Performance of a Synchronized Phasor ...

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Performance of a Synchronized Phasor Measurements System in the Brazil Power System I. C. Decker, D. Dotta, M. N. Agostini, S. L. Zimath, and A. S. e Silva.

Abstract-- This paper describes recent results of the MedFasee project aiming at the development and performance testing of a Synchronized Phasor Measurement System (SPMS) prototype and its applications for monitoring of power system operation. The prototype comprises a PDC and three PMUs installed in cities in Southern Brazil. The PMUs were tested to evaluate their performance. Results from monitoring system frequency and voltage in normal and abnormal conditions are shown. Index Terms—Phasor Measurements, PMU, power system monitoring, wide-area monitoring, SPMS, WAMS.

I. INTRODUCTION

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N the last years, economic considerations associated to the electricity market and environment restrictions have led grid operators to postpone or reduce investments. This scenario combined with the continuous load increase leads the system and its components to operate closer to their limits. Furthermore, reliable electricity supply is now essential for society, and blackouts are becoming more costly [1]. To operate the system closer to limits and still to keep a high reliability is a challenging task and requires tools that allow the prompt detection of instabilities. SCADA data provide a comprehensive record of system conditions but at a relatively slow rate [2]. New tools such as Synchronized Phasor Measurement Systems (SPMS), which use advances in communications, computers and Global Positioning System (GPS) technologies, are needed for monitoring and control to improve the security of large power systems [3]. The SPMS can capture the faster system variations enabling operators to monitor and often control critical system operating indices, which are essential for secure operation of a large power system, including static phase-angle limits (system stress), critical intermediate voltage support when operating at large phase-angle separation, dynamic/transient phasor movements indicating dynamic/transient swings among different areas and

Work partially supported by contract FNDCT CT-Energ/Finep 01.02.0039.00 and Reason Technology S.A., a manufacturing of monitoring equipment. I. C. Decker, D. Dotta, M. N. Agostini , and A. S. e Silva are with Federal University of Santa Catarina, Florianópolis, SC 88.040-900 Brazil (e-mail: [email protected],[email protected],[email protected],agui [email protected]). S. L. Zimath is with Reason Technology S. A., Florianópolis, SC 88.025500 Brazil (e-mail: [email protected]).

modal inter area oscillation frequencies and their modal damping [3]. The SPMS, sometimes referred to more generically as a Wide Area Measurement System (WAMS), are basically composed by PMUs (“Phasor Measurements Units”) connected to a Phasor Data Concentrator (PDC) and application methodologies for monitoring and control of power system real time operation [2]. The first developments in SPMS started in 1989 with the WAMS project in subsystems of the WECC (“Western Electricity Coordinating Council”) [4]. This project involved the use of GPSsynchronized measurements over a large area of that power network [5]. In the last few years, several others countries started to install SPMS in their electrical systems. The following countries are reported to have installed and integrated phasor measurement units for research or are developing prototypes: Brazil [4], Scandinavia [9], Korea [10], Taiwan [11], China [12], Japan [13], and France, Italy, Switzerland, Croatia, Greece, Mexico, South Africa [5].. In the beginning of 2003, following the experience gained in the western system, the U.S. Department of Energy (DOE) launched the Eastern Interconnection Phasor Project (EIPP) which is being executed by a work group comprising transmission owing utilities, hardware and software vendors, system operators, reliability councils and government. The EIPP seeks to improve power system reliability through wide area measurement, monitoring and control [6]. To reach this goal six task forces were created by EIPP. As the SPMS technology is so incipient, the main work developed for the task forces was the identification, description and specification of the functional requirements for components of software and hardware for SPMS. These components include the equipments (PMUs, PDC, network, etc) and the monitoring, protection and control applications. Specifically, the Performance Requirements Task Force (PRTT) has been working in a report including guidelines/requirements for a “PMU Testing Guide”. The main objective of “PMU Testing Guide” is to define a testing procedure to assess the PMUs that will be installed in Eastern Interconnection. The PMU assessment is important as shown in [7]. That work shows that PMUs of comparable accuracy can be only compared under nominal frequency operations conditions. In off-nominal frequency operation every tested PMU unit yield a different phase and magnitude for the common measured voltage

