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STARTING WITH THE NAME OF ALLAHTHE MOST BENEFICIENT AND THE

MOST MERCIFUL

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Presented bySHARIQ SHAIKHLECTURER

EED, NEDUET

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Session Test

Assignment

GroupPresentation

10 marks 8 th week

5 marks 6 th week

5 marks 3rd to 8 th week

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GANTT CHART ACCORDING TO EACH PHASE ALONG WITH THE COMPLETEPROJECT PHASE IS MADE FOR SMOOTHLY MEETING THE TIMELINE OF PROJECT

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Transmission system is necessary for:

1. Transferring the power from the bulk power sources to the maintransmission network.

2. For the main transmission network.3. For system interconnection.4. For transfer of power from main transmission network to the

distribution systems.

With the passage of electrical industry evolution, design constraints havebeen overcome up to great extent. Therefore making it possible to overcomethe previous challenges.

As power flow is proportional to the square of voltage therefore researchhad directed the focus in raising the voltage level and thus EHV systemsoriginated with some pros and cons.

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By 1970s: World was in favor of high voltage AC system with voltage level up to

220 KV and extra high voltages up to 760 KV.

By 1990s : USSR, USA and Canada developed Ultra high voltage AC system with

voltage levels of 1000 KV, 1100 KV and 1200 KV.

First commercial HVDC was introduced in 1953 and with the developmentof high power thyristor valve technology, it take place as an alternative tothe high voltage AC systems.

CHOICES

1. HVAC up to 220 KV.2. EHV AC between 400 KV and 760 KV.3. UHV AC above 760 KV.4. HVDC up to 1600 KV.

Selection depends upon the technical and economical considerations.

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Description HV AC EHV AC UHV AC

Nominal KV 132 220 275 345 400 500 750 1000 1100

Higher op.voltage

145 245 300 362 420 520 765 1050 1200

Reference values for the 3 phase AC lines

Year 1965 1969 1988 1990 1985 2000

Highest levelKV

735 756 1000 1100 1200 1600

Country Canada USA USA ITALY USSR USA

Increase in transmission voltages over the world 3 phase AC lines

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Transmission system design have four important aspects:

1. Electrical design.2. Mechanical Design.3. Structural and civil Design.4. Miscellaneous Design.

Also it includes; Sub-station design. Transmission line design. Network planning.

Some of the main Electrical aspectsare:

Choice of transmission voltage. Conductor configuration. VAR compensation. Corona loss & radio

interference. Transient stability. Abnormal operating conditions. Insulation coordination. Neutral grounding. Sub-station grounding.

Earthing electrodes. Harmonics and filters. Lightning protection. Power line communication. Radio, TV, telephone

interference.

Audible noise. ETC

DIFFERENT ELECTRICAL ASPECTS

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=

Where,V1 = sending end voltage.V2 = receiving end voltage. = Power angle.X = Line reactance.

Usually power angle is kept low to meet thetransient stability issue.

o

Line reactance X is sumof inductive andcapacitive reactance.Sometimes EHV linesuses series compensationequals to 50% of theinductive reactance. Xc =Xl / 2.

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AC transmission is subjected to power swings during the contingency conditionssuch as sudden changes in load, tripping of a fault line and fault on bus bars. All

these causes sudden swing in the load angle of the buses.

To improve the issue many measures can be adopted:1. Rapid fault clearing.2. Rapid functionality of auto re-closure.

PRINCIPLE OF EQUAL AREA

CRITERION

The kinetic energy added to rotor due tofault occurrence must be removed afterthe fault to restore synchronous speed.

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Pm2

Pe

Pm 1

A1

A2

max

This method also gives the maximum input power which can be applied whilestability is maintained.If both the areas are not equal then accelerating momentum can never beovercome.The stability limit occurs when max is at the intersection of line Pm and thepower-angle for 90 0 <

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Pm2

Pe

Pm1

APPLICATION TO SUDDEN INCREASE IN INPUT

POWER

o 1 max

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Pm2

Pe

Pm1

APPLICATION TO SUDDEN INCREASE IN INPUT

POWER

1 max o

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Suitable for long distance transmission as with EHV-AC cables length restricts between 40 to 100 Km.

It can carry 5 times more energy than AC (sameline).

Less losses. Cost of maintenance is low although capital

investment is high. More stable in transient conditions over HVAC

system.Etc.

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EHVAC is superior in forming mesh, voltage variation is easy via transformer.Network can be easily tapped at various points.

HVDC is more economical at long distance transmission.

HVDC is more stable as compare to HVAC system.

EHVAC transmission is limited by the constraints of transient stability marginand line reactance. There is no such issue on HVDC system.

In EHV voltage vary along the line due to reactive power absorption but inHVDC there is no such issue.

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EHV-AC at least require two three phase lines but HVDC deals only with onebi-pole line for majority of applications.

HVDC is not economical due to the need of intermediate sub-stations usuallyat distance of 300 Km..

HVDC can interconnect two systems operating at different frequencies andserves as asynchronous tie. Frequency disturbance do not penetrates eachother and the stability improves.

There is no skin effect in HVDC system and therefore complete utilization ofthe conductor size can be made.

Both the mentioned problems are quite less in HVDC than EHVAC for the samedistance and power transfer.

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Charging current is absent in HVDC systems.

