mse20029 - differential 87

DATE 02/04/2014 DOC. MSE20029 REV.6.5.3

DIFF-87

AUTOMATIC PROGRAM FOR THE TEST OF

DIFFERENTIAL RELAYS ANSI-IEC 87

USER MANUAL

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REVISIONS SUMMARY VISA

N. PAG. DATE

4.0.0 All 30/03/2006 Issued revision 4.0.0 Morandi

5.2.0 7, 36-37 17/11/2008 Added Save in TDMS men voice and the link with the same print report as TDMS

Rossoni

5.3.0 All 04/08/2009 Added description of latest addition to correct

problems in relay testing. Updated images.

Morandi

6.5.2 All 06/07/2013 Updated to 6.5.2 Lodi

6.5.3 14-15 2/4/2014 Updated to 6.5.3: new relays in the Library. Lodi

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PREFACE ............................................................................................................................................................ 6

1 TEST OF THE POWER TRANSFORMER DIFFERENTIAL PROTECTION ................................................... 7

1.1 Menu bar description ............................................................................................................................................... 7 1.1.1 Menu file ......................................................................................................................................................... 7

1.1.1.1 Open .................................................................................................................................................... 7

1.1.1.2 Save Results ........................................................................................................................................ 8

1.1.1.3 Save in TDMS .................................................................................................................................... 8 1.1.1.4 Notes to the test ........................................................................................................................................... 8

1.1.1.5 Print results ......................................................................................................................................... 8 1.1.1.6 Exit .............................................................................................................................................................. 9

1.1.2 Menu options .................................................................................................................................................. 9

1.1.2.1 Header ................................................................................................................................................. 9

1.1.2.2 Preferences .......................................................................................................................................... 9 1.1.3 System information ....................................................................................................................................... 11 1.1.4 Languages ..................................................................................................................................................... 12 1.1.5 Type of protection ......................................................................................................................................... 12 1.1.6 Differential Library ....................................................................................................................................... 14 1.1.7 Other icons .................................................................................................................................................... 15

1.2 Program Description .............................................................................................................................................. 18

1.3 The SYSTEM tab .................................................................................................................................................... 19 1.3.1 The Transformer selection tab ...................................................................................................................... 19

1.3.1.1 Transformer connection ............................................................................................................................ 19 1.3.1.2 The Transformer tap ................................................................................................................................. 22 1.3.1.3 The nominal values ................................................................................................................................... 23

The ........................................................................................................................................................................ 23 1.3.2 Relay characteristic tab ................................................................................................................................. 23

1.3.2.1 Nominal characteristic .............................................................................................................................. 24 1.3.2.2 Transformation method ............................................................................................................................. 30 1.3.2.3 The Monitored Contact ............................................................................................................................. 31 1.3.2.4 The I Bias formula .................................................................................................................................... 31

1.4 The Test Selection tab............................................................................................................................................. 33 1.4.1 Click and test ................................................................................................................................................ 33 1.4.2 Verify curve .................................................................................................................................................. 34 1.4.3 Harmonic Restraint ....................................................................................................................................... 35 1.4.4 Tripping time test .......................................................................................................................................... 36 1.4.5 Stability test .................................................................................................................................................. 38

1.5 The TEST tab .......................................................................................................................................................... 40 1.5.1 Fault type and side ........................................................................................................................................ 40 1.5.2 IR [pu] and Id [pu] ........................................................................................................................................ 41 1.5.3 Timer and input contacts ............................................................................................................................... 42

1.6 The RESULT tab ................................................................................................................................................... 44

1.7 Actions ..................................................................................................................................................................... 45 1.7.1 Change colors ............................................................................................................................................... 45 1.7.2 Perform table functions ................................................................................................................................. 45

2 POWER GENERATOR DIFFERENTIAL PROTECTION .............................................................................. 47

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3 AUTOMATIC TEST OF THE LINE DIFFERENTIAL PROTECTION............................................................. 48

3.1 The line differential relay: principle of operation .................................................................................................... 48

3.2 The line differential relay: how to test it ................................................................................................................... 49

3.3 The line differential relay: line setting parameters .................................................................................................. 50 3.3.1 Alpha plane characteristic; one protection test .................................................................................................... 50

3.3.1.1 Line parameters............................................................................................................................................. 50 3.3.1.2 Relay characteristic ....................................................................................................................................... 52 3.3.1.3 Test selection ................................................................................................................................................ 55

3.3.2 Alpha plane characteristic; two protections test................................................................................................... 57 3.3.2.1 Line parameters............................................................................................................................................. 57 3.3.2.2 Relay characteristic ....................................................................................................................................... 58 3.3.2.3 Test selection ................................................................................................................................................ 60

3.3.3 IR versus Id characteristic, one protection test .................................................................................................... 64 3.3.3.1 Line parameters............................................................................................................................................. 64 3.3.3.2 Relay characteristic ....................................................................................................................................... 66 3.3.3.3 Test selection ................................................................................................................................................ 66

3.3.4 IR versus Id characteristic, two protections test ................................................................................................... 68 3.3.4.1 Line parameters............................................................................................................................................. 68 3.3.4.2 Relay characteristic ....................................................................................................................................... 69 3.3.4.3 Test selection ................................................................................................................................................ 69

4 PRINT RESULTS ........................................................................................................................................... 71

5 IMPORT RESULTS TO EXCEL ..................................................................................................................... 73

APPENDIX 1: POWER TRANSFORMERS CONNECTIONS .......................................................................... 75

A1.1 Power transformer: connection YY0 ..................................................................................................................... 75

A1.2 Power transformer: connection YY6...................................................................................................................... 75

A1.3 Power transformer: connection Y 1 ..................................................................................................................... 76

A1.4 Power transformer: connection Y 11 ................................................................................................................... 76

A1.5 Power transformer: connection Y 7 ...................................................................................................................... 77

A1.6 Power transformer: connection Y 5 ...................................................................................................................... 77





A1.11 Power transformer: connection 0 ................................................................................................................... 80



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A1.15 Power transformer: connection 10 ................................................................................................................. 82


APPENDIX 2: IEC61850-8 ................................................................................................................................ 83

A2.1 Relay connection ...................................................................................................................................................... 83

A2.2 File Menu .................................................................................................................................................................. 84

A2.3 Main Area ................................................................................................................................................................. 85 A2.3.1 Exploring Goose ............................................................................................................................................... 85 A2.3.2 Filters ................................................................................................................................................................ 87 A2.3.3 Virtual Contacts ................................................................................................................................................ 88 A2.3.4 Substation files (SCD) ...................................................................................................................................... 90 A2.3.5 Virtual contact test example .............................................................................................................................. 91 A2.3.6 Publishing Gooses ............................................................................................................................................. 92

APPENDIX 3: IEC61850-9 ................................................................................................................................ 98

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Preface

The program DIFF-87 has been designed as an interactive test software for all ISA Automatic test

sets, to test Differential relays coping with the standard ANSI-IEC 87.

Theoretically, from the beginning the user is driven throughout all steps in simple way, so that he has

the feeling that everything is under control.

It is like playing with a fully manual test equipment with very powerful tools. The following are the

features available in the program:

Capability to load/save the results created as a Microsoft Access file (*MDB;*.87);

Manual and automatic test of any type of differential relays;

Capability to load a result and repeat all the tests included.

