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CASE STUDY OF DESGINING A FAULT RECORDER SYSTEM

N. Saeidi

Niroo Research Institute, IRAN

ABSTRACT

Fault recorders are reckoned among the most important components of protection systems within high voltage substations which through continuous monitoring the power system parameters and diagnosing fault situations as well as recording the parameters status before and after happening the faults, assist substation engineers to better identifying cause of faults and eliminating them. Thanks to designing them by using innovative and breakthrough hardware and software components such as high-speed processors, high capacity mass storages, accumte analog to digital converters, real time operating systems and so on, they collect huge amount of precise information from the status of power network which subsequently can he used by evaluating softwares to analysis the circumstances of fault to avoid repeating it in the future. In this paper, designing a fault recorder system is discussed as a case study of designing an embedded system with considering its specific requirements and exceptions.

KEYWORDS

Digital Fault Recorder, Fault Diagnosis, Fault Analysis, Intelligent Electronic Device, Protection and Control System

INTRODUCTION

In generation, transmission and distribution systems of electrical energy, the main objective is to deliver energy with an “assured quality to the customers. So the conditions of power are continually monitoretl in all sections of these systems to achieve this goal. By using intelligent devices and equipped tools, the protection & control section of power network supervises the running situation of the network for being able to avoid further damages or blackouts with rapid diagnosing fault and proper reacting in time of fault Occurrence and to prevent repeating faulty situations in the future by evaluating fault causes and recognizing weak points of the network and eliminating them after returning to normal operation. Due to the high costs of power system damages, which

a fault may cause, more investing for equipping protection systems is extremely justifiable and makes financial advantages and in this regard is beneficial for the power generating companies. Fault recorder is ti principal member of protection system which through its analog inputs measures lines’ voltages and currents and with regards to its predefined settings -which are desired conditions of the network - “senses” fault occurrence and records all the information related to the fault. With further analyzing, it identifies different aspects of the fault such as fault cause, fault type, fault location etc. Also, through its digital inputs it monitors the behavior of other devices in protection system especially protective relays when a fault occurs or during their abnormal operation. In fact the function of this device is to analyze the recorded data in order to quick elimination of fault causes rather than just recording raw data. Rapid and great advances in electronic chip designs and functioualities and widespread use of them in designing such devices along with applying developments in software and operating systems arena which has allowed implementing novel and sophisticated algorithms for fault diagnosis have made these devices more closer to their ideal goal which is recognition of fault at the correct time and accurate and developed analysis of fault. So this is why there is an evolution in new models of these equipments, compared with previous ones. This paper discusses designing a fault recorder system in two steps. The first step contains the hasis of a research project in Niroo Research Institute, which was ahont designing such a system based on current requirements of UHV and HV substations and explains different stages of the design. The second step includes some ideas for choosing design strategies depending on the substation in which fault recorder is supposed to be installed and expresses the author experiments from the first step for future plans.

APPLICATION NECESSITIES

The fact that protective relays have fault diagnosis and location capabilities may bring this question to the mind: in spite of large availability of theses devices in protection systems is there any need for designing and applying fault recorders? The clear answer of this misunderstanding is that fault diagnosis and analysis cannot he limited to local data gathered by relays. So for total fault evaluation in a substation all the data

0 2004 The Institution of Electrical Engineers. Printed and Dublished bv the IEE. Michael Faraday House, Six Hills Way, Stevenage, SGI 2AY

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recorded by relays must be entered in a unique database. But most of the time this is not attainable. First because of different relays, which are in the market from different manufacturers, have their own specific recording format and it is dificult or even impossible to retrieve and interpret all these formats. However, this problem may remove if all the manufactures observe a unique format like COMTRADE (See Ref. 1) for the recorded data. The second problem arises when for overall data processing it is essential for all the information acquired by protective relays to be time tagged and synchronized with an absolute time reference, because the authenticity of the recorded data is deeply depended on their time tags. This one is an obstacle in using relays information because time synchronizing between relays used in substations is usually neglected. Besides the problems mentioned above, there is a more important one that is sharing, concentrating and analyzing theses data need skillful personnel always available in substation for rapid and accurate decision making based on the information they have evaluated. Fault recorder has reduced substation engineers’ tasks via its powerful analyzer tools, which have been inserted in its hardware and software. Up to this point, it was supposed that the behavior of the relays and their data recording have been done correctly. But it is likely to happen that due to an error in the hardware or software of the relay, it performs a wrong action and therefore records data not related to a real fault or because of the limited storage capacity available in the relay its data become overwritten before reading. In these cases the recorded data cannot he relied on and it’s not worthy for evaluating. Regarding these restraints i t is necessav to have a dedicated system for monitoring of power network and checking its parameters in order to chasing and erasing faults. The functions of this so-called Fault Recorder arc:

