.Total

ABSTRACT

The main aim of our project is to deal with an analysis, modeling and control of a

two-level 48-pulse voltage source converter for high voltage DC (HVDC) system.

A Set of two-level 6-pulse voltage source converters (VSCs) is used to form a 48-

pulse converter which is operated at fundamental frequency switching (FFS). The

performance of the VSC system is improved in terms of reduced hormonics level at

FFS and THD (Total Harmonic Distortion) of voltage and current is achieved.

The performance of the VSC is studied in terms of required reactive power

compensation, improved power factor and reduced harmonics distortion.

Simulation results are presented for the designed two level multipulse converter to

demonstrate its capability. The control algorithm is discussed in detail for operating

the conveter at fundamental frequency switching.

1

Introduction

1.1.introduction

In industrial development huge amount of energy is required at increasing rates.

For nation to develop industrially, there should be utilization of energy which is

nothing but electrical energy. Proper transmission link should be provided

between industries to have efficient power supply. Thus HVDC (High Voltage

Direct Current) transmission has become a source to have an efficient and

economic transmission of power even to a very long distances, with this type of

transmission of power the growing demand of loads can be achieved.

The increasing rating and improved performance of self-commutated

semiconductor devices have made possible High Voltage DC (HVDC)

transmission based on Voltage-Sourced Converter (VSC). The principal

characteristic of VSC-HVDC transmission is its ability to independently control

the reactive and real power flow at each of the AC systems to which it is

connected, at the Point of Common Coupling (PCC). In contrast to line-

commutated HVDC transmission, the polarity of the DC link voltage remains

the same with the DC current being reversed to change the direction of power

flow.

2

VSC-HVDC Transmission System Model

1.2.VSC Control System:

Overview of the Control System of a VSC Converter and Interface to the Main

Circuit shows an overview diagram of the VSC control system and its interface

with the main circuit.

Fig(1.2a.)Overview of the Control System of a VSC Converter and Interface to the Main Circuit

The converter 1 and converter 2 controller designs are identical. The two

controllers are independent with no communication between them. Each

converter has two degrees of freedom. In our case, these are used to control:

← P and Q in station 1 (rectifier)

← Udc and Q in station 2 (inverter).

3

The control of the AC voltage would be also possible as an alternative to Q.

This requires an extra regulator which is not implemented in our model.

Fig(1.2b)High Level Block Diagram of the Discrete VSC Controller

4

Dynamic Performance:

In the next sections, the dynamic performance of the transmission system is

verified by simulating and observing the dynamic response to step changes

applied to the principal regulator references, like active/reactive power and DC

voltage. Recovery from minor and severe perturbations in the AC system.

For a comprehensive explanation of the procedure followed obtaining these

results and more, refer to the Model Information block.

System Startup - Steady-State and Step Response

Startup and P & Q Step Responses in Station 1:

5

The main waveforms from the scopes are reproduced below.

Startup and Udc Step Response in Station 2:

Station 2 converter controlling DC voltage is first deblocked at t=0.1 s. Then,

station 1 controlling active power converter is deblocked at t=0.3 s and power is

ramped up slowly to 1 pu. Steady state is reached at approximately t=1.3 s with

DC voltage and power at 1.0 pu (200 kV, 200 MW). Both converters control the

reactive power flow to a null value in station 1 and to 20 Mvar (-0.1 pu) into

station 2 system. After steady state has been reached, a -0.1 pu step is applied to

the reference active power in converter 1 (t=1.5 s) and later a -0.1 pu step is

applied to the reference reactive power (t=2.0 s). In station 2, a -0.05 pu step is

applied to the DC voltage reference. The dynamic response of the regulators are

observed. Stabilizing time is approximately 0.3 s.The control design attempts to

decouple the active and reactive power responses. Note how the regulators are

more or less mutually affected.

6

AC Side Perturbations:

From the steady-state condition, a minor and a severe perturbation are executed

at station 1 and 2 systems respectively. A three-phase voltage sag is first

applied at station 1 bus. Then, following the system recovery, a three-phase to

ground fault is applied at station 2 bus. The system recovery from the

perturbations should be prompt and stable. The main waveforms from the

scopes are reproduced in the two figures below.

