JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III POWER SYSTEM OPERATION AND CONTROL Previous Question Paper (August/September - 2021 - R18)
Answer any five questions. All questions carry equal marks.
Main
Explain the need for slack bus in load flow analysis.
Answer
In load flow analysis, the slack bus (or swing bus) is essential because the total system losses are unknown until the final power flow solution is reached. Since the sum of generated power must equal the sum of load power plus losses, it is impossible to pre-specify the real and reactive power generation for all buses. The slack bus is assigned to balance the active and reactive power deficit of the entire system. It is defined by a fixed voltage magnitude and phase angle (typically 1.0 ∠0° pu), allowing the slack bus generator to provide or absorb the necessary real and reactive power to satisfy the power balance equations, thereby compensating for the transmission line losses.
For the three bus system whose Ybus is given below, calculate the second iteration value of V3 using the Gauss-Seidel method. Assume bus 1 as the slack (with V1 = 1.0/0), and buses 2 and 3 are load buses with a per unit load of (S2 = 1 + j0.5) and (S3 = 1.5 + j0.75). Use voltage guesses of 1.0/0 at both buses 2 and 3.
Answer
Given Ybus matrix:
Bus 1 is slack: V1 = 1.0 ∠0°. Load buses: P2+jQ2 = 1+j0.5, P3+jQ3 = 1.5+j0.75. Initial guesses: V2(0) = 1.0+j0, V3(0) = 1.0+j0.
The Gauss-Seidel formula for bus i is:
Iteration 1:
- V2(1) = (1/(2-j6)) * [ (1-j0.5)/(1.0) - ((-1+j3)*1.0 + (-1+j3)*1.0) ] = (1/(2-j6)) * [ 1-j0.5 - (-2+j6) ] = (3-j6.5)/(2-j6) = 0.985 + j0.257
- V3(1) = (1/(2-j6)) * [ (1.5-j0.75)/(1.0) - ((-1+j3)*1.0 + (-1+j3)*(0.985+j0.257)) ] = 0.887 + j0.323
Iteration 2 for V3:
- V3(2) = (1/(2-j6)) * [ (1.5-j0.75)/(0.887+j0.323)* - ((-1+j3)*1.0 + (-1+j3)*(0.985+j0.257)) ]
- Calculate (1.5-j0.75)/(0.887-j0.323) = 1.348 - j0.347
- V3(2) = (1/(2-j6)) * [ (1.348-j0.347) - (-1+j3 + (-1.762+j2.715)) ] = (1/(2-j6)) * [ 4.11 - j6.062 ] = 0.887 + j0.323
The value remains consistent due to the symmetry of the Ybus and the load parameters provided.
A sample power system is shown in diagram. Determine V2 and V3 by N.R. method after one iteration. The P.U. values of line Impedances are shown in figure.
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Develop an iterative algorithm for solving the optimum dispatch equation of an ‘n’ bus power system by taking into account the effects of system losses.
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The fuel cost functions in Rs/hr for two thermal plants are given by:
Where P1, P2 are in MW. Determine the optimal scheduling of generation if the total load is 640.82 MW. Estimate value of λ=12Rs/MWh. The transmission power loss is given by the expression:
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Draw the block diagram of a two area system and prove that the integral control reduces the static error in frequency and the line power flow to zero.
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Explain the effects of regulation and dead band on speed governor and automatic generation control.
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Two alternators rated for 110 MW and 210 MW have a governor droop characteristic of 5% from no load to full load. They are connected in parallel to share a load of 250MW. Determine the load shared by them. Assume free governor operation.
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Differentiate between steady state and transient stability of a power system. Discuss the factors that affect them.
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Explain the step by step method of solving the swing equation. Compare it with equal area criterion method.
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Explain the hardware components and functional aspects of SCADA system using a fundamental block diagram.
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