800 KV HVDC for Transmission of Large Amount of Power Over Very Long Distances

2006 International Conference on Power System Technology

800 KVHVDC

FOR TRANSMISSION OF LARGE AMOUNT OF POWER OVER VERYLONG DISTANCES

Gunnar Asplund, Urban Astrdm and Victor Lescale

I. INTRODUCTION

In Asia and several other places in the world there is a growing interest to exploit distant hydro resources as a means tosupply electric power to the fast growing economies. More than 10000 MW over distances of 2000- 3000 km are typicalfor these applications. As most of the power should be transmitted from a distant generating area to a consumer area HVDCis normally more economical than HVAC. As the amount of power is so big and the distances so long the best economywill be when the voltage is as high as is technically possible. Until today the highest voltage ofHVDC in use is 600 kV, butas will be demonstrated in this paper it is now possible to build HVDC with a voltage of 800 kV.In China large hydropower resources are available in the Western part of the country and the power will be transmitted tothe industrialized regions in the Eastern and Southern areas of China.In India transfer of the hydropower generated at the Bramaputra River Basin in the North- Eastern part of India will have tobe transmitted to the southern part of the country where the power is needed.In Africa there is a great potential for power production at the basin of the Congo River near the location of Inga. Parts ofthe power is planned to be transmitted to South Africa.In Brazil vast hydropower resources are located in the Amazon region, while the power consumer centers are located alongthe eastern coast.In several investigations that have been carried out in the past, the common conclusion has been that for these big amountsof power and long distances the use of 800 kV HVDC is the most economical solution. [1], [2].In order to meet the requirements from the market, ABB is at present concluding development of equipment for 800 kVHVDC.

II. ECONOMY

The total cost for a HVDC transmission system is composed of the investment in converter stations and line and the capitalizedvalue of the losses. For a given power the cost for the stations increases with the voltage, while the line has a minimumcombined cost at a certain voltage.

1-4244-0111-9/06/$20.00c02006 IEEE.

Authorized licensed use limited to: UNIVERSIDADE DE SAO PAULO. Downloaded on March 15,2010 at 12:09:52 EDT from IEEE Xplore. Restrictions apply.

2

11000

10000 I N Power 12000 MWLine length 2000 km

9000 800 kV AC 8 lines

8000 -0500 kVDC 4 lines700a___ 800 kVDC 2 lines7000

--O- ~~~~~~~500kVAC 19 ines6000

5000

4000

30001 2 3 4 5 6 7 8 9 10

Percent line losses

Figure 1 Cost of stations, lines, compensation and losses as a function of line losses. Loss evaluation is 1400 USD/kW andseries compensation of the ac- lines is 70 percent.

A comparison of the total cost for transmitting 12000 MW over 2000 km at 500 kV AC, 800 kV AC, 500 kV DC and 800 kVDC has been done. The result is that the 800 kV DC is the most cost effective alternative because of a higher line capacity andlower line losses. The total cost for the 800 kV DC alternative is 20-25 % lower than for 500 kV DC and still much lower thanthe two AC alternatives.

III. SYSTEM ASPECTS

There are basically three alternatives to interconnect a distant generating network.

1. All by HVDC2. All HVAC3. Most by HVDC and some by HVAC

Alternative 1 gives the lowest cost alternative as has been shown above.The advantage by Alternative 2 is the possibility to feed power to many big intermediate loads. If however the intermediateloads are small Alternative 2 will be very uneconomical.Alternative 3 is the most economical alternative to both transmit big bulk power between two points and also supply manyintermediate loads with power. In this case there are some alternatives as shown below.


3

Alternative 1 Alternative 2 Alternative 3

Sendingnetwork

HVDCBack to back

Receivingnetwork

Figure 2 Alternatives of an ac connection parallel to the HVDC lines.

The first alternative shows a synchronous connection of the sending network. The ac connection will have to transmit allthe power generated that the HVDC does not transmit. As the ac- line is quite long this power is limited. It will then beimportant to let the dc lines transmit the power generated even at fault of one converter, pole or even bipole. This can bedone by the inherent overcapacity in the healthy lines. However, normally this overcapacity is limited in time why thepower balance is reached by reducing the power in the generators. In the case there is not sufficient overcapacity in theHVDC transmission over sufficient time generators might have to be tripped.

