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CHAPTER 1

INTRODUCTION TO MEDIUM VOLTAGE DRIVES

Table of Contents

Chapter -1. Introduction to Medium Voltage Drives

S.No. Name of the Sub-Title Page No. 1.1 Introduction 2

1.2 Overview of Medium Voltage drive 2

1.2.1 Industrial Applications of MV drive 4

1.3 General Block diagram of MV drive 5

1.4 Requirements and Technical Challenges in

control of MV drive 8

1.4.1 High Power converter efficiency 9

1.4.2 Harmonic minimization to satisfy

IEEE 519-1992 harmonic guidelines 10

1.5 Survey of inverter topologies used by manufacturers

in Present Market 12

1.6 Conclusions 16

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INTRODUCTION TO MEDIUM VOLTAGE DRIVES

1.1 INTRODUCTION

This chapter emphasizes the technical challenges and

requirements of modern high-power medium voltage drives, which are

foundation for the proposed research work. The general block diagram

of MV drive and functions of each block and numerous industrial

applications are presented. The main technical challenges and

requirements of MV drives such as high converter efficiency, less

switching frequency losses of the semiconductor switching devices

and less total harmonic distortion to comply with IEEE 519-1992

harmonic guidelines are discussed. Finally, a summary of inverter

topologies such as Neutral Point Clamped (NPC), Flying Capacitor (FC)

and Cascade H-Bridge (CHB) topologies along with ratings in MV Drive

products by major drive manufacturers are presented.

1.2 OVERVIEW OF MEDIUM VOLTAGE DRIVES

With the recent technical requirements of modern industry such

as higher power ratings with high converter efficiency, improved

reliability, energy saving and reduced cost leads to the extensive usage

of modern high-power medium voltage (MV) drives in many industrial

applications [1]-[5]. Though, MV drives cover power ratings from

0.4MW to 100MW at the medium voltage level of 2.3kV to 13.8kV,

majority of the installed MV drives are in 1 to 4 MW power range and

the voltage ratings of 3.3kV to 6.6kV as represented in Fig.1.1.

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Nowadays, around 85% of the total installed drives are pumps,

compressors, conveyers and fans, only 15% of the installed drives are

nonstandard as represented in Fig.1.2 (a). According to the present

market research, 97% of the installed MV motors operate at fixed

speed and 3% are variable-speed drives as shown in Fig. 1.2 (b) [6].

In majority of industrial applications, fans and pumps usually

driven by fixed speed motors, controlling of air or liquid flow is usually

achieved by conventional mechanical methods such as throttling, flow

Fig: 1.1 Voltage and power ranges of the MV drive. Source: Rockwell Automation [7]

(a) Load types for the MV drive (b) MV drive versus MV motors

Fig: 1.2 (a) & (b) MV drive market survey. Source: ABB [7]

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control valves and inlet dampers, resulting in a substantial amount of

energy loss. On other hand by using electrically driven, compressors

wellhead pumps are the current alternatives to traditional water and

gas injection for pressure augmenting requirements. Hence, usage of

electrically driven motors can significantly minimize the energy loss

and productivity can be increased by installing the MV drive [3].

1.2.1 Industrial Applications of MV drive

With the recent advancements in semiconductor technology,

switching devices like GTOs, IGBTs and IGCTs led the pace of MV

drives since 1980s. MV drives are extensively used in various

industrial applications because of the advantages such as high

efficiency, high dynamic performance, considerable savings on energy

cost, regenerative braking, four quadrant operation and increase in

productivity [7]. Some of the applications are in Table 1.1.

Table: 1.1 A summary of MV Drive industrial applications [7]

S.No Industry Applications

1 Power Feed water pumps, induced draft fans,

compressors, forced draft fans and blowers.

2 Oil & Gas

Turbo compressors, pipeline pumps,

mixers/extruders, reciprocating compressors

and centrifugal pumps.

