International Journal of Engineering Trends and Technology (IJETT) – Volume17 Number7–Nov2014

ISSN: 2231-5381 http://www.ijettjournal.org Page 331

Improvement of EHVAC by DC-AC composite Transmission System

S.Krishnakumar#1, A.Arunbalaj#2, L.Dineshbabu#3, R.Santhoshkumar#4

Assistant Professor,

Department of Electrical and Electronics Engineering Vel Tech (owned by RS trust) Engineering College

Avadi, Chennai-62, Tamilnadu, India

Abstract-Economic concerns have delayed the construction of a new transmission line. The power is often not available at the growing load centers but at remote locations. Often the regulatory policies environmental acceptability and the economic concerns involving the availability of energy are the factors determining these locations. Now due to stability considerations the transmission of the available energy through the existing ac lines has an upper limit and thus it is difficult to load long extra high voltage ac lines to their thermal limits as a sufficient margin.The flexible ac transmission system is based on the application of power electronic technology to the existing ac transmission system and this improves stability to achieve power transmission close to its thermal limit. Keywords-Power Improvement, EHVAC passage, Simultaneous DC-AC Transmission.

I. INTRODUCTION Industrial countries are experiencing increasing difficulty in finding suitable passage for new overhead transmission lines and in many cases it is simply impossible. There is an increasing pressure to provide the substantial power upgrading of existing transmission line corridors. Two possible suggestions discussed in reference.For the powerupgrading of existing ac transmission areappropriate modification to existing ac lineswithout major new construction with increasedvoltage level either ac or dc.Elimination of the old existing ac transmission linesand their substitution with new lines of EHVAC or HVDC. This would be the amount of major change. Recently proposed concept of simultaneous dc-ac powertransmission enables the long EHV ac lines to be loaded closeto their thermal limits. The conductors are allowed to carrycertain amount of dc current superimposed on usual ac. The novelty of the present work demonstrates thesubstantial power up gradation of an existing EHV ac lines byconverting it into composite dc-ac line. No alterations of conductors, insulator strings and tower structures of theoriginal EHV ac line are required in this case. II. IMPROVEMENT OF COMPOSITE DC-AC POWER

TRANSMISSION Simultaneous power transmission systems the conductors are allowed to carry dc current superimposed on ac current. DC and AC power flow independently and theadded dc power flow does not cause any transient instability. The network in Fig.1

shows the basic scheme for simultaneous dc-ac power flow through a double circuit actransmission line. The dc power is obtained by converting apart of ac through line commutated 12-pulse rectifier bridgeused in conventional HVDC and injected into the neutralpoint of the zigzag connected secondary windings of sendingendtransformer. The injected current is shared equallyamong the three windings of the transformer. The same is reconverted to ac by the conventional line commutatedinverter at the receiving end. The inverter bridge is connectedto the neutral of zigzag connected winding of the receivingend transformer. Each transmission line is connected betweenthe zigzag windings at both ends. The double circuittransmission line carries both three phase dc as well as ac power.

III. TRANSMISSION VOLTAGE SELECTION The voltage selection and insulation design of a transmission line are discussed and electrical energy can be generated centrally in bulk and transmitted economically long distances. The instant value of each conductor voltage with respect to ground becomes more in case of simultaneous dc-ac transmission system by the amount of the dc voltage superimposed on ac and more discs are to be added in each string insulator to withstand this increased dc voltage. However there is no change required in the conductor separation distance. Therefore tower structure does not need any modification if same conductor is used. Another possibility could be that the original ac voltage of the transmission be reduced as dc voltage and it is added such that peak voltage with respect to ground remain is unchanged. Therefore there would be no need to modify the towers and insulator strings. Let Vph be per phase rms voltage of original ac line and also Va be the per phase voltage of ac component of composite dc-ac line with dc voltage Vd superimposed on it. As insulators remain unchanged the peak voltage in both cases should be equal.Electric field produced by any conductor possesses dc component superimposed on a sinusoidal varying ac component. But the instantaneous electric field polarity changes its sign twice in a cycle if (Vd/Va) <√2 is insured. Under this condition the higher creepage distance requirement for insulator discs as required for HVDC lines are not needed in this case.Each conductor is to be insulated for Vmax but the line to line voltage has no dc component and VLLmax = √6Va.Therefore conductor to conductor separation distance of

International Journal of Engineering Trends and Technology (IJETT) – Volume17 Number7–Nov2014

ISSN: 2231-5381 http://www.ijettjournal.org Page 332

each line is determined only by rated ac voltage of the line. Maximum permissible dc voltage offset is that for which the composite voltage wave just touches zero in each cycle.

