Application Note
System-based Testing of Line Distance Protection with Power Swing Blocking Author Jens Baumeister |
[email protected] Date February 28, 2017 Related OMICRON Product CMC - RelaySimTest Application Area Power Swing Detection Keywords RelaySimTest, Power Swing, System-based Testing, Version v 1.0 Document ID ANS_17009_ENU
Abstract This application note describes the Power Swing Test Template of the RelaySimTest Software.
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.
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Table of content 1
Safety Instructions ................................................................................................................................4
2
About this Application Note .................................................................................................................5
3
4
2.1
General Requirements....................................................................................................................5
2.2
What this Application Note describes .............................................................................................5
2.3
Power Swing ...................................................................................................................................5
System Under Test ............................................................................................................................. 11 3.1
Application Example – Protected Line and Time Grading ........................................................... 11
3.2
Settings of the System Under Test Menu in RelaySimTest ........................................................ 12
Test Cases ........................................................................................................................................... 14 4.1
5
Suitable Test Cases ..................................................................................................................... 14 4.1.1
General Test Information ................................................................................................................ 14
4.1.2
Synchronize to TransView Feature ................................................................................................. 16
4.1.3
Test Case 1 and 2 – Stable Power Swing....................................................................................... 17
4.1.4
Test Case 3 and 4 – Stable Power Swing with Load ...................................................................... 19
4.1.5
Test Case 5 and 6 – Stable Power Swing with Fault ...................................................................... 20
4.1.6
Test Case 7 and 8 – Unstable Power Swing ................................................................................... 21
4.1.7
Test Case 9 and 10 – Unstable Power Swing with Load ................................................................ 23
4.1.8
Test Case 11 and 12 – Unstable Power Swing with Fault .............................................................. 23
List of Literature ................................................................................................................................. 24
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1
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.
DANGER Death or severe injury caused by high voltage or current. Before wiring up or rewiring the equipment always turn off each system involved to the test process. WARNING Equipment damage or loss of data caused by high voltage or current possible. Before wiring up or rewiring the equipment always turn off each system involved to the test process. Before starting a test always check that the test signals are suitable for your system under test.
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 and protection testing.
•
Good knowledge of the OMICRON CMC test sets and RelaySimTest.
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2
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 The application note describes the relay tests of the RelaySimTest predefined power swing template. It covers the following content of the test template. 1. 2. 3. 4.
Power Swing (general information) System Under Test Test Sets Configuration Test Cases
The application note does not describe wiring checks and parameter tests. To test the protection thoroughly such tests are also recommended.
2.3 Power Swing In a power grid static load flow is generated by a stationary voltage difference. In a mostly inductive grid the difference is mainly given by an angle deviation between the ends in order to transport active power while a difference in magnitude would produce reactive power. Figure 1 shows a grid with two infeeds, while Figure 2 illustrates the corresponding node voltages and the grid current.
Line 1 Z L
Infeed 1 D I
V Q1
Z Q1
A
V ZQ1
V V AE
L
Infeed 2 B
Z Q2
V ZQ2
C
V Q2
E Figure 1: Power Transmission
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C V ZQ2 V Q2
B
V
L
A
V ZQ1 D
ϑ
V AE
E
I V Q1
Figure 2: Voltages and Current
According to Figure 2 the source voltage VQ1 is leading compared to VQ2. That means there is an active power transmission from infeed 1 to infeed 2. The amount of power depends on the impedances, on the magnitudes of VQ1 and VQ2 and last but not least on the angle between them. If the active resistances of ZQ1, ZL and ZQ2 are neglected and only X as the sum of their reactive resistances is taken into consideration Equation 1 describes the amount of transmitted active power. Equation 1
𝑃 = 𝑉𝑄1 ∗
𝑉𝑄2 ∗ sin ϑ 𝑋
During normal operation of the power grid the actual load always equals the generation. But caused by load changes, switching operations and faults this balance can be disturbed. Power grids with their connected electrical machines and their control systems are dynamic systems that react on any transient disturbance by a usually damped oscillating reaction (due to the inertia of the electrical machines and the control systems) until a new stable state is reached. Any change in active power consumption causing the generators to accelerate or decelerate until the supplied real power equals the consumed real power and all oscillations have ceased due to damping elements in the system. Acceleration and deceleration of generators mean increasing and decreasing angles between there generated voltages and consequently increasing and decreasing grid currents causing oscillating power. This means a power swing takes place. The static stability limit of a generator is at a load angle of 90°. At this operating point the generator delivers its maximum active power. During dynamic processes like a power swing the load angle can be higher than 90°. As long as the electrical load of the generator is higher than the power supplied to its turbine (e. g. water or steam power) the generator can fall back to a stable condition with a load angle smaller than 90°. In that case it is a stable power swing. But it can also happen that generators do not find a stable condition after the disturbance. This means their load angle increases continuously and they fall out of step.