3 Metals Hot rolling mill drives, sectional steel mills

and blast furnace converter.

4 Mining Ore mills, bucket wheel excavators, conveyor

belts and blowers

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5 Transportation Propulsion for naval vessels, shuttle tankers

and traction drives for locomotives.

6 Cement Kiln induced draft fans, forced draft fans and

preheat tower fans.

1.3 GENERAL BLOCK DIAGRAM OF MV DRIVE

The general block diagram of MV drive is represented in Fig. 1.3,

the main parts of MV drive are rectifier, line-side filter, transformer, dc

filter, inverter and motor-side filter. Depending upon the application

and system requirements, the selection of rectifier topology, inverter

topology, transformer connections, line and motor side filters will be

varied. Specifications of supply system for MV drive are at the medium

voltage level of 2.3kV to 13.8kV.

The main function of the rectifier in MV drive is to convert ac

supply voltage to a fixed or adjustable magnitude of dc voltage.

Commonly used rectifier topologies are multi pulse diode-rectifier and

multi pulse SCR-rectifier. Among these two topologies multi pulse

Fig: 1.3 General block diagram of the MV drive [7].

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diode-rectifiers are used as front end convertors, by major high power

drive manufacturers around the world [8]-[9]. In addition to above

mentioned function, the purpose of multi pulse diode-rectifier is to

reduce harmonic content in output voltage waveform.

In general, by using six pulse rectifiers, the lower order harmonics

such as 5th and 7th are cancelled out. The %THD of the line current,

further decreases by increasing the pulse number of diode rectifier.

Compared to six-pulse diode rectifiers, 12-pulse diode-rectifiers have

better harmonic profile but does not satisfy IEEE 519-1992 harmonic

guidelines.

At present in MV drive, 18-pulse and 24-pulse diode-rectifiers are

used as front end rectifiers which have better harmonic profile and

satisfy IEEE 519-1992 harmonic guidelines. The main functions of the

phase shifting transformers are to produce required phase

displacement for harmonic cancellation, proper secondary voltage and

an electrical isolation between rectifier and utility supply. The

commonly used configurations are Y/Z and ∆/Z (zigzag) configuration

[10].

The line and motor side filters are optional and are decided by

depending on system requirements and type of converter employed. In

the case of voltage source converters the dc filter can be a capacitor,

which provides constant dc voltage to the inverter but in the case of

current source converters the dc filter can be an inductor which

reduces the ripples in dc current waveform [7].

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The main function of the inverter is to convert fixed dc supply into

adjustable magnitude of ac with adjustable frequency. The inverters

which are used in MV drive can be classified into current source

inverter (CSI) and voltage source inverter (VSI). The function of VSI is

to convert dc voltage into a three-phase ac voltage with adjustable

magnitude and frequency where as current source inverter converts

dc current to an adjustable ac current [11].

The advantages of voltage source inverter are:

1. Operates easily at no load

2. Asymmetric blocking devices can be used

3. Less interactive with load

4. Requires regenerative converter on line side

5. Highly reliable

6. Good dynamic response as compared to CSI

Inspite of above mentioned advantages, control of voltage source

inverter is complex when compared to current source inverter [12].

The advantages of current source inverter are [13]-[14]:

1. Because of inherent four-quadrant operation extra power circuit

is not required

2. More rugged

3. Control is easy when compared to VSI

4. Operating at higher power levels

5. Multimachine operating capability

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Disadvantages of CSI are:

1. Sluggish dynamic response compared to VSI

2. It cannot be used in open loop.

3. Inferior efficiency, overall cost and transient response than VSI.

4. Unable to operate at no load

5. Multimachine operation is very difficult

In general, voltage source inverter or current source inverter can be

chosen based on system requirements and applications.

Commercially, the various topologies for voltage source inverter which

are used by MV drive manufacturers are neutral point clamped (NPC),

flying capacitor (FC) and cascaded H-bridge (CHB) topologies [15].

