83.-Multiconverter Unified Power

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ABSTRACT

In order to meet PQ standard limits, it may be necessary to include some

sort of compensation. Modern solutions can be found in the form of active

rectification or active filtering. A shunt active power filter is suitable for the

suppression of negative load influence on the supply network, but if there are

supply voltage imperfections, a series active power filter may be needed to

provide full compensation. In recent years, solutions based on flexible ac

transmission systems (FACTS) have appeared. The application of FACTS

concepts in distribution systems has resulted in a new generation of

compensating devices. A unified power quality conditioner (UPQC) is the

extension of the unified power-flow controller (UPFC) concept at the

distribution level. It consists of combined series and shunt converters for

simultaneous compensation of voltage and current imperfections in a supply

feeder.

An IPFC consists of two series VSCs whose dc capacitors are coupled.

This allows active power to circulate between the VSCs. With this

configuration, two lines can be controlled simultaneously to optimize the

network utilization. An interline unified power-quality conditioner (IUPQC),

which is the extension of the IPFC concept at the distribution level. The IUPQC

consists of one series and one shunt converter. It is connected between two

feeders to regulate the bus voltage of one of the feeders, while regulating the

voltage across a sensitive load in the other feeder. In this configuration, the

voltage regulation in one of the feeders is performed by the shunt-VSC.

However, since the source impedance is very low, a high amount of current

would be needed to boost the bus voltage in case of a voltage sag/swell which is

not feasible. It also has low dynamic performance because the dc-link capacitor

voltage is not regulated.

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This paper presents a new unified power-quality conditioning system

(MC-UPQC), capable of simultaneous compensation for voltage and current in

multi-bus/multi-feeder systems. In this configuration, one shunt voltage-source

converter (shunt VSC) and two or more series VSCs exist. The system can be

applied to adjacent feeders to compensate for supply-voltage and load current

imperfections on the main feeder and full compensation of supply voltage

imperfections on the other feeders. In the proposed configuration, all converters

are connected back to back on the dc side and share a common dc-link

capacitor. Therefore, power can be transferred from one feeder to adjacent

feeders to compensate for sag/swell and interruption. The proposed topology

can be used for simultaneous compensation of voltage and current imperfections

in both feeders by sharing power compensation capabilities between two

adjacent feeders which are not connected. The system is also capable of

compensating for interruptions without the need for a battery storage system

and consequently without storage capacity limitations. The performance of the

MC-UPQC as well as the adopted control algorithm is illustrated by simulation.

CHAPTER.1

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INTRODUCTION

With increasing applications of nonlinear and electronically switched

devices in distribution systems and industries, power-quality (PQ) problems,

such as harmonics, flicker, and imbalance have become serious concerns. In

addition, lightning strikes on transmission lines, switching of capacitor banks,

and various network faults can also cause PQ problems, such as transients,

voltage sag/swell, and interruption. On the other hand, an increase of sensitive

loads involving digital electronics and complex process controllers requires a

pure sinusoidal supply voltage for proper load operation [1].

In order to meet PQ standard limits, it may be necessary to include some

sort of compensation. Modern solutions can be found in the form of active

rectification or active filtering [2]. A shunt active power filter is suitable for the

suppression of negative load influence on the supply network, but if there are

supply voltage imperfections, a series active power filter may be needed to

provide full compensation [3]. In recent years, solutions based on flexible ac

transmission systems (FACTS) have appeared. The application of FACTS

concepts in distribution systems has resulted in a new generation of

compensating devices. A unified power-quality conditioner (UPQC) [4] is the

extension of the unified power-flow controller (UPFC) [5] concept at the

distribution level. It consists of combined series and shunt converters for

simultaneous compensation of voltage and current imperfections in a supplyfeeder [6][8].

Recently, multiconverter FACTS devices, such as an interline power-flow

controller (IPFC) [9] and the generalized unified power-flow controller

(GUPFC) [10] are introduced. The aim of these devices is to control the power

flow of multilines or a subnetwork rather than control the power flow of a single

line by, for instance, a UPFC.

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When the power flows of two lines starting in one substation need to be

controlled, an interline power flow controller (IPFC) can be used. An IPFC

consists of two series VSCs whose dc capacitors are coupled. This allows active

power to circulate between the VSCs. With this configuration, two lines can be

controlled simultaneously to optimize the network utilization. The GUPFC

combines three or more shunt and series converters. It extends the concept of

voltage and power-flow control beyond what is achievable with the known two-

converter UPFC. The simplest GUPFC consists of three convertersone

connected in shunt and the other two in series with two transmission lines in a

substation. The basic GUPFC can control total five power system quantities,

such as a bus voltage and independent active and reactive power flows of two

lines. The concept of GUPFC can be extended for more lines if necessary. The

device may be installed in some central substations to manage power flows of

multilines or a group of lines and provide voltage support as well. By using

GUPFC devices, the transfer capability of transmission lines can be increased

significantly.

