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Factors affecting mechanical design of overhead lines and factors affecting span length |
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Corona , audio noise & radio interference |
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Testing transmission
line materials – Indian Standards |
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Internet websites: Simulator Line constants program |
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HOME takes you to the start page
after you have read these Topics. Start page has links to other topics.
http://powerlearn.ee.iastate.edu --Simulator for
transmission thermal limits
TOWERABC: Calculates line constants for overhead
three-phase, single-circuit or double-circuit, transmission lines and produces
contour plots of rms V, E, and H, plus sound. It can be used for 50/60 Hz, and
also for harmonic frequencies.
OVERHEAD
LINES -OBJECTIVE TYPE QUESTIONS
1. The surge impedance of a 110 kV, 3-phase transmission line is 440 ohms. The surge impedance loading of the line is
Ans.: (b)
2.The capacitance and inductance per unit length of a 3-phase line, operating at 110 kV are .01 microfarad and 2.5 mH. The surge impedance of the line is
(a) 50 ohms
(b) 500 ohms
(c) 250 ohms
Ans: (b)
3. A long transmission line is energized at then sending end and is kept open circuited at the receiving end. The magnitudes of the sending end voltage Vs and of the receiving end voltage Vr satisfy the following relationship
Ans: (c)
4. Voltage regulation of a short transmission line is
Ans: (c)
5. The capacitance of an overhead line increases with
Ans: (b)
6. Shunt compensation for long EHV lines is primarily resorted to
Ans: (a)
7. Series compensation is primarily resorted to
Ans: (b)
8. Fair weather corona loss may be computed using the empirical formula given by Peterson. According to Peterson's formula corona loss is proportional to
(a) f and V2
(b) f 2 and V
where f and V are the system frequency and voltage respectively.
Ans: (a)
9. Bundled conductors are used in EHV lines primarily for
Ans: (b)
10. There are 20 discs in the string of insulators of a 3-phase 400 kV transmission line. String efficiency is 80 %. The maximum voltage across any disc is
Ans: (b)
11. Two or three sheds or petticoats are provided in pin-type insulators in order to increase
Ans: (a)
12. Pin -type insulators are use up to
Ans: (b)
13. Insulators used for transmission line at the dead -end tower are
Ans: (c)
14. Economic studies have shown that D.C. transmission is cheaper than a. c transmission for lengths
Ans. b
15.Transmission voltages in the range 230 kV-765 kV are known as
b. Extra High Voltage
c. Ultra High Voltage
Ans. b
16. Which one of the following statements is false?
As the transmission voltage increases,
a. Corona loss decreases
b. Conductor copper loss decreases
c. Cost of insulators, transformers, switches & circuit breakers increases
Ans. c
17. The internal inductance of a solid conductor of radius r and carrying a current I is equal to
a. 0.5 I * 10-7 H/m
b. 0.5 I * 10-7 exp(-1/4)*r H/m
c. 0.5 * 10-7 H/m
Ans. c
18. Which one of the following statements is not true?
a. The GMD method of finding inductance does not apply to ACSR conductors
b. Current density in ACSR conductors is uniform
c. The GMD between two circular areas, each of different diameters, is equal to the distance between their centres.
Ans. b
19. Expanded ACSR conductors are used
a. To increase the tensile strength of the line
b. To reduce corona loss
c. To reduce I2R loss
Ans. b
20. A conductor with 19 strands, each of same diameter and each having an inductance of L Henries is used for a transmission line. The total inductance of the conductor will be
a. L/19
b. L/361
c. 19L
Ans. a
21. The line to neutral capacitance of single -phase line with conductors of radius 1cm and spaced 1m apart is equal to
a. 10-9/72 F/m
b. 10-9/36pF/m
c. 2 p210-7 F/m
Ans .a
22. In a double-circuit line with hexagonal spacing ,
a. The phases are balanced, but the conductors of each individual phase are not balanced.
b. The conductors of each individual phase are balanced , but the phases are not balanced
c. The phases, and the conductors of each individual phase are both balanced
Ans. c
23. Which one of the following statements is true?
a. Skin effect at 50 Hz is negligible for larger diameter conductors but becomes appreciable for smaller conductor.
b. Skin effect at 50 Hz is negligible whatever the diameter of the conductor.
c. Skin effect at 50 Hz is negligible for the smaller diameter conductors but becomes appreciable for the larger conductor conductor.
