Protection System Performance Evaluation

21, rue d’Artois, F-75008 PARIS http : //www.cigre.org

B5 - 208

CIGRE 2008

EVALUATION OF PROTECTION SYSTEM PERFORMANCE USING DFR AND RELAY COMTRADE FILES

M. A. M. Rodrigues*, J. C. C. Oliveira, A. L. L. Miranda, M. V. F. Figueiredo, R. B. Sollero Centro de Pesquisas de Energia Elétrica – CEPEL L. A. C. D. CASTILLO, F. D. CAMPOS, N. F. COSTA Light Serviços de Eletricidade S.A. – LIGHT S.E.S.A. BRAZIL

SUMMARY The paper proposes an automated analysis system, based on data generated by Digital Fault Recorders, capable of evaluating the protection system operation behavior against expected patterns, pinpointing possible malfunctions and indicating problems prone to rise in the future. The protection performance is evaluated focusing on the external behavior of the relays and the communication channels. This approach allows for dealing with equipment from different technologies with few changes in the evaluation software. The relay performance can also be checked against simplified models of the protection elements. Instead of developing intricate and complete models of commercial relays, which are dependent on manufacturer information, simplified models, based on classic protection theory, allows for simple but robust reasoning tasks, like comparing relay operation against an over-current directional or a line current differential algorithm. The basic idea is to use the output of these virtual relay elements as a possible warning to the protection engineer. The system was put in service at LIGHT S.E.S.A., a major distribution utility in Brazil. The authors believe that this evaluation system can be a relevant step in protection and fault analysis in LIGHT S.E.S.A. and similar companies, as it aggregates quite useful information to the automated fault pre-analysis. KEYWORDS Protection - Transmission Line - Expert System - Digital Faul Recording - Fault - WEB page

(*)[email protected]

1. INTRODUCTION 1.1. Scope and Motivation Some distribution utilities in Brazil also own and operate generation and transmission (or sub transmission) assets, usually at 138 kV and bellow. In order to optimize the utilization of the right-of-way paths in metropolitan areas, it has been quite usual to add parallel lines in these transmission systems (sometimes more than three). Proper selectivity is fundamental in the protection of these lines, limiting the protection zone to the faulted circuit, while avoiding loosing non-affected lines or busbar terminals. In fact, these busses usually supply high density load centers (residential, commercial or even industrial) and failure in attaining this requirement could impact negatively the quality of the service, reflecting in indexes like SAIDI and SAIFI, used in Brazil to impose penalties to the utility by the Regulatory Authority. On the other side, while these companies are investing in new and better protection equipment (numerical technology) some old equipment, mostly electromechanical, not designed to handle actual engineering analysis information in complex network topologies are still in service. In this context, an automated analysis system, capable of evaluating the protection operation against expected behaviors, pinpointing possible malfunctions and indicating problems prone to rise in the future, is highly desired. This paper will present the results of a joint work on this matter developed by CEPEL (Electric Power Research Center) and LIGHT S.E.S.A., a private-owned company responsible for the distribution of 72% of Rio de Janeiro State’s electricity consumption, including the City of Rio de Janeiro. Today, this is related to about 10 million inhabitants spread in an area of 10970 km². The total energy distributed by Light S.E.S.A. is around 24500GWH/year, attending 3.8 million customers. To do its job LIGHT S.E.S.A. manages about 2200 km of overhead and underground sub-transmission system in 138 kV, 96 substations, 5 hydro plants (780 MW), with 4000 employees. The sub transmission system uses protection functions like distance, current differential, overcurrent directional and teleprotections scheme. Since august 2006, a Brazilian Group - RME (Rio Minas Energia Participações) has become the shareholder of LIGHT S.E.S.A.

1.2. Implemented Solution The main objective of the evaluation system is to help the protection engineer to do his job, evaluating most of the files related to short circuit conditions and detect protection problems associated to relay or communication channels malfunctions in transmission lines teleprotections schemes. Although the basic idea is not new [1], it is interesting to note that the solution described here is flexible enough to cope with relays from distinct technologies, ranging from electromechanical to numerical. Of course, the amount of information available from numerical relays is typically much larger than the offered by old generation relays. Numerical relays can output the behavior of most of their internal elements, not only in the digital channels of disturbance recordings but also in specific files (like the event report and fault description report [1]), what makes their operation easier to understand. Static and electromechanical relays do not deliver digital files and most of their internal elements are unavailable. 2

The protection analysis system was developed on top of a previous work [2], relative to analog waveform interpretation of fault recordings. That system was enhanced by a fault recording management system, capable of storing, classifying and backing-up fault related files. The system performs both an automated fault analysis and the protection performance evaluation and stores the results (fault type, quantities values, timing information, relay performance report etc.) in a relational data-base, ready to be used on-demand. Also, a set of WEB pages were developed to help the user in selecting, among many files, those with relevant information. This is an important feature because the protection teams, although highly skillful in modern utilities, are generally small, having difficulty to manage, manually, all disturbance data generated by DFR and digital relays.

