HVDC

1 Manoj Barsaiyan

HISTORY OF HVDC

HVDC technology first made its mark in the early under-sea cable interconnections of Gotland (1954) and Sardinia (1967), and then in long distance transmission with the Pacific Intertie (1970) and Nelson River (1973) schemes using mercury-arc valves.

A significant milestone occurred in 1972 with the first Back to Back (BB) asynchronous interconnection at Eel River between Quebec and New Brunswick; this installation also marked the introduction of thyristor valves technology and replaced the earlier mercury-arc valves.

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HISTORY OF HVDC

The first 25 years of HVDC transmission were sustained by converters having mercury arc valves till the mid-1970s.

The next 25 years till the year 2000 were sustained by line-commutated converters using thyristor valves.

It is predicted that the next 25 years will be dominated by force-commutated converters. Initially, this new force-commutated era has commenced with Capacitor Commutated Converters (CCC) eventually to be replaced by self-commutated converters due to the economic availability of high power switching devices with their superior characteristics.

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RECENTLY COMPLETED HVDC PROJECTS Year Project MW Remarks

2004 Square Butte , USA 500 (Built by GE.) Control system upgrade by ABB.

2004 Three Gorges - Guangdong, China 3000 2004 CU Project, USA 1000 Control system upgrade 2003 Rapid City, USA 200 CCC 2003 Three Gorges - Changzhou, China 3000 2002 CrossSound, USA 330 2002 Murraylink, Australia 220 Land cable 2002 Garabi 2, Brazil 1100 CCC 2001 Italy-Greece 500 2000 Swepol, Sweden - Poland 600 2000 Terranora interconnector, Australia 180 Land cable 2000 Tjaereborg, Denmark 7 Land cable 2000 Eagle Pass, USA 36 1999 Gotland HVDC Light, Sweden 50 Land cable 1999 Garabi 1, Brazil 1100 CCC 1998 Chandrapur - Padghe, India 1500 1997 Leyte - Luzon, Philippines 440 1997 Hllsjn, Sweden 3 First HVDC Light

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RECENTLY COMPLETED HVDC PROJECTS Year Project MW Remarks

2011 Valhall offshore, Norway 78

2010 Lingbao II (Extension project), China 750

2010 Hulunbeir-Liaoning , China 3000

2010 Caprivi Link Interconnector, Namibia 300 HVDC Light with overhead line

2010 Xiangjiaba - Shanghai, China 6400 800 kV UHVDC

2009 Outaouais, Canada 1250

2009 Chteauguay, Qubec, Canada 1000 Control system upgrade

2009 Blackwater, USA 200 Valve cooling and control systems upgrade.

2008 Cahora Bassa, Apollo upgrade, South Africa

1920 Upgrade of: valves, filters and control system

2008 NorNed, Norway - Netherlands 700

2007 Skagerrak 1&2, Norway - Denmark 500 Control system upgrade

2007 Sharyland Asynchronous Tie,USA 150

2006 Three Gorges - Shanghai, China 3000

2006 Estlink, Estonia - Finland 350

2005 Vizag II, India 500

2005 Troll offshore, Norway 84

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Year Project MW Remarks

2015 DolWin2 900 The world's largest offshore HVDC system

2015 NordBalt 700 The longest HVDC Light cable connection

2015 North East - Agra, India 6 000 Multi-terminal 800 kV UHVDC 2014 Skagerrak 4 700 500 kV, first HVDC Light to run in bipolar

configuration with an HVDC Classic link 2013 Jinping - Sunan, China 7 200 UHVDC. The most powerful trasmission in the

world! 2013 DolWin1 HVDC Light, Germany 800 320 kV HVDC Light cables

2013 Inga-Kolwezi Upgrade, DR Congo 560 Upgrade

2012 Highgate Refurbishment 200 Valve cooling and control systems upgrade.

2012 Rio Madeira, Brazil 3150 World's longest transmission

2012 Rio Madeira back-to-back, Brazil 800

2012 Fenno-Skan 1 Upgrade 500

2012 East West Interconnector, Ireland - UK 500 200 kV HVDC Light cables

2011 Fenno-Skan 2, Sweden - Finland 800

2011 SAPEI, Italy 1000

2011 BorWin1, Germany 400 Offshore wind power

2011 IPP Upgrade, USA 2400

UPCOMING HVDC PROJECTS

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HVDC AROUND THE WORLD

EXISTING HVDC IN INDIA

RIHAND- DADRI (DELHI) 1500 MW BIPOLE

(1991)

