology and control technique of a UPQC that operates in simultaneous voltage and current control modes. In the voltage control mode it can make bus voltage at a load terminal sinusoidal against any unbalance, harmonic or flicker in the source voltage or unbalance or harmonic in the load current. In the current control mode, it draws a balanced sinusoidal current from the utility bus irrespective unbalance and harmonic in either source voltage or load current. The operation of UPQC presented in this paper is suitable for both utilities and customers having sensitive loads. From the utility standpoint, it can make the current drawn balanced sinusoidal. To accomplish this, the voltage at the point of common coupling must be of similar nature and also must contain the same amount of harmonics as the source. From the point of view of a customer, the UPQC can provide balanced voltages to their equipment that are sensitive to voltage dips. At the same time, the UPQC also filters out the current harmonics of the load. Therefore, the operation of UPQC is ideal from both viewpoints.

It is however to be mentioned that a UPQC is a very powerful device and can be operated in various other modes depending its ownership. We have known that a method of control is beneficial to both utility and customer.

2.2.4 M. Hojo and T. Funabashi “Unified Power Quality Conditioner for Dynamic Voltage Restoration And Fault Current Limitation,” IEEE Trans. Power Del., vol. 45, no. 1, pp. 205–213, Jan. 2005.

This paper deals with dynamic voltage restoration and fault current limitation by a Unified Power Quality Conditioner. When two types of voltage sag, three-phase balanced voltage sag and single phase unbalanced voltage sag, is applied to the system, the UPQC can quickly respond to the sag and compensate it well. In case of the system fault at the load side of the UPQC, the series converter of the UPQC also shows good performance.

It is confirmed by the simulation studies that the UPQC has great potential to improve power quality. It is expected that the UPQC is also effective when a distributed generator is interconnected to the distribution system.

CONCLUSION

After analyzing all these technical papers, UPQC is a very powerful device. This can solve the power quality problems. It is the best way to handle power quality problems is to mitigate equipment of distorted voltage or current at the point of common coupling. This would can harmonics are restricted from entering the distribution system. Thereby the loads connected to the system are provided with clean power. The IUPQC is to regulate the voltage at the terminals of Feeder and to protect the sensitive load from disturbances occurring upstream. The performance of the IUPQC has been evaluated under various disturbance conditions such as voltage sag in either feeder, fault in one of the feeders and load change. This may increase the cost of this device. However, the benefit that may be obtained can offset the expense.

However the performance under some of the major concerns of both customer and utility e.g., harmonic contents in loads, unbalanced loads, supply voltage distortion, system disturbances such as voltage sag, swell and fault has been described. The IUPQC has been shown to compensate for several of these events successfully. In the IUPQC configuration discussed in above paper, the sensitive load is fully protected against sag and interruption. The sensitive load is usually a part of a process industry where interruptions result in severe economic loss.

.

CHAPTER-3

CIRCUIT DESCRIPTION

CHAPTER-3

CIRCUIT DESCRIPTION

3.1 MOSFET Description:

Modern Power Electronics makes generous use of MOSFETs and IGBTs in most applications and, if the present trend is any indication, the future will see more and more

Applications making use of MOSFETs and IGBTs. Although sufficient literature is available on characteristics of MOSFETs and IGBTs, practical aspects of driving them in specific circuit configurations at different power levels and at different frequencies require that design engineers pay attention to a number of aspects.

Fig.3.1.1 structure of MOSFET and V-I characteristics

Fig.3.1.2. Shows a symbol of N-Channel MOSFET and an equivalent model of the same with three inter-junction parasitic capacitances, namely CGS, CGD and CDS.

For example the CGD, decreases rapidly as the Drain to Source voltage rises, as shown in Fig. 3.1.1 the high value of CGD is called CGDh, while the low value of CGD is termed CGDl. In addition, it also shows the internal body diode and the parasitic BJT.

Fig.3.1.2 shows a symbol of N-Channel MOSFET and an equivalent model

3.1.1 Turn-on and Turn-off Phenomena

Turn-on Phenomenon

Fig. 3.1.3 A MOSFET being turned off by a driver Fig.3.1.4 A MOSFET being on by a driver in a clamped inductive load. a clamped inductive load

To understand Turn-on and Turn-off phenomena of the Power MOSFET, we will assume clamped inductive switching as it is the most widely used mode of operation. This is shown in Fig. 3.1.3 and Fig. 3.1.4 A model of MOSFET is shown with all relevant components, which play a role in turn-on and turn-off events. As stated above, MOSFET’s Gate to Source Capacitance CGS needs to be charged to a critical voltage level to initiate conduction from Drain to Source.

