Tutorial on Busbar Protection

Busbar Protection - Can YOU handle it?

Session 3: Application of centralized
SIPROTEC 7SS85 and CT dimensioning

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Agenda

Time Titel Presenter

09:00-09:10 Welcome & opening and agenda Michaela Mönikes
09:10-09:25 Recap of the SIPROTEC 5 Configurator Nina Hühn

09:25-10:30 Learn how to configure a centralized busbar protection with DIGSI 5 Rainer Goblirsch

10:30-10:45 Break
Experience how to test a centralized SIPROTEC 7SS85 busbar
10:45-11:45 Nina Hühn
configuration with the SIPROTEC DigitalTwin
11:45-12:00 Break

12:00-12:30 Get to know more about CT dimensioning Rainer Goblirsch

12:30-12:45 „Let‘s talk about your questions!“ Wrap Up all

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Content

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 Recap of the SIPROTEC 5 Online
Configurator​
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Example Configuration for a centralized approach

• 4 feeders, 1 coupler with 2 CT‘s → 6 in total

• 2 bus zones
• Disconnector image
→ Significant Feature B: 2 zones, 4 bays incl., discIm
X

• Functions: 50BF, 50EF, Bus Coupler Diff

• Binary inputs: 8 per feeder bay, 7 per coupler bay
X

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Online Configurator

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 Learn how to configure a
centralized busbar protection
with DIGSI 5​
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Steps in DIGSI

• Create the device

• Single Line
• Function
• …

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 Experience how to test a
centralized SIPROTEC 7SS85
busbar configuration with the
SIPROTEC DigitalTwin
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Experience how to test a centralized SIPROTEC 7SS85 busbar configuration
with the SIPROTEC DigitalTwin​

• Stable conditions – disconnector image, general introduction Digital Twin

• Internal fault

• External fault with breaker failure

• Fault in the bus coupler dead zone

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Stable System – Coupler Closed
Bay01 Bay02 Coupler Bay03 Bay04
BB1

BB2

2A 2A -1A -1A
X
X

X
1A 1A

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Stable System – Coupler open
Bay01 Bay02 Coupler Bay03 Bay04
BB1

BB2

1A -1A
X
X

X
1A -1A

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Disconnector Shunt
Bay01 Bay02 Coupler Bay03 Bay04
BB1

BB2

X
X
X

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X
3-pole Internal Fault on BB2; High fault current
Bay01 Bay02 Coupler Bay03 Bay04
BB1

BB2

1A -1A
X
X

X
1A
5A -1A
5A

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1-pole Internal Fault on BB2; Small fault current
Bay01 Bay02 Coupler Bay03 Bay04
BB1

BB2

1A -1A
X
X

X
1A -1A
1A

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External Fault – Bay04
Bay01 Bay02 Coupler Bay03 Bay04
BB1

BB2

3A
1A -3A
-1A
X
X

X
1A -1A

> Start 50BF

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Fault in the dead zone of the bus coupler, 3 pole
Bay01 Bay02 Coupler Bay03 Bay04
BB1

BB2

2A 2A
-2A 1A
-1A 1A
-1A
X
X

X
1A 1A

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Fault in the dead zone of the bus coupler, 1 pole
Bay01 Bay02 Coupler Bay03 Bay04
BB1

BB2

2A 2A
-2A 1A
-1A 1A
-1A
X
X

X
1A 1A

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 Get to know more about CT
dimensioning​
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CT layout parameters

The CT requirements are quite low

In comparison to high-impedance BBP, as

▪ no special CT types (like class PX / IEC 60044-1) are required
▪ different CT types (e.g. 5Px,TPZ, Cxxx) in one scheme are possible, but should be
avoided (especially avoid mix of class P and TPZ)
▪ different CT ratios can be matched by software
▪ the CT core can be shared with other protective relays
In comparison to other protective relays,
▪ the required transient dimensioning factor ktd is as low as 0.5
(ktd > 1.2 … 5 for differential or distance relays)
7SS85 is stable up to 80 % remanence flux (IEC standard definition)
No numerical limit of max/min-CT ratio, but the weakest CT must fulfil the requirements

