This BASIC DESIGN report refers to an interconnecting transmission link between country A and Country B of maximum1200 km length, depending on the selection of the landing points. The power system analysis and the preliminary results of the economic/financial evaluation show preference for a solution which is fully or partially based on a long-distance HVDC link, therefore the design must address both types of transmission lines, namely a conventional double circuit 400 kV OHL and a +/- 500 kV bipolar line.

The Contractor’s scope of the work includes all materials, labour and services needed for the complete design, supply, testing and construction of the transmission line and shall include but not be limited to the following:

·         survey of determined and agreed route

·         right-of-way width determination

·         prepare sag/tension data for the conductors and OPGW

·         plan and profile production with located towers

·         preparation of tower schedule and material lists,

·         tower spotting, site survey and stacking

·         geotechnical Investigation

·         preliminary measurement of ground resistance

·         testing of construction materials

·         design, supply and installation of conductor and accessories

·         design, supply and installation of overhead shield-wire and accessories considering that one of the shield-wire shall be OPGW

·         design, supply and installation of OPGW repeater stations for inter-station communication

·         design, supply and erection of transmission line towers and accessories

·         design, supply and installation of transmission line tower foundations

·         design, supply and installation of insulator strings

·         design and installation of transmission line tower grounding

·         design, supply and installation of all kind of signs (including the aircraft warning lights and spheres), anti-climbing devices, bird guards and accessories

·         final inspection and testing

·         reinstatement and cleanup of site

·         remedying defects during the defects liability period of 12 months

·         as-built drawings and electronic records

 

The following items of work associated with the project do not form part of this contract for the overhead power line:

·         Acquisition of transmission line right-of-way (land owner compensation) and legal surveys.

·         Design and installation of substations including SS landing gantries.

Other project specifics, e.g. Import taxes and duties for all project materials and construction equipment including import taxes and duties among Country A and Country B, any fees by state-owned authorities of Country A and Country B related to the project execution, e.g. blasting, transport, crossing or telecommunication permission, system study and any expenses or entertainment of Client.

 

 

The 400 kV HVAC line on the Country B territory has a length of about 100 km, connecting xxxxx and xxxx 400 kV S/S. The line will be in a first stage operated at 220 kV, voltage level available at Ixxxx and xxxx S/S for the development of the sub-transmission network. The 400 kV HVAC line on the Country A territory from the hydropower plant xxxxx to xxxx is already in an tendering phase, so it is not part of the present design report. The line route is mainly situated in mountainous area, between 1000 and 2000 m a.s.l.

 

·         Line length                                                                       approx. 100 km

·         Altitude                                         1000~2000m

·         Nominal voltage                                                           400 kV AC

·         Transmission capacity                                            2000 MW(double circuit)

·         Rated lightning impulse withstand voltage                       1815 kV

·         temporary and switching over voltages         1128kV

·         Direct lightning stroke shielding                               0

·         Minimum creepage distance (pole to ground voltage)            25 mm/kV

The +/- 500 kV 1066 km long Bipolar HVDC Line from xxx to xxxx will be designed without metallic return. The ground electrode line will be designed in the engineering phases of the Project, since requiring intimate data on the converter stations.

 

·         Line length                                                                       approx. 1066 km

·         Altitude                                         800~2000m

·         Nominal voltage                                                           500 kV DC

·         Transmission capacity                                                     2000 MW

·         Rated lightning impulse withstand voltage                       1485 kV

·         temporary and switching over voltages         1050kV

·         Direct lightning stroke shielding                               0

·         Minimum creepage distance (pole to ground voltage)            43.25 mm/kV

 

Figure 2.1 �?Proposed line route

The main factors affecting the transmission line are wind, air pressure, temperature, solar radiation, isokeraunic levels and seismic disturbances.

 

(a) Rainfall

The average annual rainfall of the region is over 800 mm for a month.