Monitoring the network via measuring its parameters Providing an archive from information of occurred faults for further analysis and using this data for testing the protection devices and also to correct models used in simulation ofprotection system Supervising backup protection and protective devices Performing precise calculations and preparing information for quick troubleshooting of the network

DESIGN CONSIDERATIONS

With paying attention to the fact that the data recoded by fault recorder is considered as the basic and reference materials for future calculation, analyzing and decision making, it is necessary for this device to collect data without adding errors by itself. This principal fact affects all the design phases so that signal

conditioning of the inputs and quantizing of them, local processing and calculations and finally collecting data from all the input modules in a main processing module must he done in a safe manner. Also, the recorder should he equipped with self- checking feature in order to he very quickly aware of failure in any part of itself and then seek a remedy for the failure. This remedy can be just sending an alarm for the operator or restarting the failed module. Figure 1 shows the block diagram of the designed fault recorder. In the following sections different parts and aspects of this system are explained.

1.Modularity

The system has been designed modular as it can he seen in figure 1. Modular designing helps designer to divide and separate the functions of a system, which are especially too complicated in a fault recorder, to small components and by reducing the load of the main processing unit provides a fair distribution of system activities and tasks between units. Activity division in this system has been done in a function-oriented manner; therefore some functions such as analog inputs acquiring and processing, require more throughputs to compare with digital input scanning function. On the other hand, modularity~ makes testing and troubleshooting of the system more easily and in this way increases its reliability. A high performance and robust standard bus - VME bus - has been chosen as the backbone of the recorder. (See Ref. 2) Observing strict constraints and requirements of this well-defined bus enriches reliability of the device. This adds to the design the open architecture advantage so that future changes, customizing and tailoring the system can he done quickly by using modules available in the market from different manufactures.

2. Synchronization

Synchronization for systems with distributed parts and components is of great impofiance. Power system is an example of such systems especially in geographically large countries in which nodes of the system (i.e. substations) are located far from each other with considerable time differences. Now, when a fault occurs in a line and both fault recorders in the ends substations recognize and record it, the recorded data can be merged (e.g. in dispatching centers) for total analysis of the network just in case both recorders have been synchronized with one time reference. Different means for time synchronization have been studied for applying in this fault recorder including using modem for dialing a unique number which is belonged to a dedicated center for transmitting time information, using radio frequency transmitters and

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Analog Input Modules Digital Input Modules Periphera Module CPU Module

Figure 1 -Block Diagram of the Fault Recorder System

GPS. The last one - GPS - is selected because of its wide accessibility. The designed recorder receives a one-pulse-per-second input and 'serially transmitted time information via a GPS receiver and synchronizes itself with the global time.

3. Configuration

The capability of configuration of the recorder has been implemented in different ways and levels.

-Dedicated display and keyboard of the device and its serial polts (RS232 & RS485) for direct connecting to a computer have been selected for local configuration and for remote configuration connection of the device via modem or LAN have been considered. Also therc are two levels (supervisor level and operator level) for accessing the device resources (fault data, parameters' settings and configuration of the device).

4. Input Modules

These modules interface the device to power system and protection equipment. As it was already mentioned, special care should he taken in the design for gathering real data without adding any errors. For accomplishing this goal, in the signal conditioning of the analog input modules accurate elements (hall effect sensors) have been used and digitizing the input signals has been performed by applying precise 16-hit resolution AIDS with 256 samples per cycle rate. There are 12 input channels (6 voltage channels and 6 current channels) for each analog module and up to 3 analog modules can be inserted in the system. Each module has a powerful DSP for preprocessing the acquired

data, checking the inputs conditions with the predefined criteria and triggering the device in case of diagnosing a fault. There is a similar hut simpler way for scanning and evaluating the digital inputs. In each digital module there are 32 channels (with Ims r$solution) and maximum two digital modules can be used in the system. Input range of the modules can he easily changed by removable signal conditioning sub modules and extending the number of inputs can be done by direct connectivity and network capabilities of the device.

