How does psse determine the current injection for a constant power load during dynamic simulations during low voltages?

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The load characteric is described in Chapter 5.5 in Program Application Guide, Volume 1. The constant load is reduced at voltages below the value set by parameter PQBRAK (see General tab in Solution Parameters). The default value of PQBRAK is 0.7.

Please note that the figures in the manual are misleading. Figure 5-3a shows the constant load MVA characteristic when PKBRAK = 0.5, while Figure 5-3b shows the constant load current characteristic when PKBRAK is approximately 0.7. The two curves do thereby not correspond to each other.

Neither constant current nor constant impedance characteristic is used for constant MVA load load below breakpoint PQBRAK. Instead an elliptical function is used, whose details are not revealed in the manual.