OR Explain thermal breakdown in solid dielectrics. How this mechanism is

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Beverly Casey
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1 Subject : High Voltage Engineering (2090) ITM Universe, Vadodara Electrical Engineering Department Class : Electrical Sem : th Long Questions Sr. No Question Unit No : 0 Explain Charge Simulation method (CSM). 2 Explain CSM for a Multi-Dielectric Medium. Unit No : 02 Describe the current growth phenomenon in a gas subjected to uniform Electric fields. What is Paschen s law? 2 Discuss Paschen s Law for breakdown in Gases. Explain Corona Discharges. Discuss Vacuum Breakdown. Define Vacuum and its categories. Describe breakdown in vacuum. What are electronegative gases? Discuss the criteria for its breakdown. Why its breakdown strength is higher than other gas? 7 Describe various factors influencing breakdown in gas. Explain Corona Discharges. Discuss concept of positive and negative 8 Corona. Define Townsend s first and second ionization co-efficient. Explain 9 Townsend s discharge. 0 Explain streamer theory of breakdown in gases. Unit No : 0 Explain the various theories that explain breakdown in commercial liquid dielectrics. 2 Explain breakdown and filtration tests for liquid dielectric. Explain Purification & Breakdown tests for Liquid Dielectric. Explain cavitations & bubble theory and stressed oil volume theory of liquid dielectric. What are treeing and tracking? Explain this two phenomenon in solid Dielectrics. Explain Thermal breakdown in solid dielectric. OR Explain thermal breakdown in solid dielectrics. How this mechanism is more significant than the other mechanisms? Explain intrinsic strength of solid dielectric and explain different 7 mechanism by which breakdown in solid dielectric occurs. 8 Discuss Epoxy Resins & Polyesters as solid Dielectrics.

2 Unit No : 0 Describe, with a neat sketch the working of a Van de Graff generator. What are the factors that limit the maximum voltage obtained? A 2-stage impulse generator has 0.2 µf capacitors. The wave front 07 and the wave tail resistances connected are 800 ohms and ohms respectively. If the load capacitor is 000 pf, find the front and tail times of the impulse wave produced. Explain the different schemes for cascade connection of transformers for producing very high a.c. voltages. Determine the specific heat generated in the test specimen due to Dielectric loss if the dielectric constant and loss angle of the specimen are.8 and respectively. The electric field is 0 KV/cm at 0Hz. What is the principle of operation of a resonant transformer? How is it advantageous over the cascade connected transformers? Describe the construction, principle of operation and application of a multistage Marx s surge generator. Why is controlled triggering required in an impulse generator? Explain 7 typical trigger arrangement. 8 Explain how are switching impulse generated in Laboratory. An impulse generator has 8 stages with each condenser rated for 0. µf and 2 KV. The load capacitor available is 000 pf. Find 9 the series resistance and damping resistance needed to produce.2/0 µs impulse wave. What is the Maximum output voltage of the generator, if the charging voltage is 20 KV? Explain capacitance voltage Transformer with its schematic representation, Equivalent circuit & its phasor diagram. With help of neat sketch define front time and tail time for the impulse wave. Give allowable tolerances. Explain different circuits to produce impulse wave. Discuss their relative merits and demerits. Explain cascade connection for producing a. c. high voltage. Explain the use of isolating transformer. Explain the working of Cockcroft Walton circuit with schematic diagram. Explain with neat diagram the principle of operation of (i) series (ii) parallel resonant circuits for generating high a.c. voltages. Compare their performance. Explain Marx circuit & modified Marx circuit for Multistage impulse generator. Explain Tesla coil for producing high frequency high voltages with its derivation of output voltage. A 0 stage Cockcroft-Walton circuit has all capacitors of 0.0 µf. The secondary voltage of the supply transformer is 00KV at a frequency of 0 Hz. If the load current is ma, Determine () voltage regulation (2) The ripple () The optimum number of stages for maximum output voltage, () The Maximum output voltage

