100 MCQ OF Synchronous Generator or Alternator with Explanation

Answer.1. With fixed frequency

Explanation:

• In an alternating current electric power system, synchronization is the process of matching the speed and frequency of a generator or other source to a running network. An AC generator cannot deliver power to an electrical grid unless it is running at the same frequency as the network. If two segments of a grid are disconnected, they cannot exchange AC power again until they are brought back into exact synchronization.
• Hence alternators are usually designed with the fixed frequency of AC voltage.

Answer. 2. 32 MW, 48 MW

Explanation:-

Change in frequency from No load to full load

f = 50 x 0.05 = 2.5Hz

Full load frequency

f_fl1 = 50 – 2.5 = 47.5Hz

Since both alternators have same speed regulation then

f_fl1 = f_fl2

From the above diagram

For machine 1

(50 – f)/(80 – x) = (50 – 47.5)/40

x – 16f = 80 – 16 x 50

x- 16f = -720—–1

For machine 2

(50 – f)/(80 – x) = (50 – 47.5)/60

x – 24f = -1200 ——-2

From equation (1) and (2)

x= 48 MW

f = 60 Hz

So, machine A operates at a load of 48 MW While machine B will operate at a load of

80 – 48 = 32 MW

Answer.1 20.25

Explanation:-

The per-unit impedance is given as

$\begin{array}{l}{Z_{pu\;new}} = {Z_{pu\;old}}\left( {\dfrac{{MV{A_{base\;new}}}}{{MV{A_{base\;old}}}}} \right){\left( {\dfrac{{K{V_{base\;old}}}}{{K{V_{base\;new}}}}} \right)^2}\\\\ = 0.9 \times \dfrac{{50}}{{20}} \times {\left( {\dfrac{{33}}{{11}}} \right)^2=20.25}\end{array}$

Answer.4. Unity

Explanation:-

At unity p.f., the effect of armature reaction is merely to distort the main field; there is no weakening of the main field and the average flux practically remains the same.

At zero p.f. lagging, armature reaction is directly demagnetizing and the armature reaction weakens the main flux. This causes a reduction in the generated e.m.f.

At zero p.f. leading, armature flux is now in the same direction as the field flux and, therefore the armature reaction strengthens the main flux. This causes an increase in the generated voltage.

Answer.1. Magnetic Flux

Explanation:-

The armature leakage reactance is a parameter that accounts for that part of the magnetic flux produced by the armature that does not cross the air gap to link with the rotor winding. The flux leakage is formed by end-winding leakage, slot leakage, and tooth tip leakage fluxes.

Answer. 1. Small diameter and long axial length

Explanation:-

Smooth Cylindrical Type or Non-Salient Pole alternator rotor is used for steam-driven alternator i.e turbo alternator which runs at very high speed.

• This type of Rotor is used for steam-driven alternator i.e turbo alternator which runs at very high speed.

• The Rotor is made up of smooth solid forgings of alloy steel cylinder having a number of slots along the outer periphery.
• The field windings of the cylindrical type rotor are connected in series to the slip rings through which they are excited by the DC exciter.
• The top portion of the slot is covered with the help of steel or manganese wedges and the unslotted portion of the cylinder acts as the poles of an alternator.
• The field windings are arranged in such a way that their flux density is maximum on the polar central line.
• In the cylindrical rotor, the pole doesn’t project out from the smooth surface of the rotor hence they maintain the uniform air gap between the stator and rotor.
• Since the steam turbine runs at a very high speed, therefore, they required less number of poles hence the diameter of the rotor is small, and the axial or rotor length is large.

Answer.3. The synchronous reactance of the generator

Explanation:-

1. Sub-transient (xd”). Subtransient reactance is a value used to determine the short circuit current during the first few cycles after a short-circuit occurs. This is the short-circuit current value to be used in all short-circuit studies.
2. Transient reactance (Xd’). Transient reactance is a value used to determine the short-circuit
   current from the first few cycles up to about 30 cycles after the short-circuit occurs (depending
   upon the design of the machine). This value is often used in voltage regulation studies.
3. Synchronous reactance (Xd). Synchronous reactance is a value used to determine the short-circuit current when the steady-state condition has been reached. Steady-state is reached several seconds afier the shortcircuit occurs. This value is often used to determine the setting of the generator backup overcurrent relays.

Answer 2. Decreased

Explanation:-

Peripheral speed refers to the actual linear speed of a tooth or point of the circumference of an alternator when it is revolving under power. Peripheral speed is thus the product of the circumference of the alternator and the number of revolutions per minute.

The peripheral speed of the alternator

Ps = π × D × RPM

The peripheral speed depends on the speed as well as the diameter of the rotor. So to reduce the peripheral speed of the alternator the diameter of the rotor should be decreased.

Answer. 2. -j 1.67 pu

For symmetrical fault, the fault current is given as

I_f = E/(Zi +Zn)

Where E = Pre fault voltage Which is equal to 1

Z_i = poisitive impedance
Z_n = Neutral Impedance

Zi = 0.5j & Zn = 0.1 j

I_f = 1/(0.5j + 0.1j)

I_f = -j 1.67

Answer. 2. 1228.81 volt

Explanation:-

E.M.F Equation of an alternator is given as

E = K_c K_d √2π f Φ N_p

Or E = 4.44 K_c K_d  f Φ N_p    ………….. (since √2π = 4.44)

Where

K_c = Coil span factor

K_d = Distribution factor

Φ = Flux per pole

f = frequency

Np = Turns per phase

E = 4.44 × 0.955 × 0.966 × 50 × 25 × 10^-3 × 240

E = 1228.1 volts