Application Note
System-based Testing of Transformer Differential Protection Author Florian Fink |
[email protected] Date May 12, 2017 Related OMICRON Product CMC – RelaySimTest Application Area Transformer differential protection Keywords RelaySimTest, System Testing, Transformer differential protection Version v2.0 Document ID ANS_16001
Abstract Due to the high short-circuit power on power transformers and to guarantee the continuity of power supply, it is necessary to switch off appearing faults selectively and in a short time. The transformer differential protection can provide these functionality for different applications. It serves as the main protection of transformers in case of winding failure. The protective zone of a differential protection includes all the equipment between the current transformers on the primary and secondary side of the power transformer. To test such a protection system properly, a simulation of the transmission ratio and phase shift is necessary. This application note describes how this could be done in an easy and comfortable way using RelaySimTest. RelaySimTest offers simulation based system testing methods. To perform a test a fault scenario is calculated based on the simulation of the power system. The resulting voltages and currents for the different relay locations can be used to test the correct behavior of the differential protection system.
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 2017. All rights reserved. This application note is a publication of OMICRON.
© OMICRON 2017
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Table of content 1
Safety instructions ................................................................................................................................4
2
About this application note ..................................................................................................................5 2.1
General requirements .....................................................................................................................5
2.2
What this application note describes ..............................................................................................5
2.3
Template .........................................................................................................................................5
3
Transformer Differential Protection .....................................................................................................6
4
System under Test ................................................................................................................................8 4.1
5
Test Cases ..............................................................................................................................................9 5.1
6
Application Example – Protected area ...........................................................................................8 Suitable Test Cases ........................................................................................................................9 5.1.1
Test Case 1 – Stable Load Flow ..................................................................................................... 10
5.1.2
Test Case 2 – Fault on HV Busbar ................................................................................................. 10
5.1.3
Test Case 3 – Fault on LV Busbar .................................................................................................. 11
5.1.4
Test Case 4 – Fault on HV Winding ................................................................................................ 11
5.1.5
Test Case 5 – Fault on LV Winding ................................................................................................ 12
5.1.6
Test Case 6 – Double ground fault on LV Side ............................................................................... 12
List of literature................................................................................................................................... 13
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Safety instructions This application note may only be used in conjunction 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 with. Carefully read and understand the contents of this application note (as well as the manuals of the systems involved) before you start to work with it. Please contact OMICRON Support if you have any questions or doubts regarding the safety or operating instructions. Follow each instruction listed in the manuals particularly the safety instructions, since this is the only way to avoid danger that can occur when working at high-voltage or high current systems. Only use the equipment according to its intended purpose to guarantee safe operation. Existing national safety standards for accident prevention and environmental protection may supplement the equipment’s manual.
Only experienced and competent professionals who are trained for working in high-voltage or high current environments may perform the applications in this document. In addition 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 CMC test sets.
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About this application note
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 This application note describes how transformer differential protection systems could be tested using RelaySimTest. Therefore it shows the following content: 1. Transformer Differential Protection (general information) 2. Defining the System under Test 3. Defining Test cases The application note doesn’t describe wiring checks and parameter tests. To test the protection properly such tests are also recommended.
2.3 Template For this application note a corresponding template is installed with setup of RelaySimTest. The template Transformer Differential Protection.rstt contains the same system under test and test cases described in the application note.
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Transformer Differential Protection A transformer 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 these currents. The protected area of a transformer differential could also contain busbars and cable feeding a transformer. Differential Relay
Protected area
I1̅
I2̅
Figure 1: Protection Principle: No fault in the protected area
A significant differential current indicates a fault inside of the protected area and the protection system should switch off the protected area as fast as possible. Differential Relay
Protected area
Fault in protected area
I1̅
I2̅
Figure 2: Protection Principle: Fault inside the protected area
Some effects like measurement errors can lead to differential current even if there is no fault. To prevent unwanted tripping due to these disturbances the protection system has to be stabilized. For this reason transformer differential relays calculate a stabilization current Ibias as a function depending on the load flow through the transformer. The operating characteristic defines depending on Ibias which differential currents will trip and which not.
