Question 1: Impulse generation
(a) What are the tolerances associated with a standard lightning impulse waveform and, a standard switching impulse waveform?
(b) Define Marx's principle and explain how it is used in the operation of a multi stage impulse generator.
(c) What is the significance of the surge impedances of the phase conductors and ground conductors (shield wires), in the context of a lightning striking a transmission line and insulation coordination? Briefly explain.
(d) Explain the importance of the adjustable front and tail resistors for an impulse generator in relation to the impulse waveform generation.
(e) Explain the significance of the tolerances given, with a standard lightning impulse voltage waveform generation, considering the range of capacitance values for test objects. The capacitance values for different test objects are given below.
| Load capacitances for different test objects: |
|
| Simple post or suspension insulators |
~ 10pF |
| Bushings; simple or graded type |
~100 - 1000pF |
| Potential Transformers |
~200 - 500pF |
| Power Transformers depending on KVA rating |
~1000 - 10,000pF |
| HV Power Cables (oil-paper impregnated) |
~250 - 300pF/m |
| HV Power Cables (gas insulated) |
~60pF/m |
| Metal clad substation, SF6 insulated |
~1000, and >10,000pF |
Question 2: Measurements
(a) Discuss the main problems associated with potential dividers, and explain why capacitive dividers are preferred to resistive ones when measuring AC voltages.
(b) How the surface roughness (irregularity) influences (i) the electric field distribution and (ii) the measured voltages using spark gaps?
(c) Sphere gaps are extensively used in high voltage measurements compared to other non- uniform gaps, why?