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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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: 2 - 2.5 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

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

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

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