Power system analysis

Daffodil International University Department of Electrical and Electronic Engineering Faculty of Engineering Midterm Examination, Fall – 2026 Course Code: 0713-315 Course Title: Power System Analysis Section: A, B, C, D, E, R Level-Term: L3-T2 Teacher's Initial: DRA, AI, SMP Full Marks: 25 Exam Date: 31 October, 2026 Time: 1.5 Hours Read the Question Carefully and Answer Three Questions from Four Q1 is Mandatory (All terminologies shall be interpreted in their conventional sense. Assume any required values wherever necessary. Candidates are advised to present concise and relevant solutions — unnecessary elaboration may result in the deduction of marks) Q1. (a) Define per unit system and mention an advantage of using this system. CO-1 C2 [2] (b) A motor is rated at 30 MVA, 18 kV, with power factor of 0.6 leading based on its own rated values. The motor is connected to a power system with base values of 50 MVA and 20 kV. Determine the per-unit impedance of the motor on the new system base values. CO-1 C3 [2] (c) For the following power system, draw an equivalent impedance diagram showing all impedances in per-unit system on a 50 MVA base. Consider 20 kV as the voltage base for the generator. CO-1 C3 [7] [Figure: 1 - A power system showing Generator Eg connected through Transformer T1, bus 1, Transmission Line j10 Ω, bus 2, Transformer T2 to Load EL. Generator: 60 MVA, 20 kV, X = 12.5%; T1: 50 MVA, 20 / 230 kV, X = 10%; T2: 45 MVA, 215 / 70 kV, X = 14%; Load: 38.5 MVA, 60 kV, X = 20%] Q2. Figure: 2 contains an SLD of a Four-bus power system where impedances are given in per unit. With the assistance of admittance diagram, write the KCL equations for all buses, find diagonal and off-diagonal elements and achieve Ybus matrix. CO-2 C3 [7] [Figure: 2 - Four-bus power system SLD with impedances: between bus 1 and 2: j0.4; between bus 2 and 3: j0.3; between bus 1 and 4: j0.1; between bus 2 and 4: j0.1; between bus 3 and 4: j0.2] Q3. For the given power system in figure below, the line impedances are marked in per unit on 100 MVA base. Using the Gauss-Seidel method, calculate the approximate phasor values of the voltages at load buses 2 and 3 (P-Q buses) after performing two iterations. CO-2 C3 [7] [Figure: 3 - Three-bus system: Bus 1 (Slack Bus, V1 = 1.05∠0), connected to Bus 2 with line impedance 0.2 + j0.4, and connected to Bus 3 with line impedance 0.1 + j0.3; Bus 2 connected to Bus 3 with line impedance 0.125 + j0.25; Bus 2 load: 25 MW, 14 MVar; Bus 3 load: 15 MW, 10 MVar] Q4. (a) Define Swing Bus, PQ bus and PV bus in a power system and also mention their alternative names. CO-2 C2 [2] (b) Eliminate bus 3 from the given bus admittance matrix and form new bus admittance matrix by applying Kron method. CO-2 C3 [5] [-j9.8 0.0 j4.0 j5.0 0.0 -j8.3 j2.5 j5.0 j4.0 j2.5 -j14.0 j8.0 j5.0 j5.0 j8.0 -j18.0]