Harmonic Analysis in HVDC System

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459,ISO 9001:2008 Certified Journal, Volume 4, Issue 5, May 2014)

132

Harmonic Analysis in HVDC System

Anuradha.V

1

, Anitha.S

2

, Apoorva.D.C

3

, Priyanka.N

4,

Somashekar.B

5

1, 2, 3, 4_{Student, B.E, EEE, Dr. T. Thimmaiah Institute of Technology, K.G.F} 5_{M.Tech, Lecturer, EEE, Dr. T. Thimmaiah Institute of Technology, K.G.F.}

Abstract— Harmonics are electric voltages and currents that appear on the electric power system as a result of non-linear electric loads. Non-non-linear loads include common office equipment such as computers and printers, Fluorescent lighting, battery chargers and also variable speed drives. Harmonic components should be reduced as much as possible. According to harmonic stability problem that was caused by the non-linear of converter, the simulation model was established based on the actual equipment situation of convert station. Harmonic instability may occur in high-voltage dc (HVDC) links due to dynamic interactions between HVDC terminals and the impedance of the dc lines or cables. In recent years, the issue of harmonics compensation has got considerable attention. The main purpose of this paper is to reduce the harmonics in transmission of power in HVDC system. THD values can be calculated using FFT analysis. Simulation model is developed in MATLAB/Simulink environment.

Keywords— HVDC, harmonics, filters, rectifier, inverter, active filters, passive filters

I. INTRODUCTION

The semiconductor devices in DC converter station is a non-linear power electronic device. A large amounts of characteristic and of non-characteristic harmonics currents will be produced and injected into the AC power system when the HVDC running even the supply voltage waveform of AC side is the standard sinusoidal wave and cause voltage distortion and because of the asymmetric of the three-phase AC systems respectively. The harmonics that come from the HVDC will not only increase loss, thermal stress of equipment, reduce equipment life, interference with communications, metering, protection and control devices to work properly and in some cases, it can even lead to the collapse of the system.

Harmonics are electric voltages and currents that appear on the electric power system as a result of non-linear electric loads. Harmonic frequencies in the power grid are a frequent cause of power quality problems. Harmonic components should be reduced as much as possible.

There are two types of harmonics in electrical power systems, namely current harmonics and voltage harmonics, which are distortions to current and voltage waves respectively. On the basis of generation, there are two types of harmonics-characteristic and non-characteristic.

One of the major effects of power system harmonics is to increase the current in the system. This is particularly the case for the third harmonic, which causes a sharp increase in the zero sequence current, and therefore increases the current in the neutral conductor. This effect can require special consideration in the design of an electric system to serve non-linear loads.

The presence of harmonics leads to low system efficiency, poor power factor, increased loss and reactive power components from AC and also on the equipment present in the system and interference on the telecommunication lines.

There were many methods used for the analysis of harmonics in a LCC (Line Commutated Converter) HVDC system. Some of them are – Symmetrical component method, individual phase control, harmonic mapping, Eigen value analysis, numerical methods, etc. But each of the methods had limitations, thus, impedance model is sorted as a better analysis method for harmonics in a LCC-based HVDC system.

Basically for reduction of harmonics, filters are used. Here we use a hybrid configuration of both passive and active filters for improved power handling capacity of semiconductor devices and better performance.

Fig.i Block representation of HVDC system

II. SYSTEM MODELLING

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International Journal of Emerging Technology and Advanced Engineering

133 Basic formulae like-

P=3*V*I*cosα (assuming cos α = 0.85)

R=V/I

XL=2πf L (where XL =0.15p.u. – given)

KVA= (KW^2 + KVAR^2) ^1/2

Q=X/R (quality factor)

KVA=KW/power factor

are used to obtain the parameters for the transformers and filters.