In this paper, a new configuration of a UPQC called the multiconverter

unified power-quality conditioner (MC-UPQC) is presented. The system is

extended by adding a series-VSC in an adjacent feeder. The proposed topology

can be used for simultaneous compensation of voltage and current imperfections

in both feeders by sharing power compensation capabilities between two

adjacent feeders which are not connected. The system is also capable ofcompensating for interruptions without the need for a battery storage system

and consequently without storage capacity limitations.

CHAPTER.2

POWER QUALITY

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The contemporary container crane industry, like many other industry

segments, is often enamored by the bells and whistles, colorful diagnostic

displays, high speed performance, and levels of automation that can be

achieved. Although these features and their indirectly related computer based

enhancements are key issues to an efficient terminal operation, we must not

forget the foundation upon which we are building. Power quality is the mortar

which bonds the foundation blocks. Power quality also affects terminal

operating economics, crane reliability, our environment, and initial investment

in power distribution systems to support new crane installations. To quote the

utility company newsletter which accompanied the last monthly issue of my

home utility billing: Using electricity wisely is a good environmental and

business practice which saves you money, reduces emissions from generating

plants, and conserves our natural resources. As we are all aware, container

crane performance requirements continue to increase at an astounding rate. Next

generation container cranes, already in the bidding process, will require average

power demands of 1500 to 2000 kW almost double the total average demand

three years ago. The rapid increase in power demand levels, an increase in

container crane population, SCR converter crane drive retrofits and the large

AC and DC drives needed to power and control these cranes will increase

awareness of the power quality issue in the very near future.

POWER QUALITY PROBLEMS

For the purpose of this article, we shall define power quality problems as:

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Any power problem that results in failure or mis operation of customer

equipment, manifests itself as an economic burden to the user, or produces

negative impacts on the environment.

When applied to the container crane industry, the power issues which degrade

power quality include:

Power Factor

Harmonic Distortion

Voltage Transients

Voltage Sags or Dips

Voltage Swells

The AC and DC variable speed drives utilized on board container cranes

are significant contributors to total harmonic current and voltage distortion.

Whereas SCR phase control creates the desirable average power factor, DC

SCR drives operate at less than this. In addition, line notching occurs when

SCRs commutate, creating transient peak recovery voltages that can be 3 to 4

times the nominal line voltage depending upon the system impedance and the

size of the drives. The frequency and severity of these power system

disturbances varies with the speed of the drive. Harmonic current injection by

AC and DC drives will be highest when the drives are operating at slow speeds.

Power factor will be lowest when DC drives are operating at slow speeds or

during initial acceleration and deceleration periods, increasing to its maximum

value when the SCRs are phased on to produce rated or base speed. Abovebase speed, the power factor essentially remains constant. Unfortunately,

container cranes can spend considerable time at low speeds as the operator

attempts to spot and land containers. Poor power factor places a greater kVA

demand burden on the utility or engine-alternator power source. Low power

factor loads can also affect the voltage stability which can ultimately result in

detrimental effects on the

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life of sensitive electronic equipment or even intermittent malfunction. Voltage

transients created by DC drive SCR line notching, AC drive voltage chopping,

and high frequency harmonic voltages and currents are all significant sources of

noise and disturbance to sensitive electronic equipment

Power quality can be improved through:

Power factor correction,

Harmonic filtering,

Special line notch filtering,

Transient voltage surge suppression,

Proper earthing systems.

In most cases, the person specifying and/or buying a container crane may

not be fully aware of the potential power quality issues. If this article

accomplishes nothing else, we would hope to provide that awareness.

In many cases, those involved with specification and procurement of

container cranes may not be cognizant of such issues, do not pay the utility

billings, or consider it someone elses concern. As a result, container crane

specifications may not include definitive power quality criteria such as power

factor correction and/or harmonic filtering. Also, many of those specifications

which do require power quality equipment do not properly define the criteria.

Early in the process of preparing the crane specification: Consult with the utility company to determine regulatory or contract

requirements that must be

satisfied, if any.

Consult with the electrical drive suppliers and determine the power quality

profiles that can be

expected based on the drive sizes and technologies proposed for the specific

project.

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Evaluate the economics of power quality correction not only on the present

situation, but consider the impact of future utility deregulation and the future

development plans for the terminal.

THE BENEFITS OF POWER QUALITY

Power quality in the container terminal environment impacts the economics of

the terminal operation, affects reliability of the terminal equipment, and affects

other consumers served by the same utility service. Each of these concerns is

explored in the following paragraphs.

