Application Note

Testing the Adaptive Characteristics of a SIEMENS 7SDxxx Line Differential Protection Relay using Test Universe 2.x

Author Sameh Eldmrdash | sameh.eldmrdash@omicron.at

Date Nov 4, 2011

Related OMICRON Product CMC - Test Universe

Application Area Adaptive line differential protection function testing

Keywords Test Universe, Adaptive line differential testing, SIEMENS 7SDxxx

Version v1.0

Document ID ANS_11003_ENU

Abstract Transformer, transmission lines and cables are the most important elements in the electric power transmission networks therefore protecting those elements takes its importance in the field of testing the protective relays nowadays. Testing the differential characteristics of any transformer or line differential protection relay is one of the most important tests have to be run to check the behavior of that relay in both tripping and blocking areas of the characteristics. This application note describes how to test the differential characteristics of the 7SDxxx "line differential protection relay" from SIEMENS using the TU2.x. The SIEMENS 7SD6xx item in the protection testing library of OMICRON "PTL" is working exactly as described in this application note.

Content

1 Introduction ..................................................................................................................................... 3 2 Given Information ............................................................................................................................ 3 3 Target ............................................................................................................................................... 4 4 Solution ........................................................................................................................................... 5

4.1 Derivations ............................................................................................................................... 6 4.2 Example of Calculations............................................................................................................ 9

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1 Introduction The differential characteristics for the SIEMENS 7SDxxx relay are given in the technical manual of that relay as shown in Figure 1. This kind of characteristics is an adaptive one and can't be tested directly using OMICRON's Test Universe Software as usual with the classical characteristics.

Figure 1 : Adaptive Stabilization and Differential characteristic for 7SDxxx

With our software, only the Ibias Calculations are shown in Figure 2 are available and it is clear that that kind of adaptive characteristic is not yet supported by the Test Universe software.

Figure 2 : Available I bias Calculation Methods in Test Universe Software

2 Given Information The first given information is the bias calculation formula which is:

rSystemErroIfIfII diffStab +++= > 2211 The second given information is the CT error approximation curve which is helping to draw the points in a

correct way (See Figure 3 ).

Figure 3 : Approximation of the current transformer errors

To consider the influences of current transformer errors, each device calculates a self restraining quantity.

This is calculated by estimating the possible local transformer errors from the data of the local current

transformers and the intensity of the locally measured currents. Since each device transmits its estimated

errors to the other devices, each device is capable to form the total sum of possible errors; this sum is used

for restraint. [1]

Let's try to imagine the approximation of the current transformers error in case of having two current

transformers, one at each end of the protected object either line or a power transformer, Figure 4 shows that

imagination with 6 points resulting in five straight lines.

Figure 4 : General view of CT errors in case of two current transformers

3 Target The main target is to find the formulas controlling the calculation of the points shown in Figure 4 structuring

those pieces of straight lines to draw the whole characteristics.

Those formulas must be depending on the settings of the relays from both sides as shown in Figure 5 and its

shape should change according to the setting values from both sides.

Figure 5 : Basic principle of the differential protection for a line with two ends

In other words, the bias current calculation formula has to be converted to another formula already

supported by the Test Universe Software.

( )+= > currentsfaultII diffbias ( )21 IIIbias += Figure 6 : Conversion from a calculation method to another

4 Solution Having a look to Figure 7 in the first straight line between P1 and P2, the CT errors are taken as 1f and 2f .

In the second straight line between P3 and P4, the CT errors are taken as '1f and 2f because the Test Universe is injecting bigger value of current from the primary side more than the current injected from the

secondary side in case of the primary side is defined to be the reference side, so that is why the CT error

value at the primary side should be taken as the value in case of fault.

In the third straight line between P5 and P6, both current are too high that is why the bigger CT errors '1f

and '2f are taken in consideration at both sides.

Figure 7 : Definition of the lines between the six points

4.1 Derivations The formula mentioned in the manual how to calculate the differential current is:

rSystemErroIfIfII diffStab +++= > 2211 (1)

But the characteristics line is defined to be 45 line which mean that diffStab II =

So the formula can also be written as:

rSystemErroIfIfII diffdiff +++= > 2211 (2) System Error can be defined as 1.5 % In [2]

Our Test Universe differential module is working like that:

21 IIIX bias +== (3)

21 IIIY diff == (4) From Figure 4, it is clear that:

at Point 3,

11 KI = and at Point 2,

11 )1( KI = (5)

The same rules apply for Point 4 and Point 5, so:

at Point 5,

22 KI = and at Point 4,

22 )1( KI = (6) where %10= > Point 1: Applying that

02 =I in equations 2, 3 and 4 we get:

)1()(

1frSystemErroI

YX diff

+== >

> Point 2: From equation 4 we know that,

111 9.0)1( KKI == Applying this in equations 2,3 and 4 we get:

)1())1((

2

112 f

rSystemErrofIII diff

+

++= >

Then 21 IIIX bias +== and 21 IIIY diff == > Point 3: From equation 4 we know that,

11 KI = Applying this in equations 2,3 and 4 we get:

)1())1'((

2

112 f

rSystemErrofIII diff

+

++= >

Then

21 IIIX bias +== and

21 IIIY diff == > Point 4: From equation 5 we know that,

22 9.0 KI = Applying this in equations 2, 3 and 4 we get:

)'1())1((

1

221 f

rSystemErrofIII diff

+++= >

Then 21 IIIX bias +== and 21 IIIY diff == > Point 5: From equation 5 we know that,

22 KI = Applying this in equations 2, 3 and 4 we get:

)'1())1'((

1

221 f

rSystemErrofIII diff

+++= >

Then 21 IIIX bias +== and

21 IIIY diff == > Point 6:

rSystemErroIY diff += >> and from Figure 3 and Figure 4 it is clear that P1, P2 and P3 are lying on the same straight line and the

same for P1, P5 and P6, then it is easy to get :

156

16

mYYX =

And here are the setting values needed for the calculations as symboled in the last formulas and also with its

Digsi address:

Symbol Digsi Parameter Name Digsi ID Default Value

Idiff> 87-1 PICKUP 1210 0,3 A

Idiff>> 87-2 PICKUP 1233 1,2 A

Isec CT SECONDARY 0206 1 A

Iprim CT PRIMARY 0205 400 A

K K_ALF/K_ALF_N 0251 1 f E% ALF/ALF_N 0253 5% f' E% K_ALF_N 0254 15%

Table 1 : Symbols used and their Digsi Names, IDs and Default values

The Primary side is the side with the Digsi Paramter ID 4710 is equal to relay 1.

The Secondary side is the side with the Digsi Paramter ID 4710 is equal to relay 2 (see Figure 8).

Figure 8 : Setting Local Relay "Digsi"

4.2 Example of Calculations To make it easy we will show now an example to calculate the first two points in the characteristics in order

to help with the understanding of our method.

Taking in consideration that we have the default settings shown in Table 1 at both sides and following the

formulas derived at each point;

3316.0)05.01(

)015.03.0(11 =

+==YX

41.1)05.01(

015.0)105.0(9.03.0)19.0(212 =+++

=+== IIIX bias

39.0)05.01(

015.0)105.0(9.03.0)19.0(212 =+++

+=== IIIY diff

List of Literature

[1] SIPROTEC Differential Protection 7SD610 V4.6 Technical Manual

[2] Differential Protection Symposium 2005, Gerhard Ziegler

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1 Introduction2 Given Information3 Target4 Solution4.1 Derivations4.2 Example of Calculations