HVDC for Advanced Power Systems

8/10/2019 HVDC for Advanced Power Systems

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By:

Siddharth Joshi

Lect. EED SoT PDPU

Unit - 1

HVDC


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HVDC

Scheme

EE 431T Advances in Power System

Teaching Scheme Examination Scheme

L T P CHrs/

Week

Theory Practical Total

Mark

sMS ES IA LW

LE/

Viva

3 - - 6 3 30 60 10 - - 100


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introduction


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HVDC System


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History

First commercial application of HVDC between Swedishmainland and the island of Gotland in 1954.

Underwater link of 90 km and 20 MW.

After the advent of thyristor convertor, New Brunswick

and Quebec 320 MW back-to-back DC interconnectioncommissioned in 1972.

With reduced size, cost and improved reliability of powerelectronic converters, has made HVDC transmission more

widespread. In North America, total HVDC transmission capacity in

1987 was 14,000 MW.


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Hvdc in India

First HVDC link in India i.e. Rihan to Delhi

Highest transmission kV 600kV.

Growth 2500 MW /year world


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HVDC Projects


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Advantages

Technical Economical


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Advantages In a number of applications HVDC is more effective than AC

transmission. Examples include: Undersea cables, where high capacitance causes additional AC

losses. (e.g. 250 km Baltic Cable between Sweden and Germany)

Long power transmission without intermediate taps, for example,in remote areas; Power transmission and stabilization between

unsynchronized AC distribution systems Connecting a remote generating plant to the distribution grid

Reducing line cost: 1) fewer conductors 2) thinner conductorssince HVDC does not suffer from the skin effect

Facilitate power transmission between different countries that useAC at differing voltages and/or frequencies

Synchronize AC produced by renewable energy sources


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Disadvantage of HVAC?

With increase in distance and voltage the problemsassociated with ac systems increase. these problems areof :

Corona loss and radio interference.

Heavy supporting structure and erection difficulties .

Insulating problem.

Stability problem.

Current carrying capacity. Ferranti effect. Surge impedance loading.

Mechanical vibrations and oscillations.

Radio interference.


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Disadvantages

The disadvantages of HVDC are in conversion,switching and control.

Expensive inverters with limited overload capacity

Higher losses in static inverters at smaller transmission

distances

The cost of the inverters may not be offset by reductionsin line construction cost and lower line loss.

High voltage DC circuit breakers are difficult to buildbecause some mechanism must be included in the circuitbreaker to force current to zero, otherwise arcing andcontact wear would be too great to allow reliableswitching.


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Major Drawback

It cannot transmit reactive power

Eg. Connecting synchronous generator into

bus Ps and Qs will flow through line in AC

while in this case it will flow through links.


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Cost of HVDC Transmission Costs vary widely depending on power rating, circuit length,

overhead vs. underwater route, land costs, and AC networkimprovements required at either terminal.

For example, for an 8 GW, 40 km link laid under the English

Channel, the following are approximate primary equipment costsfor a 2 GW, 500 kV bipolar conventional HVDC link is:

Converter stations ~$170 M

Subsea cable + installation ~$1.5 M/km

So for an 8 GW capacity between England and France in four links, little

change is left from ~$1.2B for the installed works. Add another $300

$450M for the other works depending on additional onshore works

required


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Comparison of AC and DCTransmission

Economics of transmission

Technical Performance: DC transmission have

some positive features:

full control over power transmitted

the ability to enhance transient and small

signal stability associated with AC n/w

fast control to limit fault current in DC lines.

Reliability


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Economics of transmissionHVAC and HVDC

Right of Way (RoW)

Transmission towers

Conductors Insulators

Terminal equipment


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Right of Way

A transmission right ofway is an integral part of atransmission system,which includes the high-

voltage lines, towers,substations and otherequipment and facilitiesthat carry electric power.In simple terms, the right

of way is the strip of landimmediately below andadjacent to a transmissionline. Egs. Wind farms,towers etc


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Technical Performance

Stability limits

Power carrying capability is a function of

distance as figure.

Unaffected by distance and affected by current

carry capacity (thermal limit)

Voltage control

Line compensationMid point voltage is reduced if the load

is higher than surge impedance


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Why Do We Need Reactive

Power

Reactive power (vars) is required to maintain

the voltage to deliver active power (watts)

through transmission lines.

Motor loads and other loads require reactive

power to convert the flow of electrons into

useful work. When there is not enough reactive

power, the voltage sags down and it is notpossible to push the power demanded by loads

through the lines.


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Problemswith ACinterconnection

Two power systems are connected through AC ties the AGC ofboth of the systems have to coordinated using tie line powerand frequency signals.

Even with coordinated control of interconnected systems theoperation of AC ties can be problematic due to:

1. Presence of large power oscillations which can lead tofrequent tripping

2. Increase in fault levels

3. Transmission of disturbances from on systems to another.

The controllability of power flow in DC lines eliminates allabove problems.

No need for coordinated control.

Two systems have different nominated frequency cannotconnected directlyrequire the DC link.