Phase to phase, phase to ground clearance and tower size is less in HVDCtransmission system for the same voltage level EHVAC system. Also installationis simpler and cheaper due to absence of central window.

HVDC make use of less usually 2 conductors.

There is no need for intermediate VAR compensation in HVDC like in EHVAC.

HVDC system can utilize the earth return effectively.

In HVDC, system can be operated from bipolar mode to unipolar mode if one

of the pole is subjected to the permanent fault.

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HVDC control can be modify to: (a) frequency control of AC networks. (b)

Damping control for improved stability of AC networks.

Fault level in HVDC intermediate system remains unchanged at both ends asboth are operating independently. In EHVAC system, linkage of two AC systemscauses the level to increase.

Control of converter valves permit rapid power change and direction of powerflow thus transient stability improves.

Length of cable can be increased in HVDC which get limited due to chargingcurrent aspect in the EHVAC system.

In HVDC line can be operated with the constant current regulation or constantvoltage regulation. Losses in HVDC are less due to absence of excess amount of

charging current.

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In AC transmission system the PF adjusts the load angle automatically as the powerflow from the power surplus point tot the deficit point. Sometimes phase shiftingtransformers are also installed in order to control the line flow. However, thecontrol is still sluggish and difficult due to involvement of many devices dynamics.

Control of power on HVDC system is fast, accurate, bidirectional and has a widerange of magnitude limits .

Current flow depends on voltage difference V1 V2 i.e. the voltage of sending andthe receiving end respectively.

=

Where , = ( )

Both the voltages can be altered by changing the angle of the thyristor valves atrespective ends. As value of R is less therefore, small change in voltagedifference brings large change in Idc and consequently the power.

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TYPICAL AC/DC TRANSMISSION LINESTRUCTURE FOR 2000 MW

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The fault power for the three phase fault is called the short

circuit level at a particular fault point.

In HVAC system, addition of new transmission lines and tie lines usually increases theshort circuit level. This is because the addition of new parallel lines causes thereduced equivalent reactance. Therefore, SC level raises at both ends.

In HVDC systems, the fault level remains unchanged. This is because the fault currentgets restricted by the thyristor valves firing.

So with the use of Tie Lines, SC level in AC system rises causing the consequent causeof the sub-station equipment to get raised. Therefore, additional cost will be required

to be injected to the system.

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One of the design constraint in the EHVAC and HVDC system is that the coronaloss and the interference with radio transmission should be with in thespecified limits.

Corona is the visible, audible, partial electric discharge at the surface of thehigh voltage conductors.

The critical value of the voltage depends upon the humidity, pressure,temperature, pollution level in the air and the condition of the conductors

surface.

Corona voltages are different for positive and negative voltages, negativebeing more severe. The charges in DC must flow to the ground or the otherconductor because of opposite polarity. Therefore, it is characterized by thevoltage level not the voltage gradient.

The corona behavior of HVDC mono-polar is different from that of the Bi-polarHVDC. Corona effect rise in HVAC is significant than that of the HVDC.

In AC, the ionized particles move away from the conductor in half cycle beingsimilarly charged then in the next half cycle move towards the conductorwhile in DC there movement is unidirectional.

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One of the design constraint in the EHVAC and HVDC system is that audiblenoise from the HVAC or HVDC should be with in the specified limits.

A.N is basically due to the following reasons:1. Corona.2. Humming of Transformers.3. Cooling systems and other auxiliaries.

Usually, the values are measured in dB and values are regulated by the

standards. The A.N is in the range of VLF to 15 KHz .

For design purpose, testing facility is used and on the basis of opinionsfollowing results have been produced:

No complaints (Below 52.5 dB) A few complaints (52.5 dB 59 dB) Many complaints (Above 59 dB)

Audible noise from the transmission line depends on the following factors:1. Factors affecting corona.2. No of sub-conductors.3. Lateral distance between the line and point of measurement.

AN is caused by thevibrations produced in theair due to change in air

pressure.

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Transmission lines of EHVAC and HVDC should be design such that they causevary less RI and TI. Usually it should be less that 40 dB and 1mV/m at 1 MHz.

Radio and television interference occur in the range of broad cast frequencies:

RI : 0.5 MHz 1.6 MHzTI : 54 MHz 216 MHz

RI is more important in deciding the ROW.

in HVDC the triggering frequency of the thyristor creates high frequencyharmonics around 0.1 MHz to 10 MHz. and can be reduces by:

Selection proper site for sub station.By providing electromagnetic screening to the valve hall.Installing grounding wires on switch yard at proper locations.Proper selection of insulators and hardware to prevent partial discharges.

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High cost of apparatus such as AC to DC rectifiers and DC to ACinverters.

HVDC is quite much costly than that of HVAC in perspective of CB. DC voltage can not be altered with that much ease as in HVAC using

Transformer. Complexity and dependence of high technology. Several abnormal operating conditions and consequent failures. Thyristor valves control is quite complex. HVDC substation require high cost. HVDC convertor require high cooling requirement. HVDC require high number of harmonic filtering.

It is not suitable for multi-point or multi-terminal network. Etc.

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Advancement in the DC technology is in the following domains:

1. Increase in the rating of the thyristor cell that makes up thevalve.

2. Modular construction of the thyristor valves.3. Twelve pulse (and higher) operation of controllers.4. Use of forced commutation.5. Application of digital electronics and fiber optics in the control of

converters.

Load voltage Gate trigger pulse

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