Chapter 07

CHAPTER -1 INTRODUCTION

CHAPTER-1

INTRODUCTION

1.1 Introduction

ELECTRIC utilities and end users of electric power are becoming increasingly

concerned about meeting the growing energy demand. Seventy five percent of total global

energy demand is supplied by the burning of fossil fuels. But increasing air pollution, global

warming concerns, diminishing fossil fuels and their increasing cost have made it necessary

to look towards renewable sources as a future energy solution. Since the past decade, there

has been an enormous interest in many countries on renewable energy for power generation.

The market liberalization and government’s incentives have further accelerated the

renewable energy sector growth.

Renewable energy source (RES) integrated at distribution level is termed as

distributed generation (DG). The utility is concerned due to the high penetration level of

intermittent RES in distribution systems as it may pose a threat to network in terms of

stability, voltage regulation and power-quality (PQ) issues. Therefore, the DG systems are

required to comply with strict technical and regulatory frameworks to ensure safe, reliable

and efficient operation of overall network.

With the advancement in power electronics and digital control technology, the DG

systems can now be actively controlled to enhance the system operation with improved PQ at

PCC. However, the extensive use of power electronics based equipment and non-linear loads

at PCC generate harmonic currents, which may deteriorate the quality of power.

Generally, current controlled voltage source inverters are used to interface the

intermittent RES in distributed system. Recently, a few control strategies for grid connected

inverters incorporating PQ solution have been proposed. In an inverter operates as active

inductor at a certain frequency to absorb the harmonic current. But the exact calculation of

network inductance in real-time is difficult and may deteriorate the control performance. A

similar approach in which a shunt active filter acts as active conductance to damp out the

harmonics in distribution network is proposed. A control strategy for renewable interfacing

DEPARTMENT OF ELECTRICAL & ELCTRONICS ENGINEERING, MIST


inverter based on – theory is proposed. In this strategy both load and inverter current sensing

is required to compensate the load current harmonics.

The non-linear load current harmonics may result in voltage harmonics and can

create a serious PQ problem in the power system network. Active power filters (APF) are

extensively used to compensate the load current harmonics and load unbalance at distribution

level. This results in an additional hardware cost. However, in this paper authors have

incorporated the features of APF in the, conventional inverter interfacing renewable with the

grid, without any additional hardware cost. Here, the main idea is the maximum utilization of

inverter rating which is most of the time underutilized due to intermittent nature of RES. It is

shown in this paper that the grid-interfacing inverter can effectively be utilized to perform

following important functions: 1) transfer of active power harvested from the renewable

resources (wind, solar, etc.); 2) load reactive power demand support; 3) current harmonics

compensation at PCC; and 4) current unbalance and neutral current compensation in case of

3-phase 4-wire system. Moreover, with adequate control of grid-interfacing inverter, all the

four objectives can be accomplished either individually or simultaneously. The PQ

constraints at the PCC can therefore be strictly maintained within the utility standards

without additional hardware cost.

CHAPTER 2DEPARTMENT OF ELECTRICAL & ELCTRONICS ENGINEERING, MIST


LITERATURE SURVEY

The last decade has seen a marked increase on the deployment of end user equipment that

is highly sensitive to poor quality control electricity supply. Several large industrial users are

reported to have experienced large financial losses as a result of even minor lapses in the

quality of electricity supply. [21]

A great many efforts have been made to remedy the situation, where the solutions based

on the use of latest power electronic technology figure prominently. Indeed custom power

technology, the low voltage counterpart of the more widely known flexible as transmission

system (FACTS) technology, aimed at high voltage power transmission applications, has

emerged as a credible solution to solve many of the problems relating to continuity of supply

at the end user level. The various power quality Problems at the Distribution level are voltage

sag and swells, fluctuations, harmonics, flickering etc. [18]

Recently, various power electronic technology devices have been proposed especially to

be applied to medium voltage networks, generally named custom power. Custom power

concept introduced by N.G.Hingorani has been proposed to ensure high quality of power

supply in distribution networks using power electronics devices. Additionally, various

custom power devices are based on the voltage source converter technology introduced by

N.G.Hingorani and L.Gyugyi. [6]

At present, wide ranges of very flexible controllers, which capitalize on newly available

power electronics components, are emerging for custom power applications. Among these

the Distribution static Compensator (DSTATCOM) and dynamic voltage restorer (DVR),

both of them based on the VSC principle. [19]

The modeling and analysis of these custom power devices has applied for the study of

power quality by Olimpo Anaya-Lara and E Acha [7] presenting comprehensive results to

assess the performance of each device as a potential custom power application. The different

control techniques of DSTATCOM are discussed in papers [8]-[13].

Mahesh K. Mishra, Arindam Ghosh, and Avinash Joshi have demonstrated Operation of a

DSTATCOM in Voltage Control Mode. [9]



N.H. Woodley, L. Morgan, A. Sundaram introduces the prototype DVR installation and

presents early results from the demonstration project. [15]

Arindam Ghosh, and Avinash Joshi, Amit Kumar Jindal discusses the operating principles

and control characteristics of a dynamic voltage restorer (DVR) that protects sensitive but

unbalanced and/or distorted loads. They successfully given conclusion that DVR regulate the

voltage at the load terminal irrespective of sag/swell, distortion, or unbalance in the supply

voltage. [16]

Hideaki Fujita, Hirofumi Akagi demonstrates the unified power quality conditioner

(UPQC’s), which aim at the integration of series-active and shunt-active filters and results

reveal that UPQC compensate the voltage flicker/imbalance, reactive power, and harmonics.

[21]

Shairul Wizmar Wahab and Alias Mohd Yusof highlights voltage sag as one of a power

quality issue and Dynamic Voltage Restorer (DVR) is using for mitigation of voltage sag.

[17]

Zainal Salam , Tan Perng Cheng and Awang Jusoh intended to review the development of

active power filter (APF) technologies that are commonly used to mitigate harmonics in

utility power lines. The operation of common APF topologies, namely the shunt, series and

hybrid APFs are described in detail. [2]

K. Ça_atay BAYINDIR given detailed analysis on Modeling of custom power devices. [5]

Cai Rong had given Analysis of STATCOM for Voltage Dip Mitigation. [12]

S.V Ravi Kumar and S. Siva Nagaraju describes the techniques of correcting the supply

voltage sag, swell and interruption in a distributed system. Comprehensive results are

presented to assess the performance of DVR and DSTATCOM as a potential custom power

solution. [19]



CHAPTER 3

ELECTRIC POWER QUALITY MITIGATION

TECHNIQUES TO IMPROVE POWER QULITY

3.1 Introduction

Modem power systems are complex networks where hundreds of generating stations and

thousands of load centers are interconnected through long power transmission and

distribution networks. The main concern of consumers is the quality and reliability of power

supplies at various load centers where they are located at. Even though the power generation

in most well-developed countries is fairly reliable, the quality of the supply is not so reliable.

Power distribution systems, ideally, should provide their customers with an uninterrupted

flow of energy at smooth sinusoidal voltage at the contracted magnitude level and frequency.

However, in practice, power systems, especially the distribution systems, have numerous

nonlinear loads, which significantly affect the quality of power supplies.

As a result of the nonlinear loads, the purity of the waveform of supplies is lost. This ends

up producing many power quality problems. Apart from nonlinear loads, some system

events, both usual (e.g. capacitor switching, motor starting) and unusual (e.g. faults) could

also inflict power quality problems. The consequence of power quality problems could range

from a simple nuisance flicker in the electrical lamps to loss of money due to production

shutdown. [14]

3.2 Power Quality improvement methods

Here the following methods can improve power quality in the power system.

3.2.1 Motor-generator sets

Motor-generator sets are composed of the motor supplied the power system, a synchronous

generator connected with sensitive load and flywheel. The motor and generator are connected

together on a common axis, shown in Fig.3.1. The rotational energy, stored in the flywheel, can

be used to regulate the steady-state voltage and to support voltage during disturbances. The

advantages of this system are: high efficiency, low initial cost and the capability of long



duration ride through (several seconds). However, it can only be used in industrial environments

because of the limitation of its size, noise and maintenance. [12]

Fig.3.1 Motor-generator sets.

