9291_c006 - 6 Use of Oscillograph Records to Analyze System...

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6 Use of Oscillograph Records to Analyze System Performance John R. Boyle Power Sys tem Analys is Protection of present-day power systems is accomplished by a complex system of extremely sensitive relays that function only during a fault in the power system. Because relays are extremely fast, automatic oscillographs installed at appropriate locations can be used to determine the performance of protective relays during abnormal system conditions. Information from oscillographs can be used to detect the: 1. Presence of a fault 2. Severity and duration of a fault 3. Nature of a fault (A phase to ground, A – B phases to ground, etc.) 4. Location of line faults 5. Adequacy of relay performance 6. Effective performance of circuit breakers in circuit interruption 7. Occurrence of repetitive faults 8. Persistency of faults 9. Dead time required to dissipate ionized gases 10. Malfunctioning of equipment 11. Cause and possible resolution of a problem Another important aspect of analyzing oscillograms is that of collecting data for statistical analysis. This would require a review of all oscillograms for every fault. The benefits would be to detect incipient problems and correct them before they become serious problems causing multiple interruptions or equipment damage. An analysis of an oscillograph record shown in Fig. 6.1 should consider the nature of the fault. Substation Y is comprised of two lines and a transformer. The high side winding is connected to ground. Oscillographic information is available from the bus potential transformers, the line currents from breaker A on line 1, and the transformer neutral current. An ‘‘A’’ phase-to-ground fault is depicted on line 1. The oscillograph reveals a significant drop in ‘‘A’’ phase voltage accompanied with a rise in ‘‘A’’ phase line 1 current and a similar rise in the transformer neutral current. The ‘‘A’’ phase breaker cleared the fault in 3 cycles (good). The received carrier on line 1 was ‘‘off’’ during the fault (good) permitting high-speed tripping at both terminals (breakers A and B). There is no evidence of AC or DC current transformer (CT) saturation of either the phase CTs or the transformer neutral CT. The received carrier ß 2006 by Taylor & Francis Group, LLC.
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signal on line 2 was ‘‘on’’ all during the fault to block breaker ‘‘D’’ from tripping at terminal ‘‘ X ’’. This would indicate that the carrier ground relays on the number 2 line performed properly. This type of analysis may not be made because of budget and personnel constraints. Oscillographs are still used extensively to analyze known cases of trouble (breaker failure, transformer damage, etc.), but oscillo- graph analysis can also be used as a maintenance tool to prevent equipment failure.
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This note was uploaded on 03/03/2010 for the course POWER 332 taught by Professor Dr during the Spring '10 term at Ain Shams University.

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9291_c006 - 6 Use of Oscillograph Records to Analyze System...

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