1.4 REQUIREMENTS AND TECHNICAL CHALLENGES IN CONTROL OF MV DRIVES

The standard requirements for the MV drives are high efficiency,

minimum space requirement, less energy loss, low noise, easy to use,

easy to integrate, minimum downtime for repairs, high dynamic

performance, low maintenance, significant savings on energy cost,

four quadrant operation and high reliability. In addition to these

requirements, the main technical challenges in control of MV drives

are: high converter efficiency, high input power factor and maintaining

the value of %THD to comply with IEEE 519-1992 harmonic

guidelines [16].

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1.4.1 High Converter Efficiency

High converter efficiency is one of the important requirements in

high-power MV drive applications. Particularly in high-power

converters, the high switching of semi conductor switches leads to

high switching losses and further leads to poor converter efficiency.

In particular, switching losses are directly related with the modulation

strategies which have been selected in control of multilevel inverters

[17]. Hence, selection of suitable modulation strategy with less

switching losses and also with less %THD has been an important

requirement to be addressed in MV Drives.

If the device switching losses are not minimized, the cooling

requirements can be increased, which leads to increase in physical

size, manufacturing cost of the switching devices [7]. Fast switching

speed of the switching devices may cause high 푑푣 푑푡⁄ at the falling and

rising edges of the inverter output voltage waveform. The high 푑푣 푑푡⁄

may results in failure of motor winding insulation and also causes

electromagnetic emission in cables, which are connected in between

motor and inverter, which further effects the operation of sensitive

equipments [18].

On other hand, the reduction in the switching frequency leads to

increase in %THD of line and motor side waveforms. Therefore efforts

should be made to minimize %THD in output voltage waveform with

optimum switching frequency.

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1.4.2 Harmonic minimization to satisfy IEEE 519-1992 harmonic guidelines

Harmonics deteriorates the performance of an electric drive.

Particularly, switching devices used in the power electronic converters

in MV drive generate harmonics in line and motor side voltage and

current waveforms. The distorted voltage and current waveforms may

cause several problems such as motor derating, over heating of the

electrical machines, interference with communication malfunction of

utility relays and control signals which results in unwanted tripping of

computer controlled industrial process which leads to expensive

downtime and ruined product.

In addition to above effects, line side capacitors are used in MV

industrial drives to reduce %THD and for power factor compensation.

These capacitors may form LC resonant circuit with the line

inductance of the system and resonates with harmonic voltages and

currents which results in severe oscillations (or) over voltages that

may destroy switching devices in converter circuits. Hence, harmonic

minimization in multilevel inverter has been an interesting challenging

research topic since several decades [19].

For the reasons mentioned above, the Institute of Electrical and

Electronics Engineers (IEEE) and The American National Standards

Institute (ANSI) have established certain harmonic guidelines for

unbalanced and distorted voltages in the power systems for specific

applications [16] & [20]-[21]. Due to the increasing huge increment in

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the usage of power semiconductor devices in high-power converters

have prompted growing concern over the harmonic distortion in

modern power distribution systems. All the manufacturers and

industries should comply with specified limits of harmonic distortion

guidelines.

In this connection, the Institute of Electrical and Electronics

(IEEE) has established certain guidelines for harmonic regulation,

such as IEEE standard 519-1992 harmonic guidelines [16] as listed in

Table 1.2.

The converters used in high-power medium voltage drives should

satisfy these specified limits of percentage total harmonic distortion

(%THD).The percentage total harmonic distortion (%THD) can be

calculated according to equation 1.1

%푇퐻퐷 = ∑∞ 푋100 (1.1)

Where Vh is the amplitude of the hth harmonic voltage and V1 is

the amplitude of the fundamental voltage.

