Questions
8–12 questions in major PSU papers
Difficulty
Medium-Hard
Importance
Core — never skip
Overview
Power Systems is a fundamental pillar of Electrical Engineering that deals with the generation, transmission, and distribution of electrical energy. In PSU exams, this topic is critical as it consistently carries a high weightage and requires proficiency in both complex analytical calculations and conceptual protection logic. Mastery requires a deep understanding of impedance calculations, system stability, and fault currents.
Transmission Line Parameters and Per Unit System
Transmission line modeling involves calculating R, L, and C per unit length based on physical geometry, often using GMD and GMR. The Per Unit system simplifies complex network calculations by eliminating transformer turns ratios and normalizing values to a base MVA and kV.
- GMR of bundled conductors (d-bundle) = (n*r*d^(n-1))^(1/n)
- Inductance L = 2*10^-7 * ln(GMD/GMR) H/m
- Capacitance C = 2*pi*epsilon / ln(GMD/r) F/m
- Z_pu_new = Z_pu_old * (MVA_new/MVA_old) * (kV_old/kV_new)^2
- Base Impedance Z_base = (kV_base)^2 / MVA_base
Load Flow and Fault Analysis
Load flow analysis determines the steady-state operating condition of the power grid, while fault analysis is essential for sizing protective equipment. Symmetrical faults involve all phases, while unsymmetrical faults require Sequence Network analysis using symmetrical components.
- Gauss-Seidel method is simple but slow convergence
- Newton-Raphson method has quadratic convergence
- Symmetrical components: Ia = Ia0 + Ia1 + Ia2
- Single Line-to-Ground (SLG) fault: If = 3*Vph / (Z1 + Z2 + Z0)
- Line-to-Line (LL) fault: If = sqrt(3) * Vph / (Z1 + Z2)
Power System Protection
Protection systems ensure grid reliability by isolating faulty components using Relays and Circuit Breakers. Understanding relay characteristics and the physics of arc extinction in circuit breakers is vital for technical interview and exam questions.
- PSM = (Fault current) / (CT ratio * Plug setting)
- Time Multiplier Setting (TMS) adjusts relay operating time
- Distance relay: Mho relay is inherently directional
- Arc extinction in CB: Current zero crossing is mandatory
- Restriking voltage peak = 2 * Recovery Voltage (maximum)
Formula Sheet
L = 2*10^-7 * ln(GMD/GMR)
C = 2*pi*epsilon / ln(GMD/r)
Z_pu_new = Z_pu_old * (S_new/S_old) * (V_old/V_new)^2
I_fault_SLG = 3*V_th / (Z1+Z2+Z0 + 3*Zn)
R_arc_extinction = V_peak / sqrt(L/C)
PSM = I_fault / (CT_ratio * PS%)
f_resonance = 1 / (2*pi*sqrt(L*C))
Exam Tip
Always convert secondary side impedance to primary side using the square of the turns ratio before applying per-unit calculations.
Common Mistakes
- Mixing up GMR (Geometric Mean Radius) and GMD (Geometric Mean Distance) in inductance calculations.
- Forgetting to perform base changes correctly when shifting between transmission and distribution levels in Per Unit problems.
- Neglecting the phase shift introduced by Star-Delta transformers during sequence network sequence component analysis.
More Revision Notes
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