The second alternative is easier from a system point of view as the sending and receiving networks will not be synchronousand at loss of transmission capacity in the HVDC the frequency could be allowed to increase temporarily in the sendingnetwork. However, the flexibility to use the parallel ac is somewhat limited. Power cannot be transferred on the ac all theway.

Alternative three where the ac is interrupted in the middle and connected by an HVDC back to back will take care of thisproblem as now power can flow back and fourth in the ac interconnection. As the sending end and the receiving end are

asynchronous the behavior at loss ofHVDC transmission capability is as good as alternative 2.

IV. AVAILABILITY AND RELIABILITY

Transmission of 3000 - 6000 MW bulk power into heavy load-centers like for example Shanghai require very high reliability.For comparison the reliability requirements for the converter stations in the Three Gorges - Shanghai 3000MW transmission

are shown below in a table, together with the foreseen requirements for a new 6400MW transmission

Forced outage rates 3GS 6400MWSingle pole trips per year 5 4Bipolar trips per year 0.10 0.05

AvailabilityFEU 0.5%0 0.5%0


4

From the figures, it can be seen that serious improvements have to be made: Regarding single pole trips, the improvement fromfive to four would appear moderate, but the added complexity of the 800kV pole configuration speaks against a better figure.Regarding bipolar trips, the task is even harder: halving the outage rate that is state of the art requires radical improvements.One of the keywords is separation: between converter groups, and even more stringently, between poles. The two poles in eachstation are regarded as practically two stations that happen to be neighbors.

A. HVDC LinefaultsThe frequency of line faults is dependent on the length of the line. Bipolar faults can occur e.g. at tower failures or due to

icing and wind at extreme weather conditions, but are rare. The majority of the pole line faults are cleared easily within someperiods by retarding and restart. During the retard time the healthy pole compensates the power loss on the failing pole. At rareoccasions the line will stay tripped for longer periods, and will recover within a few hours. The time needed for dead linemaintenance will be added to the line unavailability.For some DC systems special arrangements have been done to increase the power availability. In the Inga-Shaba HVDC

project, the two converters in the bipole can be paralleled and the power can be transmitted on one pole line. Switching stationsalong the line allow for continued transmission even for simultaneous line faults on different segments along the line. For theItaipui HVDC project, with two bipoles, the converters can be connected in parallel to one bipole, in order to minimize the lossof power at bipole line outage.

1. CONVERTER CONFIGURATION

There are several possibilities to build a bipolar station for 800 kV with ratings from 3000 to 9000 MW. With higher ratingsmakes it necessary to have more than one converter group per pole. This will minimize the disturbances at faults and increase

Series connectedSingle twelve pulse twelve pulse groups Parallel twelve pulse groupsgroup in each pole per pole

_ K11~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~P

3000-

3000 4500 4500-6400 6000-9000

Fig. 3 Different bipolar arrangements for 800 kV HVDC

the reliability and availability of the transmission. Another reason for dividing into more groups is the transport restrictions (sizeand weight) of the converter transformers. A scheme with more than one group per pole is not a new concept, in fact it was usedin the mercury arc valve projects from the mid 60's where six pulse groups were connected in series to achieve the desiredvoltage. Each group had a by-pass breaker, should one mercury arc valve be out of order. The Itaipu + 600 kV HVDC project isthe only project with thyristor valves that has two groups per pole and the operation experience is excellent.The arrangement on the DC-yard will be almost the same as for the + 500 kV projects but with all equipment rated for + 800kV. The only "new" equipment is the by-pass arrangement with disconnectors and high-speed breakers for each group.


5

B. Control andprotectionA very important aspect has to do with ac system faults close to the inverter station: If an ac fault is close enough to the

station, it causes commutation failures in the converters. It is very essential that the converters will not block for such events,because if they do, the HVDC power will not be restored when the fault is cleared. The valves produced by the author's group

have a firing system capable of operation as soon as the ac system has enough voltage for the thyristors to start conducting, even

if the voltage was zero for a very long time before that, and the valve control system can resume operation in less than a

microsecond. This ensures that this requirement is fulfilled, and thus need no new considerations.The structure of the present control and protection system, is being revised, reflecting the different requirements on reliability

and availability and also the pole configuration. It is envisaged that, in the new control structure, the two poles will be totallyindependent and that the groups in each pole will have a minimum of interactions. Ideally, the bipole should be built as twoseparate monopoles. This should also be applied for the AC-yard configuration, with possibility to entirely disconnect the areas

that are needed for each separate pole.The philosophy of the transducers feeding the control and protection system is also being scrutinized, as is the routing of the

cables feeding signals in, and actions out.

