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Simulation of Cascaded H-BridgeConverter based DSTATCOM

PRESENTED BY

K.SURENDRA KUMAR (06A81A0248)

M.RAVI SHANKAR VUMMIDI (06A81A0220)

T.VINODH KUMAR (06A81A0257)

Under the Estemeted Guidence of

Mr.M.R.P. REDDY,MTECH,(Ph.D)


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CONTENTS

Introduction

Cascaded H-Bridge Converters

Dstatcom

Modeling and Control strategies

Simulation Studies

Conclusion


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WHY CASCADED H-BRIDGE CONVERTER BASED DSTATCOM

Voltage source converter based DSTATCOM has been established as themost preferred solution for management of reactive power in distribution

utilities and for improving voltage regulation, power factor and power quality

in industries.

For high power applications, cascaded H-Bridge converter is the most ideal

choice compared to neutral point clamped inverter and two-level inverter with

series connected power devices.

This paper presents the analysis, modeling and simulation studies on a cascaded H-

bridge converter based DSTATCOM for power factor improvement in power

distribution set-up.


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INTRODUCTION

The deployment of a large amount of electrical equipment in modem society not onlyimpacts the grids but also causes a higher demand of power quality.

The electrical companies haven't set up management system correspond to electrical

load in the customer side. And each customer improves his power quality in his own

way. All these cause the low power quality of the entire grid to these problems, theelectrical companies improve operation processes.

For high power applications the power electronics technology and the rapid

development of applications for more than power quality problems have emerged in a

variety of new equipment to improve power quality, such as, DSTATCOM, which

increase the methods to improve the quality of power grid supply.


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The DSTATCOM is a static compensator consisting of IGBT orGTO based voltage source converter to provide

voltage stabilization

power factor correction

Harmonic elimination

Load balancing

The main function of a DSTATCOM is to inject or absorb reactive

power to the grid for improving power factor and voltage regulation.

WHY DSTTATCOM


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WHY CASCADED H-BRIDGE CONVERTER

As the power rating increases, high voltage switching devices have to be used to

reduce the current rating of the converter. High voltage switching devices are

comparatively costlier and cannot be switched at high switching frequency

To overcome these problems several new inverter topologies have been used

in high voltage FACTS, custom power equipment and industrial drives

Reduced injection of harmonics

low cost.

Low voltage power switching

devices


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A FACT has been defined by the IEEE as

"A power electronic based system and other static equipment thatprovide control of one or more AC transmission system parameters to enhance

controllability and increase power transfer capability."

In general, FACTS controllers can be divided into three categories:

1. Series controllers,

2. Shunt controllers and

3. Combined series-shunt controllers.


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Shunt-Connected Controllers and Statcom

In principle, all shunt-type controllers inject additional current into thesystem at the point of common coupling (PCC). An impedance of the shunt

controller, which is connected to the line voltage, causes a variable current flow,

and hence represents an injection of current into the line.

Static var compensator (SVC)

Static synchronous compensator (STATCOM)

Static synchronous generator (SSG)


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POWER QUALITY ISSUES

Current Harmonics

Poor power factor

Voltage interruptions

Voltage sag and swell

Voltage unbalance

IMPACT OF PQ PROBLEMS ON POWER

SYSTEM COMPONENTS

Electronic Defective operation, radio interference

equipments

Power distribution

network

Excitation of system resonance, increased

level of voltage distortion


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CASCADED H-BRIDGE CONVERTERS

A cascaded H-bridge converter consists of a series of H-bridge (single-phase

full-bridge) inverters per phase.

The general function of the cascaded H-bridge converter is to synthesize a

desired voltage from several Separate DC Sources (SDCS)

Three-level cascaded H-bridgeconverter


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SDCS is connected to a single-phase-full-bridge inverter. For DSTATCOM

applications, the DC link capacitor of each H-bridge can be maintained at desiredlevel by closed loop control by drawing power from the grid

Each inverter level can generate three different voltage outputs, +Vdc, 0 and -

Vdc by connecting the dc source to the ac output side by different combinations

of the four switches, S1 , S2 , S3 and S4 .

