lecture 6 development of transmission line models professor tom overbye department of electrical and...
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Lecture 6Development of Transmission Line Models
Professor Tom OverbyeDepartment of Electrical and
Computer Engineering
ECE 476
POWER SYSTEM ANALYSIS
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2
Reading
For lectures 5 through 7 please be reading Chapter 4– we will not be covering sections 4.7, 4.11, and 4.12 in
detail
HW 2 is due now HW 3 is 4.8, 4.9, 4.23, 4.25 (assume Cardinal
conductors; temperature is just used for the current rating) is due Thursday
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Bundle Inductance Example
0.25 M0.25 M
0.25 M
Consider the previous example of the three phasessymmetrically spaced 5 meters apart using wire with a radius of r = 1.24 cm. Except now assumeeach phase has 4 conductors in a square bundle,spaced 0.25 meters apart. What is the new inductanceper meter?
2 3
13 4
b
70a
1.24 10 m ' 9.67 10 m
R 9.67 10 0.25 0.25 2 0.25
0.12 m (ten times bigger!)
5L ln 7.46 10 H/m
2 0.12
r r
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Transmission Tower Configurations
The problem with the line analysis we’ve done so far is we have assumed a symmetrical tower configuration. Such a tower figuration is seldom practical.
Typical Transmission Tower Configuration
Therefore ingeneral Dab Dac Dbc
Unless something was done this wouldresult in unbalancedphases
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Transmission Tower Examples
Source: Tom Ernst, Minnesota Power
230 kV lattice steel tower
double circuit
230 kV wood pole H-frame
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Transposition
To keep system balanced, over the length of a transmission line the conductors are rotated so each phase occupies each position on tower for an equal distance. This is known as transposition.
Aerial or side view of conductor positions over the lengthof the transmission line.
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Line Transposition Example
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Line Transposition Example
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Inductance of Transposed Line
13
m 12 13 23
0 0a
70
Define the geometric mean distance (GMD)
D
Then for a balanced 3 system ( - - )
1 1ln ln ln
2 ' 2 '
Hence
ln 2 10 ln H/m2 ' '
a b c
ma a a
m
m ma
d d d
I I I
DI I I
r D r
D DL
r r
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Inductance with Bundling
b
0a
70
If the line is bundled with a geometric mean
radius, R , then
ln2
ln 2 10 ln H/m2
ma
b
m ma
b b
DI
R
D DL
R R
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Inductance Example
Calculate the per phase inductance and reactance of a balanced 3, 60 Hz, line with horizontal phase spacing of 10m using three conductor bundling with a spacing between conductors in the bundle of 0.3m. Assume the line is uniformly transposed and the conductors have a 1cm radius.
Answer: Dm = 12.6 m, Rb= 0.0889 m Inductance = 9.9 x 10-7 H/m, Reactance = 0.6 /Mile
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Review of Electric Fields
eA
2
To develop a model for line capacitance we
first need to review some electric field concepts.
Gauss's law:
d = q (integrate over closed surface)
where
= electric flux density, coulombs/m
d = differential
D a
D
a
2
e
area da, with normal to surface
A = total closed surface area, m
q = total charge in coulombs enclosed
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Gauss’s Law Example
Similar to Ampere’s Circuital law, Gauss’s Law is most useful for cases with symmetry.
Example: Calculate D about an infinitely long wire that has a charge density of q coulombs/meter.
Since D comesradially out inte-grate over the cylinder bounding the wireeA
d 2 q
where radially directed unit vector2
D Rh qh
qR
r r
D a
D a a
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Electric Fields
The electric field, E, is related to the electric flux density, D, by
D = Ewhere
E = electric field (volts/m)
= permittivity in farads/m (F/m)
= o r
o = permittivity of free space (8.85410-12 F/m)
r = relative permittivity or the dielectric constant(1 for dry air, 2 to 6 for most dielectrics)
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Voltage Difference
P
P
The voltage difference between any two
points P and P is defined as an integral
V
In previous example the voltage difference between
points P and P , located radial distance R and R
f
d
E l
R
R
rom the wire is (assuming = )
V ln2 2
o
o o
Rq qdR
R R
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Voltage Difference, cont’d
R
R
With
V ln2 2
if q is positive then those points closer in have
a higher voltage. Voltage is defined as the energy
(in Joules) required to move a 1 coulomb charge
against an ele
o o
Rq qdR
R R
ctric field (Joules/Coulomb). Voltage
is infinite if we pick infinity as the reference point
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Multi-Conductor Case
i
1
Now assume we have n parallel conductors,
each with a charge density of q coulombs/m.
The voltage difference between our two points,
P and P , is now determined by superposition
1V ln
2
ni
iii
Rq
R
where is the radial distance from point P
to conductor i, and the distance from P to i.i
i
R
R
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Multi-Conductor Case, cont’d
n
ii=1
1 1
11
11 1
1
If we assume that q 0 then rewriting
1 1 1V ln ln
2 2
We then subtract ln 0
1 1 1V ln ln
2 2
As we more P to infinity, ln 0
n n
i i iii i
n
ii
n ni
i iii i
i
q q RR
q R
Rq q
R R
RR
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Absolute Voltage Defined
1
Since the second term goes to zero as P goes to
infinity, we can now define the voltage of a
point w.r.t. a reference voltage at infinity:
1 1V ln
2
This equation holds for any point as long a
n
iii
qR
s
it is not inside one of the wires!
