Application-oriented testing of line differential protection with a transformer...
Application Note
Application-oriented testing of line differential protection with a transformer within the protected area using RelaySimTest Author Jens Baumeister |
[email protected] Date Feb 10, 2015 Related OMICRON Product CMC, RelaySimTest, CMGPS 588 Application Area Line Differential Protection End to End testing Keywords RelaySimTest, System Testing, Line Differential Protection, End to End testing, GPS synchronization, PTP Version v1.1 Document ID ANS_14002_ENU
Abstract Due to the increasing complexity of our electrical power systems, the need for highly selective protection is increasingly being fulfilled by the use of line differential protection. Sometimes a transformer is included in the protected area of the line differential protection system. To test such a protection system thoroughly, a distributed end-to-end test with synchronized injection can be utilized. This application note describes how this could be done in an easy and comfortable way using the OMICRON RelaySimTest software. RelaySimTest offers simulation based system testing methods. To perform a test a fault scenario is calculated based on the simulation of the power system network. The resulting voltages and currents for the different relay locations can be used to test the correct behavior of the differential protection system. For this reason RelaySimTest offers the possibility to control several distributed and time synchronized CMC test sets.
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General information OMICRON electronics GmbH including all international branch offices is henceforth referred to as OMICRON. The product information, specifications, and technical data embodied in this application note represent the technical status at the time of writing and are subject to change without prior notice. We have done our best to ensure that the information given in this application note is useful, accurate and entirely reliable. However, OMICRON does not assume responsibility for any inaccuracies which may be present. OMICRON translates this application note from the source language English into a number of other languages. Any translation of this document is done for local requirements, and in the event of a dispute between the English and a non-English version, the English version of this note shall govern. All rights including translation reserved. Reproduction of any kind, for example, photocopying, microfilming, optical character recognition and/or storage in electronic data processing systems, requires the explicit consent of OMICRON. Reprinting, wholly or partly, is not permitted. © OMICRON 2015. All rights reserved. This application note is a publication of OMICRON.
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Content 1
Safety instructions ................................................................................................................................4 1.1
2
3
4
Introduction ............................................................................................................................................5 2.1
General requirements .....................................................................................................................5
2.2
What this application note describes ..............................................................................................5
2.3
Line differential protection...............................................................................................................5
System under Test ................................................................................................................................7 3.1
Application example – Protected area ............................................................................................7
3.2
Settings of the “System under Test” menu in RelaySimTest ..........................................................8
Test cases ..............................................................................................................................................9 4.1
Application example – Grid topology ........................................................................................... 10
4.2
Suitable test cases ....................................................................................................................... 10
4.3
5
Requirements to use this application note ......................................................................................4
4.2.1
Test case 1 – Charging, magnetization and load current ................................................................ 12
4.2.2
Test case 2 and 3 – Single infeed, fault on busbar ......................................................................... 12
4.2.3
Test case 4 – Double infeed, fault on line 1 (high voltage site of the transformer) .......................... 13
4.2.4
Test case 5 – Double infeed, fault on the low voltage site of transformer inside protected area ..... 13
4.2.5
Test case 6 – Double infeed, phase to ground faults on the low voltage site of the transformer .... 14
4.2.6
Test case 7 – Single infeed B, fault on line 1 (high voltage site of the transformer) ........................ 14
4.2.7
Test case 8 – Single infeed A, fault on the low voltage site of transformer inside protected area ... 15
Entering the test cases in RelaySimTest ..................................................................................... 15 4.3.1
Duplicate function ........................................................................................................................... 16
4.3.2
Vary parameters ............................................................................................................................. 16
4.3.3
Measurement and assessment ....................................................................................................... 16
Test sets configuration ...................................................................................................................... 18 5.1
Test setup .................................................................................................................................... 18
5.2
The test sets configuration in RelaySimTest and the Test Set Remote Agent ........................... 19
6
Performing the test ............................................................................................................................. 22
7
List of literature................................................................................................................................... 23
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1
Safety instructions
1.1 Requirements to use this application note This application note may only be used in combination with the relevant product manuals which contain all safety instructions. The user is fully responsibility for any application that makes use of OMICRON products. Instructions are always characterized by a symbol even if they are included to a safety instruction. DANGER Death or severe injury caused by high voltage or current if the respective protective measures are not complied. Carefully read and understand the content of this application note as well as the manuals of the involved systems before starting its practical application. Please contact OMICRON before you continue the process if you do not understand the safety instructions, operating instructions, or parts of it. Follow each instruction mentioned there especially the safety instructions since this is the only way to avoid danger that can occur when working at high voltage or high current systems. Furthermore, only use the involved equipment according to its intended purpose to guarantee a safe operation. Existing national safety standards for accident prevention and environmental protection may supplement the equipment’s manual.