Va = Vph/2 and Vd = Vph/√2 (1)

IV. WORKING Working of prime mover steam Turbine is governed by fly ball governor mechanism which is governed to run the synchronous generator to control the frequency of the system. The Automatic voltage regulator (AVR) is used to regulate the voltage of the power system network. AVR is a fast acting loop. With the help of zigzag transformer the DC supply is superimposed on AC supply with is then transmitted through EHVAC transmission line by which thermal limit and stability limit is improved. One third of the AC supply is converted to DC supply by using 12 pulse converters (Rectifier) and this DC supply is injected into zigzag transformer neutral point to superimpose the AC with DC. In the inverter circuit the DC supply is retrieved to AC and it is connected to infinite bus as shown in Fig 1.

Fig 1.Model of composite DC-AC transmission In pure HVDC system capacitance of transmission line cannot be utilized to compensate inductive VAR as the dc line voltage is constant with time. The rectifier and inverter bridges consumes lagging VAR for their operation [4]. This VAR requirement increases with gate firing angle of thyristors. The VAR of the converter in addition to lagging VAR of load is to be supplied by synchronous condenser or static capacitor. In dc-ac power transmission the superimposed voltage varies with time and the transmission line capacitance appears as shunt admittance to converter and in parallel to the load. For long EHV line when receiving end power is less than its natural load there is an excess of line charging. Qs is negative and Qr is positive. This huge amount of leading VAR compensates partly or fully the lagging VAR requirement of converter and load. But it remains latent in HVDC transmission.

V. POWER IMPROVEMENT OF EHV AC TRANSMISSION LINE

The total power transfer through the double circuit line before conversion is,

P/total = 3Vph2Sinδ1/X (2)

X is the transfer reactance per phase of the double circuit line or of single circuit line if considered for conversion and δ1 is the power angle between the voltages at the two ends. To keep sufficient stability margin, δ1 is generally kept low for long lines and its value exceeds with the increasing length of line the load ability of the line is decreased[4]. An approximate value of δ1 may be computed from the loadability curve by knowing the values of Surge Impedance Loading (SIL) and transfer reactance X of the line.

P/total = 2.M.SIL (3) Where M is the multiplying factor and its magnitude increases with the length of line. The value of M can be obtained from the load ability curve. The total power transfer through the composite dc-ac line

Ptotal = Pac + Pdc = 3Va2Sinδ2/X + 2VdId (4) The power angle δ2 between the ac voltages at the two ends of the composite line may be increased to a high value due to fast control ability of dc component of power. Approximate value of ac current per phase per circuit of the double circuit line may be computed as

Ia = V (Sinδ/2)/X (5) The dc current order for rectifier and inverter is adjusted on-line as:

Id=3√(Ith

2 – Ia2) (6)

Power up gradation in percent or increase in powertransfer capability of a composite dc-ac line may be defined as Power Up gradation = Power transfer by composite ac dc line Power transfer by pure ac line - Power transfer by pure ac line

% Power Up gradation= (PTOTAL - P’TOTAL) / P’TOTAL (7)

VI. SIMULATION The proposed simultaneous dc-ac power transmission scheme shown in Fig.1 has been simulated in steady state mode using MATCAD software package. A synchronous machine feeding power to infinite bus via a double circuit three-phase 400 KV, 50Hz, 450Km, ac transmission line is considered. The 2750(5x550)MVA, 24.0KV synchronous machine isdynamically modeled with a field coil on d-axis and a damper coil on q-axis with the frame of reference based on rotor.The converters on each end of dc link are modeled as line commutated two six- pulse bridges. Their control system

International Journal of Engineering Trends and Technology (IJETT) – Volume17 Number7–Nov2014

ISSN: 2231-5381 http://www.ijettjournal.org Page 333

consists of constant current (CC) and constant extinction angle (CEA) and voltage dependent current order limiters (VDCOL) control. The converters are connected to ac buses via Y-Y and Y-Δ converter transformers. Each bridge is a compact representation of a DC converter which includes a built in 6-pulse Graetz Converter Bridgecan be inverter or rectifier an internal Phase Locked Oscillator (PLO), firing and valve blocking controls and firing angle (α)/extinction angle measurements. It also includes built in RC snubber circuits for each thyristor. The controls used in dc system are those of CIGRE Benchmark modified to suit at desired dc voltage. AC filters at each end on ac sides of converter transformers are connected to filter out 11th and 13th harmonics. These filters and shunt capacitor supply reactive power requirements of converters.The 400 kV ac line is converted to composite dc-ac transmission line as per equations. The ac voltage and dc voltage components are taken as

Va= 120 kV and Vd= 160 Kv (8)

It has been observed from computation as well as simulation at various transmission angles at a particular length studiesof the line that the maximum power transfer by composite dc-ac line occurs at power angle of 600.Table I shows the simulation results for power transfer of composite dc-ac power transmission through different lengths of lines at transmission angle of 55o.[7] It can be observed that there is a substantial upgradation of power transfer capacity of the line by simultaneous ac-dc power transmission as compared to ac transmission alone.