TALCHER - KOLAR 2500 MW BIPOLE

(2001)

BALIA - BHIWADI 2500 MW BIPOLE (Under

Construction )

NER AGRA 6000MW AT +/- 800KV DC ( Proposed)

VINDHYACHAL 2 X 250 MW BACK TO

BACK(1989)

CHANDRAPUR 2 X 500 MW BACK TO

BACK(1997)

VIZAG 2 X 500 MW BACK TO BACK(1999)

SASARAM 1 X 500 MW BACK TO

BACK(2002)

ADVANTAGES OF HVDC

Why HVDC rather than HVAC?

Long distances make HVDC cheaper

Improved link stability

Fault isolation

Asynchronous link

COMPARISON OF AC-DC

TRANSMISSION

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EVALUATION OF TRANSMISSION COSTS

The cost of a transmission line comprises of the capital investment required for the

1. actual infrastructure (i.e. Right of Way (RoW),

2. towers, conductors, insulators and terminal equipment)

3. and costs incurred for operational requirements (i.e. losses).

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AC, DC COMPARISION

With the dc option, since there are only two conductors (with the same current capacity of 3 ac conductors), the power transmission losses are also reduced to about two-thirds of the comparable ac system.

The absence of skin effect with dc is also beneficial in reducing power losses marginally.

Corona effects tend to be less significant on dc than for ac conductors.

The other factors that influence line costs are the costs of compensation and terminal equipment. dc lines do not require reactive power compensation but the terminal equipment costs are increased due to the presence of converters and filters.

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COST COMPARISON OF AC AND DC TRANSMISSION

VOLTAGE CONTROL

Voltage control in ac lines is complicated by line charging and voltage drops. The voltage profile in an ac line is relatively flat only for a fixed level of power transfer The voltage profile varies with the line loading.

The maintenance of constant voltage at the two ends requires reactive power control as the line loading is increased.

Although dc converter stations require reactive power related to the power transmitted, the dc line itself does not require any reactive power.

LINE COMPENSATION

Line compensation is necessary for long distance ac transmission to overcome the problems of line charging and stability limitations. The increase in power transfer and voltage control is possible through the use of line compensation.

In the case of dc lines, such compensation is not needed.

PROBLEMS OF DC TRANSMISSION

The application of dc transmission is limited by factors such as:

1. High cost of conversion equipment,

2. Inability to use transformers to alter voltage levels,

3. Generation of harmonics,

4. Complexity of controls.

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ADVANCES IN DC TECHNOLOGY

Increase in the ratings of a thyristor cell that makes up a valve,

Modular construction of thyristor valves,

Twelve-pulse (and higher) operation of converters,

Use of forced-commutation , and

Application of digital electronics and fiber optics in the control of converters.

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APPLICATIONS OF DC TRANSMISSION Underground or underwater cables

In the case of long cable connections over the breakeven distance of about 40-50 km, dc cable transmission system has a marked advantage over ac cable connections.

The recent development of Voltage Source Converters (VSC) and the use of rugged polymer dc cables, with the so-called HVDC Light option, is being increasingly considered.

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UNDERWATER CABLES

LONG DISTANCE BULK POWER TRANSMISSION

Bulk power transmission over long distances is an application ideally suited for dc transmission and is more economical than ac transmission whenever the breakeven distance is exceeded.

The breakeven distance is being effectively decreased with the reduced costs of new compact converter stations possible due to the recent advances in power electronics

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ASYNCHRONOUS INTERCONNECTION OF AC SYSTEMS

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In terms of an asynchronous interconnection between two ac systems, the dc option reigns supreme. There are many instances of BB connections where two ac networks have been tied together for the overall advantage to both ac systems.

In the future, it is anticipated that these BB connections will also be made with VSCs offering the possibility of full four-quadrant operation and the total control of active/reactive power coupled with the minimal generation of harmonics.

