A Zone-Switching Application for the SEL-487B Relay threephase

Application Guide Volume IV AG2010-02
A Zone-Switching Application for the
SEL-487B Relay
David Costello, Derrick Haas, and Ariana Amberg
INTRODUCTION
The SEL-487B Bus Differential and Breaker Failure Relay provides differential protection for as
many as six single-phase zones and includes circuit breaker failure protection for up to six threephase
circuit breakers. Advanced zone selection logic in the SEL-487B uses disconnect auxiliary
contacts to automatically reconfigure bus protection zones during disconnect switch operation.
This application guide discusses how to configure the SEL-487B for a simple zone-switching bus
differential application in a basic two-bus station.
This application guide uses ACSELERATOR QuickSet ® SEL-5030 Software to create a settings
file. ACSELERATOR QuickSet is a Microsoft ® Windows ® -based program that allows for creating,
editing, exchanging, downloading, and uploading relay settings for SEL devices. It is available
for download on the SEL website at http://www.selinc.com/softwaresolutions/. The relay settings
discussed in this application guide are also provided in an ACSELERATOR QuickSet file that is
available for download with this application guide at http://www.selinc.com. This settings file can
serve as a starting point for a simple zone-switching application.
INITIAL CONFIGURATION OF THE SEL-487B
Because of the capabilities of the SEL-487B, it is important to consider the overall protection
requirements for the specific application. An SEL-487B worksheet is available online at
http://www.selinc.com/sel-487b.htm. This worksheet may be used to determine the protection
options required for a specific application, as well as to define the application settings.
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Table 1 is from the SEL-487B worksheet and lists the protection functions that are used in this
application. This example only uses basic bus differential protection.
Table 1 Standard Protection Functions
Protection Function
CT (current transformer) ratio mismatch smaller than 10:1
Circuit breaker status logic
Disconnect monitor logic
Differential protection
Dynamic zone selection logic
Sensitive differential protection
Zone supervision logic
Zone-switching supervision logic
Coupler security logic
Circuit breaker failure protection
Instantaneous overcurrent protection
Time-overcurrent protection
Phase voltage elements
Zero- or negative-sequence voltage elements
Selection
Yes
Yes
Yes
Yes
Yes
Yes
No
No
No
No
No
No
No
No
CONFIGURATION OF CURRENT INPUTS
Because busbar protection applications differ substantially from station to station, we show a
basic station application that excludes all protection functions except busbar protection. Figure 1
is a one-line diagram of the application. The bus zone includes seven three-phase circuit breakers.
Note that the ratios of the CTs are not all the same.
Figure 1
Application One-Line Diagram
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None of the current inputs to the SEL-487B are preassigned, which provides maximum flexibility
in applying the relay. The SEL-487B can accommodate 18 individual current inputs. For this
application, the bus differential protection requires all 18 inputs: Phase A, B, and C currents from
each of the breakers (the phase currents from Breakers 1 and 2 are paralleled and share relay
inputs).
Because of the number of current inputs on the SEL-487B and the fact that this application has
more than six breakers, CTs from Breakers 1 and 2 are paralleled to one input on the SEL-487B.
Each of these breakers serves a radial load only. Do not parallel source and load CTs to the same
inputs of the relay, and do not parallel two sources to the same input of the relay. For external
faults, the paralleled connection of a source and load, or two sources, would reduce restraint
current and make the relay less secure.
There are three three-phase 161 kV breakers on Bus A: Breakers 1, 2, and 3. There are four
three-phase 161 kV breakers on Bus B: Breakers 5, 6, 7, and 8. A motor-operated switch
(MOS 4) connects Bus A to Bus B.
Faults on Bus A trip Breakers 1, 2, and 3 when MOS 4 is open and Breakers 1, 2, 3, 5, 6, 7, and 8
when MOS 4 is closed. Faults on Bus B trip Breakers 5, 6, 7, and 8 when MOS 4 is open and
Breakers 1, 2, 3, 5, 6, 7, and 8 when MOS 4 is closed.
Table 2 shows the external CT connections corresponding to the current input assignments in the
SEL-487B. The order and grouping of the current inputs are flexible. In this example, the threephase
currents from each breaker are grouped on each row. The inputs are also grouped by phase
in each column.