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Disadvantages of DC

transmission

a) The difficulty of breaking DC current whichresults high cost of DC breakers.

b) Inability to use transformers to change

voltage levels.c) High cost of conversion equipment.

d) Generation of harmonics which require AC

and DC filters, adding the cost of converterstations.

e) Complexity of control.

Over the years passed


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Over the years passed.

Except (b) has been over come Development of DC

breakers

Modular constructions of

thyristor valves

Increase the rating ofthyristor cell that make

up a valve.

Twelve pulse operation of

converters.

Use of metal oxide gap

less arrestors

Application of DE and

fiber optics

a) The difficulty of breaking

DC current which results

high cost of DC breakers.

b) Inability to use transformers

to change voltage levels.

c) High cost of conversion

equipment.

d) Generation of harmonics

which require AC and DC

filters, adding the cost of

converter stations.

e) Complexity of control.


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Reliability

Performance of thyristor is more reliable than

mercury arc valves.

Further developments in devices, control and

protection is much more reliable.


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HVDC outage statistics


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Applications of DC

transmission

As per above comparison following

applications are noted here:

1. Long distance bulk power transmission.

2. Underground of underwater cables.

3. Asynchronous interconnection of AC system

operating at different frequencies.

4. Control and stabilizations of power flows in

AC ties in an integrated power system.


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Long distance bulk power transmission.

Underground of underwater cables.

First these two advantageseconomicadvantages, break even distance, etc. moreover

various alternative that is used to solve the

problem of AC transmission, i.e. phase shifter,

static var systems, series capacitors, singlepole switching etc.


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Components of HVDCsystems


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hvdC COmprises Of..


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Converters


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Converters

They perform AC/DC and DC/AC conversion

They consist of valve bridges and transformers

Valve bridge consists of high voltage valves connected in a 6-pulse

or 12-pulse arrangement The transformers are ungrounded such that the DC system will be

able to establish its own reference to ground

Smoothing reactors

They are high reactors with inductance as high as 1 H in series

with each pole They serve the following:

They decrease harmonics in voltages and currents in DC lines

They prevent commutation failures in inverters

Prevent current from being discontinuous for light loads

Harmonic filters

Converters generate harmonics in voltages and currents. Theseharmonics may cause overheating of capacitors and nearbygenerators and interference with telecommunication systems

Harmonic filters are used to mitigate these harmonics


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Reactive power supplies

Under steady state condition conditions, the reactive power consumed by the

converter is about 50% of the active power transferred

Under transient conditions it could be much higher

Reactive power is, therefore, provided near the converters

For a strong AC power system, this reactive power is provided by a shunt

capacitor

Electrodes

Electrodes are conductors that provide connection to the earth for neutral.They have large surface to minimize current densities and surface voltage

gradients

DC lines

They may be overhead lines or cables

DC lines are very similar to AC lines

AC circuit breakers

They used to clear faults in the transformer and for taking the DC link out of

service

They are not used for clearing DC faults

DC faults are cleared by converter control more rapidly


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In HVDC converter transformer also consumes

reactive power it supplies from;

AC filters.Shunt compensators.

Excessive reactive power from network.

Static compensator (SVS).

Synchronous Condenser.


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Converters

SmoothingReactors

Harmonic filtersReactive Power

Supplies


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Typesof DC links

It uses one conductorThe return path is provided by ground or water

Use of this system is mainly due to cost considerations

A metallic return may be used where earth resistivity is too high

This configuration type is the first step towards a bipolar link.


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Types of DC links


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It uses two conductors, one positive and the other

negative.

Each terminal has two converters of equal ratedvoltage, connected in series on the DC side.

The junctions between the converters is grounded

Currents in the two poles are equal and there is noground current.

If one pole is isolated due to fault, the other pole

can operate with ground and carry half the ratedload (or more using overload capabilities of its

converter line).


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Types of DC links


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It has two or more conductors all having the

same polarity, usually negative.

Since the corona effect in DC transmission

lines is less for negative polarity, homo-polarlink is usually operated with negative polarity.

The return path for such a system is through

ground.


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Valve Characteristics


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Converter Characteristics

Three phase full wave bridge - rectifier

Analysis of three phase full


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Analysis of three phase, full

wave bridge circuit


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Sum / Difference of Trigonometric Functions Formulas.

7. sin A + sin B = 2 sin [ (A + B) / 2 ] cos [ (A - B) / 2 ]

8. sin A - sin B = 2 cos [ (A + B) / 2 ] sin [ (A - B) / 2 ]

9. cos A + cos B = 2 cos [ (A + B) / 2 ] cos [ (A - B) / 2 ]

10. cos A - cos B = - 2 sin [ (A + B) / 2 ] sin [ (A - B) / 2 ]

Product of Trigonometric Functions Formulas.

11. 2 sin A cos B = sin (A + B) + sin (A - B)

12. 2 cos A sin B = sin (A + B) - sin (A - B)

13. 2 cos A cos B = cos (A + B) + cos (A - B)

14. 2 sin A sin B = - cos (A + B) + cos (A - B)


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Values of eac over


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which period of

time???


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Double Commutation Failure


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Date post:	02-Jun-2018
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HVDC for Advanced Power Systems

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