3 A 200 kva, 20V/0 kv, 0 Hz, testing transformer has an 8% leakage reactance and a 2% winding resistance. An insulator of capacitance 20 nf is to be tested at 00 kv using this transformer as part of the 9 resonance circuit. Determine the value of the inductance (Q-factor=) required to obtain resonance. Determine also the value of the input voltage required. A six-stage impulse generator designed to generate the standard waveform (.2/0 µs) has a per stage capacitance of 0.0 µf to be used to test transformers with an equivalent winding to earth capacitance of nf. A peak output voltage of 0 kv is required for testing the 20 transformer. The wave front time is to be defined based on 0% and 90% values. (a) With the aid of appropriate calculations select the values of the resistive elements in the circuit to produce the required waveform. State any assumptions made. Explain why a steep fronted surge waveform is more likely to cause 2 damage to the line end turn insulation of a transformer winding rather than elsewhere. A ten stage Cockcroft-Walton circuit has all capacitors of 0.0 F. The secondary voltage of the supply transformer is 00 kv at a frequency of 22 0 Hz. If the load current is ma, determine (i) voltage regulation (ii) the ripple (iii) the optimum number of stages for maximum output voltage (iv) the maximum output voltage. Unit No : 0 Explain how a sphere gap can be used to measure the peak value of Voltages. What are the parameters and factors that influence such voltage measurement? Explain with neat diagram the principle of operation of an electrostatic 2 Voltmeter. Discuss its advantages and limitations for high voltage measurements. Discuss the different methods of measuring high d.c. voltages. What are the limitations in each method? What are the different types of resistive shunts used for impulse current measurements? Discuss their characteristics and limitations. Which are different factors that affect the spark over voltage of Sphere gap? Discuss it in brief. Give the basic circuits for measuring peak voltage for a.c. and impulse. Explain how they differ. Explain how a sphere gap can be used to measure the peak value of 7 voltages. What are the parameters and factors that influence such voltage measurement? 8 Explain series capacitor peak voltmeter. Explain potential dividers for impulse voltage measurement by CRO. 9 Suggest arrangement to minimize errors. Explain with neat diagram the principle of operation of an electrostatic 0 Voltmeter. Discuss its advantages and limitations for high voltage measurements.

4 Show that the deflecting torque of an electrostatic voltmeter is proportional to the product of the square of the applied voltage and the rate of change of capacitance. Also draw a diagram of Abraham Voltmeter and describe its operation principle. Explain briefly, with the aid of suitable diagrams, why a potential divider connected at the output of an impulse generator needs to be 2 matched to the cable connecting it to an oscilloscope and how matching may generally be achieved. You may select any specific type of potential divider as an example. Describe with the aid of suitable diagrams how the dielectric loss in a lossy capacitor is measured in comparison with a standard loss less capacitor using the x-y mode in the oscilloscope. Show that the area of the ellipse displayed is proportional to the loss. Describe Hall generator for measuring high dc currents. Unit No : 0 What is meant by insulation co-ordination? How are the protective devices chosen for optimal insulation level in a power system? 2 Explain with neat sketches the mechanism of lightning discharge. Explain the lightning mechanism including stepped leader stroke and power return stroke with appropriate diagram. Write a comprehensive note on metal oxide arrestors. Unit No : 07 Discuss High Voltage Schering Bridge. Explain the partial discharge tests for high voltage cables. How fault is 2 located? What are partial discharges and how are they detected under power frequency operating conditions? Discuss measurement of dielectric constant and loss tangent of capacitor. Explain partial measurement test for insulation quality assessment. Figure Q. shows a high Voltage Schering Bridge used in a particular measurement. The values of the components at balance are shown on the diagram. Determine the value of the unknown capacitor and its loss tangent. All necessary equations must be derived.

5 Unit No : 08 Explain High voltage Test on Insulator. 2 Explain briefly impulse testing of power transformer. Explain classification of HV Laboratories. Explain test facilities and testing equipments in HV laboratories. Write a short note on design and layout of HV laboratory.

Fatima Michael College of Engineering & Technology

Fatima Michael College of Engineering & Technology ANNA UNIVERSITY AFFILIATED COLLEGES BE EEE SEMESTER VI EE2353 - HIGH VOLTAGE ENGINEERING UNIT IOVER VOLTAGES IN ELECTRICAL POWER SYSTEMS 1. What is surge arrester? 2. Name the sources of switching surges.