Comparisons of the Two Modes of Operation

As a comparison between the two different modes of voltage injection, let us first consider the constant reactance mode of voltage injection with a compensation level of 50%. Choosing V² / X as the base power, the power-angle characteristic reaches a maximum of 2.0 per unit at a load angle π / 2. Now | V_Q | in constant voltage mode is chosen such that the real power is 2.0 per unit at a load angle of π / 2. This is accomplished using (10.29) where we get

per unit

The power-angle characteristics of the two different modes are now drawn in Fig. 10.16 (a). It can be seen that the two curves match at π / 2. However, the maximum power for constant voltage case is about 2.1 per unit and occurs at an angle of 67° .

Fig. 10.16 (b) depicts the line current for the two cases. It can be seen that the increase in line current in either case is monotonic. This is not surprising for the case of constant reactance mode since as the load angle increases, both real power and line currents increase. Now consider the case of constant voltage control. When the load angle moves backwards from π /2 to 67° , the power moves from 2.0 per unit to its peak value of 2.1 per unit. The line current during this stage decreases from about 2.83 to 2.50 per unit. Thus, even though the power through the line increases, the line current decreases.