code: def _p_iteration(self, P, Bp_solver, Vm, Va, pvpq):
          
          dVa = -Bp_solver.solve(P)
  
          # Update voltage.
          Va[pvpq] = Va[pvpq] + dVa
          V = Vm * exp(1j * Va)
  
          return V, Vm, Va
docs: Performs a P iteration, updates Va.