Only experienced and competent professionals that are trained for working in high voltage or high current environments may perform this application note. Additional the following qualifications are required: •
authorized to work in environments of energy generation, transmission or distribution and familiar with the approved operating practices in such environments.
•
familiar with the five safety rules.
•
good knowledge of OMICRON CMC test sets, RelaySimTest and CMGPS 588.
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2
Introduction
2.1 General requirements Before you get started with this application note, read the “Getting Started” manual [1] of RelaySimTest. Please make sure that you also have a good knowledge about the CMC test system.
2.2 What this application note describes The application note describes how End-To-End relay tests of a line differential protection system with a transformer inside of the protected area could be performed using the predefined RelaySimTest template. Therefore it shows the following content. 1. 2. 3. 4.
Line Differential Protection (general information) Defining the System under Test Defining Test cases Defining the Test sets configuration
The application note does not describe single end tests, wiring checks and parameter tests. To test the protection thoroughly such tests are also recommended.
2.3 Line differential protection A line differential protection system compares the current flowing into the protected area with the current flowing out of the protected area. Under normal conditions there should be nearly no difference between this currents. A high differential current (Idiff) indicates a fault on the line. The protection system should switch off the line as fast as possible if a fault occurs.
Figure 1: Protection principle: No fault on the line (left); fault on the line (right)
Some effects like capacitive currents and measurement errors lead to a relatively small differential current even if there is no fault. To prevent unwanted tripping due to these influences the protection system has to © OMICRON 2015
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be stabilized. For this reason many relays calculate a bias- or stabilization current Ibias. Depending on this a relay operating characteristic defines which differential currents have to lead to a trip and which not.
Figure 2: Example of a differential operating characteristic
If a transformer is inside the protected area, the protection system has to take its ratio and vector group into account. Furthermore the stabilization has to be increased, because the magnetization current and the tap changer position will increase the differential current calculated by the protection system. The protected area is defined by the current transformer at the beginning and the end of the line. That means a line differential protection system provides 100% selectivity for this area, but no back-up protection for any other object. Faults outside of the protected area should not lead to a trip of the differential protection system.
Figure 3: Transformer inside the protected area: no fault (left); fault outside of the protected area (right)
Due to the fact that the line ends are distanced from each other, there has to be one relay on each end of the line. For the comparison of the currents a communication between the ends is necessary. With this communication the measured current values are transmitted to the relay at the remote end. This is often realized by optical fiber. Two synchronized CMCs are necessary to test such a distributed system with two ends (see chapter 5.1 ). © OMICRON 2015
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Figure 4: Line differential protection system
For more information about line differential protection see [2].
