unit-5 hvdc transmission

Darshan Institute of Engineering & Technology, Rajkot

[email protected]

9429050495

Electrical Engineering Department

Advanced Power Electronics (APE)GTU # 2170906

Unit-5

HVDC Transmission

2Prof. Hardik K. Lakhani #2170906 (APE) Unit 5 – HVDC Transmission

Outline

Background

Historical Development

Working Principle of HVDC

Rapid Growth in HVDC

HVDC Schemes

Monopolar Link

Bipolar Link

Homopolar Link

Equipments required for HVDC

12-pulse converter for HVDC System

Comparison of HVDC with EHVAC

Limitations

Prof. Hardik K. Lakhani 3#2170906 (APE) Unit 5 – HVDC Transmission

HVDC : Background

Growing Power Sector in India

Present Installed Capacity : 1,60,000MW

Present Peak Demand : 1,00,000MW

By 2020, Installed Generation : 4,50,000MW

Anticipated Peak Demand : 3,50,000MW

Present per capita electricity consumption : 500 units

By 2020, Consumption : 1000 units

HVDC was first developed in 1950s

When 3-Ф AC Power transmission over long distance & through cables became difficult

HVDC seemed to be a viable solution

Requires less space & suitable for long distance bulk power transmission


HVDC : Background

The first commercially projects launched in this field were

Subsequent developments in high power electronic static devices made

Power Conversion Uncomplicated

Cost Effective

Many projects have come up all over world

To transmit power over long distance (>500 km)

To Interconnect 2 different frequency AC systems

Name Length of Cable Transmission Power

Moscow - Kashira 100 Km 30 MW

Sweden – Gotland Island 98 Km 20 MW


Historical Development

Originally, Power Generation and transmission was through Direct Current

In 1882, the first electric central station was built by Thomas A. Edison in NY

Capable of supplying DC at 110 V

It had Edison bipolar DC generators driven by steam engines

For high voltage, the size of commutator increase whichrestricts the peripheral speed of machine

Due to limitation of size & cost of machines on one handand advent of transformers, poly phase ckts. & inductionmotors on other hand, AC power system gained muchpopularity

When the length of the line is more than 500 km, reactivepower generated by AC line is more than its powertransmitting capacity

Reactive power requirements of long AC DC Transmission Lines


Rapid Growth in HVDC

Innovations in the developments of DC conversion technology

Replacement of mercury valves with thyristor valves

Thyristor valve offers

• Low maintenance requirement

• Low Power Loss

• Free from arc backs

Progressive increase in V & I ratings of Thyristors

12-pulse mode of operation has brought economy in filter requirements

Cooling arrangements for SCRs to enhance power handling capability

Control operations by microprocessor


Working Principle of HVDC

Rectifier Inverter

ACBus - 1

ConverterTransformer - 1

Idc

ACBus - 2


R

Generated A.C. supplyis fed to the convertertransformer - 1 wherethe voltage level of theA.C. supply is changed.

Converter station -1 orrectifier converts A.C.into D.C. and then thepower is beingtransferred on the D.C.link.

Converter station -2 orinverter converts D.C.into A.C and then withhelp of transformer-2converted A.C. voltageis stepped down.

Thereafter power isdelivered to the loadcenters via distributionnetworks. Here, R is theinternal resistance ofthe conductor.

A.C. bus -1 is transferring power to A.C. bus-2


Types of HVDC System

Rectifier Inverter

ACBus - 1


Id

Id

ACBus - 2

ConverterTransformer - 2 Id1

Id2

Id2

+ Id1

ACBus - 1

ACBus - 2

Rectifier Inverter

Id1

Id2

Id2

- Id1

ACBus - 1

ACBus - 2

Rectifier Inverter

Monopolar Link

Bi-polar Link

Homopolar Link


Monopolar HVDC Link

It has a single conductor of negative polarity and uses earth or sea for the return path ofcurrent. Sometimes the metallic return is also used.

The main disadvantage of this link is; if link stops working then there will not be any powertransmission between the stations until it is repaired.

Rectifier Inverter

ACBus - 1


Id

Id

ACBus - 2



Bipolar HVDC Link

The Bipolar link has two conductors one is positive, and the other one is negative to the earth.

The mutual or ground point is maintained at the mid-potential.

Each terminal of a bipolar system has two converters of equal voltage ratings connected inseries.

Id1

Id2

Id2 - Id1

ACBus - 1

ACBus - 2

Rectifier Inverter

If both neutrals are grounded then two polesoperate at equal current and there is noground current.

In the event of fault in one conductor, theother conductor with ground return can beused up to half the rated load or power withrated current of the pole.


