Insulator
Q. What is an insulator?
Ans 1 : An insulator is a material that does not conduct electrical current. Insulating
materials include paper, plastic, rubber, glass and air.
Ans 2 : The current doesn’t flow through the substance is known as an insulator.
==>> The five(05) types of insulators are:
1. Suspension insulators
2. Pin insulators
3. Strain Insulators
4. Stay Insulators
5. Shackle Insulators
1). What is the difference between pin and post type insulator?
Ans :Pin type is used up to 33KV system whereas the post type is used in high & low
voltage systems.
Disc Insulator :
Q. What is a Disc Insulator?
Ans : Definition: An insulator that is made up of high-grade wet processes like
brown-green glazed is known as disc insulator.
==>> Types of Disc Insulators :
Ans : Disc insulators are classified into 2 types which include
1. Suspension type
2. Strain type.
Suspension insulator :
The conductor can be hanged under the support point. These types of insulators are
available in two types.
1. Connected cap type
2. Interlinking/ Hewlett type insulators.
Strain Type Insulator :
This kind of insulator is also known as a tension insulator. These insulators are
utilized in overhead electrical wiring for supporting overhead power lines & radio
antennas.
Q. How are disc insulators made?
Ans :These are made with a high-grade wet process like Brown Green Glazed.
Q. Advantages and Disadvantages of Disc Insulator :
The advantages of disc insulators include the following.
1. This type of insulator has a normal voltage rating like 11KV, so a suspension
string can be designed with a set of discs.
2. In suspension insulators, if any of the discs is damaged then it can be changed
much simply.
3. Mechanical pressures on this insulator are less due to the hanging line on an
elastic suspension string.
4. This kind of insulator is used at any high voltage by connecting discs separately
Repair is reasonable as simply damage units can change.
5. It protects from noise, electricity, and heat.
6. It gives support to the overhead conductor.
7. In a substation, it protects the transformer, switchgear, etc.
The disadvantages of disc insulators include the following.
1.The string of this insulator is expensive compared with post and pin type insulators.
2.It needs more height for the supporting structure and also to maintain similar
ground permission of the current conductor.
3.It needs a cross arm with a high length.
4.The tower’s weight must be high to withstand the insulator’s weight.
==>> Advantages & Disadvantages of Wall Insulation:
Advantage—Energy Efficiency.
Advantage—Friendly to the Environment.
Advantage—Noise Reduction.
Disadvantage—Rain Penetration.
Disadvantage—Health Risks.
==>> Advantages and Disadvantages of Glass Insulators:
Advantages :
1. The glass insulators have higher resistance to breaking as compared to the
ceramics and its mechanical compressive strength is 1.5 times higher than ceramics.
2. The electrical resistance is much higher than that of ceramic insulators (between
500 kV/cm and 1000 kV/cm).
3. The thermal expansion coefficient of glass insulators is small, and its relative
deformation is very low due to temperature variation. Also, glass insulators before
cracking are completely crushed, so it is easy to detect faulty insulators on the floor.
Contrary to ceramic insulators, any crack or hole created during the manufacturing
process is detectable in the glass insulators.
4. Due to transparency, some or sun rays pass from the glass insulator, thereby
decreasing the degradation of the insulator.
Disadvantages :
1. The mechanical durability of glass insulators against bending forces is slightly less
than that of ceramics.
2. By a strong impact all insulators will break.
3. Any impurity in composition or any temperature variations of the environment
during storing or operation leads to breaking the insulator.
4. The glass insulators absorb contaminations easier than other types of insulators.
5. The moisture easily distilled on the surface of the glass insulators.
==>> Advantages and Disadvantages of Chemicals Insulators :
Advantages :
1. Expands quickly after it is applied and fits easily into a home’s crevices.
2. Great for crawlspaces, knee walls, basement rim joists, bonus rooms, and more.
3. Acts as an air sealant, keeping air from escaping through small gaps and holes.
4. High R-value of up to R-6.9 per inch
5. Water resistant
6. Long-lasting
7. Sag-resistant
Disadvantages :
1. Should be installed by professionals
2. More expensive than other types of insulation
3. Must vacate your home for at least 12 hours during installation
4. Poorly mixed chemicals can lead to health risks and ineffective insulation.
Corona
Q. What is the corona effect?
Ans 1 : A corona discharge is an electrical discharge caused by the ionization of a
fluid such as air surrounding a conductor carrying a high voltage.
Ans 2 : The ionization of air surrounding the high voltage transmission lines causing
the conductors to glow, producing a hissing noise, is called Corona Discharge or
Corona Effect.
(Note : Corona effect mostly occurs at the sharp point of insulators. )
==>> Corona Formation: (Why corona happens)
Ans : Air is not a perfect insulator, and even under normal conditions, the air
contains many free electrons and ions. When an electric field intensity establishes
between the conductors, these ions and free electrons experience force upon them.
Due to this effect, the ions and free electrons get accelerated and moved in the
opposite direction.