With the revision 6.5.2, and with DRTS XX test sets, the following features are available.

Preliminary selection of the type of differential relay: Transformer, Generator, Line differential.

Possibility to synchronize two devices in line differential tests with the internal IRIG-B or GPS synchronization.

Possibility to perform tests using the IEC61850-8 communication messages.

Possibility to perform tests using the IEC61850-9 protocol.

Handling of the TRANSCOPE option.

The program starts going to TDMS and selecting the type of differential relay to be tested.

This manual deals with the test of: transformer differential relays (two and three windings), generator

differential relay, and automatic test of line differential relays. The manual for the manual test of the

line differential relay is in another document, named MSE20081.

We discuss first the test of transformer differential relays; the test of the other two types is described

taking reference to the first one.

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1 Test of the power transformer differential protection

Once the transformer test has been selected, the following window is displayed.

On the top, there are two bars: the menu bar and the toolbar.

1.1 Menu bar description

1.1.1 Menu file

1.1.1.1 Open

This menu item allows loading a result file that contains all the tests performed and the relative relay

settings (graphics included). Results are stored into a database file (Microsoft Access file).

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1.1.1.2 Save Results

This menu item allows saving a result file containing all the tests performed and the relative relay

settings. The standard dialog window appears, to allow the user select the name of the file to be saved.

1.1.1.3 Save in TDMS

This menu item allows exporting the result file into the database of TDMS program. When you click

this selection, a window with the network topology defined inside TDMS will appear. Prior to saving,

in the TDMS database you must have created this relay. This window allows you to select the relay in

the database, and to assign a name to the result file. By pressing the Cancel button, the result will not be saved.

1.1.1.4 Notes to the test

By selecting this menu, it is possible to add comments regarding the tests performed. These notes will

be included in the results file, and can be printed.

1.1.1.5 Print results

This menu item is discussed in the relative chapter about printing.

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1.1.1.6 Exit

Closes the application.

1.1.2 Menu options

1.1.2.1 Header

By choosing this menu item, the user is asked to fill data that identifies the conditions of the test. They

will be printed on the results, allowing the traceability of the test performed. In detail, please insert the

Substation name (Plant name), the Line name (Feeder), data regarding the relay such as Relay

Manufacturer, Relay Model and Serial Number. Also fill the Operator and Instrument Serial

Number fields. The other fields are to be considered optional.

Click on Load logo to change the logo to be printed on the test report, or if the program is not able to

find the predefined logo. Reasons could be that the file no longer exists or the .INI file is corrupted.

1.1.2.2 Preferences

This menu item loads a window where the operator can specify general parameters that will influence

the way test are carried out or results are displayed

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The frame CT Ratio allows using appropriately a CT on the current outputs (specifically designed for the options IN1-CDG and HPB400). It allows therefore selecting which is the

CT ratio, by selecting the primary side and secondary side currents. The selection is

performed by choosing the appropriate Primary/Secondary side values from the menu. The

last selection allows choosing on which current outputs the CT is connected. NOTE: once

input the ratio, the test set will generate the primary current, and the screen will display the

secondary current, which is the one actually applied to the relay. In addition, test results are

saved with secondary side data.

In some instances, you want to test a converter, such as a clamp, converting high current into low current or high current into low voltage. The frame Measure Conversion constants

allows inputting the converter ratio; as a result, measurements displayed on the measuring

board are displayed as primary side values, instead of secondary side values.

The test set optionally has the capability of generating low-level voltages (called zero power outputs), which are proportional to the current and to the voltage to be generated. The frame

Zero Power allows defining the parameters of these outputs; it is be enabled only if the

option is available, if the instrument is connected and if the round cable carrying these signals

is fit: this is because the test set detects the presence of the cable.

The programming of the output extends the test set current and voltage ranges: for instance,

you can program for the current a ratio of 100 A on the primary side. The important thing is

that, when you use the program, you can input the primary current and voltages, while the test

set generates the low voltages on the zero power outputs.

The frame Time delay allows the user to display trip times and delays in seconds or in cycles. In the latter case, it must be indicated the reference frequency to be used for the cycle

calculation.

The frame Power Line Synchronization allows generating the current and voltage outputs in phase with the mains. This can be very useful if you want to leave, for instance, voltage

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connections, and to generate currents in phase with them. The selection is not available on the

DRTSXX series.

The frame Output Colors is disabled on this page. The access to the color selection is performed going to the graphic part of the winding, and right clicking on it: the following

window shows up.

Tested points 1, 2 and 3 refer to tests L1, L2 and L3. Left click on a color to access the color

selection.

The frame GPS & IRIG-B Injection delay allows delaying the signal generation, when the test set is synchronized with the optional internal GPS or IRIG-B. The purpose of this delay

is to compensate a different time delay with respect to the other test set used for the test. The

selection is not used for the test of differential transformer protections.

The last two selections, Impedance and V Supply, refer to the DRTS 66 series. In these test sets, when the test starts, the test set measures the burden impedance and computes the

corresponding supply voltage. If checked, you can input these parameters; however, take care

to verify that the current output is not distorted.

By pressing OK, the parameters are saved, while with Cancel any change is discarded.

1.1.3 System information

Its a panel showing information regarding the connected instrument, (RAM, Flash EPROM etc.).

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A picture of the connected instrument appears (in the example, DRTS.64), and the booster, if

connected. It also shows ISA contact details. The important information is the firmware revision

number, to be compared to the latest available in the WEB site: if the version is not the last one, please

upgrade it.

1.1.4 Languages

This menu gives the capability to change the language of the software without closing the program.

The active language will be saved by quitting the program.

1.1.5 Type of protection

It is possible to change the selection performed on TDMS; for the transformer protection, it is

possible to select the type of transformer.

2 winding transformer

The transformer has two windings: primary and secondary.

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The manual refers generally to this type of protection

3 winding transformer

The transformer has three windings: primary and two secondary.

A menu appears beside the connection menus to let the user choose the 3rd

winding type, Y or

D.

Generator protection

For this kind of protection, setting parameters are very few.

Line differential protection

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A picture appears together with a description of warnings to consider when testing such a

protection. The description of this test is given in the manual named MSE20081.

1.1.6 Differential Library

This menu item opens the Differential Library. The Library is a list of special programs that ease the

configuration of the relays according to the diff-87 Automatic program parameters.

When pressing the Differential Library menu, the following window appears:

The window displays the list of the Special Differential Programs that are available; they are the

followings.

SIEMENS type 7UT512: differential 87T transformer protection, two windings.

AREVA MICOM P546: differential 87L line protection.

ABB RED670: differential 87L line protection.

ABB REG670: differential 87G generator protection.

ABB RET670: differential 87T 2 or 3 windings transformer protection.

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SCHWEITZER SEL 387: differential 87T 2 or 3 windings transformer protection.

ABB SPAD346: differential 87T 2 windings transformer protection.

To run a special program, select it and the press the button Open. The relative window will open and

allow the user to insert the parameters of the relay under test.

Purpose of the selection is to input the relay settings in the format foreseen by the particular relay, and

to transform these data into the settings requested by the Differential program. For example, when

selecting the Sel387.exe program, the following window opens.

