Continuing with our discussion over angular separation measurement, the first method is “Offline simulation study”.
Offline simulation study (Mathematical model) : In this method, the actual system is represented by means of a mathematical model. In this entire power system is modeled with power system nonlinear differential-algebraic equations (DAEs) based on Differential Transformation.
There are two types of studies used by power system engineers: load flow analysis (power flow analysis) and transient stability analysis. ?One important fact is that for all types of angle measurement, there will always be a reference node whose angle will be used for calculating the angle difference between two nodes.
Let us discuss now on angular separation is calculated by Load flow (steady state analysis) and Transient stability analysis.?
- Load Flow Analysis: In load flow analysis one Bus/node is taken as Slack Bus ( V=1, δ=0). This bus in general is a generating bus.?Now w.r.t. to slack bus angle, rest of the angles will be calculated using Kirchhoff's law application on power flow equation.?This slack bus absorbs all uncertainties arising from the solution so even moderate amounts of uncertainty will have an impact on the calculated angle. Therefore, angular separation (power angle) between two nodes in load flow will depend on the scenario which is under study base case, accuracy of system modeling and associated assumptions taken during the study. These angles will represent only one scenario which is under study. For any change in angel with any event, next scenario will be simulated and change in angle will be calculated.
- Transient Stability Study: In transient analysis, nonlinear differential-algebraic equations (DAEs) based on Differential Transformation are solved.?Here one machine or weighted average of all machine’s angle is taken as reference and such practice depending on the solver and method used. ?W.r.t to this machine angle, angle variation of all nodes during any power system events can be simulated and checked. Here also angular separation (power angle) between two nodes will depend on the scenario which is under study base case, accuracy of system modeling and associated assumptions taken during the study.
Usage by System operator:
- System operator uses the angular separation from offline study to ensure that during the envisaged scenarios, there is no violation of angular separation and other various electrical parameters. It helps them in planning contingency measures well in advance for such scenario which can be automatic or manual in nature.
- Offline study also provides a good overview to system operators that under which scenario overall system wide power angle can be maximum. It provides information on the maximum allowable angle in the system based on worst case study. In addition, it helps in accessing the maximum allowable angle in a corridor/flow gate considering reliability criteria(N-1, N-2, N-1-1 etc.) as well as adjacent nodes.
- Offline study also helps in accessing the upper and lower limit of angle for adjacent nodes/flowgates/system for utilization in online angular separation monitoring tools to alert operator in real-time.?
However, such study and associated scenario preparation take its own time. This time can be further reduced based on the preparation of study base case directly using real-time data (load/generation/Vsch/other power system component availability) available from the SCADA/EMS system at control centers. Using this, offline study output can provide a good accurate picture of angular separation in the power system under monitoring. This aspect will be further discussed in EMS/SE based angular separation monitoring in control center.
In the next discussion, let us have a look at Synchrocheck relay/Synchrocheck trolley based monitoring and usage of power angle monitoring.?
Note : Figures used for display has been quoted with their reference/source.
