4.2 Advanced Differential Example
4.2.7 Testing the Operating Characteristic
This test verifies the operating characteristic function and settings of the differential protection. This is accomplished by the proper calculation of the Idiff/ Ibias pairs resulting in the appropriate test currents for all fault loops.
The test module Diff Operating Characteristic has three test methods:
Shot Test: A test allowing specific shots in the Idiff/ Ibias plane, to verify the tolerances defined by the manufacturer.
Search Test: A test for the exact determination of the characteristic shape and its tolerances.
Static Output: A test for the Idiff/ Ibias pairs in terms of the actual test currents for an internal fault with numerical or vector diagram entry of vector magnitude and angle input.
Test the operating characteristic of the differential protection using the Shot Test with the B-C (L2-L3) and A-N (L1-E) fault types, the static output test, and the Search Test with the ABC (L1-L2-L3) fault type.
Insert the test modules in the OCC test document and define the test points and report format. This should effectively verify and document the Differential Operating Characteristic for each fault loop within the tolerances previously specified for the protection device.
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Step 1: Embedding the test module into the test document
1. Position the cursor in the test document after the Diff Configuration object.
2. Select IN S E R T | TE S T MO D U L E and then "OMICRON Operating Characteristic", or click the appropriate toolbar icon.
Figure 4-21:
Diff Operating Characteristic Test View, General tab
Step 2: Defining the test report
Select PA R A M E T E R S | RE P O R T. A dialog box appears where you can define the scope of the report.
A detailed description about defining test reports can be found in the "Concept"
manual’s section 5.2 ”Test Reports”.
Select VI E W | RE P O R T to display the test report.
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Step 3: Entering the general test parameters
1. Select the General tab of the Diff Operating Characteristic test module.
2. Select or verify that the options "Ignore Nominal Characteristic for Search Test" and "Prefault / Apply" are NOT selected now.
3. Enter a value of 0.5 s for the prefault time and 0.5 In for the prefault current.
Test Method
In relays where the exact characteristic is not known, it can be determined by the search function. If a nominal characteristic has been entered, it would be ignored for this purpose.
Prefault
The inclusion of a prefault state can be necessary under certain conditions (such as, when the differential protection function is enabled only after energizing the transformer or for verifying proper operation under conditions of magnetizing inrush for a short time). Care should be taken when applying these settings.
Figure 4-22:
General tab of the Diff Operating Characteristic module
91 A fault for the operating characteristic test is simulated in the protected zone based on the reference winding with Idiff/ Ibias pairs. From this, test currents are applied to the selected winding pairs: one in-feed is the network on the reference winding itself; the other in-feed is the network on the second winding or a selectable one for a three-winding transformer. Therefore, for a three-winding transformer, one winding has zero current. The different Idiff/ Ibias calculated currents are generated by the change in the network in-feed parameters.
Step 4: Using the Shot Test
The Shot Test is used to verify that the operating characteristic is within a specified tolerance. The number of test points can be determined by the test engineer, and can thus be limited to a minimum. The marked tests points in the Idiff/ Ibias plane are tested for tripping or not tripping the relay. It would be logical to test every characteristic point with two shots, one at the positive tolerance and another at the negative.
The tolerance band is created from right angles of the corner points from the positive and negative tolerance of the differential current and the stabilizing bias current. The defined expected characteristic line is shown with the tolerance band.
Testing B-C (L2-L3) fault with the Shot Test Use the Shot Test for the fault type B-C (L2-L3).
1. Click the Shot tab in the Diff Operating Characteristic module.
2. Select the fault type B-C (L2-L3) and enter the test points for the B-C Shot Test.
The first point is: Idiff= 0.25; Ibias= 0.5. Enter these values and click AD D. In a similar manner, enter all of the data points provided in table 4-7.
Table 4-7:
Shot test data points for
B-C (L2-L3). Idiff Ibias
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3. Start the test by selecting TE S T | ST A R T/ CO N T I N U E. 4. Change to the Report View.
