4.2 Advanced Differential Example
4.2.3 Starting Diff Harmonic Restraint from the OCC
Start the OMICRON Control Center from the Start Page by clicking OP E N
EM P T Y DO C U M E N T. Insert Diff Harmonic Restraint into the OCC document by selecting the menu item IN S E R T | TE S T MO D U L E. . . | O M I C R O N DI F F HA R M O N I C RE S T R A I N T.
4.2.4 Setting up the Test Object
For configuration of your relay under test, the correspondingly named software function Test Object is used. Open Test Object with the pull-down menu item PA R A M E T E R S | TE S T OB J E C T. Alternatively, click the Test Object toolbar icon. In Test Object browse through, access and edit the test object
parameters.
A detailed description of Test Object and the closely related subject "XRIO" can be found in the "Concept" manual’s section 3 ”Setting up the Test Object”.
In this example we want to import the test object parameters from a file.
Step 1: Inserting a test object and defining the device settings
1. In the OCC, select IN S E R T | TE S T OB J E C T to open the dialog box for the test object-specific data.
2. In Test Object, select FI L E | IM P O R T and import the file Schweitzer SEL 587_Getting Results Example.rio. This loads the parameters for the protection device.
This .rio file is stored in ...OMICRON Test Universe installation path\Test Library\Test Objects_XRIO\Schweitzer.
3. Check/specify the device settings.
4. Check/define the differential protection parameters as described in the following steps 2 to 6.
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Step 2: Defining the settings for the protected object
Figure 4-3 shows the standard page for the protected object in the Differential Protection Parameters dialog box.
In this example, the protected object is a transformer.
Make sure that the parameters for the primary and the secondary windings are correctly entered. In this case, the starpoint grounding of the primary is "no"
while it is "yes" for the secondary, as shown in figure 4-3.
Figure 4-3:
Protected Object page in the Differential Protection Parameters dialog box.
The name of the windings can be changed to provide a more meaningful designation.
The starpoint grounding is important for the current distribution for a single pole grounding error (figure 4-4).
The Vector Group setting is for the phase correction of the line currents through the Protected Object. This data usually comes from the boiler plate information.
However, it may have a different terminology from what is used here.
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Table 4-1:
Vector group terminology
The selection of the single-phase fault type in a test module does not mean that a ground current (zero-sequence current) is simulated correctly. Indeed, the parameter Starpoint Grounding is critical here. This means that a zero-sequence current can only flow in a winding if the starpoint is effectively grounded to
• the selected fault side for the Diff Configuration module
• the reference side of the Diff Operating Characteristic or the Diff Trip Time module.
Figure 4-4:
Single pole error and current distribution with a grounded starpoint.
Test with zero-sequence current.
Figure 4-5:
Single pole error and current distribution with a non-grounded starpoint.
Test without zero-sequence current.
In the second case, it is assumed that the zero sequence current comes from the other side, meaning that the circuit is grounded at the other point.
Object Type Setting Phasor Reference
HV LV HV LV HV LV
D 0° Y 0° D Y0 0° 0°
D 0° Y 30° D Y1 0° 30° lag
D 0° Y330° D Y 11 0° 30° lead
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Step 3: Defining the settings for the current transformers
Figure 4-6 shows the standard page for the CT in the Differential Protection Parameters dialog box.
In this example, the current transformers (CT) scale the transformer current down to a level suitable for the relay.
Figure 4-6:
CT (current
transformer) page in the Differential Protection Parameters dialog box
When the option for "Use Ground Current Measurement Inputs" is selected, the zero sequence current for each winding is simulated on the configured current output.
Enter the nominal values of the ground current transformer, if these are connected to the relay, the zero sequence current is measured from the differential relay, and the zero sequence current is used in the calculation.
Note: One transformer starpoint has to be grounded in order to check the box for the ground current measurement input.
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Step 4: Defining the settings for the protection device
Figure 4-7 shows the Protection Device page of the Differential Protection Parameters dialog box. Enter the appropriate settings based on figure 4-7.
The following table contains the settings required for the differential protection device.
