Application Note
Testing of Restricted Earth Fault Protection with OMICRON Advanced Differential
Author Daniel Wiesner |
[email protected] Date Dec 07, 2010 Related OMICRON Product CMC Test Sets Application Area Restricted Earth Fault relays, r elays, Advanced Differential Keywords Restricted Earth Fault, REF, Restricted Ground Fault, RGF, Differential Version v1.0 Document ID ANP_10011_ENU ANP_10011_ENU
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Content 1
What is Restricted Earth Fault Protection and Why is it Necessary? ........................................... 3
2
Testing with OMICRON Advanced Differential............................................................................... 5 2.1 Introduction............................................................................................................................... 5 2.2 Connection CMC – REF Relay .................................................................................................. 5 2.3 Advanced Differential Test Object Settings............. ................................................................... 6 2.3.1 2.3.2 2.3.3 2.3.4
Protected Object Tab ...................................................................................................................6 CT Tab ........................................................................................................................................7 Protection Device Tab ..................................................................................................................8 Characteristic Definition Tab.........................................................................................................9
2.4 Advanced Differential Tests .....................................................................................................10 2.4.1 2.4.2 2.4.3
Differential Protection Function ................................................................................................... 10 Diff Configuration Module ........................................................................................................... 10 Diff Operating Characteristic Module .......................................................................................... 10
Please use this note only in combination with the related product manual which contains several important safety instructions. The user is responsible for every application that makes use of an OMICRON product.
OMICRON electronics GmbH including all international branch offices is henceforth r eferred to as OMICRON. © OMICRON 2010. All rights reserved. This application note is a publication of OMICRON. All rights including translation reserved. Reproduction of any kind, for example, photocopying, microfilming, optical character recognition and/or storage i n electronic data pro cessing systems, requires the explicit consent of OMICRON. Reprinting, wholly or in part, is not permitted. The product information, specifications, and technical data embodied in this application note represent the technical status at the time of writing and are subject to change without pri or notice. We have done our best to ensure that the information given in this application note is useful, accurate and entirely reliable. However, OMICRON does not assume responsibility for any inaccuracies which may be present. OMICRON translates this application note from the source language English into a number of other languages. Any translation of this document is done for local requirements, and in the event of a dispute between the English and a nonEnglish version, the English version of this note shall govern.
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1
What is Restricted Earth Fault Protection and Why is it Necessary? For certain line-to-earth faults inside a transformer the transformer overall differential protection is not sensitive enough to detect them. One main reason for this limited sensitivity is the square dependency of the differential current depending on the fault location on the winding, caused by the transformation between LV and HV side of the transformer. Furthermore it is necessary to consider magnetization effects, tap changer errors and f urther sources of errors.
Figure 1: Diff Operating Characteristic
Because of the necessary zero-sequence elimination, also residual currents inside the transformer will be eliminated, which further reduces the sensitivity in regard to line-to-earth faults. Due to these reasons some actual line-to-earth faults with l ow differential current (especially faults near the transformer starpoint) may not be recognized by the differential protection function as a fault. In order to enhance the overall differential protection, a Restricted Earth Fault (REF) protection can be applied in parallel. The basic REF principle is to balance one current measured in the starpoint of a transformer winding against the vectorial sum of the three phase currents of the same winding. This gives maximum sensitivity without having to consider effects of the magnetic coupling of two windings. The actual REF protected area is from the line CTs to the starpoint CT of the corresponding transformer side. So the REF function can only protect one side of a transformer. For the other side an additional second REF function may be installed if both sides are starpoint grounded.
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Figure 2: REF Principle
As the residual current is measured at the starpoint of the same side, there is no LV/HV transformation and hence the dependency of differential current on fault location is linear which results in a much larger protected zone of REF protection in comparison with Diff protection.
Figure 3: Diagram showing Dependency of Differential Current on Fault Location
Actual line-to-earth faults between the fault location 0.2 – 0.55 (yellow area) are not detected by the Diff function but by REF.
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2
Testing with OMICRON Advanced Differential
2.1
Introduction OMICRON Test Universe modules do not offer explicit REF support for testing and test result evaluation. Of course it is possible to use a State Sequencer module, possibly combined with LinkToXRIO, but it would be beneficial to have the operating characteristic displayed, like in the Differential modules. This Application Note is an approach to use the Differential modules for REF testing.
