ESSAY ANSWERS PLEASE 1.1. What are the highest AC and DC transmission voltages c
ID: 2248798 • Letter: E
Question
ESSAY ANSWERS PLEASE
1.1. What are the highest AC and DC transmission voltages currently in use in the world (Hint: Look up UHV transmission lines) and where are they located? Read about HVAC and HVDC transmission and give two advantages and two disadvantages of each system. Give one example in New York State which uses HVDC transmission link? (Co-relate this with HW-1 Problem 2-c-ii, which asks about constant interface flow).
1.2. Mention at least five differences between power transformer and distribution transformer? For both these transformers, what is the approximate load point (% of full load) where the transformers are designed to achieve maximum efficiency? Why?
1.3. Compare and mention the advantages and disadvantages of overhead transmission lines and underground transmission lines (cables)? What is the most important difference between overhead transmission lines and underground cables in terms of line parameters (R, L, and C)? Will this impose any limitations for the underground cables?
Explanation / Answer
1.1 the hightest AC tranmission voltage in the world now is UHVAC that is 1000KV. the hightest DC tranmission voltage in the world is 800KV.
HVAC tranmission : advatnages :
(1) in HVAC tranmission system the voltages can be increased or decreased based on our requirements.
(2) the circuit breaker cost of the system in HVAC is cheaper
disadvatnages:
(1) the number of conductor required for the HVAC is more with the increase the level of voltages ,so the cost of the system will be increased.
(2)there is telephone interference in the HVAC system.
HVDC : advatnages :
(1) there no skin effect in HVDC system ,and less corona losses are in the HVDC when compared to HVAC.
(2)it does not have the stability problem and synchorizing problem
disadvatnges:
(1) for conversion of the DC voltages the converter are needed , so the cost of the system is increased .
(2) there is limited switches and circuit breaker are available .
(1.2) difference between the power transformer and distribution transformer :
(i) the power power transformer are generally operates at the full load capacity at all time , wherea as the distribution transformer will operates depending upon the load in the system .
(ii)the power transformer usually consists of one primary and one secondary winding , where as the distribution transformer consists of one primary and secondary tapping i.e more secondaries.
(iii)the flux density of the power transformer is more than the distribution transformer
(iv)the rating of the power transformer is used at the generating station which high value of MVA and high value of voltages , wherea as the distribution transformer is used at the distribution side which less value of MVA and low value of voltages
(v)the load fluctuation in the power transformer is less and independent of the time , where as the load fluctuation is more in distribution transformer and dependent on the time .
the maximum efficiency of the power transformer is more because there is fixed core and copper losses but where as the distribution transformer consists of fixed core losses and varying copper losses due to the load change,so the effciency for the distribution transformer is calcualted in terms of ALL-DAY efficiency.
(1.3) overhead transmission line :
advantages :
(I) the conductor size required will be less for the over head transmission line whenn compared to under grouung transmission for same amount of power .
(ii)the faultin the tranmission lines are easily detected.
(iii) the insuation required for the overhead tranmission line is less
disadvatnages :
(i) these will faces problem due to the interference with the neighbour communication lines .
(ii) public safety is less.
(iii) these are able to hazards to the lighting discharge
underground :
advantages :
(i)there is fewer problems due to theh lighting discharge.
(iii) there is more public safety .
(iii) there is interference problem due to the communication lines
disadvatanges :
(i) the insulation required for the under groung cables are much more .
(ii) the conductor size is also very high .
the surge impendence for a loss less over head transmission line i.e square root of L/C is 400ohms but where as the under grounf system it is 40 ohms
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