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Figure 3: Example of a differential operating characteristic
The protected area is defined by the installation points of both current transformers. The protection system has to take the ratio and vector group of the transformer into account. That means a transformer differential protection system provides 100% selectivity for the protected area, but no back-up protection. Faults outside of the protected area should not lead to a trip of the differential protection system. Differential Relay
Fault outside of the protected area
Protected area
I1̅
I2̅ IF̅
Figure 4: Protection Principle - Fault outside the protected area
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System under Test
4.1 Application Example – Protected area Figure 5 shows the transformer with its differential protection system which is used as an example.
Figure 5: Example protected transformer (CT: Current Transformer, CB: Circuit Breaker)
CB settings: Type: Trip time: Close time:
3-pole 20 ms 50 ms
CT settings: CT A: CT B:
250/1 A 2500/1 A
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Test Cases
5.1 Suitable Test Cases This chapter describes test scenarios that fit the application. Figure 6: Suitable test cases gives an overview of the different test cases, while the following chapters describe them in detail. Because each protection scheme is very individual, these test cases are just examples for tests. They should explain testing methods with RelaySimTest in general. Suitable test cases are: Stability Tests: 1. Stable load flow 2. Fault on HV Busbar 3. Fault on LV Busbar Faults in Protected Area: 4. Fault on HV Winding 5. Fault on LV Winding 6. Double Ground Fault on LV Side
2
4
5
3
1
1
1
1
Figure 6: Suitable test cases
For the faults in the different test cases 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, you may need to add more fault types. 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. Sometimes the behavior of the protection system depends on the prefault condition e.g. load current. However, for this example, this distinction is not considered.
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5.1.1 Test Case 1 – Stable Load Flow This test cases checks that the differential protection does not trip, if load flow passes the protected area. The inrush of the transformer is not simulated by the network model.
Figure 7: Stable Load flow
→ This test case is the initial test. If it fails there could be a wiring or configuration error (e.g. wrong vector group of the transformer or CT direction/ratio). 5.1.2 Test Case 2 – Fault on HV Busbar These test cases checks that the differential protection does not trip, if a fault occurs outside of the protected area. The relay will also see a differential current for phase-ground faults although there is no infeed on the secondary side. This is due to the fact the Y-vector group is grounded the zero sequence component must be eliminated by the relay. > In test case 2 a fault occurs on busbar A.
Figure 8: Test case 2 – Fault on primary busbar
→ The fault current doesn’t flow through the protected area. There is a differential current on the HV side of the transformer must be compensated by the protection system. Therefore the differential protection is not allowed to trip.
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5.1.3 Test Case 3 – Fault on LV Busbar These test cases shows that the differential protection does not trip, if a fault occurs outside of the protected area. > In test case 5 fault occurs on busbar B.
Figure 9: Test Case 5 – Fault on secondary busbar
→ The fault current flows through the protected area. There is no differential current. Therefore the differential protection is not allowed to trip. 5.1.4 Test Case 4 – Fault on HV Winding This test case checks that a fault on the on the high voltage side of the transformer leads to a trip of the differential protection. >
The fault inception angle is 0° to challenge the protection with a high DC component.
Figure 10: Test Case 3 – Fault on primary winding
→ A fault on the primary winding side leads to a differential current. Therefore the protection system has to trip.
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5.1.5 Test Case 5 – Fault on LV Winding This test case checks that a fault on the on the low voltage side of the transformer leads to a trip of the differential protection. > The fault inception angle is 0° to get high transients
Figure 11: Test Case 4 – Fault on secondary winding
→ In case of a phase-phase fault a fault on the secondary winding side leads to a differential current. Therefore the protection system has to trip. Because of the delta winding the low voltage side of the transformer is galvanically isolated against the primary system. In the case of phase-ground faults the system is not able to trip (except there is an extra protection system to detect and trip these faults). 5.1.6 Test Case 6 – Double ground fault on LV Side These test cases checks that the differential protection does trip, if a double ground fault occurs inside or inand outside of the protected area. It is a realistic scenario that a ground fault outside of the protected area can cause a fault at the secondary winding. Winding insulation can fail because of higher phase-neutral voltage in case of a ground fault. > In test case 6 ground fault occurs on busbar B and after a few cycles a ground fault occurs at the secondary winding. The inception angle of the fault is set to 90°. This is the voltage peak that can often lead to an insulation breakdown.
Figure 12: Test Case 6 – Double ground fault on low voltage side of the transformer
→ Double ground fault inside or in- and outside of the protected area leads to a differential current. Therefore the protection system has to trip.
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List of literature [1] Getting started with RelaySimTest; OMICRON electronics GmbH; 2017 [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
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