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Transmission of Electric Power at Extra High Voltage
2. What are the limitations of HVAC transmission ?
EHV transmission lines are used to transmit large blocks of power over long distances.
It is used to supply power in those areas having large fuel shortage
Following factors are needed to be considered while designing EHV transmission line
- Size of load demand
- Cost of conductors
- Voltage regulation
- Cost of tower and insulators
- System stability
- Thermal capacity
Selection of conductor depends on the current carrying capacities
In case of tower design, tower size is calculated from the insulation requirement
EHV transmission lines are most commonly used due to:
- Electrical loss are less
- Improvement in transmission efficiency
- Voltage regulation improvement
- More flexible towards system and load growth
- Power transmission capacity is large
- Right of ways (RoWs) required is less
- Life of cable insulation is large
Limitations of high voltage transmission line :
1) Large cost is needed for converter substations.
2) For reliable power supply, continuous communication is needed between all terminals of power system which is not possible.
3) Larger harmonics are presents in voltage and current waveforms which are producing Larger power losses.
4) Electro-corrosion occurs in earthing system.
5) Heating of transmission lines due to line losses causes lower value of thermal limit. 6)For longer transmission line, stability of power system is a major factor.
7) Transformer has limited amount of power transfer which depends on current carrying capacity of transmission line.
Design Consideration of EHV Lines
Following factors are considered while designing of EHV lines
Method of Earthing : EHV lines are need to have solid permanent connection with ground. This allows the user to use lower rated lighting arresters.
Voltage level : Voltage level selection depends on the future good growth, loading level of transmission line.
Conductor : Transmission line conductors are selected depending on :
(1) Power transmission capability needed
(2) Power losses
(3) Number of strands
(4) Surge impedance loading limit
Surge Impedance Loading Formula
Z and Y are series impedance and shunt admittance per Km respectively.
Transmission loss : It need to be considered at varying load factors
Right of Ways (RoW) : These are based on the voltage of transmission line, cost required for Rows, safety level and environmental factors etc.
Aesthetics : Visual appearance due to transmission lines should be as minimum
as possible.
Reliability : Reliability level should be as large as possible to ensure each customer have access to electricity.
Clearances : While designing EHV
transmission line financial clearances, technical clearances and forest clearances are need to be considered.
Power system stability : To improve power system stability we have to take few measures like use of bundled conductors, placing series capacitors in power system, use of low reactance alternators and transformers.
Uses of proper type of towers, suspension strings are absolutely necessary depending on voltage level.
Voltage regulations : Keep as possible as low voltage regulation.
Insulation co-ordination : Proper insulation co-ordination results into reliable operation of transmission line, less damage of equipments etc.
Radio : interferences are need to be minimum.
Insulator failure : Insulator can easily operate at normal voltage level. During the operation due weather effects insulator developes flashover cracks on the surface. Due this during flashover insulator may get damaged. To have proper operation insulator maintenance should be done regularly.
Environmental aspects while designing EHV lines :
(1) Due to high voltages of transmission line, corona is produced with hissing sound increasing the noise pollution.
(2) For erection of transmission line, same times trees are need to cut.
(3) Also during erection of tower, there is a need of digging the earth for tower footing which affects the stability of soil near it.
(4) Soil erosion is a major problem due transmission line erection.
(5) Largest climate change occurs due to energy loss during electricity transmission.
(6) Restrictions on land use : Building and structures are not permitted in transmission line areas due to electrical safety.
Bundled conductor in design of transmission line :
In bundled conductor, two or more strands are used in one phase. These are mainly used in system having voltages larger than 230 kV.
Due to bundling of conductors, reactance of bundle conductor is reduced because of self GMD which helps to transmit larger /maximum power.
Advantages of bundle conductors
- Reduced reactance
- Lesser corona loss
- Lesser radio interface
- Surge impedance is lower.
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