Synchrophasorapplication for protection scheme
Electrical power systems in Southamerica are mainly characterized by long high-voltage transmission lines,interconnecting power plants or substations and loads, carrying large amounts of power. These topological features urge the study of angular and voltage instability in both, normal and critical conditions.In order to ensure security standards in these kind of power systems, new means of detection and protection schemes must be developed so that transient instability, regarding angular and voltage, could be avoided.
This paper consistsin the detection and mitigation of angular and voltage instability by using synchrophasorschemes to face extreme power system contingencies.
The case under study analyzes the generation outage of 1360MW in the northern area of SIC Chilean power system (featured by long 500kV transmission lines in south-north direction), which represents the 30% of the total power installed meaning an extremely severe event.
As a result of the fault event being analyzed, SIC subsystem gets divided into two subsystems from Charrúa substation to the north (central-northern area) and to the south (southern area). After transients simulations, it was concluded that such large amount of electrical power outage induces an angular instability between the central and northern areas in the SIC, mainly caused by the interaction between thermal generating units in the northern area and long 220kV transmission lines that connect to substations located in the central area.
In other words, due to the unbalance between generation and load, synchronous machines located in the northern increased their injected power and thus the transferred north to central power boosted from 30% to 75%. As a result, source angle differences started to grow causing a synchronous decoupling between both subsystems followed by voltage drops before 300ms having occurred the fault event.
The figure below depicts the instability phenomena mentioned above, related to the synchronous decoupling between northern and central subsystems.
The second after having cleared the fault, the northern and central subsystems frequencies presents a difference greater than 1.5Hz. The lack of generation in the central-northern area initially induces the increase of injected power by the remaining units in the north. Therefore, power transferred becomes greater which directly affects load angles in busbars.
Because of the high participation of thermal generating units in the zone, they are very likely to be in service during frequent operation scenarios which improve power systems instability chances due to their initial electrical power contribution. The following picture shows the load angle difference between the most representative busbars in the power subsystem, due to the electrical power transient contribution of generating units in the northern area.
The angular instability cannot be avoided by the control systems of generating units. As an alternative, although lowering the power transference through 220kV links could be performed by automatic generation shedding (AGS), it would be impractical because of the lack of generation in the central-northern subsystems. This action would induce greater unbalance causing sub-frequency instability issues. Moreover, contingencies under study are located 900km away from de operation and control center making it difficult to take control actions fast enough. On the other hand, due to the low occurrence probability of the fault event, installing new primary compensation equipment (SVC, STATCOM, etc.) does not represent a technical-economic feasible solution.
Mitigation scheme proposed
The angular instability between northern and central subsystemscould be avoided by the disconnection of a 220kV transmission double circuit link. For this purpose, conventional distance protection schemes are not fast enough to avoid voltage and frequency getting dangerous operating levels. Therefore, the most suitable solution that could foresee angular instability problems is the application of a synchrophasors scheme.
The implementation of this scheme is achieved by installing a real-time monitoring system that measures the busbar angular difference at both ends of the transmission line. In order to establish the necessary thresholds and conditions for the fastest angular instability detection, several operation scenarios are studied. To that end, two conditions that must occur simultaneously are set: angular difference and its rate of change. This allows to clearly determine which fault events cause the central-northern subsystem angular instability.
The following picture shows, for different fault events analyzed, the evolution of angular difference (delta) vs rate of change (Sf). As can be seen, stable cases do not reach tripping zone which allows the 220kV link to keep interconnecting central and northern subsystems as a unique. On the other hand, cases that induce angular and voltage instability are fast detected by the synchrophasor protection scheme allowing the split of central and northern subsystems.
Basing on these results, tripping times lower than 300ms positively improve central and northern subsystems leading to stable operating conditions.
- Electrical studies regarding defense schemes against severe contingencies concluded in the need to perform a method that could rapidly detect and take decisions in order to avoid angular/voltage instability.
- Due to the fault type and its location, it is not possible to implement traditional methods (generation and load automatic shedding schemes) to avoid the power system collapse.
- Protection devices installed in the systembusbars did not result fast enough to detect instability and take actions before power system collapse.
- Synchrophasor protection schemewas designed and located in two critical busbars, in order to split the electrical subsystem in two once the instability phenomena was detected. Nodes chosenenable, simultaneously, the instability detection before it succeeds and later stabilization of both subsystems split.
- The synchrophasorprotection scheme proposed ensures the prediction of instability conditions, achieves the fault event detection and control measures. Proper actions are taken in order to return the stability conditions required by the power system. Particularly, the study case analyzed proposes the split in two power subsystems so that angular and voltage stability would be sustained by their own control and protection schemes.