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Required data for CT dimensioning (IEC 60044 terms)

System data: Iscc max maximum short-circuit current

CT data: Ipn Rated primary current

Sn rated CT power
Rb rated resistive burden
Kssc rated symmetrical short-circuit current factor
Rct secondary winding resistance

CT circuit data: R´b connected burden (Rleads + Rrelay)

Notes:
▪ CT data according to ANSI, IEEE, BS standard should be converted to IEC
▪ Rct as a dominant parameter should be given or measured;
a “20 % Sn assumption” could lead to an overdimensioning of the CT
▪ with 5 A CTs attention should be payed to the resistance of the leads >

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Why is Ktd = 0,5 for 7SS85 ?
8 Infeed
e.g. 2500/1
6
Tsat
4
feeder
2 e.g. 400/1

0
0 20 40 60 80 100 120 140 160 180 200
-2

-4

▪ The limit of stability is given by Idiff = k•Istab* (=tripping characteristic)

▪ With k=0.5 we get at Tsat: Idiff (max)= Iscc ; k•Istab (max) = 0.5•2•Iscc = Iscc = Idiff

▪ The CT must be able to transmit the half of the maximum

short-circuit current without saturation (ωTsat  90 degrees)
→ Ktd = 0,5
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Accurate designation of stabilising factor k

▪ Only in the steady state of Iscc the k-factor has an influence on the k -factor

stability 0,85

▪ For ωTsat  90° the default setting k=0.65 can remain

0,80

0,75

0,70

▪ For ωTsat < 90° a simulation (e.g. by CTDim) or calculation of the CT

k-factor
0,65

behavior is necessary to determine the required k-factor 0,60

0,55

0,50

0,45
ωTsat Tsat /ms Tsat /ms
kth ¹ 0,40

(deg) (60Hz) (50Hz)

0,00 1,00 2,00 3,00 4,00 5,00 6,00 7,00 8,00

Tsat/ms (50Hz)

90 4.2 5 0.50
72 3.3 4 0.53
54 2.5 3 0.62
39 1.8 2.15 0.80

1) the setting should include a safety margin (10 %)

practical limit therefore (50Hz):
kth=0.80 -10 % =0.72 -> Tsat*= 2.4ms

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CT dimensioning criterias

Required Kssc = Ktd * Iscc max / Ipn

Rb + Rct
Effective (Operational) K´ssc (ALF) = Kssc
R´b + Rct

Effective (Operational) K´ssc > Required Kssc

&
Iscc max / Ipn ≤ 100 (limit of measuring range)

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Example:

Transformer: 600/1, 5P10, 15VA l mm2 50m

Rleads = 2r = 2•0.0179• = 0,45 Ω
600/1 89 BB Sec. winding resistance: 4 A m 4mm2
Leads: 50m, 4mm2 CU
Relay burden: 0,1 Rb = Sn / Isn2 = 15 VA / (1A)2 = 15 Ω
Iscc max = 30kA

Rb + Rct
Issc max
Effective K’ssc = Kssc *
Required K’ssc = Ktd * R´b + Rct
Ipn

15 + 4
30 kA
= 10 * = 42
= 0,5 * = 25 0,55 + 4
600A
Effective K’ssc > Required K’ssc
& 30KA / 600 = 50 < 100

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CT correctly dimensioned
CT dimensioning with CTDim

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Contact
Published by Siemens 2023

Smart Infrastructure
Electrification & Automation
Mozartstraße 31 C
91052 Erlangen
Germany
For the U.S. published by
Siemens Industry Inc.
100 Technology Drive
Alpharetta, GA 30005
United States

© Siemens 2023
Subject to changes and errors. The information given in this
document/video only contains general descriptions and/or performance
features which may not always specifically reflect those described, or which
may undergo modification in the course of further development of the
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are expressly agreed upon in the concluded contract.
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