(b) Temperatures

Mean max. temperature 36C Degree

Mean min. temperature 10C Degree

Minimum temperature -5C Degree

Maximum temperature 41C Degree

average annual temperature 20C Degree

Loading temperature 75ºC

Highest temperatures generally occur between December and February, lowest between June and August.

(c) Humidity

Mean relative humidity 60 to 90 %

(d) Isokeraunic Level

An isoceraunic level (storm) of 70 days/year shall be considered for design purposes.

(e) Maximum Solar Radiation

For design purposes a solar radiation value of 1.180 W/m2 shall be considered

(f) Earth quake loading

For design purposes an earthquake loading of 0.1 g shall be assumed.

(g) Wind load

On structures: 86 daN/m²

On conductor & earthwire 52 daN/m²

On single insulator 60 daN/m²

Maximum wind speed 103 km/h (28.8 m/s) at -5C Degree temperature.

 

 

All towers shall be self supporting lattice tower type as per the principle view given in Annex of this document. The following types of towers, body and leg extensions are assumed:

 

The towers will be double circuit. Vertical geometry is preferable.

Two full transpositions of the phases are required for the line within the route. The tension tower type, 0⁰ - 30⁰, shall be used as transposition tower.

The safety factors for tower design shall be as follows:

·         for  normal condition                                                        2.00

·         for broken wire condition                                            1.25

·         for maintenance and stringing condition                                    1.50

·         for tower dead weight                                                  1.00

 

For the towers the following materials are defined:

·         structural steel acc. to EN 10025:           mild steel          S235JR *)

                                                                    high tensile steel       S355J0 *)

·         bolt connection                                           acc. to DIN 7990 *)

·         bolts                                                     acc. to DIN 267 part 10: class min. 5.6

·         nuts                                                      acc. to EN 24034 or DIN 555 *)

·         plain washers                                             acc. to DIN 126 *)

·         spring washers                                           acc. to DIN 127 *)

*) Alternative Standards may be applied based on tower manufacturer.

 

Minimum design Criteria shall be as follow:

 

·         Stresses                                    Ultimate stresses are allowed in design

·         Buckling                                    In accordance to “ASCE 10: Guide for design of  

                                                  Steel Transmission Towers�?or similar

·         Tension on net section                     1.00 Fy

·         Bearing                                               1.80 Fy < Ultimate tensile strength

 

·         Stresses                                    Ultimate stresses are allowed in design

·         Shear                                         0.60 Fu

·         Bearing                                               1.00 Fu

Fy=Yield point of steel member materials;        Fu=Ultimate tensile stress of bolt material

 

The minimum diameter and number of bolt at each connection shall be as follow:

·         Diameter of bolt                                         16mm

·         Number of bolt for stressed members            2

Note:  When the computed stress is less than one-half of the capacity of a single bolt, a single bolt connection will be acceptable.

 

The ratio between unsupported length of a member and fie relevant radius of gyration, L/r, shall not exceed the following:

 

·         Main members                                           120

·         Bracings                                             200

·         Redundant members                                250

·         Tension members                                     350

 

Minimum thickness and size of members shall be as follow:

·         Leg and main compression members in

        crossarm and shield-wire peak                                               6 mm

·         Other members-stressed bracings                                     5 mm

·         Secondary members without stress                                  4 mm

·         Gusset plates                                                                5.5 mm

·         Equal angle section                                                     L 40x40xt mm

·         Unequal angle section                                                L 45x30xt mm

 

Maximum length of members shall not exceed eleven (11) meters.

