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MODE CHANGE IMPLEMENTATION MODEL IN POWER SYSTEM AUTOMATION: A CASE STUDY Roger P.Siqueira, Keiko V.O. Fonseca CPGEI /UTFPR – Federal University of Technology – Paraná Av. Sete de Setembro 3165, 80230-901 Curitiba-PR Brazil.
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Abstract. This work approaches the problem of meeting time requirements of an Electricity Utility under overload. Specifically, it evaluates the effects of data acquisition mode changes on the data network behavior when executing the DNP protocol. Simulation results show the efficiency increase in network resources utilization through mode change techniques at the system automation. Also, they allowed quantifying important characteristics of the system under test, as data packet sizes, channel rates, processing time of RTUs in the SCADA system.
1. Introduction An electrical power system of an electricity utility presents real-time requirements to its safe operation. Requirements violations can impose outages and widespread blackouts across entire regions. One particular requirement is bounded time to message exchanges or communication service executions. Generally, communication systems of electricity utilities run master-slave protocols to control and share the access to the physical medium. At these protocols, slave devices report their data only when requested by a master device [Clarke, Reynders, Wright, 2004]. The protocol execution requires the configuration of slave scan cycles to acquire data periodically and/or to assign priorities to slave messages. Particular to power systems, protocols like DNP (Distributed Network Protocol) (DNP, 1997) and IEC 60870-5-101 (IEC 870, 1995) specify support to data acquisition mode. Specifically, the DNP, at run time, allows mode changes by enable/disable unsolicited messages, changes on message types and scan cycles. This paper evaluates the mode change models proposed at [Pedro, 1999] adapting them to the particular case of a SCADA system of electrical substations running DNP.
2. The control system of an electricity utility and the DNP protocol SCADA integrate, geographically dispersed, a master station, Remote Terminal Unities (RTU), Communication System and the Human-Machine interfaces. A typical SCADA has a master station and a number of RTUs connected through a variety of communication channels. The communication system configuration should consider the RTUs, the number of points each RTU supports, the information update rate about the RTU location; devices and technology available at the RTUs, the medium access control (MAC) protocol, and characteristics of the physical medium [Gaushell, Block, 1993]. At the Substations(SE), the supervision level usually comprises a Control Unit(CU or master station); the RTUs connected to the power devices and the microprocessed
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power relays as integrated RTUs. The CU centralizes the RTU communication inside the SE acting as a protocol converter for the higher supervisory levels[Zambenedetti, Frisch et al, 2000] and running the control algorithms. Usual protocols to RTU access at the studied system1 are Harris/5000[Harris, 2005] and DNP. At each SE, links use RS232 point-to-point or DNP over UDP/IP. Error control is let to DNP or Harris protocols. Inside the SE, a serial distributor connects a RTU to a serial port. Several RTUs(up to 10 per distributor) can be connected to the same serial port. The CU–Operation Center link can be a serial point-to-point, a radio, optical fiber or a twisted pair link. Typical time requirements of a RTU are the bounded data age (10s) and the broadcast scan period ( Load ↓ Normal load Event avalanche
Reference system (no mode changes) Measurements at the real system/ simulation Data collected from company logs/ simulation
Implementing mode changes Simulation/experimental models
execution mode -> Mode change mode ↓
Initial & final stable system mode
Unsolicited report
Simulation/experimental models Simulation/experimental simulation/experimental
Master slave #2 Master-slave #2
Transition system mode Analytical model Analytical Analytical
Load scenarios covered event generation from 10 to 100% of the supervised points per RTU and allowed the investigation of the % of event changes that causes performance degradation on the communication system relating them to protocol choices, message sizes and sequences, physical medium and bit rate. The collected data fed the simulation models. A simulation tool [ASE2000] provided models for master, slave nodes and channel monitoring of DNP messages. The master simulation allows for the selection of messages characteristics and slave addressing. The slave simulation allows for the configuration of RTU messages, number of supervised points, event generation patterns, response mode, among others. The 3 evaluated modes were: (#1) changes to the DNP “unsolicited report mode”; at master slave communication mode, (#2) changes on DNP objects reading and (#3) on message characteristics and DNP objects. Modes (#2 and #3) tested basically changes on periodical reading and characteristics changes of some variables. At mode #2, the system read only variables were changes occurred. Also, the sporadic reading with timestamp for each point and status field for each point was changed to periodical reading with relative timestamp. Finally, at mode #3, changes were done on message scan periods without affecting the supervisory control demands. The initial and final mode change steps were evaluated with the simulation tool. The tool does not allow dynamical changes on configuration, then the transition step was evaluated through an analytical model [Pedro,1999]. The computed task characteristics were: wi , the worst case response time of a task i; Yi the time spent since the arrival of the mode change request until the introduction of a new task or a modified task.
