1. BRIEF ON THE REPORT OF CIGRE WG B4-45 –
TECHNOLOGICAL ASSESSMENT OF 800KV HVDC APPLICATIONS
(Authors: R N Nayak, Mohammed Rashwan, R P Sasmal)
R N NAYAK
UHV Symposium Delhi, Jan 2009

2. PRESENTATION SUMMARY Background: Formation of WG
Converter Configuration
Ground electrode station
Insulation Co-ordination
External Insulation
Reliability and Availability
Interference Levels
Conclusion 2

3. Background
WG14.32 formed to review the current state of HVDC converter stations up to 600kV and requirement to expand the technology to voltages above 600kV and specifically to 800kV.
Several studies and meetings confirmed 800 kV HVDC transmissions - a feasible voltage step (IEEE and Cigré in the late 80’s, Cigré 2002, Power Grid Corporation of India Ltd. Workshop in Delhi, February 2005)
New WG B4-45 formed in 2005 for “ Technological Assessment of 800kV HVDC Applications “ 3

4. Background The main driving forces for 800 kV HVDC systems:
Cost of power losses on overhead lines.
Need for Bulk power evacuation over very long distances.
Technological constraints of other EHV options.
Right of way constraints.
Techno-economic drive necessitates development of 800 kV HVDC 4

5. Converter Configuration
Decided by Utilities / planners
The amount of power to be transmitted
The transmission distance
Staging consideration of the project
Location of converter station
The amount of power to be transmitted at the different stages of the project
Reliability and availability requirements
Loss evaluation
Size and weight of the converter transformers for transport 5

6. Converter Configuration
3,000 MW
3,750 A
400 kV
800 kV
800 k V
1,875 A
Possible 800 kV Arrangements- Series and Parallel
Two similar rating parallel 12 pulse converters per pole
Two dissimilar rating parallel 12 pulse converters per pole.
One single 12 pulse converter per pole
Two similar MW rating series connected 12 pulse converters per pole
Two dissimilar MW rating series connected 12 pulse converters per pole
Converter Configuration 6

7. Converter Configuration
Series converters
The advantages/disadvantages
one of the valve groups insulated for 400 kV only;
only one among four transformer groups have full insulation at 800 kV
In case, one 12 – pulse bridge fails,
half of the pole power can still be transmitted (no ground return),
voltage level will be half of nominal
losses would be high;
No practical staging scheme; installation at full power done at once;
4 spare units needed per station unless provisions made for 600 KV and 800 KV units to fit in the space of 200 KV and 400 KV units
3,000 MW
3,750 A
800 kV
600 kV
400 kV
200 kV
12 x 300 MVA 7

8. Converter Configuration Parallel converters
Advantages / disadvantages:
loss of a converter means still the operation at 800 KV; with unbalanced ground current
Metallic return can not be used unless the same polarity parallel converter is removed from service
The staging in power, possible by installing one 12 – pulse bridge for each pole, later on, a second one
Possible to make rectifiers and inverters at different location i.e Multi-terminal stations.
2 spare units required, as minimum, per station;
3,000 MW
800 kV 8

9. Ground Electrode Earth Electrode station Design Criteria
Life expectancy : 50 yrs
Unbalance current during normal operation
Pole outage condition
Due consideration for parallel /multi-terminal operation
Possibility of Metallic return to avoid ground current
Selection of Suitable site
Close to HVDC terminal to reduce cost
Soil resistivity upto the depth of 10 kms. 9

10. Magnetotelluric measurement
Ground Electrode
Magnetotelluric measurement
Natural sources located in the magnetosphere and ionosphere,
Earth being conducting natural sources, induce secondary fields in the earth.
Vector nature of electromagnetic fields enables to estimate the tensor form of the resistivity structure by measuring five components time series data consisting of three magnetic (Hx, Hy , Hz ) and two electric (Ex, Ey ) components.
Magnetotelluric (MT) measurement based on natural electromagnetic (EM) fields & it delineate the electrical structure of the earth
Natural EM fields contain a wide spectrum of signals
Deeper resistivity information by recording low frequency content of the signal for a longer duration of MT time series recording 10

11. Typical result of Soil Resistivity
Ground Electrode
Typical result of Soil Resistivity 11

12. Insulation Co-ordination
Arrester arrangement for series Converters
neutral
800 kV DC D 10 E1 V 3 V2 72 V1 92
valve hall boundary C 2 1 71 8
AC Bus - A 52 62 51 61 91 A2 E2
400 kV DC 82 M SR
Converter transformer arrester “A2”
Converter group arrester type “C1” and “C2”
Mid point arrester type “M”
Smoothing arrester type “SR”
AC Bus
Provide higher safety and reliability to the equipment. 12

13. Insulation Co-ordination
Arrester arrangement for parallel Converters
Converter transformer arrester “A2”
Converter group arrester type “C”
Mid point arrester type “M”
Smoothing arrester type “SR” E1 9
valve hall boundary V2 7 V1 C AC -
Bus 1 A 5 6 A2 81 M
800 kV DC D 10 SR
neutral E2 82
Provide higher safety and reliability to the equipment. 13

14. Insulation Co-ordination
Insulation levels depends upon:
Particular layout,
arrester arrangement
arrester data
system parameters
Typical Minimum Insulation Values:-
SIWL
kVpk
1600
LIWL
1900
DC withstand test voltage kV
1200
Polarity reversal test voltage
1020

15. External Insulation External insulation Creepage distance
Pollution level
Surface material of insulators or equipment housing
Shed profile
Corrections for Altitude
Converter station equipment: The conditions well defined
Transmission line: conditions vary along the route ( 2000 – 3000 km) as the line pass through all kinds of terrain, including polluted areas and high altitudes > 1000 m. 15

16. Availability and Reliability
Established procedure of calculation of availability and reliability for HVDC projects already established and being monitored and reported Worldwide
Characteristics of a reliable HVDC project:
long continuous operations without fault
being fault tolerant, that is, being able to recover from faults quickly
only partial and acceptable loss on major faults
well integration in the AC system. 16

17. Availability and Reliability
Typical forced outage rate used as design base in the recent HVDC projects:
Pole outage : 5 – 6 per year per pole
Bipole outage: 0.1 per year
Possible target values with series valve group :
12 pulse bridge (Converter) outage: per year per converter
Pole outage : 2 – 3 per year per pole
Bipole outage: 0.1 per year or less
Possible target parallel valve group converters : 17

18. Interference Levels FOR HVDC TRANSMISSION LINE
Electric Field : 25kv/m – 30 kV/m
Ion current Density : 100 na/ m2
Minimum conductor height: 18 / 20 mtrs
Magnetic Field limit
Occupational exposure: 200 mT
Public Exposure: 40 mT
FOR HVDC TERMINAL
Electric Field : 30kv/m
Ion current Density : 100 na/ m2

19. Conclusion
Availability and reliability of Large HVDC system plays a major role in system stability, Needs proper planning for converter configuration
Experience gained from the initial 800 kV HVDC projects must be suitable incorporated in future projects
R & D activities must be continued to reduce the overall cost of the HVDC systems
Converter transformer design, wall bushing and external insulations needs special care during design. 19

20. THANK YOU 20