The charged particles during their motion collide with one another and also with the
very slow moving uncharged molecules. Thus, the number of charged particles goes
on increasing rapidly. This increases the conduction of air between the conductors
and a breakdown occurs. Thus, the arc establishes between the conductors.
==>> Factors Affecting Corona :
01. Conductor Size
02. Line Voltage
03. Spacing Between Conductors
04. Atmosphere
05. Frequency
==>> Advantages & Disadvantages of Corona Effect :
Advantages of Corona Effect :
01. It reduces the transient effect produced in the line, the transient effect is
produced due to surge which increases the line voltage but this increase in voltage
corona will reduce it by dissipating as heat or light.
02. Due to corona the air surrounding the conductor acts like a conductor, suppose
suddenly current increases in the line then that current will go through the air and
thus protect the conductor from damage.
Disadvantages of Corona Effect
01. Power losses will occur and efficiency will be reduced.
02. Corona produces the ozone gas, ozone gas produces some acid in the line and
due to this acid corrosion is formed in a conductor.
Disadvantages of corona effect in transmission line
The corona effect has following disadvantages such as …
1. Results in power loss, so ultimately efficiency will be reduced.
2. It creates interference in the neighboring communication line.
3. Hissing noise would cause earache for those who lived near to the transmission
line.
4. Ozone gas formation leads to polluting the atmosphere.
Q. What is the Corona Effect in Transmission Lines? How do Engineers
Overcome it?
Ans : There is a hissing noise with violet glow phenomenon termed as corona effect
which is commonly observed in high voltage transmission lines. The corona effect
leads to high voltage drop and energy loss along with release of ozone gas. There is
a need to be aware of this phenomenon and its effects on the transmission system.
Q. How Corona Effect is Reduced:
Ans : It has been observed that the intense corona effects are observed at a working
voltage of 33 kV or above. On the sub-stations or bus-bars rated for 33 kV and
higher voltages, highly ionized air may cause flash-over in the insulators or between
the phases, causing considerable damage to the equipment, if careful designing is
not made to reduce the corona effect.The corona effect can be reduced by the
following methods:
1. By Increasing Conductor Size.
2. By Increasing Conductor Spacing.
3. By Using Corona Ring.
==>> Q. Factors affecting corona :
The following are the factors affecting the corona :
1. Effect of supply voltage – If the supply voltage is high corona loss is higher in the
lines. In low-voltage transmission lines, the corona is negligible, due to the
insufficient electric field to maintain ionization.
2. The condition of the conductor surface – If the conductor is smooth, the electric
field will be more uniform as compared to the rough surface. The roughness of the
conductor is caused by the deposition of dirt, dust and by scratching, etc. Thus,
rough lines decrease the corona loss in the transmission lines.
3. Air Density Factor – The corona loss is inversely proportional to air density factor,
i.e., corona loss, increase with the decrease in density of air. Transmission lines
passing through a hilly area may have higher corona loss than that of similar
transmission lines in the plains because in a hilly area the density of air is low.
4. Effect of system voltage – Electric field intensity in the space around the
conductors depends on the potential difference between the conductors. If the
potential difference is high, electric field intensity is also very high, and hence corona
is also high. Corona loss, increase with the increase in the voltage.
5. The spacing between conductors – If the distance between two conductors is
much more as compared to the diameter of the conductor than the corona loss
occurs in the conductor. If the distance between them is extended beyond certain
limits, the dielectric medium between them decreases and hence the corona loss
also reduces.
==>> Important points:
1. Disruptive voltage is the minimum voltage at which the breakdown of air occurs
and corona starts.
2. Visual critical voltage is the minimum voltage at which visible corona begins.
==>> Q.Corona Loss :
Corona loss is the other major type of power loss in transmission lines. Essentially,
corona loss is caused by the ionization of air molecules near the transmission line
conductors. These coronas do not spark across lines, but rather carry current (hence
the loss) in the air along the wire.
==>> Q.Corona loss in transmission line :
1.Corona appears in the transmission line when the surface voltage gradient at the
line conductor reaches the breakdown stress. Due to corona, heat and bluish light
produce. There is a loss of power and energy dissipation. This loss is known as the
corona loss.
The efficiency of the transmission line decreased due to corona loss.
2.There is also a minor effect on the voltage regulation of the transmission line . But
this is always negligible .
3.In normal and fair atmospheric conditions , the corona loss can vary between few
kW / km lengths of the conductor in Extra high voltage transmission line .
4.Particularly in a thunderstorm , the corona loss is very high . It can be hundreds of
kW / km in bad weather conditions .
Sag
==>> Q. What is Sag?
Ans : The difference in level. between points of supports (towers or utility poles) and
the lowest point on the conductor is called sag.
==>> Q. Why is Sag provided in the Transmission Line?
Ans : The sag is as a result of the tensioning of the line and must not be too low
otherwise the safety clearances may not be met. Also, the sag had to be such that it
caters for ice loading in the winter of temperate climates.