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1. Economic Impact

The economic impact of power quality is the foremost incentive to container

terminal operators. Economic impact can be significant and manifest itself in

several ways:

a. Power Factor Penalties

Many utility companies invoke penalties for low power factor on monthly

billings. There is no industry standard followed by utility companies. Methods

of metering and calculating power factor penalties vary from one utility

company to the next. Some utility companies actually meter kVAR usage and

establish a fixed rate times the number of kVAR-hours consumed. Other utility

companies monitor kVAR demands and calculate power factor. If the power

factor falls below a fixed limit value over a demand period, a penalty is billed in

the form of an adjustment to the peak demand charges. A number of utility

companies servicing container terminal equipment do not yet invoke power

factor penalties. However, their service contract with the Port may still require

that a minimum power factor over a defined demand period be met. The utility

company may not continuously monitor power factor or kVAR usage and

reflect them in the monthly utility billings; however, they do reserve the right to

monitor the Port service at any time. If the power factor criteria set forth in the

service contract are not met, the user may be penalized, or required to take

corrective actions at the users expense. One utility company, which suppliespower service to several east coast container terminals in the USA, does not

reflect power factor penalties in their monthly billings, however, their service

contract with the terminal reads as follows:

The average power factor under operating conditions of customers load

at the point where service is metered shall be not less than 85%. If below 85%,

the customer may be required to furnish, install and maintain at its expense

corrective apparatus which will increase the Power factor of the entire

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installation to not less than 85%. The customer shall ensure that no excessive

harmonics or transients are introduced on to the [utility] system. This may

require special power conditioning equipment or filters. The IEEE Std. 519-

1992 is used as a guide in Determining appropriate design requirements.

The Port or terminal operations personnel, who are responsible for

maintaining container cranes, or specifying new container crane equipment,

should be aware of these requirements. Utility deregulation will most likely

force utilities to enforce requirements such as the example above. Terminal

operators who do not deal with penalty issues today may be faced with some

rather severe penalties in the future. A sound, future terminal growth plan

should include contingencies for addressing the possible economic impact of

utility deregulation.

b. System Losses

Harmonic currents and low power factor created by nonlinear loads, not

only result in possible power factor penalties, but also increase the power losses

in the distribution system. These losses are not visible as a separate item on

your monthly utility billing, but you pay for them each month. Container cranes

are significant contributors to harmonic currents and low power factor. Based

on the typical demands of todays high speed container cranes, correction of

power factoralone on a typical state of the art quay crane can result in a reduction of system

losses that converts to a 6 to 10% reduction in the monthly utility billing. For

most of the larger terminals, this is a significant annual saving in the cost of

operation.

c. Power Service Initial Capital Investments

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The power distribution system design and installation for new terminals,

as well as modification of systems for terminal capacity upgrades, involves high

cost, specialized, high and medium voltage equipment. Transformers,

switchgear, feeder cables, cable reel trailing cables, collector bars, etc. must be

sized based on the kVA demand. Thus cost of the equipment is directly related

to the total kVA demand. As the relationship above indicates, kVA demand is

inversely proportional to the overall power factor, i.e. a lower power factor

demands higher kVA for the same kW load. Container cranes are one of the

most significant users of power in the terminal. Since container cranes with DC,

6 pulse, SCR drives operate at relatively low power factor, the total kVA

demand is significantly larger than would be the case if power factor correction

equipment were supplied on board each crane or at some common bus location

in the terminal. In the absence of power quality corrective equipment,

transformers are larger, switchgear current ratings must be higher, feeder cable

copper sizes are larger, collector system and cable reel cables must be larger,

etc. Consequently, the cost of the initial power distribution system equipment

for a system which does not address power quality will most likely be higher

than the same system which includes power quality equipment.

2. Equipment Reliability

Poor power quality can affect machine or equipment reliability and

reduce the life of components. Harmonics, voltage transients, and voltagesystem sags and swells are all power quality problems and are all

interdependent. Harmonics affect power factor, voltage transients can induce

harmonics, the same phenomena which create harmonic current injection in DC

SCR

variable speed drives are responsible for poor power factor, and dynamically

varying power factor of the same drives can create voltage sags and swells. The

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effects of harmonic distortion, harmonic currents, and line notch ringing can be

mitigated using specially designed filters.

3. Power System Adequacy

When considering the installation of additional cranes to an existing power

distribution system, a power system analysis should be completed to determine

the adequacy of the system to support additional crane loads. Power quality

corrective actions may be dictated due to inadequacy of existing power

distribution systems to which new or relocated cranes are to be connected. In

other words, addition of power quality equipment may render a workable

scenario on an existing power distribution system, which would otherwise be

inadequate to support additional cranes without high risk of problems.

4. Environment

No issue might be as important as the effect of power quality on our

environment. Reduction in system losses and lower demands equate to a

reduction in the consumption of our natural nm resources and reduction in

power plant emissions. It is our responsibility as occupants of this planet to

encourage conservation of our natural resources and support measures which

improve our air quality

CHAPTER.3

UNIFIED POWER QUALITY CONDITIONER

The provision of both DSTATCOM and DVR can control the power

quality of the source current and the load bus voltage. In addition, if the DVR

and STATCOM are connected on the DC side, the DC bus voltage can be

regulated by the shunt connected DSTATCOM while the DVR supplies the

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required energy to the load in case of the transient disturbances in source

voltage. The configuration of such a device (termed as Unified Power Quality

Conditioner (UPQC)) is shown in Fig. This is a versatile device similar to a

UPFC. However, the control objectives of a UPQC are quite different from that

of a UPFC.