3.2.2 Transformer-based solutions

Electronic tap changer, shown in Fig.3.2, can be installed on a dedicated transformer for the

sensitive load. The load voltage can be maintained by change the turns ratio of the transformer while

the input voltage varies. The secondary winding, connected with the load, is separated in a number of

sections. Each section is connected or disconnected through a fast static switch. Therefore, the

secondary voltage can be regulated in steps. In general, thyristor-based switches that can only be

turned on once per cycle are used. Thus, this solution introduces a time delay, at least one half cycle,

into the dip compensation. [12]

Fig 3.2 Electronic tap changer.



3.2.3 The Static Transfer Switch (STS)

The Static Transfer Switch (STS) is used to transfer the load from the primary source to

alternative source when a voltage dip is detected in the primary source. The load is transferred

between the two sources by a static switch, which is composed of two anti- parallel thyristors per

phase. The STS includes two three-phase static switches, as shown in Fig 3.3. The transfer time of the

STS ranges from 1/4 to 1/2 cycle of the fundamental frequency. Thus, the loads only suffer the

voltage dip for this transfer time, which most of the loads can tolerate. The main disadvantage of the

STS is that the continuous conducting of the thyristors causes a considerable conducting loss,

especially in high power applications, and the healthy source experiences a 1/4 or 1/2 cycle voltage

notch. [12]

Fig.3.3 Static transfer switch.

3.2.4 Uninterruptible Power Supply

An Uninterruptible Power Supply (UPS) consists of a diode rectifier, an inverter and an energy

storage, which is usually a battery, connected to the dc link. The scheme is shown in Fig.3.4. Under

normal conditions, the power is transferred from the power system to the load while the ac voltage is

rectified and then inverted. The battery works in standby mode and keeps the dc voltage constant.



However, the battery releases the energy to maintain the dc voltage when a voltage dip or interruption

occurs in the power system. Depending on the capacity of the battery, the load can be supplied for

several minutes or hours.

The UPS is one of the most common solutions for low power loads like computer due to its low

cost, simple operation and control. This solution is not suitable for the high power loads because of

the high costs associated with the losses of additional energy conversions and maintenance of the

battery. [12]

Fig.3.4 Uninterruptible power supply.

3.2.5 Static VAR compensator

SVC is one of the most commonly used solution to voltage control and reactive power control in

past years as well as capacitor banks. It can also perform some other tasks such as improvement in ac

system stability, damping power oscillations, voltage flicker mitigation, over voltage limitation at

load rejection, etc. SVC topology may consist of TCR (Thyrisor controlled reactor) and TCR

(Thyristor controlled capacitor).A SVC contains both these elements and in addition harmonic filters.

Fig 3.5 shows simple topology of SVC. [29]



Fig 3.5 simple one line diagram of SVC

3.2.6 Solution of Harmonic mitigation using power filters

Power filters are classified into two types namely passive filters and Active Filters

3.2.6.1 Passive filters

Harmonic distortion in power distribution systems can be suppressed using two approaches

namely, passive and active powering. The passive filtering is the simplest conventional solution to

mitigate the harmonic distortion. Although simple, the use of passive elements does not always

respond correctly to the dynamics of the power distribution systems. Over the years, these passive

filters have developed to high level of sophistication. Some even tuned to bypass specific harmonic

frequencies.

Conventional passive filters consist of inductance, capacitance, and resistance elements configured

and tuned to control harmonics. Figure 3.6 shows common types of passive filters and their

configurations



Single – tuned 1st-orderHigh-pass 2nd-orderHigh-pass 3rd-orderHigh-pass Figure 3.6 Common types of passive filters and their configurations

The notch filter is connected in shunt with the power distribution system and is series-

tuned to present low impedance to a particular harmonic current. Thus, harmonic currents are

diverted from their normal flow path through the filter. Another popular type of passive filter

is the high-pass filter (HPF). A HPF will allow a large percentage of all harmonics above its

corner frequency to pass through. The first-order, which is characterized by large power

losses at fundamental frequency, is rarely used. The second-order HPF is the simplest to

apply while providing good filtering action and reduced fundamental frequency losses. The

filtering performance of the third-order HPF is superior to that of the second-order HPF.

However, it is found that the third-order HPF is not commonly used for low-voltage or

medium-voltage applications since the economic, complexity, and reliability factors do not

justify them.

Although simple and least expensive, the passive filter inherits several shortcomings.

The filter components are very bulky because the harmonics that need to be

suppressed are usually of the low order.

Furthermore the compensation characteristics of these filters are influenced by the

source impedance. As such, the filter design is heavily dependent on the power

system in which it is connected to.

Passive filters are known to cause resonance, thus affecting the stability of the power

distribution systems.

Frequency variation of the power distribution system and tolerances in components

values affect the filtering characteristics. The size of the components become

impractical if the frequency variation is large.



As the regulatory requirements become more stringent, the passive filters might not be

able to meet future revisions of a particular Standard. This may required a retrofit of new

filters. [2]

3.2.6.2 Active Power Filters

Remarkable progress in power electronics had spurred interest in APF for harmonic

distortion mitigation. The basic principle of APF is to utilize power electronics technologies

to produce specific currents components that cancel the harmonic currents components

caused by the nonlinear load.

APFs have a number of advantages over the passive filters. First of all, they can suppress

not only the supply current harmonics, but also the reactive currents.

Moreover, unlike passive filters, they do not cause harmful resonances with the power

distribution systems. Consequently, the APFs performances are independent on the power

distribution system properties. On the other hand, APFs have some drawbacks. Active

filtering is a relatively new technology, practically less than four decades old. There is still a

need for further research and development to make this technology well established.

An unfavorable but inseparable feature of APF is the necessity of fast switching of high

currents in the power circuit of the APF. This results in a high frequency noise that may

cause an electromagnetic interference (EMI) in the power distribution systems. In general,

APFs are divided into three main categories, namely shunt APF, series APF and hybrid APF.

[2]

3.2.6.2.1 Shunt Active Power Filter

This is most important configuration and widely used in active filtering applications. A

shunt APF consists of a controllable voltage or current source. The voltage source inverter

(VSI) based shunt APF is by far the most common type used today, due to its well known

topology and straight forward installation procedure. Figure 3.7 shows the principle

configuration of a VSI based shunt APF. It consists of a DC-bus capacitor (Cf), power

electronic switches and interfacing inductors (Lf).

Shunt APF acts as a current source, compensating the harmonic currents due to nonlinear

loads. The operation of shunt APF is based on injection of compensation current which is

equals to the distorted current, thus eliminating the original distorted current. This is



achieved by “shaping” the compensation current waveform (if), using the VSI switches. The

shape of compensation current is obtained by measuring the load current (iL) and subtracting

it from a sinusoidal reference. The aim of shunt APF is to obtain a sinusoidal source current

(is): is= iL- if

Fig 3.7 Principle configuration of a VSI based shunt APF

Suppose the nonlinear load current can be written as the sum of the fundamental current

Component (iL,f) and the current harmonics (iL,h) according to

iL = iL,f + iL,h (3.1)

then the injected compensation current by the shunt APF should be

if = iL,h (3.2)

the resulting source current is

is= iL - if =iL,f (3.3)

which only contains the fundamental component of the nonlinear load current and thus free from

harmonics. Figure 3.8 shows the ideal source current when the shunt APF performs harmonic filtering

of a diode rectifier. The injected shunt APF current completely cancels the current harmonics from

the nonlinear load, resulting in a harmonic free source current.



Fig 3.8 Shunt APF harmonic filtering operation principle

From the nonlinear load current point of view, the shunt APF can be regarded as a varying shunt

impedance. The impedance is zero, or at least small, for the harmonic frequencies and infinite in

terms of the fundamental frequency. As a result, reduction in the voltage distortion occurs because the

harmonic currents flowing through the source impedance are reduced. Shunt APFs have the

advantage of carrying only the compensation current plus a small amount of active fundamental

current supplied to compensate for system losses. It can also contribute to reactive power

compensation. Moreover, it is also possible to connect several shunt APFs in parallel to cater for

higher currents, which makes this type of circuit suitable for a wide range of power ratings. [2]

3.2.6.2.2 Series Active Power Filter

The series APF is shown in Figure 3.9. It is connected in series with the distribution line through a

matching transformer. VSI is used as the controlled source, thus the principle configuration of series

APF is similar to shunt APF, except that the interfacing inductor of shunt APF is replaced with the

interfacing transformer.



