7sd52 7sd62 configuration

AP P L I C AT I O N N O T EPA 13

01/2001, Rev. A of 14

�� Power Automation 1Postfach 4806 � D-90026 NürnbergTel: (+49 911) 433-8275 � Fax: (+49 911) 433-8301 � [email protected]

Line differential settings for a two terminalconfiguration with 7SD52 / 7SD610

1.0 INTRODUCTION 1

2.0 CURRENT PRACTICE 3

3.0 NEW OR ADVANCED PRACTICE/APPLICATION 3

4.0 APPLICATION EXAMPLE 3

4.1 Settings in DIGSI 4 for the Device Configuration 4

4.2 Settings for the relay on the left line-terminal 4

4.3 Settings for the relay on the right line-terminal 7

4.4 Common settings in both relays 8

5.0 CONCLUSIONS – ONLY 3 PARAMETERS MUST BE SET 11

6.0 BENEFITS OF APPLICATION 12

7.0 APPENDIX 13

1.0 Introduction

This app-note explains, what settings are necessary for the line differential relay87L SIPROTEC (7SD522/7SD523 and 7SD610) with digital communication in atwo terminal-line configuration. It show´s how easy it is, if you want to set therelay for it´s main function 87L. It is assumed in the example that the CT´s havedifferent CT-ratios. It´s quite common today, that existing CT´s designed forelectromechanical or static relays have different datas from those which areinstalled in a new switch-gear. Different CT-ratios (allowed is 1:8) and differentCT-types are allowed with this relays.

The two terminal configuration is shown in fig. 1. The CT on the left terminal hasa ratio of 400:5 (old one) and the CT on the right terminal the ratio 600:1 (newone). Both CT´s are from TPX-type or from type 10P, that means cost effective

AP P L I C AT I O N N O T EPA 13

01/2001, Rev. A of 14


CT´s for protection applications.This CT-type is fully sufficient for the differentialprotection. It´s assumed also, that the CT´s do not saturate for the maximumexternal short circuit current divided by the nominal current of the CT´s or theaccuracy limiting factor is at least 30 for each CT.

Fig. 1: Differential configuration

400:5 600:1

87L 87L

8 km (5 miles)singlemode


2.0 Current Practice

Existing electromechanic relay design allows CT´s from the same type and fromthe same rated power. The accuracy limiting factor or knee point voltage of theCT´s must have more or less equal values. Different CT ratios are not allowed orforce to use matching CT´s on one side. This often deteriorate the stability of thedifferential configuration, because of the additional burden of the matching CT´sand results in an insensitive setting for the trip threshold.

Due to the use of summation CT´s no phase segregated tripping is possible andalso no clear indication of the faulty phases.

The exchange of the currents between the line terminals runs over pilot wirecables. The maximum distance is limited therefore to 16 miles (max. 25 km).

3.0 NEW OR ADVANCED PRACTICE/APPLICATION

With numerical design of 7SD52 / 7SD610 must of the restrictions known fromclassical design do not exist any more. Different CT-ratios and different CT typesare allowed. No huge CT´s with high rated power are required for the appication,because the relays offer low burden (0,1 VA for 1A, 0,5 VA for 5A). ExistingCT´s and new installed CT´s can be used together in a differential configuration,as long as the minimum CT-requirements are fullfiled. This allows easyrefurbishment using parts of the existing primary installation in the future. Thissaves money.

The communication between the line terminals run interference free over fibreoptic cables or over a digital communication system with N*64 kBit/s. Thecommunication aspects are not part of this application note. It´s only explainedhow to set the relevant communication parameters, to come to a runningdifferential system.

4.0 Application Example

The relays are ordered as line differential relays without comprehensiveadditional functions (e.g. autoreclosure, multiterminal extention) and for threepole tripping. This is the most frequent application in praxis. The distancebetween the two line terminals is 5 miles (8 km) and a 110 kV cable is between.The communication between the relays runs over dedicated monomode(singlemode) fiber cables. The inbuilt 1300 nm interface in the relays is requiredwhich allows in one option a distance of 10 km (6 miles) and in the other option adistance of 35 km (22 miles). The cost effective 10 km option is sufficient for


most of the lines and cables in urban areas. In general the modules can beexchanged by the customer.