Homopolar HVDC Link

It has two or more conductors with same polarity, usually negative, and they always operatewith ground return.

In the event of fault in one conductor, the whole converter can be connected to a healthy poleand can carry more than half the power by overloading but at the expense of increased lineloss.

However this is not possible with the bipolarsystem due to the use of graded insulationfor negative and positive poles. Whencontinuous ground currents are inevitable,homopolar system is preferred.

The additional advantage is lower corona lossand radio interference due to negativepolarity on the lines.

Id1

Id2

Id2

+ Id1

ACBus - 1

ACBus - 2

Rectifier Inverter


Main Elements of HVDC

Equipments Required for HVDC System

Conv. tranf.

AC Filter

ShuntCapacitor

Conv. tranf.

AC Filter

ShuntCapacitor

DC filter

DC line

Smoothingreactor

- DC line

AC Bus AC Bus

Communication channel

Control system

Id1

Id2

1. Converter transformer2. Converter unit3. Converter valves4. A.C. filters5. Shunt capacitors6. Smoothing reactors7. D.C. filters8. D.C. cables9. Control system

A typical Bipolar HVDC System



Basically, 12-pulse converter is used for the rectification purpose in the HVDC link.

To generate 12-pulse D.C. output the transformer with one primary and two secondaries isrequired.

Converter transformer connection should be Y/Y and Y/Δ.

The insulation system of the transformer has to withstand against short time over voltage, A.C.voltage and D.C. with polarity reversal.

1. Converter Transformer

°δ=0

°δ=30

(a) For 12 - pulse rectifier

For 12 Pulse Converter


°δ=0

°δ=30

vd

Cd

ia

Llk ia

id

Llk

Ls iA

' '

Ai i i= +

a a


Multi-pulse converter, specifically 12-pulse converter is used mostly in HVDC transmission.

Two number of three phase converter (6-pulse) are connected in series manner to get the 12pulse output.

Converter unit is build by series connection of thyristor valves to get high voltage blockingcapability.

Advantage of having multi-pulse converter reduces harmonics in the A.C. source current. Forexample, 12-pulse converter only produces 12k1 (k = 1,2,3…) order harmonics.

2. Converter Unit

For 12 Pulse Converter



N-number of thyristor or IGBT valves areconnected to form a module.

The design of valve is based on modularconcept.

The valves can be packaged into single-valve,double-valve or quadruple-valve.

Modern valves have an excellent performancerecord and very small losses.

3. Converter ValveDevelopment of

HVDC Valves



Conventional HVDC converters always have a demand for reactive power.

At normal operation, a converter consumes reactive power in an amount that corresponds toapproximately 50 % of the transmitted active power.

The least costly way to generate reactive power is in shunt connected capacitor banks. Some of thesecapacitor banks can then be combined with reactors and resistors to form filters providing lowimpedance paths for the harmonics in order to limit them from entering into the AC network.

Two types of A.C. filters are used;

Tuned filter

Damped filter

To suppress specific order harmonic in the source current tuned filter is connected.

Low order harmonics would be eliminated by this type of filter i.e. 11th, 13th order.

To suppress higher order harmonics, damped filter is used. It has capability to suppress wide range ofharmonics. i.e. 23rd and 25th; both together.

4. A.C. Filters



The main objectives of the reactor are: To smooth the ripple current in D.C.

To reduce the risk of commutation failures bylimiting the rate of rise of the D.C. line current attransient disturbances in the A.C. or D.C. systems.

Prevention of resonance in the D.C. circuit.

Reducing harmonic currents including limitation oftelephone interference.

5. Smoothing Reactor



They are connected to A.C. systems to fulfill the demand of reactive power generated by theconverter operation.

6. Shunt Capacitors



Compared to three conductors in three phase A.C. system only two conductors are needed forbipolar HVDC link.

For same power handling capacity, the size of conductor in D.C. is small than in A.C.

7. D.C. Cables



This type of filter is connected to filter the D.C. harmonics.

Normally a filter to eliminate 24th order harmonics is designed for HVDC transmission.

8. D.C. Filters

9. Control System

It produces the firing pulses for the valve of theconverter unit.

It also takes care of maintaining power transferprocess under disturbances in D.C. link.



A twelve-pulse bridge is effectively two six-pulse bridges connected in series on the D.C. sideand arranged with a phase displacement between their respective A.C. supplies so that some ofthe harmonic voltages and currents are cancelled.

vd

id

°δ=30

°δ=03 phase

A.C.supply

The phase displacement between the twoA.C. supplies is usually 30° and is realized byusing converter transformers with twodifferent secondary windings (or valvewindings).

Usually one of the valve windings is star(wye)-connected and the other is delta-connected.

