Centralized Remedial Action Schemes – The Next Line of Defense for Power Systems
By Benjamin Coalson, Amos Ang, and Manuel Avendaño
The interconnection of large numbers of renewable generating plants in remote, environmentally sensitive areas presents challenges to the reliable operation of the power grid. Constructing new transmission infrastructure to mitigate reliability concerns is generally costly, time-consuming, and often not viable due to the environmental constraints. To support a clean energy future, minimize environmental impacts, reduce construction and operation costs, and ensure grid reliability, Southern California Edison (SCE) has implemented the Centralized Remedial Action Scheme (CRAS).
SCE Transmission Network Challenges
The deregulation of California’s electricity market in 1996, coupled with the aggressive state-mandated portfolio standards for renewable energy sources, have resulted in a large increase of independently owned generating plants seeking interconnection to the SCE grid. The nature of these plants, mainly wind turbine and solar photovoltaic facilities, is such that the prime locations are in remote areas (where open space, wind, and sunlight are plentiful) connected to the major load centers in the greater Los Angeles, California basin by a handful of transmission corridors. As more generation comes online, the total power flowing on these transmission corridors reaches or exceeds available capacity, causing overloads and other adverse conditions during contingency outages. These conditions require some form of mitigation in order to avoid damage to equipment and maintain a stable power grid.
Mitigation comes in two flavors, SCE can either build transmission lines or install transmission system Remedial Action Schemes (RAS). Building new transmission lines are very expensive and it is a long drawn-out process. Installing RAS is relatively inexpensive and can be accomplished in a shorter time frame. The objective is the same which is to protect system reliability and keep the system safe. RAS keep the system safe by being the next line of defense after the protection system has activated to clear a fault. RAS actions would then keep the system in a secured operating state so that reliable operation is maintained. In fact, almost all bulk power lines bringing generation or imports into the greater Los Angeles basin load area, as depicted in Figure 1, are being monitored and controlled by local RAS schemes.
Figure 1. Power flows (red arrows) to the Los Angeles basin load area.
SCE has implemented 20 RASs across its service territory. These individual systems of relays and logical algorithms are designed to deliberately and automatically disconnect generators, load, or both under certain identified system conditions. Each RAS is currently designed as a stand-alone system, including all the relays, instrumentation, and other equipment necessary for the RAS to function as designed.
Although stand-alone RASs have worked in the past these configurations do not represent an effective solution going forward. As the total number of RASs increases, and the interaction between RASs in the same substations occurs more frequently, it becomes significantly less feasible to implement the needed RAS in a timely, cost-effective, and operationally sound manner. Figure 2 shows SCE’s forecast of RAS deployment and depicts the complexity of these RAS having to interact with new and existing RASs. As the new generation interconnections drive the need for more and increasingly complex RASs, currently implemented technologies are hitting their limitations to address how multiple RASs interact under a wider range of circumstances.
Figure 2. Deployment of Stand-Alone RASs in SCE.
Centralized Remedial Action Scheme (CRAS)
SCE believes a Centralized Remedial Action Scheme, or CRAS, is the more feasible solution going forward for three primary reasons:
- CRAS can elegantly handle the RAS complexity that will be required to interconnect large numbers of geographically clustered generation projects and avoid excessive tripping of generators and customer load;
- CRAS will accelerate schedules for implementing RAS and expedite the work necessary to bring new generators online; and
- The costs of CRAS vs. Stand-alone RAS are significantly offset by the avoided costs of multiple sets of relays that are required when common contingencies or generating units are involved in multiple stand-alone RASs.
CRAS differs from a traditional stand-alone RAS because the logic to mitigate a problem is controlled at a single central processor, rather than within the relays themselves. This overcomes the limitation on logic complexity that is inherent to the relays’ native firmware, enabling more precise management of the interdependency between generation and transmission. CRAS improvements over stand-alone RASs include:
- Logic processing (analytics) being at the control centers instead of in the substation, which translates to significantly greater ease of change of logic processing
- Greatly improved architecture leading to better design for protecting transmission assets
- Better system awareness and stability, enabling global reach across incorporated RASs and the ability to incorporate external system data including synchrophasors. This also helps prevent cascading failures between interrelated RASs
- Enhanced automation and simulation capabilities, allowing more comprehensive testing, debugging, and event analysis. An added value of this improvement is a significant reduction in testing labor via automated test procedures.
Future CRAS-Enabled Advanced Applications
SCE sees CRAS as a platform for programmatic approaches to control the grid. Beyond common RAS functionality, SCE envisions using the CRAS platform for improving grid resiliency and public safety. The ability to process synchrophasor data is being investigated for use in detecting broken transmission conductors and interrupting line flow before the conductor hits the ground, eliminating the ignition of wildfires. CRAS’s flexible logic processing is also anticipated to one day enable automated Blackstart procedures and programmatic load restoration via next-generation Smart Meters. Other advanced applications may also include (see Figure 3): Grid Battery Control, Voltage and VAR Control, and Dynamic Line Ratings.
Figure 3. CRAS-Enabled Advanced Applications.
This article edited by Pardis Khayyer