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signal. These measurement errors must be taking into account to allow the connection between PMUs of different manufactures. This paper describes new results of a research project on SPMS in Brazil, the MedFasee project. The main goal of this paper is to show performance of a SPMS prototype comprising three PMUs and one PDC, under normal and abnormal system conditions. The paper is organized as follows. In Section 2, the MedFasee project is presented and the main software and hardware components of the SPMS prototype are described. In Section 3, tests performed in PMU by monitoring frequency and voltage in nominal and off-nominal conditions are described. In Section 4, the performance of the SPMS prototype in monitoring the system under normal and abnormal conditions is presented. Finally, in Section 5 and 6, the future developments and main conclusions are, respectively, presented. II. MEDFASEE PROJECT The MedFasee project was started in 2003 aiming at the development of a phasor measurement system prototype and applications for power systems monitoring and control. The prototype was installed in the end of 2004 and, since then the frequency and contingencies in the Southern Brazil system, have been monitored. A. SPMS Prototype The SPMS prototype is composed by one PDC and three PMUs. The three PMUs were installed in laboratories of three universities in Southern Brazil: Federal Center of Technological Education of Parana (CEFET) in Curitiba, Federal University of Santa Catarina (UFSC) in Florianópolis and Catholic University (PUC) in Porto Alegre (Figure 1). The PMUs measure the instantaneous three-phase distribution voltage. The PMU is connected to the Internet through an ethernet network interface and sends the phasors to the PDC located in the Electrical Systems Planning Research Laboratory (LabPlan) at UFSC. In Figure 1 the geographical location of the PMUs in Brazil is shown.

1) PMU The PMUs were entirely designed and built as part of the MedFasee project. To implement the main PMU functions, phasor calculation and transmission to the PDC, the voltage and current samples, acquired synchronously with the GPS reference, are processed by Discrete Fourier Transform (DFT), and formatted in data frames, using the IEEE Std. 1344 format [15]. Each PMU has a GPS receiver to synchronize the samples, so that the phasor angles measured by all PMUs in the system are in the same time reference. The PMUs have eight analogue channels (four for voltage and four for current), and 16 digital channels. The PMU generated data are continuously sent to the PDC, at a maximum rate of 60Hz, using an Ethernet link (UDP/IP protocol). This rate and an angle precision of 0.1 electric degrees are suited for the analysis of long term dynamic phenomena [15]. 2) PDC The PDC receives and correlates time-tagged phasor data sent by all the PMUs in the system. It has the following main functions [3]: a) Acquisition of the phasors, continuosly sent by the PMUs, handling of transmission errors; b) Storage of phasors in a central database; c) Support for real time system monitoring; d) Support for offline study functions, making available old phasors; These functions are designed and implemented in computing routines using the Object Oriented Modeling paradigm and C++ programming language. As the PDC needs to support real time applications it is necessary to rank the routines priorities. For example, the task of phasors acquisition has a higher priority than a request from the offline study application. To solve this problem a real time environment needed to be implemented. The PDC was built using the GNU/Linux operating system which does not have native real time support. The real time support is enabled in GNU/Linux applying a patch to the GNU/Linux kernel. There are two main packages for this finality: RT-Linux and RTAI. The latter was chosen since it presents a better support for object-oriented programming tasks. 3) Network The PMUs and PDC are connected by ethernet using the Internet network. The Internet connection was chosen due to its availability and the facilities provided to manage the PMUs remotely. The phasors are sent by the PMUs using the UDP/IP protocol and the remote administration is performed by the SSH (Security Shell) application.