Fig 3.1

CONTROL OBJECTIVES OF UPQC

The shunt connected converter has the following control objectives

1. To balance the source currents by injecting negative and zero sequence

components required by the load

2. The compensate for the harmonics in the load current by injecting the

required harmonic currents

3. To control the power factor by injecting the required reactive current (at

fundamental frequency)

4. To regulate the DC bus voltage.

The series connected converter has the following control objectives

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1. To balance the voltages at the load bus by injecting negative and zero

sequence voltages to compensate for those present in the source.

2. To isolate the load bus from harmonics present in the source voltages, by

injecting the harmonic voltages

3. To regulate the magnitude of the load bus voltage by injecting the required

active and reactive components (at fundamental frequency) depending on the

power factor on the source side

4. To control the power factor at the input port of the UPQC (where the source

is connected. Note that the power factor at the output port of the UPQC

(connected to the load) is controlled by the shunt converter.

Operation of UPQC

The operation of a UPQC can be explained from the analysis of the idealized

equivalent circuit shown in Fig. 14.16. Here, the series converter is represented

by a voltage source VC and the shunt converter is represented by a current

source IC. Note that all the currents and voltages are 3 dimensional vectors with

CHAPTER.4

VOLTAGE SOURCE CONVERTERS (VSC):

A voltage-source converter is a power electronic device, which

can generate a sinusoidal voltage with any required magnitude, frequency and

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phase angle. Voltage source converters are widely used in adjustable-speed

drives, but can also be used to mitigate voltage dips. The VSC is used to either

completely replace the voltage or to inject the missing voltage. The missing

voltage is the difference between the nominal voltage and the actual. The

converter is normally based on some kind of energy storage, which will supply

the converter with a DC voltage. The solid-state electronics in the converter is

then switched to get the desired output voltage. Normally the VSC is not only

used for voltage dip mitigation, but also for other power quality issues, e.g.

flicker and harmonics.

The voltage source rectifier operates by keeping the dc link voltage at a

desired reference value, using a feedback control loop as shown in Fig. 12.36.

To accomplish this task, the dc link voltage is measured and compared with a

reference VREF. The error signal generated from this comparison is used to

switch the six valves of the rectifier ON and OFF. In this way, power can come

or return to the ac source according to dc link voltage requirements. Voltage VD

is measured at capacitor CD. When the current ID is positive (rectifier

operation), the capacitor CD is discharged, and the error signal ask the Control

Block for more power from the ac supply. The

Control Block takes the power from the supply by generating the appropriate

PWM signals for the six valves. In this way, more current flows from the ac to

the dc side, and the capacitor voltage is recovered. Inversely, when ID becomesnegative (inverter operation), the capacitor CD is overcharged, and the error

signal asks the control to discharge the capacitor and return power to the ac

mains. The PWM control not only can manage the active power, but also

reactive power, allowing this type of rectifier to correct power factor. In

addition, the ac current waveforms can be maintained as almost sinusoidal,

which reduces harmonic contamination to the mains supply. Pulsewidth-

modulation consists of switching the valves ON and OFF, following a pre-

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established template. This template could be a sinusoidal waveform of voltage

or current. For example, the modulation of one phase could be as the one shown

in Fig. 12.37. This PWM pattern is a periodical waveform whose fundamental is

a voltage with the same frequency

of the template. The amplitude of this fundamental, called VMOD in Fig. 12.37,

is also proportional to the amplitude of the template.

To make the rectifier work properly, the PWM pattern must generate a

fundamental VMOD with the same frequency as the power source. Changing

the amplitude of this fundamental

Fig 4.1

Operation principle of the voltage source rectifier.

FIGURE A PWM pattern and its fundamental VMOD. and its phase-shift with

respect to the mains, the rectifier can be controlled to operate in the four

quadrants: leading power factor rectifier, lagging power factor rectifier, leading

power factor inverter, and lagging power factor inverter. Changing the pattern

of modulation, as shown in Fig. 12.38, modifies the magnitude of VMOD.

Displacing the PWM pattern changes the phase-shift. The interaction between

VMOD and V (source voltage) can be seen through a phasor diagram. This

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interaction permits understanding of the four-quadrant capability of this

rectifier. In Fig. 12.39, the following operations are displayed: (a) rectifier at

unity power factor; (b) inverter at unity power factor; (c) capacitor (zero power

factor); and (d) inductor (zero power factor). In Fig. 12.39 Is is the rms value of

the source current is . This current flows through the semiconductors in the

same way as shown in Fig. 12.40. During the positive half cycle, the transistor

TN connected at the negative side of the dc link is switched ON, and the current

is begins to flow through TN .iTn.. The current returns to the mains and comes

back to the valves, closing a loop with another phase, and passing through a

diode connected at the same negative terminal of the dc link. The current can

also go to the dc load (inversion) and return through another transistor located at

the positive terminal of the dc link. When the transistor TN is switched OFF, the

current path is interrupted, and the current begins to flow through diode DP,

connected at the positive terminal of the dc link. This current, called iDp in Fig,

goes directly to the dc link, helping in the generation of the current idc . The

current idc charges the capacitor CD and permits the rectifier to produce dc

power. The inductances LS are very important in this process, because they

generate an induced voltage that allows conduction of the diode DP. A similar

operation occurs during the negative half cycle, but with TP and DN

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Changing VMOD through the PWM pattern.