Fig 3.9 Principle configuration of a VSI based series APF

The operation principle of series APF is based on isolation of the harmonics in between the

nonlinear load and the source. This is obtained by the injection of harmonic voltages (Vf) across the

interfacing transformer. The injected harmonic voltages are added/subtracted, to/from the source

voltage to maintain a pure sinusoidal voltage waveform across the nonlinear load. It is controlled in

such a way that it presents zero impedance for the fundamental component, but appears as a resistor

with high impedance for harmonic frequencies components. That is, no current harmonics can flow

from nonlinear load to source, and vice versa.

Series APFs are less common than their rival, i.e. the shunt APF. This is because they have to

handle high load currents. The resulting high capacity of load currents will increase their current

rating considerably compared with shunt APF, especially in the secondary side of the interfacing

transformer. This will increase the I2R losses. However, the main advantage of series APFs is that

they are ideal for voltage harmonics elimination. It provides the load with a pure sinusoidal

waveform, which is important for voltage sensitive devices (such as power system protection

devices). With this feature, series APF is suitable for improving the quality of the distribution source

voltage. [2]

3.2.6.2.3 Hybrid Active Power filter

In this scheme, a low cost passive high-pass filter (HPF) is used in addition to the conventional

APF. The harmonics filtering task is divided between the two filters. The APF cancels the lower order



harmonics, while the HPF filters the higher order harmonics. The main objective of hybrid APF,

therefore is to improve the filtering performance of high-order harmonics while providing a cost-

effective low order harmonics mitigation.

There are various hybrid APFs but the two most prominent ones are shown in Figure 4.10. Figure

3.10 (a) is the system configuration of the hybrid shunt APF. Both the shunt APF and passive filter

are connected in parallel with the nonlinear load. The function of the hybrid APF can thus be divided

into two parts: the low-order harmonics are cancelled by the shunt APF, while the higher frequency

harmonics are filtered by passive HPF. This topology lends itself to retrofit applications with the

existing shunt APF.

Figure 3.10 (b) shows the system configuration of hybrid series APF, in which the series APF is

coupled to the distribution line by an interfacing transformer. The shunt passive filter consists of one

or more single-tuned LC filters and/or a HPF. The hybrid series APF is controlled to act as a

harmonic isolator between the source and nonlinear load by injection of a controlled harmonic voltage

source. It is controlled to offer zero impedance (short circuit) at the fundamental frequency and high

impedance (ideally open circuit) at all undesired harmonic frequencies. This constrains all the

nonlinear load current harmonics to flow into the passive filter, decoupling the source and nonlinear

load at all frequencies, except at the fundamental [2].

Fig 3.10 Hybrid APFs: (a) combination of shunt APF and shunt passive filter and

(b) Combination of series APF and shunt passive filter



CHAPTER 4

FACTS AND CUSTOM POWER DEVICES

4.1 Introduction

Flexible AC Transmission Systems, called FACTS, got in the recent years a well known

term for higher controllability in power systems by means of power electronic devices.

Several FACTS-devices have been introduced for various applications worldwide. A number

of new types of devices are in the stage of being introduced in practice. In most of the

applications the controllability is used to avoid cost intensive or landscape requiring

extensions of power systems, for instance like upgrades or additions of substations and power

lines. FACTS-devices provide a better adaptation to varying operational conditions and

improve the usage of existing installations

The usage of lines for active power transmission should be ideally up to the thermal

limits. Voltage and stability limits shall be shifted with the means of the several different

FACTS devices. It can be seen that with growing line length, the opportunity for FACTS

devices gets more and more important. The influence of FACTS-devices is achieved through

switched or controlled shunt compensation, series compensation or phase shift control. The

devices work electrically as fast current, voltage or impedance controllers. The power

electronic allows very short reaction times down to far below one second.

The development of FACTS-devices has started with the growing capabilities of power

electronic components. Devices for high power levels have been made available in

converters for high and even highest voltage levels. The overall starting points are network

elements influencing the reactive power or the impedance of a part of the power system.

4.2 Basic types of FACTS Controllers

In general, facts controllers can be divided into four categories:

1. Series controllers

2. Shunt controllers



3. Combined series-series controllers

4. Combined series-shunt controllers

4.2.1 Series Controllers

The series controller could be a variable impedance, such as capacitor, reactor etc.

Power electronics based variable source of main frequency, sub synchronous and harmonic

frequencies to serve the desired need. In principle, all series controllers inject voltage in

series with the line. Even variable impedance multiplied by a 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.

The main applications are:

1. reduction of series voltage decline in magnitude and angle over a power line,

2. reduction of voltage fluctuations within defined limits during changing power transmissions,

3. Improvement of system damping response. damping of oscillations,

4. limitation of short circuit currents in networks or substations,

5. Power flow adjustments.[6]

4.2.2 Shunt Controllers

As in the case of series controllers, the shunt controllers may 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 line

voltage causes a 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

controllers only supplies or consumes variable reactive power. These shunt devices are

operating as reactive power compensators. The main applications in transmission,

distribution and industrial networks are:

1. Reduction of unwanted reactive power flows and therefore

reduced network losses.



2. Keeping of contractual power exchanges with balanced reactive power.

3. Compensation of consumers and improvement of power quality especially with huge

demand fluctuations like industrial machines, metal melting plants, railway or

underground train systems. [6]

4.2.3 Combined Series-Series Controllers

This could be a combination of separate series controllers, which are controlled in a

coordinated manner, in a multi line transmission system or it could be a unified controller in

which series controllers provide independent series reactive compensation for each line but

also transfer real power among the lines via the power link. The real power transfer

capability of the unified series-series controller, referred to as interline power flow controller,

makes it possible to balance both the real and reactive power flow in the lines.[6]

4.2.4 Combined Series-Shunt Controllers

This could be a combination of separate shunt and series controllers which are controlled

in a coordinated manner, or a unified power flow 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 in the line with a 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. [6]

4.3 Advantages of FACTS devices

The benefits of utilizing FACTS devices in electrical systems can be summarized as

follows

1. Control of power flow as ordered.

2. Increase the loading capability of the lines to their thermal capabilities.

3. Increase the system security through raising the transient stability limit, limiting

short circuit currents and overloads and damping the electromechanical

oscillations of power systems and machines.



4. Reduce reactive power flows, there by improving active power flows. [6]

4.4 What is Custom Power?

Custom power is the employment of power electronic or static controllers in medium and

low voltage distribution systems for the purpose of supplying a level of reliability and/or

power quality that is needed by electric power customers sensitive to power quality

variations. In other words custom power is intended to protect the customers from

interruptions and voltage reductions originating in the utility system as well as those

transferred to customers from other customers via the utility system and even internal

disturbances.

Custom power devices, or controllers, include static switches, active filters, DVRs,

injection transformers, energy storage modules that have the ability to perform current

interruption and voltage regulation functions in a distribution system to improve reliability

and/or power quality.

In a Custom Power system customer receives specified power quality from a utility or a

service provider which includes an acceptable combination of the following features:

No (or rare) power interruptions

Magnitude and duration of voltage reductions within specified limits.

Magnitude and duration of over voltages within specified limits.

Low harmonic voltage.

Low phase unbalance

The need for the Custom Power concept arises from the fact that:

1. Most of the interruptions and voltage reductions occur in the utility system on account of

lightning faults on transmission and distribution lines, trees touching the wires, equipment

failure, switching, etc. Voltage sags may also be a consequence of large load changes

affecting customers own equipment or affecting other equipment via the utility system.



2. Impulses, switching surges and over voltages affecting the insulation, would most likely

result from lightning strikes and switching events in the transmission and distribution system.

3. Temporary over voltages lasting from several cycles to several seconds would largely

result from large load changes, capacitor switching, transformer switching, excessive

leading-VARs during light loads, etc. in the utility system.

4. Voltage unbalances in a three-phase supply would occur mostly due to large unbalanced

loads on a utility's distribution lines and long lines with unbalanced phase impedances.

5. Harmonics would most likely be the consequence of high harmonics in the customer load,

or the saturation of a utility's transformers. These harmonics would then be amplified by the

natural resonances in the utility system and/or the customer system. [5]

4.5 Custom Power Devices

Just as FACTS controllers improve the reliability and quality of power transmission

systems, the custom power enhances the quality and reliability of power delivered to

customer.Custom Power Devices are intended for improving the power quality of distribution

networks against disturbances such as

sags, swells,

Transients, harmonics.