See the ordering code for the SIPROTEC 7SD610 relay in the appendix of thisapplication. This datas must be also set in DIGSI in the substation manager, ifyou want to insert and initialize a new relay.

4.1 Settings in DIGSI 4 for the Device Configuration

To activate only the differential protection function all other functions must bedisabled. The relay in this application works as a pure line differential relay withno additional functions running parallel. Very often the backup or emergencyovercurrent protection (50, 50N, 51, 51N, 50G, 51G) is used in parallel to thedifferential protection (backup mode) or if the differential protection fails, due to aloss of the data connection (emergency mode).

Figure 2: Device configuration for both relays if only 87L is activated

4.2 Settings for the relay on the left line-terminal

The CT primary rated current must be set. Here it´s 400 A according ourexample in fig. 1. Furthermore the relay must know, if it´s a 1 A or 5 A input. Thedefault is the value of the ordering code, so normaly no change is necessary.This information is e.g. used if the settings are done in secondary values.CT-inputs can be changed from 5 A to 1 A by jumpers in the relay, if this relayhas been ordered in a 1A version. How to do this is clearly written in the manual.One jumper also indicates the status of the inputs. It indicates wheather theinputs are set to 1 A or to 5 A. If this indication is different from the selection


done in DIGSI 4 for the current inputs an alarm in the Event-Log-Buffer showsthis and the relay refuse to work with this inconsistencey. The alarm is:

00192 Error1A/5Awrong Error:1A/5Ajumper different from setting

The CT-starpoint may be towards line or towards busbar. The direction isautomatically considered for the differential measurement. If the physicallyconnected CT-starpoint is equal with those of the setting the differential currentmeasured during commissioning must be equal with the charge current of theline or cable (here approx. 50 A). Otherwise the differential current is two timesthe load current divided by the the full scale current. Changing this parameter forone line end turns around the current direction there. Another indication for arestraint operation is an angle of approx. 180 degrees for each phase betweenthe line terminals. Differential and restraint current, local and remote current andalso the phase angle between the currents are available as operational values inthe relay. This allows easy commissioning and an easy check for the polaritysetting.

Figure 3: Settings for the current transformer (CT)

The default setting for the rated frequency for relays for Europe or most of therest of the world is 50 Hz, except US-ordering option. Here it´s set to 60 Hz.

Figure 4: Setting for the rated frequency

The CT-datas define the maximum measurement error of the CT up to nominalcurrent (default is 5%) and for short circuit currents (default is 10%). A shortcircuit currents in the default setting is a currents higher then nominal current.


The measurement error up to nominal current is 5%, which is much more asseen for a real CT. The error for short circuit currents (> nominal current) is set to10%. This is the error of a 10P or TPX class CT at it´s accuracy limiting factor ornear at it´s kneepoint voltage. This factory settings include much saftety magin.

The default settings can remain unchanged for all CT´s used worldwide forprotection applications. If this setting should be changed special knowledgeabout CT´s and there measurement errors is required. In special applications,where the relays shall be set very sensitive and very good CT´s are used, e.g. inthe transmission level, the datas may be changed. How to adapt the settings toreal CT datas is written in the manual of the devices.

With the default settings one is on the save side for a restraint differentialoperation during external faults and the standard settings are designed andtested by the manufacturer with it´s long experience with differential relays andthere measurement quantities and effects.

Remain the default settings for Breaker.

Figure 5: Default settings for CT-measurement errors

In a configuration each relay has an ID-number which can be set in the rangefrom 1-65535. This house-number of the relay must be set to a different value ineach relay. It allows the relay to identify the other relay clearly over the fiber linkor in a communication system. It helps to identify reflected datas in acommunication system, because the relay receives telegrams with it´s own ID-number and this immediatelly blocks of the differential protection. It´s usefullduring commissioning, because with test loops in the communication path onecan check the comms-links. Also in this case the relay receives it´s own ID andindicates:

3217 Prot.Int1: Own datas receivedDisplays the receiption the own telegrams

Especially in comms-systems where a lot of relays are working over the samesystem this setting process avoids wrong data connections between relays byaccident. The ID of the other relay is checked all the time during operation.