Voltage Step on AC System 1

The AC voltage step (-0.1 pu) is applied at t=1.5 s during 0.14 s (7 cycles) at

station 1. The results show that the active and reactive power deviation from the

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pre-disturbance is less than 0.09 pu and 0.2 pu respectively. The recovery time

is less than 0.3 s and thesteady state is reached before next perturbation

initiation.The fault is applied at t=2.1 s during 0.12 s (6 cycles) at station 2.

Three-Phase to Ground Fault at Station 2 Bus:

Note that during the three-phase fault the transmitted DC power is almost halted

and the DC voltage tends to increase (1.2 pu) since the DC side capacitance is

being excessively charged. A special function (DC Voltage Control Override)

in the Active Power Control (in station 1) attempts to limit the DC voltage

within a fixed range. The system recovers well after the fault, within 0.5 s. Note

the damped oscillations (around 10 Hz) in the reactive power.

8

The industrial growth of a nation requires increased consumption energy,

particularly electrical energy. This has lead to increase in the generation and

transmission facilities to meet the increasing demand. The generation can be

increased to the required level but the problem is in transmission due to the

thermal limit, because the transmission line load ability is fixed up to 60% of

the power to be transmitted.

9

Disadvani8stages of HVAC transmission

2.1.Disadvani8stages of HVAC transmission:

The following are the disadvantages in HVAC transmission lines:

• Thermal limit

• Corona loss

• Skin effect

• Ferranti effect

• Economics of transmission

• Technical performance

• Reliability

Thermal limit:

Thermal limits usually determine the maximum power flow for lines. Thermal

power flow limits on overhead lines are intended to limit the temperature

attained by the energized conductors and the resulting sag and loss of tensile

strength.

The amount of power that can be sent over a transmission line is limited. The

origins of the limits vary depending on the length of the line. The increase in

thermal limit of the transmission system increases the cost of insulation and

also it increases the cost of Transformers, Switch gear and other terminal

apparatus.

Corona loss:

A corona is a process by which a current, perhaps sustained, develops from an

electrode with a high potential in a neutral fluid, usually air, by ionizing that

10

fluid so as to create a plasma around the electrode. The ions generated

eventually pass charge to nearby areas of lower potential, or recombine to form

neutral gas molecules.

Skin effect:

The skin effect is the tendency of an alternating electric current (AC) to

distribute itself within a conductor so that the current density near the surface of

the conductor is greater than that at its core. That is, the electric current tends to

flow at the "skin" of the conductor. The skin effect causes the effective

resistance of the conductor to increase with the frequency of the current. Skin

effect is due to eddy currents set up by the AC current.

The skin effect has practical consequences in the design of radio-frequency and

microwave circuits and to some extent in AC electrical power transmission and

distribution systems. Also, it is of considerable importance when designing

discharge tube circuits.This effect can be minimized by using ACSR

(Aluminum conductor steel reinforce) conductors which has the property to

minimize the skin effect. But it increase the cost compare to normal conductors.

Ferranti effect:

The Ferranti Effect is a rise in voltage occurring at the receiving end of a long

transmission line, relative to the voltage at the sending end, which occurs when

the line is charged but there is a very light load or the load is disconnected.This

effect is due to the voltage drop across the line inductance (due to charging

current) being in phase with the sending end voltages. Therefore both

capacitance and inductance are responsible for producing this phenomenon.The

Ferranti Effect will be more pronounced the longer the line and the higher the

voltage applied. The relative voltage rise is proportional to the square of the line

length.Due to high capacitance, the Ferranti Effect is much more pronounced in

underground cables, even in short lengths.

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HVDC TRANSMISSION

3.1. Hvdc transmission:

Modern DC power transmission is relatively a new technology which made a

modest beginning in the year 1954. The advent of thyristor valve and relater

technological improvements over the last 18 years has been responsible for the

acceleration of the growth of HVDC technology is still undergoing many

changes due to continuing innovations directed at improving reliability and

reducing cost of converter stations. The latest development of multi-terminal

system operation has increased the scope of application of HVDC systems.

However, the growth in the knowledge on HVDC technology remains limited.

When the number and size of dc system are small, it was common to consider

HVDC power transmission as too specialized and fit only to be taken up by the

manufacturers and consultants. With the growth of HVDC systems there is now

a greater awareness among engineers from utilities, regarding the potential of dc

transmission from the point of view of interaction with ac systems. Some of

these interactions are beneficial, while others may pose problems unless

investigated thoroughly during the design stage and solutions incorporated to

overcome the adverse effects. While it is true that the HVDC systems are quite

reliable and converter control allows flexibility in the system operation.