The incorporation of the classical devices, such as D-STATCOM (Distribution Static

Synchronous Compensator), DVR (Dynamic Voltage Restorer), and UPQC (Unified Power

Quality Conditioner), means a continuous control, with a very fast response time, and they

allow and assure an improvement in the wave quality of the power supply.

A DSTATCOM can compensate for distortion and unbalance in a load such that a

balanced sinusoidal current flows in the feeder. 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 UPQC can perform the functions of both DSTATCOM and

DVR.The following terminologies useful for a better study of UPQC. [1]



4.5.1 Dynamic Voltage Restorer (DVR)

The DVR is a powerful controller that is commonly used for voltage sags mitigation at

the point of connection. The DVR employs VSC, coupling Transformer and DC energy

storage capacitor and the coupling transformer connected in series with the ac system, as

illustrated in Fig 4.1. The VSC generates a three-phase ac output voltage, which is

controllable in phase and magnitude These voltages are injected into the ac distribution

system in order to maintain the load voltage at the desired voltage reference.

Fig 4.1 Schematic representation of the DVR

The DVR is a solid state dc to ac switching power converter that injects a set of three

single phase ac output voltages in series with the distribution feeder and in synchronism with

the voltages of the distribution system. By injecting voltages of controllable amplitude, phase

angle and frequency (harmonic) into the distribution feeder in instantaneous real time via a

series injection transformer, the DVR can restore the quality of voltage at its load side

terminals when the quality of the source side terminal voltage is significantly out of

specification for sensitive load equipment.



4.5.1.1 Principle of operation of Series-connected VSC (DVR)

The basic idea involved in principle of operation is to inject a voltage ec(t) of desired

amplitude, frequency and phase between the PCC and the load in series with the grid voltage.

A typical configuration of the DVR is shown in Fig.4.2. Figure 4.3 shows a simplified single-

line diagram of the system with DVR.The DVR can be represented as a voltage source with

controllable amplitude, phase and frequency. The DVR is mainly used for voltage dip

mitigation. The device maintains the load voltage el(t) to the pre-fault condition by injecting

a fundamental voltage of appropriate amplitude and phase. Figure 4.4 shows the phasor

diagram of the series injection principle during voltage dip mitigation, where Ec is the phasor

of the voltage injected by the compensator, Il is the phasor of the load current and where Ψ is

the angle displacement between load voltage and current. In order to be able to restore both

magnitude and phase of the load voltage to the pre-fault conditions, the DVR has to inject

both active and reactive power. The voltage dip mitigation capability of this device depends

on the rating of the energy storage and on the voltage ratings of the VSC and the injection

transformer.

Fig. 4.2 Single line diagram of series connected VSC



Fig.4.3 Simplified line diagram of series connected VSC

Fig. 4.4 Phasor diagram of voltage mitigation using series VSC

The reactive power exchanged between the DVR and distribution system is internally

generated by the DVR without any ac passive reactive components, i.e. reactors and

capacitors. For large variations in the source voltage, the DVR supplies partial power to the

load from a rechargeable energy source attached to the DVR dc terminal. The DVR, with its

three single phase independent control and inverter design is able to restore line voltage to

critical loads during sags caused by unsymmetrical L-G, LL, L-L-G, as well as symmetrical

three phase faults on adjacent feeders or disturbances that may originate many miles away on

the higher voltage interconnected transmission system. Connection to the distribution

network is via three single-phase series transformers there by allowing the DVR to be applied

to all classes of distribution voltages. At the point of connection the DVR will, within the

limits of its inverter, provide a highly regulated clean output voltage.

4.5.2 Distribution STATCOM (DSTATCOM)



The DSTATCOM is basically one of the custom power devices. It is nothing but a

STATCOM but used at the Distribution level. The key component of the DSTATCOM is a

power VSC that is based on high power electronics technologies.

The Distribution STATCOM (shown in Fig 4.5) is a versatile device for providing

reactive compensation in ac networks. The control of reactive power is achieved via the

regulation of a controlled voltage source behind the leakage impedance of a transformer, in

much the same way as a conventional synchronous compensator. However, unlike the

conventional synchronous compensator, which is essentially a synchronous generator where

the field current is used to adjust the regulated voltage, the DSTATCOM uses an electronic

voltage sourced converter (VSC), to achieve the same regulation task. The fast control of the

VSC permits the STATCOM to have a rapid rate of response.

The DSTATCOM is the solid – state based power converter version of the SVC.

Operating as a shunt – connected SVC, its capacitive or inductive output currents can be

controlled independently from its connected AC bus voltage. Because of the fast switching

characteristic of power converters, the DSTATCOM provides much faster response as

compare to SVC. DSTATCOM is a shunt connected, reactive compensation equipment,

which is capable of generating and or absorbing reactive power whose output can be varied

so as to maintain control of specific parameters of the electric power system. DSTATCOM

provides operating characteristics similar to a rotating synchronous compensator without

mechanical inertia, due to the DSTATCOM employ solid state power switching devices it

provides rapid controllability of the three phase voltages, both in magnitude and phase angle.



Fig 4.5 Schematic representation of the DSTATCOM

In addition, in the event of a rapid change in system voltage, the capacitor voltage does

not change instantaneously; therefore the DSTATCOM reacts for the desired responses. For

example, if the system voltage drops for any reason, there is a tendency for the DSTATCOM

inject capacitive power to support the dipped voltages.

4.5.2.1 Principle of operation of Shunt-Connected VSC

The basic idea of the shunt-connected VSC is to dynamically inject a current i r(t) of

desired amplitude, frequency and phase into the grid. The typical configuration of a shunt-

connected VSC is shown in Fig.4.6. The device consists of a VSC, an injection transformer,

an ac filter and a dc-link capacitor. Energy storage can also be mounted on the dc link to

allow active power injection into the ac grid.

The line impedance has a resistance Rg and inductance Lg. The grid voltage and current

are denoted by es(t) and ig(t), respectively. The voltage at the point of common coupling

(PCC), which is also equal to the load voltage, is denoted by eg(t) and the load current by il(t).

The inductance and resistance of the ac-filter reactor are denoted by Lr and Rr respectively.

Figure 4.7 shows a simplified single-line diagram, where the VSC is represented as a current

source. Amplitude, frequency and phase of the current ir(t) can be controlled. By injecting a

controllable current, the shunt-connected VSC can limit voltage fluctuation leading to flicker

and cancel harmonic currents absorbed by the load, thus operating as an active filter. In both

cases, the principle is to inject a current with same amplitude and opposite phase as the



undesired frequency components in the load current, so that they are cancelled in the grid

current.

These mitigation actions can be accomplished by only injecting reactive power. A shunt-

connected VSC can also be used for voltage dip mitigation. In this case, the device has to

inject a fundamental current in the grid, resulting in an increased voltage amplitude at the

PCC, as shown in the phasor diagram in Fig.4.8. The voltage phasor at PCC is denoted by

Eg, Zg is the line impedance, Es,dip is the grid voltage phasor during the dip and Ψ is the

phase-angle jump of the dip. From the diagram it is possible to understand that when the

shunt-connected VSC is used to mitigate voltage dips, it is necessary to provide an energy

storage for injection of active power in order to avoid phase-angle jumps of the load voltage.

If only reactive power is injected, it is possible to maintain the load voltage amplitude Eg to

the pre-fault conditions but not its phase. Therefore, the voltage dip mitigation capability of a

shunt-connected VSC depends on the rating of the energy storage and on the rating in

current.

Fig. 4.6 Single line diagram of shunt connected VSC



Fig.4.7 Simplified line diagram of shunt connected VSC

Fig 4.8 Phasor diagram of voltage dip mitigation using shunt VSC

4.5.3 Unified Power Quality Conditioner (UPQC)

Poor power quality in a system could be due to different factors such as voltage sag,

voltage swell, voltage outage and over correction of power factor and unacceptable levels of

harmonics in the current and voltage. Modern solution for poor power quality is to take

advantage of advanced power electronics technology.

Recent research efforts have been made towards utilizing a device called unified power

quality conditioner (UPQC) to solve almost all power quality problems. The main purpose of

a UPQC is to compensate for supply voltage flicker/imbalance, reactive power, and

harmonics. In other words, the UPQC has the capability of improving power quality at the

point of installation on power distribution systems or industrial power systems. The UPQC,

therefore, is expected as one of the most powerful solutions to large capacity loads sensitive

to voltage flicker/imbalance.