Test the error types B-N (L2-E) and C-N (L3-E) in a similar manner.
Note: The test should be copied in the OCC test document before changing the settings of the copies to the new fault types. Before a new test can be run, the old results have to be cleared.
Figure 4-23:
Shot Test tab of the Diff Operating Characteristic module
Figure 4-24:
Results of the B-C (L2-L3) fault test for the test document
93 When switching the error type, the type of zero sequence current elimination can sometimes lead to changes in the characteristic line. This is because of the different current distributions of the individual lines (refer to section ”Step 4:
Defining the settings for the protection device” on page 73").
The line selective operation for protective devices: the relay trips whenever the Idiff and Ibias pairs of the three phases lie in the trip range. Figure 4-25 shows how the characteristic line shifts if the individual phases have different current quantities.
Figure 4-25:
Shifting of the trip characteristic line (simultaneous test on two different points in the Diff-Bias plane, in particular by single pole ground faults)
For example, mark a test point on the characteristic boundary at the second line segment in the stabilizing (biasing) region. Reference the defined test point to the size of the phase current. The other value pairs will lie in the trigger region.
This effect is automatically compensated for and the shape of the changed characteristic line displayed.
Testing A-N (L1-E) fault with the Shot Test
Use the Shot Test for the fault type A-N (L1-E) by copying the last instance of the test module. In this manner, the general parameters remain the same.
1. Single-click the last embedded test module (Diff Operating Characteristic module) within the test document.
2. (Optional) Select ED I T | OB J E C T > OP E N/ PR O P E R T I E S. . . and the View tab.
3. (Optional) Change the settings to display it as an icon and press O K . 4. Select ED I T | CO P Y to copy the test module.
5. On a new line in the test document, select ED I T | PA S T E to insert this copy of the test module.
6. Start the OMICRON Diff Operating Characteristic test module.
0
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7. Clear the results of the last test using TE S T | CL E A R.
8. Select the Shot tab in the Diff Operating Characteristic module.
9. In a similar manner, enter all of the data points provided in table 4-8. The asterisk (*) in the table indicates that it has changed from the value from table 4-7.
Table 4-8:
Shot test data points for A-N (L1-E).
10.Run the test using TE S T | ST A R T/ CO N T I N U E. 11.Change to the Report View.
Figure 4-26:
Results of the A-N (L1-E) fault test for the test document
bc
Idiff Ibias
0.25 0.50
0.40 1.00
0.40 2.00
0.75 *2.75
1.00 *4.25
2.00 *5.75
2.50 *7.25
3.00 *7.75
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Step 5: Using the static output test
Under certain circumstances it can be useful to know what the Idiff/ Ibias pairs mean in terms of the actual test currents for an internal fault. It might be necessary to output currents for longer periods of time. The currents do not correspond to errors that lie outside of the protection range, but rather to differential current that is not zero. The OMICRON Diff Configuration module is not suitable for this test. Moreover, the test quantity output is limited in time in the Diff/Bias characteristic field by the setting for the maximum test time.
For this particular test, the static output feature is available with vector magnitude and angle input as well as a vector diagram. The current test point from the Diff/Bias plane is used. The test point and fault type selected in the Shot tab will be automatically be used if the Static Output Test View is activated.
Note: The test equipment is not reset by the trip signal of the protection device.
Possible high current overloads of the protection device may occur.
Copying the test module within the test document
The static output test of the Diff Operating Characteristic test module can be inserted quickly into the test document by copying the last instance of the test module. This way, the general parameters remain the same.
1. Single-click the last embedded test module (Diff Operating Characteristic module) within the test document.
2. (Optional) Select ED I T | OB J E C T > OP E N/ PR O P E R T I E S. . . and the View tab.
3. (Optional) Change the settings to display it as an icon and press O K . 4. Select ED I T | CO P Y to copy the test module.