3.2.Comparison of ac & dc transmission:

The relative merits of two modes of transmission (ac & dc) which need to be

considered by a system planner are based on the following factors:

• Economics of transmission

• Technical performance

• Reliability

Economics of power transmission:

The cost of transmission line includes the investment and operational costs. The

investment includes costs of Right of Way (ROW), transmission towers,

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conductors, insulators and terminal equipment. The operational costs include

mainly the cost of losses. The characteristics of the insulators vary the type of

voltage applied. For simplicity, if it is assumed that the insulator characteristics

are similar for ac & dc and depend on the peak level of the voltage applied with

the respect to the ground. Then it can be shown that for lines designed with the

same insulation level, a dc line carry as much power with two conductors (with

positive and negative polarities with respect to ground) as an ac line with three

conductors for the same size. This implies that for a given power level dc line

requires less ROW, simpler and cheaper towers and reduced conductor and

insulation costs. The power losses are also reduced with dc as there are only two

conductors. The absence of skin effect with dc is also beneficial in reducing

power losses marginally. The dielectric losses in case of power cables is also

very less for dc transmission.

The corona effects tend to be less significant on dc conductors than for ac and

this also leads to the choice of economic size of the conductors with dc

transmission. The other factors that influence the line cost are the cost of

compensation and terminal equipment. Dc lines do not require compensation

but the terminal equipment costs are increased due to the presence of the

converter and filters.

3.3.Technical performance:

The DC transmission has some positive feature which are lacking in AC

transmission. These are mainly due to the fast controllability of power in DC

lines through converter control.

The following are the advantages:

• Full control over power transmitted.

• The ability to enhance transient and dynamic stability in association AC networks.

• Fast control to limit fault currents in DC lines. These make it feasible to avoid

DC breakers in two terminal DC links.

In addition, the DC transmission overcomes some of the problems of AC

transmission. These are described further:

13

Stability limits:

The power transfer in AC lines is dependent on the angle difference between the

voltage phasors at the two ends. For a given power level, these angle increases

with distance. The maximum power transfer is limited by the considerations of

steady state and transient stability.

Voltage control:

The voltage control in AC lines is complicated by the line charging and

inductive voltage drops. The voltage profile in an AC line is relatively flat only

for the fixed level of power transfer corresponding to surge impedance loading

(SIL). The voltage profile varies with the line loading. For the constant voltage

at the line terminals, the mid point voltages reduced for the line loading higher

then SIL and increase for loading less than SIL. The maintenance of constant

voltages at the two ends requires reactive power control from inductive to

capacitive as the line loading is increased. The reactive power requirements

increase with the increase in the line lengths.

Although dc converter stations require reactive power related to the line

loadings, the line itself does not require reactive power. The steady state

charging currents in ac lines pose serious problems in cables this puts the break

even distance for the cable transmission around 40 km.

Reliability:

The reliability of dc transmission systems is quite good and comparable to that

of Ac systems. An exhaustive record of existing HVDC links in the world is

available from which the reliability statistics can be computed. It must be

remembered that the performance of the thyristor valves is much more reliable

than mercury arc valves and further development in devices control and

protection is likely to improve the reliability level for example the development

of direct light triggered (LTT) is expected to improve reliability because of the

elimination of the high voltage pulse transformers and auxiliary supplies for

14

turning on the device. Both energy availability and transient reliability of

existing dc systems with thyristor valves is 95% or more.

3.4. Applications of DC transmission:

The detailed comparison of ac & dc transmission in terms of economics and

technical performance leads to the following areas of application for dc

transmission.

• Long distance bulk power transmission.

• Underground or underwater cables.

• Asynchronous interconnections of ac systems operating at different

frequencies or where independent control of systems is desired.

• Control and stabilization of power flows in ac ties in an integrated power system.

Disadvantages of DC transmission:

The scope of application of DC transmission is limited by the following factors:

• The difficulty of breaking dc currents which results in high cost of dc breakers.

• Inability to use transformers to change the voltage levels.

• High cost of conversion equipment.

• Generation of harmonics which require ac & dc filters, adding to the cost of

converter stations.

• Complexity of control.

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TYPES OF HVDC SYSTEMS

There are different types of HVDC systems which are as follows:

4.1.Mono-polar hvdc system:

In the mono-polar configuration, two converters are connected by a single pole

line and a positive or a negative DC voltage is used. In Fig. There is only one

Insulated transmission conductor installed and the ground or sea provides the

path for the return current.