Unified Power Quality Conditioner (UPQC) for non-linear and voltage sensitive loads has

following facilities.

It eliminates the harmonics in the supply current, thus improves utility current

quality for nonlinear loads.

UPQC provides the VAR requirement of the load, so that the supply voltage

and current are always in phase, therefore, no additional power factor

correction equipment is necessary.

UPQC maintains load end voltage at the rated value even in the presence of supply

voltage sag.

The voltage injected by UPQC to maintain the load end voltage at the desired value

is taken from the same dc link, thus no additional dc link voltage support is required

for the series compensator.

The UPQC consists of two three phase inverters (VSI) connected in cascade in such a

manner that one inverter is connected in parallel with the load. Second Inverter is connected

in series with the supply voltage through a transformer (Figure 4.9). [5]

The main purpose of the shunt compensator is to compensate for the reactive power

demanded by the load, to eliminate the harmonic components of

nonlinear loads in such a way that the source current is sinusoidal and

balanced. This equipment is a good solution for the case when the voltage

source presents distortion and a harmonic sensitive load is close to a

nonlinear load.

The series compensator is operated in PWM voltage controlled mode. It

injects voltage in quadrature advance to the supply voltage (current) such

that the load end voltage is always maintained at the desired value. The

two inverters operate in a coordinated manner.

.



Fig 4.9 Basic Block Diagram of UPQC

If UPQC is connected between two feeders then the new conditioner developed which is

called Interline Unity Power Quality conditioner (IUPQC).This IUPQC comes under series-

shunt facts device.

CHAPTER 5

INTERLINE UNIFIED POWER QUALITY CONDITIONER (IUPQC)

5.1 Introduction



Voltage-source converter based custom power devices 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 discussed extensively. 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

capacitor. One of the VSCs is connected in series with a distribution feeder, while the other

one is connected in shunt with the same feeder. It is also possible to connect two VSCs to

two different Feeders in a distribution system. [1]

5.2 IUPQC to Control power Quality

The series and shunt connected forms the basic principle for the operation of UPQC as it

is the back to back connection of the series and shunt connection of the VSCs. If the UPQC

device is connected between two feeders fed from different substations then it is called as

interline Unified Power Quality Conditioner(IUPQC).IUPQC cam improve the power

quality by injecting voltage in to any feeder from the DC link Capacitor. This whole

operation is controlled by controlling the two voltage souse converters (VSC) connected

between the two feeders in the Electrical distribution system. [1]

5.3 Structure and Control

The IUPQC shown in Fig. 5.1 consists of two VSCs (VSC-1 and VSC-2) that are

connected back to back through a common energy storage dc capacitor. Let us assume that

the VSC-1is connected in shunt to Feeder-1 while the VSC-2 is connected in series with

Feeder-2. Each of the two VSCs is realized by three H-bridge inverters. Inverters are



supplied from a common single dc capacitor and each inverter has a transformer connected at

its output.

Fig 5.1 Structure of IUPQC Controller

The complete structure of a three-phase IUPQC with two such VSCs is

shown in Fig.5.2. The secondary (distribution) sides of the shunt-

connected transformers are connected in star with the neutral point being

connected to the load neutral. The secondary winding of the series-

connected transformers are directly connected in series with the bus B-2

and load L-2. The ac filter capacitors Cf and Ck are also connected in each

phase to prevent the flow of the harmonic currents generated due to

switching. The six inverters of the IUPQC are controlled independently. [1]



Fig. 5.2 Complete structure of an IUPQC

5.4 Structure of a VSC in IUPQC

The schematic structure of a VSC in IUPQC is shown in Fig. 6.3. In this structure, each

switch represents a power semiconductor device (e.g.MOSFET,IGBT) and an anti-parallel

diode as shown in Fig.5.3.

Fig 5.3 Schematic structure of VSC

All the inverters are supplied from a common single dc capacitor Cdc and each inverter

has a transformer connected at its output. The six inverters of the IUPQC are controlled

independently. The switching action is obtained using output feedback control. [1]

5.4.1 Basics of Voltage Source Converter



Generally Pulse-Width Modulated Voltage Source Inverter (PWMVSI) is

used. The basic function of the VSI is to convert the DC voltage supplied

by the energy storage device into an AC voltage. In the DVR power circuit

step up voltage injection transformer is used. Thus a VSI with a low

voltage rating is sufficient. The common inverter connection method for

three phase DVRs are 3 phase Graetz bridge inverter as shown in Fig 5.4

This is referred to as two-level since the phase output voltage waveform

consists of two output levels; +Vd and 0 Volts. [18]

Fig. 5.4 Three phase voltage source converter

It is made of six valves each consisting of a gate turn off device (GTO) paralleled with a

reverse diode, and a DC capacitor. An AC voltage is generated from a DC voltage through

sequential switching of the GTOs. The DC voltage is unipolar and the DC current can flow in

either direction.

Controlling the angle of the converter output voltage with respect to the AC system

voltage controls the real power exchange between the converter and the AC system. The real

power flows from the DC side to AC side (inverter operation) if the converter output voltage

is controlled to lead the AC system voltage. If the converter output voltage is made to lag the

AC system voltage the real power will flow from the AC side to DC side (rectifier

operation). Inverter action is carried out by the GTOs while the rectifier action is carried out

by the diodes. Two switches on the same leg cannot be on at the same time.



Controlling the magnitude of the converter output voltage controls the reactive power

exchange between the converter and the AC system. The converter generates reactive power

for the AC system if the magnitude of the converter output voltage is greater than the

magnitude of the AC system voltage. If the magnitude of the converter output voltage is less

than that of the AC system the converter will absorb reactive power.

5.4.1.1 H bridge inverter

In the H bridge inverter, four switches are used. When it used for

multilevel arrangement especially for high voltage application, it is

commonly called as chain circuits. For fundamental switching each switch

is on for a duty cycle of 50% and shown in Figure 5.5. 18]

Figure 5.5 H-bridge inverter configuration

The Inverter contains four switches S1-S4, each comprising a semi-conductor device and

an anti-parallel diode as indicated in fig 5.5. Power semiconductor device can be a power

MOSFET for low power applications.

The Inverter is supplied by a DC source with a voltage of Vdc.The switch of each leg

usually have complementary values, e.g: when S1 is ON, S4 is OFF and vice versa. Further

the switches are operated in pairs. When the switches S1 & S2 are ON, S3 & S4 are OFF.

Similarly when S3 & S4 are ON, S1& S2 are OFF. However a small time delay is provided



between the turn OFF a pair of switches and turning ON the other pair. This period is called

‘Blanking Period’, is provided to prevent the DC source from being short circuited.

Consider for example the transition when S3 and S4 are turned OFF and S1 & S2 are

turned ON.Durng this period if the switch S1 gets turned ON before the switch S4 turns OFF

completely, then it will connect the two leads of the DC source directly. It is therefore

mandatory that switch S4 turns OFF completely before the switch S1 is turned ON. To

ensure this, the blanking Period (dead time) is used. Continuity of the current during the

blanking period is maintained by the anti-parallel diode. [26]

5.4.2 PWM-control scheme

There are many forms of modulation used for communicating information. When

communication is by pulses was introduced, the amplitude, frequency and pulse width

become possible modulation options. In many power electronic converters pulse width

modulation (PWM) the most reliable way of reconstructing a desired out put voltage wave

form. For a reliable signal representation it is necessary that the frequency of switching be

significantly higher than that of the desired signal.

Modulation in power electronics is a process of a switched representation of a wave form.

The switched representation is more efficient form. Ideally the switched signal can handle

high power with efficiency 100%. [26]

5.4.2.1 Sinusoidal PWM for H-Bridge Inverter

The schematic diagram for generation of PWM control signal is shown in fig 5.6.

It contains a carrier signal and a modulating signal. The magnitudes of these two signals

are compared through an analog comparator. The PWM control signal is set HIGH when

modulating signal has a higher numerical value than the carrier signal and is set LOW when

the carrier signal has a higher numerical value.

This implies that in the H-Bride inverter, switches S1 & S2 (fig 6.5) are closed when

modulating signal is higher than the carrier and switches S3 & S4 are closed when carrier is



higher than the modulating signal. Since the out put voltage of the Inverter for such an

arrangement is +Vdc or –Vdc, this is called bipolar voltage switching.