3
System under Test
3.1 Application example – Protected area The following figure shows the line and the transformer with its line differential protection system that is used as example in the RelaySimTest template. System under Test 50 Hz
Bay A1 110 kV 200 A
A
Bay B1 20 kV 1 kA
CB A 3-pole trip time = 50 ms close time = 100 ms
Trip 110 V Relay A1
CT A 200 A / 1 A
B
CB B same as CB A
Y
D5
communication connection
CT B 1 kA / 1 A
Trip 110 V Relay B1
Figure 5: Example protected area (CT: Current Transformer, CB: Circuit Breaker)
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3.2 Settings of the “System under Test” menu in RelaySimTest The parameter of the substations with their bays and relays according to the example (see chapter 3.1 ) are in the System Under Test Menu. The number colors in Figure 6 correspond to the colors of Figure 5 to show where the different parameters are defined. Comments: 1. Bay A1 and B1: This menu contains the trip and close time of the circuit breaker (CB). With this information the binary outputs of the CMC could simulate the behavior of the CB. However the test template doesn’t use this feature. Furthermore the Bay menu item contains the sub menu Instrument transformers that includes the CT direction setting (see Figure 7). 2. Relay A1 and B1: This menu item contains the test voltage and current limits. Relay manuals specify a voltage and a current that does not cause damage to the relay. The limits of RelaySimTest should be adapted according to the current and voltage limits of the relay manual. Nevertheless it is important to do breaks between the tests to ensure that the relays are not stressed too much!
Figure 6: System under test menu
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Figure 7: Instrument transformers
4
Test cases Figure 8 shows the Test cases Menu with the sub menus Model power system (1), Design test scenario (2) and test steps (3). At first the test template includes only one test case. Chapter 4.2 describes further test cases, while chapter 4.3 shows some useful features to implement them.
Figure 8: Test cases menu
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4.1 Application example – Grid topology The protected area of the example is part of the grid that is shown in Figure 9.
Figure 9: Grid topology
A Phase angle of minus 150° for infeed 2 simulates a no load condition due to the vector group of the transformer (Yd5).
4.2 Suitable test cases This chapter describes test scenarios that fit to the application. Figure 10 gives an overview of the different test cases, while the chapters 4.2.1 to 4.2.7 describe them in detail. In all test cases the relay trip command of relay A and B is observed. Chapter 4.3.3 describes how to implement the trip time measurement and assessment.
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Figure 10: Suitable test cases (1: Charging, magnetization and load current, 2 and 3: Fault on busbar, 4 and 5: Fault inside the protected area, 6: Double phase to ground fault (isolated grid), 7 and 8: Fault inside the protected area with only one infeed)
For the faults in the different test cases (except test case 6, see chapter 4.2.5 ) at least the following Fault types should be used: > > >
L1-N L2-L3 L1-L2-L3
Depending on the relays under test, on the relay’s parameter and on the grid where the protection system is used, it can be necessary to add more fault types. The Initial state of a fault is inactive to get transients in the beginning. The faults of test cases 7 and 8 are exceptions, because high transient currents are not wanted for these tests.
Figure 11: Example for a fault with Initial state "Inactive"
The nominal trip time of the differential protection is 0 s, therefore the simulation time after a fault or switching event is at least 0.5 s. Hence the protection system has enough time to show its reaction on the event.
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Sometimes the behavior of the protection system depends on the prefault condition. For example it might be different if there is a load current during the prefault state. However for this example this distinction is not considered. Some of the following test cases are used to test the protection system with particularly high currents (Test case 2 to 6) and some are used to test it with particularly small currents (Test cases 7 and 8). This is realized by using certain fault conditions like a certain fault location, inception angle or fault resistance. For example the fault resistance RF is set to 0Ω in test cases where the fault currents should be high. However the impedances of the infeeds are not changed. In a real grid these impedances vary due to the different grid topologies that are used. For this reason those test cases which should lead to high currents can be further improved by using the minimum infeed impedances of the real grid. On the other hand test cases which should lead to small currents can be improved by using the maximum infeed impedances. A Siemens 7SD610 relay system was used to test the RelaySimTest template that belongs to this application note. For more information about this relay see number [4] of the bibliography. 4.2.1 Test case 1 – Charging, magnetization and load current This test case should show that the differential protection does not trip, if the charging and magnetization current of the line capacitances and the transformer flows. Furthermore it should show that the differential relays do not trip if a load current flows through the protected area. >
At first the line is switched off – the circuit breaker (CB) on both sites are open.