Fig: 3.1.5 Turn on sequences of MOSFET

The waveforms drawn in Fig. 3.1.5 show variation of different parameters with respect to time, so as to clearly explain the entire turn-on sequence. From time zero to t1, (CGS+CGDl) is exponentially charged with a time constant T1 until Gate-to-source voltage reaches VGS (th). In this time period, neither the Drain voltage nor the Drain current are affected, i.e. Drain voltage remains at VDD and Drain current has not commenced yet. This is also termed turn-on delay. Note that between 0 to t1, as VGS rises, IGS falls exponentially, more or less like a mirror image of VGS, because from the point of view of circuit analysis, it is an RC Circuit. After time t1, as the Gate-to-Source voltage rises above VGS (th), MOSFET enters linear region as shown in Fig. 3.1.1 at time t1, drain current commences, but the Drain to Source voltage VDS is still at VDD while the MOSFET is carrying full load current. During the time interval, t2 to t4, VGS remains clamped to the same value and so does IGS. This is called the Miller Plateau Region.

During this interval most of the drive current available from the driver is diverted to discharge the CGD capacitance to enhance rapid fall of Drain to Source voltage. Only the external impedance in series with VDD limits drain current. Beyond t4, VGS begins to exponentially rise again with a time constant T2.

During this time interval the MOSFET gets fully enhanced, the final value of the VGS determining the effective RDS (on). When VGS reaches its ultimate value, VDS attains its lowest value, determined by

VDS= IDS * RDS(on) .

The Turn-off Phenomenon:

The turn-off phenomenon is shown in Fig. 3.1.6. As can be expected, when the output from the Driver drops to zero for turning off MOSFET, VGS initially decays exponentially at the rate determined by time constant T time 0 to t1. Notice that the drain current ID remains unchanged during this time interval, but the Drain Source voltage VDS just begins to rise. From t1 to t2, VDS rises from IDS * RDS (on) towards its final off state value of VDS(off), where it is clamped to the DC Bus voltage level by the diode in the clamped inductive switching circuit being studied. This time interval also corresponds to the Miller region as far as the gate voltage is concerned as mentioned above, which keeps VGS constant. During the next time interval, the VGS begins to fall further below VGS (th). CGS is getting discharged through any external impedance between gate and source terminals. The MOSFET is in its linear region and drain current ID drops rapidly towards zero value. Remember that the Drain Voltage VDS was already at its off state value VDS (off) at the beginning of this interval. Thus at t4, the MOSFET is fully turned off.

Fig: 3.1.6. MOSFET turn off sequence

3.2 INVERTERS:

Inverter is power electronic circuit that converts a direct current into an alternative current power of desired magnitude and frequency with the use of appropriate transformers, switching and control circuits. The inverters find their application in modern ac motor and uninterruptible power supplies. Static inverters have no moving parts and are used in a wide range of applications, from small switching

power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries. The electrical inverter is a high-power electronic oscillator. It is so named because early mechanical AC to DC converters was made to work in reverse, and thus was “inverted”, to convert DC to AC. The inverter performs the opposite function of a rectifier.

3.2.1 Classification of Inverters:

Based on the source used

• Voltage source inverter

• Current source inverter

Based on switching methods

• Pulse width modulation inverters

• Square wave inverters

Based on switching devices used

• Transistorized inverter

• Thyristorized inverter

Based on the inversion principle

• Resonant inverter

• Non- Resonant inverter

3.2.2 Advantages of inverters:

• Small leakage current during off stage

• Low voltage drop during ON stage

• Faster turn ON and turn OFF

• Small control power to switch from one state to other

• High forward current and blocking voltage capabilities.

• High dv/dt and di/dt ratings

3.2.3 Application of Inverters:

• Adjustable speed ac drives,

• UPS static VAR compensators

• Active filters

• Flexible AC transmission system

•

3.3 RECTIFIER:

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which is in only one direction, a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components. A device which performs the opposite function (converting DC to AC) is known as an inverter. When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is merely one of usage, i.e., the term rectifier describes a diode that that is being used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode.

3.3.1 Half -Wave Rectification:

In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a one-phase supply, or with three diodes in a three-phase supply.

3.4.1 Circuit and voltage wave of Half wave rectifier

The DC output voltage of a Half wave rectifier can be calculated by using the following two ideal equations:

Vrms = VSpeak/2 ……………… (3.4.1)

VDC = VSpeak/π ……………… (3.4.2)

3.2.2 Full Wave Rectification:

A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient. However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. Four diodes arranged this way are called a diode bridge or bridge rectifier.

