HVDC

HVDC TRANSMISSION SYSTEM

Dr. B. R. Parekh Department of Electrical Engineering

Birla Vishvakarma MahavidyalayaVallabh Vidyanagar - 388120

[email protected]

mailto:[email protected]

The History • 1880s: DC network• 1890s: AC transmission• 1889: Thury HVDC system in Italy (630 kW, 14 kV

DC, 120 km)• 1941: Elbe-Project in Berlin - 60 MW, 200 kV, 115

km buried cable link (incomplete)• 1951: Moscow-Kashira project (shut down )• 1954: world's first HVDC transmission link using

mercury-arc valve – between Sweden & Gotland -20 MW, 200 A and 100 Kv.

• 1961: 160 MW HVDC transmission link across the English Channel (shut down in 1984)

• 1964: Volgograd-Donbass project, Russia – 750 MW, 400 kV

• 1970: Pacific Intertie, USA - largest mercury arc valve - 1600 MW at ± 400 kV (ABB & GE)

• 1967: one mercury arc valve in Gotland HVDC link was replaced with thyristor

• 1970s & 80s: several HVDC projects using thyristor

The History……

• 1989: first Indian project - Sileru-Barsoor - 400 MW, 200 kV, 196 km

• 1991: the first multiterminal scheme - Quebec - New England project - 1100 km, 450 kV, 2000 MW

• 1992: the second Indian project - Rihand, 814 km, 500 kV, 1500 MW

• 1997: the first IGBT scheme in Sweden - 10 km, 180 kV, 3 MW (VSC HVDC)

• The largest of all HVDC schemes to date is the 6300-MW Itaipu HVDC link in Brazil

The History……

Thury HVDC system

• It was developed by the Swiss engineer Rene Thury. • It used series-connected motor-generator sets to

increase voltage. • Each set was insulated from ground and driven by

insulated shafts from a prime mover. • The line was operated in constant current mode, with

up to 5000 volts on each machine. • Some machines had double commutators to reduce the

voltage on each commutator. • Thury systems up to 100 kV operated until 1930s, but

high maintenance and high energy loss of rotating machinery denied a commercial success

The mercury-arc valve

• The mercury-arc valve was the workhorse of DC transmission for a long time since the end of the 1920s.

• Unlike the electrical machines mercury-arc valve design was empirical. As a result, attempts to increase its voltage rating failed.

• The problem was however solved in 1929 • The grid controlled mercury arc valves became

available for power transmission during the period 1920 to 1940

The Thyristor based HVDC

• In the spring of 1967, one of the mercury-arc valves used in the Gotland HVDC link was replaced with a thyristor valve

• Semiconductors have been used in all subsequent HVDC links

• Initially, thyristor valve converter stations were either air-cooled (indoor) or oil-cooled (outdoor)

• Today, all HVDC valves are water-cooled • Bulk transmission is likely to rely on thyristor

technology for more years to come since it is reliable, and has low cost and low losses

• Thyristor based HVDC depends on the correct functioning of the AC system

• It requires a minimum Short Circuit Power from the connected AC grid.

• It cannot feed power into a network which lacks generation completely or which has little or very remote generation

• A common measure of the adequacy of this is the Short Circuit Ratio (SCR) that relates the Short Circuit Power to the Rated Power of the HVDC transmission

• SCR should be at least 2.5–3.0. • Thyristor based HVDC does not transmit reactive

power

The Thyristor based HVDC

HVDC lines & B/B Links in India

D-R : Delhi – Rihand (1500 MW, 500 kV), P-C : Padgha – Chandrapur (1500 MW, 500 kV)S-B : Sileru – Barasur (400 MW, 200 kV)K-T : Kolar – Talcher (2000 MW, 500 kV)

V : Vindhyachal (500 MW Sa : Sararam (500 MW)C : Chandrapur (1000 MW) G : Gajuvaka (500 MW)

HVDC - The Basic Scheme

The Components

• Converter Transformers– Steps up AC voltage and provides isolation

• Series Converters– Converts AC to DC or DC to AC

• Smoothing Reactor– Reduce harmonics and transients on DC side

• AC Filter and DC Filter– Reduce harmonics

• Shunt Capacitors– Reactive power generation

• Control System– Performance control

CONVERTER TRANSFORMER DESIGN REQUIREMENTS

• Insulation has to withstand

– AC voltage, – Short time over voltage, – Superimposed DC voltage, and,– Polarity reversal

• AC voltage stress distribution is inversely proportional to dielectric constant, but, DC voltage stress distribution is proportional to resistivity

• Stress distribution under polarity reversal is capacitive in the beginning, but gradually becomes resistive.

