hvdc transmission

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1 TransGrid Solutions Inc.www.transgridsolutions.com

HVDC Transmission at 800KV



There two questions that we need to answer:

Is HVDC transmission at 800 KV feasible ?

Is HVDC transmission at 800 KV doable ?

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The answer to both questions is :

YES

However there are challenges



HVDC Transmission LineConverter StationsTestingReliabilityAC System Requirements

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HVDC Transmission Line at 800 KV

DC Line InsulationDC Line ClearancesCorona Performance of the dc line


Configuration and Layout

The possible configurations :One single twelve pulse converter per poleTwo similar MW rating series connected twelve pulse converters per poleTwo dissimilar MW rating series connected twelve pulse converters per poleTwo similar rating parallel twelve pulse converters per poleTwo dissimilar rating parallel twelve pulse converters per pole

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The choice of a particular configuration will depend on:EconomicsReliability and availabilitySize and weight of the converter transformers and restrictions for transport


One Twelve Pulse converter per pole

• The weight and dimensions of the converter transformers are a problem

• The outage of a pole (one twelve pulse converter impacts the system to a great extent.

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Two Similar Twelve Pulse converters in series per pole

In this case for 800 KV there will be two groups in series each is rated for 400 KV and the same power rating.

• The weight and dimensions of the converter transformers is acceptable.

• The outage of one twelve pulse converter does not cause the loss of large amount of power.

• Has an advantage in staging for example starting at +/- 400 KV. Specially if the blocks of power are equal steps Although at higher losses.


Two Similar Twelve Pulse converters in series per pole

• If there is a problem during a certain period with the insulation either on the line or in the stations the operation can continue at 400 KV.

• We need all the bypass switch gear and all the de-block and inter-blocking logic

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TO DC LINE

NBGS

TO ELECTRODE LINE

SMOOTHING REACTO

BY PASS ISOLATOR

DC FILTER

ISOLATOR

THROUGH WALL

VALVE ARRESTERS

GROUP ARRESTER

BY PASS BREAKER

DC BUSHINGS

VALVES

TRANSFORMER

BUSHINGS

400 KV

800 KV


Two Dissimilar Twelve Pulse Groups in series per pole

In this case the two groups in series per pole each has different voltage rating and obviously power rating. For example for 800 KV the first group can be rated for 600 KV and the next group for 200 KV.

• The weight and dimensions of the converter transformers is acceptable.

• The outage of the large group will have to be taken into consideration. Although it is still better than the single converter per pole.

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Two Dissimilar Twelve Pulse Groups in series per pole

• Has an advantage in staging if unequal power blocks are needed during the different stages.

• If there is a problem during a certain period with the insulation either on the line or in the stations the operation can continue at for example 600 KV.

• We need all the bypass switch gear and all the de-block and inter-blocking logic. In this case the inter-blocking is a little more complicated than two similar groups.

• The spare transformers are not the same.


TO DC LINE

NBGS

TO ELECTRODE LINE

SMOOTHING REACTO

BY PASS ISOLATOR

DC FILTER

ISOLATOR

THROUGH WALL

VALVE ARRESTERS

GROUP ARRESTER

BY PASS BREAKER

DC BUSHINGS

VALVES

TRANSFORMER

BUSHINGS

600 KV

800 KV

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Two Similar Twelve Pulse groups in Parallel

In this case actually we have two bipoles each consists of one twelve pulse group per pole and the two similar polarity poles are connected in parallel. All converters are rated for 800 KV and same current.

• The weight and size of the converter transformers are not a problem

• The outage of one twelve pulse group does not cause the loss of a large amount of power. However it leads to unbalanced operation and ground current until the circuit is reconfigured.


Two Similar Twelve Pulse groups in Parallel

• Has an advantage in staging for example starting with one bipole only. Specially if the blocks of power are equal steps.

• During staging the losses are lower than series groups.

• Reduced Voltage operation is only within the tap range and allowable firing angle control range.

• We do not need the bypass switch gear. However the paralleling and de-paralleling controls are more involved than the controls of series groups.

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800 KV

- 800 KV

All Valve groups areEqual in MW rating.The current ratings areThe same.


Two Dissimilar Twelve Pulse groups in Parallel

In this case actually we have two bipoles each consists of one twelve pulse group per pole and the two similar polarity poles are connected in parallel. However the two bipoles are not equal in current ratings.

