FOUR PHASE AC TRANSMISSIONA term paper submitted for partial fulfillment of degree of

BACHELOR OF TECHNOLOGY IN Electrical & Electronics Engineering BYKASARAGADDA KRISHNA CHAITANYA-Y8EE248 GANGINENI VENKATA BHARAT-Y8EE230 JAMPANI KARTHIK VERMA-Y8EE240 KOTTETI CHANDRAMOULI MADHAV-Y8EE258 MANDLA VENKATRAO-Y8EE268Under the esteemed guidance of

CHELAPATI SUDHIR BABU ASSISTANT PROFESSOR EEE department, KLCE.

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING KONERU LAKSHMAIAH COLLEGE OF ENGINEERING (AUTONOMOUS) Green Fields, Vaddeswaram, (via) K.C. Works P.O. - 522 502, Guntur District, Andhra Pradesh. Approved by A.I.C.T.E. , Affiliated to Acharya Nagarjuna University Accredited by N.B.A. & N.A.A.C ISO 9001-2000 Certified 2010-2011

KONERU LAKSHMAIAH COLLEGE OF ENGINEERING (AUTONOMOUS)DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

CERTIFICATEThis is to certify that this project report entitled FOUR PHASE AC TRANSMISSION is a bonafide work submitted by the following students in the academic year 2010-2011 under my guidance and supervision. K. KRISHNA CHAITANYA Y8EE248 G.V.BHARAT Y8EE230 J.KARTHIK VERMA Y8EE240 K.CHANDRAMOULI MADHAV Y8EE258 M.VENKATRAO Y8EE268

Project Guide CHELAPATI SUDHIR BABU

Head of the Department Dr. M.VENU GOPALA RAO

ACKNOWLEDGEMENTI express my sincere thanks and gratitude to the Department Head, Dr. M Venu Gopala Rao for his excellent supervision and guidance. I wish to express deep sense of gratitude to my internal guide Asst. prof. CH.SUDHIR BABU for his co-operation, encouragement and timely suggestions and making this paper a successful one. Project AssociatesK.KRISHNA CHAITANYA-Y8EE248 G.V.BHARAT-Y8EE230 J.KARTHIK VERMA-Y8EE248 K.CHANDRAMOULI MADHAV-Y8EE258 M.VENKATRAO-Y8EE268

TABLE OF CONTENTSABSTRACT 1.INTRODUCTION 2.CONSIDERATION OF 4 PHASE TRANSMISSION 3.COMPARISON OF ALTERNATIVES FOR THREE GORGES-WUHAN-SUZHOUTRANSMISSION SYSTEM 1. Technical characteristics: 2. Reliability characteristics 3. Environmental factors 4. Estimation of economic effectiveness 4.ALTERNATIVES OF CONVENTIONAL POWER LINKS 5.FOUR PHASE VOLTAGE FROM A FOUR BRIDGE INVERTER 1. INTRODUCTION 2. OPERATION 3. AVAILABLE SIMULATED OUTPUTS 6. REFERENCES

ABSTRACTNew type 4-phase AC transmission systems (TS's) allow radically to improve technical, reliable, environmental and economic characteristics of bulk power transmission over long and very-long distances. The 4-phase AC with 90' phase shft is the basis of these 4-phase TS's. Practical application of 4-phase TS's requires making of phase converting transformers to transform the 3-phase AC into the 4-phase AC and vice versa that offers no fundamental difficulty. Effectiveness of application of 4-phase TS's is illustrated by solving a problem of power transmission from The Three Gorges Hydroelectric Plant to East China over distance upwards of 1000 km. The 4-phase 110/ 3 kV TS has unquestionable advantage on the strength of technical-and-economic, reliable and environmental characteristics in comparison with the k750 kV DC TS, double-circuit 1150 kV AC TS. singlecircuit 1150 kV heightened SIL TS for making of Power Link "Three GorgesWuhan-Suzhou".

