Q3.1 (5 marks) Discuss and explain the conditions required to be fulfilled befor
ID: 2293845 • Letter: Q
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Q3.1 (5 marks) Discuss and explain the conditions required to be fulfilled before synchronising a generating unit to a power system. Use phasor diagrams, if convenient. 03.2 (5 marks) The synchroscope employs six light bulbs, two in series in each phase. Explain why the designer used of six, instead of three bulbs Q3.3 (2 marks) A transmission line having a characteristic impedance of 300 ohms is cut into two unequal sections, the shorter section being one-third the length of the original line. The characteristic impedance of the this section is (a) 100 ohms (b) 200 ohms (c) 300 ohms (d) None of the above. Choose the correct answer and explain why the chosen answer is correct.Explanation / Answer
Solution 3.1 Four necessary conditions must fulfill before synchronizing the generator with the power system. All are listed below
Phase sequence phase sequence of three phase alternator must be same as the power system. This problem comes in picture when installation or after maintenance connection with the power system.
Voltage magnitude.RMS voltage of incoming alternator must be same as the RMS voltage with the power system to be connected. when the voltage magnitude less then the power system voltage alternator absorb more reactive power for the power system.
when the voltage of the alternator is more than the power system to be connected high reactive power flow from the alternator to the grid.
Frequency- frequency of incoming alternator must be same as the frequency of power system. if not same results high acceleration and deacceleration in prime mover that increases transient torque.
Phase angle- phase angle between incoming generator voltage and voltage of the power system
Solution 3.2 -Reson two use two bulbs in series with each phase- the oldest and simplest means of determining if the incoming voltage is in phase with the running voltage is using two lamps connected in series across the phase wires of the incoming and running voltage signals. When the two voltages are the same magnitude and angle, the voltage across the lamps will be zero, and they will go out. When the two voltages are 180 degrees out of phase, the lamps will have the sum of the two voltages across them and burn brightly. An operator will initiate breaker close when the two lamps are at their dimmest.
Solution 3.3- option (c)300 ohm
Characteristic impedance or surge impedance of a uniform transmission line is the ratio of amplitude voltage and current of a single wave propagating along the line in one direction in the absence of reflections in the other direction. Characteristic impedance is determined by the geometry and materials of the transmission line and, for a uniform line, is not dependent on its length
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