Table 2
SEL-487B Current Input Assignments
Terminal Phase A Phase B Phase C CT Ratio
1 I01 I02 I03 2000/5Y
2 I01 I02 I03 2000/5Y
3 I04 I05 I06 2000/5Y
5 I07 I08 I09 2000/5Y
6 I10 I11 I12 600/5Y
7 I13 I14 I15 2000/5Y
8 I16 I17 I18 2000/5Y
This application requires six separate bus zones—one per phase on each bus. Table 3 shows the
bus zone assigned to each current input.
Table 3
SEL-487B Bus Zone Assignments
Bus Zone Description Terminals
1 Bus A, Phase A differential I01, I04
2 Bus A, Phase B differential I02, I05
3 Bus A, Phase C differential I03, I06
4 Bus B, Phase A differential I07, I10, I13, I16
5 Bus B, Phase B differential I08, I11, I14, I17
6 Bus B, Phase C differential I09, I12, I15, I18
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Figure 2 shows how to program the terminal-to-bus-zone connections using ACSELERATOR
QuickSet.
Figure 2 ACSELERATOR QuickSet Settings to Connect Relay Terminal I01 to Bus Zone 1
ALIASES
Alias settings can be used in the SEL-487B to rename current and digital inputs to something
significant to the user. The alias used is arbitrary and intended to provide easy identification for
metering, logic, and event reports. In this application guide, aliases are not used in order to allow
the user to cross-reference standard Relay Word bit names in logic.
ZONE SWITCHING
A disconnect auxiliary contact provides the zone selection logic with the information required to
dynamically assign current inputs to the appropriate differential elements. In this example, the
SEL-487B is set such that Bus Zones 1 and 4 merge when MOS 4 closes. Bus Zones 2 and 5
merge when MOS 4 closes. Bus Zones 3 and 6 merge when MOS 4 closes. Figure 3 shows the
settings to connect Bus Zone 1 to Bus Zone 4.
Figure 3 ACSELERATOR QuickSet Settings to Connect Bus Zone 1 to Bus Zone 4
The SEL-487B includes disconnect switch monitor logic. This logic requires both normally open
(89A) and normally closed (89B) disconnect auxiliary contacts. In this application example, the
disconnect monitor logic is not used. An 89A contact is not wired to the relay; only an 89B
contact is used.
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The 89B contact is used to initiate the merging of bus zones by way of the zone selection logic. It
is critical to the security of the relay that the bus zones be merged before arcing begins as MOS 4
closes and that the zones be unmerged after arcing ends as MOS 4 opens. As Figure 4 shows, an
89B contact provides this functionality. Do not attempt to use an 89A contact alone for initiating
zone selection logic, as relay security will be compromised during MOS arcing.
Figure 4
Motor-Operated Disconnect Switch Operation
DIFFERENTIAL PROTECTION
Because the relay accepts current inputs from CTs with a 10:1 ratio mismatch, the calculations for
the differential elements are performed on per-unit values. The relay uses the highest CT ratio
(CTR MAX ) of the installed CT ratios as a reference value in converting the input currents from
amperes to per-unit values.
CTR
MAX
• INOM
TAPnn = (1)
CTRnn
where:
TAPnn = TAP value for each terminal to convert current from amperes to per unit
(nn = 01 through 18).
CTR MAX = highest CT ratio of the terminals used in the terminal-to-bus-zone settings.
I NOM = nominal CT secondary current (1 or 5 amperes).
CTRnn = CT ratio of the specific terminal.
Using the TAPnn values, the relay calculates the current in per-unit values for each terminal as
follows:
Inn
InnCR = pu
(2)
TAPnn
where:
InnCR = per-unit current for Terminals I01 through I18.
Inn = current in amperes for Terminals I01 through I18.
pu = per unit.
The minimum sensitivity of the differential element is defined by the setting O87P and the CT
ratios. For this application, assume the O87P setting is 0.82 per unit.
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Using (1) and (2), we can derive a simple expression for the minimum sensitivity in primary
amperes. See (3). For this application, an O87P setting of 0.82 per unit translates to a minimum
sensitivity of 1640 primary amperes, based on TAP and CT ratio.