The program shows the parameters, as they appear in the Sel387 User Manual, so the user can read the

settings from the relay, and insert them in this window. Once the data input is completed, press the

button: you come back to the test program, but all the relevant settings have been input;

so, do not modify the settings that you find in System: Transformer and Relay characteristic.

.

1.1.7 Other icons

The complete set of icons is the following one.

We have already explained the first six icons; let us study the other ones.

Connect . Press this icon when the test set is ready for the connection to the PC: a communication icon is displayed; then, the icon is shown as pressed. With this command, the

test set communicates to the test program its configuration in terms of available options. In

our instance, the test set is fully optional.

Digital input types . Pressing this icon, the following selection window is accessible.

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Digital inputs are divided into groups with the same common ground: for instance, two groups for the

DRTS X series, and six groups for the DRTS XX series. It is possible to apply different selection to

the groups, or the same selection to all inputs.

Selections are:

Contacts: pressing the down arrow, the following is displayed.

This window refers to DRTS XX; for other test sets, there are two groups of isolated contacts.

The other two selections, Threshold and Debounce, can be programmed with separate values

for each input group, or the same for all inputs (all contacts).

Threshold. There are three type of selections.

o Dry contact: there is no voltage on the contact, which is polarized by the test set;

o Voltages, from 5 V to 100 V. In this case, it is possible to select the threshold, i.e. the voltage below which the input will be considered zero.

o Type of wetting voltage, DC or AC. With an AC voltage input, the accuracy of the measurement decreases, because the program must ignore the zero crossings caused

by the AC voltage itself. To solve the problem, the program takes the value of the

debounce equal to 2 ms (see here after).

Debounce. This field allows to program the time duration of bounces, in s: any change beyond this time is taken as an actual change of the input.

At the end of programming, press OK. These parameters will be applied to all tests: they

cannot be changed from a test to another one.

Apply VDC only . Pressing this icon the test set generates the auxiliary DC voltage. NOTE: with the DRTS XX series, it is possible to generate the DC voltage supply acting on

the local control: see the corresponding manual.

Generate pre-fault values . Pressing this icon the test set generates all the pre-fault values. NOTE: the program requests to press this icon before the first test execution.

Start . Pressinng this icon the test is started.

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Stop . Pressinng this icon the test is stopped, and outputs return to the pre-fault values (not to zero).

Reset . Pressinng this icon the test is stopped, and outputs return to zero. NOTE: this includes the auxiliary DC voltage supply. NOTE: with the DRTS XX series, it is possible to

avoid resetting the DC voltage supply, even when the reset icon is pressed: see the

corresponding manual.

IEC 61850-8 . This selection allows testing the GOOSE messages exchanged between the relay and the system. For more details, please refer to the corresponding Appendix.

IEC61850-9 . This selection allows testing the relay without generating currents and voltages, but sending instead digital data on the optical fibre link. For more details, please

refer to the corresponding Appendix.

TRANSCOPE . This selection is available only with the DRTS66 series test sets, when the option is available. As pressed, the TRANSCOPE program is opened, while THE

Overcurrent test program stays active. This allows monitoring and recording test currents

generated by the program, and the corresponding relay response.

For more details, please refer to the TRANSCOPE software manual.

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1.2 Program Description

Let us start now the study of how tests are performed.

As usual in ISA test programs, there are two sides, left and right, to control the test and to display

results in graphical format. In the left side, there are 4 tabs:

1. Test: It is selected during the test; 2. System: Allows inputting all transformer and relay settings; 3. Test selection: Allow to choose the type of test to be performed; 4. Results: At the end of the test, here you store all results.

The System tab is the one open at the beginning; it allows inputting the relay settings:

Transformer connection;

Nominal relay characteristic ;

Nominal parameters (Vnom, Fnom, etc).

Without these parameters, there is no way to perform the test!

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1.3 The SYSTEM tab

1.3.1 The Transformer selection tab

1.3.1.1 Transformer connection

The selection is performed in the following part of the window.

Before starting, we recall that the vector group of the transformer is defined as follows.

IA is the primary current phase 1

Ia is the secondary current phase 1

All differential transformer connections can be set according to:

The type of connection for windings 1 and 2 (primary and secondary);

The polarity (the red circle in the figure);

The sequence of phase connections.

Transformer connections for Windings 1 and 2 can be: Y, DAB or DAC. This selection, plus the

polarity, sets the transformer groups, as follows.

Yy 0, 2, 4, 8, 8, 10;

Dy 1, 3, 5, 7, 9, 11;

Yd 1, 3, 5, 7, 9, 11;

Dd 0, 2, 4, 6, 8, 10.

Connection DAB means:

positive terminal A connected to

negative terminal B

Connection DAC means:

positive terminal A connected to

negative terminal C

Ia

IA

30

130

30

30

aA

IIG

A

B

C c

b

a

n N

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Enter the connection Y, DAB, DAC in the Connection input field: left for the primary, right for the

secondary.

On both sides, you have the three choices, which are independent between them.

As you change the Connections, the corresponding vector group is displayed.

The selection of the polarity is performed left clicking on the dots by the side of the transformer

schematic. For instance, the following polarity selection corresponds to the vector group Yd7.

If you left click on the white spot to the right of the primary side winding, you change your selection

to Yd1.

You select the same vector group if you left click on the white spot to the right of the secondary side

winding.

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Last, the selection of phases is performed by left clicking on the letters telling the phase. For instance,

we have now the selection Yd1.

If the primary side phase sequence is not the right one, left click on A: the sequence is changed as

follows (BCA).

If you click again, you get the sequence CAB; pressing again, you come back to ABC. In a similar

way, you can change the phase sequence on the secondary side: abc, bca, cab.

The program will automatically check if the connection is correct (for instance, you cannot enter a

Yd2 connection).

Summarising, vector groups are selected as follows:

If the transformer has three windings, the corresponding selection window allows setting the type of

winding: Y or D.

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The reason of the reduced number of selections is that the second secondary is dedicated to serving

the substation itself; so, there is no need of phase rotation compensation.

1.3.1.2 The Transformer tap

The selection of the tap is performed in this part of the window.

The starting principle of the differential relay transformer is that the power on the primary side is

(almost) equal to the power on the secondary side; the difference being some percent. Assuming that,

during a fault, the voltage does not change very much, the power difference is a function of the current

difference. However, it is clear that the two currents, primary and secondary, as measured by the

relay, are not equal, because:

The power at both windings is the same; so, the current is inversely proportional to the voltage;

The two CTs on the two sides do not have the same ratio. In the differential relays, by TAP we mean a parameter, which accommodates for these differences.

There are two TAPs: one on the primary side, the other one on the secondary side. They are calculated so that:

I primary(CT) * TAP primary = I secondary(CT) * TAP secondary,

Where by I primary (CT) we mean the primary current after the CT.

In our program, you can either input the two TAPs directly (typical for older relays), or the program

can calculate them.

In the first instance, check the two taps and input their values, after having checked

In the second instance, TAPs are computed according to the nominal power Pn, primary and

secondary voltages V1n and V2n, the CTRatio, the nominal CT secondary current In:

n

n

ICTRV

PTap

***3 111

n

n

ICTRV

PTap

***3 222

These values are the p.u. nominal currents, at relay level after CTs. They are fundamental when

calculating the test currents to apply to the relay.

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1

1*3 V

PI n

2

2*3 V

PI n

1

11

"

CTR

II

2

22

"

CTR

II

As these parameters are input, the corresponding computed TAPs are displayed.