Figure 12: Test case 1
> >
Afterwards CB A is closed, while CB B stays open. Due to the line capacitances a charging current and due to the transformer a magnetization current flows.
The differential protection system measures this current as differential current, but it should not trip, because there is no fault on the line. > >
After 0.5 s the CB B is closed too. The phase angle of infeed 2 is varied in 5 test steps between -170 and -130°, while the phase angle of infeed 1 stays at 0°. > A load current flows depending on the phase angle difference between infeed 1 and 2. This current flows through the protected line. There is no differential current (except the load and magnetization current). Therefore the differential protection is not allowed to trip.
4.2.2 Test case 2 and 3 – Single infeed, fault on busbar These test cases should show that the differential protection does not trip, if a fault occurs outside of the protected area.
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> >
In test case 2 a fault occurs on busbar A. Only infeed 2 is modeled, it will feed the fault current that flows through the protected line.
Figure 13: Test case 2
> > >
In test case 3 a fault occurs on busbar B. Only infeed 1 is modeled to feed the fault current. The fault inception angle of both test cases is 0° to get high transients.
The fault current flows through the protected line. It differs between test case 2 and 3 due to the different source impedances of infeed 1 and 2. There is no differential current (except the charging and magnetization current). Therefore the differential protection is not allowed to trip. 4.2.3 Test case 4 – Double infeed, fault on line 1 (high voltage site of the transformer) This test case should show that a fault on the protected line on the high voltage site of the transformer leads to a trip of the differential protection. The height of the fault current depends on the fault location. Hence the fault is placed on different Fault locations. > >
Fault locations: 0%, 50%, 100% of line 1. The fault inception angle is 0° to get high transients.
Figure 14: Test case 4
>
A fault on line 1 leads to a differential current. Therefore the protection system has to trip.
4.2.4 Test case 5 – Double infeed, fault on the low voltage site of the transformer inside the protected area This test case should show that a phase to phase and a three-phase-fault on the low voltage (LV) site of the transformer (inside of the protected area) leads to a trip of the differential protection. Furthermore it should show that a phase to ground fault at the location does not lead to a relay trip. > >
The fault occurs on the node where the LV-site of the transformer is connected to the grid. The fault inception angle is 0° to get high transients.
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Figure 15: Test case 5
A phase to phase and a three-phase-fault on the low voltage site of the transformer (inside the protected area) leads to a differential current. Therefore the protection system has to trip. The relays are not allowed to trip if a phase to ground fault occurs, since the fault is in an isolated grid (delta winding of the transformer and infeed 2 has an insulated grounding).
4.2.5 Test case 6 – Double infeed, phase to ground faults on the low voltage site of the transformer This test case should show that the relays trip, if two phase to ground faults occur on the transformer low voltage site. > > >
At first there is only a L1-N fault on busbar B. Its Initial state is active, hence it is always during this test case. After 0.5 s a second phase to ground fault occurs – a L2-N fault on the node where the LV transformer site is connected to the grid.
Figure 16: Test case 6
In the beginning there is only one phase to ground fault (L1-N) in an isolated grid, therefore the relays are not allowed to trip. The second phase to ground fault is in another phase (L2-N), therefore it leads to a differential current. Now the relays have to trip.
4.2.6 Test case 7 – Single infeed B, fault on line 1 (high voltage site of the transformer) This test case represents a fault on line 1 that is characterized by a particularly small differential current. The test is similar to test case 4 but with the following differences: > >
Only infeed 2 is modeled to get a small differential current. The fault is placed at the beginning of the line (Fault location 0% of line 1) to have a high impedance from the system feeding the fault to its location (infeed 2, 100% of the protected line).