C. Auxiliary systems

Station service power is being restructured, with proper separation between the associated poles and groups, and proper

management of incoming supplies via the circuit configurations and control and protection. The physical power cable routing isalso under scrutiny and rules are being defined.The valve cooling systems are also being provided with proper separation between poles and groups: one cooling system per

12-pulse group, and with attention against human errors.

In the fire protection systems the main areas of review have to do with ensuring secure yet reliable sensing, and with theactions the protective systems can cause, directly and secondarily.

V. EQUIPMENT DEVELOPMENT

A. GeneralIn this section a summary of the R&D status, early June 2006, of the different 800 kV HVDC apparatus is presented. Since the

main focus for 800 kV development has been on converter transformers, bushings and external insulation, also these issues are

in focus for this presentationThe equipment affected by the increased voltage level is of course limited to apparatus connected to the pole bus, such as

converter transformers, wall bushings, thyristor valves, DC-voltage divider etc. The main part of the equipment within theconverter station is not exposed by DC, such as AC yard apparatus, control and protection and auxiliary systems.

B. Test levelsFor 800kVDC stations, the basic ideas for insulation coordination are the same as those applied for lower voltages; i.e. to have

equipment with withstand characteristics above the expected stresses. Then, as is normal in medium or high voltage, theexpected stresses are controlled by a combination of arresters and shielding. The difference for 800kVDC is that it iseconomically beneficial to control the expected stresses to an even higher degree, and to revise the steps leading from theexpected stresses to the desirable insulation withstand; i.e. the insulation margins.Insulation coordination studies has been performed for the dc side of an 800kV HVDC transmission system, by different

institutions, including ABB. The data for the system has been assumed based on the best available estimates, with regard topreliminary design of the equipment expected for such an installation. Further, as the study progressed, it became apparent thatone fine adjustments to the configuration would yield significant benefits: Splitting the smoothing reactor function in two equalinductances, one at the neutral, and one at the pole.The different studies performed end up with very similar results, and the test levels used for design of the 800 kV equipment

are summarized below:

Test levels (kV)DC

Equipment SI LI ACrms DC Polarityreversal

Transformer 1518 1744 900 1250 970Valve sideTransformerbushing 1518 1744 900 1250 970Valve sideMultiple 1518 1800 NA 1040 NA


6

C. Station insulators

The subject of station insulators is covered in another paper in this conference. However, it has been found that all outdoorinsulation in the DC-yard, including post insulators for air core smoothing reactors, can be done by using composite insulators.This has been verified by seismic studies of the different apparatus. This means, that by utilizing the water repellant propertiesof composite insulators, the total height of the 800 kV insulators will be about the same as what is used for 500 kV porcelaininsulators.

D. Converter transformersA simplified transformer prototype has been manufactured, including all the insulation details for an 800 kV converter

transformer. The transformer prototype has been tested:* DC withstand 1250 kV* AC withstand 900 kV

The tests were successfully passed.

Fig 4. transformer prototype in the test laboratory

E. Transformer bushingA prototype of the transformer bushing for the highest 6-pulse group has been produced, fig. 4, and the initial testing done so

far is:

* DC withstand 1456 kV* AC withstand 1032 kV

The complete type and routine test of transformer bushing together with the transformer prototype is planned to be completedwithin short.

thyristor (3 hs)valve, top toground

1000Wall bushing 1518 1800 (one 1235 1030

minute)SmoothingreactorAcross NA 2160/n NA NA NATo earth 1546 1950 NA NA NA


7

Fig 5. Testing of transformer bushing

F. Wall bushingsThe design and manufacturing of the 800 kV wall bushing is completed, the bushing has passed the pressure testing and the

dielectric testing will start within short.

G. Miscellaneous pole equipmentThe status for the prototypes for the remaining equipment is summarized as:

* Pole arrester: Design and manufacturing completed, RI testing completed* RI capacitor: Design completed, manufacturing ongoing* Pole disconnector: design completed, development tests completed, manufacturing ongoing* Voltage divider: Design completed, manufacturing ongoing* By pass breaker: Design completed, manufacturing ongoing

The equipment as above will be delivered during the summer 2006 to be installed in a long term test circuit at STRI, Ludvika.