Four-quadrant operation of the force-commutatedrectifier: (a) the PWM force-commutated rectifier; (b) rectifier operation at unity power factor; (c) inverter

operation at unity power factor; (d) capacitor operation at zero power factor;

and (e) inductor operation at zero power factor.

Under inverter operation, the current paths are different because the currents

flowing through the transistors come mainly from the dc capacitor CD. Under

rectifier operation, the circuit works like a Boost converter, and under inverter

operation it works as a Buck converter. To have full control of the operation of

the rectifier, their six diodes must be polarized negatively at all values of

instantaneous ac voltage supply. Otherwise, the diodes will conduct, and the

PWM rectifier will behave like a common diode rectifier bridge. The way to

keep the diodes blocked is to ensure a dc link voltage higher than the peak dc

voltage generated by the diodes alone, as shown in Fig. 12.41. In this way, the

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diodes remain polarized negatively, and they will conduct only when at least

one transistor is switched ON, and favorable instantaneous ac voltage

conditions are given. In Fig. 12.41 VD represents the capacitor dc voltage,

which is kept higher than the normal diode-bridge rectification value

nBRIDGE. To maintain this condition, the rectifier must have a control loop

like the one displayed in Fig.

Fig 4.2

Current waveforms through the mains, the valves, and the dc link.

VOLTAGE SOURCE INVERTER

Single-phase voltage source inverter can be found as half-bridge and full-

bridge topologies. Although the power range they cover is the low one, they are

widely used in power supplies, single-phase UPSs, and currently to form

elaborate high-power static power topologies, such as for instance, the multi cell

configurations that are reviewed The main features of both approaches are

reviewed and presented in the following.

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Types of VSI:

Half-Bridge VSI:

The power topology of a half-bridge VSI, where two large capacitors are

required to provide a neutral point N, such that each capacitor maintains a

constant voltage=2. Because the current harmonics injected by the operation of

the inverter are low-order harmonics, a set of large capacitors (C. and C) is

required. It is clear that both switches S. and S cannot be on simultaneously

because short circuit across the dc link voltage source vi would be produced.

There are two defined (states 1 and 2) and one undefined (state 3) switch state

as shown in Table. In order to avoid the short circuit across the dc bus and the

undefined ac output voltage condition, the modulating technique should always

ensure that at any instant either the top or the bottom switch of the inverter leg

is on.

shows the ideal waveforms associated with the half-bridge inverter shown in

Fig. 14.2. The states for the switches S. and S are defined by the modulating

technique, which in this case is a carrier-based PWM.

The Carrier-Based Pulse width Modulation (PWM) Technique: As

mentioned earlier, it is desired that the ac output voltage. Va N follow a given

waveform (e.g., sinusoidal) on a continuous basis by properly switching the

power valves. The carrier-based PWM technique fulfils such a requirement as it

defines the on and off states of the switches of one leg of a VSI by comparing a

modulating signal vc (desired ac output voltage) and a triangular waveform vD

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(carrier signal). In practice, when vc > vD the switch S. is on and the switch is

off; similarly, when vc < vD the switch S. is off and the switch S is on. A

special case is when the modulating signal vc is a sinusoidal at frequency fc and

amplitude ^vc , and the triangular signal vD is at frequency fD and amplitude

^vD. This is the sinusoidal PWM (SPWM) scheme. In this case, the modulation

index ma (also known as the amplitude-modulation ratio) is defined as

and the normalized carrier frequency mf (also known as the frequency-

modulation ratio) is

. vaN is basically a sinusoidal waveform plus harmonics, which features:

(a) the amplitude of the fundamental component of the ac output voltage ^vo1

satisfying the following expression:

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will be discussed later); (b) for odd values of the normalized carrier frequency

mf the harmonics in the ac output voltage appear at normalized frequencies fh

centered around mf and its multiples, specifically,

Where k . 2; 4; 6; . . . for l . 1; 3; 5; . . . ; and k . 1; 3; 5; . . .for l . 2; 4;

6; . . . ; (c) the amplitude of the ac output voltage harmonics is a function of the

modulation index ma and is independent of the normalized carrier frequency mf

form f > 9; (d) the harmonics in the dc link current (due to the modulation)

appear at normalized frequencies fp centered around the normalized carrier

frequency mf and its multiples, specifically,

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where k . 2; 4; 6; . . . for l . 1; 3; 5; . . . ; and k . 1; 3; 5; . .for l . 2; 4; 6; . . .