• Initially oil gets stressed more than the solid.

• But resistivity of solid being much more, the solid insulation gets stressed much.

• Partial discharges under DC stress in oil, in cellulose and at oil-cellulose interface weaken the insulation strength


• Harmonic content is much higher than that of conventional transformer

• It causes:– Additional winding loss – High leakage flux– Stray flux that causes eddy loss in winding, tank

and steel structure

• These losses create local hot spot

• Asymmetry in valve firing leads to DC magnetising

component (excessive loss in core due to presence of

DC)


• Arcing under AC and DC conditions are different

• DC withstand voltage of bushing is less than one-third of AC

• On load tap changer should have much wider range

• Tolerance on impedance of transformer decides the design and cost of converter. Accepted tolerance is 2-3% against a conventional 10%


• Tests:

– Long duration DC voltage test (60 minutes)

– Short duration DC voltage test (2 minutes )

– Polarity reversal test

– Switching impulse test

– Lightning impulse test


HVDC Converters

• Since transmission voltage is higher than breakdown voltage of semiconductor, HVDC converters are built using large numbers of semiconductors in series

• The string is commonly referred to as a 'valve' • Rectifier and inverter machinery are not different• Converter transformers, are often three physically

separate single-phase transformers, – to isolate the station from the AC supply, – to provide a local earth, and – to ensure the correct eventual DC voltage

• The basic configuration uses six valves, a 3 phase bridge connection

• But, with a phase change only at every sixty degrees, considerable harmonics remain on the DC side.

• Use of 12 valves (twelve-pulse system) instead, can reduce – current harmonics on AC side– voltage ripples on DC side

• This is accomplished by connecting one 6-pulse converter through a Y-Y transformer and another through a -Y transformer

HVDC Converters

dc-side voltage waveforms as a function of

Vd repeats at six times the line frequency

Conclusions

• Thyristor converters provides controlled transfer of power between the line frequency ac and adjustable-magnitude dc

• By controlling , transition from rectifier to inverter mode of operation can be made and vice versa

• Thyristor converters are mostly used at high-power levels

• Thyristor converters inject large harmonics into the utility system

A 12 pulse Bipole Scheme

• The two 6-pulse converters are connected in parallel on the AC side, and in series on the DC side (for high voltage DC)

• With twelve valves there is a phase change at every 30 degrees, producing 12 ripples in one cycle

• Pure DC is produced with the help of smoothing reactor and DC filter

• The number of harmonics present in 12 pulse scheme is just half of that in a 6 pulse scheme

HVDC Converters

• Fourier analysis gives AC current harmonics as:

h = 6k 1 for 6 pulse operation, and,

h = 12k 1 for 12 pulse operation

where h is the order of harmonics, and,

k is an integer

• Order of DC voltage harmonics,

h = 12k

HVDC Converters

• DC Voltage per pole in a 12 pulse system,

• Vd = Vd1 + Vd2

VLL = RMS line voltage applied to each of the 6 pulse converters, and,

Ls = per phase leakage inductance of transformer ref to converter side

• For > 90°, the valve operates as an inverter, sending power from DC to AC side

HVDC Converters

ds

LL IL

V

3cos

23V V d2d1

The Itaipu HVDC Transmission – 2 parallel bipoles

VAR Demand• Line frequency, line voltage-commutated converters

operate at lagging power factor– AC phase control introduces a phase shift between

current and voltage

– Commutation process introduces a further

displacement (when current commutates from one

phase to another there is line to line short circuit

through transformer impedance and it absorbs

reactive power)• For optimum power transfer, Q and Id should be

minimised - to accomplish these, should be small in the RECTIFIER MODE (10° - 20°)

HVDC Converters

• Active and reactive powers in the RECTIFIER

MODE of a 6 pulse converter are:

• For a 12 pulse converter P and Q per pole will be

double of these

• Q is normally 50% –60% of P

cos35.1I V P dd1 dLL IV

sin35.1 Q dLL IV

HVDC Converters

• In INVERTER MODE P and Q will flow in opposite

directions

is delay angle, is extinction angle and u is overlap

angle

should be as small as possible to minimise Q and Id

HVDC Converters

sin35.1 Q dLL IV

cos35.1 P dLL IVu 180

Converter Configurations

Monopole and earth return

– one of the terminals of the rectifier is connected to ground and the other to a transmission line.

