Code No: 52111/MT
M.Tech. I-Semester Examinations, February-2007.
HIGH VOLTAGE D.C. TRANSMISSION
(Common to Power System Control and Automation, Electrical Power
Engineering, Power Electronics and Power Engineering and Energy
Time: 3 hours Max. Marks: 60
Answer any FIVE questions
All questions carry equal marks
- – -
1.a) Give a neat schematic diagram of a bi-polar HVDC system and explain
the purpose of each equipment in it.
b) A double circuit 3-phase line is converted into 3 circuit D.C homopolar
line by bundling the conductors in the later. Compare the power transfer
capacity of each circuit. Assume that the insulation levels and the
percentage line losses are the same in both the circuits.
2.a) Explain the operation of cascaded two, 3-phase circuits for 6 pulse
operation and obtain the expressions for the following:
(i) D.C output voltage (ii) Peak inverse voltage (iii) R.M.S current of
each secondary of the transformer (iv) Volt-ampere rating of the
b) Also compare the above values with that of 3-phase, 6-pulse, Graetz’s
3.a) Develop an equivalent circuit of a converter working as rectifier with
delay angle ‘α’ and overlap angle µ and hence show that the equivalent
resistance of the converter is 3WL/Π, where W is the angular frequency
and L is the series effective inductance of the converter per phase.
b) Also deduce the equivalent circuit of an inverter.
4.a) Bring out the differences in power control in HVDC and EHVAC systems.
Explain the factors that can be used for power control in HVDC link.
b) Explain the need for compounding both rectifier and inverter stations
with constant current control. How do you determine the current margin
between the two stations.
5. What do you understand by characteristic harmonics in HVDC system?
Using Fourier analysis, obtain an expression for nth harmonic voltage on
the DC side of the converter system.
6.a) With the help of block diagram of control hierarchy, explain the control
philosophy of HVDC system.
b) Explain in detail the IPC and EPC modes of control of firing angle. What
are the advantages of EPC scheme over other method?
7.a) What are the limitations of constant power control in HVDC system?
b) A HVDC link delivers DC power at 500 kV at the inverter end with
constant current controller at rectifier end set at 1000A. The equivalent
resistances of the rectifier and inverter stations are 15Ω and 18Ω
respectively. The d.c. resistance of the line is 20Ω. If the AC voltage at
the rectifier is 400 kV, find the percentage of tap changer required to
maintain the current constant in the d.c. link without gate control. Also
find the value of delay angle of the rectifier to maintain the above current
in the absence of tap-changer.
8. Write short note on any TWO of the following:
(a) Commutation failures and “by pass valve”
(b) Reactive power compensation and A.C voltage control
(c) Planning and execution of HVDC system.
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