2. System Requirements 2.1. File processing requirements LIGHT S.E.S.A. has an automated system developed to deliver most of its digital fault recordings to the central office. These files are organized hierarchically (as in a directory tree) where substations are in the first level and DFRs (or relays with fault recording capability) are in the second. In this configuration, the contents of the first line of the COMTRADE Configuration File [3] are ignored and the DFR that produced the file is identified by the directory tree it belongs to. This system does not use COMNAMES [4] or other convention to identify the files by their names. As some files still need to be retrieved manually, the automatic fault analysis system, associated to this work, must provide a way to check for misplaced files. Although COMTRADE standard [3] has, in the configuration file, a field to determine the equipment being monitored, it is seldom used. Furthermore, line impedance parameters are not included in COMTRADE files (these values can be given in the COMTRADE’s information file, but it is a manufacturer-dependent approach). This information must be made explicitly available to the fault analysis system, either in files or in a database. In the absence of this information only very superficial analysis can be performed. If the description of monitored equipment is available and if it is related to topological data (like equipment connections, equipment location etc.) the event files can be listed in a database for retrieving with elaborated filters and views. However, this data is usually available only in nonelectronics or in non-formatted formats.

2.2. Fault Analysis Requirements In LIGHT S.E.S.A. transmission system, the availability of analog and digital DFR channels is determined in such way that there are always phase and ground current information from all transmission lines and three single-phase busbar voltages. On the other hand, transmission lines and underground cables protected by digital relays normally record all information related with its operation. In some protection schemes, like current differential (F.87), normally applied to 138 kV underground cables, recordings are usually limited to the currents, not voltages. This limitation impacts the analysis of DFR data because sometimes the lack of voltage information may prevent the use of algorithms like directional current or fault location. Anyway, the analysis must be robust enough to work with the data available. 3

Usually, DFR devices trigger their records independently from the relay operation, by voltage deviation, over current or other abnormal condition. Relays, on the other side, usually trigger their internal fault recording only when a trip is issued. This means that not all disturbances are recorded by relay fault recording functions (there are lines monitored solely by relays). To tackle with this problem, some relay recording functions are adjusted to trigger in such a way that a disturbance will be recorded even for faults behind the protective line or when some specific protection function starts. Another situation usually found in protection systems is the lack of time synchronization among relays located in distinct substations. This brings a number of consequences. For instance, in the case of repetitive faults occurring within minutes from each other it is not trivial to correlate recordings from both terminals of the transmission line, because the DFR clocks may be many minutes apart.

2.3. Protection Analysis Requirements In this paper, two protection systems are used to illustrate the concept: distance teleprotection based on directional blocking overreaching scheme, associated to overhead transmission lines and current differential function, associated to underground transmission cables. Directional overcurrent relays are used as backup protections for both overhead and underground lines, but in most of cases, distance relays are used for back up overhead lines as well. The reported evaluation system was implemented using the logical channels in fault recordings which correspond to relay operations. The information availability, mainly in relays, varies according to the current technology, setting philosophy and so on. As a consequence, for each of these configurations, distinct digital outputs are available. When the recording is analyzed there will be distinct channels for each configuration, and the analysis must be restricted to the available signals. For examples, when analyzing distance teleprotection based on the directional blocking overreaching protection scheme for overhead lines implemented by static or electromechanical relays like RAZOA™ and KDAR™ relays, the analysis uses the channels in the fault recording corresponding to trip, blocking reception and blocking sending, because those are the only signals connected to the DFR that generates the recording. When dealing with numerical relays like Micom P442™ and Siprotec 7SA611 (which generate the fault recordings internally), a larger set of internal information may be directed to digital channels, like those indicating zone detection, making the analysis more rich and easier to understand. Another issue is the understanding of relay operation. Although many sophisticated relay models have been proposed, the algorithms implemented in real relays are usually known only by the manufacturer. So, what is proposed here is to simulate simple, generic relay functions (like distance, differential and overcurrent) and to compare the output of these simulated relays to the real ones. However, such comparison is used only as an alert to the user, should a gross discrepancy occurs, as these simplified models are not accurate enough to assure an eventual relay malfunction.