– The earthed terminal may or may not be connected to the corresponding connection at the inverting station by means of a second conductor

– If no metallic conductor is installed, current flows in the earth between the earth electrodes at the two stations, and causes:

• Electrochemical corrosion of pipelines • Water pollution• A net magnetic field due to unbalanced current

– Modern monopolar systems carry typically 1500 MW by overhead lines or 600 MW by underground cables

Bipolar

– A pair of conductors is used, each at a high potential with respect to ground, in opposite polarity.

– Since these conductors must be insulated for the full voltage, transmission line cost is higher than a monopole with a return conductor.

– Bipolar systems may carry as much as 3000 MW at 800 kV


Back to back

• In a back to back system there are no overhead lines or cables separating the rectifier and the inverter.

• So the DC current can be kept high and the DC voltage low, which favours a compact design

• HVDC back-to-back stations are used for– coupling of electricity mains of different frequency

(as in Japan) – coupling two networks of the same nominal

frequency but no fixed phase relationship – different frequency and phase number


Systems with transmission lines

• The most common configuration - Two stations connected by means of a dedicated HVDC line.

• There are Multi-terminal HVDC links, connecting more than two points

• The configuration of multiple terminals can be series, parallel, or hybrid.

• Parallel configuration is used for large capacity stations, and series for lower capacity stations

• The largest multi-terminal HVDC system in the world is the 2000 MW Quebec - New England Transmission system opened in 1992. In 2003 it is converted into 5 terminal HVDC system.


• Converters are expensive • Overload capacity is not high• VAR compensation at terminals (thyristor schemes)• At smaller transmission distances hvdc losses are

higher than those of ac lines • For small distances cost of converters may not be

offset by reductions in line construction cost • In contrast to ac systems, realizing multiterminal

systems is complex• Expanding existing schemes to multiterminal

systems is also difficult

HVDC Drawbacks

• Controlling power flow in a multiterminal dc system requires good communication between all the terminals; power flow must be actively regulated by the control system instead of by the inherent properties of the transmission line

• Construction of new lines needs fresh right of way which is a difficult issue

• Emergence of FACTS has partially eclipsed hvdc schemes. FACTS devices have a an edge over HVDC since the former can be retrofitted into existing lines

HVDC Drawbacks

• Submarine cables

– high C causes additional AC losses in cables, but It has minimal effect for DC

– the charge and discharge current of C causes additional I2R power losses while carrying AC

– Dielectric losses are extra

Applications & Advantages

Submarine FMI (Flat Mass Impregnated) cable

• Long lines for bulk power transmission without intermediate 'taps' – connects remote power plant to the distribution grid – reduces Corona loss : space charge formation helps

HVDC to have half the loss per unit length of HVAC carrying the same amount of power

– for a given power rating the DC voltage is lower than the peak AC voltage

– voltage determines insulation thickness and conductor spacing

– thus HVDC can carry more power per conductor, which can lower costs

– HVDC lines have better electromagnetic compatibility when compared to HVAC


• Interconnections of power systems operating at different frequencies

• Capacity addition of existing lines when extra wires are difficult or expensive to install


• Tie line connecting unsynchronised AC distribution systems – prevents cascading failures from propagating from

one line to another, yet allowing power import or export

– damps out power oscillations and increases system stability margin

– sends required amount of power in any desired direction

– no transfer of disturbance from one side to the other – if required the two regions can operate at different

frequencies – HVDC tie line helps optimum power flow through each

of the two transmission lines– fast control actions in electronic HVDC scheme

enhances stability in power system


The recent developments

• Capacitor commutated converters, • Continuously tuned AC filters, • Active DC filters, • Air-insulated outdoor HVDC valves • Voltage source converter using IGBT• 800 kV as transmission voltage

– the most cost effective alternative for long distance bulk power transmission

– total losses are 50 % higher for the 600 kV

Capacitor Commutated Converter

CCC HVDC converter consumes less reactive power as the converter includes a series capacitor

Chandrapur-Padghe HVDC transmission project showing Active Filter connection

View of Second 1 x 500 MW HVDC Block

HVDC

Documents

thyristor based hvdc

gotland hvdc link

thury hvdc system

transmission voltage

dc voltage

arc valve

ac voltage

hvdc converters