All towers shall be self supporting lattice tower type as per the principle view given in Annex of this document. The following types of towers, body and leg extensions are assumed:

 

The safety factors for tower design shall be as follows:

·         for  normal condition                                                        2.00

·         for broken wire condition                                            1.25

·         for maintenance and stringing condition                                    1.50

·         for tower dead weight                                                  1.00

 

For the towers the following materials are defined:

·         structural steel acc. to EN 10025:           mild steel          S235JR *)

                                                                    high tensile steel       S355J0 *)

·         bolt connection                                           acc. to DIN 7990 *)

·         bolts                                                     acc. to DIN 267 part 10: class min. 5.6

·         nuts                                                      acc. to EN 24034 or DIN 555 *)

·         plain washers                                             acc. to DIN 126 *)

·         spring washers                                           acc. to DIN 127 *)

*) Alternative Standards may be applied based on tower manufacturer.

 

Minimum design Criteria shall be as follow:

 

·         Stresses                                    Ultimate stresses are allowed in design

·         Buckling                                    In accordance to “ASCE 10: Guide for design of  

                                                 Steel Transmission Towers�?or similar

·         Tension on net section                     1.00 Fy

·         Bearing                                               1.80 Fy < Ultimate tensile strength

 

·         Stresses                                    Ultimate stresses are allowed in design

·         Shear                                         0.60 Fu

·         Bearing                                               1.00 Fu

Fy=Yield point of steel member materials;   Fu=Ultimate tensile stress of bolt material

 

The minimum diameter and number of bolt at each connection shall be as follow:

·         Diameter of bolt                                         16mm

·         Number of bolt for stressed members            2

Note:  When the computed stress is less than one-half of the capacity of a single bolt, a single bolt connection will be acceptable.

 

The ratio between unsupported length of a member and fie relevant radius of gyration, L/r, shall not exceed the following:

 

·         Main members                                           120

·         Bracings                                             200

·         Redundant members                                250

·         Tension members                                     350

 

Minimum thickness and size of members shall be as follow:

·         Leg and main compression members in

        crossarm and shield-wire peak                                               6 mm

·         Other members-stressed bracings                                     5 mm

·         Secondary members without stress                                  4 mm

·         Gusset plates                                                                5.5 mm

·         Equal angle section                                                     L 40x40xt mm

·         Unequal angle section                                                L 45x30xt mm

 

Maximum length of members shall not exceed eleven (11) meters.

 

Sizes and Characteristics of conductors shall be as follow:

 

Maximum allowed stresses will be calculated considering relevant factors of safety based on the rated tensile strength of the conductor and shield-wire/OPGW:

                                                           conductor                 shield-wire/OPGW

·         EDS                                    20%                       20%

·         Max. tension                    4                         5

 

Conductor’s clearances, swings and distances live metal to earthed metal shall be as follow:

HVAC

·         Min. clearance from Lightning and Switching impulses                   3400 mm

HVDC

·         Min. clearance from Lightning and Switching impulses                   5370 mm

·         Min. clearance in case of maximum wind velocity                       1800 mm

 

 

Clearances to obstacles shall be as follow:

Characteristics of Insulator Strings

 

 

 

Safety Factors for Insulators and Fittings shall be as follow:

 

·         normal condition            3.12        

·         exceptional loads           1.56          

. All foundations shall be designed to withstand uplift, settlement and overturning, when subjected to the applied system loading. Foundation design criteria shall be as per the table below:

Table 7.1 Foundation types

 

·         Maximum allowable foundation depth below ground level                       6.00 m

·         Minimum allowable foundation depth below ground level                        3.50 m

·         Allowable settlement of each footing under

simultaneously applied working load                                          30 mm

·         Allowable uplift of each footing under

simultaneous applied working load                                                      30 mm

 

The foundations shall be concrete foundations. The following types are recommended to be used:

·         Soil class                                   Foundation type

·         Class 1                        Anchor, Auger

·         Class 2                        Reinforced Concrete Pad and Chimney, Earth Grillage

·         Class 3                          Reinforced Concrete Pad and Chimney

·         Class 4                          Reinforced Concrete Pad and Chimney

·         Class 5                          Reinforced Concrete Pad and Chimney

Special foundations shall be used for special soil conditions or for other special circumstances, for example raft, pile or anchor foundation.