Table: 1.2 Voltage Distortion Guidelines for Power Systems[22]

S.No. Voltage Level *Dedicated Power System

Generated Power

System

1 Medium voltage (2.4kV to 69kV)

8% 5%

2 High voltage (115kV and above)

1.5% 1.5%

* A dedicated power system is one supplying only converters or loads

that are not affected by voltage distortion

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1.5 SURVEY OF INVERTER TOPOLOGIES USED BY MANUFACTURERS IN PRESENT MARKET

In order to meet the technical requirements & challenges in MV

drives, various inverter topologies with different ratings and

modulation control schemes are available in the present market.

However, each inverter topology has its own unique features but also

have drawbacks as mentioned in chapter 2.

As mentioned earlier, in commercial market, there exists three

main topologies for multilevel inverters such as neutral point clamped

(NPC) topology, flying capacitor (FC) topology, and the cascaded H-

Bridge topology and effectively implemented by the major drive

manufacturing companies in the world.

Table 1.3 provides, summary of various topologies and power

ranges by major drive manufacturers in the world [23].

In neutral point clamped topology, three-level NPC has been widely

accepted by several manufacturers such as ABB which uses this

topology in both their ACS 1000 and ACS 6000 series [6]. Flying

capacitor topology, four-level flying Capacitor is most preferred

topology, if the switching frequency is high and one of the

manufacturers of drives such as Alstom, has adapted this topology for

VDM 6000 series [24].

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Table: 1.3 Summary of the inverter configurations used in MV Drive in present market [23]

S.No. Inverter Configuration Power Range (MVA)

Manufacturer

1 Two-level voltage source

inverter 1.4-7.2 Alstom(VDM5000)

2 Three-level neutral point

clamped inverter

0.3-5 ABB(ACS1000)

3-27 (ACS6000)

3-20 General

Electric(Innovation Series MV-SP)

0.6-7.2 Siemens

(SIMOVERT-MV)

0.3-2.4 General Electric-

Toshiba (Dura-Bilt5 MV)

3 Multilevel cascaded H-

bridge inverter

0.3-22 ASI Robicon

(Perfect Harmony)

0.5-6 Toshiba

(TOSVERT-MV)

0.45-7.5 General Electric

(Innovation MV-GP Type H)

4 NPC/ H-bridge inverter 0.4-4.8 Toshiba

(TOSVERT 300MV)

5 Flying-capacitor inverter 0.3-8 Alstom

(VDM6000 symphony)

6 PWM current source

inverter 0.2-20

Rockwell Automation

(Power Flex 7000)

7 Load commutated inverter

>10 Siemens

(SIMOVERT S) >10 ABB(LCI)

>10 Alstom

(ALSPA SD7000)

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The cascaded H-bridge topology is one of the emerging topology

which uses low-voltage blocking devices (e.g. 1700V IGBTs) to achieve

higher operating voltage and power levels. In this topology, present

manufacturers uses three to six equal H-bridge cells per phase, which

results in a seven to thirteen levels in the output phase voltage

waveform. This type of topology has been used by leading

manufacturer Siemens to develop the product named as Perfect

harmony.

ACS 6000 model MV drive developed by ABB with the

specifications of 3.3kV, 36MW along with various parts such as line

supply unit, control unit, inverter unit, dc link and water cooling unit

are represented in Fig. 1.4.

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Fig:

1.4

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1.6 CONCLUSIONS

This chapter provides the foundation for the problem formation

to address the technical challenges and requirements of MV drive. A

brief overview of MV drive and functioning of each block and various

industrial applications are discussed. In addition to that main

technical challenges and the requirements of MV drives such as high

converter efficiency and less %THD to comply with IEEE 519-1992

harmonic guidelines are discussed. Finally, the various inverter

topologies and power ratings used by leading MV Drive manufacturers

in the world are listed in detail.

In subsequent chapters, above mentioned technical challenges are

clearly addressed by selecting a suitable multilevel inverter

configuration, modulation strategy to control multilevel inverter.

Further the challenges in solving non linear transcendental SHE

equations set formulated in control of chosen multilevel inverter

configuration are addressed. The simulation results are

experimentally validated.