As has been discussed earlier, the network as simulated in the Power Flow engine is the critical link between machine models. Because of this (and because of our user’s familiarity with the oneline display in the Power Flow Focus), in the DS Focus, the oneline and the resulting power flow solution have again been brought forth as central for simulation interaction and visualization. You will see this first-hand as you enter the DS Focus, where the oneline is presented with the results of the pre-simulation power flow solution. This solution is unique and has subtle differences from the Power Flow Focus solution (documented just above). Given that DS is a real-time simulation, there is a need to know what is displayed on the oneline and how it benefits the user in the DS Focus.
2.8.1 Normal Oneline Display
The oneline display can be summarized as follows:
1. After entering DS Focus, the oneline displays the resultant power flow solution that represents the initial conditions of your simulation. As noted above, if induction motors are present, the results on the oneline may change slightly from results seen in the Power Flow Focus.
2. After a simulation is performed, the oneline displays the resultant network conditions present at the end of the simulation.
3. After right clicking on a switching device and selecting either “Open and Resolve PF” or
“Close and Resolve PF”, the oneline is resolved and displays newly updated initial conditions for your simulation while including your selected switching device status change.
4. After resetting a simulation using the Simulation Reset button, the power flow resolves;
the oneline displays the initial conditions of your simulation, taking into account whatever initial condition network changes you made in 3.
Simulation Reset
5. When a new script is selected from the Script Load drop down list box, the power flow resolves; the oneline displays the initial conditions of your simulation, taking into account whatever initial condition network changes you made in 3.
6. When DS Options are changed, the power flow resolves; the oneline displays the initial conditions of your simulation, taking into account whatever initial condition network changes you made in 3.
Thus, while in DS focus, the user has the ability to change the steady-state initial conditions used to perform a simulation. You may ask “why?” Among other reasons, this feature is essential if one wishes to perform a quick and easy motor starting simulation. Typically, a user will enter a new focus with their system model set up as it is normally running. This means that the DS Engine is ready to run a simulation based upon the steady-state initial conditions that it has been supplied. But before starting a motor, it needs to be set offline, and the system needs to be resolved in order to supply a new steady-state condition. To accomplish this, one could go back to the DB Focus, open the motor breaker, re-enter the DS Focus, and then double click on the open motor breaker. A simulation will thus be triggered which runs steady state for a pre-set time (according to the DS Options dialog – Double Click settings), closes the motor breaker, and then runs to a pre-set simulation end time.
From this example, we also see that the fundamental behavior of a double-click action on a breaker has changed in the DS Focus. In the Power Flow and Short Circuit Foci, a double-click action on a breaker changes the steady-state condition of the system (before solving the power flow, and before application of the fault). In DS, double-click actions perform simulations, similar to the Harmonics Focus. Double clicking on a bus in Harmonics causes a frequency scan to be performed. Double clicking on a bus in DS Focus causes a bus fault simulation to be performed.
So, a method is needed to change the steady-state system in DS Focus, without having to go back and forth between DB and DS Foci. The method selected is that of right mouse clicking on the switching device, and selecting “Open and Resolve PF” or “Close and Resolve PF”. After that action, the system is resolved and set up according to the third method listed above. Now, for a newly opened motor breaker, for example, a simple double click on the breaker is all that is needed to start the motor.
2.8.2 Stepping Oneline Display
One enhanced feature of the oneline, that we believe has not been done in any dynamic simulator to date, is the ability to see system wide network conditions during a dynamic simulation. To do this, however, one would need to freeze the simulation in time so that an update in the display can be made. This is accomplished through the stepping feature in DS.
The stepping feature makes use of the following button:
Simulation Step
This button is similar to the Simulation Run button:
Simulation Run
but only runs the simulation up to a point in time defined by the Tools / DS Option / Step Run Delta Time. This parameter basically defines how long the simulation will run before it is paused, and updates the oneline with the condition of the entire network. Multiple presses of the Simulation Step button will thus propel you through the simulation (defined by the presently selected script) one moment at a time, while pausing and allowing you to view conditions in the entire network.
For example, the default value of Step Run Delta Time is 0.1 seconds. Just after entering DS Focus or after a Simulation Reset, if the Simulation Step button is pressed with this setting, the simulation will run from 0.0 to 0.1 seconds, update the plot window for that brief time-span in the simulation, and then update the network conditions on the oneline. If pressed again, the simulation will run from 0.1 to 0.2 seconds, update the plot window, and then update the oneline window. This continues as you press the Simulation Step button until you reach the end of the simulation.
Figure 28. Example of system with script stepped several times into the simulation.
We suggest trying this feature at least once to expose yourself to the concept of updated oneline results during a dynamic simulation. Users with a history of using other stability simulation software may skip over this feature, as it is unique and is not a standard offering in other DS software. The power of this feature is addicting, as you find how fast you can move around the oneline to observe system conditions during a contingency event at any frozen moment in time.
An example of a system with a motor start simulation that is being stepped is shown in Figure 28. The Step Run Delta Time is set to 0.1 seconds, and so the simulation has been stepped several times to get to about 1.3 seconds. Notice the conditions of the system on the oneline, and how both temporary bus under-voltage and transformer overload are highlighted in red, according to the present settings in the Power Flow Options. Also, notice that the selected script is “Start Motor 3”, which was specifically written to start the motor at 1.0 second, and run to 3.0 seconds. The x-axis of the DS Plot Window is set to the maximum time found in the script. To see results for a step run simulation, plot channels must be defined. The example in Figure 28 has plot channels defined to monitor MCC-3 bus voltage, motor current and transformer current.
A quick pan of the oneline up to the generator bus (Figure 29) shows that the system voltage is just fine at 0.985 pu at this time in the simulation.
Figure 29. Figure 28 panned up to the generators bus.