Fig. 1. SPMS geographical location

The main hardware components and the prototype functionalities are described in the sequel:

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development of graphical applications and the mathematical treatment.

Fig. 2. Network performance (data lost in percentage).

C. SPMS Architecture The architecture of the SPMS system is divided in four main layers: 1) Data acquisition: The PMUs are located in strategic points to measure voltage and current. The phasors are calculated and sent to the PDC. 2) Data Management: The phasors sent by the PMUs are correlated in a uniform data stream. 3) Data Services: This layer includes the set of services required for supplying data for the different applications. 4) Applications: This is the layer where the monitoring, control and protection applications are executed. Figure 3 shows the architecture of the SPMS.

Figure 2 shows the phasors loss (in percentage) in a typical workday (Tuesday). The worse period is about midday when up to 2% of the data sent did not arrive at the PDC. The same behavior was found in all week days. In holidays and weekends the data loss is almost 0%. The measured transmission delay in the Internet is about 130 ms. In the local 100 Mbps ethernet network (LabPlan network) this delay falls to 30 ms. These results support the choice of the Internet for network connection. 4) Database The database is an independent process in the PDC, and it is accessed by the storage routines through specific database functions[3]. The external applications are connected directly to the database. The database structure was designed to facilitate the data access and stores up to seven days of continuous data of all PMUs in the system. The database is circular; new data replaces the oldest data. The database was implemented using the MySQL software for GNU/Linux. The MySQL database fulfills the requirements of the SPMS prototype. However, further studies and developments on Real Time Databases are necessary to improve the PDC capacity. B. Suport for Monitoring Applications Facilities for monitoring applications using the PDC phasors, were developed and implemented. These facilities are divided in two modules [14]: 1) Real Time Module This module supports the monitoring of real time data provided by the PDC. The application shows the real-time phasors arriving in the PDC. 2) On-Line Module This module allows the monitoring of the phasors kept in the PDC database. The main screen allows access to the database and graphics plotting. This screen enables the user to choose which phasors he wants to observe. One of the phasors can be chosen as the system reference. The user can still choose which measurements to observe: voltage module, voltage angle, frequency, in either, time or frequency domain graphics. Due to the characteristics of this module it was developed in Matlab. This environment facilitates the

Fig. 3. SPMS Architecture of the MedFasee Project

III. PMU PERFORMANCE TESTS In [7] results of a comparative test carried out on four PMUs from different manufacturers are reported. The test results have shown that it is possible to combine these units in applications involving steady state and slow varying dynamic conditions at the fundamental frequency. This requires that the measured magnitude and phase offsets be corrected in the units or be taken into account in the application program [7]. However, at off-nominal frequency operation, even PMUs with correction algorithms yield different phase and magnitude offsets at different frequencies [7]. These measurement errors will directly affect the performance of SPMS monitoring and control applications. As the measurements do not represent the real system state in offnominal operating conditions an operator, which is using a SPMS monitoring application to follow the system variation, could make wrong decisions because he is using information that do not reflect the real system condition. This kind of error also has a special effect under emergency control applications where there is not much time to take into account measurement errors. These errors arise mainly because the existing Synchrophasors Standard IEEE 1344 [15] does not require phase or magnitude correction for off-nominal operation [7].

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A. Test Settings As remarked in section II, the prototype PMUs measure the instantaneous three-phase distribution voltage. Therefore the tests realized in this work emphasized the performance of the voltage measurement under nominal and off-nominal conditions. Two tests based on [7] were realized and described in the sequel. B. Balanced three phase voltages at nominal frequency This test compares de performance of the PMU under balanced three phase voltage conditions in a range from 10% to 120% of the nominal voltage rating in steps of 10% at nominal frequency [7]. For every voltage step a three second window of data was aligned according to the time stamp provided by each PMU unit. The aligned phasor magnitudes were compared against a reference value measured at every voltage step using standard laboratory measuring instruments. As in [7] no attempt to calibrate the laboratory instruments was made. The following results only measure accuracy with respect to the laboratory instruments used. 1) Magnitude Comparison In the Figure 4 the deviation of the measurement phasor magnitude with respect to the reference value is shown.