5. Main Processing Module

This module is the most critical section of the system and its main functions are:

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Supervising the overall operation of the system Collecting data from the input modules after occurrence of a fault and organizing the acquired data in a file Post processing of the data for calculating the virtual and logical channels Handling printer for drawing curves and printing values of the input signals when a fault occurs Sending fault information in the different formats for using in analyzer tools Managing mass storage of the device Sending alarms in case of internal failures or user defined situations Sending configuration parameters of the input modules Supporting interface capabilities of the device (RS232RS485, Modem, LAN)

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It is obvious that all these tasks must he done properly and simultaneously. So a real-time multi-task operating system has been chosen for the system and software of this module has been implemented based on this operating system. This is another feature, which guarantees reliability of the recorder.

6. Peripheral Module

This module supports interfering the device to its local display and keyboard and receives GPS pulse and data and synchronizes the input modules.

techniques for fault diagnosis and fault location are amongst these opportunities. As the substation CTs and PTs, which provide inputs for fault recorder, make some degree of deviation in input signals, calculating more accurate compensation coefficients should not be neglected. Adding a limited capability of changing the firmware of the modules from a computer running the system’s software can he useful for enhancing the device feature with low cost. Enrichment of analyzing capabilities of the software leads to more accurate conclusion, so organizing a lihraq from the occurred faults data and “training” the software for using these experiences for future analysis is a good opportunity which needs more efforts.

7. Power Supply CONCENTRATING OR DISTRIBUTING

This is simple hut important component of the device and has been considered with self-monitoring capability so that in case of power down it warns the main processing unit and switches to the dedicated battery.

8. Software

Although with capability of sending the recorded data out in COMTRADE format different analyzer tools receiving this format can he used for evaluating the fault data, a software package has been implemented for assuring use of the recorded data in a powerful tool, which doesn’t waste the accuracy of the data. Some important features of this software are:

Uploading the fault data from the device in a selective manner Drawing of input signals curves with many graphical facilities Configuring the device and uploading the current configuration Calculating different criteria (RMS, Rate of change, THD, Sequences, etc) and drawing the results of calculation Performing advanced calculation such as fault location Managing and organizing fault data in a database with importiexport capability of different formats

IDEAS FOR FUTURE DEVELOPMENT

Designers and manufactures of fault recorders ought to consider a required level of capabilities in their products without taking into account that these Capabilities may exist in the other protection devices and this certainly has overhead costs because the desigh of these devices becomes more complicated. If there were a generally accepted standard for + r i n g data ‘and features between different manufacturers’ products the design of theses devices could he simpler and then more economically reasonable. Also, with using computers, which nowadays are available in many substations, most of the functions of the main processing unit can he passed to a computer. This eliminates requirements for powerful CPU and operating system hut needs fast and safe transmission of data between the recorder and the computer and can be done by implementing a network like field bus for the substation.

ACKNOWLEDGEMENT

The author wishes to thank all the people in Electronics, Control and Instrumentation Department of Niroo Research Institute for their valuable advices during this research plan especially those who cooperated as the team of “Designing a Fault Recorder System” project and believes without their appreciable efforts this work could not he done.

REFERENCES

Regarding sampling rate and accuracy, the hardware of I . IEEE Publication, 1991,”IEEE Standard Common new fault recorders has been reached to an acceptable - Format for Transient Data Exchange or sometimes over designed- stage but there are plenty (COMTRADE) for Power Systems”, IEEE C37.111 of oppomnities for innovation in software parts of ’ 2. IEEE Publication, 1987, “”IEEE Standard for’A theses devices. Using wavelet theory for phasor Versatile Backplane Bus: VMEhus”, ANSliIEEE calculation, new fuzzy logic and nemal network Std 1014