 

The effective types shall be established in accordance with the recommendations of the geological test report performed by the Contractor.

Normally, to reduce huge earth works, leg extensions shall be used.

Both solutions, namely with and without leg extensions, shall be specified, the selected design being subject of Engineer's approval.

Adequate access roads for the approach of each tower shall be constructed for erection and maintenance purposes.

 

        

 

                                     Figure 8.1  HVAC Towers

Figure 8.2  HVDC Towers

 

 

 

 

 

 

 

 

I.          General information of the Converter

The xxx-Country B Power Systems Interconnection Project is from xxxx Converter Station in Country A xxx to xxxx Converter Station in Country B side.

Current transmission capacity�?x500MW

Current rated d.c. voltage�?00kV

Current rated d.c. current�?000A

HVDC Power-line distance: 1066km

II       Main electrical circuit

1.       Converter valve:

Converter valve: one group of 12 pulsating converter and thyristor-valves for per pole，and two groups for one converter station.

Converter transformer: single-phase three-winding.

2.       DC switchyard connection:

The HVDC switchyard incorporates all equipment required for conversion, operation, protection and control of the DC power. It is the typical layout for The HVDC switchyard.

3.      AC switchyard connection

xxxx Converter Station (Country A): For the 400 kV outdoor switchyard, a 1½ breaker configuration was proposed.

xxx Converter Station (Country B): For the 400 kV/220kV outdoor switchyard, a 1½ breaker configuration was proposed. For the 35kV outdoor switchyard ,a sectionalized single busbar was proposed.

4.      AC filter/shunt filter

AC filters/shunt filters:2 groups for the station,4 filters for one groups single-bus.

III      Main equipment specification

1.      Short circuit current:

Short-circuit current for HVAC: 40kA  (proposed)

Short-circuit current for HVDC: 24kA   (proposed)

2.      Converter valve

The recommended configurations for the valve: one group of 12 pulsating converter and thyristor-valves for per pole，and two groups for one converter station. It’s proposed that size of 4-inches for each thyristor at rated current 1kA, quadrivalve in suspension structure, indoor type and air-insulated plus water cooling.

3.       Converter transformer

The recommended configurations for The converter transformers: Single-phase three-winding. 420/�?kV  (The voltage of  the converter transformers�?a.c. side)�?00MVA (single transformer’s rated capability).

Transformers of each pole of each station, a total of 6 units, additional a separate one spare for one converter station.

4.                        Smoothing reactor

The recommended configurations for The smoothing reactors: 2000A(rated d.c.), 3 reactors in series connected to the pole DC-bus for the per pole per station .100mH per reactor.

3 reactors of each pole of each station. a total of 6 units, additional a separate one spare for one converter station.

5.                        The main equipments of Switchyard

The equipments of HVDC Switchyard: outdoor

The main equipments of HVAC Switchyard:outdoor

1.       The layout of Valve hall and converter transformer area

It’s adopted that One group of 12 pulsating converter and thyristor -valves for per pole，Quadrivalve in suspension structure .and Single -phase three-winding for The converter transformers.3 transformers array on the straight line，and located next to the AC line side of the valve hall . 12 bushings inserts into the valve hall and each 12-pulse group is connected to a separate secondary winding of the converter transformer. Secondary winding connections in delta and star arrangements provide for a 30°phase shift.

2.       HVDC Switchyard

Outdoor, suspension bus

3.       HVAC Switchyard

HVAC 400kV Switchyard: outdoor, suspension bus, 1½ breaker.

HVAC 220kV Switchyard: outdoor, suspension bus, 1½ breaker.

HVAC 35kV Switchyard: outdoor, support bus,1½ breaker, sectionalized single busbar ,arrayed on the straight line.

4.       HVAC Filters Switchyard

outdoor, suspension bus,arrayed on the straight line for the filters.