Table 4-2:
Parameters for the
protection device Parameter Value Description
Idiff> 0.30 IN Idiff minimum pick-up
value
Idiff>> 3.0 IN Idiff instantaneous pick-up value
1st segment of characteristic element
Idiff = Idiff> operating characteristic
2nd segment of characteristic element
Idiff/ Ibias= 25%
Knee-point = 2.0 Ibias
operating characteristic
3rd segment of characteristic element
Idiff/ Ibias= 50% operating characteristic
Reference winding Winding 1 of the protection device (here HV side)
Reference winding for Idiff/ Ibias calculation
Standardize using Protected Object Nominal Current
Reference value for calculating test quantities Zero Sequence
Elimination
IL - I0 Method for zero sequence
elimination
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Figure 4-7:
Protection Device page of the Differential Protection Parameters dialog box
Reference winding:
The currents measured by differential relays are different in their absolute value and phase under normal operation and cannot be used directly for the
calculation of the Idiff and Ibias values.
Therefore, the protection relay has to define a reference winding to normalize the currents to the same phase shift and eliminate the zero-sequence current.
In order to be able to test the operating characteristic this reference has to be defined to the test module.
In principle, it makes no difference which side of the transformer is defined as the reference side, but the current distribution in the single phases and their absolute values and phase shifts are different for each reference winding depending on the vector group for single-phase and two-phase faults.
List box for the Ibias calculation of the protection device
Reference Winding is the winding used for the test current reference calculation of Idiff and Ibias pairs. This is for testing the operating characteristic and the trip time characteristic;
the fault will always be placed on this side.
Maximum duration of test currents being output, if the differential relay does not trip.
Delay Time is the time between two automatic test steps or shots.
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Figure 4-8:
Measured currents with nominal operation
Ibias Calculation:
The Ibias quantity - sometimes referred to as the stabilizing or restraint quantity - is used to compare against the Idiff quantity for the tripping decision. The calculation method for this quantity Ibias has to be determined from the relay manufacturer and cannot be set arbitrarily. Knowing this setting is of critical importance for the test of the operating characteristic.
Note: Presently, only relays can be tested that calculate the Ibias and Idiff values from currents which are zero-sequence and vector group compensated.
Table 4-3:
Calculation methods for
the biasing quantities Method Manufacturer
Siemens K1 = 1, GEC, SEL K1 = 2 AEG K1 = 2
AEG three-winding K1 = 3 conventional relays K1 = 1 GE Multilin SR 745 K2 = 1
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Standardize Using:
The absolute value (standardization) of the currents to be compared takes place at the protected object nominal current or the current transformer nominal current of the most powerful winding (depending on the relay manufacturer).
Note: These parameters are essential and have to be determined from the relay manufacturer if they are not known. Arbitrary specification leads to undefined results.
Figure 4-9:
Measured currents for nominal operation of a delta winding transformer.
Numerical relays directly measure the phase currents; zero-sequence
elimination, absolute value, and vector group compensation are computationally performed in the relay.
Transformer model:
The transformer model represents the simulation of the ideal response of the transformer. This means, that vector group, ratio and current transformer data are taken into account for the calculation of the test quantities. At the moment, the test can only be run for relays that work phase-selective and with
symmetrical bias windings.
"No Transformer Model" means that these parameters are not used. The test then corresponds to the traditional test of conventional relays after the interposing transformers.
InomInterposingTransformer InomTransformer
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Figure 4-10:
Connection of the test devices to a
conventional differential relay with symmetrical bias winding and test with deactivated transformer model
Zero Sequence Elimination:
For transformers with a delta-wye grounded winding configuration (whether the delta is a power winding or phantom winding), a zero-sequence current will flow in the grounded winding for a single-phase fault. Because this zero sequence current does not flow in the delta winding, the currents into the protective relay have to be compensated or corrected for the unbalance caused by the phase angle displacement across the transformer. The phase angle correction can be accomplished by interposing auxiliary current transformers (traditional method) or physical jumpers within the relay (some E/M and static relays) or
computationally by the relay (most digital relays). The method used for zero sequence elimination is critical to the test. Therefore, it is necessary to select the type of Zero Sequence Elimination used.
The following describes the essential aspects of this setting.