1.INTRODUCTION

In the whole world production, transmission and distribution of electricity are based on the 3-phase AC. In the field of bulk power transmission over long and very-long distances technological progress is bound up with mastering of UHV, perfection of AC power transmission lines as well as application of DC transmission systems (TS's) New elaboration's of 3-phase overhead lines allow to increase their natural power two times and more with respect to traditional ones and correspondingly to raise their capacity . But considerable increase of singlecircuit transmission capacity is admissible only when to solve a problem of reliability. In the main reliability of UHV TS's depends on line faults among which overwhelming quota (98%) is single phase ones. More than 50% of these faults are arcing ones that can be cleared by single-phase auto reclosing without interrupting the remaining two phases. In case of stable single phase faults it is possible to raise reliability by means of implementation of short-term (within 20 minutes) and long-duration (within 20-40 hours) 2-phase post-emergency conditions. Accordingly TS must be equipped with balanced devices. Concentration of bulk power in single-circuit transmission line sharpens a problem of electric and magnetic field effects as well. As a result economic effectiveness of bulk transmission systems is appreciably reduced when it measures into account in order to meet reliable and environmental requirements. One of directions of raising the TS reliability is founded on application of DC TS's which allow to keep 67% and 60% af transmission capacity operating accordingly under short-term and long-duration unipolar conditions.

However, high costs of converter substations decrease noticeably efficiency of this way For the problem of bulk power transmission over long and very-long distances to solve it is advisable to turn to the 4- phase AC since it allows radically to improve economic, reliable and environmental characteristics of TS's . At present time in China The Three Gorges Transmission Project is being studied to transmit some of power from The Three Gorges Hydroelectric Plant on The Yantze River to the city of Shanghai over distance upwards of 1000 km Comparative analysis of four alternatives for The ThreeGorges Power Link (+750 kV DC TS, 1150 kV double-circuit AC TS, 1150 kV single-circuit AC TS, 4-phase 1 150/3 kV AC TS) allow to make certain of that the 4-phase 1150/3 kV AC TS has the unquestionable advantage

2.CONSIDERATION OF 4 PHASE TRANSMISSIONThe 4-phase AC with 90' phase shift (Fig. 1) is the basis of the 4-phase AC TS's. Principal distinctive properties of the 4- phase AC TS are as follows: It is fitted with phase-converting transformers in order to transform the 3phase AC into the 4-phase AC and conversely. Phase conductors of the 4-phase overhead line are placed on towers so that they form two independent symmetrical 2-phase systems in every one of them phase currents and voltages are opposite in order to increase SIL and decrease environmental factors. In case of emergence of the stable single-phase faults the 4-phase TS is switched to a 3-phase scheme to transmit 100% and 85% nominal power accordingly under short-term and long-duration post-emergency conditions

The transmission tower layout is represented as shown below

Converting the 3-phase AC into the 4-phase AC and vice versa is a problem solved in essence if one means well-known schemes of converting the 3phase AC (a,b,c) into 2-phase AC (,) and inversely, e.g. the :Scott's scheme proposed by the American engineer C.Scott in the end of the past century. The 4-phase AC is simpler all obtained from the 3-phase AC by means of two phase-converting transformers. The first transformer converts the 3phase AC(a,b,c) into the 2-phase AC (, ) with the 90' shift and the second one does the 3- phase AC into the 2-phase AC(,) with the opposite direction of phases so that there is in total the 4-phase symmetrical AC. A scheme of phase-converting transformer T1 differ from the scheme of transformer T2 only by opposite coupling of the 2-phase windings

A typical scott connection can be shown as

If phase voltages of 3-phase and 4-phase lines as well as their bundle phase constructions are the same the natural power of the 4-phase line is 1.5 times that of the 3-phase line. By substitution of double-circuit 3-phase line for 4-phase line (their total cross-section being equal) the natural power of the 4-phase line is two times and more in comparison with that of the single-circuit 3-phase line 4-phase TS is alternative with respect to a traditional double- circuit 3-phase TS, a heizhtened SE single-circuit 3-phase TS as well as DC TS According to

estimate 4-phase lines have costs 20% less than 3-phase ones on condition that they have equal SE, and the same effects of electric and magnetic fields. On the other hand substations of 4-phase TS can have costs 10-20% more than the conventional 3-phase ones because of use of phase-converting transformers instead of the traditional 3-phase autotransformers It follows from this that the so-called economic crossover length of 4-phase lines is very much less than for DC TS's and may be under 200-300 km for UHV Application of 4-phase TS's instead of DC ones gives the largest effect. It is explained that 4-phase line and DC one have approximately the same costs in case of equal transmission capacity Further 4-phase TS's owing to realization of 3-phase schemes to transmit 85100% rated power is more reliable than DC TS's as the latter allow to transmit by unipolar schemes only 60-67% rated power. But if positive qualities of DC TS is ensured at the expense of more that twofold increase of converter substation costs in comparison with 3-phase AC substations the raise in the costs of 4-phase substations as stated above is much less.