The corresponding circuit diagram for which the simulation model is developed is as given below:

Fig.ii. Circuit diagram of a HVDC transmission system

The above circuit diagram consists of a three- phase generator source of 345Kv at frequency of 50Hz which is connected to the converter transformer for stepping down voltage to 211KV for transmission over long distance. Further a 12-pulse thyristor bridge converter is connected, which is called as rectifier for converting AC to DC while the converter at the other end is referred as inverter for vice-versa conversion. The rectifier-end has firing angle (α) control for ignition of the thyristor while the inverter-end contains the extinction/gamma control (δ).

A. Three Phase Converter

The converter transformers are constructed by three phase three winding on same core material by connecting primary and secondary windings as either wyes or deltas.

Fig.iii. Twelve pulse converter

B. Pulse Generator

In this paper, a discrete twelve pulse generator is used to fire the thyristors of HVDC rectifier built with two six pulse bridges. The four inputs of a pulse generator are alpha firing angle (in degrees), other three inputs are phase to ground synchronizing voltages. There are two outputs of a pulse generator, one connected to wye secondary winding of transformer and other connected to delta winding of secondary transformer. Similarly for gamma angle control at the inverter-side, a twelve-pulse firing control and discrete gamma measurement subsystem is used for the two extinction/gamma angle triggering for the inverter.

C. Filters

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International Journal of Emerging Technology and Advanced Engineering

134

III. SIMULATION CIRCUIT AND RESULTS

The corresponding simulation model is as shown below:

Fig.iv. Simulation model of a single pole HVDC transmission system

A 12 pulse HVDC converter has been considered at both the ends. Two thyristor bridges of six pulse is use to build converter in series connection. The thyristor is fired at different firing angle (α) values which vary from 0-90 degrees but the optimal value for the system is taken as 19 degrees. A feeder consisting of RLC elements is used to connect source to both rectifier and inverter. Here the harmonic compensation has been done using AC filters which comprises of two single tuned and a high pass filter , tuned to 11th, 13th and 24th part of fundamental frequency respectively. Also a capacitor bank of 150MVAR reactive power. Further a DC line of 300km distance is considered with appropriate line resistance and inductance. The respective pulse generator components are used for triggering of pulses at the rectifier and inverter ends of the system.

The subsystem constructed for the AC filters is as shown below:

Fig.v. Subsystem components of the AC filters

The subsystem components used in the gamma control block is as given below:

Fig.vi. Subsystem components of the gamma control

In the above subsystem shown, two discrete gamma measurement blocks from the SimuLink Library is being used – one for the star(Y) and delta (Δ) bridge connections of the transformers and the constant given as the gamma angle is 160degrees , considering the general equation for extinction/gamma angle (γ) –

γ =180°- α- commutation angle

(where the commutation angle is assumed as 180° and

α=19°)

The simulation results for the simulation model is as given below:

1. Waveforms For Rectifier Side – Without Filters

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International Journal of Emerging Technology and Advanced Engineering

135 2. Waveforms For Rectifier Side – With Filters

Peak value is 325kv and current is 1080A

3. Waveforms For Inverter Side – Without Filters

4. Waveforms For Inverter Side – With Filters

IV. FFT ANALYSIS

The FFT analysis is performed for the outputs at the inverter end of the system to finally analyze the effect of the insertion of the filters into the system.

The THD value of the above FFT analysis is found to be 78.43% at the fundamental frequency of 50Hz.

V. CASE STUDY –PRACTICAL VALUES

A similar simulation model has been developed

considering a single pole HVDC terminal at

POWERGRID, KOLAR (2000MW-bipole +_500KV) converter station in located in Karnataka. According to the given equipment and its parameters, the corresponding simulation model using SimuLink is as shown below:

Fig.viii. Simulation circuit for the single pole terminal at POWEGRID Kolar converter station

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International Journal of Emerging Technology and Advanced Engineering

136 A. AC FILTERS (RECTIFIER)

Fig.ix. Subsystem of AC filters used at rectifier terminal

The components used in this subsystem are-

 Capacitor bank of reactive power of 66MVAR

 Double tuned filter of 120MVAR at tuned frequencies

of Fr1=11*50Hz and Fr2=13*50Hz; quality factor, Q=150

 Single tuned filter of 97MVAR with tuning frequency

of 13*50Hz and Q=150

 Shunt reactor(L) of 72.6MVAR

B. AC FILTERS (INVERTER)

Fig.x. Subsystem of AC filters used at inverter terminal

The components used in this subsystem at the inverter end consists of:

