Introduction to Substation Design TADP 542

Transcription

1 Introduction to Substation Design TADP 542 Substation Structures - Design Loads Instructor: Dr. Yenumula Prasad Transmission & Distribution Program

2 Substation Structures- Design Loads References Substation Structure Design Guide (2008), ASCE Manuals and Reports on Engineering Practice No. 113, American Society of Civil Engineers NESC (2007), National Electric Safety Code, The Institute of Electrical and Electronics Engineers, NY

3 Substation Structures - Design Loads Development of design loads Analysis Design

4 Substation Structures- Design Loads Dead Loads Equipment Operating Loads Terminal Connection Loads for Electrical Equipment Extreme Wind Loads Extreme Ice and Concurrent Wind Loads Earthquake Loads Short Circuit Forces Wire Tension Loads Construction and Maintenance Loads

5 Design Loads Dead Loads Weight of structure Weight of supporting equipment Weight of accessories

6 Design Loads Equipment Operating Loads Dynamic loads created during operation of equipment Eg. Switches and circuit interrupting devices Equipment manufacturer provide loading These loads shall be combined with other load cases

7 Design Loads Terminal Connection Loads for Electrical Equipment Forces and moments created by Jumpers Taps Jumpers and taps shall be designed using same load cases used for strain bus Hardware of jumpers and taps create additional moments

8 Extreme Wind Loads ASCE-7 (Minimum Design Loads for Buildings and Other Structures) provides guidance on extreme wind loading. NESC-2007 adopted the same guidelines Wind force (F in pounds) = (V) 2 (k z )(G RF ) (I) (C f )

9 Extreme Wind Loads Basic wind speed (V) 3-second gust wind speed in mph At 33 ft above ground In flat and open country terrain (Exposure Category C) 50-year return period Wind maps provided in ASCE-7 and NESC Section-25

10 Extreme Wind Loads Terrain Exposure Coefficient (k Z ) Terrain and height effects Exposure categories B (urban and suburban areas) C (Flat & open terrain) D (Flat and unobstructed coastal areas) With height, wind force increases Category B provides minimum wind force and Category D provides maximum wind force

11 Extreme Wind Loads Gust response factor (G RF ) Account for dynamic effects of gust on wind response of substation structures Importance factor (I FW ) Adjusts level of structural reliability = 1.0 for utility structures for NESC Can be greater than 1 for important substation structures

12 Extreme Wind Loads Force Coefficient (C f ) Ratio of resulting wind force per unit area in the direction of wind to applied wind pressure Drag coefficient = 1.0 for round objects such as conductors and cylindrical structures such as poles

13 Extreme Ice and Concurrent Wind Loads ASCE-7 and NESC provide ice maps Extreme ice with concurrent 3-sec gust wind Ice is glaze 50-year return period

14 Earthquake Loads Seismic or earthquake loads Calculated based on governing building code or IEEE Standard 693

15 Short Circuit Forces Short Circuit Forces Short-circuit currents produce electromagnetic fields which produce forces on bus Short circuit forces depend on Conductor spacing Magnitude of short-circuit current Type of short-circuit Degree of short-circuit asymmetry Support flexibility Corner and end effects IEEE Guide for the design of substation rigid bus structures provide an equation

16 Wire Tension Loads Tension loads resulting from phase conductors and shield wires under extreme weather events such as extreme wind extreme ice and concurrent wind Wires attached to the station deadends and strain bus Station deadends Full tension deadends Slack-span deadend

17 Loading Combinations & Load Factors Load factors introduced to account for uncertainty in loadings especially weather related events Electrical substations are unique Substations uninhabited, low hazard to human life in the event of failure Load factors differ from those suggested for design of buildings

18 Loading Combinations & Load Factors Unlikely that two extreme events such as earthquake & high wind and earthquake & extreme ice would occur simultaneously Short circuit and operating loads may occur with high winds or earthquakes Many utilities use their own combinations and factors ASCE manual 113 provided combinations of load cases and load factors for the design of substation structures

19 NESC Load Cases Applicable to US Electric Utilities Applicable to Station deadends which support wires that extend outside substation yard Governed by sections 24, 25, and 26 of NESC (2007) code NESC specified three weather load cases: Combined ice and wind district loading (Rule 250 B) Extreme wind loading (Rule 250 C) Extreme ice with concurrent wind loading (Rule 250 D)

20 NESC Weather Load Cases

21 NESC Load Cases NESC recommend load factors and strength factors for different grades of construction For transmission lines, Grade B recommended The load factors for district loads (rule 250B loads) are more than unity. Load factor for vertical load = 1.5, load factor for transverse load due to wind = 2.5 and load factor for transverse load due to wire tension = 1.65 Load factors both extreme wind (rule 250C loads) and extreme ice with concurrent wind (rule 250D loads) are unity.

22 Reliability vs. Weather Loading Minimum extreme wind loading & extreme ice with concurrent wind loading - 50 year return periods The probability of an event having a return period T to occur in a given year is equal to 1/T For example, if a structure is designed for a 50-year return period, the probability of load exceedence in any one year is 1/50 = 2%. Over a 50-yr life span, the probability of load exceedence increases to 64% and over a 100-yr lifespan, it increases to 87%.

23 Reliability vs. Weather Loading Select return period of line (i.e. reliability) based on importance of substation The factors to convert 50-year load to higher return periods are presented in the Reliability based design of utility pole structures, ASCE, 2006.

24 Wind Load Factors Source: Reliability based design of utility pole structures, ASCE, 2006.

25 Ice Thickness Factors Source: Reliability based design of utility pole structures, ASCE, 2006.

26 Illustrated Example If the basic extreme wind speed (50-year return period) for a transmission line located in IOWA state is 90 mph, determine wind speed for 100, 200 and 400 year return periods. Solution: 100 year return period basic wind speed = 90 mph x 1.07 = 96.3 mph 200 year return period basic wind speed = 90 mph x 1.14 = mph 400 year return period basic wind speed = 90 mph x 1.20 = mph

27 Illustrated Example If the radial ice thickness (50-year return period) for a transmission line located in New York state is 1 inch with concurrent 40 mph wind speed. Determine ice thickness for 100, 200 and 400 year return periods. 100 year return period ice thickness = 1 inch x 1.25 = 1.25 inches 200 year return period ice thickness = 1 inch x 1.50 = 1.50 inches 400 year return period ice thickness = 1 inch x 1.85 = 1.85 inches Note: The associated wind speed of 40 mph remains the same for all return periods.

28 Loads for Deflection Analysis Structure deflections critical in addition to structure stresses ASCE manual 113 recommends minimum loads 5-year mean recurrence interval peak gust wind speed 5-year mean recurrence interval peak ice thickness Higher loads shall be considered without any load factors