Fig 3.3.2 circuit and output of the full wave rectifier

3.4 TRANSFORMER

Transformer refers to the static electromagnetic setting which can transfer power from one circuit to another one. In AC circuits, AC voltage, current and waveform can be transformed with the help of Transformers. Each transformation is usually to transfer from one circuit to another one by the way of electromagnetism, but it has no direct relation with this circuit.

An elementary transformer consists of a soft iron or silicon steel core and two windings. The winding are insulated from both the core and each other. The core is built up of soft iron or silicon steel laminations to provide a path of low reluctance to the magnetic flux. The windings connected, to the supply main is the primary and to the load circuit is the secondary. Although in the actual construction the windings are usually wound one over the other.

When the primary winding is connected to an AC supply mains, a currents flows through it. Since this winding links with an iron core, so current flowing through this winding produce an alternating flux in the core. Since flux is alternating and links with the secondary winding also, so induces an e.m.f. in the secondary winding.

The frequency of induced e.m.f. in secondary is the same as that of the flux or that of the supply voltage. The induced e.m.f. in the secondary winding enables it to deliver current to an external load connected across it. Thus the energy is transformed from primary to secondary winding by means of electromagnetic induction without any changing frequency.

Isolation transformers provide galvanic isolation and are used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected together.

Suitably designed isolation transformers block interference caused by ground loops. Isolation transformers with electrostatic shields are used for power supplies for sensitive equipment such as computers or laboratory instruments..

Isolation transformers are commonly designed with careful attention to capacitive coupling between the two windings. The capacitance between primary and secondary windings would also couple AC current from the primary to the secondary. High frequency transformer is considered to reduce the size of the circuit.

3.4.1 Applications

In electronics testing and servicing an isolation transformer is a 1:1 (under load) power transformer used for safety and simplicity. Replacing the line-frequency transformer with a high-frequency isolated dc-dc converter would make the energy storage system more compact and flexible.

3.5 CAPACITOR

Capacitor is used as an interface between the two back to back connected inverters and the voltage across it acts as the dc voltage source driving the inverters. Thus, the capacitor that acts as an energy storage device is shared between the two bi-directional converters. The voltage across the capacitor is held constant at the required value using closed loop control. In order to assure the filter current at any instant, the DC voltage, VDC must by at least equal to 3/2 of the peak value of the line AC mains voltage. The capacitor is to be so designed so as to provide DC voltage with acceptable ripples

3.6 Basic types of loads

Unbalanced loads: The loads connected to the line are said to unbalanced if they draw the currents which are not in the phase difference as source voltage. In a three-phase system, load imbalance could be caused by unevenly distributed single-phase load or by balanced three-phase load running at a fault condition, such as phase open or short fault. The source imbalance may be caused by a large load imbalance and non-uniform source output impedance. An unbalanced load may show up as different load current rms levels among phases, or same load current rms levels but different phase shift, or both. There are several existing ways to define the imbalance. They could be grouped into two methods. The first definition is based on the differences between the maximum per-phase load and the minimum per-phase load. A similar definition is also given another similar definition was adopted in, and expressed as

Max(line to line load) – Min (line to line load)

%Unbal =

Total three phase load

Based on symmetrical component representation, IEC gives the definition of “degrees of unbalance in a three-phase system” in as “ratios between the R.M.S. values of the negative sequence [dissymmetry] or zero sequence [asymmetry] coordinate and the positive sequence co-ordinate.”

Non linear load: The most common linear loads in power electronics system are resistors, inductors and capacitors. The most common nonlinear loads are diode rectifier, thyrisitor chopper, arc furnace, and switching mode power supply. A linear load could be defined as a linear relationship between the voltage across and the current through the load or their derivatives. Although there is no explicit mathematical description for nonlinear loads, they could be described as “a load that draws a non-sinusoidal current wave when supplied by a sinusoidal voltage source”

Several typical diode rectifier loads are studied, including a three-phase diode rectifier without DC link filter, three-phase diode rectifier with only capacitor as the DC link filter, three-phase rectifier with L/C as the DC link filter, and three single-phase rectifiers. Because of their simplicity, reliability and low cost, they are the most popular topologies for front-end AC/DC conversions. All the rectifiers are assumed to have a 277 V (line-to-neutral) three-phase source and an output power of 150 kW with a resistive load. Other nonlinear loads, such as a thyristor rectifier may lead to larger harmonic currents, thus even worse situations. Simple nonlinear loads, such as rectifiers, are enough to point out the impacts of nonlinear loads.

Such as

1. Three-phase diode rectifier without DC link filter.

2. Three-phase diode rectifier with capacitor DC link filter.

3. Three-phase diode rectifier with L/C DC link filter.

Sensitive load: The load which will be readily respond to the disturbances even for small duration and will effected early towards the non linearity in the system. Hence we have to keep the load with better safety measures.