Monitoring of Angular Separation in Power system : Basic concepts : Part 1

 Monitoring of Angular Separation in Power system : Basic concepts

In an AC power system, power angle can be monitored by measuring voltage phasor or specifically say then its positive sequence voltage angle measurements. Positive sequence components represent the balanced power system.  In view of that, its angle is used for angular separation monitoring purposes as even during disturbance it provides a good overview of the angular separation between two nodes of the system.

Now let us learn about the methods for angular separation (power angle) monitoring in an AC power system.  Grid operator across the world uses in general five ways for power angle monitoring.

These methods have come over time and are being utilised by system operator based on technological advancement in area of instrumentation, computing and communication. When we are talking about usage by the system operator then it means that angle of power system measured from a substation located far across has to be brought to the Control centre where system operator is working for its monitoring and usage. 

This will in general involve four stages : 

• Measurement at the substation (field-level values)

• Field level transformation into digital information to sent to control centre

• Communication media through which information will be communicated to control centre

• Control Centre processing of field information into usable information

Now let us learn about each of these methods one by one :

1.    Offline simulation study for angular separation monitoring

Also called as Mathematical model-based measurement. 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:

1. 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.
2. 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.
3. 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.

Reference :

• S.K.Soonee, S.R.Narasimhan, R.K.Porwal, S.Kumar, Rajesh Kumar, Vivek Pandey, “Application of phase angle measurement for real time security monitoring of Indian electric power system- an experience”, CIGRE Paris 2008 https://e-cigre.org/publication/C2-107_2008-application-of-phase-angle-measurement-for-real-time-security-monitoring-of-indian-electric-power-system--an-experience