IL-I0 (Computational Zero Sequence Elimination)
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Figure 4-11:
Zero-sequence elimination IL-I0, reference side = primary side (HV)
In the primary winding flows a zero-sequence current.
The secondary side, which is feeding in here, is zero-sequence current free, because the delta winding eliminates it.
Calculation method in the relay:
Phase current
Zero-sequence current:
From this, the following calculated phase currents I ´L are obtained for the differential values:
79 Note: Because of the selected type of zero-sequence elimination, the relay now detects a biasing quantity in all 3 phases and gets less sensitive by the factor 3/
2 for the operating characteristic test.
YD interposing transformer:
(Zero Sequence Elimination by a Delta Winding). A delta winding represents a short-circuit to zero-sequence current. This means, if such a winding is present, no zero-sequence current will flow at the in-feed side for a single-phase fault at the grounded side. The following figure shows that the interposing transformer circuit with the delta winding performs the zero-sequence elimination. For conventional relays, these interposing transformers are present as hardware; for different numerical relays, they are simulated by the software. This information needs to be taken from the relay documentation or has to be determined from the manufacturer. side = secondary side (LV)
Conventional relays are connected at the zero-sequence system free side, various numerical relays also calculate the differential and biasing currents with reference to this side. (See dotted rectangle in figure.)
Entering the characteristic is done with lines, which have to be derived from the relay parameters.
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Step 5: Defining the characteristic lines
Figure 4-13 shows the standard page for characteristic definition in the Differential Protection Parameters dialog box.
Figure 4-13:
Characteristic Definition page in the Differential Protection Parameters dialog box.
The initial data is taken from the general relay parameters.
1. Start with the second line, because the lines Idiff= Idiff> = 0.30 (first line) and Idiff= Idiff>> 3.00 (last line) are automatically added.
The second line has Idiff/ Ibias= slope1 = 25% to the intersection with the third line. The knee-point is Ibias= 2.0.
Table 4-4:
Calculation of the end point of the 2nd line
81 3. Enter the 3rd line Idiff/ Ibias= slope 2 = 50%. The line needs to be defined to
the intersection of line Idiff>>= 3.0.
Table 4-5:
Calculation of the end point of the 3rd line (the intersection with Idiff>>).
Idiff3- Idiff2/ Ibias3- Ibias2= 0.5 3.0 - 0.5 / Ibias3- 2.0 = 0.5 2.5 = 0.5 Ibias3- 1.0 3.5 = 0.5 Ibias3 Ibias3= 7.0
4. Click the AD D button to get this data into the table of defined lines.
The characteristic definition should look similar to what is presented in figure 4-14.
Figure 4-14:
Characteristic Definition page of the Differential Protection Parameters dialog box with two lines defined
Start point End point
Idiff2 = 0.50 Idiff3 =3.0
Ibias2 = 2.0 Ibias3 = 7.0
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Step 6: Defining the parameters for the harmonic restraint
Figure 4-15 shows the standard page for the harmonic settings in the Differential Protection Parameters dialog box.
Entering the harmonic restraint settings is done directly as a percentage of the target harmonic. Multiple harmonics may be selected and set with their
individual settings and tolerances.
Figure 4-15:
Harmonic settings in the Differential Protection Parameters dialog box.
Harmonic selection and setting
Allowed Tolerances
Idiff data is automatically provided
Ixf/ Idiff= f(Idiff)
Graphic is updated when the "Add" button is pressed.
1. Enter the harmonic settings as listed in table 4-6.
Table 4-6:
Harmonic restraint parameters
2. Click the AD D button to get this data into the image for the harmonic settings.
The Idiff data are automatically provided from the Protection Device settings.
Parameter Setting #1 Setting #2
Harmonic 2nd 5th
Restraint 15% 25%
Tol Relative 5% 5%
Tol Absolute 1% 1%
Time Delay 0.00s 0.00s
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4.2.5 Configuring the Hardware
Configure the hardware according to the wiring described in section 4.2.2 ”Wiring Between Relay and CMC/CMA”.
A detailed description of Hardware Configuration can be found in the
"Concept" manual’s section 4 ”Setting Up the Test Hardware”.