2.2
Connection CMC – REF Relay To simulate the line CTs, connect the CMC current output block A to the related three line CT inputs of the relay. Connect the three current outputs of the CMC output block B in parallel and connect the resulting single current to the starpoint CT input of the relay. With this connection it is not necessary to change the wiring during testing of the different line t o earth fault types (L1-E; L2-E; L3-E)
Figure 4: Connection Diagram CMC\Relay
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2.3
Advanced Differential Test Object Settings
2.3.1 Protected Object Tab 1) Protected Object: Generator For testing the REF function no zero-sequence elimination shall be considered and hence the setting Generator is appropriate. 2) Enter the nominal settings according to the power system data (same as in relay settings). As the REF protected zone consists of just one transformer winding, enter the same settings for both sides. 3) Starpoint Grounding: Yes (both sides) As the protected transformer winding is starpoint grounded, select for both sides Starpoint Grounding Yes.
Figure 5: Protected Object Tab Settings
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2.3.2 CT Tab Enter the CT settings according to the power system data (same as in relay settings). The “Primary CT” settings are related to the line CTs and the “Secondary CT” settings to the starpoint CT. The option “Use Ground Current Measurement inputs (CT)” shall be disabled.
Figure 6: CT Tab Settings
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2.3.3 Protection Device Tab Figure 7 shows the settings of a MiCOM P633 relay displayed in the software S&R-103 and the corresponding settings in the Advanced Differential Test Object.
Figure 7: Relay settings (P633) to Protection Device Tab
Ibias Calculation: The calculation of the biasing current is handled differently by the various relay manufacturers. You should find the used calculation in the relay manual. Test Max: "Test Max" restricts the maximum shot output time to speed up the test – if no trip is registered during this shot time then it is assumed that the relay will not trip for the given current output. It should be set slightly larger than the maximum expected relay trip time including time tolerance. For an instantaneous differential trip function it may be set way below 1 s (e.g. 200 ms). Reference Winding: The reference winding is the winding used for measuring currents and phase angles, i.e. the side where the trip characteristic applies (the currents of the other side are recalculated by the relay to match the reference side regarding e.g. the CT ratios). Reference Current: Select if the calculated “Protected Object Nominal Current” or the “Current Transformer Nominal current” shall be used as “Reference Current” as given by the relay type or relay setting. The P633 refers to the protected object nominal current, not to the nominal CT data. Delay Time: The delay time is the time in between consecutive tests to give the relay time to reset. Diff Current Settings Enter data in accordance with the relay settings. Diff Time Settings Enter data in accordance with the relay settings. You may add the relay processing time.
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Current Tolerances Enter these settings in accordance with the relay manufacturer's data sheets. Time Tolerances Enter these settings in accordance with the relay manufacturer's data sheets.
2.3.4 Characteristic Definition Tab Enter the operating characteristic according to the relay settings.
Figure 8: Relay settings (P633) to Characteristic Definition Tab
Start point and End point Enter Ibias/Idiff pairs at "Start point” and "End point" and add this segment to the characteristic by clicking the "Add" button. After a segment was added to the characteristic, its Ibias / Idiff end point is used as the start point for the next segment. Slope The slope is calculated from the start and end points.
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2.4
Advanced Differential Tests
2.4.1 Differential Protection Function Please make sure that the Differential function of the relay does not interfere with t he REF function since this setup does not feed the other winding(s) thus causing a trip of the Diff function whenever its pickup value is reached. Either route the Diff and the REF trip signals to separate outputs, or temporarily disable/block the Diff function. 2.4.2 Diff Configuration Module Use the Diff Configuration module to test that the relay does not trip under any through fault condition. The module also checks if the CT at the starpoint grounding is connected in the right direction. After configuring the module and starting the test, enter the measured values from the relay into the module (Test Tab) if needed for your test documentation and perform a manual assessment. If the relay trips during the test execution, the test ends automatically and is classified as “Failed”. In t his case the test setup has t o be corrected before further tests will have a chance to succeed. Repeat the test f or all phase-to-earth loops. 2.4.3 Diff Operating Characteristic Module As with a usual differential protection, the REF operating characteristic can be tested with either Shot or Search Test, to verify the proper REF tripping and stabilization above and below the displayed characteristic curve. Fault type “L1-E”, “L2-E” and “L3-E” may be performed.
Figure 9: Diff Operating Characteristic Module
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Shot Test The Shot Test is used to verify that the operating characteristic is within a specified tolerance, i.e. by placing test shots close to but outside the tolerance band of the characteristic. The number of test points can be determined by the test engineer. The marked tests points in the I diff / Ibias plane are tested for tripping or not tripping the relay. Search Test A test for the exact determination of the characteristic shape and its tolerances. Test points for each given bias current are automatically generated while intelligently varying the related differential current until t he characteristic boundary is found with the precision given by the specified resolution and tolerance.
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