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Three Conductor Case
A
BC
Assume we have three infinitely long conductors, A, B, & C, each with radius r and distance D from the other two conductors. Assume charge densities suchthat qa + qb + qc = 0
1 1 1 1ln ln ln
2
ln2
a a b c
aa
V q q qr D D
q DV
r
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Line Capacitance
j
1 11 1
For a single line capacitance is defined as
But for a multiple conductor case we need to
use matrix relationships since the charge on
conductor i may be a function of V
i i i
n
n
q CV
q C C
q
1
1n nn n
V
C C V
q C V
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Line Capacitance, cont’d
In ECE 476 we will not be considering theses
cases with mutual capacitance. To eliminate
mutual capacitance we'll again assume we have
a uniformly transposed line. For the previous
three conductor exam
aa a
ple:
q 2ince q = C
ln
a
a
V V
S V CDVr
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Bundled Conductor Capacitance
1
1cb 12
Similar to what we did for determining line
inductance when there are n bundled conductors,
we use the original capacitance equation just
substituting an equivalent r
Note fo
adius
r t
( )
he
Rn
nrd d
b
capacitance equation we use r rather
than r' which was used for R in the inductance
equation
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Line Capacitance, cont’d
1
m
13
m
1cb 12
-12o
For the case of uniformly transposed lines we
use the same GMR, D , as before.
2
ln
where
D
R ( ) (note r NOT r')
ε in air 8.854 10 F/m
n
mcb
ab ac bc
n
CDR
d d d
rd d
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Line Capacitance Example
Calculate the per phase capacitance and susceptance of a balanced 3, 60 Hz, transmission line with horizontal phase spacing of 10m using three conductor bundling with a spacing between conductors in the bundle of 0.3m. Assume the line is uniformly transposed and the conductors have a a 1cm radius.
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Line Capacitance Example, cont’d
13
13
m
1211
c 11
8
(0.01 0.3 0.3) 0.0963 m
D (10 10 20) 12.6 m
2 8.854 101.141 10 F/m
12.6ln
0.09631 1
X2 60 1.141 10 F/m
2.33 10 -m (not / m)
cbR
C
C
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Line Conductors
Typical transmission lines use multi-strand conductors
ACSR (aluminum conductor steel reinforced) conductors are most common. A typical Al. to St. ratio is about 4 to 1.
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Line Conductors, cont’d
Total conductor area is given in circular mils. One circular mil is the area of a circle with a diameter of 0.001 = 0.00052 square inches
Example: what is the the area of a solid, 1” diameter circular wire? Answer: 1000 kcmil (kilo circular mils)
Because conductors are stranded, the equivalent radius must be provided by the manufacturer. In tables this value is known as the GMR and is usually expressed in feet.
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Line Resistance
-8
-8
Line resistance per unit length is given by
R = where is the resistivityA
Resistivity of Copper = 1.68 10 Ω-m
Resistivity of Aluminum = 2.65 10 Ω-m
Example: What is the resistance in Ω / mile of a
-8
2
1" diameter solid aluminum wire (at dc)?
2.65 10 Ω-m1609 0.084
0.0127m
mR
mile mile
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Line Resistance, cont’d
Because ac current tends to flow towards the surface of a conductor, the resistance of a line at 60 Hz is slightly higher than at dc.
Resistivity and hence line resistance increase as conductor temperature increases (changes is about 8% between 25C and 50C)
Because ACSR conductors are stranded, actual resistance, inductance and capacitance needs to be determined from tables.
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ACSR Table Data (Similar to Table A.4)
Inductance and Capacitance assume a Dm of 1 ft.
GMR is equivalent to r’
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ACSR Data, cont’d
7L
3
3 3
X 2 4 10 ln 1609 /mile
12.02 10 ln ln
12.02 10 ln 2.02 10 ln
m
m
m
Df L f
GMR
f DGMR
f f DGMR
Term from table assuminga one foot spacing
Term independentof conductor withDm in feet.
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ACSR Data, Cont.
0C
6
To use the phase to neutral capacitance from table
21X -m where
2 ln
11.779 10 ln -mile (table is in M -mile)
1 1 11.779 ln 1.779 ln M -mile
m
m
m
CDf Cr
Df r
Df r f
Term from table assuminga one foot spacing
Term independentof conductor withDm in feet.
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Dove Example
7
0.0313 feet
Outside Diameter = 0.07725 feet (radius = 0.03863)
Assuming a one foot spacing at 60 Hz
12 60 2 10 1609 ln Ω/mile
0.03130.420 Ω/mile, which matches the table
For the capacitance
a
a
C
GMR
X
X
X
6 41 11.779 10 ln 9.65 10 Ω-mile
f r
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Additional Transmission Topics
Multi-circuit lines: Multiple lines often share a common transmission right-of-way. This DOES cause mutual inductance and capacitance, but is often ignored in system analysis.
Cables: There are about 3000 miles of underground ac cables in U.S. Cables are primarily used in urban areas. In a cable the conductors are tightly spaced, (< 1ft) with oil impregnated paper commonly used to provide insulation
– inductance is lower – capacitance is higher, limiting cable length
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Additional Transmission topics
Ground wires: Transmission lines are usually protected from lightning strikes with a ground wire. This topmost wire (or wires) helps to attenuate the transient voltages/currents that arise during a lighting strike. The ground wire is typically grounded at each pole.
Corona discharge: Due to high electric fields around lines, the air molecules become ionized. This causes a crackling sound and may cause the line to glow!
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Additional Transmission topics
Shunt conductance: Usually ignored. A small current may flow through contaminants on insulators.
DC Transmission: Because of the large fixed cost necessary to convert ac to dc and then back to ac, dc transmission is only practical for several specialized applications
– long distance overhead power transfer (> 400 miles)– long cable power transfer such as underwater– providing an asynchronous means of joining different power
systems (such as the Eastern and Western grids).
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DC Transmission Line
+/- 400 kV HVDC lattice tower
Source: Tom Ernst, Minnesota Power