Ans. c
24. Which one of the following statements is true?
a. Resistance of a conductor decreases and the internal inductance increases as the frequency is increased
b. Resistance and internal inductance of a conductor both increase with increase of frequency
c. Resistance of a conductor increases and the internal inductance decreases as the frequency is increased
Ans. c
25. The surge impedance of a double-circuit power transmission line is
a. 40 ohms
b. 200 ohms
c. 400 ohms
d. 800 ohms
Ans. b
26. The surge impedance of a telephone line is
a. 50 ohms
b. 75 ohms
c. 200 ohms
d. 400 ohms
Ans. b
26. Bundle conductors are preferred in EHV transmission lines because
a. It is easy to fabricate thin conductors and combine them to make a bundle
b. Inductance of the line is reduced, and the corona loss, and radio & TV interference is minimized.
c. Tower height is reduced and hence transmission cost is low.
Ans. b
27. Inductive interference between power & communication lines can be minimized by
a. Increasing the spacing of power line conductors
b. Transposing power line conductors
c. Transposing communication line conductors
d. Either b or c.
Ans. d
28. The percentage regulation of an overhead transmission line can be zero when the load power factor is
a. Lagging
b. Unity
c. Leading
Ans. c
29. Which one of the following statements is true?
a. Skin effect increases the resistance of a conductor ,but proximity effect decreases the resistance
b. Both skin effect and proximity effect increase the resistance of a conductor
c. Both skin effect and proximity effect increase the internal inductance of a conductor
Ans. b
30. A transmission line having parameters A1, B1, C1, D1 is in parallel with another having parameters A2, B2, C2, D2. The overall " A " parameter of the combination is
a. A1A2 +B1C2
b. (A1B2 +A2B1)/(B1+B2)
c. C1+C2 + (A1-A2)(D2-D1)/(B1+B2)
Ans. b
31. Disruptive critical voltage is
a. Equal to
b. Greater than
c. Less than
visual critical voltage for corona on an overhead line.
Ans. c
32. Corona loss
a. Increases
b. Decreases
c. Does not change
the switching voltage on a transmission line.
Ans. b
33. Handling of the telephone receiver may become dangerous due to
a. Electromagnetic induction
b. Electrostatic induction
c. Both electromagnetic and electrostatic induction
from a power line.
Ans. b
34. Mutual inductance between a three -phase power line and a telephone line due to third harmonic current in the power line is
a. The algebraic sum of mutual inductances from individual phase wires
b. The arithmetic sum of mutual inductances from individual phase wires
c. Zero
Ans. b
35. Dielectric strength of mechanically sound porcelain is
a. 10 kV/cm
b. 22 kV/cm
c. 65 kV/cm
d. 100 kV/cm
Ans. c
36. Dielectric strength of glass is
a. 22 kV/cm
b. 60-66 kV/cm
c. 140 kV/cm
d. 240 kV/cm
Ans. c
37. Which one of the following statements is true?
a. For 11 kV insulators, the ratio of wet spark-over voltage to working voltage is 8.3
b. For 66 kV insulators, the ratio of dry spark-over voltage to working voltage is 8.3
c. For 11 kV insulators, the ratio of dry spark-over voltage to working voltage is 8.3
