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Q1) Define effective and Short Circuit Ratio& Explain the Magnetic field affects.
Solution-
Effective short Circuit ratio is defined as the difference between the short Circuit ratio and the ratio of the reactive power supplied by capacitors banks and filters and the rated dc power of the convertor.
Magnetic field affects -
Most e.h.v. transmission lines consist of one 3-phase circuit on a tower with horizontal
Configuration of the 3 phases, Based on Maxwell's method of images, we can calculate the magnetic field generated at
any point in space in the vicinity of the 3-phase line. In most applications, the field intensity at
ground level is the most important quantity. But the equations derived will be very
Q2) Short notes on primary Shock currents.
Solutions - (a) Primary Shock Currents. These cause direct physiological harm when the current
exceeds about 6-10 mA. The normal resistance of the human body is about 2-3 kilohms so that
about 25 volts may be necessary to produce primary shock currents. The danger here arises
due to ventricular fibrillation which affects the main pumping chambers of the heart. This
results in immediate arrest of blood circulation. Loss of life may be due to (a) arrest of blood
circulation when current flows through the heart, (b) permanent respiratory arrest when current
flows in the brain, and © asphyxia due to flow of current across the chest preventing muscle
contraction.
The 'electrocution equation' is i2t = K2, where K = 165 for a body weight of 50 kg, i is in
mA and t is in seconds. On a probability basis death due to fibrillation condition occurs in 0.5%
of cases. The primary shock current required varies directly as the body weight. For i = 10 mA,
the current must flow for a time interval of 272 seconds before death occurs in a 50 kg human
being.
(b) Secondary Shock Currents. These cannot cause direct physiological harm but may
produce adverse reactions. They can be steady state 50 Hz or its harmonics or transient in
nature. The latter occur when a human being comes into contact with a capacitively charged
body such as a parked vehicle under a line. Steady state currents up to 1 mA cause a slight
tingle on the fingers. Currents from 1 to 6 mA are classed as, 'let go' currents. At this level, a
human being has control of muscles to let the conductor go as soon as a tingling sensation
occurs. For a 50% probability that the let-to current may increase to primary shock current,
the limit for men is 16 mA and for women 10 mA. At 0.5% probability, the currents are 9 mA for
men, 6 mA for women, and 4.5 mA for children.
Q3)bundle spacing explain
Solutions -In almost all cases, the sub-conductors of a bundle are uniformly distributed on a circle of
radius R. There are proposals to space them non-uniformly to lower the audible noise generated
by the bundle conductor, but we will develop the relevant geometrical properties of an Nconductor
bundle on the assumption of uniform spacing of the sub-conductors
The spacing between adjacent sub-conductors is termed 'Bundle Spacing' and denoted by
B. The radius of the pitch circle on which the sub-conductors are located will be called the
'Bundle Radius', denoted as R.
Q4) field of point charge and its properties
Solutions –
The properties of electric field of almost all electrode geometries will ultimately depend on that
of a point charge. The laws governing the behaviour of this field will form the basis for extending
them to other geometries which shows the source point S1 where a point
charge + Q coulombs is located. A second point charge q coulomb is located at S2 at a distance
r metre from S1. From Coulomb's Law, the force acting on either charge is
F = Q.q /4 e0e r r2
Q4)Aeolian vibrations-
Solutions The resulting oscillation or vibrational forces cause fatigue of conductor and supporting
structure and are known as aeolian vibrations. The frequency of detachment of the Karman
vortices might correspond to one of the natural mechanical frequencies of the span, which if
not damped properly, can build up and destroy individual strands of the conductor at points of
restraint such as at supports or at bundle spacers. They also give rise to wave effects .
Q5)corona inception gradient
Solutions - Corona-inception gradients on conductors under impulse conditions on cylindrical
conductors above a ground plane are equal to those under power frequency but crest values
have to be used in Peek's formula. The increase in effective radius will in turn change the
capacitance of the conductor which has an influence on the voltage coupled to the other phaseconductors
located on the same tower. The increased coupling factor on mutually-coupled
travelling waves was recognized in the 1930's and 40's under lightning conditions. At present,
switching surges are of great concern in determining insulation clearance between conductor
and ground, and conductor to conductor.
Section 2
Q2)basic function of extra high voltage transmission
Solutions –
The basic functions of extra high voltage transmission is to deliver the transmission of current without the loss of heating due the high current by maximizing the voltage the transmission due high current is reduced
This is happen when current is high due to the implication of ir2 heating .heating is done due to current and thus the high voltage transmission reduced the heating and in this way it reduces not only the heating but also protect the conductors and increase the life span of conductors . it reduces the extra cost of conductor because for the transmission of high current more conductors are required .