4 Results and Conclusions A lot of experiments were done to establish the network behavior at the masterslave communication mode under several load scenarios. Through simulation, we check if sets of task related to the transmission/reception of DNP object are schedulable. Simulation results were validated comparing them with those from experimental and analytical models. All test/load/mode change combination and results are presented at
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(Siqueira, 2006). Mode change models were subject of very interesting academic works but practical applications of the reported models are not of our knowledge. We study their implementation models on a real system, an automated supervisory control system of an electricity utility substation. Until now, the results showed that under event avalanche, mode changes led to better performance values compared to the original system since they decrease the data network overload in acontrolled way. The analysis of the mass volume of collected data allowed us to understand particular behaviors of the target system, like influences of the MAC protocol configuration, processing time of automation equipment and desired (or undesired) features of the communication interfaces offered with them. The results were fully incorporated by the electricity utility and applied to the specifications of new equipments and communication devices, as well to new policies for communication systems.
References ASE2000. Communication Test Set. Applied Systems Engineering, Inc. Version 1.40. Clarke, G.; Reynders, D.; Wright, E. (2004), “Practical Modern SCADA Protocols: DNP, 60870.5 and Related Systems”. Burlington, UK: Elsevier Curtis, Ken (2005), “A DNP3 Protocol Primer”, http://www.dnp.org/, Rev. A, 20 March DNP (1997): Basic Four Document Set : Subset Definitions, Data Link Layer, Transport Functions, Data Object Library. DNP Users Group, Canada Ethereal. Network Protocol Analyzer, G. Combs. V0.10.13. http://www.ethereal.com. Gaushell, D.; Block, W. (1993), “Scada Communication Techniques and Standards”. In: IEEE Computer Applications in Power. V. 6, Issue 3, p. 45-50, July Harris (2005). Harris Inc. Site. http://www.harris.com. IEC 870 (1995): IEC-870-5 Standard (Inter. Electro Tech. Commission, TC 57, WG 03) ONS (2003) - (in portuguese) Operador Nacional do Sistema Elétrico– Submódulo 10.19. “Requisitos de Telesupervisão para a Operação”. Brasil Pedro, P. S. M. (1999) “Schedulability of Mode Change in Flexible Real-Time Distributed Systems”. York (UK), Computer Science Dr. Thesis, University of York. Real, J.; Crespo A. (2004) “Mode Change Protocols for Real-Time Systems: A Survey and a New Proposal”. Real-Time System. Netherlands, n. 26, p.161-194 Siqueira, R. P.; Fonseca, K. V. O et al (2004) “A Mode Change Implementation for Real Time System: A Case Study to Control and Operation of Remote Stations of Electric Power System”. Annals of VI INDUSCON, Brazil, UDESC Siqueira, R: (2006) Um Modelo de Implementação de Mudança de modo em Sistemas de Automação de Energia, MsC thesis, CPGEI/UTFPR, Brasil, December Zambenedetti, V.C.; Frisch, et al. “Desenvolvimento de Interface do Protocolo DNP3 para o Sistema de Automação de Subestações e Redes de Distribuição da COPEL”. ANEEL Project Report (in Portuguese). Curitiba: LACTEC/COPEL, 2000.