2) Phase Comparation Figure 5 shows the phase difference between the measured data and the reference. Phase Measurement Accuracy 0,1 0,08 0,06 0,04 0,02 Error

To correct these problems the new Synchrophasors Standard PC 37.118 [16] (under revision) defines measurement requirements, compliance verification and accuracy limits that takes into account the off-nominal operation. Discussions promoted by PRTT on this issue, can be found in [8] . Following [7], tests of the prototype PMUs were carried out. The main goal was to evaluate the performance of the PMU under off-nominal operating conditions. This is important, since the SPMS prototype has been monitoring events in the Southern Brazil system as described in this paper.

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Fig. 5. Phase error

C. Unbalanced (single phase) voltage at offnominal frequency This test is intended to evaluate the performance of the PMU under unbalanced and off-nominal frequency operation. The unbalanced condition is simulated by applying a single phase voltage (phase A) to the PMU unit. The frequency is varied in a range between 55 and 65 Hz. 1) Magnitude Comparison The variation of the phasor magnitude with respect to the off-nominal frequency is shown in Figure 6. Magnitude versus Frequency 0,02500% 0,02000%

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Fig. 6. Magnitude error under off-nominal frequency

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Fig. 4. Magnitude error

Figure 4 shows that the phasor magnitudes measured by the PMU presents a good performance with errors lower than 0.35% with respect to the reference valor.

From this figure it can be concluded that the phasor magnitude measured by PMU is not affected by off-nominal frequencies in the range 55 and 65 Hz. It is a clear indication that the measuring algorithms correct their final results for offnominal conditions. IV. SPMS APPLICATIONS The development of phasor measurement applications for

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monitoring is part of the MedFasee project. This section describes SPMS prototype performance under normal and abnormal operating conditions. A. Monitoring and Analysis of Frequency Oscillations Several examples of frequency oscillations monitoring in the Southern Brazil 60 Hz power system based on measured data from the SPMS prototype are discussed below. In Figure 7, the behavior of voltage frequency measured by the PMU in Curitiba at heavy load (between 21h and 21h: 30min), is presented. Periods of 30 minutes, on Wednesday 30/11/2005, and on Thursday 31/11/2005, were analyzed. This graphics shows frequency oscillations with large magnitude in periods of heavy load.

identification of oscillation modes, using system real data, without the need of simulations with complex models. 1) Disturbance Detection – Outage of a 765kV Transmission Line An important event was registered by the SPMS prototype in October 04, 2005. At 20h38min, circuit two of a three circuit 765 kV transmission line Itaipu/Ivaiporã (shown in Figure 9) was tripped.

Fig. 9. Brazil South/Southeast Power System

In Figure 8, the frequency spectrum of the system frequency, at the heavy load period, is shown.

In the sequel, at 20h40min, circuits one and three of the same line were tripped. This caused the loss of 13 generators including 8 Itaipu generators, with a total generation loss of 6.920 MW. There was a disconnection between the Northen/Southern and Southeastern/Northeastern regions of the Brazilian System. The first stage of the SPS (Special Protection Scheme) was activated with load shedding of approximately 2.842 MW. Figure 10 shows the frequency evolution from 20h35min until 21h00min.