The star-star bridge produces six pulseoutput and star-delta also delivers six pulseoutput voltage.



With twelve valves connecting each of thetwo sets of three phases to the two D.C.rails, there is a phase change every 30°, andharmonics are considerably reduced.

For this reason the twelve-pulse system hasbecome standard on almost all line-commutated converter HVDC systems.

It normally does not require any LC filters orpower factor compensators, which leads tothe elimination of possible LC resonancesdue to fewer ripples produced by theconverter operation.

(a) Three phase supply

(b) Output voltage of bridge-1

(c) Output voltage of bridge-2

(d) Output voltage of 12-pulse converter

VS

Vd1

Vd2

Vd


Comparison of AC & DC Transmission

Comparison must be done based on following 3 factors Economics of power transmission

Technical Performance

Reliability


Economics of Power Transmission

DC transmission of bulk power over long distance has certain distinct advantages over convention AC powertransmission

In DC transmission, L & C of the line has no effect on the power transfer capacity of line and line drop

No leakage or charging current of the line under steady state conditions

The cost of terminal equipment is more in DC lines than in AC lines

Break-even distance is one at which the cost of two systems (AC / DC) is same


Cost subdivision for a typical 2000 MW HVDC Scheme


Technical Performance

Full control over power transmitted in either direction

The ability to improve the transient & dynamic stability of AC system when embedded with DClink

Fast control to limit the fault currents in DC lines

A DC link can be used as an asynchronous tie which can tie down the small variation in systemfrequency of different systems

Only 2 conductors per circuit rather than 3

Towers carry less conductor weight

Hence, Smaller in Size & less cost


Reliability

HVDC is more reliable because during fault it supplies at least 50% power

Stability of Power System : Power transfer capacity remains same throughout the distance in DC system where as it is not same in AC

system

Corona, Radio Interference & Skin Effect : Corona loss& radio interference are lower in DC

There is no skin effect in the conductor of a DC system

Cable for Underground or Sea transmission : DC cables are subjected to less over current stresses

With same conductor size the power transmitted in DC is about 2.5 times that with AC


Limitations of HVDC System

Due to generation of harmonic in converter operation, non sinusoidal current will flow in aconverter transformer in the AC side, causing audio frequency telephone interference

Huge filters are required on both AC & DC sides to suppress the harmonics

Reliable multi-terminal DC systems are yet to be established because of lack of HVDC circuitbreakers

Complexity of control

High cost of conversion equipment

Instability to use transformers to change the voltage levels

More maintenance

Not economical for short distance


Comparison

Sr. No. Characteristics EHVAC Link HVDC Link

1 Power transfer abilityLower, limited by power angle andthe reactance

High, limited by temperature rise

2 Control of power flow Slow & difficult Fast, accurate & bi-directional

3 Frequency DisturbanceCommunicated between thesystem

Reduced

4 Transient performance Poor Excellent

5 Power swing Continue for long time Damped quickly

6 Fault Levels Get added after interconnectionRemains unchanged afterinterconnection

7 Frequency Conversion Not possible Possible


Comparison


8 Number of conductors3 conductors are used. Hencehigher volume of conductormaterial is required

Only 2 conductors are used. So,remarkable saving is obtained inthe conducting material

9 Stress on the insulator Very High Very Low

10 Corona effectIs present which createsinterference with surr-oundingcommunication lines

Is absent which does not createinterference with surroundingcommunica-tion lines

11 Skin Effect Is present Is absent

12 Ferranti Effect Is present Is absent

13Substation maintenance

Very easy Very difficult


Comparison


14 Simplicity of line Is complicated Is very simple

15 Voltage regulationPoor regulation because ofcapacitive & inductive reactance

Better regulation becaus-e ofabsence of capaci-tive & inductivereact-ance

16 Reactive power loss Is present Is absent because f is zero

17 Overload capacity Is highBecause of low overload capacityof converters, capacity of thesystem is low

18 Power factorLagging because net reactance ispresent which is mostly inductive

Power factor is unity

19 System Stability Less stable , less reliable Stability is more

20 Intermediate SubstationsIt is required in AC system at every300 m distance for the compensationof reactive power

Not required


References

Ned Mohan, Tore M. Undeland and William P. Robbins, “Power Electronics – Converters,Applications and Design”, John Willey & sons, Inc., 3rd ed., 2003.

Muhammad H. Rashid, “Power Electronics - Circuits, Devices and Applications”, Prentice Hall ofIndia, 3rd ed., 2009.

Muhammad H. Rashid , “Power Electronics Handbook”, Elsevier, 3rd ed., 2011.

P.C.Sen, “Modern Power Electronics ”, S. Chand and Co. Ltd., New Delhi, 2000.

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