Note:

In some relays, such as SEL-387, the transformer TAPs take into account the connection style of the

relay. If the relay is Y-connected, the formula is the one above. If it is-connected, the TAPs will be

the above, multiplied by 3 . Therefore, for a correct calculation of the taps, the program provides the

box . By selecting it, the calculated tap is also multiplied accordingly.

1.3.1.3 The nominal values

Vnom [V]: This is the nominal PT secondary voltage. It is important in case the relay has a voltage restraint characteristic, and it might be interesting to see how the characteristic

changes as a function of the nominal voltage.

Fn [Hz]: The nominal frequency.

Imax [A]: The maximum current, not to be exceeded during the test. Max Err %: If the current error is more than specified, the test result is fail; else, it is pass.

Test @ 2x 45A With DRTS.6 or DRTS 66, if the test is performed with two currents, it is

possible to put in parallel the currents I1-I2-I3 and I4-I5-I6, so that the maximum test current

is increased.

Vdc [V]: With the auxiliary DC voltage output it is possible to energize the relay.

Ramp Vdc: The relay DC supply voltage is filtered by a capacitor. If the step application of the voltage produces an overload on the Vdc generator, it is possible to ramp it.

1.3.2 The Relay characteristic tab

When selecting this tab, the following window becomes accessible.

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In this window it is possible to define the characteristic of the relay we will test.

1.3.2.1 Nominal characteristic

There are many types of relay characteristics. To ease the test, the following icons allow an easy input

of the settings of a given characteristic.

Six choices are available; of these, the first one allows drawing any characteristic, with a point-by-

point approach.

Point-by-point.

After this selection, you access the following table.

To the right, the window displays the characteristic diagram, which matches the selections of the

table.

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The characteristic curve can be defined by entering the coordinates of each point, one by one, in the

setting table. The operation is the following.

o You can modify any value by entering it; o You can Delete a point by selecting it, and then pressing the relative button. o You can Add a point by pressing the relative button. The point is added after the

programmed ones: if you want to insert a point, clear all points after the desired one.

o For each point you enter the coordinates IR and the Slope (in percent): they define a line passing through IR with a specific Slope. Id is computed as a consequence of the

other two parameters; the cell background is gray. By definition, Slope = 0 is an

horizontal line, while Slope = 999 is a vertical line.

o You can Clear all points, and start from scratch. o The Redraw button is not necessary: the diagram is refreshed as soon as you modify a

parameter.

Let us consider the example in the figure, in order to understand how to edit a characteristic.

Point n. 1

The characteristic always starts from IR1=0, so that you cannot edit IR1 for this point.

Id can be any value providing it is positive here Id1 = 0.3.

Slope1 = 0: horizontal line.

Point n. 2

The first is a horizontal line that goes from IR1 = 0, with a slope of 0, to IR2.

Id2 cannot be edited, as it is function of IR2 and of the slope; it is:

3.0)(*100

121

12 IRIRSlope

IdId

Then the characteristic will continue with a Slope2 = 25%

Point n. 3

1 2 3

4 5

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The second line goes from IR2 up to IR3 = 2,5

Id3 cannot be edited, as it is function of IR3: it is:

675.0)(*100

232

23 IRIRSlope

IdId

Then the characteristic will continue with a Slope = 200%

And so on for all points. Just remember that: NP = NL + 1; where:

NP is number of points;

NL is the number of lines.

Other selections. After the point-by-point definition, there are 5 types of macros, corresponding to 5 different types of characteristic. Click on the relative icon to display a frame where you can enter the

setting values:

Bias restraint characteristic: the most common characteristic, for static relays.

Two bias restraint characteristic: the most common, for new digital relays.

Two bias with knee point: not very common, but to be taken into account.

Duobias characteristic: specific curve for VA-Teck Duobias Relay.

GET60 Universal relay: specific curve for GE T60 relays.

Bias Restraint Characteristic

The user must define the differential current pick-up and the slope.

Input parameters are:

PkUp: it is the differential pick-up current

SL1%: Slope of the restraint differential current

OS: Offset at origin: the slope is a straight line, starting from the IR offset; this is normally zero, but for some Alstom relay is a positive or negative value (see the drawing).

Doc. MSE20029 Rev. 6.5.3 Page 27/101

Max IR: Maximum Restraint current.

Max Id: Maximum allowed Differential current (or unrestrained current).

Enable Max Id for: enables the maximum differential current for the specified phases.

Two Bias Restraint Characteristic


PkUp: It is the differential pick-up current.

SL1%: Slope of the 1st current differential restraint.

SL2%: Slope of the 2nd current differential restraint.

Base point: Starting point of the 2nd current restraint characteristic.

OS: Offset at origin: the slope is a straight line starting from the offset; this is normally zero, but for some Alstom relay is a positive or negative value




NOTE: there are two more definitions for the second slope starting point; they are:

The intersection of the second slope with the axis IR, whose value is IR1;

The intersection of the second slope with the axis Id, whose value is Id2.

Id

IR OS

IR1 Base point

Id2

Id

IR OS

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So, if you know the value of IR1 or of Id2, you have to compute the corresponding value of IB

before proceeding. The computation formulas are the following ones.

Given:

OS = Offset; pu;

S1 = first slope, in percent;

S2 = second slope, in percent;

IR1, in pu,

Id2, in pu,

It is possible to compute the corresponding IB, as follows:

If you want, you can also compute IR1 and Id2 from IB:

( )

( )

Two Bias Restraint Characteristic with a Knee Point


PkUp: it is the differential pick-up current.



Base point: Starting point of the 2nd current restraint characteristic.

OS: Offset at origin: the slope is a straight line starting from the offset; this is normally zero, but for some Alstom relay is a positive or negative value.

Max IR: Maximum Restraint current

Max Id: Maximum allowed Differential current (or unrestrained current)


DuoBias Characteristic

Doc. MSE20029 Rev. 6.5.3 Page 29/101


PkUp: Differential pick-up current.


B: Base point of the 2nd current restraint characteristic.

OS: Offset at origin: the slope is a straight line starting from the offset; it is normally zero.

Max IR: Maximum Restraint current

Max Id: Maximum allowed Differential current (or unrestrained current)


The curve above point B follows this formula:

Trip if :

slope BiasM

limit slope Bias

2

2

)21(

:

2

1

2222

Bias

22

B

BMBK

III

X

Where

KXI diff

GE T60 Universal relay


PkUp: It is the differential pick-up current.



Break1: End point of the 1st current restraint characteristic.

Doc. MSE20029 Rev. 6.5.3 Page 30/101

Break2: Starting point of the 2nd current restraint characteristic

OS: Offset at origin: the slope is a straight line starting from the offset; it is normally zero.




For all types of curve, the following selection serves to choose the transformation method and the I

bias formula.

We have not yet considered the fact that the primary and secondary currents are phase shifted

according to the transformer power group. Now we have to consider it, because the differential relay

operates correctly only if the two currents are in phase.

The conventional electromechanical or static electronic devices of the differential protection were

electromechanical devices, which compensated the vector shift, with appropriate connection of the

current transformer secondary coils.