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3. Automatic Digital Fault Recording Analysis FILE MANAGEMENT SERVICE

REPOSITORY WEB INTERFACE DATABASE

Figure 1 : Simplified diagram of the Fault Recording Management System.

The Fault Recording Management System was built on Windows™ platform and it is composed of the following components, as depicted in Figure 1:


Files Repository: it is the directory where the fault recordings are stored. In this case only one repository is used. Its hierarchical structure, as explained earlier, is created by a third part software responsible for retrieving files from remote substations. The files are stored in COMTRADE format or compacted with Pkzip format.




File Management Service: this component detects when a new file arrives in the repository and it calls a number of analysis algorithms in order to fill the database accordingly. The File Management Service performs cyclically the following operations: register new files into the database, quarantine analysis (to filter out files not good enough for analysis), fault analysis, protection performance evaluation and other minor tasks (log managing, automatic backup etc.).




Database: It is the repository of all data used by the system, except for the recording files. The database stores topological and power system configuration data (DFRs, relays, busbars, transmission lines and subtations), connection of register channels to power system quantities, the rules for protection analysis, the analysis results and the software configuration. The relational database is implemented under Microsoft SQL Server™.




Web Interface: A set of HTML dynamic webpages (developed in C#.Net) helps browsing the repository contents, bearing facilities such as filters (by date, company, substation and busbar), displays and graphics.

The WEB Interface implements screens for the interaction of the users with the system database data. These interfaces allow for:
Entering and consulting topological and power system confguration data (DFRs, relays, busbars, transmission lines and subtations).
Entering and consulting connection of register channels to power system quantities.
Entering and consulting the rules for protection analysis.
Consulting the analysis results.
Entering and consulting the software configuration.

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4. Fault analysis The fault analysis algorithm is activated by the File Management Service when a file has passed the quarantine phase. Fault analysis will use signal processing techniques to classify the event registered in the digital oscillogram. The results are stored synthetically in the database. Fault analysis main objectives are: to determine if a fault have occurred and, if so, what is the fault type and the fault location. Fault analysis is performed in a number of steps, summarized below:


Signal processing: using information in the database, determine the channels monitoring the same transmission line to determine time periods candidates to span the power system states (like fault pre-fault, fault and pos-fault periods) during the event. These periods are tested for pruning noisy segments. If approved values that characterize the period are calculated. By using these values, the period may be considered a normal period (on regime), abnormal period (not faulty, but not entirely healthy), faulty, null (no currents), invalid (could not classify the period).




Event diagnosis: based on the sequence of periods found, heuristic methods are used to determine the registered event. The results may be, for instance: non-faulty, faulty with successful reclosing, faulty with line opened.

A typical event presents a pre-fault, a fault and a post-fault segment. During the pre-fault the system is healthy, although some small abnormalities may appear, indicating system deterioration. The post-fault period is, basically, determined by the opening of the line or the continuity of its operation, in a possibly new point of operation (value of currents and power delivered). Besides, the recording may not exhibit the pre-fault period, as in the case of a delayed trigger or because of triggering during reclosing action or line switch in. Post-fault behavior will be determinable only when the recording length is appropriate. Furthermore, the algorithm has a special behavior when no voltages are available in the oscillogram. If more than one recorder monitor the busbar at which the transmission line is connected, then the voltage can be obtained from other recordings in that busbar, bearing the same date. If voltages are not available at all, faults are analyzed solely from the current channels.

5. Protection performance analysis Protection performance analysis is then carried out using two approaches:


Coherency analysis: by checking the state of digital channels at the trip instant against patterns of expected states.




Comparative analysis: by comparing fault analysis results against simulations of relay elements using simple protection functions, like directional determination and current differential.

The logical behavior of the protection system is analyzed as follows: information from one terminal can be used to assert relay operation while information from more than one terminal can be used to assert the operation of protection system and communication channels as well. In such cases the WEB interface facilitates time synchronization of two or more digital oscillograms. 6

When performing fault recording based analysis the origin of the data must be considered. Files produced by relays reproduce directly the internal states of the device and do not rely on external contacts or wiring. Usually, in this case, there are more signals available on the recording. On the other hand, the DFR equipments are necessary for old devices and are very important, even when have digital relays are available, because DFR provide an overall view of the contributions from all substation transmission lines to the fault analyzed.