Fig 5.6 Generation of PWM control signal

The most popular form of PWM synthesis is the Sinusoidal PWM (SPWM).In an SPWM

the modulating signal is sinusoidal and carrier signal is a triangular wave. The frequency of

the modulating signal is chosen to be the fundamental frequency of the output waveform to

be synthesized. [26]

The converter output voltage can be controlled using various control techniques. Pulse

Width Modulation (PWM) techniques can be designed for the lowest harmonic content.

When sinusoidal PWM technique is applied turn on and turn off signals for GTOs are

generated comparing a sinusoidal reference signal Vr of amplitude Ar with a triangle carrier

waveform Vc of amplitude Ac as shown in Fig 6.6 The frequency of the triangle waveform

establishes the frequency at which GTOs are switched.

Consider a phase-leg as shown in Fig. 5.7(a) In this case Vr>Vc results in a turn on signal

for the device one and gate turn off signal for the device four and Vr<Vc results in a turn off

signal for the device one and gate turn on signal for the device four.



Fig. 5.7 PWM inverter (a) A phase leg, (b) PWM waveforms

The fundamental frequency of the converter output voltage is determined by the

frequency of the reference signal. Controlling the amplitude of the reference signal controls

the width of the pulses. The amplitude modulation index is defined as ratio of Ar to Ac

m = Ar/Ac (5.1)

For m≤1 the peak magnitude of the fundamental frequency component of the converter

output voltage can be expressed as

V=m Vdc/2 (5.2)



Fig 5.8 Voltage Control block Diagram

The aim of the control scheme is to maintain constant voltage magnitude at the point

where a sensitive load is connected, under system disturbances. The control system only

measures the rms voltage at the load point. The VSC switching strategy is based on a

sinusoidal PWM technique which offers simplicity and good response. Besides, high

switching frequencies can be used to improve on the efficiency of the converter, without

incurring significant switching losses. An error signal is obtained by comparing the reference

voltage with the rms voltage measured at the load point.

The PI controller process the error signal and generates the required delay angle to drive

the error to zero, i.e., the load rms voltage is brought back to the reference voltage. In the

PWM generators, the sinusoidal V control signal is phase modulated by means of the delay

angle. The modulated signal Vcontrol is compared against a triangular signal (carrier) in

order to generate the switching signals for theVSC valves. [27]

5.5 System Description

An IUPQC connected to a distribution system is shown in Fig 5.9. In this figure, the

feeder impedances are denoted by the Pairs (Rs1, Ls1) and (Rs2, Ls2). It can be seen that the

two feeders supply the loads L-1 and L-2. The load L-1 is assumed to have two separate

components—an unbalanced part (L-11) and a non-linear part (L-12). The currents drawn by



these two loads are denoted by il11and il12 respectively. We further assume that the load L-2 is

a sensitive load that requires uninterrupted and regulated voltage. The shunt VSC (VSC-1) is

connected to bus B-1 at the end of Feeder-1, while the series VSC (VSC-2) is connected at

bus B-2 at the end of Feeder-2. The voltages of buses B-1 and B 2 and across the sensitive

load terminal are denoted by Vt1, Vt2 and Vl2 respectively.

The aim of the IUPQC is two-fold:

1. To protect the sensitive load L-2 from the disturbances occurring in the system by

regulating the voltage Vt2.

2. To regulate the bus B-1 voltage Vt1 against sag/swell and or disturbances in the system.

Due to the presence of unbalanced and non-linear load L-1, the voltage is both unbalanced

and distorted. Also, the load L-11 causes an unbalance in the current. While load L-12 causes

distortion in the current.

. Fig. 5.9 Typical IUPQC connected in distribution 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 Vt2 across the sensitive load. Due to the presence of

unbalanced and non-linear load L-1, the voltage is both unbalanced and distorted. Also, the



load L-11 causes an unbalance in the current, while load L-12 causes distortion in the current

.These waveforms can be improved using the IUPQC. [1]

5.6 IUPQC Operation

The shunt VSC (VSC-1) holds the voltage of bus B-1 constant. This is accomplished by

making the VSC-1 to track a reference voltage across the filter capacitor Cf . The equivalent

circuit of the VSC-1 is shown in Fig. 5.8 (a) in which u1.Vdc denote the inverter output

voltage where Vdc is dc capacitor voltage and u1 is switching action equal to + n1 or –

n1where n1is turns ratio of the transformers of VSC-1. In Fig 6.8(a), the inverter losses and

leakage inductance of the transformers are denoted by Rf1 and Lf1 respectively. All system

parameters are referred to the line side of the transformers.

Defining the state vectors

(5.3)

The state space model of the voltage source converter -1 is written as

(5.4)

y= vt1 =Hx1 (5.5)

Where

(5.6)



(5.7) H= [1 0] (5.8)

(5.9)

Fig 5.8 (a) Single-phase equivalent circuit of VSC-1

u1c is the continuous time equivalent of u1. The system given here is descretized and is

written in input–output form as

A1(Z-1)y1(k)=B1(Z-1)u1c(k)+C1(Z-1)η1(k)

(5.10)

Where η1is a disturbance which is equal to ish.A pole shift controller is used to

determine the switching action u1 from u1c .and is used to track a reference signal y1ref(k).

The reference y1ref(k) is the desired voltage of the bus B-1.The peak of this

instantaneous voltage is pre-specified and its phase angle is adjusted to maintain the power



balance in the System. To set the phase angle, we note that the dc capacitor must be able to

supply VSC-1 while maintaining its dc bus voltage constant by drawing power from the ac

system .A proportional controller is used for controlling the dc capacitor voltage and is given

by δ1=K1 (Vdcref - Vdcav) (5.11) Where Vdcref is the average voltage across the dc capacitor over a cycle Vdcav is its set

reference value and K1 is the proportional gain. It is to be noted that the average voltage is

obtained using a moving average low pass filter to eliminate all switching components from

the signal. The equivalent circuit of the VSC-2 is shown in Fig. 5.8(b) and is similar to the

one shown in Fig. 5.8 (a) in every respect.

Fig 5.8 (b) Single-phase equivalent circuit of VSC-2

Defining a state and input vector, respectively, as

x2T= [Vk if2] (5.12)

z2T= [u2c is2] (5.13)

and the state space model for VSC-2 is given as

(5.14) y2 =Vt2=Hx2 (5.15)



where F2 and G2 are matrices that are similar to and F1 and G1 respectively. The

discrete-time input–output equivalent of is given as

A2(Z-1)y2(k)=B2(Z-1)u2c(k)+C2(Z-1) η2(k) (5.16)

Where η2 is a disturbance which is equal to is2.A pole shift controller

is used to determine the switching action u2 from u2c. A separate pole-shift

controller is used to determine the switching action so as to track the reference signal.

From Fig. 5.8(b), the purpose of the VSC-2 is to hold the voltage across the sensitive load

L-2 constant. Let us denote the reference load L-2 voltage as Vl2. Then the reference yref(k)

is computed by the application of Kirchoff’s voltage law as

yref (k) = v* l2 - vt2 (5.17)

Now demonstration of the operation of the IUPQC is through simulation using MATLAB.

[1]



CHAPTER 6

SIMULATION AND RESULTS

6.1 Introduction

In the analysis of IUPQC, the device is connected between two feeders; at the end of

feeder-1 non linear load and unbalanced loads are connected and at the end of feeder -2

sensitive load is connected. When the power system with these loads and without any

conditioner, develops poor power quality. This is in the form of voltage Sags (Swells). When

the conditioner like IUPQC is connected to the power system, it compensates the voltage

sags and maintains power quality. Now, the performance of IUPQC has been evaluated

considering various disturbance conditions. All the simulation work is done on the

MATLAB/SIMULINK

6.2 MATLAB/SIMULINK model of distribution system without IUPQC

This work presents a new connection for a UPQC called interline UPQC

(IUPQC). Two feeders called Feeder-1 and Feeder-2, which are connected

to two different substations, supply the system loads L-1 and L-2. The

purpose of the IUPQC is to hold the voltages and constant against voltage

sag/swell, temporary interruption in either of the two feeders. It has been

demonstrated that the IUPQC can absorb power from one feeder (say

Feeder-1) to hold constant in case of a sag in the voltage. This can be

accomplished as the two VSCs are supplied by a common dc capacitor.

