High Voltage Engineering

(a) What are the major emerging electrical energy storage technologies? [4] (b) Discuss the use of FACTS and HVDC technologies for the development of Nepal power sector. [2+2] (c) Explain the concept of touch potential and step potential with necessary figures. How can these be prevented and their effects minimized?

a) What will be the no-load sending-end voltage and the voltage rise from the sending-end to the receiving-end for a $50\text{ Hz}$, $300\text{ km}$ long line if the receiving-end voltage is $220\text{ kV}$? Solve by considering both the long line and nominal $\pi$-circuit line models. Also show the voltage rise by the phasor diagram. Resistance of the line is negligible. [8] b) An overhead transmission line has $l=1\text{ mH/km}$ and $c=11.11\text{ nF/km}$. The conductor plus ground resistance amounts to $0.4\Omega \text{/km}$. (i) Taking a single-phase representation, calculate (a) the velocity of propagation, (b) the surge impedance, (c) the attenuation factor for $400\text{ km}$, and (d) the maximum value of open-end voltage.

a) A tower has a $40\text{-}\Omega$ footing resistance and two ground wires each with $Z_g=500\Omega$. The lightning stroke surge impedance is $Z_s=400\Omega$. For lightning current surge $=50\text{ kA}$, crest, calculate the tower top potential (i) considering all impedances, (ii) neglecting the ground wire and stroke surge impedances, (iii)The line-to-ground has a coupling factor between line and ground conductors $K_f=0.2$. The tower-footing resistance is $40\Omega$. Using tower top potential obtained in (i) above, find the voltage experienced by the insulator string. [8] b) A $400\text{ kV}$ line has the following details: $N=2$, $r=0.0318\text{ m}$, $\text{B}=\text{bundle spacing}=0.4572\text{ m}$, height $\text{H}=13\text{ m}$, phase separation $\text{S}=12\text{ m}$ in horizontal configuration. Surface voltage gradients are $16.815\text{ kV/cm}$ and $16.015\text{ kV/cm}$ for the center phase and for outer phases, respectively. The R-O-W provided is $15\text{ m}$ from the outer phases at ground. Is the design satisfactory if the RI level is to be limited to $40\text{ dB}$ at $1\text{ MHz}$ at the edge of R-O-W? Use CIGRE formula: ${\text{RI}}_i(\text{dB})=3.5g_m+6d-33{\log }_{10}(D_i/20)-30$

a) A 3-phase $750\text{ kV}$ horizontal line has minimum height at midspan of $12\text{ m}$, sag of $12\text{ m}$, and phase spacing of $15\text{ m}$. Conductors are $4\times 0.035\text{ m}$ with bundle spacing of $45.72\text{ cm}$. Calculate the capacitance and charge matrices. [10] b) Will there be a sustained discharge in a gas with an electrode separation of $4\text{ cm}$ if its primary and secondary ionization coefficients are $2.43\text{/cm}$ and $6.823\times 10^{-4}$? [3] c) An air bubble of $0.5\text{ mm}$ thickness is embedded in a solid insulation of $1.0\text{ cm}$ having a relative dielectric constant of $3.0$. The voltage applied across the solid is $30\text{ kV}$ (rms) of $50\text{ Hz}$. Calculate the voltage at which internal discharge will occur if the break down strength of the air is $30\text{ kV (peak)/cm}$.

(a) Describe the major breakdown mechanisms in solid dielectrics. [8] (b) What are the general tests that are carried out on high voltage equipment? Explain with clean circuit diagram the method of measurement of high voltage using resistance divider and micro-ammeter.