 Capacitor bank of reactive power 69MVAR

 Two double tuned filters of 120MVAR and 97MVAR

reactive power both at tuning frequencies of 11th and 13th of fundamental frequency of 50Hz

Waveforms:

Fig.xi. Waveforms at rectifier end without filters

Peak voltage = 390kv and current= 1050A

Fig.xii. Waveforms at rectifier end with filters

Peak voltage =400kv and current=1020A

Fig.xiii. Waveforms at inverter end without filter

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International Journal of Emerging Technology and Advanced Engineering

137

Fig.xiv. Waveforms at inverter end with filter

Peak voltage=340Kv and current =1000A

VI. CASE STUDY –THD VALUES

The THD value is found almost same as 78.43% with lot of reduction in the harmonic distortions.

VII. DCLINE CURRENT WAVEFORM

Fig.xv. Waveforms of DC line current

VIII. CONCLUSION

This paper accounts the harmonic analysis in a basic HVDC transmission system and the importance of using filters for the reduction of the harmonic distortions in the system to improve the efficiency and reduce various ill effects due to the harmonics. On observing the various waveforms with and without the insertion of filters, we may conclude that on connecting the harmonic filters the voltage values is increased and optimized while the high current values are reduced. The THD value obtained is found to be 78.43% and also the case study in done to observe the effect of filters in a practical HVDC converter system using LCC and three phase three winding transformers.

REFERENCES

[1] ―Modeling and Analysis of DC-Link Harmonic Instability in LCC

HVDC Systems‖ by Hanchao Liu ,Member ,IEEE and Jian Sun, Member,IEEE

[2] ―Harmonic stability analysis of HVDC system based on Short circuit

ratio‖ by He Xingqi,Member, IEEE and Chen Ce ,Member,IEEE

[3] IEEE Recommended Practices and Requirements for Harmonic

Control in Electrical Power Systems, IEEE Std.519-1993,New York: IEEE,1993

[4] IEEE Guide for Analysis and Definition of DC Side Harmonic

Performance of HVDC Transmission Systems, IEEE Power Engineering Society, IEEE 2003

[5] ―Harmonic Compensation of HVDC Rectifier using Shunt Active

Filter‖ by Shashank Srivastava, Rahul Kumar, Satendra Pratap Singh, Nitin Singh , Members, IEEE

[6] ―Study on Harmonic Losses of Inductive Filtering Converter

Transformer (IFCT) in HVDC System‖ by Dechang Yang , IEEE , Yong Li ,Christian Rehtanz ,Longfu Luo ,Jiazhu Xu , Members IEEE

[7] ―Harmonics in HVDC Links, Part I – Sources‖ by Mohamed H.

Okbai, Mohamed H. Saied, M Z. Mostafa, and T. M Abdel-Moneim , Members , IEEE

[8] ―Direct Current Transmission – Volume 1‖ by Edward Wilson

Kimbark, Fellow, IEEE

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International Journal of Emerging Technology and Advanced Engineering

138 AUTHORS

ANURADHA. V pursuing (8 th-sem)

B.E (Electrical & Electronics

Engineering) in Dr. T. Thimmaiah Institute of Technology, K.G.F. VTU

PRIYANKA.N pursuing (8 th-sem)

APOORVA.D.C pursuing (8 th-sem)

ANITHA.S pursuing (8 th-sem) B.E (Electrical & Electronics Engineering) in

Dr. T. Thimmaiah Institute of

Technology, K.G.F. VTU

SOMASHEKAR. B received B.E degree (Electrical & Electronics Engineering) in Golden Valley Institute of Technology,

K.G.F in 1998 under Bangalore

University and M. Tech (VLSI & Embedded Systems) from BMS, VTU in 2010.