In our example the ID for the left relay is 1 (factory setting). Furthermore therelay has an index (number of the relay in a configuration) which is here set to 1.A wrong setting for the ID-numbers (equal values or different values in eachrelay) or for the index (equal values) is indicated in the event log buffer and as adefault on an LED with a red alarm indication. This clearly helps to identifywrong settings for the ID-numbers and the index (parameter 1710).

3234 DT unequalMeans that the ID´s set in one relay are not the same as in the other relay

3487 Equal IDs Means the ID´s are not set to a different value in each relay

The relay with index 1 time-synchronizes the other relay/relays. That´s thedefault setting for Time Synchronization. If an external time synchronizationunit is used it should be connected to relay with index 1. The other relay/relayshave now the same time tagging as this relay.

Also independent time tagging for each relay can be set under TimeSynchronization in DIGSI 4. This is not recommended, because it makes thefault and event log analysis difficult.

Figure 6: Default settings for the differential topology

4.3 Settings for the relay on the right line-terminal

The settings for Power System, Breaker and CT Datas are the same as for theleft relay. The CT datas, which are set under Transformers are differentaccording fig. 1.


Figure 6: Settings for the current transformer (CT)

One parameter of the Differential Topology must be changed. It´s the index ofthe second relay, which must be set to two. This is a need, otherwise theconfiguration can not operate. This is checked by the relays. Relays must havedifferent ID-numbers, but the same values for 1701/1702 in each relay (becausethe local relay must know this value from the remote side, to identify it) and musthave different index (1 and 2 for a two line-terminal configuration).

Figure 7: Settings for the differential topology for the right relay

4.4 Common settings in both relays

The following settings must have the same value in both relays. A very importantparameter is 1104 Measurement Full Scale Current. All percentage operationalmeasurements like the local and remote currrent (the per unit values) and thedifferential- and restraint current is rated to this quantity. The same value mustbe set for parameter 1104 in each relay of a differential topology. Normally thehigher primary rated current of the CT´s is set in the relays, which is thereference for all relays. This value is checked by the relays during the loginprocedure after the data connection is established. If it´s different the differentialfunction can not work (no common reference) and is blocked. An alarm isindicated in the event log buffer and indicated on LED 7 (factory I/O masking).


For a line with equal CT-ratios parameter 1104 is set to the same value asparameter 205.

The full scale voltage is only from interest, if voltage transformers are connectedto the VT-inputs of the relay. It´s must be set to the same value for both lineterminals (here 110 kV according to the nominal voltage of the line).Remain the factory setting of 400 kV, if you don´t want to make any settings hereor no VT´s are connected. A default setting has no influence to other functions.

Figure 8: Common current and voltage settings

It´s not a must to have equal settings for the differential setpoints at each lineterminal. But it´s recommended to do it, to get the same tripping reaction.Because each line terminal see the same quantity for the differential andrestraint current the tripping behaviour is the same. No intertrip is normallyrequired. It´s also recommended to set the parameters in DIGSI 4 in primaryvalues. So each line end can be set to the same values, especially when the CT-ratios are different.

The sensitive differential setpoint 1210 I-DIFF> is calculated according thecharge current of the cable or set to a minimum value, which results from theCT´s transient behaviour. For 10P or TPX CT´s a minimum setting of 30% forparameter 1104 is recommended, which is the factory setting.

The charge current caused by the capacitance of the line/cable is a permanentdifferential current during normal operation. I-DIFF> should be set 2,5-3 times ofthis steady state charge current. For charge currents less than 10% of Full ScaleCurrent (here 60 A) the charge current has no influence of the setting, so it´s0,3*600A = 180A.

The charge current is calculated as follow:

IC = 3,63 · 10-6 · UN · fN · COp' · l [1]

IC Primary charge current in AU N Nominal voltage of the line/cable in kV


f N Rated frequency in HzCOp’ Capacitance of the line or cable in nF/km (typ. line 8 nF/km, cable 250 nF/km)l Length of the line/cable in km

In our example we have a relativ long cable with 8 km (5 miles), which is seldomin praxis. Even for this long cable the value for IC is only:

IC = 3,63 · 10-6· 110 kV · 60 Hz · 250 nF/km · 8 km = 48 A

This value is below 60 A, so for I-DIFF> the factory setting is a good choise.