The principle is that the Y connected current transformers on the delta side of the transformer do not

shift the currents flowing out of the transformer. Instead, the delta connected current transformers on

the Y side of the transformer cause a phase shift. Additionally, the delta connection of the current

transformers eliminates the zero sequence current component flowing on the grounded Y side of the

transformer. As on the delta side no zero sequence current can be detected, this compensation is

unavoidable for the correct operation of the differential protection.

If a phase-to-ground fault occurs at the Y side of the transformer, then a zero sequence current flows

on the grounded Y side, but, on the delta side, no out-flowing zero sequence current can be detected.

Without the elimination of the zero sequence current component, the differential protection generates

a trip command in case of an external ground fault. However, if the connection group of the current

transformers on the Y side is delta, then no zero sequence current flows out of the group. So the

problem of zero sequence current elimination, in case of external ground fault, is solved.

1.3.2.2 Transformation method

The numerical differential protection devices (e.g. ABB RET or PROTECTA DTD) apply a numerical

matrix transformation for modeling the delta connection of the current transformers. In the practice, it

means cyclical subtraction of the phase currents. We define these type of relays as performing

Transformation Method .

The numerical protective differential algorithms offer also the possibility to eliminate the zero

sequence current from the Y side of the transformer. In this transformation, the Y side is the reference

side, which means that the reference currents are the real currents flowing in the phase coils.

Consequently, the currents flowing out on the delta side must be transformed to real coil currents.

We define these type of relays as performing Transformation Method Y.

Doc. MSE20029 Rev. 6.5.3 Page 31/101

It is therefore important to select the appropriate method, depending on the algorithms performed by

the relay under test. The box Transformation Method

has two options: Y and . Select the one applicable. With the Y type, a further selection is available.

The information can be found in the Relay User manual.

1.3.2.3 The Monitored Contact

A differential relay normally operates with three differential elements, corresponding to phases A,B,C.

This means that, for each type of fault, three elements are stimulated and contribute to define the trip

of the relay. On a variety of relays, it is also possible to monitor separately the differential elements,

and therefore have 4 trip commands: one for each individual differential element, and one that

represents the OR function of the three individual commands.

Normally the trip command 1+2+3 (general trip, OR of the individual commands) is the one

monitored, but if the user connects an individual trip output of the relay to the instrument trip contact,

it is possible to verify its characteristic and not the general tripping characteristic by selecting the

monitored element from the menu.

1.3.2.4 The I Bias formula

We define IR as the Restraint current, and it is normally computed as the average between currents I1

and I2, where:

I1: transformer primary side pu current;

I2: transformer secondary side pu current.

Depending on the relay manufacturer, the Restraint current is calculated as:

21 IIIR : this formula is used by Siemens relays

2

21 III R

: this is the standard formula.

Doc. MSE20029 Rev. 6.5.3 Page 32/101

3

21 III R

: this is used by some of GE relays for three windings transformer.

21 III R : this is used on Ret316 relays.

Some more formulas are available.

The differential current is always defined as: 21 IIId

Doc. MSE20029 Rev. 6.5.3 Page 33/101

1.4 The Test Selection tab

These are the possible selections.

1.4.1 Click and test

This means that you can select a test point by clicking on the graph with the mouse.

Press OK to confirm the selection;

Select a test point on the graph and left click with the mouse;

Test currents are injected immediately after clicking.

Note

3: Select the test point

and click

1: Select the fault type 2: Select the maximum

time Tmax

Stability test:

If you click here only IR will

be generated

Internal fault:

If you click here only Id

will be generated

Doc. MSE20029 Rev. 6.5.3 Page 34/101

The results will be displayed in different colors (according to Color Setups) depending whether the

relay tripped or not within the selected maximum time.

1.4.2 Verify curve

The nominal differential current Id is verified as function of IR.

The test is performed from IRstart to Irstop. The number of points is defined by Step. The nominal

Id value is automatically calculated as

Id = f ( IR )

To help you see where the test points will fall over the curve, the relative points are displayed.

By pressing OK, the test points are added to the test table, and displayed on the diagram. For instance,

with the above settings, the diagram would look like the following one.

Press the Start button to initiate the test.

A binary search method is applied in order to find the Id threshold.

The measured Id value is placed into the column ID of the test table.

Doc. MSE20029 Rev. 6.5.3 Page 35/101

The trip time is measured and placed in the T (s) column.

The Error is calculated in % of ID nom and placed into Err% column.

Pass / Fail indication is then written into the relative column.

The Monitored Contact allows to decide which differential element is being monitored and

connected to the trip contact of the instrument. It can be any individual differential element trip

command of the relay or the general trip command (OR of the individual elements).

For instance, with the above settings, the test result could be the following one.

1.4.3 Harmonic Restraint

The differential transformer relay is restrained from tripping if the harmonic content of the currents

exceeds the programmed limits. This function serves to ignora the huge transient differential current

which is generated when the transformer is powered on, and which is caused by the magnetization

currents.

Doc. MSE20029 Rev. 6.5.3 Page 36/101

The programming window allows you to program the harmonic you want to restrain, and the

corresponding threshold.

Enable the harmonic to be restrained;

Enter the harmonic restraint value in %;

Press OK to write the relative lines into the test table.

The output waveform is made of the fundamental plus the harmonic to be tested. At each injection the

amount of the distortion is modified; the search ends when the criteria described here below is met.

If we indicate:

If: the RMS value of the fundamental at 50 or 60 Hz;

Ih: the RMS value of the harmonic;

h: harmonic order, we have:

IRMS: the total RMS value of the injected current is 22

fhRMS III

H%: the harmonic percentage distortion is 100*22

%

fh

h

II

IH

The search method is the following one.

1. Set IR =0 and calculate the relative Id current; 2. Set Id output = 2 * Id (to be sure the relay will trip) at nominal frequency; 3. Calculate the output current I1, I2, and I3; 4. Set If as the maximum between I1, I2, and I3; 5. Inject If: the relay should trip, otherwise the current is increased until the relay trips; 6. Set the harmonic % value to be added to the nominal frequency;

7. Calculate the rms value of the harmonic component 2

%

2

%

100

*

H

IHI

f

h

;

8. Calculate the total RMS value as defined above, 22 fhRMS III ;

9. Update the waveform according to this formula:

10. ))***2sin(*)**2sin(*(*2)( hfIfIti hf

well we actually use the harmonic module to calculate everything

Should the relay trip, the harmonic % is not sufficient to lock the operation: H% is increased and the test start from point #6 again;

Shouldnt the relay trip, the harmonic % is sufficient to lock the operation: H% is decreased and the test start from point #6 again.

11. Interactions continue until the harmonic % threshold is found with the desired accuracy.

1.4.4 Tripping time test

This allows measuring the tripping time for different values of Id, and to verify that they do not

change with ID (this is the nominal behaviour). The value of IR is computed accordingly. The

programming window is the following one.

Doc. MSE20029 Rev. 6.5.3 Page 37/101

In this test, the diagram has coordinates time versus Id, instead of the usual Id versus IR. For instance,

with the above programming values, the following is the corresponding t Id diagram.

With the same settings, if you press OK, the following is the tests table.

Pressing Start, the following is a possible test result.

Note that if you select the standard representation on the Id versus IR diagram you get this result.

Doc. MSE20029 Rev. 6.5.3 Page 38/101

You may appreciate that the trip delay is not visible here.