Ans. c
38. Which one of the following statements is true?
a. Flash-over tests are performed on all insulators
b. Routine tests are performed on 1/2 percent of insulators supplied
c. Design tests are done on 1/2 percent of insulators supplied
Ans. none of the above
39. Which one of the following statements is true?
a. The spark-over voltage is less than the puncture voltage
b. The spark-over voltage is greater than the puncture voltage
c. The spark-over voltage is equal to the puncture voltage
Ans. a
40. Which one of the following statements is true?
The sheds of an insulator should be shaped
a. To conform to the electrostatic tube of force and the body should be shaped to conform to the equipotential surfaces
b. To conform to the equipotential surfaces and the body should be shaped to conform to the electrostatic tubes of force
c. To conform to the equipotential surfaces and so also the body
Ans. b
41. In a suspension insulator, the mechanical stresses on the conductor are
a. Increased
b. Reduced
c. The same
Ans. b
42. The string efficiency of the insulator can be increased by
a. Increasing the number of strings in the insulator
b. Increasing the ratio , capacitance to earth/capacitance per insulator
c. By the correct grading of various capacitances
d. Decreasing the number of strings
Ans. c
43. String efficiency of insulators for wet flash-over is
a. Less
b. More
c. The same
as that for dry flash-over
Ans. b
44. The potential across insulator discs can be equalized by having
a. The same capacitance for each unit
b. The highest capacitance for the lowest unit and decreasing progressively the capacitance of other units
c. The lowest capacitance for the lowest unit and increasing progressively the capacitance of other units
Ans. b
45. Grading ring serves the purpose of
a. Equalizing the voltage distribution across discs
b. An arcing shield
c. Both equalizing the voltage distribution and acting as an arcing shield
Ans. c
· Voltage level
· Conductor type & size
· Line regulation & voltage control
· Corona & losses
· Proper load flow & system stability
· System protection
· Insulation co-ordination
· Right of way
· Mechanical design
· Sag & stress calculation
· Conductor composition
· Conductor spacing
· Insulator /conductor hardware selection
· Structural design
· Structure types
· Stress calculations
Gauge sizes decrease as the wire increases in size.
Number of strands = 3 n2 -3n + 1
where n = number of layers including the single central strand.
The following conductors are used.
AAC-all aluminum conductor
AAAC-all aluminum alloy conductor
ACSR-aluminum conductor steel re-inforced
ACAR-aluminum conductor alloy re-inforced
R = r l/A
R2/R1 = (T0 +T2)/ (T0 +T1)
R2 = Resistance
at temperature T2
R1 = Resistance
at temperature T1
T0 = Constant
= 234.5 for annealed copper of 100% conductivity
=241 for hard drawn copper of 97.3% conductivity
=228 for hard drawn aluminum of 61% conductivity
Skin effect is function of conductor size, frequency and resistance of conductor material.
Discuss the proximity effect, stranding and spiraling of conductors
Line inductance - one phase & 3-phase
Single-phase overhead line
Voltage drop in a single-phase line due to loop impedance
= 2 l (R + j w m 0 ln (Dm/Ds)/2p) I
l= line length, m
R= resistance of each conductor, m
Dm= equivalent or geometric mean distance (GMD) between conductor centres
Ds= Geometric mean radius(GMR), or self-GMD of one conductor
= 0.7788 r for cylindrical conductor
r= conductor radius
I = current
L= 2 x 10 -7 ln (Dm/Ds ) H/m
Three-phase overhead line
(unsymmetrical spacing)
Dab +Dbc +Dca
Equivalent equilateral
spacing=Deq = Dm = (Dab DbcDca) 1/3
In practice , conductors are transposed.
Transposition is carried out at switching stations
Average inductance per phase
L=2 x 10 -7 ln (Deq/Ds ) H/m
Line capacitance, 1-phase & 3-phase
Single-phase overhead line
Cab = 2 p e 0e r/ ln (D/r) (F/m)
The capacitance to neutral for a two- wire line is twice the line-to-line capacitance, Cab.
Three-phase overhead line
Line-to-neutral capacitance
Cn = 2p e 0e r/ ln (Deq/r) (F/m)
Charging current /phase =jv Cn Vph (A/m)
Effect of ground on capacitance of 3-phase line
The capacitance of a 3-phase transposed line considering ground effect is given by
Cn = 2p e 0e r/ [ln (Deq/r) -ln (h12 h23 h31/h11h22h33)] (F/m)
where h12= distance between conductor 1 and image of conductor 2, etc. Effect of ground is to increase the capacitance.
Equivalent circuit for short transmission line (up to 80 km)
Note that bold symbols indicate complex quantities.
Vs =Vr + Ir Z
Is = Ir = I
Draw a phasor diagram for a short line with inductive load and with capacitive load, using Vr as the reference phasor.
Show that
Vs = SQRT[(Vr + IR Cos f r +(or -) IX Sinf r) 2 + (IX Cos f r +(or -) IR Sinf r)2]
+ sign above is for lagging p. f
- sign above is for leading p.f
f
r = angle between Vr & Ir
f
s = angle between Vs & Is
d = f s-f r = load angle
tand = (IX Cos f r +(or -) IR Sinf r)/ (Vr + IR Cos f r +(or -) IX Sinf r)
Vs = AVr + BIr
Is = CVr +DIr
For a short line, A=1, B=Z, C=0, D=1
Line Efficiency (pu)= Vr I Cos f r/ Vs I Cos f s
Voltage regulation (pu)=(Vs-Vr)/Vr
= (VrNL- VrFL)/
VrFL
= [I(R Cos f r -(or+) XSinf
r)]/ VrFL
Equivalent circuit for medium length line
A T or a p network is formed depending upon how the series impedance or the shunt admittance is lumped at a few points. See Fig.3
The ABCD parameters of the nominal-T network are:
A = 1+ZY
B = Z (1+ZY/4)
C = Y
D= A
The ABCD parameters of the nominal-p network are:
A = 1+ZY/2
B = Z
C = Y (1+ZY/2)
D= A
Nominal -T and Nominal-p networks are not equivalent electrically, as may be verified by using the Y-D transformation.
