CONTINUOUS-TIME MODEL OF INVERTER-BASED FACTS DEVICES FOR PSCADIEMTDC
5.3 Continuous-Time Inverter Model In PSCADIEMTDC
This thesis, in its entirety, has developed all its inverter-based FACTS device models in the simulation package PSCADIEMTDC. The PSCADIEMTDC software program was originally developed for studying HYDC systems. However, over the years many models and capabilities have been added and at present PSCADIEMTDC is now a versatile tool to study AC as well as DC power systems problems [56]. PSCADIEMTDC comprises two modules: the EMTDC computing engine itself and the PSCAD design environment in which the power system to be simulated is constructed graphically from a block diagram library of pre-coded components and models; the parameters of each model are entered by the user in the PSCAD environment via dialogue boxes that are associated with each component's block diagram.
PS CADIEMTDC, however, also offers the facility for users to design their own customized components in the form of user-defined models written in FORTRAN code. Introducing a new, user-defined model to the program requires three main development steps.
1. Designing a PSCAD functional block diagram representation, together with the necessary parameter dialogue boxes, for the new model; these will then appear in the block diagram component library;
2. Carrying out the necessary programming within PSCAD to allow interfacing of the new component's functional block diagram to the model itself in the EMTDC computing engme;
3. Programming the necessary equations of the model in a subroutine that will be called by the EMTDC computing engine at each time step during a simulation.
Within a user-defined component in PSCAD, it is possible to define programmable current and voltage sources to supply the electrical output terminals of the model, with the equations for these current and voltage sources then written in FORTRAN code. It is also possible to define, as inputs to the user-defined model, either control variables or electrical measurements (voltages and currents) from an external network. Similarly, the user-defined models are able to supply the external circuit with control variables and electrical measurements as outputs if so desired.
The user-defined continuous-time inverter model of a three-level, 24-pulse voltage sourced inverter is defined to have eight electrical terminals and two control input terminals as shown schematically in Fig. 5.2.
At the dc electrical terminals the voltage Vdc applied to the inverter is measured as an input to the model while at the AC terminals the currents ia, hand ic in the inverter's three-phases are also measured as inputs. Finally, the variables y and e, from a high-level inverter controller are imported as control inputs to the user-defined model.
The facility that allows users to design their own customized components in PSCADIEMTDC, as previously mentioned, is a program within PSCADIEMTDC called the Component Wizard [80].
The continuous-time inverter model described in the previous section has been developed as a user-defined model in PSCADIEMTDC by using this program. The Component Wizard aided in creating a continuous-time inverter functional block whose property is divided into three main sections, namely, the Graphics, Parameter and SectionsNode sections. The Graphics section allows the user to create the visual graphics of the new component; a simplified inverter functional block similar to that in Fig. 5.2 is created in this section. The Parameter section allows the user to enter any required parameters related to the functionality of the component.
This particular section is left blank as it is not a design requirement for the simplified inverter model. The SectionsNode section holds all the necessary code for the designed component that to be calculated by the EMTDC simulation engine at each time step. The code for current
injection,Idc , which is responsible for representing the charging and discharging characteristics of the dc capacitor, is placed in this particular section (i.e. equations (5.5) and (5.6)). The necessary parameters and variables are then passed in between the SectionsNode section and a separate subroutine that contains the code for the ac voltage sources (based on the equations (5.1), (5.2), (5.3), and (5.4)). The subroutine is then called by the EMTDC computing engine for simulation.
At each simulation step, these equations (coded in FORTRAN) are solved and used to calculate the correct electrical values of the inverter model, namely, Vu, Vb and Vc and Idc ' Finally, the user-defined voltage and current sources in the inverter model are updated with these calculated values of Vu, Vb and Vc and Idc and these are passed to the main EMTDC solution of whatever external electrical network is connected to the user-defined inverter model (on both its dc and ac sides).
t r
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Fig. 5.2: Schematic representation of the continuous-time inverter model developed in PSCADIEMTDC.
The full details of the design of the continuous-time inverter model with all its components and embedded code in PSCADIEMTDC are shown in Appendix El.
The following section compares some of the SSSC, STATCOM and UPFC time domain results produced by the simplified PSCADIEMTDC model developed in this chapter with the results from the detailed models in Chapter Three and Chapter Four. Appendix E2 then also presents a more detailed performance study of the SSSC, using the simplified model, to determine if the SSSC's characteristics at subsynchronous frequencies are retained in the simplified model.