If the sag is large, and the line becomes heavily loaded, then the sag will further
increase and breach the safety clearances. Similarly, if the sag is low, then when the
line contracts in the winter, low sag will indicate a high tension, and as a result of this
contraction, the line may snap. Sag is inversely proportional to the tension of the line,
and is given by the formula below. For high tensions, the sag should be small. For
low tensions, the sag should be high.
less than 66kV - 20 feet (6.1m)
66kV to 110kV - 21feet (6.4m)
110kV to 165kV - 22feet (6.7m)
Greater than 165kV -23feet (7.0m)
FACTOR AFFECTTING SAGGING IN A TRANSMISSION LINE
The factors that affect sagging of transmission lines are: Temperature: As the
temperature increases, the outstretched conductor length will increase by an amount
equal to:
ΔL =αTS…………………………..(i)
α = the coefficient of thermal expansion
T = the temperature increase in •C
S = the span length in meters
This will result in increase in sag and decrease in conductor tension.
==> Age: Conductor sag over time may increase due to the effects of strand settling
in and metallurgical creep. A higher tension may be used when the conductor is first
erected to allow for ―settling in of conductor strands and for subsequent metallurgical
creep of the conductor material
==> WIND: A wind load on the conductor will increase the apparent weight of the
conductor resulting in an in increase in tension. The increase in tension will increase
the cable length due to elastic stretch by an amount given by given by:
ΔL = (To − T) / EA ………………………..
(ii)
To = the initial tension in Newton
T = the final tension
E = the coefficient of elasticity
A = the cross section of the conductor in meters.
This increase in resultant load will result in an effective sag in an inclined direction
with both horizontal and vertical components.
# Pole movement: Any movement of pole tops due to stay relaxation etc will have
the effect of introducing additional length into the span
# Ice: Ice build up on the conductor will increase the apparent diameter and weight
of the conductor. This is not an issue in Queensland however the same approach
can be used for calculating loads and sags if bird diverters are installed along a
span
#. Conductor load per unit length, span
#. Conductor tension
#. Level of supporting towers.
Another important factor is the required safety clearance of conductors. Safety
clearances are of the following :
1. Vertical Clearance: Distance between conductors and ground.
2. Horizontal Clearances: Distance between conductors and distance between
conductor and structure.
HVDC
==>> The full form of HVDC is high-voltage, direct current.
==>> HVDC Transmission:
Definition: High voltage direct current (HVDC) power systems use D.C. for
transmission of bulk power over long distances.
==>>Economic Distance For HVDC transmission lines :
DC lines are cheaper than the AC lines, but the cost of DC terminal equipment is
very high as compared to AC terminal cables (shown in the graph below). Thus, the
initial cost is high in HVDC transmission system, and it is low in the AC system.
The point where two curves meet is called the breakeven distance. Above the
breakeven distance, the HVDC system becomes cheaper. Breakeven distance
changes from 500 to 900 km in overhead transmission lines.
Advantages of HVDC transmissions :
1.A lesser number of conductors and insulators are required thereby reducing the
cost of the overall system.
2.It requires less phase to phase and ground to ground clearance.
3.Their towers are less costly and cheaper.
4.Lesser corona loss is less as compared to HVAC transmission lines of similar
power.
5.Power loss is reduced with DC because fewer numbers of lines are required for
power transmission.
6.The HVDC system uses earth return. If any fault occurs in one pole, the other pole
with ‘earth returns’ behaves like an independent circuit. This results in a more flexible
system.
7.The HVDC has the asynchronous connection between two AC stations connected
through an HVDC link; i.e., the transmission of power is independent of sending
frequencies to receiving end frequencies. Hence, it interconnects two substations
with different frequencies.
8.Due to the absence of frequency in the HVDC line, losses like skin effect and
proximity effect does not occur in the system.
9.It does not generate or absorb any reactive power. So, there is no need for
reactive power compensation.
10.The very accurate and lossless power flows through the DC link.
Disadvantages of HVDC transmission :
1.Converter substations are placed at both the sending and the receiving end of the
transmission lines, which result in increasing the cost.
2.Inverter and rectifier terminals generate harmonics which can be reduced by using
active filters which are also very expensive.
3.If a fault occurs in the AC substation, it may result in a power failure for the HVDC
substation placed near to it
4.Inverter used in Converter substations have limited overload capacity.
5.Circuit breakers are used in HVDC for circuit breaking, which is also very
expensive.
6.It does not have transformers for changing the voltage levels.
7.Heat loss occurs in the converter substation, which has to be reduced by using the
active cooling system.
8. The HVDC link itself is also very complicated.
==>> Types of an HVDC System
The different types of an HVDC system are explained below : (3 types)
1. Back-to-Back HVDC Station
2. Two Terminal HVDC System
3. Multiterminal DC (MTDC) System
There are two type of MTDC Systems
!) Series MTDC System
!!) Parallel MTDC System