The continual expansion of the meshed interconnected operation within the Central European System's Union for the Coordination of Transmission Electricity (UCTE) has intensified the need to “see or monitor” the transmission system beyond one's own national boundary to fully understand the reasons for fast power-flow changes at certain times.
Wide-area monitoring (WAM) uses high-resolution measurements synchronized by GPS timing. The measurements from remotely located substations create a comprehensive system overview. Currently, swissgrid ag (Laufenburg, Switzerland) has implemented a power oscillation monitoring (POM) system that is under development in a test operation. The POM makes it possible to detect the excitation of one of the two major interarea oscillation modes that exist in the UCTE power system.
At the end of January 2006, for example, schedule changes exceeding 5 GW between individual systems were noted. These power changes resulted in frequency deviations of more than 0.15 Hz for several minutes (Fig. 1) because of inaccurate ramping. With the frequency deviations shown, inadvertent power flows in the whole system could occur, which subsequently may cause cascaded events.
WAM is based on at least once per cycle high-resolution measurements synchronized by GPS timing signals. By using reliable telecommunication channels, they can be computed and displayed in real time. These measurements from remotely located substations allow for the creation of a detailed system overview. Also, post processing of these signals opens the door to a new dimension of wide-area system control and protection.
This is in stark contrast to previous methods of power system monitoring, which was performed with SCADA systems that deliver measurements or estimated values in 5- to 20-second intervals. A steady-state snapshot of the transmission system gives an overview of topology, power flows, voltage profiles and power-frequency controller operation. The information displayed covers the complete national operation area and only partial sections of the neighboring systems.
The use of WAM measurements enables system dynamic behavior to be permanently monitored and, when coupled with smart computations algorithms, an early-warning system against dangerous system operation is established. This practice started in 2003 in Switzerland, which occupies a strategic position in the UCTE, and is now the subject of the latest system and functionality expansion.
Switzerland has a power-transfer load through the Swiss transmission system that is equivalent in magnitude to the system load itself. Thus, corridor monitoring is a very important task. Phase measurement units (PMUs) enable the exact measurement of voltage phase difference along the corridor. Based on only two measurements, the system loading and the complete topology between the two substations can be monitored. Figure 2 shows the complete signal chain from the feeder up to the SCADA system.
Data communication between the substation and the data concentrator consists only of positive sequence phasor values for current, voltage and the determined frequency. Each measurement package has an individual time stamp. For the communication itself, standardized protocols are used. The acquired data are stored in an OPC database with a time resolution of 100 msec.
Dedicated information is piped from the data concentrator to the control room. This information consists of either alarms or other calculated values from the different permanent running applications. The main challenge is to extract important information from analyzing electromechanical dynamics of the whole system and send corresponding alarms to the control-room operators. In addition, the planning departments could benefit from calibrating their dynamic models on exports of synchronized high-resolution measurements on different feeders.
The impact of events far from the well-monitored system is shown in Fig. 3. In the event of losing a section of the transmission system during the Italian import of several thousand megawatts through the systems of Slovenia, Switzerland and France, an automatic higher loading of the rest of the interconnection lines will occur. In this particular mode of operation, more than an additional 500 MW will flow through the Swiss system on the four main 380-kV north-south transmission lines. This additional load flow, about 100 MW for each line, correlates with a stepwise increase of the voltage phase-angle difference from the northern to the southern system border of 2.5 degrees.
The highly interconnected UCTE power system has reached a critical size. Poorly damped interarea oscillations recorded in 2005 have shown that dynamic system behavior has to be carefully observed to prevent unfavorable system situations. Therefore, the measurements available from the Swiss data concentrator were used for the extraction of dynamic system information together with many other recordings on the complete UCTE power system. Figure 4 gives an overview of the current synchronous interconnected system and the location of the on-line connected PMUs. For Switzerland, these are positioned in Bassecourt Substation on the northwest border, Mettlen Substation in the center of the transmission system, and Lavorgo, Soazza and Robbia substations on the southern border. Based on bilateral agreements with ELES (Slovenia) and HTSO (Greece), two other substations at Divaca and Ag. Stanfos are equipped to complete the system.
The first successful application of the on-line dynamic system monitoring was performed during the critical phase of the resynchronization of the first and second UCTE zones on Oct. 10, 2004. At that time, a permanent modem link from Zagreb (Croatia) to Laufenburg (Switzerland) enabled the resynchronization team to be sure that, during the weak interconnection time window, no instabilities occurred.