• The weight and size of the converter transformers are not a problem

• The outage of the large twelve pulse group has to be considered. It also leads to unbalanced operation and ground current until the circuit is reconfigured.

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Two Dissimilar Twelve Pulse groups in Parallel

• Staging can be done in unequal steps.• During staging the losses are lower than series

groups.• Reduced Voltage operation is only within the tap

range and allowable firing angle control range.• We do not need the bypass switch gear. However

the paralleling and de-paralleling controls are more involved than the controls of series groups.


800 KV

- 800 KV

1

2

3

4

5

6

7

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Groups 1,2,3, and 4are same MW rating.

Groups 5,6,7, and 8have lower MWrating

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To examine the impact of the configuration on the ac system Let us consider two cases:Case 1

The power to be transmitted is only 6000 MW at +/- 800 KV the dc current will be 3750 Amps.If we consider each pole has only one 12 pulse converter, then the outage of a converter means the loss of 3000 MW and with 10% over load the loss will be 2700 MW, this represents 45 % of the power.It is hard to see the AC system can withstand this.



If we consider each pole has two 12 pulse converters in series, then the outage of a converter means the loss of 1500 MW and with 10% over load the loss will be 1050 MW, this represents 17.5 % of the power.It is clear that the two 12 pulse converters in series per pole are preferred here.The same benefits are obtained from two 12 pulse converters in parallel

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Case 2The power to be transmitted is now 18000 MW with three bipoles at +/- 800

KV. Each bipole will be rated at 6000 MW. The dc current will be 3750 Amps in each bipole.If we consider each pole has only one 12 pulse converter, then the outage of a converter means the loss of 3000 MW and with 10% over load the loss will be 1500 MW, this represents 8.3 % of the power.This looks now manageable. If each pole has two 12 pulse converters in series, then the loss of a converter means the loss of 1500 MW, and with 10% overload there will be no loss at all.Obviously, two 12 pulse converters in series per pole will reduce the impact.The same benefits are obtained from two 12 pulse converters in parallel



One argument in the debate of two converters in series per pole versus one converter per pole will always be what happens in the case of a dc line fault.Obviously it does not make a difference, although with two converters in series one can argue that in the event of restart failures, the pole can be operated at 400 KV with one converter only.

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To compare two converters in series against two in parallel per pole and assuming the staging of the project , the following can be said :

In parallel converters we have to deal with ground currents for the loss of one converter.The controls for parallel groups are more complicated than series groups.In parallel converters it takes longer to isolate a faulted converter using the de-paralleling sequence.



.With parallel converters we will be at 800 KV right away compared to operating at lower voltage in the series case.This means in the parallel option the losses on the transmission line are lower during the first stage for the same power transmitted because of the lower currentDuring DC line faults we can not reduce the voltage to half in the parallel option compared to the series converters but we can reduce it to some value.

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PHASE 2DELTA

QUADRI

PHASE 1DELTA

PHASE 1STAR

VALVE 1

BUSHING

800 kV BUS

800 kV WALL

VALVE VALVE

PHASE

STAR 2

PHASE 3DELTA

2 3

PHASE 3STAR

400 kV WALLBUSHING

400 kV BUS

QUADRIQUADRI



800 kV BUS

DOUBLEVALVE

1

800 kV WALLBUSHING

DOUBLEVALVE

2

DELTA BRIDGE

DOUBLEVALVE 3

DOUBLEVALVE

400 kV WALL

DOUBLEVALVE

2

400 kV BUS

STAR BRIDGE

DOUBLEVALVE

1 3

BUSHING

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Converter Stations at 800 KV

Designing and constructing a converter station at 800 KV is impacted by the following equipment and factors:Converter TransformersExternal InsulationHVDC Thyristor valvesExternal Measuring Devices


Converter Stations at 800 KV

Switching Devices ( Disconnects)Mitigation of Audible Noise from EquipmentConfiguration and Layout of the stationSize of the major equipment

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Converter Transformers

One of the most critical components in an HVDC station are the converter transformers. Past experience with converter transformer performance and reliability has been adequate. Although there have been failures of converter transformers, the conclusions from theCigre WG that looked into these failures were as follows:



The failures are concentrated in a small number of projects and since the design of the transformers in one projects is the same then obviously if there is a design problem it will appear in all unitsThe challenge is how to prevent these failures in the future. The WG recommended the tightening of the specifications, design review, and thorough testing.