Fig. 8. Curitiba, PR, Frequency Spectrum

Fig. 10. Voltage frequency at three locations

An oscillation mode near 0.02Hz, corresponding to a period of approximately 50 seconds, can be observed. This mode appears with evidence in all load periods (heavy, middle, low). The use of the phasors obtained from the SPMS prototype allows many analyses in real time, including the

Figure 10 shows that at 20h40min26s the frequency started to fall reaching the lower limit of 58.25Hz in Porto Alegre, at 20h40min30s. The frequency recovery started at 20h40min33s and at about 20h43min33s the frequency reached 59.6 Hz. Approximately at 20h56min the frequency returned to the nominal value. To show the SPMS prototype capability, it is

Fig. 7. Curitiba, PR, Frequency Voltage

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shown, in Figure 11, the moment where circuit two (Itaipu/Ivaiporã) was tripped and the subsequent attempt to reconnect the circuit.

696 MW in the southern most state (RS) area. Figure 13 shows the Brazilian System frequency calculated from the angular variation registered at the PMU located in Florianopolis. At 14h43min50s, an oscillatory process started in the frequency leading to an overfrequency of 60.178 Hz, eight seconds after the event (14h43min58s). The frequency returned to the nominal value approximately three minutes after the event start, at 14h47min.

Fig. 11. Voltage frequency zoom

At 21h06min, all loads and the Northern/Southern Interconnection were restored. At 21h30 min, circuit one (Itaipu/Ivaiporã) was reconnected. However, at 21h52min, circuit one was tripped again and it was followed by a generation tripping of 1800 MW in Itaipu. In this event there was no load shedding and the protection scheme kept all circuits of the Itaipu/Ivaiporã line open. The frequency evolution of this event is shown in Figure 12.

Fig. 13. Voltage frequency evolution at Florianópolis, SC

In Figure 14, the angular difference between the voltages measured by the PMUs installed in Curitiba and in Florianópolis, during the event, is shown. The fast oscillations during the event can be observed again. The angular difference between the points fell to approximately 1.8 degrees as a result of the active power flow redistribution in the network.

Fig. 12. Voltage frequency evolution in the second Itaipu event

Figure 12 shows that at 21h50min50s the frequency started to fall reaching the lower limit of 58.4Hz. After three minutes the frequency returned to the nominal value. 2) Disturbance Detection – Outage of a 230 kV Substation On August 26, 2005, at 14h43min58s, part of an important 230 kV substation (Cidade Industrial), in Porto Alegre, was tripped. The result of this event was a generation loss of 215 MW and a load shedding of 38 MW. The resulting undervoltage cause a natural load reduction of approximately

Fig. 14. Angular difference between Florianópolis, SC and Curitiba, PR

The data sent by the PMU located in Porto Alegre, during the event, did not arrive at the PDC as a consequence of an Internet connection failure between UFSC and PUC. The failure duration was of approximately 1 min, although the PMU kept registering the data since it was connected to a no-

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break system. In Figure 15 the voltage magnitude monitored by the PMU in Porto Alegre is shown. It can be observed the magnitude improvement during the system recovery (about 10% of the nominal value).

Internet has proved a good choice for the project but reliability can be an issue for industrial applications. The use of private networks by the utilities can be a reliable alternative. Monitoring of nominal and off-nominal conditions were accomplished by the SPMS prototype. The measured data allowed the analysis of the low-voltage frequency and identification of a natural oscillation mode with a period of approximately 50 seconds in the Brazilian Interconnect system. The prototype robustness was tested with the capture of important system events. The events were registered with high precision and enabled the analysis of the disturbance effects at specific points of the low-voltage system. Although the observation was at low-voltage it allowed the observation of the dynamics of the whole system. The authors believe that a new generation of tools using the SPMS data will facilitate the operation of the Brazilian Power System and the MedFasee Project can contribute for that goal. VII. ACKNOWLEDGMENT