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If the power swing is stable, it is not necessary to disconnect generators or loads. To stop an unstable power swing it is necessary to split the grid at certain predefined nodes to get a stable condition again, where the actual load equals the generation. The impedances of the grid shown in Figure 1 appear if the voltages of Figure 2 are divided by the current. Figure 3 shows the impedances.
X
C Z Q2 B ZL
Z Load A
Z Q1
ϑ
E R
D Figure 3: Impedance Plane
Figure 1 assumes node A as location of a distance protection relay. During a power swing the load impedance ZLoad measured by the distance relay changes depending on the voltage angle changes of VQ1 and VQ2. If both source voltages have the same magnitude, the impedance ZLoad would move on a line, while angle ϑ changes. Figure 4 illustrates this. The figure shows also a tripping zone of the distance protection function. It is possible that during a power swing the impedance measured by the relay moves into such a tripping zone. But the distance protection function should not trip during a power swing, if there is no grid fault.
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X
C distance protection tripping zone
Z Q2 B
ZL Z Load A
Z Q1
E
E
Z Load I
ϑ
E R
D Figure 4: ZLoad calculated by relay A changes during a power swing
But how does the relay distinguish between a stable power swing, an unstable one and a fault? While the impedance change during a power swing is rather slow, the impedance vector jumps directly into the tripping zone at a fault occurrence. At a stable power swing the measured impedance enters the distance protection zones from one side, then turns around and leaves at the same side. During an unstable power swing the impedance crosses the X axis completely and leaves the zones on the other side. Therefore the angle ϑ increases continuously. Figure 5 illustrates an unstable power swing. The moment when ZLoad crosses the dashed red line, angle ϑ is 180° - this is the theoretical limit for a stable power swing.
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X
C Z Q2 B
ZL E
E
ϑ
Z Load A Z Q1
I
E R
D Figure 5: Load Impedance during an Instable Power Swing
The relay manufacturers developed different algorithms to decide which case is present. The relay could use for example a power swing detection zone. This zone surrounds the tripping or the starting zones of the distance protection. If the impedance is calculated within this area for a given number of measurement cycles, it is recognized as slowly changing and thus a power swing. Another possibility is to determine the time the trajectory needs to move through this area. Position and size of this frame are important parameters and can be set for some protection relays.
X
C distance protection tripping zone
Z Q2 B
power swing detection zone
ZL
E Z Load
A
Z Q1
I
E
ϑ
E R
D Figure 6: Power Swing Detection Zone © OMICRON 2017
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There are two different power swing functions in modern numerical relays: 1. Power Swing Blocking: The whole distance protection or only assigned zones are blocked when a power swing occurs. 2. Power Swing Tripping: The relay trips after detecting an unstable power swing. Whether any one of these functions is used or if even both functions are used depends on the requirements at the corresponding grid node where the distance protection relay is installed. As example for this Application Note it is assumed that the distance protection relay uses only the first function (Power Swing Blocking). For more information about power swing see [2] and [3].
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3
System Under Test
3.1 Application Example – Protected Line and Time Grading The following figure shows the power system protected by a distance protection relay with power swing blocking that is used as example in the RelaySimTest template.
Z1
85%
Z2
Figure 7: Example – Line protected by Distance Protection Relay using Power Swing Blocking
Figure 7 shows that the distance relay protects the line with Zone 1 (Z1) and 2 (Z2), while Figure 8 illustrates the time grading of the distance protection. The first zone has no time delay, which means the nominal trip time is 0 s. In the second zone the nominal trip time is 400 ms.
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Z2
0.4 s 0s
Z1 A
Figure 8: Time Grading
Figure 9 shows distance protection zones of the example in the impedance pane using secondary values.
Figure 9: Distance Zones - Impedance Pane
3.2 Settings of the System Under Test Menu in RelaySimTest The power system parameters according to the example (see chapter 3.1 ) are in the submenu Power system of the menu System Under Test. A click on an element like “Infeed A” opens the associated input mask on the right side of the screen.