Voltage regulation (pu)= ((Vs/A)
- VrFL)/ VrFL
Long line equations (above 240 km)
The solution of the voltage wave equation using the initial conditions is
V = (Cosh g x) Vr + (Z0 Sinhg x) Ir
I = (Y0 Sinhg x) Vr
+ (Cosh g x) Ir
g = sqrt (yz) = a + jb
a = attenuation constant pu length
b = phase-shift constant pu length
y = shunt admittance pu length
z = series impedance pu length
Z0 = surge impedance = sqrt (z/y); Y0 =1/Z0
Vs = AVr + BIr
Is = CVr +DIr
where
A = Coshg l
B = Z0Sinhg l
C = (1/Z0) Sinh g l
D = A
l= line length
Line
The exact equivalent p circuit and the exact equivalent T circuit for a long line are shown in Fig.4
The elements of the p circuit are obtained from
Zp = B = Z0Sinhg l = (Z Sinhg l)/ g l
Yp /2 = (A-1)/B = ( Coshg l - 1)/ Z0Sinhg l = (tan(g l/2).Y/2)/(g l/2).
The elements of the T circuit are obtained from
ZT/2 = (A-1)/C = (Coshg l-1)/ ((1/Z0) Sinh g l)
ZT = 2 Z0 tanh (g l/2) = (Z tanh (g l/2))/ (g l/2)
YT = C= (1/Z0) Sinh g l = (Y Sinh g l)/ g l
Surge impedance loading of lines
Incident and reflected voltages on long lines
Vs = (1/2) (Vr +Ir Zo) ea l e jb l
+ ((1/2) (Vr -Ir Z0) e-a l e -jb l
Is = (1/2) (VrYo +Ir) ea l e jb l
+ ((1/2) (VrYo -Ir) e-a l e -jb l
The first and second terms in each of the above equations refer to the incident and reflected voltages respectively.
The wavelength is defined by
l = 2p /b
The velocity of propagation n of the waves is given by
n = l f
l = 6000 km at 50 Hz.
When the line is terminated in its surge impedance Zr = Zo, there is no reflected wave. (Infinite line)
Surge Impedance Loading (SIL) of a transmission line
SIL = [Vr (L-L) (in kV)]2/Zo' (MW)
where Zo' = sqrt(L/C)
SIL is a measure of the maximum power that can be delivered over a line. The following factors affect the maximum power:
· Line length
· Terminal apparatus impedances
· All other factors that affect stability.
To increase SIL, kVr can be increased and Zo reduced by using series compensation.
The distinction between maximum power and SIL should be mentioned.
The parameter A = Cosh g l decreases with increase in line length. In such cases Vr is considerably greater than Vs, when the line is charged but unloaded. In underground cables, the effect is much more pronounced, even in short lengths. It is called the Ferranti effect. Discuss the effects of shunt compensation and reactive loading.
Advantages of bundled conductors
· Reduced line reactance
· Reduced voltage gradient
· Increased corona critical voltage, and therefore, less corona power loss, audible noise, and radio interference.
· Reduced amplitude & duration of high frequency conductor vibration
Disadvantages of bundled conductors
· Increased ice & wind loading
· Inspection more complicated ,spacers required
· Increased clearance requirements at structures
· Increased charging kVA which may be a disadvantage at light loads
Ds = GMR of subconductors
d = distance between two sub-conductors
Dsb = GMR of bundled conductor
Dsb = (Dd) 1/2 (For a 2-conductor bundle)
Dsb = (Dd2) 1/3 (For a 3-conductor bundle)
Dsb = (Dd3) 1/4 (For a 4-conductor bundle)
Average inductance per phase of a bundled conductor,
L= 2 x 10 -7 ln (Deq/ Dsb), H/m
Deq = (D12 D23 D31)
Dij = spacing between phase i and phase j
Factors affecting mechanical design of overhead
lines
Factors
affecting span length
There are five kinds of stresses on lines & supports
Sag and tension
analysis of overhead lines
Required clearances:
The data for the following clearances of different voltage levels should be known.