BACK-TO-BACK STATION

It is a plant in which both static inverters and rectifiers are in the same area, usually in the same building. The length of the direct current line is kept as short as possible. HVDC back-to-back stations are used for coupling of electricity mains of different frequency (as in Japan; and the GCC interconnection between UAE [50 Hz] and Saudi Arabia [60 Hz] under construction in 20092011)

coupling two networks of the same nominal frequency but no fixed phase relationship (as until 1995/96 in Etzenricht, Drnrohr, Vienna, and the Vyborg HVDC scheme).

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In large interconnected systems, power flow in ac ties (particularly under disturbance conditions) can be uncontrolled and lead to overloads and stability problems thus endangering system security.

Strategically placed dc lines can overcome this problem due to the fast controllability of dc power and provide much needed damping and timely overload capability.

STABILIZATION OF POWER FLOWS IN INTEGRATED POWER SYSTEM

RENEWABLE ELECTRICITY SUPERHIGHWAYS

A number of studies have highlighted the potential benefits of very wide area super grids based on HVDC. A study concludes that a grid covering the fringes of Europe could bring 100% renewable power (70% wind, 30% biomass) at close to today's prices. There has been debate over the technical feasibility of this proposal[28] and the political risks involved in energy transmission across a large number of international borders.[29]

In January 2009, the European Commission proposed 300 million to subsidize the development of HVDC links between Ireland, Britain, the Netherlands, Germany, Denmark, and Sweden, as part of a wider 1.2 billion package supporting links to offshore wind farms and cross-border interconnectors throughout Europe. 31

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Presently the number of dc lines in a power grid is very small compared to the number of ac lines. This indicates that dc transmission is justified only for specific applications.

Although advances in technology and introduction of Multi-Terminal DC (MTDC) systems are expected to increase the scope of application of dc transmission, it is not anticipated that the ac grid will be replaced by a dc power grid in the future. There are two major reasons for this:

First, the control and protection of MTDC systems is complex and the inability of voltage transformation in dc networks imposes economic penalties.

Second, the advances in power electronics technology have resulted in the improvement of the performance of ac transmissions using FACTS devices.

HVDC PRESENT STATUS

HVDC PRESENT & FUTURE

The longest HVDC link in the world is currently the Xiangjiaba-Shanghai 2,071 km (1,287 mi) 6400 MW link connecting the Xiangjiaba Dam to Shanghai, in the People's Republic of China.

In 2012, the longest HVDC link will be the Rio Madeira link connecting the Amazonas to the So Paulo area where the length of the DC line is over 2,500 km (1,600 mi). ABB will provide two 3,150 megawatt HVDC converter stations, and at 600 KV.

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TRANSMISSION LINE SYSTEMS

AC DC

Maximum voltage

in operation kV 800 +/- 600

Maximum voltage

under development kV 1000 +/- 800

Maximum power

per line in

operation

MW 2000 3150

Maximum power

per line under

development

MW 4000 6400

Advantages of HVDC

No (capacitive) charging currents

Grid coupling (without rise of short-circuit current) No stability problems (frequency) Higher power transfer No inductive voltage drop No Skin-Effect High flexibility and controllability

Disadvantages of HVDC

Additional costs for converter station and filters Harmonics requires reactive power Expensive circuit breakers Low overload capability

TYPES OF HVDC SYSTEMS

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MONOPOLAR LINK

In a common configuration, called monopole, one of the terminals of the rectifier is connected to earth ground. The other If no metallic conductor is installed, current flows in the earth between the earth electrodes at the two stations. Therefore it is a type of single wire earth return.

A metallic return can also be used where concerns for harmonic interference and/or corrosion exist.

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Hvdc monopolar schematic

BIPOLAR LINK

A bipolar link has two conductors, one positive and the other negative. Each terminal has two sets of converters of equal rating, in series on the dc side. The junction between the two sets of converters is grounded at one or both ends by the use of a short electrode line. Since both poles operate with equal currents under normal operation, there is zero ground current flowing under these conditions.

Monopolar operation can also be used in the first stages of the development of a bipolar link. Alternatively, under faulty converter conditions, one dc line may be temporarily used as a metallic return with the use of suitable switching.

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Hvdc bipolar schematic

HOMOPOLAR LINK

In this type of link two conductors having the same polarity (usually negative) can be operated with ground or metallic return.

Due to the undesirability of operating a dc link with ground return, bipolar links are mostly used.