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For HVDC converter transformers for 800 KV dc the following should be considered:Currently IEC 61378-2 is accepted as the standard for both the routine and type test requirements on converter transformers as well as the acceptance criteria



IEC 61378-2 should be examined again with respect to certain issues:The sequence of the dielectric testsMore frequent gas in oil analysis during testingThe dc polarity reversal tests on the valve winding

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IEC 61378-2 should be examined again with respect to certain issues:PD measurements during testing and the allowable levelThe AC applied voltage tests on the valve winding with PD measurements



The top bridge 800 KV dc converter transformer valve winding insulation levels based on a single phase unit as discussed in Cigre WG14.32 report are:VW Lightning impulse level chopped to ground 1990 KVVW Lightning impulse level to ground 1990 KVVW switching impulse level to ground 1891 KVVW Lightning impulse level across winding 1050 KVVW switching impulse level across winding 803 KVVW 60 minutes dc withstand 1223 KVVW polarity reversal withstand 834 KV

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One of the critical areas in the converter transformer design is the insulation barriers around the oil end of the valve winding bushings. At 800 KV dc proper design review of this area is recommended.


External InsulationThe performance of external insulation in HVDC converter stations is

recognized to be a critical factor in determining the reliability of the transmission. Of all the different types of insulators used in a converter station wall bushings have been specially prone to flashovers. Many of the flashovers on wall bushings have occurred in conditions during which flashovers would not have been anticipated on the basis of contamination level or the degree ofwetting that can cause a flashover. Other than wall bushings other insulators such as transformer bushings, reactor bushings, measuring devices housings and to a much lesser degree post insulators have also experienced flashovers.

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External InsulationThe mitigation of the current flashover problems in existing HVDC

systems:Continuous cleaning and washing of porcelain insulators, bushings, housings of measuring devicesThe application of silicone grease to porcelainsThe application of RTV coatings to porcelainsThe use of booster shedsThe use of composite bushings with silicone rubber housingsThe use of silicone rubber housings for measuring devicesThe elimination of the wall bushings on the valve winding side by using the transformer bushings through the wall. This reduces the number of wall bushings to only one on the dc side on the high voltage side.The use of indoor dc switch yards, we still need one bushing.


External InsulationFor 800 KV converter station, it is essential to reduce as much as possible the number of wall bushings and in polluted areas the use of indoor dc switch yards may be the answer.The reduction in the number of bushings can be accomplished by eliminating the bushings on the valve winding side and using thetransformer bushings through the wall. Although this will impact the size of the building specially for series connected valve groups and when single phase two winding transformers are used.For the dc side bushing the use of composite silicone rubber type bushing is recommended. These types of bushings have been in use in several HVDC systems and have proved to have a higher reliability from aflashover point of view compared to porcelain bushings. A composite bushing with silicon rubber sheds rated for 600 KV dc was tested at IREQ in the early 90sType testing of a composite bushing with silicone rubber housing for 800KV should be performed under both unequal wetting and pollution conditions.

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External InsulationFor measuring devices, voltage dividers and dc current measuringshunts, it is also recommended to use silicone rubber housings.As far as the length for the outdoor insulators it will be determined by the requirements for of the specific creepage distance not from the air clearance point of view.For indoor equipment there is no change necessary from the classical 19mm/KV currently used. The approximate length will be5 meters for 800 KV and therefore the length of the insulators will be determined by the air clearance required by the switching surge.It seems that for 800 KV the answer for less than clean environment conditions, the answer for the dc side yard is indoors.


Thyristor ValvesThe design of the thyristor valves for 800 KV is not going to be any different than the current design, meaning it will be still water cooled and air insulated. It will probably be still modular in design.The actual determination of whether to use a quadrivalve design or other wise is going to be determined by the type of transformers used,for example single phase two windings or single phase three windings, the economic size of the valve hall as far as height ,width and clearances. The choice of one or two valve groups in series will impact the dimensions of the valves. The impact of EMI should be consideredIn the case of seismic requirements the increase in the height and weight of a quadrivalve design has to be taken into account.Although with proper design and grading the voltage stresses per level are the same as in the current designs, still the top of the valve group is at 800 KV and proper grading to ground is required.

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Testing

The issues associated with testing are:Test levels.The availability of testing facilities for the higher test levels as well as the larger size of the 800 KV equipment.Careful consideration during in particular the valve testing the representation of the surroundings of the valve.

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