1. Clearance of conductors passing by buildings
2. Minimum clearances of conductors above ground or rails
3. Crossing clearances of wires carried of wires carried on different supports
4. Horizontal clearances at support between line conductors based on sags.
Sag and tension analysis:
Factors affecting sag are:
1. Conductor load per unit length
2. Span
3. Temperature
4. Conductor tension
5. Level at supports
Conductor load depends on
1. Weight of conductors
2. Weight of ice or snow on conductors
3. Wind blowing against wire
Effect of change in temperature:
If the conductor stress is constant and if the temperature changes, the change in length is
Dl = lo. a.Dt
Dt = t1-to= change in temperature
Dl = l1-lo = change in length
a = Coefficient of linear expansion of conductor per deg. C. If temperature is constant while conductor stress changes (i.e. loading), the change in length is
Dl = lo. DT/MA
DT =T1-To= change in tension in kg
M= modulus of elasticity of conductor
A = Metal cross-section of conductor.
Consider the following in sag & tension calculations:
1. Supports at same level ( I) Catenary method ,( ii) Parabola method
2. Supports at different levels (unsymmetrical spans)
3. Effect of ice
4. Effect of wind
Line location
1. Profile & plan of right-of-way
2.
Templates for locating structures
These are used to provide the following
a. Maintenance of proper clearance from conductor to ground and to crossing conductors
b. Economic layout
c. Proper grading of structures
d. Prevention of excessive insulator swing or uplift at structures.
If an alternating potential is applied to two wires whose spacing is large in comparison with the diameter and the potential difference is gradually increased, a point will be reached when a faint luminous glow of violet colour will appear, and a hissing sound will be heard. This phenomenon is known as Corona. The formation of corona is accompanied by a loss of power. It causes non-sinusoidal nature of current and interference with neighbouring communication circuits.
Corona formation takes place due to ionization of a layer of air immediately surrounding the conductor. For air under ordinary conditions near sea level & without impurities, the value of potential gradient at which ionization takes place can be taken as 30kV/cm (peak).
Interference with communication circuits may be due to both electromagnetic and electrostatic action, the former producing currents, which are superposed on the true speech currents, thereby setting up distortion and the latter raising he potential of the communication circuit as a whole.
Disruptive Critical Voltage
The maximum potential gradient , gr is maximum at the surface of the conductor is:
gr = V/(r ln (d/r)).
For visual corona at normal temperature & pressure,
V= 30 (r + 0.3 Ö r) ln (d/r) kV (peak)
Conditions affecting corona:
· Line voltage
· Ratio d/r
· Contour of the surface
· State of the surface
Considering the above factors , the critical disruptive voltage to neutral becomes
Vc = m0g0 d r ln (d/r)
m0= irregularity factor
g0=disruptive critical voltage gradient for air in kV at NTP (21.1 kV/cm ,RMS)
d =air density factor =392 b/(273+t)
b=atmospheric pressure in cm of Hg
t=temperature in deg. C
The visual critical voltage is given by
Vv = m0g0 d r (1+ 0.3/sqrt(rd ))ln (d/r)
Power loss due to corona
Corona formation results in power loss. Peek's formula for corona loss is:
P= 241 [(f+25)l/d ]sqrt(r/d) (Vph - Vc)2 10-5 kW/ph
where Vph and Vc are the effective phase and critical disruptive voltages , f is the frequency of the system, l= length in km.
Peterson's formula for corona loss is :
P = 0.000021 f V2 F
/[log10(d/r)]2
P = power loss in kW per km of conductor under fair weather conditions.
f = frequency, Hz
V = line to ground voltage
D = spacing between conductors
R = radius of the conductor
F = corona factor determined by test
Audio Noise
When corona is present on the conductors, EHV lines generate audible noise, which is especially high during polluted weather. The noise is broadband , which extends from very low frequency to about 20 kHz. Corona discharges generate positive & negative ions, which are alternately attracted & repelled by the periodic reversal of polarity of the a.c excitations. Their movement gives rise to sound-pressure waves at frequencies of twice the power frequency and its multiples, in addition to the broadband spectrum which is the result of random motions of the ions. Audible noise can become a serious problem from 'psychoacoustics ' point of view, leading to insanity due to loss of sleep at night to inhabitants residing close to an EHV line.