5.1. Coherency analysis As the development of patterns of expected states is a very complex task, an expert system tool was used to aid system development. The CLIPS [5] software was chosen for this. CLIPS has a special language to code the rules needed to evaluate the protection coherence analysis. Furthermore, the CLIPS software was designed so that rules can be stored in the database for ease changing, instead of being written in the source code. The WEB page interfaces enable access to the tables containing these rules. This feature is intended to couple with ongoing refurbishment projects. The rules for coherency analysis were developed from the knowledge acquired from LIGHT S.E.S.A. protection specialists. As an example, consider a 138 kV underground transmission line protected by current differential relays (ANSI 87). Table 1 shows two test examples: one where only the trip signal is available and the other where there are also the 87 function signal, the transfer trip transmission and reception signals and the 87 function blocking (due to loss of communication channel or manual switching). Table 1: Hierachical evaluation rules. Protection Set

Evaluation set Differential relay with Trip signal only Relay descrpt.: BPDLTS123_87, SMTLTS123_87, ...

One terminal, current differential protection set

Differential relay with Tranfer Trip transmission to remote terminal (TT local), Tranfer Trip reception (TT rem.), function 87 (F87), function 87 blocking (Block87), and trip. Relay descrpt.: PERLTS136_87, BNPLST136_87, GVDLTS160_87 ...

Diagnosis

Test patterns

The relay 87 did not issue a trip

Trip

=

0

The relay 87 did issue a trip

Trip

=

1

F87 Trip TT local TT Rem. Block87

= = = = =

1 1 0 0 0

F87 Trip TT local TT Rem. Block87 F87 Trip TT local TT Rem. Block87

= = = = = = = = = =

0 1 0 1 0 0 0 0 1 0

Relay 87 tripped due to function 87

Relay 87 tripped due to Transfer Trip reception

Unexpected relay trip

•••

Rules are organized as follows:


Protection set: major division, which may span a number of evaluation sets. Protection sets used in this work are: one terminal, current differential protection set (ANSI 87), one terminal, distance protection set (ANSI 21), one terminal, overcurrent 7

protection set (ANSI 51), two terminal current differential protection set (ANSI 87), two terminals, distance protection set (ANSI 21).


Evaluation set: minor division that includes all evaluations of a given set of relays, whose available digital signals are the same and whose logic is the same. In Table 1 it can be seen that there are two sets for differential current protection. The first one has only the trip signal available, and corresponds to electromechanical relays HCB (from Westinghouse). The second set has more signals available and corresponds to relays MiCOM P541, from AREVA or relays 7SD511 from Siemens. Other evaluation sets exist for distance protection, with one terminal and with two terminals.




Diagnosis: For each pattern, one diagnosis is applicable.




Test pattern: The states of each digital signal.

5.2. Protection modeling Comparing fault analysis results against simulations of relay elements using simple protection functions, like directional determination and current differential is a simple yet effective way to check fault evolution and relay operation. Of course, more complex simulations can be introduced, enhancing the results, if better modeling information is available. In this work the following functions were simulated using classical approachs:


Current Differential Function (ANSI 87): implemented using an operation/restriction characteristic.




Current Direcional Function (ANSI 67): modeled using the symmetrical component approach

6. Conclusions The paper presented an automated system developed by CEPEL and LIGHT S.E.S.A for protection performance evaluation from oscillographic registers. It was developed to cope with the information limitations imposed (and possibilities offered) by relays from distinct technologies. The system is already operating at LIGHT S.E.S.A. The limited experience till now suggests a performance as good as expected.

7. Bibliography [1] [2]

[3] [4] [5]

X. Luo, and M. Kezunovic, “Fault Analysis Based on Integration of Digital Relay and DFR Data” (Power Engineering Society - PES – Meeting, June 2005, San Francisco, CA). M.A.P. Rodrigues, M.A.M. Rodrigues, A.L.L. Miranda, S.S Diniz, M.V.F. Figueiredo, “A system for automated oscillogram analysis at LIGHT” (VII Seminário Técnico de Proteção e Controle - STPC (June/2003), Rio de Janeiro, Brazil), in Portuguese. IEEE Std C37.111-1999, “IEEE standard common format for transient data exchange (COMTRADE) for power systems,” Mar. 1999. IEEE Standard PC37.232 – Recommeded Practice for naming time sequence data files COMNAMES – Draft 4 (2004) C Language Integrated Production System (CLIPS) – www.ghg.net/clips/CLIPS.html (NASA)

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