To obtain +Vdc switches S1 and S4 are turned on. Turning on switches S2 and

S3 yields Vdc. By turning on S1 and S2 or S3 and S4 the output voltage is 0.

The number of output phase voltage levels in a Cascaded Inverter is defined by

m=2s+1; where s is the number of dc sources.

WORKING


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Comparison between Two-Level and Three-level converters

Harmonics in output waveform

Capacitance Requirement

Main Device Losses

High Power-Rating Capability

the switch voltage of the two-level converter is 3 / 2times that of the three-level converter


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DSTATCOM

Shunt compensation device

Regulates the bus voltage by injecting the reactive power during the pulsed load

operation

Functions

vReactive Power Compensation

vVoltage Regulation

vUnbalance Compensation (for 3-phase systems)


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Operating Principals of the Statcom

The basic principle of reactive power generation by a voltage converter is the same as that of a

conventional rotating synchronous machine. The basic voltage source converter scheme for reactive power

generation is shown schematically in the form of a single line diagram

Power circuit of DSTATCOM

DC input voltage source , provided by the charged capacitor C, the converter produces a set of

controllable three phase output voltages of the same frequency as that of the AC power system .

Each output voltage can be controlled both in magnitude and phase angle, which is coupled to the

corresponding AC system voltage through a relatively small (0.15 0.2 p.u.) tie reactance (which in

practice may be provided by per phase leakage inductance of the coupling transformer).


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MODELING AND CONTROL

The DSTATCOM basically consists of three main parts:

cascaded H-bridge converter with separate DC capacitors

The coupling inductors

a closed loop controller.

The coupling inductor in each phase serves both as a converter output-current filterand an inductive coupler between the mains phase and the respective converter phase

voltage

CONTROLSTRATEGY

The control strategy for DSTATCOM involves the measurement of three phase

voltages and currents at the incoming substation and DC link voltage only.


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The cascade H-bridge converter based DSTATCOM


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POWER FLOW IN A DISTRIBUTIONSTATCOM

The source power has two components, viz. instantaneous active power and instantaneous

reactive power. Similarly, the load power and inverter power also comprise of active and reactive

components

For full reactive power compensation of the load, the inverter has to supply reactive power of

the same magnitude, but have opposite sign. Thus, in such a case, the reactive power drawn from

the source is zero.

Power flow in a Distribution STATCOM


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The control block diagram of the cascaded three-level

Converter-based DSTATCOM

The three- phase source voltages (Vsa, Vsb, Vsc) are applied to three-phase Phase Locked

Loop (PLL) to synchronize the three-phase voltages at the converter output with the zero crossings

of the fundamental component of the supply phase voltages.

The PLL provides the synchronous reference angle required by the abc-dq0 (and dq0-abc)

transformation. The three phase source currents (isa, isb, isc) are converted into equivalent direct

axis and quadrature axis component currents (id, iq)


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PWM strategy for the 3-level inverter

The important advantages of the proposed 3-level inverter is that it can beoperated as a 2-level inverter in the lower output voltage range.

This is accomplished by comparing the modulating sine wave with only one

triangular carrier wave for the generation of PWM signals in the lower outputvoltage range and with two triangular carrier waves in the higher output voltage

range


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SIMULATION STUDIES

To verify the control performance, the proposed controller with the three-level cascaded inverter based DSTATCOM is simulated.

The line-to-line voltages of H-bridge cascaded inverter


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The DSTATCOM is switched with a steady load of 500 KVA .At 0.2 seconds an

additional load 500 KVA with power factor of 0.8 lag is switched on. At 0.3

seconds the additional load is switched off .

Source phase voltage and source Current waveforms

.


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DC link voltage (Vdc) and direct and quadrature axis source

currents (Id &Iq)


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It is observedthatIt is observedthat

The net reactive power drawn from the source is zero in

steady state and transient conditions


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Conclusion and future scope

The paper presents the principle of operation of cascaded H-bridge converter andsimulation studies on cascaded converter based DSTATCOM. It is observed that the

DSTATCOM is capable of maintaining the net reactive power drawn from the source

both during steady state and transient operating conditions.

The study presents the application of a DSTATCOM as a possible solution to power

Quality problems

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