Radio Interference (RI)
Pulse type corona discharge from transmission line conductors gives rise to interference to radio broadcast in the range of 0.5 MHz to1.6 MHz.
Electromagnetic Effect
The emf induced in the communication circuit due to neighbouring power circuit depends on its distance with respect to the power line. The net emf induced due to electromagnetic coupling with a 3-phase line is small since the phasor sum of induced emfs tends to zero. However, the presence of certain harmonics would cause seriously high induced emfs. This problem is more serious these days since the power line current is not sinusoidal because of he use of static controllers.
Electrostatic Effect
The communication line may acquire dangerously high potential due to electrostatically induced charges. The interference between power & communication lines can be reduced considerably by transposing the conductors of both power & communication lines.
The communication line may require electrostatic shielding to overcome electrostatic interference.
Materials & types of insulators
The insulators used in connection with overhead systems employing bare conductors are composed almost invariably of glazed porcelain. Glass has also been used for medium voltages . The porcelain used should be ivory white ,sound, free from defects and thoroughly vitrified .
There are three types of insulators for overhead lines:
1. Pin-type
2. Suspension type
3. Strain type
a) What is the difference between "puncture voltage "and "spark-over voltage?
b) What is the difference between arcing distances under "wet "and 'dry' conditions?
c) What is the "tracking distance?
d) Tabulate the ratio of spark-over voltage to working voltage for different voltage levels.
e) What are the merits of suspension insulator string?
· Each insulator is designed for a comparatively low working voltage, usually about 11 kV, and the insulation for any required system voltage can be obtained by using a "string' of such insulators.
· In the event of failure of an insulator, on unit , instead of the whole string, has to be replaced.
· The mechanical stresses are reduced.
· In the event of an increase in the operating voltage of the line , this can be met by adding the requisite number of units in each string.
What is the difference between suspension & strain insulators?
Potential distribution over a string of insulators
1. Draw the equivalent circuit of string of three insulators.
2. Show how would you determine the potential distribution across the above string.
Model questions
1. " An insulator for overhead line should be designed so that it will spark-over before it will puncture". Why?
2. Why is wet S.O.V less than dry S.O.V?
3. What is the effect of pollution on S.O.V?
String efficiency
= S.O.V of a string of n insulators/ ( n * S.O.V of one insulator)
The string efficiency depends on the ratio= capacitance per insulator/capacitance to earth.
Methods of improving string efficiency
The string efficiency can be improved by the following methods:
· By increasing the ratio
m = insulator self-capacitance/capacitance to earth
This would require long cross-arms and hence is not economical.
· Grading of the units.
This approach requires units of different sizes. Hence it is not generally preferred. The self-capacitance of the lowest unit has to be maximum and as we move upward , the self-capacitance should decrease progressively.
· Static shielding
The voltage distribution is controlled in this method by the employment of a grading or guard ring, which usually takes the form of a large metal ring surrounding the bottom unit and connected to the metal work at the bottom of this unit , and therefore to the line . This ring , or shield , has the effect of increasing the capacitances between the metal work and the line.
The string efficiency increases with the guard ring.
Here special features of the transformer bushing may be explained.
What is the effect of surface leakage resistance on the potential distribution across a string of insulators?
What is the effect of corona on string efficiency?
Distribution System Planning (Moduled.xls)
This Excel spreadsheet module demonstrates the basics of distribution system planning. We select the proper conductors and the numbers of shunt capacitors for compensation subject to the requirements on voltage regulation, losses and fixed and operating costs. We specify the customer demands either in power or in impedance. We specify the operating costs for losses. We also specify the capital costs for various conductor line building and for capacitor placements. We have to select the best combination of conductors and capacitors to minimize cost over a certain period, normally one-year
Type Tests – Tests carried out to prove conformity with the specifications. These are intended to prove the general qualities and design of a given type of manufactured item.
Routine Tests-Tests carried out on each part/item manufactured to check parameters (as per requirements0, which are likely to vary during production.
Acceptance Tests- Tests carried out on samples taken at random from offered lot of manufactured item for the purpose of acceptance of lot.
