11-06-2013, 12:51 PM
SEMESTER EXAMINATIONS FOR HIGH VOLTAGE DC TRANSMISSION
HIGH VOLTAGE DC.pdf (Size: 55.18 KB / Downloads: 15)
Answer any five questions
All questions carry equal marks
1. Discuss the different factors that favour DC transmission.
2.a) Derive the equation for power flow in an HVDC link. How are the losses estimated?
b) An HVDC link delivers DC power with the AC line voltage to the rectifier being 180kV and that at the inverter being 165kV. Taking α = 150, γ = 200, Rc1 and R=10 Ω and 5 Ω. Calculate the DC voltage at both the ends and the current in the DC link.
3.a) Explain the necessity of VDCOL control used in HVDC systems
b) An HVDC link delivers DC power at 500kV at the inverter end with constant current controller at the rectifier end set at 1000A. The equivalent resistances of the rectifier and inverter station are 15Ω and 18 Ω respectively .The DC resistance of the line is 20 Ω. If the AC voltage at the rectifier is 400kV, find the percentage of tap changer required to maintain the current constant in the DC link without gate control. Also find the value of delay angle of the rectifier to maintain the above current in the absence of a tap changer.
4.a) Mention the basic requirements of control philosophy adopted in HVDC systems. Explain the use of phase locked oscillator in pulse phase control scheme.
b) An HVDC link delivers DC power with AC line voltage to the rectifier being 230 kV and that at the inverter being 220 kV. The current order of the rectifier is 1000A and that of the inverter is 900A. Estimate the delay/extinction angle of the rectifier /inverter when the DC voltage at the inverter end is 250kV. Assume the DC resistance of the line as 20 Ω.
5.a) Identify the various sources for generation of harmonics in HVDC systems and mention the various adverse effects caused due to the presence of harmonics.
b) An HVDC link is supplying DC power of 1000MW at ±500kV at the inverter end. Estimate the value of the eleventh harmonic current when α =100 and μ = 200.
6.a) What are the methods of eliminating or suppressing harmonics with HVDC system? Give a comparison using passive filter network.
b) Obtain the potential of the shore electrode w.r.t remote earth when carrying a current of 100A into the boundary of land sea, when the resistivity of land ρ2 =100 Ω-m and that of sea ρ1 is 0.20 Ω-m. Take the radius of electrode as 1 m and slope of the land with horizontal plane as α = 0.1 radian. Also compute the efficiency of shore electrode.
7.a) What are the methods adopted for limiting overvoltages in DC systems
b) Discuss the different aspects and limitations pertaining to
i) Using an AC system and
ii) DC system for power transfer
8. Explain the terms
a) Firing angle delay
b) Commutation delay as applied to HVDC converters
Sketch the voltage and current waveforms for Graetz’s circuit when
i) α=300
ii) α=1200
iii) γ = 150 (neglect overlap)