A homopolar link has the advantage of reduced insulation costs, but the disadvantages of earth return outweigh the advantages.

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Homopolar Link

MAIN COMPONENTS OF HVDC

SYSTEM

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CONVERTER

CONVERTER

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CONVERTER

During the period (about) 1950-1990s, HVDC systems used the CSC configuration almost exclusively.

From about 1990 onwards, the alternative VSC became economically viable due to the availability of new self-commutating high-power switches (such as GTOs and IGBTs) and the computing power of DSPs to generate the appropriate firing patterns.

However, at present VSC are still limited to below 250 MW capacity due to commercial and practical limitations of the electronic switches.

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COMPARISIONN OF CONVERTERS

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COMPARISIONN OF CONVERTERS

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6 PULSE CSC

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6 PULSE CSC

12-PULSE CONVERTOR BRIDGE

Y

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VOLTAGE SOURCE CONVERTER VSC

Different kinds of Pulse Width Modulation (PWM) techniques can be employed to operate the VSC in inverter mode to provide a sinusoidal output to the ac system. The advantages of the VSC are:

Rapid control of active as well as reactive power,

It provides a high level of power quality,

The technology lends itself to the following types of applications:

Low power (less than 250 MW) HVDC transmission (commercially referred to as HVDC Light),

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VOLTAGE SOURCE CONVERTER VSC

VOLTAGE SOURCED CONVERTERS (VSC) The development of insulated gate bipolar

transistors (IGBT) and gate turn-off thyristors (GTO) has made smaller HVDC systems economical.

These may be installed in existing AC grids for their role in stabilizing power flow without the additional short-circuit current that would be produced by an additional AC transmission line.

The manufacturer ABB calls this concept "HVDC Light

Siemens calls a similar concept "HVDC PLUS" (Power Link Universal System).

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VOLTAGE SOURCED CONVERTERS (VSC) They have extended the use of HVDC down to

blocks as small as a few tens of megawatts and lines as short as a few score kilometres of overhead line.

There are several different variants of Voltage-Sourced Converter (VSC) technology: most "HVDC Light" installations use pulse width modulation but the most recent installations, along with "HVDC PLUS", are based on multilevel switching. The latter is a promising concept as it allows reducing the filtering efforts to a minimum. At the moment, the line filters of typical converter stations cover nearly half of the area of the whole station.

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Worldwide VSC HVDC PROJECTS

POSSIBILITIES FOR TRANSMISSION SYSTEMS FOR HIGH POWER

Hybrid

Connection

Alternating Current (AC)

Direct Current (DC)

Hybrid AC / DC - Connection

Source: SIEMENS

The converter transformers adjust the supplied ac voltage to the valve bridges to suit the rated dc voltage.

The transformer for a 12-pulse bridge has a star-star-delta three-winding configuration, or a combination of transformers in star-star and star-delta connections.

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CONVERTER TRANSFORMERS

Converter operation generates harmonic currents and voltages on the ac and dc sides, respectively. On the ac side, a converter with a pulse number of p generates characteristic harmonics having the order of np1 (n=1,2,3,).

AC filters are installed to absorb those harmonic components and to reduce voltage distortion below a required threshold. Tuned filters and high pass filters are used as ac filters.

On the dc side, the order of harmonics is np. DC filters, along with dc reactors, reduce the harmonics flowing out into the dc line. DC filters are not required in cable transmission or back-to-back schemes.

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HARMONIC FILTERS

AC filters are passive circuits used to provide low impedance shunt paths for ac harmonic currents. Both tuned and damped filter arrangements are used. In a typical 12-pulse station, filters at the 11th and 13th harmonics are required as tuned filters.

Damped filters (normally tuned to the 23rd harmonic) are required for the higher harmonics.

The availability of cost-effective active ac filters will change the scenario in the future..

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AC FILTERS

These are similar to ac filters and are used for the filtering of dc harmonics. Usually a damped filter at the 24th harmonic is utilized.

Active dc filters are increasingly being used for efficiency and space-saving purposes.

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DC FILTERS

DC SMOOTHING REACTOR

A sufficiently large series reactor is used on the dc side of the converter to smooth the dc current and for converter protection from line surges.