Testing transmission line materials – Indian Standards
ACSR
Conductors for 400 kV and above IS: 398 (Part 5) –1992
TYPE TESTS
|
No. |
Type Test |
Purpose |
|
1 |
Visual examination |
To verify good workmanship and surface finishing of the conductor |
|
2 |
Measurement of diameter of individual aluminium & steel wires |
To measure actual diameter of each strand to check that it is within specified limits |
|
3 |
Measurement of lay ratio of each layer |
To measure actual lay ratio of each layer strand to check that it is within specified limits |
|
4 |
Breaking load test (on complete conductor) |
To measure actual breaking load of complete conductor to check that it is within specified limits |
|
5 |
Ductility test(for galvanized steel wires only). Torsion & elongation test |
To record fracture of strand- number of complete twist shall not be < 18/16 for sample before /after stranding respectively Elongation shall not be < 3.5% |
|
6 |
Wrapping test |
For no-break observation in aluminium/galvanized steel wire (strand) after wrap/unwrap process |
|
7 |
Resistance test |
To check resistance of aluminium strand at 20 0C |
|
8 |
Galvanizing test (for galvanized steel wires only) |
To check uniformity of zinc coating ( 4 dips of 1 min. each in CuSO4 solution of sp. Gravity 1.186). The weight of zinc coating shall not be less than specified value. |
|
9 |
Surface condition test |
To verify cylindrical shape and relative movement of strands under tension condition of 50% of ultimate breaking load of the conductor. (Applicable to conductors of nominal aluminium area 100 sq. mm and above) |
|
10 |
Corona test |
To check corona extinction voltage not less than specified value. |
|
11 |
Radio interference voltage test |
To check RI voltage level within limits. |
ROUTINE TESTS: shall be same as Acceptance Tests and shall be carried out on each coil.
ACCEPTANCE TESTS:
same as Type test Nos. 1-8 given above.
SAMPLING Criteria:
Earth Wires (Galvanized Strands for Earthing ) IS:
12776-1989
The tests under TYPE , ROUTINE, and ACCEPTANCE
category are not specified in the Indian Standard. However, the following tests shall be carried out on the selected samples.
|
No. |
Test |
Purpose |
|
1 |
Breaking load test |
To verify
strength/measure ultimate breaking load of galvanized wires separately to be
within limits |
|
2 |
Elongation test |
For elongation to be
within limits |
|
3 |
DC Resistance test |
Actual resistance of wire
to be within limits |
|
4 |
Wrapping test |
To verify capacity to
withstand wire twisting. No break in wire after wrap/unwrap process |
|
5 |
Galvanizing test |
To check uniformity of
zinc coating. No permanent copper deposition after 4 dips of 1 min. each in
CuSO4 solution of sp. gravity 1.186. The weight of zinc
coating shall not be less than
specified value. |
|
6 |
Torsion test |
To verify capacity to
withstand torsion of wire. No break in wire after process. |
REJECTION & RE-TEST: If test sample fails any of the tests, three further samples from same lot , out of which, one sample from same drum of original sample , be selected and the tests repeated on all three samples.
Porcelain Disc Insulators for
Overhead Lines with Nominal Voltage > 1000 V, IS 731-1989
TYPE TESTS
|
NO. |
Type test |
Purpose |
|
1 |
Visual examination |
For workmanship /surface defects |
|
2 |
Verification of dimensions |
For ensuring dimensions as per requirement and approved drawing |
|
3 |
Visible discharge test |
For measuring visual corona |
|
4 |
Impulse voltage withstand test |
To check ability of the insulator housing to withstand voltage stresses under dry and wet conditions |
|
5 |
Wet power frequency voltage withstand test |
To check ability of the insulator housing to withstand voltage stresses under wet conditions |
|
6 |
Temperature cycle test |
To check capability of the insulators to withstand thermal stresses |
|
7 |
Electro-mechanical filing load test |
To check capability of the insulators to withstand under combined electrical and mechanical stresses (For string insulator units –Type B only) |
|
8 |
Mechanical failing load test |
To check capability under mechanical stresses (For string insulators of Type A and those of Type B to which electro-mechanical failing load test is not applicable, and for rigid insulators only) |
|
9 |
24 Hours mechanical strength test |
To check capability to withstand the electrical stresses ( for insulators- Type B only) |
|
10 |
Puncture test |
To check capability to withstand the electrical stresses (for insulators –Type B only) |
|
11 |
Porosity test |
To confirm non-porous nature of product |
|
12 |
Galvanizing test |
To determine the uniformity and thickness of zinc coating |
NOTES:
1. A radio interference test is under consideration
2. Type tests are normally carried out once and unless otherwise agreed to, test certificates giving results of type tests , made on not less than two insulators identical in all essential details with those to be supplied, are regarded as evidence of compliance. The tests should be carried out in the order mentioned below:
a) On both insulators: Tests 1-6
b) On first insulator: Tests 9,7,8,11
c) On second insulator: Tests 10 &12.