5. On a new line in the test document, select ED I T | PA S T E to insert this copy of the test module.
6. Start the OMICRON Diff Operating Characteristic test module.
7. Clear the results of the last test using TE S T | CL E A R.
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Testing the static output
Test the relay with a 3-pole error A-B-C (L1-L2-L3).
1. Select TE S T | ST A T I C OU T P U T. . . to open the Static Output dialog box.
2. Set Idiff to 0.5 I/In and set Ibias to 2.0 I/In.
3. Select fault type A-B-C and the actual test currents for the knee-point setting are displayed.
4. Click O N / O F F to activate / de-activate the outputs to the protection device.
5. Click CL O S E to exit. Close the Diff Operating Characteristic module and return to the OCC document.
Figure 4-27:
Static Output dialog box of the Diff Operating Characteristic module
Calculated test currents to be output according to the set Idiff / Ibias values and the selected fault type
Vector diagram of the currents
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Figure 4-28:
OMICRON Diff Operating Characteristic test module inserted into a test document
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Step 6: Using the Search Test
The Search Test is used for the exact determination of the operating
characteristic shape and its tolerances. When testing conventional or analog electronic relays with non-linear characteristic shapes, this method for determination of the operating characteristic is advantageous.
The number of test points is automatically determined by the algorithm from the specified resolution and the tolerance. The tolerance band is constructed by rectangles, which result from the positive/negative tolerance of the differential and biasing currents. The tolerance band is displayed for a set nominal characteristic.
Copying the test module within the test document
The Search Test of the Diff Operating Characteristic test module can be inserted quickly into the test document by copying the last instance of the test module. In this manner, the general parameters remain the same.
1. Click the last embedded test module (Diff Operating Characteristic module) within the test document.
2. (Optional) Select ED I T | OB J E C T > OP E N/ PR O P E R T I E S. . . and the View tab.
3. (Optional) Change the settings to display it as an icon and press O K . 4. Select ED I T | CO P Y to copy the test module.
5. On a new line in the test document, select ED I T | PA S T E to insert this copy of the test module.
6. Start the OMICRON Diff Operating Characteristic test module.
7. Clear the results of the last test using TE S T | CL E A R.
99 Configuring the Search Test
1. Select the Search tab in the OMICRON Diff Operating Characteristic test module.
2. You may enter each search line manually by entering a value for Ibias and then clicking the AD D button. The test point is displayed by a vertical line.
The algorithm searches along this vertical line for the active trip value.
If you choose this method of data entry, add test points from 0.5 to 7.5 with a step size of 1.0. This should produce eight test lines.
Note: If the AD D button is disabled, you need to first remove the test points from the Shot Test.
3. A quicker method of data entry is provided by clicking the AD D SW E E P
button.
Enter 0.5 as start value and 7.5 as end value.
Then enter 1.0 as the step size and click AD D T O TA B L E. All eight test lines are now entered in the test table.
4. Now make sure the fault type is selected as ABC (L1-L2-L3).
Figure 4-29:
Input of the Search Test points for the Diff Operating Characteristic test
Numeric input of the test line
Adding or removing test points to/from
the list
Marked test lines in the graphic control
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Running the Seach Test
1. Run the Search Test by selecting TE S T | ST A R T/ CO N T I N U E on the pull-down menu.
Figure 4-30:
Search Test results of the Diff Operating Characteristic test
2. Select VI E W | RE P O R T VI E W on the pull-down menu.
3. Test the error types B-N (L2-E) and C-N (L3-E) in a similar manner.
Note: The test should be copied in the OCC test document before changing the settings of the copies to the new fault types. Before a new test can be run, the old results have to be cleared.
Idiff nominal and actual values
Assessment of the test results:
+ = Passed x = Failed
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Figure 4-31:
Test document containing three different OMICRON Diff Operating
Characteristic tests
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