The performance of the IUPQC has been evaluated through simulation

studies using MATLAB/SIMULINK.



Fig 6.1 shows the distribution system without IUPQC is simulated in

MATLAB/SIMULINK. Here two blocks are considered one is source and second is load.

Three phase AC voltage source is used in MATLAB/SIMULINK. This source is connected

to three phase RL branch which gives the line resistance and reactance. Here the supply

is connected to the load L-1.and again the load L-1 is assumed to have

two separate components—an unbalanced part (L-11) and a non-linear

part (L-12).Here Bridge operation is considered. In bridge operation because of continuous

switching operation of the switches harmonics are developed in both supply side and load

side.

Fig 6.1 Simulink model of distribution system without IUPQC

Due to presence of the unbalanced and non-linear load the system response gives poor

power quality. This can be observed by seeing the response of the distribution system.

Fig.6.2 shows the three phase line voltage across unbalanced load without IUPQC is

considered, here because of the unbalanced condition, all the three phase voltages are

distorted and maintaining poor power quality. When no IUPQC is connected to the system,



there is no matter of introducing capacitive storage voltage in to the line, so the waveforms

are distorted.

Fig 6.3 shows three phase voltage across non-linear load without IUPQC .Due to

continuous switching operation of the thyristors, the voltage across the bridge circuit are

distorted.

Fig 6.4 shows three phase line current passing in to the load. These are also unbalanced.

So the main objective of IUPQC is to regulate these distorted unbalanced voltages and

currents.

Fig 6.2.Three phase line voltages across unbalanced load without IUPQC



Fig 6.3: Three phase line voltages across non-linear load without IUPQC

Fig 6.4: Three phase currents passing in to the load without IUPQC

6.3 MATLAB/SIMULINK model of distribution system with IUPQC

In this present work an IUPQC connected to a distribution system is

shown in Fig.6.5. Two feeders called Feeder-1 and Feeder-2, which are

connected to two different substations, supply the system loads L-1 and L-

2. Load L1 has two components one is unbalanced load (L11) and other is

non-linear load (L12).Load L2 is sensitive load. The IUPQC is connected

between two buses, and the IUPQC take care about maintaing constant

voltage at supply side of feeder1 and voltage across the sensitive load.

The IUPQC shown in Fig. 6.5 consists of two VSCs (VSC-1 and VSC-2) that are

connected back to back through a common energy storage dc capacitor.VSC-1 is connected

in shunt to Feeder-1 while the VSC-2 is connected in series with Feeder-2. Each of the two

VSCs is realized by three H-bridge inverters. In Voltage source converters, each switch is a

MOSFET with an anti-parallel diode. All the inverters are supplied from a common single dc



capacitor and each inverter has a transformer connected at its output. The secondary

(distribution) sides of+ the shunt-connected transformers are connected in star with the

neutral point being connected to the load neutral. The secondary winding of the series-

connected transformers are directly connected in series with the bus-2 and load L-2. The ac

filter capacitors Cf and Ck are also connected in each phase to prevent the

flow of the harmonic currents generated due to switching.

Fig 6.5 Simulink model of distribution system with IUPQC

The figure 6.6 represents the SIMULINK block of shunt connected VSC used in the

IUPQC device. It consists of three H-bridge inverters with twelve switching devices



(MOSFETS). The shunt VSC has three step-down identical transformers (1MVA, 11/3Kv).

The triggering pulses for the gate terminals are obtained from the PWM controller circuit

respectively.

The figure 6.7 represents the SIMULINK block of series connected VSC in the IUPQC

device. It consists of three H-bridge inverters with twelve switching devices (MOSFETS).

The series VSC has three step-up identical transformers (1MVA, 3/11Kv). The triggering

pulses for the gate terminals are obtained from the PWM controller circuit respectively.

Fig 6.6 Shunt connected VSC



Fig. 6.7 Series connected VSC

The aim of the IUPQC is two-fold:

1. To protect the sensitive load L-2 from the disturbances occurring in the system by

regulating the voltage

2. To regulate the bus voltage to which loads L-11 and L-12 are connected against

sag/swell and or disturbances in the system.

To analyze these, two conditions are considered, one is sag occurred in

feeder-1.Second is sag occurred in second feeder-2.

When the system with IUPQC as shown in the figure 7.5 is considered, all the voltages

and currents of the system are balanced without any distortions. This is because the capacitor

discharging the stored voltage in to the line by using controls circuit. It is assumed that

initially the capacitor is uncharged, and this capacitor acts as the conditioner for the distorted

and unbalanced voltages.

When the system without IUPQC is considered, because of the harmonics due to non

linear load it gives poor power quality. When the IUPQC is connected, it works as



conditioner to maintain constant voltage across sensitive load and voltage at supply side. The

conditioned voltages are shown in the following figures. Fig 6.8 shows three phase line

voltages across unbalanced load with IUPQC. Fig 6.9 shows three phase line voltages across

non-linear load with IUPQC. Fig 6.10 Three phase currents passing in to the load with

IUPQC. It is observed from all figs that all responses are maintaining values prior to

disturbance; hence IUPQC improves the power quality.

Fig 6.8 Three phase line voltages across unbalanced load with IUPQC



Fig 6.9 Three phase line voltages across non-linear load with IUPQC

Fig 6.10 Three phase currents passing in to the load with IUPQC

6.4 System with Voltage Sag in Feeder-1

System with IUPQC and having sag in feeder-1 as shown in Fig. 6.11. Now to analyze

system performance against sag, sag is applied at feeder-1 by using timer and circuit

breakers. Here by changing the line impedance value which is less than existing value can

produce sag in the voltages. This logic is applied by using timer.



Fig. 6.11 System with IUPQC and having sag in feeder-1

Two circuit breakers are connected to lines by different impedance values. When the

timer is 1, then only one circuit breaker will conducts. Like this, sag is applied in the feeder.

With the system operating in the steady state, voltage sag occurs in which peak of the

supply Voltage Vs1, reduces from their nominal value of 9 kV. The trends in the other two

phases are similar. The dc capacitor voltage Vdc, drops as soon as the sag occurs. If the bus

voltage remains constant, the load power also remains constant. However, since the source



voltage Vs1 has dropped, the power coming out of the source has reduced. In order to supply

the balance power requirement of the load, the Vdc drops.

To offset this, the angle δ1 retards such that the power supplied by the source increases.

As the sag is removed, both the voltage Vdc and phase angle δ1 returns to their steady state

values. In order to supply the same load power at a reduced source voltage, the feeder current

increases.

6.4.1 System response without IUPQC during sag in feeder-1

When the system without IUPQC is considered i.e. without connecting the IUPQC to the

distribution system of fig.6.11, there is no conditioning of the voltages and currents. Fig7.12

represents the source voltage and it is balanced because of that sag is applied after the source.

So it is a balanced three phase voltage. The following figures show the results of the system

without IUPQC.

Now sag is introduced in feeder 1. Fig 6.13 is voltage across the unbalanced load. Sag is

occurred from 0.1 sec to 0.2 sec. Also the voltage across the bridge circuit (non linear load)

also has the sag between 0.1 sec and 0.2 sec as shown in fig 6.14.Line current shown in fig

7.15 has the sag between 0.1sec and 0.2 sec.

This fig.6.12 represents the input voltage given to the feeder-1 at the first bus.

Fig. 6.12 Vabc_11, kV (voltage at first bus on feeder-1)

From the fig6.13, it is clear that there is a dip in the unbalanced voltage during the sag conditions and it is found at third bus of feeder-1.



Fig. 6.13 Vabc_31, kV (voltage at third bus on feeder-1)

From the fig 6.14 it is clear that there is a dip in the non linear load voltage during the sag condition at the fourth bus of feeder-1

Fig. 6.14 Vabc_41, kV (voltage at fourth bus on feeder-1)



There is dip in the current waveform also at the second bus of feeder-1 shown in fig 7.15

Fig. 6.15 Iabc_21, A (current at second bus on feeder-1)

6.4.2 System response with IUPQC during sag in feeder-1

Here the system responses with IUPQC are discussed. Here the charged capacitor is

discharging depending on the control circuit. Depending on the sag value the controller is

injects the voltage in to the line. All the following figures show the results of the system with

IUPQC.

It is clear from the following waveform that the sag in the unbalanced voltage is

completely removed in the presence of the IUPQC.