3. Type tests shall be carried out and certified by the manufacturer or by an agreed independent authority.
ROUTINE TESTS:
|
No. |
Routine test |
Purpose |
|
13 |
Visual examination |
For workmanship /surface defects |
|
14 |
Mechanical routine tests |
T o confirm withstanding mechanical stresses during normal conditions ( for string insulator units only) |
|
15 |
Electrical routine tests |
T o confirm withstanding electrical stresses during normal conditions ( for Type B string insulators and rigid insulators) |
ACCEPTANCE TESTS: Test Nos. 2, 6, 9, 7, 8, 10, 11, 12
SAMPLING CRITERIA: IS : 731, 1987
|
FOR DIMENSIONS
& TEMPERATURECYCLE TESTS |
|||||||
|
Lot size, N |
First sample
size, n1 |
Second sample
size, n2 |
Permissible
failure, a |
First rejection
number, r1 |
Second rejection
number, r2 |
||
|
Up to 1000 |
8 |
8 |
0 |
2 |
2 |
||
|
1001- 3000 |
13 |
13 |
0 |
2 |
2 |
||
|
3001-10000 |
20 |
20 |
0 |
2 |
2 |
||
|
10001 & above |
32 |
32 |
1 |
3 |
4 |
||
|
FOR MECHANICAL,
ELECTRO-MECHANICAL and POROSITY TESTS |
|||||||
|
Lot size, N |
FOR MECHANICAL, ELECTRO-MECHANICAL and POROSITY TESTS |
FOR GALVANIZING
& PUNCTURE TESTS |
|||||
|
First sample
size, n1 |
Second sample
size, n2 |
First sample
size, n1 |
Second sample
size, n2 |
||||
|
Up to 1000 |
5 |
5 |
3 |
3 |
|||
|
1001- 3000 |
8 |
8 |
5 |
5 |
|||
|
3001-10000 |
13 |
13 |
7 |
7 |
|||
|
10001 & above |
20 |
20 |
12 |
12 |
|||
|
NOTE: The samples selected shall be divided approximately into three parts and subjected to the applicable tests in the following order. |
|||||||
|
Parts of sample |
Tests on string
insulator units |
Tests on Rigid
insulators |
|||||
|
Type A |
Type B |
Type A |
Type B |
||||
|
First & second part |
5, 6,8,11 &9 |
5, 6,7 or 8 (whichever is applicable), 9 (when specified) &11 |
5, 6, 8, & 11 |
5, 6, 8, & 11 |
|||
|
Third part |
5, 6, 10 & 12 |
5, 6, 10 & 12 |
5, 6, 10 & 12 |
5, 6, 10 & 12 |
|||
Other
relevant Indian Standards are the following:
AAA Conductors [All Aluminium Alloy Stranded Conductors ] IS: 398 (Part 4) –1994
ACSR Conductors IS: 398 (Part 2) –1994
Pre-formed armour rods for conductor IS: 2121 (Part I) –1981
Repair Sleeve and Mid-span joints for conductors IS: 2121 (Part II) –1981
Stockbridge Vibration Dampers for overhead Lines IS: 9708-1993
Spacer and Spacer Dampers for Twin Horizontal Conductors IS:10162-1982
Earth-wire accessories- Suspension and Tension Clamps; Mid-span Compression Joints; Repair Sleeves and Flexible Copper Bonds IS2121(Part-3)-1992
Cross-by Clips IS2121(Part-3)-1992
H Frame/Tower Structural Steel[rail, Girder, Angle etc.] IS:2062
Stay wires[Hot-dipped galvanized Stay Brand IS:2141-1992
Porcelain Insulators-Bus Post for system nominal voltage > 1000 V IS: [2544-1973]
Step Bolts and Their Nuts for steel structures –IS: 10238 –1982