. Additional important issues are: (a) for small values of mf (mf < 21), the

carrier signal vD and the modulating signal vc should be synchronized to each

other(mf integer), which is required to hold the previous features; if this is not

the case, sub harmonics will be present in the ac output voltage; (b) for large

values of mf (mf > 21), the sub harmonics are negligible if an asynchronous

PWM

technique is used, however, due to potential very low-order sub

harmonics, its use should be avoided; finally (c) in the over modulation region

(ma > 1) some intersections between the carrier and the modulating signal are

missed, which leads to the generation of low-order harmonics but a higherfundamental ac output voltage is obtained; unfortunately, the linearity between

ma and ^vo1achieved in the linear region does not hold in the over modulation

region, moreover, a saturation effect can be observed

The PWM technique allows an ac output voltage to be generated that

tracks a given modulating signal. A special case is the SPWM technique (the

modulating signal is a sinusoidal) that provides in the linear region an ac output

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voltage that varies linearly as a function of the modulation index and the

harmonics are at well-defined frequencies and amplitudes.

These features simplify the design of filtering components. Unfortunately, the

maximum amplitude of the fundamental ac voltage is vi=2 in this operating

mode. Higher voltages are obtained by using the over modulation region (ma >

1); however, low-order harmonics appear in the ac output voltage.

Square-Wave Modulating Technique:

Both switches S. and S are on for one-half cycle of the ac output period.

This is equivalent to the SPWM technique with an infinite modulation index

ma. Figure 14.5 shows the following: (a) the normalized ac output voltage

harmonics are at frequencies h . 3; 5; 7; 9; . . . , and for a given dc link voltage;

(b) the fundamental ac output voltage features an amplitude given by

and the harmonics feature an amplitude given by

where the angles a1, a2, and a3 are defined as shown. The angles are found by

means of iterative algorithms as no analytical solutions can be derived. The

angles a1, a2, and

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are plotted for different values of in Fig. 14.7a. The general expressions

to eliminate an even N 1 .N 1 . 2; 4; 6; . . .) number of harmonics is

where a1, a2; . . . ; aN should satisfy a1 < a2 < _ _ _ < aN


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Fundamental magnitude control (N 1 . 3), the equations to be solved are:

where the angles a1; a2; a3, and a4 are defined as shown in Fig.b. The angles

a1; a2, a3 and a4 are plotted for different values of The general

expressions to

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eliminate an odd N -1 (N 1 . 3; 5; 7; . . .) number of harmonics are given by

Full-Bridge VSI:

The power topology of a full-bridge VSI. This inverter is similar to the

half-bridge inverter; however, a second leg provides the neutral point to the

load. As expected, both switches S1. and S1 (or S2. and S2) cannot be on

simultaneously because a short circuit across the dc link voltage source vi

would be produced. There are four defined and one undefined

The undefined condition should be avoided so as to be always capable of

defining the ac output voltage. In order to avoid the short circuit across the dc

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bus and the undefined ac output voltage condition, the modulating technique

should ensure that either the top or the bottom switch of each leg is on at any

instant. It can be observed that the ac output voltage can take values up to the dc

link value vi , which is twice that obtained with half-bridge VSI topologies.

Several modulating techniques have been developed that are applicable to full-

bridge VSIs. Among them are the PWM (bipolar and unipolar) techniques.

Bipolar PWM Technique:

States 1 and 2 (Table) are used to generate the ac output voltage in this

approach. Thus, the ac output voltage waveform features only two values,

which are vi and vi. To generate the states, a carrier-based technique can be

used a sine half-bridge configurations where only one sinusoidal modulating

signal has been used. It should be noted that the on state in switch S. in the half-

bridge corresponds to both switches S1. and S2 being in the on state in the full-

bridge configuration.

Similarly, S in the on state in the half-bridge corresponds to both

switches S1 andS2. being in the on state in the full-bridge configuration. This

is called bipolar carrier-based SPWM. The ac output voltage waveform in a full-

bridge VSI is basically a sinusoidal waveform that features a fundamental

component of amplitude ^vo1that satisfies the expression

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Fig. Chopping angles for SHE and fundamental voltage control in half-bridge

VSIs: (a) fundamental control and third, fifth, and seventh harmonic

elimination; (b) fundamental control.

Thus, the amplitude of the fundamental component and harmonics in the

ac output voltage are given by

It can also be observed in Fig. 14.12c that for a1 . 0 square wave operation is

achieved. In this case, the fundamental a output voltage is given by

where the fundamental load voltage can be controlled by the manipulation of

the dc link voltage

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CHAPTER.5

MODELLING OF THE UPQC

Figure shows the equivalent single-phase representation of the UPQC.

Figure . Equivalent single-phase representation of the UPQC.