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Figure 17: Test case 7
>
A fault resistance of 5 Ohm is used to reduce the fault current. (Number [3] of the bibliography shows how to estimate an arc resistance for a 110kV grid.) > The initial state of the fault is “Active” to suppress transients. The fault has to lead to a trip, because there is a fault inside of the protected area even if the fault current is small. 4.2.7 Test case 8 – Single infeed A, fault on the low voltage site of the transformer inside the protected area This test case represents a fault on the low voltage site of the transformer inside the protected area that is characterized by a particularly small differential current. The test is similar to test case 5 but with the following differences: >
Only infeed 1 is modeled to get a small differential current.
Figure 18: Test case 8
>
A fault resistance of 5 Ohm is used to reduce the fault current. (Number [3] of the bibliography shows how to estimate an arc resistance for a 110kV grid.) > The initial state of the fault is “Active” to suppress transients. A phase to phase and a three-phase-fault must lead to a trip, because there is a fault inside of the protected area even if the fault current is small. The relays are not allowed to trip if a phase to ground fault occurs, since the fault is in an isolated grid.
4.3 Entering the test cases in RelaySimTest The predefined test template that corresponds to this application note includes only test case 1. The other test cases of chapter 4.2 can be realized in the Design test scenario menu (see Figure 8 number 2). This chapter shows some useful features to make it easier to implement them.
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4.3.1 Duplicate function All described test cases use nearly the same power system model. Therefore it is meaningful to use the Duplicate test case function instead of the Add test case button to create another test case. Afterwards it is not much effort to adapt the duplicate to the new test case.
Figure 19: Duplicate Function
4.3.2 Vary parameters All test cases with faults should be performed with different fault types (s. chapter 4.2 ). But it is not necessary to add them step by step. It’s much easier to use the Vary parameters button of the Test steps tab and to choose there the Fault type as varied parameter. Afterwards a list with the different fault types is available, where all selected fault types can be added by one click. Not only the Fault type but also other parameters like the Phase angle of an infeed can be chosen as varied parameter to create different tests in a fast and easy way.
Figure 20: Variations
4.3.3 Measurement and assessment The Define measurements menu of the Test steps tab (see Figure 8 number 3) defines the start and stop event for the trip time measurements. > >
The “start measuring event” is the beginning of the corresponding fault, if the fault has to lead to a trip. The “start measuring event” is the beginning of the simulation (0s) or the beginning of the corresponding fault for tests where the relays are not allowed to trip.
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>
In both cases the trip command is observed. Therefore the moment when the trip signal becomes active is the “stop measuring event”. > The trip time should be measured for relay A and B.
Figure 21: Measurement
The Set assessment condition menu (next to the Define measurements menu) defines the assessment of the test steps. For test points where the relays have to trip the min/max values have to be entered (option “Custom min/max values”, see Figure 17). For those test points where the relays are not allowed to trip the option “No stop event occurs” is the assessment condition.
Figure 22: Custom min/max values
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5
Test sets configuration
5.1 Test setup Due to the distribution of the protection system an end-to-end test of a line differential protection system with two ends requires two CMCs. RelaySimTest offers the possibility to control both CMCs with one main application via internet. For this reason two computers with internet access are necessary – one at the local and one at the remote end. The local PC runs the RelaySimTest main application, the remote PC just a proxy application which takes care of network connection issues and announces the test device to the controlling software application at the other end. This proxy application is the OMICRON Test Set Remote Agent. How to configure RelaySimTest and the Test Set Remote Agent for such an application is described in the next chapter. Figure 23 illustrates the end to end test setup. To perform a test the main application of RelaySimTest on the local PC calculates a fault scenario based on the simulation of the power system network. It calculates the voltages and currents not only for its own end but also for the remote end. The results are used for the end to end test where the main application controls the local and the remote CMC. To perform an end to end test for a line differential protection system synchronized injection of test currents is necessary. The synchronization ensures that both test sets – the local and the remote one – start the test at the same time. This is very important since any inaccuracy can result in an unwanted differential current and thus in an unexpected relay behavior. For this reason RelaySimTest supports the use of CMGPS 588 an antenna-integrated GPS controlled time reference to synchronize the starting point of a CMC test process. Each end needs its own CMGPS 588 (see Figure 23). It delivers a time signal using PTP (Precision Time Protocol). For more information about PTP see [5].
