This is part 3 in a series on staged fault testing.
In this series, we’ve explored why staged fault testing is essential for grid reliability; we’ve reviewed the equipment and procedures necessary to perform a staged fault test; and we’ve outlined the best methods to demonstrate results of a staged fault test.
Now we’ll outline data analysis strategies that electric utilities can use to understand the results of their staged fault tests. We recommend three tiers of data analysis:
Tier 1 – Review results from the line being tested.
Tier 2 – Review impacts on other lines on the system.
Tier 3 – Compare results to anticipated results in models.
In this week’s post, we demonstrate the results of a simulated staged fault test on Line 6 in Figure 1, illustrated below. Let’s take a closer look.
Exploring a Simulated Staged Fault Test
Staged fault tests, when performed properly, post minimal risk to component damage and continued system operation. This means that ground switches should not be used to initiate a staged fault test. Every staged fault test needs to be intentional and self-clearing because the fault must be automatically removed from the electric grid within 100 milliseconds. This is true regardless of whether newly installed circuit breakers and protective relaying schemes operate as designed.
Figure 1 shows a sample electric energy grid with a new, 230 KV transmission line shown in blue (Line 6). Line 6 will be accepted for use after staged fault test results are accepted. Two staged fault test locations are recommended: one on Line 6 near Bus 3, and another on Line 6 near Bus 5. The results of these two test simulations are discussed below.
Figure 1 shows a 230 KV electric energy grid. Line 6, a new line, is ready for staged fault testing.
Tier 1 Data Analysis: Review Line Being Tested
Tier 1 staged fault data is used to verify that protective relaying schemes associated with Line 6 are operating as designed. In this case, Tier 1 is verifying that the primary relays, secondary relays, and circuit breakers at both terminals of Line 6 are operating as intended. Line 6 cannot be accepted for use before Tier 1 data is assessed, and any errors, omissions, or discrepancies are resolved.
Tier 2 Data Analysis: Review Other Lines on the System
Tier 2 staged fault data is used to verify that primary and secondary relays at terminals of other lines actuate as designed. In this example, Tier 2 data analysis will review the lines and buses surrounding Line 6.
Tier 2 data is associated with protective relaying schemes that should not operate when staged fault tests are performed. Tier 2 data needs to be assessed quickly, within a few days of the staged fault testing. However, errors, omissions, or discrepancies involving Tier 2 can be addressed after a new transmission line is placed in service.
Tier 3 Data Analysis: Compare Results to Model
Before intentional, staged fault tests are performed, the configuration of the electric grid and expected results need to be understood, documented, and tabulated via a model. Tier 3 staged fault data is used to verify the accuracy of short circuit models.
Tier 3 data needs to be assessed within a few weeks of testing. Errors, omissions, or discrepancies involving Tier 3 data can be uncovered after the new transmission line is placed in service.
Tier 1: Line 6 Staged Fault Data Analysis
The following data outlines the results of a simulated, three phase staged fault on Line 6. The actual fault location is on a 230 KV overhead span near Bus 3. In the following tables, Zone 1, Zone 2, and DCB Trip are impedance type, forward looking relays. DCB Block are impedance type, reverse looking relays.
Tier 1 data reveals that Line 6 cannot be energized until the discrepancy involving the Zone 1 relays at Bus 5 is resolved. The recommendation should be to resolve and retest.
Tier 2: Staged Fault Impact on Line 2
The following data outlines the impact on Line 2 during the staged fault on Line 6. The actual fault location is on the 230 KV overhead span near Bus 3. Tier 2 data does not indicate other discrepancies.
Tier 3: Staged Fault Current Data (Model Comparison)
The following data compares calculated short circuit current values to Test Values for a three phase staged fault on Line 6. Tier 3 data indicates that the impedance of Line 6 is less than the value entered in the model and used to specify the settings of Zone 1 relays.
Staged Fault Test Findings
By analyzing the data in the three charts above, the following findings can be identified:
Zone 1 operation on Line 6 at Bus 5, along with higher-than-calculated current flow from Bus 5 to the fault, is an indication that the impedance of Line 6 is less than the value used to set Zone 1 relays. Line 6 should not be accepted for service until this discrepancy is resolved.
Protective relays associated with other lines are performing as designed.
Improve Safety and Reliability with Staged Fault Testing
By analyzing staged fault test results using the data analysis tiers described above, electric utilities can address issues with protective relaying schemes as new lines are prepared for energization. Staged fault testing also verifies that protective relaying schemes associated with nearby facilities are operating as designed.