The distorted supply voltage vs at the PCC can be represented by the sum

of two voltages, vf (fundamental) and vh (harmonics). The nonlinear load is

modeled by a current source iL composed of both fundamental and harmonics

that will be changed with different loads. The supply current is denoted by is

and the voltage across the nonlinear load is denoted by vL. The voltage vz in

Figure is the voltage drop across the line impedance Rl + jwLl. The series active

filter of the UPQC is modeled by a series Voltage Source Inverter (VSI) with

Lse and Cse as the second order low-pass interfacing filter and Rse as the losses

of the series VSI. The shunt active filter of the UPQC is represented by a shunt

VSI with Lsh and Csh as the second order low-pass interfacing filter and Rsh as

the losses of the shunt VSI. iCsh is the leakage capacitor current of the shunt

low-pass interfacing filter. represent the switching voltages

across the series and the shunt VSI outputs of the UPQC respectively.

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CHAPTER.6

PROPOSED MC-UPQC SYSTEM

A. Circuit Configuration

The single-line diagram of a distribution system with an MC-UPQC is shown in

Fig.

Fig. Single-line diagram of a distribution system with an MC-UPQC.

As shown in this figure, two feeders connected to two different

substations supply the loads L1 and L2. The MC-UPQC is connected to two

buses BUS1 and BUS2 with voltages of and , respectively. The shunt

part of the MC-UPQC is also connected to load L1 with a current of . Supply

voltages are denoted by while load voltages are Finally,

feeder currents are denoted by and load currents are Bus

voltages are distorted and may be subjected to sag/swell. The load L1

is a nonlinear/sensitive load which needs a pure sinusoidal voltage for proper

operation while its current is non-sinusoidal and contains harmonics. The load

L2 is a sensitive/critical load which needs a purely sinusoidal voltage and must

be fully protected against distortion, sag/swell, and interruption. These types of

loads primarily include production industries and critical service providers, such

as medical centers, airports, or broadcasting centers where voltage interruption

can result in severe economical losses or human damages.

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B. MCUPQC Structure

The internal structure of the MCUPQC is shown in Fig.

Fig. Typical MC-UPQC used in a distribution system.

It consists of three VSCs (VSC1, VSC2, and VSC3) which are connected

back to back through a common dc-link capacitor. In the proposed

configuration, VSC1 is connected in series with BUS1 and VSC2 is connected

in parallel with load L1 at the end of Feeder1. VSC3 is connected in series with

BUS2 at the Feeder2 end. Each of the three VSCs in Fig. 2 is realized by a

three-phase converter with a commutation reactor and high-pass output filter as

shown in Fig.

Fig. Schematic structure of a VSC.

The commutation reactor and high- pass output filter are connected

to prevent the flow of switching harmonics into the power supply. As shown in

Fig, all converters are supplied from a common dc-link capacitor and connected

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to the distribution system through a transformer. Secondary (distribution) sides

of the series-connected transformers are directly connected in series with BUS1

and BUS2, and the secondary (distribution)side of the shunt-connected

transformer is connected in parallel with load L1. The aims of the MC-UPQC

shown in Fig are:

1) to regulate the load voltage against sag/swell and disturbances in the

system to protect the nonlinear/sensitive load L1;

2) to regulate the load voltage against sag/swell, interruption, and

disturbances in the system to protect the sensitive/ critical load L2;

3) to compensate for the reactive and harmonic components of nonlinear load

current .

In order to achieve these goals, series VSCs (i.e., VSC1 and VSC3) operate as

voltage controllers while the shunt VSC (i.e., VSC2) operates as a current

controller.

C. Control Strategy

As shown in Fig., the MC-UPQC consists of two series VSCs and one shunt

VSC which are controlled independently. The switching control strategy for

series VSCs and the shunt VSC are selected to be sinusoidal pulse width-

modulation (SPWM) voltage control and hysteresis current control,

respectively. Details of the control algorithm, which are based on the dq

method [12], will be discussed later. Shunt-VSC: Functions of the shunt-VSC

are:

1) to compensate for the reactive component of load L1 current;

2) to compensate for the harmonic components of load L1 current;

3) to regulate the voltage of the common dc-link capacitor.

CHAPTER.7

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POWER-RATING ANALYSIS OF THE MC-UPQC

The power rating of the MC-UPQC is an important factor in terms of cost.

Before calculation of the power rating of each VSC in the MC UPQC structure,

two models of a UPQC are analyzed and the best model which requires the

minimum power rating is considered. All voltage and current phasors used in

this section are phase quantities at the fundamental frequency. There are two

models for a UPQCquadrature compensation (UPQC-Q) and inphase

compensation (UPQC-P). In the quadrature compensation scheme, the injected

voltage by the series- VSC maintains a quadrature advance relationship with the

supply current so that no real power is consumed by the series VSC at steady

state. This is a significant advantage when UPQC mitigates sag conditions. The

series VSC also shares the volt ampere reactive (VAR) of the load along with

the shunt-VSC, reducing the power rating of the shunt-VSC.