Reduces short circuit etc
The main objective of EHV transmission is to deliver the transmission of power safely without having the power loss, heating loss , conductor loss and problem related to above mentioned .
It also reduces the affect of corona loss
In this way ehv transmission deliver safe power to station to substation and to the home and industries .
Q2)effects of resistance in conductors
Solutions -Conductors used for e.h.v. transmission lines are always stranded. Most common conductors
use a steel core for reinforcement of the strength of aluminium, but recently high tensile
strength aluminium is being increasingly used, replacing the steel. The former is known as
ACSR (Aluminium Conductor Steel Reinforced) and the latter ACAR (Aluminium Conductor
Alloy Reinforced). A recent development is the AAAC (All Aluminium Alloy Conductor) which
consists of alloys of Al, Mg, Si. This has 10 to 15% less loss than ACSR. When a steel core is
used, because of its high permeability and inductance, power-frequency current flows only in
the aluminium strands. In ACAR and AAAC conductors, the cross-section is better utilized
effects of resistance in conductor it causes heating effects and due to heating effects it causes the loss of conductors due to the phenomenon of heating.
Resistance is the property which prohibits the flow of current in the conductor in this way it causes the ir2 heating and it damages the conductors slowly and slowly .
It causes the extra cost of conductor replacement.
Q)4 capacitance of long objects
Solutions –
Electrostatic induction to adjacent lines such as telephone lines can be determined by Maxwell's
Potential Coefficients and their inverses. If ground resistance and inductance are to be
considered, Carson's formulas given in Chapter 3 are used. However, for a long object such as
a lorry or vehicle parked parallel to a line under it, an empirical formula for its
Fig. 7.1 Calculation of capacitance of long object located near an e.h.v. line.
capacitance due to Comsa and René is given here. The object is replaced by an equivalent
cylinder of diameter D and height h above ground as shown below. Figure gives the actual
dimensions of the object where a = length of object, b = width, v = height, t = height of tyres.
Then, h = t + v – 0.5 b, and D = b. Other dimensions of line are shown on the figure. The
capacitance of the vehicle, including end effects, is
C = 1 2 a.C C ...(7.1)
where C1 = ln( / ) , pF /m
4
Q5) cost of transmission lines and equipments
Solutions -COSTS OF TRANSMISSION LINES AND EQUIPMENT
It is universally accepted that cost of equipment all over the world is escalating every
year. Therefore, a designer must ascertain current prices from manufacturer of equipment
and line materials. These include conductors, hardware, towers, transformers, shunt reactors,
capacitors, synchronous condensers, land for switchyards and line corridor, and so on. Generating
station costs are not considered here, since we are only dealing with transmission in this book.
In this section, some idea of costs of important equipment is given (which may be current in
2005) for comparison purposes only. These are not to be used for decision-making purposes.
(1US$ = Rs.50; 1 Lakh = 100, 000; 1 Crore = 100 Lakhs = 10 Million = 107).
(a) High Voltage DC 400 kV Bipole
Back-to-back terminals : Rs. 50 Lakhs/MVA for 150 MVA
Q6)maximum charge conditions on three phase lines
Solutions
Several examples of conductor configurations used on transmission lines following world-wide
practice are given in the Table 4.1. The maximum surface voltage gradients are also indicated.
These are only examples and the reader should consult the vast literature (CIGRE Proceedings,
etc.) for more details. Most conductor manufacturers use British units for conductor sizes and
the SI units are given only for calculation purposes. These details are gathered from a large
number of sources listed in the bibliography at the end of the book.
The conductor sizes given in the table are not the only ones used. For example, the
following range of conductor sizes is found on the North American continent.
345 kV. Single conductor— 1.424, 1.602, 1.737, 1.75, 1.762 inches dia
2-conductor bundle—1.108, 1.165, 1.196, 1.246 inches dia.
500 kV. Single conductor — 2.5 inches dia
2-bundle—1.602, 1.7, 1.75, 1.762, 1.82 inches dia (ACAR).
3-bundle—1.165 inches dia.
4-bundle—0.85, 0.9, 0.93 inches dia.
735-765 kV 4-bundle—1.165, 1.2, 1.382 inches dia.
GRADIENT FACTORS AND THEIR USE
From the Mangoldt (Markt-Mengele) formula given in Section 4.5, it is observed that the
maximum surface voltage gradient in the centre phase of a horizontal 3-phase ac line is a
function of the geometrical dimensions and the maximum operating voltage V. As shown in
Table 4.1, the maximum operating voltages show a wide variation. It is therefore advantageous
to have a table or graph of the normalized value called the 'gradient factor' in kV/cm per kV or
V/m per volt or other units which will be independent of the voltage. The gradient factor is
denoted by gf = Ecm/V .