Fig.4.1.Mono polar HVDC system

4.2. Bi-polar hvdc systems:

This is the most commonly used configuration of HVDC transmission systems.

The bipolar configuration, shown in Fig. below uses two insulated conductors as

Positive and negative poles. The two poles can be operated independently if

both Neutrals are grounded. The bipolar configuration increases the power

transfer capacity.

Under normal operation, the currents flowing in both poles are identical and

there is no ground current. In case of failure of one pole power transmission can

continue in the other pole which increases the reliability. Most overhead line

HVDC transmission systems use the bipolar configuration.

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Fig.4.2. Bipolar HVDC system

4.3.Homo-polar hvdc systems:

In homo polar configuration, two or more conductors have the negative polarity

and can be operated with ground or a metallic return. With two Poles operated

in parallel, the homopolar configuration reduces the insulation costs. However,

the large earth return current is the major disadvantage.

Fig.4.3Homo-polar hvdc system

HVDC conversion is implemented mostly by monopolar or bipolar configurations of 12-

pulse series connected thyristor converters. In such case the resulting high contents of 12-

pulse converter related harmonics can couple into near by telephone lines and cause noise

in the communication network. This may also cause mal-function of protective relaying

and circuit breakers. To avoid such undesirable harmonic effects, tuned passive filters have

been employed on the AC side of the converter. To reduce the harmonic distortion in VSC

based HVDC systems without using conventional filters, there are three feasible solutions.

These are through the use of multi-pulse converter, the multi-level converter, and the pulse

width modulation (PWM) technique. The PWM converter should switch many times within

17

one power cycle to synthesize its output waveform, therefore its switching loss is

reasonably high, and this greatly limits its development in high power applications.

Magnetic coupled multi-pulse converter has two or more bridges and develops the staircase

voltage waveform by varying transformer turns ratio with zigzag connections. In these

multi-pulse converter circuits the converter bridges are operated at fundamental frequency

switching (FFS) thus reduce the switching losses substantially. Pulse number can be

increased in multiples of six, and an increase in every six pulse VSC reduces the harmonics

in the system proportionally. The THD of stepped voltage of two level multi-pulse VSC

converters are given in Table .From this table, it is clear that VSC only with pulse number

48 qualified, where the THD is less than 5%. A 48-pulse voltage source converter is

already reported for STATCOM applications. The reason for choosing 48-pulse operation

in this work is that a48-pulse voltage source converter will give THD which is required to

value less than 5% ,compared to other pulse number such as 30 and36 as shown in Table.

Pulse

number. 6 12 18 24 30 36 42 48 96

%VSC

voltage

THD

30.9 15.2 10.1 7.5 6.1 5.4 4.3 3.75 1.8

Standard voltage THD of two-level multi-pulse converters

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STATIC SYNCHRONOUS COMPENSATOR

5.1.STATCOM (Static synchronous compensator):

It is a device connected in derivation, basically composed of a coupling

transformer that serves of link between the electrical power system (EPS) and

the voltage synchronous controller (VSC), that generates the voltage wave

comparing it to the one of the electric

system to realize the exchange of reactive power. The control system of the

STATCOM adjusts at each moment the inverse voltage so that the current

injected. In the network is in quadrature to the network voltage, in these

conditions P=0 and Q=0. In its most general way, the STATCOM can be

modeled as a regulated voltage source Vi connected to a voltage bar Vs through

a transformer.

Fig(5.1a)Equivalent circuit of a STATCOM system

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The figure shows the equivalent circuit of a STATCOM system. The GTO

converter with a dc voltage source and the power system are illustrated as

variable ac voltages in this figure. These two voltages are connected by a

reactance representing the transformer leakage inductance.

STATCOM converters are designed to mitigate the described phenomena with

solutions based on Power Converter System

(PCS) platforms providing the following control features:

• Power factor correction (cos phi control)

• Voltage control

• Active harmonics cancellation

• Flicker mitigation

• Unsymmetrical load balancing

5.2.Application of the three-phase STATCOM in voltage stability:

Voltage stability is one of the biggest problems in power systems. Engineers

and researchers have met with the purpose of discussing and trying to

consolidate a definition regarding to voltage stability, besides proposing

techniques and methodologies for their analysis. Most of these techniques are

based on the search of the point in which the system’s Jacobian becomes

singular; this point is referred as the point of voltage collapse or maximum load

ability point.