VI. LONG TERM TESTING

On order to verify the long term behavior of the 800 kV HVDC equipment, all relevant pieces of equipment will be installedin a long term test circuit, and energized at 855 kV DC, for at least half a year. The test circuit will include a "valve hall" wherethe temperature will be kept at 60° C, to simulate the actual operating conditions. The transformer bushing will protrude insidethe "valve hall" and be connected to the wall bushing that will be installed in the wall. The remaining equipment will beinstalled outdoors, together with the voltage generator and a prototype of the air core smoothing reactor. The layout for the testcircuit is given in fig 6.


8

Fig 6. Long term test circuit1. Transformer prototype2. Wall bushing3. Optical current transducer4. Voltage divider5. Pole arrester6. Smoothing reactor prototype7. RI Capacitor8. Disconnector9. Voltage divider, test equipment10. By pass breaker11. Voltage divider, test equipment12. Transformer, test equipment

The civil works for the test circuit is ongoing, and the test operation is planned to start during autumnm 2006

VII. STATION DESIGN

A. Valve hallsThe most decisive factor for the design of the valve hall is whether to use double valves or quadruple valves. Both options are

possible for 800 kV. Also, a new compact thyristor module suitable for 6" thyristors that is 5000 more compact has beendeveloped.In order to keep the transport dimensions within acceptable limits, single phase two winding transformers is the only realistic

alternative for a 6400 MW converter. A proposed valve hall arrangement utilizing quadruple valves is presented in fig. 5. Thislayout also gives very good separation between different poles and between converter groups, as is recommended due to thehigh reliability requirements.


9

Polarea

Fig 7. 800 kV converter station

B. Indoor DCyardIn areas with high pollution level, or in case there is a possible but uncertain future increase of pollution level, indoor DC yard

is an attractive alternative.

Fig 8. 800 kV converter station with indoor DC yard

The buildings for the DC yard will have the dimensions LxWxH -125x0x30 m.

VIII. CONCLUSIONS

800 kV HVDC is economically attractive for bulk power transmission of 6000 MW and even 9000 MW per bipole overdistances up to 3000 km. With the present progress of R&D converter equipment for 800 kV HVDC will be qualified withinshort. With proper separation and proper structure of the control and protection and auxiliary systems, the reliability andavailability will be as good as, or even better than, for converters at lower voltages.

IX. REFERENCES

[1] HVDC Converter Stations for Voltages Above 600 kV, EPRI EL-3892, Project 2115-4, Final report February 1985[2] HVDC Converter Stations for Voltages Above ±600 kV, Cigre' Working Group 14.32, December 2002[3] Power Transmission with HVDC at Voltages Above 600 kV, Urban Astrdm, Lars Weimers, Victor Lescale and GunnarAsplund, 2005 IEEE/PBS Transmission and Distribution Conference & Exhibition: Asia and Pacific August 14-18, 2005Dalian, China


10

X. BIOGRAPHIES

Gunnar Asplund was born in Stockholm, Sweden 1945. He got his MS in Electrical Engineering at the University ofLund in 1969 and is since 2005 Honorable Doctor of Technology at the Royal Institute of Technology in Sweden.

His employment experience is with ASEA and later ABB. He has worked in the fields of high voltage testing, thyristorvalve development, project management, commissioning of the Itaipu HVDC project in Brazil, later manager of systemstudies and engineering at ABB in Ludvika, Sweden. He is since thirteen years R&D manager of HVDC within ABB.

Urban Astrom was born in Njurunda, Sweden 1946. He received his M.Sc degree in physical engineering from the university of Uppsala, Sweden 1973. In 1974 he joined ABB 's HVDC department and has worked with design, development and testing of control equipment,thyristor valves, valve cooling and converter transformers. From 1995 to 2000 he was manager of theHVDC Converter Valve Development department, when he joined the Three Gorges- Changzhou project team as commissioningmanager. Since 2004 he has been manager for the 800 kV HVDC development project

Victor Lescale Victor F. Lescale was born in Mexico in 1944. Graduated as an Electrical Engineer from the University of Mexico 1966. Hehas more than 30 years of engineering experience, in, among other fields, protection relays and control, high and extra high voltageinstallation commissioning, power system planning, special projects, HVDC control, HVDC system design, and in international HVDCproject engineering and management.


800 KV HVDC for Transmission of Large Amount of Power Over Very Long Distances

Documents