Significance of SCR

Significance of SCR :

1. For low value of SCR, the value of X_S is more hence the drop IaXS is more. Hence the machine requires large changes in the field current (excitation) for the small changes in the load, to keep terminal voltage constant.

2. A low value of SCR indicates smaller air gap and poor regulation due to large I_aX_S

3. The synchronous power is inversely proportional to X_S. This is the power which keeps alternators in synchronism during parallel operation and maintains the stability. Any disturbances from equilibrium conditions are compensated by synchronizing power. For low value of SCR, X_S is very large and synchronizing power is very low. As synchronizing power decreases, tendency of alternators to remain in synchronism decreases. This decreases the stability. Thus low SCR puts the stability limit.

Key Point : Lower the value of SCR, low is the stability limit. 

4. The SCR can be increased by increasing the air gap but this needs more        to obtain same e.m.f. Hence the pole size increases which increases the overall size and cost of the machine.

5. Practically the SCR value is selected between 0.5 to 1.2.

Example 2.13.1 Show that the short circuit ratio of an alternator is the reciprocal of the per unit value of the synchronous reactance at rated value.

Solution : The Short Circuit Ratio (SCR) is defined as the ratio of the field current required to produce rated voltage on open circuit to the field current required to produce rated armature current with the armature terminals shorted while the machine is mechanically rim at N_S.

Consider O.C.C and S.C.C. shown in the Fig. 2.13.2.

Thus short circuit ratio is reciprocal of X_S adjusted to saturation at rated value.

Example for Practice

Example 2.13.2 A 2200 V, 50 Hz, 3-phase, star connected alternator has an effective resistance of 0.5 Ω per phase. A field current of 30 A produced the full-load current of 200 A on short circuit and a line to line e.m.f. of 1100 V on open circuit determine.

i) The power angle of the alternator when it delivers full-load at 0.8 p.f. (lagging).

Ii The SCR of the alternator.

[Ans.: S = 14.35°, SCR = 0.3189]

Review Question

1. Explain the significance of short circuit ratio.