Minimum sensitivity (in primary amperes) = O87P • TAPnn • CTRnn (3)
Set O87P low enough to ensure operation of the differential element for an internal bus fault
when fed from the weakest source. Simulate a bus fault under minimum source conditions, and
ensure that the minimum sensitivity is below the minimum fault current, including margin.
Based on the SELOGIC ® control equations IqqBZpV (Terminal-qq-to-Bus-Zone-p connections)
and BZpBZrV (Bus-Zone-p-to-Bus-Zone-r connections), the SEL-487B determines the
following:
• Bus zone(s) to be included in each protection zone.
• Active terminals to be included in the associated protection zone.
• Terminals to trip for differential and breaker failure protection operations.
• Differential elements to run.
After the relay detects a switching operation (e.g., status change in ZNpIqq), it asserts the
corresponding zone-switching operation bit, ZSWOPp. Figure 5 shows the processing sequence
that the relay follows to determine whether to run the differential element. Placeholders p and r
refer to Bus Zones 1 through 6, and qq refers to Terminals 01 through 18. See Figure 5 and
Figure 6.
Figure 5
Flow Logic Diagram for Determining Terminals for Each Zone
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Start
No
87Zn
asserted?
Yes
Read terminals to trip within
Zone n from ZNpIqqT
Generate trip for the
selected terminals 87BTRkk
kk = 01...18
End
Figure 6 Flow Logic Diagram for Generating Trip Outputs 87BTRkk (kk = Terminals 01 Through 18)
The sensitive differential element 87STn (n = 1 through 6) detects CT open- or short-circuit
conditions. The factory default setting for sensitive differential elements is 10 percent of zone
differential element pickup values. This setting of 10 percent allows sensitive differential
elements to respond to CT problems, even under low-load conditions. With sensitive differential
element supervision enabled (E87SSUP = Y), the time-delayed outputs of the sensitive
differential elements (87STn) block the zone differential elements (87Rn) from operating.
For this application, we route the time-delayed output of the sensitive differential element to
output OUT201. Connect OUT201 to SCADA (supervisory control and data acquisition) for
remote indication. If blocking the zone differential elements is not desired, set E87SSUP = N.
Note that the operation of the sensitive differential element (87STn) is independent of the
E87SSUP setting. Thus, OUT201 still asserts for CT problems when E87SSUP = N.
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TRIP OUTPUT LOGIC
Figure 7 identifies trip logic needed for this application. Each SEL-487B terminal has a trip Relay
Word bit, 87BTRkk. Because this is a three-pole trip application, we can combine the Phase A, B,
and C trips into one output for each breaker. In this example, TR01 represents the trip logic for
Breaker 1.
Figure 7
SEL-487B Trip Logic
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Figure 8 shows the output contact programming for this application. OUT101, for example, is the
trip output contact for Breaker 1 and is set equal to TRIP01. TRIP01 asserts when TR01 is a
logical 1, which occurs for Phase A, B, or C trips to Breaker 1. These trips are the result of the
87BTRkk Relay Word bits, which in turn are the result of the terminal and bus zone logic and
differential elements.
Figure 8
SEL-487B Output Logic
REPORT SETTINGS
The report settings do not affect the differential protection, but they do provide valuable
information for evaluating relay operations. The SEL-487B includes both SER (Sequential Events
Recorder) and event report (oscillography) files. Note that the default SER settings do not include
any points. For this example, only relevant Relay Word bits for this application were included in
the SER settings and the list of digital elements for event reports.
CONCLUSION
This application guide provides an introduction to setting the SEL-487B for a simple bus zoneswitching
differential application. The settings developed in this guide may be used as a starting
point to create settings for a specific bus differential application.
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FACTORY ASSISTANCE
We appreciate your interest in SEL products and services. If you have questions or comments,
please contact us at:
Schweitzer Engineering Laboratories, Inc.
2350 NE Hopkins Court
Pullman, WA 99163-5603 USA
Telephone: +1.509.332.1890
Fax: +1.509.332.7990
www.selinc.com • info@selinc.com
© 2010 by Schweitzer Engineering Laboratories, Inc.
All rights reserved.
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the trademark or registered trademark of their respective
holders. No SEL trademarks may be used without written
permission.
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U.S. and Foreign patents.
*AG2010-02*
SEL Application Guide 2010-02 Date Code 20100421