1.4.5 Stability test

This test allows verifying that the relay is stable in case of a through fault. In this event, both the

transformer and the relay are interested by a high through current, theoretically without producing

differential current. However, due to the CT errors, the transformer taps, and many other reasons, the

relay may sense some differential current. Nevertheless, the relay should remain stable and block any

possible trip.

To perform such test it is enough performing a shot test at Idiff = 0. However, a specific test has been

defined in order to make it clear from the result handling point of view.

The Monitored Contact allows to decide which differential element is being monitored and

connected to the trip contact of the instrument. It can be any individual differential element trip

command of the relay or the general trip command (OR of the individual elements).

For instance, with the above settings, the diagram would be the following one.

You may note that all tests do not have any Id component. The following is the corresponding test

result.

Doc. MSE20029 Rev. 6.5.3 Page 39/101

The nominal trip delay is the timeout: the relay should not trip. On the diagram, this is shown with

black dots.

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1.5 The TEST tab

After having programmed all tests, the TEST window is the following one.

1.5.1 Fault type and side

The fault type selection (single phase, phase to phase, three-phase) depends on the type of instrument

and the transformer connection. Look at this table for reference.

Nr of currents

available in the test

instrument

Transformer

connection Type of fault selectable On which winding

3 YY L1 or L2 or L3 Winding 1

3 YD Not possible

3 DY Not possible

3 DD Not possible

6 YY-YD-DY-DD L1 or L2 or L3

L12 or L23 or L31

L123

Winding 1 and / or Winding 2

If the test equipment only has three output currents, the relay must be tested phase by phase. In this

case, only 2 out of 3 currents are used. So that:

For a YY connection, we perform a single phase test on Winding 1;

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For other types of connection or type of faults, a six currents instrument must be used;

The only exception is the single-phase test of an YD transformer on the Y side, because actually three currents are used. However, it would be necessary to perform connections as follows.

Fault L1: I1, I2 and I3 are to be connected to IA, Ia, Ic;

Fault L2: I1, I2 and I3 are to be connected to IB, Ia, Ib;

Fault L3: I1, I2 and I3 are to be connected to IC, Ib, Ic.

Whatever the selected test type (see the Test Selection tab chapter), before starting any test we get the

following dialog windows, describing how to connect the test equipment to the relay under test:

For 3-phase equipment: For 6-phase equipment:

UTB UTS DRTS DRTS-3

ART-3

DRTS + AMI-3 or AMIV-3

DRTS-3 + AMI-33 or AMIV-33

DRTS-6

DRTS 64, 66

The 1st dialog shows how to connect an instrument with only 3 output currents.

The red dot represents the positive terminal of the generator, either 1 or 2;

The brown dot represents the neutral IN of the generator: the two points are joined together.

The 2nd

dialog shows how to connect a 3-phase instrument and booster (6 output currents).

BOOSTER on Winding 1: o The red dot represents the positive terminal of the generators 1, 2 and 3; o The brown dot represents the neutral IN.

DRTS on Winding 2: o The red dot represents the positive terminal of the generators 1, 2 and 3; o The brown dot represents the neutral IN.

The 3rd

dialog shows how to connect a 6-phase instrument.

The red dot represents the positive terminal of the generators 1, 2, 3, 4, 5 and 6

The brown dot represents the neutral IN: the two points are joined together.

1.5.2 IR [pu] and Id [pu]

These two fields just copy the mouse position in the graph. The restraint current IR (X axis) and the

differential current Id (Y axis) can be edited and then added to the test list to be injected. We give this

option in order to set specific test points: they cannot be reached with the mouse.

The values of IR and Id define the values of the currents to be injected, according to the following

information:

Doc. MSE20029 Rev. 6.5.3 Page 42/101

Transformer connection;

Transformer taps;

Fault type;

Fault side.

Currents to be injected in the various outputs, both amplitude and argument, are displayed as vectors

and are shown in the following table.

1.5.3 Timer and input contacts

In this frame youll define:

The input contact to be used: C1 to C8 (DRTS X) or C1 to C12 (DRTS XX);

The status: normally open or closed (NO or NC);

The digital input type: dry or wet. The contacts group change with the different type of test sets: two groups with DRTS X series; six groups, with DRTS XX series.

The prefault time T pre: it can be used to insert a time delay between one test and the other, and therefore to slow down the pace of testing.

The maximum injection time T max this must be greater than the nominal tripping time of the relay (for instance, twice, with a minimum of 0.5 s).

By clicking on the square box in the centre, it is possible to define the hardware status of the

contacts. This section applies to DRTS ,DRTS3, DRTS6, DRTS 66 and ART100. The following

window appears:

Debounce.

Doc. MSE20029 Rev. 6.5.3 Page 43/101

The debounce is the time during which the instrument waits for the input to become stable: this

serves to avoid stopping the test because of noise, and also to ignore contact bounces. When a

transition is sensed the debounce counter starts: any opposite transition before the debounce expires

resets the counter. Only if the input does not change for the programmed duration the input is

accepted, and the timer stopped. Default value: 500 us. With electronic inputs the value can be

programmed to 0; with heavy contacts the debounce is better programmed to 2000 us (maximum).

The time measurement is not affected by this selection.

Type There are three selections about the type of contact: Dry; Polarized by a d.c. voltage; Polarized by an

a.c. voltage. With selection dry, next selection is not applicable. With selection a.c. voltage, the

debounce is forced to 2000 us to overcome the zero crossings of the input.

Threshold When the contact is selected Polarized, it is also possible to select the threshold voltage: all inputs

less than 80% of the selected value are made equal to zero. This serves to avoid noise on the line, or

in case there is a protective resistor in parallel to the contact driving the coil, so that the Open contact

does not correspond to zero volts. Available thresholds: 5 V (logic input); 24 V; 48 V; 100 V. This

selection should match the wetting voltage.

If dry input is selected and a voltage is applied the instrument does not suffer, provided that the input

is no more than 220 V. If a voltage is selected and the input is not polarized, the trip is not sensed.

If a wrong voltage threshold is selected:

. Threshold higher than the voltage: the trip is not sensed;

. Threshold lower than the voltage: the contact could be found closed when it is open.

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1.6 The RESULT tab

Here, are usual, you can open previous results, save, print and delete all results.

By clicking on the result table it is also possible to select multiple rows and delete them by choosing

the menu item that appears after the click.

It is also possible to select the columns to be displayed in the results grid.

Select here the

fault type to show

Representation IR-

Idiff or

Idiff-Time

Select the monitored

contact to show

Select here the

fault side to show

Doc. MSE20029 Rev. 6.5.3 Page 45/101

1.7 Actions

Different actions are possible when you test a relay.

For instance you may change the colors of the graph: background, nominal characteristic, tested and

not tested points.

You can repeat a test that you feel wasnt performed correctly because the relay wasnt energized. So you can:

1.7.1 Change colors

Just right click on the graph to get the setup colors windows here displayed. Press the button where

you want to change color and get into the other window:

1.7.2 Perform table functions

By left clicking on the Test Table, a popup menu appears. The enabled items depend on the status of

the test table:

If the table is empty, by clicking on the header of the table, the only available item is Select columns to show

If more than one row is selected, the items Comment to the test, Load fault values with line values and Load line values with Fault values are not enabled, otherwise all item are enabled.