However, after this latest extension step of the UCTE, poorly damped interarea oscillations were repeated. Using three different input signals — frequency, voltage phase-angle difference and active power — the system damping parameters were permanently extracted and recorded (Fig. 5).
WAM enables system operators to permanently dispose of stability indices, in order to receive warning signals in the case of poor system damping. Secondly, by permanently recording stability indices, a correlation between dangerous operating conditions and related system loading or topology configuration can be deduced.
POWER OSCILLATION MONITORING
As mentioned previously, swissgrid ag's POM can detect the excitation of one of the two major interarea oscillation modes existent in the UCTE power system. The most visible is the east-west mode reflected by active power swings in the east-west direction that can be measured on the tie lines connecting areas on this axis or the frequency at the Eastern system margin. As input for recording this mode, the comparison between the frequency in Switzerland and Greece is used. Similar results can be obtained by using the voltage phase-angle difference between Greece and Switzerland. The second mode, which reflects the north-south interarea oscillation, is also monitored by swissgrid ag by using, as input, the active power flows of two 380-kV tie lines oriented in the north-south direction as part of the import corridor from the North to Italy.
The timely high-resolution measurements (every 100 msec is one measurement set) are subsequently processed in such a way that as a result of an on-line parameter estimation together with a modal analysis, three main parameters describing the system damping are calculated and stored in the measurement database: damping factor, oscillation amplitude and oscillation frequency. The most recent interarea oscillation observed in the UCTE power system is shown in Fig. 6 and the corresponding oscillation monitoring tool output is presented in Fig. 7.
By using both indices, namely the damping factor and the oscillation amplitude, together with a timer allows creation of a reliable oscillation alarm, which triggers acquiring additional recordings or for performing system topology improvements. Currently, this signal is used only for monitoring purposes, but in the future it may be integrated in special protection schemes or enhanced control loops.
Rapid and frequent changes of power-flow patterns have to be managed by transmission system operators with the help of enhanced system monitoring tools. One possible solution is a WAM approach in order to enable control-room operators to react faster. A dedicated aggregation of global information of the system creates intelligent alarms containing comprehensive information related to highly meshed systems. Based on post-processing of measurements, coordinated actions can be initiated in order to prevent cascaded events.
Current developments in data acquisition, reliable and fast telecommunication systems combined with increased computation power enables power engineers to implement new comprehensive monitoring and control schemes, which increase power system security with faster reaction. However, the most challenging process will be to combine the existing SCADA/EMS systems with the latest modern techniques to create tools for the control of the increasing interconnected power systems throughout the world.
Initially, the WAM approach is used as a support for decisions, but after further improvements in a second step, the same technology has to be integrated into an automatic control system or into new special protection schemes. As PMU information is received from the most important substations, these measurements also represent the basis of an emergency backup system and might be used correspondingly. In addition, these measurements might be used as a permanent functionality quality check for the on-line state estimator and other EMS functions.
The current system was developed in close cooperation with ABB, the manufacturer of the PMUs and related software packages. The authors wish to acknowledge and thank the ABB project team led by Dr. Joachim Bertsch.
Ing. Walter Sattinger earned his electrical engineering degree from the University of Stuttgart in 1988 and his Doktor-Ingenieur PhD degree from the same university in 1995, after which he joined DigSILENT (Germany) as a consulting engineer. In 2003, Sattinger moved to ETRANS Ltd., now named swissgrid ag, where he works in the System Planning and Studies department as a power system analysis expert. He serves as the project engineer on the interface between system planning and system operation. His specific responsibilities include dynamic system studies, wide-area monitoring and line thermal monitoring. He is a member of various UCTE and CIGRÉ working groups, task forces and technical committees. firstname.lastname@example.org
Rudolf Baumann has almost three decades of experience in network control, and communication and information technology. He started his career with Brown Boveri & Co. as a development engineer commissioning network control systems around the world before leading the company's engineering group for SCADA systems. Baumann worked for Elektrizitäts-Gesellschaft Laufenburg, leading the Information Technology department from 1986 to 1999 prior to joining ETRANS AG in 2000. At ETRANS AG, now named swissgrid ag, he is responsible for overall project management, strategic and business development, and development of the Swiss market system. Since June 2005, Baumann has been responsible for operations at swissgrid ag and COO. email@example.com
Philippe Rothermann earned his electrical engineering degree from the University of Applied Science of Friburg in 2004 and joined ETRANS, now named swissgrid ag, in 2005. As a power system analysis expert, he is a member of the System Planning and Studies department, where he works on dynamic system studies, wide-area monitoring, line thermal monitoring and load flow optimization. Philippe.Rothermann@swissgrid.ch