The series and shunt compensation are able to increase the maximum transfer

capabilities of power network .Concerning to voltage stability, such compensation has

the purpose of injecting reactive power to maintain the voltage magnitude in the

nodes close to the nominal values, besides, to reduce line currents and therefore the

total system losses. At the present

time, thanks to the development in the power electronics devices, the voltage

magnitude in some node of the system can be adjusted through sophisticated and

20

versatile devices named FACTS. One of them is the static synchronous compensator

(STATCOM).

Typical STATCOM applications:

• Utilities with weak grid knots or fluctuating reactive loads

• Unbalanced loads

• Arc furnaces

• Wind farms

• Wood chippers

• Welding operations

• Car crushers & shredders

• Industrial mills

• Mining shovels & hoists

• Harbor cranes

5.3.Advantages of STATCOM with VSC converter:

• Continuous and dynamic voltage control

• High dynamic and very fast response time

• Enables grid code compliance

• Maximum reactive current over extended voltage range

• High efficiency

• Single phase control for unbalanced loads

• Small footprint

• Enhanced ride-through capability

• High reliability and availability

21

BIBILIOGRAPHY

[1] J. Arrillaga, Y. H. Liu and N. R. Waston, “Flexible Power Transmission,

The HVDC Options,” John Wiley & Sons, Ltd, Chichester, UK, 2007.

[2] Gunnar Asplund Kjell Eriksson and kjell Svensson, “DC Transmission

based on Voltage Source Converter,” in Proc. of CIGRE SC14

Colloquium in South Africa 1997, pp.1-8.

[3] Y. H. Liu R. H. Zhang, J. Arrillaga and N. R. Watson, “An Overview of

Self-Commutating Converters and their Application in Transmission and

Distribution,” in Conf. IEEE/PES Trans. and Distr.Conf. & Exhibition,

Asia and Pacific Dalian, China 2005.

[4] B. R. Anderson, L. Xu, P. Horton and P. Cartwright, “Topology for VSC

Transmission,” IEE Power Engineering Journal, vol.16, no.3, pp142-

150, June 2002.

[5] G. D. Breuer and R. L. Hauth, “HVDC’s Increasing Poppularity”, IEEE

Potentials, pp.18-21, May 1988.

[6] IEEE Standard 519-1992, IEEE Recommended Practices and

Requirements for Harmonic Control in Electrical Power Systems, IEEE

Inc., New York, 1993.

[7] M.S. EL-Moursi and A. M. Sharaf, “Novel controllers for the 48-pulse

VSC STATCOM and SSSC for voltage regulation and reactive power

compensation”, IEEE Trans. on Power Systems, vol.20, no.4, pp.1985-

1997, Nov-2005.

[8] Zhengping Xi and S. Bhattacharya, “Magnetic Saturation in

Transformers used for a 48-pulse Voltage-Source Converter based

STATCOM under Line to Line System Faults”, in. Prof of IEEE Power

Electronics Specialists Conference, 2007, PESC 2007, IEEE, 17-21 June

2007, pp.2450–2456.

[9] Makoto Hagiwara, Hideaki Fujita and H. Akagi, “Performance of a Self-

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Fault Condition,” IEEE Trans. on Power Electronics, vol.18. no.1,

pp.278-285. Jan-2003.

22

[10] Makoto Hagiwara and Hirofumi Akagi, “An Approach to Regulating

the

DC-Link Voltage of a Voltage Source BTB system during power flow.

CONTENTS

TOPIC PAGE NO.

CHAPTER-1.INTRODUCTION

1.1.Introduction 2

1.2.VSC control system 3-9

CHAPTER-2.HVAC SYSTEM

2.1.Disadvantage of hvac system 10-11

CHAPTER-3.HVDC TRANSMISSION

3.1.HVDC transmission 12

3.2.comparision of ac and dc transmission 12-13

3.3.Techinical performance 13-15

3.4. Applications of dc transmission 15

CHAPTER-4.TYPES OF HVDC systems

4.1.Mono-polar HVDC system 16

4.2.Bi-polar HVDC system 16-17

4.3.Homo-polar HVDC system 17-18

CHAPTER-5.STATCOM

5.1.STATCOM 19-20

5.2.Applications of statcom 20-21

5.3.Advantages of statcom 21

CHAPTER-6.CONCLUSION

6.1.Conclusion 22

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