The tasks to be performed to the test table are:

delete selected tests: deletes the selected rows from the test table regardless if they have already been executed or still to be tested

Doc. MSE20029 Rev. 6.5.3 Page 46/101

Mark as executed: this function puts a mark * beside the test number of the selected rows. In this way it is possible to treat a not executed test as it has been executed and therefore save it

into a database.

Repeat selected tests: repeats the tests corresponding to the selected rows

Comment to the test: The following window appears:

In the Message box, it is possible to define a comment to the test that must be displayed prior to the test execution. The Show this message Checkbox allows the user to decide whether to display the message when this test is executed. There is also a Notes box, where other notes can be inserted. Message and Notes are stored together with the test data and can contain also pictures.

By pressing the OK button, if the Show this message checkbox is set, an icon will appear on the first column of the test table beside the test number. This means the comment is active,

and it will be shown when the test is executed.

Store all tests: transfers all the test that have been executed (with the mark *) from the test table to the result table

Delete all tests: deletes all the rows in the test table

Set columns to show: opens up a window, showing the columns available in the test table and allowing the user to select which columns should be displayed.

Doc. MSE20029 Rev. 6.5.3 Page 47/101

2 Power generator differential protection

This protection is a subset of the transformer different ial protection. After opening

TDMS, select Differential 87 and then Differential 87G.

The window is the following one.

The difference with respect to the transformer differential protection is visible in the

connection schematic, and in the associa ted settings. The generator has three

windings, which, on one side, are connected together; the output is a delta

configuration. As a consequence, settings are reduced to the secondary side

characteristics; if the tap is defined, it is all you need to proc eed with the test .

All other characteristics: nominal values, characteristic curve, are the same as those

seen for the differential transformer protection. Also the available tests are the

same.

The other difference is found in the Test page, which is th e following one.

In the Fault type and side selection, the only selection is on the secondary side.

The rest of testing, results, printing is the same as with the differential transformer

protection: please, refer to the former chapters.

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3 Automatic test of the line differential protection

This protection is a subset of the transformer differential protection. After opening

TDMS, select Differential 87 and then Differential 87 L (Automatic) .


Before studying it and the meaning of the parameters, let us study the line

differential relay, and how it can be tested.

3.1 The line differential relay: principle of operation

Line Differential Relays are used to protect HV lines (typically, HV cables) as well as the most

common Distance Relays. But they work in a different way:

Distance relays measure voltages and currents in order to calculate the ratio Z=V/I: when this ratio is within a value that corresponds to certain protected zone, the relay operates the circuit

breaker and open the line on its proper side. The other side of the line is opened by another

distance relay. The two relays normally exchange some information (direction of the fault, for

instance) but usually they work standing alone.

A line differential relay is made of two different relays installed at both ends of the line. They have to exchange information regarding the point-by-point measurement of the two currents.

The difference is computed by both relays, on a continuous analogue signal (older models), or on a

digitally sampled signal (recent models), where the link is performed by optical fibres rather than

copper wires.

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During the normal operation, currents I1 and I2 have (nearly) the same value. The little difference is

due to the capacitance to earth and between conductors. So the difference

I1 I2 0 During a fault, currents I1 and I2 have different values. The result is a high differential current:

ID = I1 I2

We underline the difference between distance relays and line differential relays. Distance relays

measure currents and voltages, and then they compute the fault impedance. In this process, the

information related to the angles at the two ends is lost. For line differential relays, the difference is

point-by-point: two currents having the same amplitude but different phases can cause the relay

intervention.

3.2 The line differential relay: how to test it

Since the two relays are installed at the two ends of the line, the test must be performed with two test

sets, by two operators. Beside this, since the two relays sense the fault exactly at the same time, also

the two equipment must inject the fault exactly at the same time... and with a zero phase angle

difference. In order to do this, it is necessary:

1. A very high accuracy synchronization device. This could be a GPS or an IRIG-B signal, providing a time tag with an accuracy down to 20 s.

2. The reaction time of the instrument at the two ends must be the same. 3. The test equipment must be able to control the phase angle at the injection. We call this the

Absolute Angle control.

For the first point, the solution changes as a function of the test set.

With all test sets unless the DRTS XX series, the device to be used is the external GPS option;

With the DRTS XX series there are two options: internal GPS or internal IRIG-B synchronization.

For the second point, there is the possibility that the two equipment are of different model, different

synchronization mean, or even different make. This may lead to a difference in the reaction time of

the equipment with respect to the synchronism. However, the software is able to compensate small

differences, providing that:

The reaction time difference is less than 15 ms;

The reaction time of each single equipment does not vary more than 300 s.

This last point is very important. Consider that every 55 s correspond to the error of 1; 300 s correspond to 6, and to an amplitude error of 0.5%. The error follows the cosine; so:

An error of 500 s implies a percentage error of 1.3 %;

An error of 750 s implies a percentage error of 2.8 %;

An error of 1000 s implies a percentage error of 5 %.

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The automated test of the differential relay is performed by the test program, which, once the type

of test is selected, generates a series of synchronized tests. At the end of the test, it is possible to

examine the relay characteristic, as already performed with power transformer differential relay

testing.

3.3 The line differential relay: line setting parameters

Let us come back now to the study of the setting parameters.

Parameters change as a function of the first selections available, that are:

Normal (Id versus IR) characteristic, or Alpha plane characteristic,

Test one protection or test two protections. In all, four combinations.

3.3.1 Alpha plane characteristic; one protection test

This combination is selected as follows.

3.3.1.1 Line parameters

The One protection test selection allows performing the test of the relays installed at two different sites, with two test sets and two operators. Each test set uses three currents in all, so it may be of

any type. Three currents are injected at the meantime at both ends of the line. The connection

schematic is the following one.

RELAY A

TEST

SET A

TRIP CURRENTS

PC A

GPS

IRIG-B

RELAY B

TEST

SET B

TRIP CURRENTS

PC B

GPS

IRIG-B

TX RX

TX RX

EXT

GPS

EXT

GPS

C5 C5

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If the synchronization is performed using the external GPS, then its output is connected to the C5

test set input. For DRTS XX test sets, you may have the external GPS; otherwise, you may have the

GPS or IRIG-B which are inside the test set.

Relays communicate via the TX, RX optical fibre links.

The Line selection parameters are the following ones.

The two currents on the two sides should be reversed by 180. The selection + 180 ,

available at both ends, is there in order to reverse the currents: just check on one side

only.

Selections and are fundamental for the correct test execution. Before starting the test, operators must agree: one has to select the side A (the program assigns to it the local

current), pressing it; the other one must select the side B , (the program assigns to it the

remote current); else the test would be completely wrong.

The EXTERNAL GPS or SYNCH synchronization can be: external GPS, or internal (DRTS XX series). The selection is performed here.

You can select GPS external. The selection SYNCH stays for internal GPS or IRIG-B

(DRTS XX series). After selected SYNCH, the corresponding icon becomes accessible.

Press it: the following window opens.

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As you open it, the synchronization is not enabled. Select what you have in your test set, IRIG-B

or GPS (internal): the window is the same for the two options. For the IRIG-B synchronization

you will connect the test set to the plant using the optical fibre provided; for the GPS

synchronization, you have to connect the antenna to the rear of the test set, and then to locate it

so that the sky is visible at least for two thirds; else, the synchronization could be impossible.