COMPARISON OF ALTERNATIVES FOR THREE GORGES-WUHAN-SUZHOU-TRANSMISSION SYSTEM: 1. Technical characteristics: The aluminum cross-section of bundle phase (pale) for each alternative was determined on the basis of current economic density. For AC versions the natural power of lines was chosen so that steady-state stability margin of both transmission section "Three Gorges-Wuhan" and "Wuhan-Suzhou" was equal to at least 20%. When calculating it was assumed that 3-phase fault level MVA at 500 kV busbars was 15000 MVA and 25000 MVA accordingly for Wuhan and Suzhou substations. Concerning the Three Gorges substation it was considered that the forced-action automatic voltage regulators of generators maintain a constant voltage at 500 kV busbars. Installed reactive power of shunt reactors in UHV systems must ensure degrees of compensation of 100- 120 per cent of the line charging. The double-circuit 1150 kV AC TS has the most total crosssection and its steady-state stability margin are considerably more than the normative rate. But technical constraints arising when determining of bundle phase construction don't allow to draw these characteristics nearer to corresponding ones of other versions.

2. Reliability characteristics: When emerging of stable single-phase faults following measures to raise reliability take: Transition to unipolar scheme for DC TS. Switching-off the faulted section line for double-circuit 1150 kV AC TS. Realization of2-phase scheme for single-circuit 1 150 AC TS. Transition to 3-phase scheme for 4-phase 1150163 kV TS.

On initial stage it is advisable to compare reliable characteristics of alternatives using simple method of approach and evaluating such important reliable ones of Power Link as reduction of transmission capacity under short-term and long- during post emergency conditions with respect to nominal capacity. The double-circuit 1150 k.V AC TS and the 4-phase C. Environmental factors 115O/3 kV TS have the best reliable characteristics which dont practically differ. The single-circuit 1150 kV PLC TS and the +750 kv DC TS have similar reliable characteristics too but which is considerably worse than ones of previous alternatives. 3. Environmental factors: The environmental effects of UHV overhead lines is conditioned by corona, electric and magnetic fields, by necessity of land occupation under towers and cutting area along line route, passing through large tracts of forest. Width of sanitary-protective zone determines limits of buildings near by transmission lines According to the norm in force in Russia electric stress on the limits of that zone needn't exceed the value 1 kV/m. Efficiency of use of cross dimensions of overhead lines was evaluated as ratio of transmitted power (for section "Three Gorges-Wuhan") to cross dimensions (distance between side phases) of line. Guyed towers of single-circuit 1 150 kV heightened S E line occupy the greatest area of land Double-circuit 1150 kV transmission line has the most wide cutting area and the greatest width of sanitary-protective zone. Note should be taken that for this altemative minimum distance between circuits in unstraitened terrain circumstances accounts for 100 m Such constraint is bound up with necessity to decrease induction potentials of conductors and earth wires of parallel line being under construction in the second place.

Alternatives

charatceristicsPhase(pole) T E C H N I C A L constructionThree gorgesWuhan schuzou

750 kv Double DC TS circuit ac 5xac-1300 8xac-300 5xac-1200 8xac-300 10600 10600 16000 16000 16200

Single Four circuit ac phase ac 17xac-240 9xac-330 11xac-300 8xac-300 8800 7350 16000 10800 12800 8800 7600 16000 12000

Three gorges

SIL, MW

Wuhan schuzou

Total substation capacity MW Installed reactive power of react Mvar of balancing devices Steady state stability margin% 3 gorges Wuhan

35 60 33 33 40 40 670 17 63 5 17 15 1450 72 176

20 25 24 34 38 40 4000 100 70

20 25

R E L I B L E E C O L O G

Dec. tr capacity in 3 gorges post emergency conditions Wuhan Long duration post emergency condition 3 gorges Wuhan

15 16 580 15 61

Occupation of lang under suspension tower sq.mAverage area of land, occupied under permanent use per 100km of line route. hectare

Average wdth of cuttmg area, m

I C A L E C O N O M I C

Width of samtary-protective zone, m Effect. Use of cross dimensions in MW/m Annual losses of energy Twh Lines Substations Total Cost of transmission lines million$ 3 gorges Wuhan Total

110 350 0.86 1.25 2.11 169 316 485 1360 1845 (100)

290 85 0.95 0.69 1.64 291 583 874 610 1484 (80.4)

165 270 1.26 0.51 1.77 218 358 576 748 1324 (71.7)

145 380 1.30 0.57 1.87 174 302 476 578 1054 (57.1)