Figure 23: Scheme of an end-to-end test
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5.2 The test sets configuration in RelaySimTest and the Test Set Remote Agent This chapter assumes that the test set up according to chapter 5.1 is already done: > > > > >
The local PC is running RelaySimTest, the remote PC the Test Set Remote Agent. Both have Internet access. The wiring between the CMCs and the relays is already done. The local and the remote CMC are already switched on and synchronized via CMGPS 588. If the connections between the CMCs and the PCs are done by Ethernet, it has to be ensured that both CMCs are associated to the local PC.
The Test sets configuration menu (local PC) defines the CMCs and their configuration used for the tests. At first the test template includes two general CMCs as shown in Figure 24. The label on the left site of the CMC icon shows that the first CMC belongs to substation A and the second one to substation B.
Figure 24: Test sets configuration menu of RelaySimTest (local PC)
After a click on the “Choose test set” button on the right site of the CMC icon, a new window opens and offers the CMC that is connected to the local PC. A click on the local CMC selects it.
Figure 25: Local CMC is selected
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The Test Set Remote Agent on the remote PC has to open an Internet session before RelaySimTest can control the remote CMC. The upper part of the Test Set Remote Agent shows the remote CMC. If the correct CMC is not already selected, it has to be chosen by using the “Change test set” button. A click on “Grant remote access” announces the remote CMC on an Internet server. After a short time the Test Set Remote Agent displays a session ID for the Internet session. The software offers also the possibility to use a session password, but this is optional.
Figure 26: Test Set Remote Agent (remote PC)
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A click on “Connect to remote test set” in the Test sets configuration menu of RelaySimTest (local PC) opens a window to enter the session ID from the remote end (see Figure 27).
Figure 27: Connect to remote test set (local PC)
Afterwards RelaySimTest displays both CMCs – the local and the remote one. Figure 28 illustrates this. The test sets are ready for time synchronized injection due to the use of CMGPS 588. The green watches next to the CMC icons indicate that.
Figure 28: Local and remote CMC connected to RelaySimTest
The Getting Started manual of RelaySimTest [1] describes how the wiring between the CMCs and the relays can be configured in the Test sets configuration menu.
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6
Performing the test Before a test is started it is strongly recommended to do a wiring check. The execute buttons start the tests. There are different execute buttons - “Execute all” and “Execute selected”. What they mean depends on the menu where they are: >
If a test case is open a click on the “Execute selected” button executes only the selected test step. A click on the “Execute all” button runs all test steps of the test case sequentially (see Figure 29 top). > In contrast to this in the Test Manager menu the “Execute selected” button runs all selected test cases, while “Execute all” runs all test cases (see Figure 29 bottom). Especially after tests with high currents it is meaningful to interrupt the test sequence with the Stop button. This ensures breaks between the tests to avoid too much stress for the relays.
Figure 29: “Execute selected” and “Execute all” button for a certain test case (top figure) and in the Test Manager menu (bottom figure)
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7
List of literature [1] Getting started with RelaySimTest; OMICRON electronics GmbH; 2014 [2] “Numerical Differential Protection: Principles and Applications”; second edition; Gerhard Ziegler; Publicis MCD; 2012 [3] “Digitaler Distanzschutz: Grundlagen und Anwendungen”; second edition; Gerhard Ziegler; Publicis MCD; 2008 (English version is also available) [4] „SIPROTEC Differential Protection 7SD610 V4.6“, SIEMENS [5] “Implementation and Transition Concepts for IEEE 1588 Precision Timing in IEC 61850 Substation Environments”; B. Baumgartner, C. Riesch, M. Rudigier; OMICRON electronics GmbH
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