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Synchronization accuracy

In this report, the Working Group examined the meaning of synchronization and the means to achieve it. They tackled the question of how accurate time synchronization needs to be and how accurately it can be done. It is fortunate that the synchronization accuracy required for event analysis can be achieved with reasonably priced equipment and careful engineering.

Event reconstruction using data from intelligent electronic devices (IEDs)
Event reconstruction using data from intelligent electronic devices (IEDs)

Reconstructing an Event, Sequence of Events (SOE)

When an event occurs on the power system, recorders installed at different locations may be triggered at different moments to record specific information that reflects different aspects of the same event during the course of the event.

Normally, the records obtained by different recorders can be transferred to a central control office for further analysis. If different recorders are synchronized to the same time reference, then the records provided by different recorders may be correlated with each other to reconstruct the event and thus provide a clearer and more complete picture of the entire event.

The input data that may be needed for analyzing the event usually include both analog data such as phase currents, residual current, phase voltages, and residual voltage and digital data such as primary and backup relay trip, breaker open/close position, breaker failure contact, carrier start, and carrier received contacts, etc.

The following is an example of using the records from different recorders to reconstruct an event. A sample power system is shown as follows, in which control area 1 is exporting energy to control area 2 at a rate of 500 MW. Digital Fault Recorders (DFRs) are installed at bus 1, 2 and 3.

Distance relays are installed at each bus. An under-frequency relay is installed at bus 3.

Example of using the records from different recorders to reconstruct an event
Example of using the records from different recorders to reconstruct an event

Suppose that a fault occurred on the tie line between bus 1 and 2, and the digital fault recorders had the following information:

DFR 1

  • The distance relay at bus 1 tripped at 13:20:03.011 (13:20, with seconds 3 and milliseconds 11)
  • The breaker at bus 1 opened at 13:20:03.071

DFR 2

  • The distance relay at bus 2 tripped at 13:20:03.014
  • The breaker at bus 2 opened at 13:20:03.085

DFR 3

  • The underfrequency load shedding relay at bus 3 tripped at 13:20:08.030
  • 400 MW of Load 3 at bus 3 is shed at 13:20:08.050

Sequence of events

Based on the records, it can be determined that the following sequence of events occurred:

  1. A fault occurred on the tie line.
  2. The distance relay at bus 1 tripped at 13:20:03.011, and the breaker at bus 1 operated correctly and opened the line after 60 ms after receiving the tripping signal.
  3. The distance relay at bus 2 tripped at 13:20:03.014, and the breaker at bus 2 operated correctly and opened the line after 71 ms after receiving the tripping signal.
  4. Since the imported power is cut off, control area 2 has a shortage of power. The under-frequency load shedding relay at bus 3 tripped at 13:20:08.030, and 400 MW of Load 3 was shed at 13:20:08.050 to maintain the system normal frequency.

By taking advantage of the voltage and current waveforms, the time taken by the relay to issue a tripping signal from the fault inception moment can also be calculated, which can be used to evaluate the relay performance.

The recorded system frequency locus during the event can also be used to evaluate the performance of the under-frequency load shedding relay.


Non-Fault Events and Wide Area Disturbances

In reporting circuit outages for wide area disturbances such as a blackout, the most precise time to use for the circuit interruption may be the current-zero. This assumes that a record is triggered which captures the last phase current-zero. It further assumes that if a disturbance recorder is used, it’s recording the current waveform and not just an RMS signal; though the current-zero on the RMS record will be fairly accurate.

This also assumes that sampling and recording rates are sufficient to provide the desired accuracy. A recording rate on the order of 720 Hz (12 samples per cycle at 60Hz) should provide sufficient accuracy; while recording rates as low as 240 Hz (4 samples per cycle) have proven to be useful for event reconstruction many times in the past.

If all three phases are available, then the last phase to go to zero should be reported. If only one phase is recorded and it’s the first phase interrupted, then the time reported would be for the phase recorded. In either case, the report should include an indication of whether it is the last phase or the only phase available.

Using the current zero method also requires analysis of each record, and assembly of a sequence of events from these records.

It may be possible with many EMS/SCADA systems to record Sequence of Events (SOE) times at a central location. However, these times will be based upon breaker auxiliary contacts or auxiliary relays that introduce additional delays and time reporting variations. If SOE data are recorded, it should include the time that the device actually operated, not the time that the EMS/SCADA system polled the status point.

Polling time and polling sequences can significantly skew the recorded breaker operating times.

Title:Fault event reconstruction using data pulled from intelligent electronic devices (IEDs) – Final Report of IEEE Power System Relay Committee Working Group I11; Approved by Relay Practices Subcommittee
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Fault event reconstruction using data pulled from intelligent electronic devices (IEDs)
Fault event reconstruction using data pulled from intelligent electronic devices (IEDs)

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