5.4 STEADY STATE RESPONSE

The purpose of automatic load frequency control (ALFC) is to maintain the desired output of a generator unit while controlling the frequency of the larger interconnection. The ALFC also helps to keep the net interchange of power among pool members at pre-determined values. One of the objectives of the loop is to maintain constant frequency in spite of floating loads.

Output → ∆f

Inputs → ∆P_C, ∆P_D

where ∆P_C = Change in speed changer setting, and

∆P_D = Change in load demand.

The speed changer has a fixed setting, i.e. ∆P_C = 0 and the load demand changes.

This is called as free governor operation.

(i) Let us assume that the speed changer has a fixed setting ∆P_C (S) = 0.

Assume that there is a small step of change of load demand

Fig 5.5 Single area control with ∆P_C (S) = 0

K_T, K_P are fixed for turbine and power system. K_g is the speed governor gain. It can easily adjustable by varying the lengths of various links.

(ii) There is a step change in speed governor setting and the demand remains constant, i.e.,

Fig 5.6 Single area control with ∆P_D (s) = 0

If the speed changer setting is changed by ∆P_C and the load demand changes by ∆P_D, the steady state frequency change is obtained by superposition theorem, i.e.,

Fig 5.7

R → Speed Regulation

D → Demanding coefficient

Typical values of D = 0.01 MW/Hz, R = 3.

∆ƒ ≈ –R∆P_D