CMC-Testing of Restricted Earth Fault Protection With OMICRON Advanced Differential

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© OMICRON Page 1 of 12

Application Note

Testing of Restricted Earth Fault Protection withOMICRON Advanced Differential

Author Daniel Wiesner | [email protected]

Date Dec 07, 2010

Related OMICRON ProductCMC Test Sets

Application AreaRestricted Earth Fault relays, Advanced Differential

KeywordsRestricted Earth Fault, REF, Restricted Ground Fault, RGF, Differential

Versionv1.0

Document ID ANP_10011_ENU

mailto:[email protected]:[email protected]:[email protected]:[email protected]


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© OMICRON 2011 Page 2 of 12

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

Protected Object Tab ...................................................................................................................6

2.3.2

CT Tab ........................................................................................................................................7

2.3.3

Protection Device Tab ..................................................................................................................8

2.3.4

Characteristic Definition Tab.........................................................................................................9

2.4 Advanced Differential Tests .....................................................................................................10 2.4.1

Differential Protection Function ................................................................................................... 10

2.4.2

Diff Configuration Module ........................................................................................................... 10

2.4.3

Diff Operating Characteristic Module .......................................................................................... 10

Please use this note only in combination with the related product manual which contains several important safetyinstructions. The user is responsible for every application that makes use of an OMICRON product.

OMICRON electronics GmbH including all international branch offices is henceforth referred 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, opticalcharacter recognition and/or storage in electronic data processing 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 technicalstatus at the time of writing and are subject to change without prior notice.

We have done our best to ensure that the information given in this application note is useful, accurate and entirelyreliable. 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. Anytranslation of this document is done for local requirements, and in the event of a dispute between the English and a non-English 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 notsensitive enough to detect them.

One main reason for this limited sensitivity is the square dependency of the differential current depending onthe 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 further sources oferrors.

Figure 1: Diff Operating Characteristic

Because of the necessary zero-sequence elimination, also residual currents inside the transformer will beeliminated, which further reduces the sensitivity in regard to line-to-earth faults.

Due to these reasons some actual line-to-earth faults with low differential current (especially faults near thetransformer 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 beapplied 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 transformerside. So the REF function can only protect one side of a transformer. For the other side an additional secondREF 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 andhence 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 Difffunction 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 wouldbe 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 therelay. Connect the three current outputs of the CMC output block B in parallel and connect the resultingsingle 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 to earth faulttypes (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 settingGenerator 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 bothsides.

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 thecorresponding 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 relaymanufacturers. 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 registeredduring this shot time then it is assumed that the relay will not trip for the given current output. It should be setslightly larger than the maximum expected relay trip time including time tolerance. For an instantaneousdifferential 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 P633refers 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 SettingsEnter data in accordance with the relay settings.

Diff Time SettingsEnter data in accordance with the relay settings. You may add the relay processing time.


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Current TolerancesEnter 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 pointEnter Ibias/Idiff pairs at "Start point” and "End point" and add this segment to the characteristic by clickingthe "Add" button. After a segment was added to the characteristic, its Ibias / Idiff end point is used as thestart point for the next segment.

SlopeThe 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 the REF function since this

setup does not feed the other winding(s) thus causing a trip of the Diff function whenever its pickup value isreached.Either route the Diff and the REF trip signals to separate outputs, or temporarily disable/block the Difffunction.

2.4.2 Diff Configuration Module

Use the Diff Configuration module to test that the relay does not trip under any through fault condition. Themodule 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 duringthe test execution, the test ends automatically and is classified as “Failed”. In this case the test setup has tobe corrected before further tests will have a chance to succeed. Repeat the test for 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 orSearch Test, to verify the proper REF tripping and stabilization above and below the displayed characteristiccurve. 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 placingtest shots close to but outside the tolerance band of the characteristic. The number of test points can bedetermined by the test engineer. The marked tests points in the Idiff / Ibias plane are tested for tripping or nottripping the relay.

Search Test A test for the exact determination of the characteristic shape and its tolerances. Test points for each givenbias current are automatically generated while intelligently varying the related differential current until thecharacteristic boundary is found with the precision given by the specified resolution and tolerance.


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CMC-Testing of Restricted Earth Fault Protection With OMICRON Advanced Differential