About this previous year paper
SECTION A – Short Answers (2 × 10 = 20)
(a) Differentiate between renewable and non-renewable energy
| Parameter | Renewable Energy | Non-Renewable Energy |
|---|---|---|
| Source | Naturally replenished (sun, wind, water) | Finite sources (coal, oil, gas) |
| Example | Solar, wind, hydro, biomass | Fossil fuels, nuclear |
| Pollution | Clean and eco-friendly | High carbon emissions |
| Availability | Infinite supply | Limited, exhaustible |
(b) What is a diversity factor?
Diversity Factor=Sum of individual maximum demandsMaximum demand on system\text{Diversity Factor} = \frac{\text{Sum of individual maximum demands}}{\text{Maximum demand on system}}Diversity Factor=Maximum demand on systemSum of individual maximum demands
It measures load variation among consumers.
Higher diversity factor → better system utilization.
(c) What is skin effect?
When AC current concentrates near the surface of a conductor, reducing effective cross-sectional area and increasing resistance with frequency.
Negligible in DC.
(d) Is corona more in AC or DC?
Corona is more pronounced in AC because: Voltage changes polarity → ionization–deionization cycles. Higher effective voltage gradient at conductor surface.
(e) What do you mean by sag template?
A graphical tool used by engineers to determine sag and tension in overhead conductors over varying span lengths and tower heights. It ensures safe clearance and economical conductor length.
(f) Define the term sag.
Sag is the vertical distance between the highest point of the support and the lowest point of the conductor.
S=wL28TS = \frac{wL^2}{8T}S=8TwL2
where
www = weight per unit length, LLL = span, TTT = tension in conductor.
(g) What do you mean by self GMD and mutual GMD?
Self GMD (GMR): Geometric Mean Radius of a single conductor bundle.
Mutual GMD (GMD): Geometric mean distance between conductors of different phases.
Used in inductance and capacitance calculation of transmission lines.
(h) Main causes of insulation failure Over-voltage due to lightning or switching surge
Mechanical stress and vibration Moisture and dirt deposition
Thermal aging Manufacturing defects or poor maintenance
(i) Underground cable losses Dielectric losses in insulation.
Conductor (I²R) losses. Sheath and eddy current losses.
Charging current losses at high voltage.
(j) Difference between overhead lines and underground cables
| Aspect | Overhead Lines | Underground Cables |
|---|---|---|
| Installation | Open-air, supported on poles | Buried under ground |
| Cost | Low | High |
| Fault detection | Easy | Difficult |
| Maintenance | Easy | Complex |
| Appearance | Unsightly | Aesthetic |
| Use | Long distance | Urban/underground systems |
SECTION B – Descriptive Questions (Any 3 × 10 = 30)
(a) Equipments used in a substation Bus bars
Circuit breakers Isolators
Transformers Instrument transformers (CTs, PTs)
Surge arresters Control and protection relays
Grounding system
(b) Kelvin’s Law for Conductor Size
States that total annual cost = capital cost + energy loss cost, and is minimum when these two are equal.
Used to determine economic cross-section of transmission conductor.
(c) Factors affecting sag Weight of conductor
Span length Tension applied
Wind and ice loading Temperature variation (thermal expansion)
(d) Inductance of double circuit single-phase line
L=2×10−7H/mlnDmr′L = \frac{2 \times 10^{-7}}{\text{H/m}} \ln\frac{D_m}{r'}L=H/m2×10−7lnr′Dm
You substitute given D1,D2,D_1, D_2,D1,D2, and diameter values to compute per km inductance.
(e) Types of insulating materials Solid: Paper, mica, porcelain
Liquid: Mineral oil, silicone oil Gaseous: Air, SF₆ gas
Synthetic: Epoxy, Teflon, PVC
SECTION C – Long Questions (Any 1 from each)
3(a) Daily Load Curve Calculation
Given data allows:
Max Demand = 70 MW Units Generated = Area under curve = Σ(P×t)
Average Load = Total Units / 24 hrs Load Factor = Average Load / Max Demand
4(a) Types of Supply Systems
DC 2-wire, DC 3-wire, AC 1-phase, AC 3-phase (3-wire and 4-wire)
Comparison: 3-phase 3-wire requires less conductor material for same power compared to DC 2-wire.
4(b) Medium Transmission Line (T-model)
Used for lines 80–250 km long.
Considers distributed R, L, C parameters as lumped halves at each end.
ABCD parameters derived using nominal T-circuit:
A=D=1+YZ2,B=Z(1+YZ4),C=YA = D = 1 + \frac{YZ}{2},\quad B = Z(1+\frac{YZ}{4}),\quad C = YA=D=1+2YZ,B=Z(1+4YZ),C=Y
5(a) String Efficiency
Given: Vs=15kVV_s = 15 kVVs=15kV, ratio k=CCs=8k = \frac{C}{C_s} = 8k=CsC=8.
Use capacitive voltage distribution to find
Vtotal, η=nVmeanVmax×100V_{total},\ \eta = \frac{nV_{mean}}{V_{max}} \times 100Vtotal, η=VmaxnVmean×100
6(a) Catenary Method for Sag
Sag curve forms a catenary under self-weight.
Equation:
y=T0w[cosh(wxT0)−1]y = \frac{T_0}{w}\left[\cosh\left(\frac{wx}{T_0}\right) - 1\right]y=wT0[cosh(T0wx)−1]
Used for long spans where parabolic approximation isn’t accurate.
7(a) Inter-sheath Grading of Cables
Reduces dielectric stress by inserting metallic sheaths at intermediate potentials.
Practical difficulty: Complex manufacturing and insulation leakage between sheaths.
7(b) Economical Diameter of Cable
For 66 kV, 3-phase system with peak stress E=50 kV/cmE = 50\text{ kV/cm}E=50 kV/cm:
r=VmE,D=2rr = \frac{V_m}{E}, \quad D = 2rr=EVm,D=2r
Calculate for 66 kV (rms) → Vm=66×103×2/3V_m = 66 \times 10^3 \times \sqrt{2}/\sqrt{3}Vm=66×103×2/3.