Cost of substations in us $ million Total costs in million us $

4. Estimation of economic effectiveness: Economic evaluation of alternatives of Power Link "Three Gorges-Wuhan-Suzhou" with transmission capacity of 8000 MW at sending end and 6000 MW at receiving one is based on data and Table. When annual energy losses was estimated the annual load factor of Power Link made up of the order of 60% Capital costs of lines was estimated on the basis of specific transmission line cost per km. Capital expenditures of DC and AC substations was evaluated according to specific cost per kW of installed converter power and specific cost per kVA of the volt-amperes rating of autotransformers or phaseconverting transformers for AC substations. Specific cost for AC substations took into account costs of shunt reactors. Costs of balancing devices was allowed for version of the single- circuit 1150 kV TS. It

results from Table 1 that the 4-phase 1150/3 kV TS has the minimum summary investment costs which constitutes less than 60%- of the total costs of the most expensive altemative DC TS. Versions of double-circuit and single-circuit 1150 kV TS's take a intermediate position. Note should be taken of that fact that the +750 kV DC TS has as well the maximum annual energy losses which are more by 1020% that for AC altematives.

4.ALTERNATIVES OF CONVENTIONAL POWER LINKS:

In China The Three Gorges Hydroelectric Station has been building what will be the world's largest Plant At peak load, 26 hydrogenerators will generate IS 6 GW Average of many years electricity production is evaluated by 84 TWh The earliest units is planned to commission in 2010 The Three Gorges Hydroelectric Plant will be an organic part of Central China Power Network where roughly two thirds power of this Plant will be used It is supposed to transmit this power over 500 kV transmission lines About one third power from The Three Gorges Plant is planned to transmit to the main load centers in East China Power Network over long distance more than 1000 km This problem is more effectively solved by virtue of application of 1150 kV AC TS's now being built in Russia. Let us assume that find substation will be located in East China Power Network near the city of Suzhou (100 km to the East of Shanghai) and receive approximately 6000 MW In the vicinity of the city Wuhan is installed intermediate substation with received power of 2000 MW. Thus The Three Gorges substation will send the 8000 MW The Power Link "Three Gorges-Wuhan-Suzhou" with total length of line 1080 km and transmitted power of 8000 MW at sending end and 6000 MW at receiving one has the most efficient following alternatives.

First alternative is based on DC. It consist of Three Gorges sending (8000 MW), Wuhan intermediate receiving (2000 MW) and Suzhou receiving (6000 MW) converter substations interconnected with two sections of 750 kV bipolar line. Second and third versions of Power Link are formed with application of 1150 kV AC TS's. Second variant will have, in its final stage, sending, intermediate and receiving substations The main power equipment of these substations is 500/1150 kV, 2000 MVA autotransformer banks and 1150 kV,

900 MVAR shunt reactor groups each from single-phase units In the middle of the section "Wuhan-Suzhou" there is a switching point Over total length double-circuit overhead lines are provided ensuring high level of reliability. With the purpose of improvement technical-and-economic characteristics third alternative of Power Link is founded on 1150 kV heightened SIL lines. Single-circuit 1150 kV TS is equipped with balancing devices in order to realize 2-phase post-emergency conditions in case of emergence stable singlephase faults. At last fourth version is made up from the 4-phase TS of 1150/3 kV (phase voltage). Shunt reactors, circuit breakers and other apparatuses, developed for 3-phase 1150 kV AC TS's can be used to 4-phase 1150163 kV TS's as they have single- phase design. It is necessary to develop only phaseconverting transformers for transforming 500 kV 3-phase AC into 4-phase 1150/1/3 kV AC and vice versa. There is a stock of time more than 12 years before Power Link "Three Gorges- SUzhoul' will begin to build that it is quite enough to develop and manufacture phase-converting transformers.

5. FOUR PHASE VOLTAGE FROM A FOUR BRIDGE INVERTER

A. INTRODUCTION: A novel method (AC-DC-AC)to produce symmetrical four-phase voltage is presented in this paper, The main circuit is the four-bridge topology and the control method is the three-dimensional voltage space vector control. This method has the merits of novel design, high conversion efficiency, good waveform quality and the method is easily to be realized.The power circuit topology of realizing four phase power transmission is three-phase four-bridge arms inverter. In this paper, firstly, the four-phase inverter mathematical model is analyzed, Via the analysis, the phase voltage and line voltage switching function expressions and their conversion matrixes are got, The method of describing fourphase symmetrical sine voltage with three-dimensional space vector is analyzed, Four-phase / three-phase coordinate conversion matrix is derived, through the threedimensional space vector analyses, these are reached: (a) The movement locus of four-phase symmetrical voltage parameters is a ellipse;(b) The control model of four-phase symmetrical voltage may be three-dimensional space vector pulsewidth modulation, lastly, these proposed techniques have been verified by simulation and experimental results. The four-phase power transmission is the easiest multiphase output mode to be realized, compared with the three-phase power transmission, Four-phase power transmission can enhance transmission power density remarkably, save electrical wiring corridor, reduce the cost of investment in transmission unit capacity. At present, the main technique of realizing four-phase power output is to adopt