The sag in the nonlinear load voltage is also completely removed in the presence of IUPQC shown in fig 6.17.

Fig. 6.17 Vabc_41, kV (voltage at fourth bus on feeder-1)




The current waveform at the second bus of feeder-1 is found to be sinusoidal.

6.5 System having Voltage Sag in Feeder-2

Now Sag is applied in feeder-2.Here also by changing the line reactance values sag is

applied in feeder-2 With the system operating in the steady state, Feeder-2 is subjected to

voltage sag in which the peak of all three phases of the supply voltage reduces from their

nominal value of 9.0 kV.The load L-2 voltage remains balanced and sinusoidal.



Fig.6.19 System with IUPQC and having sag in feeder-2



6.5.1 System response without IUPQC during sag in feeder-2

When the system without IUPQC is considered i.e. without connecting the IUPQC to the

distribution system of fig.6.19 there is no conditioning of the voltages and currents.Fig7.20

represents voltage of the feeder-2, because of the sag in feeder 2 all the three phase voltages

are contains sag between 0.1 sec and 0.2 sec. Fig 7.21 is line current of feeder 2 having the

sag between 0.1sec and 0.2 sec.

Following fig 6.20 shows that there is a dip in voltage at second bus of feeder-2.

Fig. 6.20 Vabc_22, kV (voltage at second bus on feeder-2)

There exists a dip even in the current waveform at the second bus of feeder-2 shown in fig 6.21.




6.5.2 System response with IUPQC during sag in feeder-2

Here the system responses with IUPQC are discussed. Here the charged capacitor is

discharging depending on the control circuit. Depending on the sag value the controller is

injects the voltage in to the line. All the following figures show the results of the system with

IUPQC.

IUPQC remove the voltage sag and restore the original values across sensitive loads as shown in following fig.


The current waveform still found to be sinusoidal at the input side as shown in fig 6.23.



Fig. 6.23 Vabc_12, kV (voltage at first bus on feeder-2)

6.6 System response during Faults

The performance of the IUPQC is tested when faults occurs in Feeder-2 at sensitive load

without and with IUPQC.

6.6.1 System response without IUPQC during Faults in feeder-2

In order to know the sytem performance without IUPQC L-G, L-L-G, 3-phase faults are created in sytem and simulated.

6.6.1.1 System response without IUPQC during L-G Fault in feeder-2

Simulink Diagram during L-G Fault is as shown in Fig 6.24.By setting Timer, fault is created during 0.05s to 0.1s in Phase-a.

Fig 6.24 simulink Diagram during L-G Fault without IUPQC

The system response is shown in Fig 6.25 when L-G fault occurs at

0.01 s such that the a-phase of B-32 bus voltage becomes zero. When the

fault occurs, the power fed to load

L-2(sensitive load) by Feeder-2 is reduced.



Fig 6.25 Voltage across Sensitive Load (B-32 bus voltages) Vabc_b32 without IUPQC

6.6.1.2 System response without IUPQC during L-L-G Fault in feeder-2

Simulink Diagram during L-L-G Fault is as shown in Fig 6.25.By setting Timer, fault is created during 0.05s to 0.1s in Phase-a and phase-b

Fig 6.26 simulink Diagram during L-L-G Fault without IUPQC

The system response is shown in fig 6.27 when L-L-G fault occurs

during 0.01s to 0.5s such that both a and b-phases of bus voltage to

which sensitive load is connected become zero.



Fig 6.27 Voltage across Sensitive Load Vabc_b32 without IUPQC

6.6.1.3 System response without IUPQC during 3-phase Fault in feeder-2

Simulink Diagram during 3-Phase Fault is as shown in Fig 6.28.By setting Timer; fault is created during 0.05s to 0.1s in 3-phases.

Fig 6.28 simulink Diagram during 3-phase fault without IUPQC

Now, the system performance has been tested when a three phase

fault occurs during 0.01s to 0.5s in Feeder-2 at sensitive load such that

the voltage becomes zero. The system response is shown in Figs.6.29.



When the fault occurs, the power fed to sensitive load by Feeder-2

becomes zero.

Fig 6.29 Voltage across Sensitive Load Vabc_b32 without IUPQC

6.6.2 System response during Faults with IUPQC



Simulink diagram with IUPQC during faults is as shown in fig 6.30. IUPQC can restore

voltage values to prior to faults at sensitive load.

Fig 6.30 simulink Diagram with IUPQC during faults

6.6.2.1 System response with IUPQC during L-G Fault in feeder-2



Here the system response with IUPQC is shown in Fig6.31.It shows balanced sinusoidal

waveform. Here the charged capacitor is discharging depending on the control circuit.

Depending on the fault level value the controller is injects the voltage in to the line. To

meet the power requirement of the load L-2, the dc capacitor starts

supplying this power momentarily so that the L-2 load voltages remain

balanced throughout the fault period.

Fig 6.31 Voltage across Sensitive Load Vabc_b32 with IUPQC during L-G fault

6.6.2.2 System response with IUPQC during L-L-G Fault in feeder-2

Here the system responses with IUPQC are shown in Fig6.32.IUPQC improve voltage

quality. Here the charged capacitor is discharging depending on the control circuit.

Depending on the fault level value the controller is injects the voltage in to the line.



Fig 6.32 Voltage across Sensitive Load Vabc_b32 with IUPQC during L-L-G fault

6.6.2.3 System response with IUPQC during 3-phase Fault in feeder-2

When the 3-fault occurs, the power fed to sensitive load by Feeder-2

becomes zero. To meet the power requirement of the load, the dc

capacitor starts supplying this power momentarily. This causes the dc

capacitor voltage to drop and to offset the voltage drop, the angle

retards. As a result, power is drawn from the source through Feeder-1 and

supplied to both the loads in Feeder-1 and Feeder-2.

Here the system responses with IUPQC are shown in Fig6.33. Depending on the fault

level value the controller is injects the voltage in to the line.



Fig 6.33 Voltage across Sensitive Load Vabc_b32 with IUPQC during 3-phase fault



CONCLUSIONS

The present work illustrates the operation and control of an interline Unified Power

Quality Conditioner (IUPQC). The device is connected between two feeders coming from

different substations. An unbalanced and non-linear load L-1 is supplied by Feeder-1 while a

sensitive load L-2 is supplied through Feeder-2. The main aim of the IUPQC is to regulate

the voltage at the terminals of Feeder-1 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 and fault in one of the feeders. In

case of voltage sag, the phase angle of the bus voltage in which the shunt VSC is connected

plays an important role as it gives the measure of the real power required by the load. The

IUPQC can mitigate voltage sag in Feeder-1 and in Feeder-2 for long duration. The IUPQC

discussed is capable of handling system in which the loads are unbalanced and distorted.

From above discussion, it has been observed that an IUPQC is able to protect the

distribution system from various disturbances occurring either in Feeder-1 or in Feeder-2. As

far as the common dc link voltage is at the reasonable level, the device works satisfactorily.

The angle controller ensures that the real power is drawn from Feeder-1 to hold the dc link

voltage constant. Therefore, even for voltage sag or a fault in Feeder-2, VSC-1 passes real

power through the dc capacitor onto VSC-2 to regulate the voltage Vl2. Finally when a fault

occurs in Feeder-2 or Feeder-2 is lost, the power required by the Load L-2 is supplied

through both the VSCs. This implies that the power semiconductor switches of the VSCs

must be rated such that the total power transfer through them must be possible. This may

increase the cost of this device. However, the benefit that may be obtained can offset the

expense.

In the IUPQC configuration, the sensitive load is fully protected against sag/swell and

interruption. The sensitive load is usually a part of a process industry where interruptions

result in severe economic loss. Therefore, the cost of the series part of IUPQC must be

balanced against cost of interruptions. It is expected that a part of IUPQC cost can be

recovered in 5–10 years by charging higher tariff for the protected line. Furthermore, the

regulated bus can supply several customers who are also protected against sag and swell. The



remaining part of the IUPQC cost can be recovered by charging higher tariff to this class of

customers. Such detailed analysis is required for each IUPQC installation. In conclusion, 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, and fault has been studied. The IUPQC has been shown to compensate for

several of these events successfully.

Future scope

Unified power quality conditioner can be extended for compensation of voltage/current in

multibus/multifeeder systems.



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Chapter 07

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