A Real-world Implementation of Centralized RAS System

by J. Wen, P. Arons, E. Lee, Southern California Edison , USA

Stage 1 - Conceptualization and Technology Validation

SCE began to build a pilot feasibility testing system in 2007 to validate the concept and available technologies for a centralized RAS system. With the notion of centralized logic came a series of questions about the concept that were asked and answered systematically and conclusively, so that the key functional attributes became an integral part of the development of the system.

The first question arose regarding the need for speed of the RAS operation. RASs are expected to provide rapid automatic mitigation to relieve the system from dynamic stability, post-transient voltage violations or thermal overloads, which range from the transient timeframe of milliseconds to steady-state timeframe of seconds and minutes. After a careful review of SCE’s current standalone RAS practice and an examination of the system reliability and economic needs, 50 milliseconds (ms) was chosen as the target throughput time for the monitoring relay to detect a line open, the controller to process the data, logic, and the mitigation relay to close the contact. The breakdown estimation of 50 ms is as follows:

 

 

 

 

 

Following the establishment of the speed performance requirement, the next question was whether it was feasible to achieve the speed performance across a large geographical area. The test results of the communication, together with the test results of the relay and controller, proved that the 50 ms throughput time could be achieved. The 19 ms for each one-way communication leaves sufficient margins for possible delays.

  • Communication Solution: SCE has more than 7,000 miles of high-speed communication circuits connecting all major bulk power system substations and generating plants in its grid network. The strong communication backbone and extensive network coverage has made possible a wide area application. The IEC 61850-8-1 Generic Object Oriented Substation Events (GOOSE) messaging was chosen as the transport mechanism after the evaluation of various communication protocols. It allows reliable publication and recognition of protection information in milliseconds on a LAN, and the message publishers and subscribers to be configured according to the IEC 61850 XML-based Substation Configuration Language (SCL) through a standard tool-based process as described in IEC 61850-6. Applying IEC 61850 over large scale wide area monitoring and protection network, covering the entire service territory of SCE, is the first attempt in the industry.

Another major new desirable feature is the GOOSE ability to directly send analog data values so that these data values can be easily translated into engineering units as needed. The pilot tests showed a result of 20-22.5 ms of transporting GOOSE over 660 miles (far enough to cover the most remote location) including multiple hops from fiber to microwave and back and therefore, proved that the centralized concept and desired speed performance targets were feasible.

  • Controller Platform: In order to decouple the dependencies between RAS central controller implementation and potential hardware/firmware environment, unlike traditional RAS implementation, SCE decided to develop the centralized RAS algorithms and data processing modules on a standard Intel-based computer platform. This decoupling allowed SCE to continue taking full advantages of future advancements in the ICT industry. The Microsoft Windows was chosen for convenience. And, the product developed by SISCO, called “Unified Analytic Platform (UAP)”, was used as the core platform for central controller. The UAP is model based, consisting of GOOSE interface model which receives and decodes the incoming GOOSE and formats and sends the outgoing GOOSE, the analytics model which hosts the RAS logic, and historian model which interfaces with the historian. The model based approach facilitates independence between the various components, and is easy to expand as system grows. The pilot tests showed a result of less than 4 milliseconds total for data processing, computation, and RAS logic execution, when the controller handled normal system conditions. Experiments also moved into whether the speed could be sustained if the controller were stressed. The test scenarios included a total of 400 devices, assuming 5 each at 80 substations was simulated, with 20% of the devices reported and 8 data items changed in each of the GOOSE messages.

In the pilot feasibility testing configuration, an existing SCE RAS was also simulated. In the lab set-up, the monitoring relays were deployed in the field, and the controller and mitigation relays were hosted in the lab. Both functionality and performance tests were conducted. The test result showed the data processing and logic execution could be completed within 4 ms that were budgeted within the 50 ms estimation. With the foundational attributes of centralized logic, the ability to move analog data, sufficient speed and bandwidth, standardized communication protocols and equipment with appropriate functionality, the basic concept of centralized RAS was considered viable.

The development of the pilot feasibility testing system has been largely shared with the WECC RASRS members and industry experts. Valuable feedback was obtained through various professional meetings and conferences. Further refinements followed with respect to triple-redundancy at the controller for high system availability, two-out-of-three voting logic at the mitigation relay for high dependability, and demarcation of the equipment failures for high maintainability, etc.
The subject of testing and quality assurance was also extensively investigated. Various types of testing were developed that would be implemented into the system with the intent that the reliability of the system would be improved and would alleviate the workload on maintenance personnel. The different levels of testing including site acceptance test, commissioning test, maintenance test, and trial operation as discussed in IEC 61850-4 should be performed as part of an overall quality assurance process for the centralized system.
Each of these refinements set the stage for subsequent centralized system development.


 

 

 

 

 

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