multiphase

generator

or

three-phase

converted

to

four-phase

power

transformer.The power transformer has the structure to be complex, volume huge and so on questions , Producing the fourphase power source by use of inverter is the application of power electronics in power transmission which has the prospects for development. At present, the domestic and foreign researches on this aspect are very few This paper proposes a new method (AC-DC-AC) of converting threephase to four-phase via inverter, The main circuit topology structure of the four-phase four-leg inverter is showed in Fig.1,which is similar to the three-phase fullbridge inverter, through adding one leg and one phase load to the three-phase full-bridge inverter, through proper control, four-phase voltage can be output. Using this method has structure simply , easy to realize and so on Merits which should be paid full attention.This kind of topology's inverter only suits the balancing load, regarding the non-balancing load, needs to add another bridge arm, but this is not the scope which this article discusses . Because the vector control has the high DC voltage use ratio, reduction switching frequency merits and so on, this article uses the space vector control method to analyze how the three-phase four bridge arms produce the fourphase power source.This article reasons vector distribution, output voltage waveform emphatically and so on.

B. OPERATION:

The basic circuit diagram involved for four phase output voltage is as shown below

The states of four-leg a,b,c,d can be expressed as switch function Sa,Sb,Sc,Sd, if neglected the influence of deadtime, supposing the upper switch is on while the lower switch is off and vice versa ,the switch function can be given by :1 (the upper switch is on) or 0 (the upper switch is off ).If defining the voltage of p point in figure 1 to be Vdc, o point to be 0,so Vao=SaVdc Vbo=SbVdc Vco=ScVdc Vdo=SdVdc,the output voltage of each bridge arm can be given by the product of switch function and power supply voltage Vdc, the output voltage of the four-

phase inverter is the combination of switch function Sa,Sb,Sc.Sd. Because the value of switch function is 0 or 1,the total state number of the inverter is 24=16,Each state corresponds to a set of certain transient values of phase voltage and line voltage. For each state, there is an expression for the line voltage:

In the condition of balance load : Vab =Van -Vbn; Vbc = Vbn -Vcn; Vcd= Vcn -Vdn; Vda =Vdn -Van ; Van +Vbn +Vcn +Vdn = 0; (3) can be deduced ,By combining (2) and (3) , (4) can be deduced.

Thus the corresponding relations between switch function and line voltage & phase voltage are deduced. Because the states of inverter can be expressed as switch function, the corresponding relation is virtually the mathematical model of the four-phase four-leg inverter.

AVAILABLE SIMULATED OUTPUTS:

Using the global source we have an available outputs with certain conditions in the simulation data as follows. The simulated output voltages, SPWM operation,with 400V DC input, 10mH filter inductor, 100uF filter capacitor, 10 load resistance for each phase ,The frequency of the sinusoidal modulation signal was 400Hz, the frequency of the triangle carrier wave was 2.5kHz. As can be seen, the four-phase structure can produce four-phase symmetrical sinusoidal voltages as shown below.

Some other results are also available when simulated by changing some of the parameters as listed below

The four-phase symmetrical sinusoidal output voltages experimental waveform of the four-bridge inverter is showed below, SPWM operation, with 400V DC input, 2mH filter inductor, 55uF filter capacitor, 10 load resistance for each phase ,The frequency of the sinusoidal modulation signal was 50Hz, the frequency of the triangle carrier wave was 9.6kHz.The modulation ratio is 0.6,. 100V/div in Ordinate axis, Simulation results and experimental result verify that the method producing four-phase symmetrical output voltages under three-dimensional space vector control is correct and feasible.

6.REFERENCES

1) 2) 3) 4) 5)

IEEE JOURNAL WIKIPEDIA ELECTRICAL MACHINES BY P.S.BHIMBRA ELECTRICAL MACHINES BY J.B.GUPTA POWER ELECTRONICS BY P.S. BHIMBRA