VII. CONCLUSION The present simultaneous dc-ac transmission is feasible to load these tie lines close to thermal limits specified in the data sheets. Here the conductors are carrying superimposed dc current with ac current. The added dc power flow is flawless and is not the cause of any transient instability. Simulation studies are being made for the co-ordinated control and also individually the control of ac and dc power transmitted through the lines. There is no physical alteration in insulator strings, towers and arresters of the original line.There issubstantial gain in the loading capability of the line .There is a master controller which controlsthe overall current that is flowing in the lines so in case of fault also the current is limited andstability is enhanced . Table1. SIMULATION RESULTS AT 55O TRANSMISSION ANGLE FOR DIFFERENT LINE LENGTHS

Line Length(km)

100 200 500 1000

Pac (MW)

550 450 400 328

Pdc (MW)

1520 1615 1646 1660

Pt (MW)

2070 2065 2046 1988

Qacs (MVAR) 229 160 110 72

Qacsr(MVAR)

-320 -240 -169 -132

Ia (KA)

0.897 0.7157 0.5941 0.508

Id (KA)

4.772 4.993 5.153 5.131

Power Upgrade%

66.5% 94.9% 115% 130%

REFERENCES

[1]Clerici A, ParisL and Danfors P.,"HVDC conversion of HVAC line to provide substantial power upgrading,” IEEE Transaction on Power Delivery, 6(1), 1991, pp.324-333. [2]Clerici A, Valtorta G and Paris L,“AC and DC substantial power upgrading of Existing OHTL Corridors,” AC DC Conference, London, 1991, pp.220-225. [3]Brierley R H et al,“Compact Right of Ways with multi-Voltage Towers” IEEE Transaction on Power Delivery, Vol.6 No.4,October1991,pp.1682-1689. [4] P. S. Kundur, “Power system stability and control”, New York:McGrawhill Inc., 1994. [5]H.Rahman and BH Khan “Power Upgrading of Transmission Line by Combining dc-ac Transmission” IEEE Transaction on Power System, Paper ID N0.TPWRS-0864, Vol.22,No.1,February 2007, Digital Object Identifier 10.1109/TPWRS.2006.887895,pp459-466. [6]H. Rahman and B. H. Khan, Senior Member, IEEE, “Power Upgrading of TransmissionLine by Combining AC–DC Transmission”, IEEE Transactions on Power Systems, Vol.22, No. 1, February 2007. [7]PrabhaKundur-power system stability and control TataMcgraw Hill edition, New Delhi 1993, 11th reprint 2011. [8]N.G.Hingorani,“FACTS—flexible A.C. transmission system,” in Proc. Inst. Elect. Eng.5th. Int. Conf. A.C. D.C. Power Transmission. [9]Padiyar.”HVDC Power Transmission System.”New Age International Publishers, New Delhi, 2nd revised edition 2012.

International Journal of Engineering Trends and Technology (IJETT) – Volume17 Number7–Nov2014

ISSN: 2231-5381 http://www.ijettjournal.org Page 334

KRISHNAKUMAR.S-received the BE Electrical and Electronics engineering from Pallavan college of engineering, kanchipuram, Tamilnadu and M.E Embedded System Technologies from Vel techMultitech engineering college Avadi, Tamilnadu. At present he is currently working as an Assistant professor in the department of Electrical and ElectronicsEngineering at Vel tech engineering college in Avadi, Chennai, Tamil Nadu.

A.ARUNBALAJ-received B.E Electrical and Electronics engineering from I.F.E.T college of college, villupuram, Tamilnadu and M.E Power system engineering from college of engineering Guindy Anna University, Chennai,Tamilnadu.He is currentlyworking as an Assistant Professor in Vel tech engineering college, Chennai, Tamil Nadu.

L.DHINESHBABU-received B.E electrical and electronics engineering from Kamaraj College of Engg& Tech, Virudhunagar. Tamilnadu and M.Tech Power system engineering from Kalasalingam University, Krishnankoil, Tamilnadu.He is currently working as anAssistant Professor in Vel tech engineering college, Chennai, Tamil Nadu.

R.SANTHOSHKUMAR- received B.E Electronics and communication Engineering from Arulmigumeenakshiamman College of Engineering and Technology India and M.E embedded system technologies fromVelmultitech engineering college,India. He is currentlyworking as an Assistant Professor in Vel tech engineering college, Chennai, Tamil nadu,India.