The reactor is usually designed as a linear reactor and may be connected on the line side, on the neutral side, or at an intermediate location.

Typical values of the smoothing reactor are in the 300600mH range for long-distance transmission and about 30mH for a BB connection.

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DC SMOOTHING REACTOR

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A line commutated converter in steady-state operation consumes reactive power of about 60% of the active, or dc, power transferred.

The shunt capacitors installed at the converter ac bus supply the reactive power required to maintain the converter ac bus voltage. To achieve satisfactory power factor the shunt capacitors are normally subdivided and switched by circuit breakers as the dc power varies. Some or all of the shunt capacitors are normally configured as ac harmonic filters

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SHUNT CAPACITORS

This is usually modified ac equipment and used to interrupt only small dc currents (i.e., employed as disconnecting switches).

Dc breakers or metallic return transfer breakers (MRTB) are used, if required, for the interruption of rated load currents.

In addition to the equipment described above, ac switchgear and associated equipment for protection and measurement are also part of the converter station.

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DC SWITCHGEAR

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HVDC CONTROLS

In a typical two-terminal dc link connecting two ac systems the primary functions of the dc controls are to:

Control power flow between the terminals,

Protect the equipment against the current/voltage stresses caused by faults, and

Stabilize the attached ac systems against any operational mode of the dc link.

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HVDC CONTROLS

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HVDC CONTROLS

Limit the maximum dc current.

Due to a limited thermal inertia of the thyristor valves to sustain overcurrents, the maximum dc current is usually limited to less than 1.2 pu for a limited period of time.

Maintain a maximum dc voltage for transmission.

This reduces the transmission losses, and permits optimization of the valve rating and insulation.

Minimize reactive power consumption.

This implies that the converters must operate at a low firing angle. A typical converter will consume reactive power between 50-60% of its MW rating. This amount of reactive power supply can cost about 15% of the station cost, and consume about 10% of the power loss.

PROTECTION

SCHEMES

PROTECTION AGAINST OVER CURRENTS

Faults and disturbances can be caused by either malfunctioning equipment or insulation failures.

First, these faults need to be detected with the help of monitored signals.

Second, the equipment must be protected by control or switching actions.

Since dc controls can react within one cycle, control action is used to protect equipment against overcurrent and overvoltage stresses and minimize loss of transmission. In a converter station, the valves are the most critical (and most expensive) equipment that need to be protected rapidly because of their limited thermal inertia.

CURRENT EXTINCTION (CE)

CE can occur if the valve current drops below the holding current of the thyristor.

This can happen at low current operation accompanied by a transient leading to current extinction. Because of the phenomenon of current chopping of an inductive current, severe over voltages may result.

The size of the smoothing reactor and the rectifier Imin setting help to minimize the occurrence of CE.

COMMUTATION FAILURE (CF)

The detection of a CF is based on the differential comparison of dc current and the ac currents on the valve side of the converter transformer.

During a CF, the two valves in an arm of the bridge are conducting. Therefore, the ac current goes to zero while the dc current continues to flow.

SHORT CIRCUITS ON DC LINE

An internal bridge fault is rare as the valve hall is completely enclosed and is air-conditioned. However, a bushing can fail, or valve cooling water may leak, resulting in a short circuit. The ac breaker may have to be tripped to protect against bridge faults.

The fast-acting HVDC controls (which operate within one cycle) are used to regulate the dc current for protection of the valves against ac and dc faults.

PROTECTION AGAINST OVERVOLTAGES

The typical arrangement of metal-oxide surge arrestors for protecting equipment in a converter pole is used.

In general, the ac bus arrestors limit over voltages entering from the ac bus; similarly, the dc arrestor limits Over voltages entering the converter from the dc line.

The ac and dc filters have their respective arrestors also.

Critical components such as the valves have their own arrestors placed close to these components.

SURGE ARRESTORS

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HVDC CONVERTER STATION DESIGN

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HVDC CONVERTER STATION DESIGN

THE HVDC CLASSIC CONVERTER STATION

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THE CCC* CONVERTER STATION

MTDC OPERATION

Most HVDC transmission systems are two-terminal systems.

A multi terminal dc system (MTDC) has more than two terminals.

There are two possible ways of tapping power from an HVDC link, i.e., with series or parallel taps.

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MTDC

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THANK YOU

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