code: def _get_voltage_angle_var(self, refs, buses):
          
          Va = array([b.v_angle * (pi / 180.0) for b in buses])
  
          Vau = Inf * ones(len(buses))
          Val = -Vau
          Vau[refs] = Va[refs]
          Val[refs] = Va[refs]
  
          return Variable("Va", len(buses), Va, Val, Vau)
docs: Returns the voltage angle variable set.