Once the synchronization is achieved, the message disappears, and the

timer will display the absolute (Greenwich) time. Now, both operator can agree for the first test

to be started at a given time, and input it. Allow for at least two minutes after the current time!

The Add one day is for those living after the day change line. The

can be 30 s or 60 s: this sets the pace at which tests will

be performed, one after the other, at both ends, until all tests are over.

CT Ratio . These are the primary currents respectively at sites A and B; the secondary side current is the same for both sites, and it is programmed here,

with the other nominal parameters. The selection is provided for the case that

primary currents are not the same.

3.3.1.2 Relay characteristic

The Alpha plane selection refers to a recent line of differential relays (for instance, SEL 311L), where the relay characteristic, instead of being represented in the usual IR Id plane, is displayed with the coordinates Re(IR/IL) and Im(IR/IL), where:

o Re( ) and Im( ) are the real and the imaginary components of the vector IR/IL; o IR is the remote current; o IL is the local current.

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According to the manufacturer, recent studies show that this characteristic is less prone to false

interventions than the standard Id versus IR characteristic.

Having checked Alpha plane characteristic, the diagram is the following.

The characteristic means that the relay trips for all values outside the dotted area: the outside is

the protected area. In fact, the nominal situation is IL = IR, with a phase shift of 180. Studying

the characteristic, you may see that it is limited by the following lines.

An external arc, corresponding to the maximum value of the ratio (four in the example);

Two straights in the first and fourth quadrant starting from zero, having a small angle with respect to the Im axis. Purpose of this is to avoid unwanted trips caused by minor angle errors;

An internal arc, which cannot be zero because it is the reciprocal of the external arc, exchanging IR and IL (if the external arc has a radius of 4, the internal one has a radius of 1/4 = 0.25).

We will discuss this kind of test and its results.

Once the selection is performed, when you select the Relay characteristic tab, you get the following window.

Relay characteristic parameters are:

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Restraint radius; it is the external limiting radius. As explained, the internal limiting radius is the reciprocal of the external one.

Angular restraint: it is the total angle of the no-trip area: see the following drawing.

The angular restraint.

Test settings are a table of three lines and three columns. The meaning of lines is the following.

Phase: it is the phase current, and refers to faults: L1, L2, L3, L123.

Negative sequence: it is simulated generating two currents which generate the programmed reverse sequence value.

Zero sequence: it refers to this type of fault; it is simulated generating three currents in phase, which generate the programmed zero sequence value.

NOTE: there is a problem when performing these tests, and it is that the relay intervention

could be caused by the phase protection when testing the Negative sequence one. It is

therefore necessary to enable a function at a time.

The meaning of columns is the following.

Pick-up current: it is the relay setting; the relay does not trip if the difference between the local and the remote currents is less than the pick-up.

Minimum current: based upon the relay characteristic and the pick-up current, the program computes the value of the minimum current, which guarantees that, when the

test current is higher than the minimum, the relay will trip in all the protected area.

Test current: during the test, the test current will be the one generated by the local test set, while the remote test set will generate the computed value for remote the test current, which changes at each test. The value of the test current can be any, but the

relay will trip only if the test current is more than the minimum current; else, for some

point, it will be impossible to perform the test, because the relay will not trip.

For instance, if I pick-up is 2 A and the test current is 1 A, the relay will not trip for

differential currents less than 2 A, no matter what is the restraint radius.

Having set-up all the parameters, we can proceed with the testing, selecting the type of test.

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3.3.1.3 Test selection


Unlike the transformer differential protections, the only available test is the Verify curve. Test

parameters are:

Start angle: it is the first angle at which the characteristic will be tested, on the external and on the internal restraint radius;

Stop angle: it is the last test angle, on the external and on the internal restraint radius. Usually, it is symmetrical with respect to the start angle.

Step: it is the angle increment between two tests.

Inner or outer radius. This selection allows to define which are the test points: the ones of the external radius for the A (or local) relay and inner for the B (remote) relay

or vice versa . Note that the selection

outer for one side corresponds to the selection inner on the other side. The selection must be the same for both ends.

Perform only border test. If this box is not checked, the program will search the actual limits, with the criteria of stopping the search when the result accuracy is

one tenth of the programmed relay tolerance. The process can take some time, according to

the relay characteristic deviation from nominal values.

By border test, we mean performing tests at the minimum and at the maximum of the tolerance. Test result is pass if:

o At the minimum of the tolerance (inside the diagram) the relay does not trip, and: o At the maximum of the tolerance (outside the diagram) the relay trips.

With this selection, the diagram looks like the following one.

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Test. Press OK and go to the Test table. If the selection is Search, the following test table is displayed.

For the Alpha plane characteristic curve, relevant parameters are:

Fault type;

Trip delay;

IP1 = test current, phase 1. This is the local test current;

IS1 = test current, phase 1. This is the remote test current;

|IR/IL|: this is the measured module of the ratio of remote divided by local current;

Fi(IR/IL): it is the angle of the vector IR/IL.

|IR/IL| nominal: this is the nominal module of the ratio of remote divided by local current.

NOTE: the Pass/Fail selection is not operational with the Alpha plane characteristic.

If the Border test is selected, the Test table looks like the following one.

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For the same number of points, the number of tests is twice as compared to the search test. Test

result parameters are the same as above.

NOTE: when a result is obtained, the diagram displays two points: the outer and the inner circle, at

angles symmetrical with respect to 180.

3.3.2 Alpha plane characteristic; two protections test

This combination is selected as follows.

3.3.2.1 Line parameters

The Two protection test selection allows performing the test of the relays before installing them at their sites, using a single six-current generator, such as DRTS.6, DRTS 64, DRTS 66. The relays

are taken to the same place and connected between them, as they will be in the plant. Thanks to the

six current generation, there is no need to synchronize with an external mean: the test set itself

ensures the synchronization. The connection schematic is the following one.

Relays communicate via the TX, RX optical fibre links.

The Line selection parameters are the following ones.

RELAY A

TEST

SET

TRIP CURRENTS

PC

RELAY B

TRIP CURRENTS

TX RX

TX RX

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The two currents on the two sides should be reversed by 180. The selection + 180 ,

available at both ends, is there in order to reverse the currents: just check on one side

only.

Monitor 2 contacts. We have two relays, and therefore two trips. The program operates as usual, using one of the two relay contacts as the trip input. However, it is also interesting to

verify that also the other relay trips, nominally with the same delay. The test is performed by

checking the box, and selecting the test set input for the second contact.

The test program verifies that the second contact trips within the programmed maximum

time; if not, the operator is alerted by a warning message.

CT Ratio . These are the primary currents respectively at sites A

and B; the secondary side current is the same for both sites, and it is programmed here,

with the other nominal parameters. The selection is provided for the case that

primary currents are not the same.

3.3.2.2 Relay characteristic

The Alpha plane selection refers to a recent line of differential relays (for instance, SEL 311L), where the relay characteristic, instead of being represented in the usual IR Id plane, is displayed with the coordinates Re(IR/IL) and Im(IR/IL), where:

o Re( ) and Im( ) are the real and the imaginary components of the vector IR/IL; o IR is the remote current; o IL is the local current.

According to the manufacturer, recent studies show that this characteristic is less prone to false

interventions than the standard Id versus IR charac

mse20029 - differential 87

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