Fig. 15. Voltage magnitude at Porto Alegre, RS

V. FUTURE DEVELOPMENTS The prototype described in this paper has been working for a year monitoring important variables of Southern Brazilian Electrical system during normal and abnormal conditions. The Medfasee involves further research and developments. Although the prototype was installed in the distribution system, many transmission system phenomenon could be observed. However, the connection at the distribution system level makes the analysis more complex since transient components due to switchings in the distribution system add to frequency components associated to power oscillations at the transmission system level. Therefore, the next phase of the project comprises the installation and tests of a SPMS in the EHV (Extra High Voltage) Brazilian transmission system. The PMU developed in the project is compliant with the Synchrophasor Standard PC 37.118, but the PDC is still under development to make it compliant with that standard. Monitoring applications were giving special attention in this paper. However other applications are being developed such as model improvement using SPMS, fault location emergency control and control applications aiming the smalldisturbance angle stability. VI. CONCLUSIONS This paper described the performance of a SPMS prototype developed by the MedFasee Project. The PMUs were purposely installed in geographically distant cities of Southern Brazil. The concerns about the PMUs operations at offnominal conditions were taken into account. The performance tests have shown that the PMUs are able to monitor system events (abnormal operation). Performance results of SPMS under Internet have shown its capability in providing network connection for SPMS. The

The authors gratefully acknowledge the contributions of Professor F. Neves, from CEFET-PR, and Professors F. B. Lemos and A. Manzoni, from PUC-RS, and their laboratory staff for their cooperation to support PMUs installation. VIII. REFERENCES [1]

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8 Measurements”. Power Tech Conference Proceedings. IEEE – Bologna. 2003. [14] I. C. Decker, et al., “Phasor Measurement Development and Applications in Brazil,” presented in First International Conference in Electrical Engineering. Coimbra, Portugal, 2005. [15] IEEE Standard for Synchrophasors for Power Systems, IEEE Standard 1344-1995, 1995. [16] IEEE Standard for Synchrophasors for Power Systems, IEEE Standard PC37.118-2005, Jun. 2005. (under revision)

IX. BIOGRAPHIES Ildemar Cassana Decker received his B. Sc. from the Catholic University of Pelotas, RS., Brazil. He obtained his M.Sc. (1984) and D.Sc. (1993) degrees in Electrical Engineering from Federal University of Santa Catarina and Rio de Janeiro, Brazil, respectively. From 1980 to 1985 he worked in Federal University of Santa Maria, RS. Since 1985 he has been Associate Professor of the Federal University of Santa Catarina, in Department of Electrical Engineering. His general research interest is in the area of computer methods for power systems analysis and control and high performance scientific computing.

Daniel Dotta received his B. Sc. and M.Sc. degrees in Electrical Engineering from the Federal University of Santa Catarina, SC., Brazil. Since 2004 he has been developing his Ph.D. in Federal University of Santa Catarina, in Department of Electrical Engineering. His general research interest is in the area of modeling and object-oriented programming for power systems analysis and control and high performance scientific computing.

Marcelo Neujhar Agostini received his degree in Electrical Engineering from Federal University of Santa Maria in 1996. He worked as a research engineer at the same institution before starting postgraduate studies. He obtained his D.Eng. degree in Electrical Engineering from the Federal University of Santa Catarina in 2002. Currently he works at this university as a researcher engineer. His general research interest are phasor measurements, software engineering applied to Electric Power Systems, Object-Oriented Modeling, Electric Power Systems Modeling, Electric Power Systems Dynamics and High Performance Scientific Computing.

Sergio Luiz Zimath received his degree in Automation and Control Engineering from Federal University of Santa Catarina in 1997. Since 1995, he has been with Reason Technology where he was responsible for the development of the Digital Fault Recorder model RPIV, GPS Based time references among other products. Since 2005 he is in charge of the Research Projects Department, involved in the study of new technologies.

Aguinaldo Silveira e Silva received his degree in Electrical Engineering from Federal University of Parana in 1977, and the M.Sc. and Ph.D degrees in Electrical Engineering from Federal University of Santa Catarina, in 1982 and UMIST, UK, in 1990, respectively. Since 1980, he has been with Federal University of Santa Catarina. His main research interests are in the area of power systems dynamics and control applications.