Fig. shows the phasor diagram of this scheme under a typical load power factor

condition with and without a voltage sag.

Fig. Phasor diagram of quadrature compensation. (a) Without voltage sag. (b)

With voltage sag.

When the bus voltage is at the desired value , the series-injected

voltage is zero [Fig.(a)]. The shunt VSC injects the reactive component of

load current , resulting in unity input-power factor. Furthermore, the shunt

VSC compensates for not only the reactive component, but also the harmonic

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components of the load current . For sag compensation in this model, the

quadrature series voltage injection is needed as shown in Fig. (b). The shunt

VSC injects in such a way that the active power requirement of the load is

only drawn from the utility which results in a unity input-power factor. In an

inphase compensation scheme, the injected voltage is inphase with the supply

voltage when the supply is balanced. By virtue of inphase injection, series VSC

will mitigate the voltage sag condition by minimum injected voltage. The

phasor diagram of Fig. explains the operation of this scheme in case of a voltage

sag.

Fig. Phasor diagram of inphase compensation (supply voltage sag).

CHAPTER.8

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SIMULATION RESULTS

The proposed MC-UPQC and its control schemes have been tested through

extensive case study simulations using PSCAD/ EMTDC. In this section,

simulation results are presented, and the performance of the proposed MC-

UPQC system is shown.

A. Distortion and Sag/Swell on the Bus Voltage

Let us consider that the power system in Fig. 2 consists of two three-phase

three-wire 380(v) (rms, L-L), 50-Hz utilities. The BUS1 voltage contains

the seventh-order harmonic with a value of 22%, and the BUS2 voltage

contains the fifthorder harmonic with a value of 35%. The BUS1 voltage

contains 25% sag between and 20% swell between

. The BUS2 voltage contains 35% sag between

and 30% swell between . The

nonlinear/sensitive load L1 is a three-phase rectifier

load which supplies an RC load of 10 and 30 F. Finally, the critical load L2

contains a balanced RL load of 10 and 100mH.

The MCUPQC is switched on at t=0.02 s. The BUS1 voltage, the

corresponding compensation voltage injected by VSC1, and finally load L1

voltage are shown in Fig.

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Fig. BUS1 voltage, series compensating voltage, and load voltage in Feeder1.

In all figures, only the phase a waveform is shown for simplicity. Similarly, the

BUS2 voltage, the corresponding compensation voltage injected by VSC3, and

finally, the load L2 voltageare shown in Fig.

Fig. BUS2 voltage, series compensating voltage, and load voltage in Feeder2.

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As shown in these figures, distorted voltages of BUS1 and BUS2 are

satisfactorily compensated for across the loads L1 and L2 with very good

dynamic response. The nonlinear load current, its corresponding compensation

current injected by VSC2, compensated Feeder1 current, and,

finally, the dc-link capacitor voltage are shown in Fig.

Fig. Nonlinear load current, compensating current, Feeder1 current, and

capacitor voltage.

CHAPTER.9

CONCLUSION

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In this paper, a new configuration for simultaneous compensation of voltage and

current in adjacent feeders has been proposed. The new configuration is named

multi-converter unified power-quality conditioner (MC-UPQC). Compared to a

conventional UPQC, the proposed topology is capable of fully protecting

critical and sensitive loads against distortions, sags/swell, and interruption in

two-feeder systems. The idea can be theoretically extended to

multibus/multifeeder systems by adding more series VSCs. The performance of

the MC-UPQC is evaluated under various disturbance conditions and it is

shown that the proposed MC-UPQC offers the following advantages:

1) power transfer between two adjacent feeders for sag/swell and interruption

compensation;

2) compensation for interruptions without the need for a battery storage system

and, consequently, without storage capacity limitation;

3) sharing power compensation capabilities between two adjacent feeders which

are not connected.

INDEX

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CHAPTER.1 INTRODUCTION

03

CHAPTER.2 POWER QUALITY

05

2.1 power quality problems

2.2 Benfits of power quality

CHAPTER.3 UNIFIED POWER QUALITY

CONDITIONER 15

CHAPTER.4 VOLTAGE SOURCE CONVERTERS

21

CHAPTER.5 MODELLING Of UPQC

45

CHAPTER.6 PROPOSED MC-UPQC SYSTEM

49

CHAPTER.7 POWER-RATING ANALYSIS OF THE MC-

UPQC 54

CHAPTER.8 SIMULATION RESULTS

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CHAPTER.9 CONCLUSION

69

CHAPTER.10 REFERENCES

70

TABLE OF FIGURES

Fig 3.1 : Unified Power Quality Conditioner 15

Fig 4.1 : Voltage Source Rectifier 22

Fig 4.2 : Voltage Source Inverter 26

Fig 5.1 : Equivalent single-phase representation of the UPQC 45

Fig. 6.1 : Single-line diagram of a distribution system with anMC-UPQC 49

Fig. 6.2 : Typical MC-UPQC used in a distribution system 50

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83.-Multiconverter Unified Power

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