The praxis is, that only for cables longer than 6 km (UN up to 110 / 132 kV)and overhead lines longer than 100 km the check with the formula isrequired. Otherwise remain the factory settings. So in minimum for 95% ofall applications the factory settings can be used.

When switching on a very long cable or line from one side, high charge peakvalues appear during the first milliseconds. To avoid any pickup of the sensitivedifferential trip stage 1210 I-DIFF> a second setpoint 1213 I-DIFF>:Valueunder switch on condition can be set for this situation to a higher value. Therelay monitors always, if no current is flowing and use this setpoint instead of1210 I-DIFF> for a duration set in parameter 1132 (default is 100 ms) afterswitching on the line. Then after 100 ms the setpoint 1213 is lowered to thevalue of 1210 I-DIFF>, if they are set to different values.

1213 setpoint must be higher then 4 times steady state charge current calculatedaccording formula [1]. In our example it´s:

1213 I-DIFF>: Value ... = 4 · 48 A = 192 A

For cables shorter 6 km and lines shorter 100 km parameter 1210 and 1213have the same value and you can remain the factory setting for both parameters.

The 1233 I-DIFF>> parameter is set for the fast current comparision algorithm,which use instead of the I-DIFF> trip stage a short 5 ms filtering window. Thisresults in tripping times of 16-25 ms (depends on the baud rate of the data link,16 ms for 512 kBit with direct fiber connection).I-DIFF> trip stage use a one cycle filtering window and the tripping time istherefore 30-35 ms. The longer filtering window suppress DC-components in thesummation for the differential current and allows the sensitive setting for 1210and 1213.

Fault currents above 720 A are tripped by the relays in 16 ms (with direct fiberconnection). Please remain this parameter unchanged for line or cables. Only


with transformers in the protection zone or networks grounded with a Petersoncoil special considerations about this setting are required.

Figure 9: Settings for the differential protection in primary values for both relays

If secondary values shall be used for the settings of the differential protection thefollowing quantities must be set in the left relay:

1210 I-Diff> (sec, left side) = 180 A / 400 A * 5 A = 2,25 A1213 I-Diff> (sec, left side) = 192 A / 400 A * 5 A = 2,4 A1233 I-DIFF>> (sec, left side) = 720 A / 400 A* 5 A = 9 A

In the right relay the settings are:

1210 I-Diff> (sec, right side) = 180 A / 600 A * 5 A = 0,3 A1213 I-Diff> (sec, right side) = 192 A / 600 A * 5 A = 0,32 A1233 I-DIFF>> (sec, right side) = 720 A / 600 A = 1,2 A

No other settings are necessary for the differential protection function.

5.0 Conclusions – Only 3 parameters must be setTo use the relays with the differential protection function 87L, even if differentCT-ratios and types are used, require only the setting of the:

0205 CT-Rated-Primary-Currentin each relay (normally the same value with equal CT-ratio)

A common current reference is set in1104 Full Scale Currentto the same value for each relay (normaly 0205 = 1104)

The differential setpoints remain unchanged for almost every line or cable.

The relay-index


1710 Local relay of one relay must be set to 1 and to 2 for the other relay.

The defaults for the communication parameter are set for a fiber opticconnection. With comms-system between the baudrate for the communicationinterface must be set.

6.0 Benefits of Application

With the differential relays 87L SIPROTEC easy to set relays are provided bySIEMENS.

Different CT-types and CT-ratios can be used. The mismatch of the CT-ratio canbe 1:8, which covers all practically known cases until now. The newmeasurement technique of the relay consider this conditions adaptively, so theuser only have to set datas, which are well known from it´s CT´s or the line/cableor remain the factory setting.

For the communication between the relays less datas are required. Thesupervision of the connection is an inherent part of the relays design. Switchingeffects in the comms-system are adaptively considered in the restraint current,which is calculated by the relay from the CT-datas and from measured commscharacteristics


7.0 Appendix

Figure 10: Ordering code of the left relay


Figure 10: Ordering code of the right relay

7sd52 7sd62 configuration

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