day 2-a-electrical characteristics of transmission line
TRANSCRIPT
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Electrical Characteristics of Transmission Lines and
Cables
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Overview• The purpose of the transmission lines are used
– to connect electric power sources to electric power loads– to interconnect neighboring power systems
• Since transmission line power losses are proportional to the square (VL2)of the load current, therefore high voltages are used to minimize losses and voltage drop.
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Voltage level of the transmission system• High-voltage transmission lines or cables for long-
distance bulk power transfers. • Standard voltage levels include particularly 380 KV
Other standard levels are 230, 132 and 110 kV.• Medium and low-voltage lines and cables are used
for transmission over short distances and distribution circuits. Standard levels are 66,33, 24,13.8 and 11 kV.
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Transmission Line Structures• Overhead transmission lines are supported by
towers that are typically built of either wood or steel• Transmission line tower design is governed by many
factors such as:– Voltage level– Conductor size– Minimum clearance
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Transmission System Characteristics• Transmission line shunt capacitance (charging)
produces reactive power proportional to the square of the voltage
• Since transmission line reactive power varies over the load cycle, we can state:– Transmission line production = V2B (relatively constant)– Transmission line consumption = I2X (variable)– Line shunt susceptance, B = C– Line series reactance, X = L
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Surge Impedance Loading (SIL)• We are often interested in the loading where
production equals consumption• For an incremental length of line of reactance x and
susceptance b, we set V2b = I2x, and solve for the surge impedance:– Z0 = V/I = √(x/b) = √(l/c)
• Then surge impedance loading:– P0 = V2/Z0
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Transmission line parameters• Most important parameters are:
– Series resistance and reactance– Shunt susceptance
• Series resistance affects of:– Losses– Loadability (thermal and sag limits)
• Resistance can be ignored for high voltage lines
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Transmission line parameters• An equation for inductive reactance is:
– x = l = 2 10-4 ln (GMD/GMR) Ω/km– Where:
- power system radian frequency– GMD – geometric mean distance between phases:
GMD = (dab + dac + dbc)1/3
– GMR – geometric mean radius (obtained from conductor tables), GMR 0.8r where r is the conductor radius
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Transmission line parameters• For bundled conductors (several subconductor per
phase) with spacing s between adjacent subconductors, the equivalent GMR is: – GMRequiv = [n x GMR[s/(2sin/n)]n-1]1/n
• For two and three conductor bundles, the equivalent GMRs are: – Two conductor, √(s x GMR)– Three conductor, 3√(s2 x GMR)
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Transmission line parameters• Reducing the reactance by reduce the phase
spacing (GMD) and/or increase the equivalent GMR• GMRequiv is reduced mainly by increasing the number
of subconductors
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Transmission line parameters• A corresponding equation for shunt
susceptance is:– b = c = 10-6/[18ln(GMD/r)] S/km (siemens/km)
• For bundled conductors:– requiv = [n x r [s/(2sin/n)]n-1]1/n
• The charging reactive power is:– Qchg = V2b
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Transmission line parameters• Reduced phase spacing and bundled conductors
reduce line inductance and reactance, and increase line capacitance and susceptance.
• This increases the surge impedance loading and effective transmission capability
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Cables• Cable parameters are very different• Close spacing, inductive reactance is lower and
capacitance is higher• For example a 380 kV cable has:
– Inductive reactance – 0.09-0.16 Ω/km– Charging reactive power – 13 MVAr/km
• Caused by high charging power, a key parameter of cables is the critical length
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Cables• Critical Length
– The length at which the charging power equals the cable thermal capacity
• For Extra High Voltage (EHV) cables, the critical length around 25km
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End of Presentation