JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Tech III Year II Semester POWER SYSTEMS ANALYSIS Previous Question Paper (August/September - 2024 - R16)
Note: i) Question paper consists of Part A, Part B. ii) Part A is compulsory, which carries 25 marks. iii) In Part B, Answer any one question from each unit.
Part A
What is primitive impedance matrix?
Answer
A primitive impedance matrix is a diagonal matrix that represents the individual impedances of the elements (branches) of a power system network when they are considered in their isolated, uncoupled state. It serves as the fundamental building block for constructing the system-wide bus impedance or admittance matrices.
Key characteristics include:
- It is a square matrix of size b × b, where b is the total number of branches in the network.
- Since each element is considered independently, the matrix is diagonal, meaning all off-diagonal elements are zero.
- The diagonal entries represent the self-impedance of each individual branch.
In this matrix, each element zi corresponds to the impedance of branch i. If mutual coupling exists between branches, the primitive impedance matrix becomes a full matrix with non-zero off-diagonal elements representing the mutual impedances between those specific branches.
How the Zbus is modified when a branch of impedance Zb is added from a new bus-p to the existing bus-q?
Answer
When a new bus p is added to an existing system and connected to an existing bus q through a branch impedance Zb, the size of the Zbus matrix increases from n × n to (n+1) × (n+1). The modification process follows these steps:
- The existing n × n impedance matrix remains unchanged as the top-left submatrix of the new Zbus.
- The new row and column (representing the new bus p) are calculated based on the existing elements of the q-th column of the original matrix.
- The diagonal element for the new bus p is given by: Zpp = Zqq + Zb
- The off-diagonal elements for the new row and column are equal to the corresponding elements in the q-th column of the original matrix: Zip = Zpi = Ziq for i = 1, 2, ..., n
This results in the new bus impedance matrix having the form:
How is the swing bus selected in a load flow study?
Answer
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What is the necessity of power flow studies?
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The approximation to the N-R method is based on two factors for decoupled load flows. What are they?
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What are the assumptions are considered for fast decoupled load flow method?
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How can select the base quantities?
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Define positive, negative and zero sequence components.
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Distinguish between steady state and transient stability.
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State the assumptions made in the solution of the swing equation.
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Part B
Develop the relation between bus admittance matrix, bus incidence matrix and primitive admittance matrix.
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For the network shown in figure 1, draw the graph and tree. Also determine the Ybus matrix by direct inspection method. All the mentioned values are impedances in p.u.
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Using the method of building algorithm find the bus impedance matrix for the network shown in figure 2.
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Discuss the bus classification and their significance in details.
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Derive the static load flow equations of a power system. Explain the N-R method of solving the load flow equations.
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Three bus power system given in figure 3. Bus (1) slack bus, Bus (2) PV bus, Bus (3) PQ bus. Carry out one iteration of load flow solution by Gauss-Seidal method. Take Q limits at generator bus 2 as 0 <= Q <= 4.
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Compare the Gauss-Seidel and Newton-Raphson methods for power flow solution.
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Derive the expression for DC load flow method? List out its assumptions.
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For the power system data are given in tables, determine the voltage profile using Decoupled load flow method at the end of first iteration.
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What is per unit system. Write different expressions that may useful in per unit system for single phase and three phase systems. Derive the necessary expressions.
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A power system has three generating units each rated at 7500kVA with 15% reactance. The plant is protected by a tie-line system with reactance’s rated at 7500 MVA and 6%. Determine the fault kVA when a short circuit occurs on one of the sections of bus bars. If the reactors were not present what would be the fault kVA.
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Derive an expression for power in a 3-phase circuit in terms of symmetrical components.
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A 3-phase generator noted 20 MVA, 12 kV has a solidly grounded neutral. The sequence impedances of the alternator are Z1= j0.4, Z2= j0.35 and Z0= j0.05 p.u. What values of (i) resistance and (ii) reactance must be placed in general neutral for a LG fault of zero fault impedance to the rated line current?
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What are the factors affecting the stability of a power system? Explain.
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A double circuit line feeds an infinite bus from a power station. If a fault occurs on one of the line and the line is witched off, derive an expression for the critical clearing angle.
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Give flow chart for transient stability analysis of a multi machine power system and explain each step.
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Consider the system is shown in figure 4. A three phase fault occurs at point ‘P’ as shown at the midpoint on line 2. Determine the power angle characteristics for the system with fault persisting.
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