1) a) The three phase ratings of the three winding transformer are
Primary: Y connected, 110 kV, 20 MVA
Secondary: Y connected 13.2kV, 15 MVA
Tertiary: Δ connected, 2.1 kV, 0.5 MVA
Three short circuit tests carried out on this transformer provides the following results
i) Primary excited, secondary shorted: 2290 V, 52.5 A
ii) Primary excited, Tertiary shorted: 1785V, 52.5 A
iii) Secondary excited, Tertiary shorted: 148 V, 328 A.
Determine the p.u. impedances of star connected single phase equivalent circuit for a base of 20 MVA, 110 kV in the primary circuit.
b) Describe the requirement for the power system analysis in planning and operation.
2) a) A 3- phase transmission line is feeding from the 23/230 kV transformer at its sending end. The line is supplying a 150 MVA, 0.8 pf load through a step down transformer of 230 / 23 kV. The impedance of the line and the transformer is (18+j60) ?. The sending end transformer is energized from the 23 kV supply. Find the tap setting for every transformer to manage the voltage at the load at 23 kV.
b) Deduce the formula that is used in the above problem.
3) A salient pole generator without dampers is rated as 20 MVA, 13.8 kV, and has the direct axis sub transient reactance of 0.25 pu. The negative and zero sequence reactance are 0.35 and 0.10 pu respectively. The neutral of the generator is solidly grounded. Find the sub transient current in generator and the line to line voltage for sub- transient conditions when a single line to ground fault occurs at the generator terminal with the generator operating unloaded at rated voltage. Neglect the resistance.
4) The Y bus matrix for a network is given below
Y bus = {-j5.833 j 2.5 j 3.333
j 2.5 - j 7.5 j 5
j 3.333 j 5 -j 8.333}
Find the complex bus bar voltage at the end of first iteration, by using the gauss seidel method. Bus 1 is a slack bus with V1=1.05?0. Bus 2 : PV bus, V2=1.0 pu , Pg =3 pu, Bus 3: PQ bus, P3=4 pu and Q3 = 2 pu.
5) Write down the algorithm steps for loading the flow solution by using the Newton Raphson method.
6) Explain the equal area criterion for transient stability analysis of the system for the modification in the mechanical input.
7) A synchronous generator having a reactance of 1 pu is connected to an infinite bus through a transmission system with a reactance of 0.7 pu. The generator is running on no load with a voltage of 1.1 pu. Consider H = 4.5 MW-s/ MVA. The voltage of infinite bus is 1?0 pu and its frequency is 50 Hz. Determine the frequency of the natural oscillation if machine is abruptly loaded to
i) 60 % and
ii) 75 % of its maximum power limit.
Neglect the resistance and the machine damping.