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Figure 10: Power System
A double click on the protection relay opens the corresponding configuration menu. It contains details like general settings and device connections as well as the description of signal in- and outputs. The distance protection relay is connected to VT and CT A as well as to CB A (trip signal).
Figure 11: Protection relay
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4
Test Cases
4.1 Suitable Test Cases 4.1.1 General Test Information According to [4] a power swing in the European UCTE-grid (Union for the Co-ordination of Transmission of Electricity) occurs with a frequency of 0.2 to 1.5 Hz. Therefore the test template that corresponds with this application note uses a power swing frequency of 1 Hz. For another grid the frequency has to be adapted. When a power swing is induced with both ends reacting in a differing manner (e. g. swing speed, magnitude) the voltage difference between the infeeds will vary accordingly, driving a similarly varying current. For easier interpretation it is assumed that infeed A is not swinging at all while infeed B performs a phase slip. The phase slip causing a stable power swing is defined by the slip angle and the slip time period. Both parameters are defined in the power system of the corresponding Test Case. At the beginning of each test case the switch at infeed B is open. After a certain time it is closed to start the power swing. The time of this close event depends on the slip time period and on the power system frequency: 𝑡𝑒𝑣𝑒𝑛𝑡 = 𝑇𝑆𝑙𝑖𝑝𝑇𝑖𝑚𝑒𝑃𝑒𝑟𝑖𝑜𝑑 − 10 ∗ 𝑇𝑓−𝑝𝑒𝑟𝑖𝑜𝑑 tevent is the time of the close event TSlipTimePeriod is the slip time period Tf-Period is the time period of the power system’s frequency RelaySimTest does not show the first ten periods after the simulation start in order to start at steady state conditions for a given power system. Hence the displayed simulation time 0 s is indeed simulation time 10*Tf-period (200 ms for a 50 Hz system). The starting point of a power swing period is therefore defined by the formula standing above.
Figure 12: Phase slip parameters
For a stable power swing the phase slip of infeed B is enabled, its slip time period is 1000 ms to realize a power swing frequency of 1 Hz. The power swing starts at the beginning of the simulation, after the slip time period the switch at infeed B is opened to stop the stable power swing. The slip angle of infeed B defines the maximum phase difference between both ends that occurs during the slip time period.
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Figure 13: Test scenario – stable power swing
To create an unstable power swing these two parameters are not used, instead a frequency difference of 1 Hz between the two infeeds defines the unstable power swing frequency. After one power swing period (1 s) the switch at infeed B is opened to stop the unstable power swing. Sometimes it can be necessary to test with multiple power swing cycles, in that case the opening of this switch has to be adapted. To keep the template simple, test cases with faults use only the following Fault types: > > >
L1-N L2-L3 L1-L2-L3
Depending on the relay under test, on the relay’s parameters and on the grid where the protection system is used, it can be necessary to add more fault types. All test cases should have an adequate simulation time to cover the whole test scenario and to ensure that the relay has enough time to show its reaction on the test. The test template uses by default the following binary relay signals: > > > > >
Distance Protection Trip Distance Protection Pickup Power Swing Detection Power Swing Unstable Power Swing Trip
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The following chapters describe the different test cases of the template in detail. To use the template power system and test cases have to be adapted to the respective application, because each protection system is very individual. In all test cases the distance protection trip command of the relay is observed. Chapter Error! Reference source not found.describes how to measure and to assess the trip times. 4.1.2 Synchronize to TransView Feature The OMICRON TransView software is useful to analyze the test results. Here it is possible to display the impedance and power trajectories resulting from the test voltages and currents. The feature Synchronize to TransView in RelaySimTest offers the export of the signals.
Figure 14: Synchronize to TransView Button
It is also possible to display the distance zones in the TransView Circle Diagrams, if they are available as RIO file for example in the Menu Test Object of the OMICRON Test Universe. The RIO file can be exported from the Test Universe Test Object as follows: Select File in the Test Object main menu. Select Export … in the submenu. Select RIO file type and enter %TEMP%\SIMULATIONTEST.RIO as file name. The RIO file will be saved in the same location where the RelaySimTest signal export is saved (C:\Users\UserName\AppData\Local\Temp). Save the file.
Figure 15: RIO export from the menu Test Object of the OMICRON Test Universe
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Arrangement: In this application note the impedance resulting from the test voltages and currents is called ZLoad. 4.1.3 Test Case 1 and 2 – Stable Power Swing These test cases should show, that the relay detects a power swing. Furthermore it should show that its power swing blocking function prevents the tripping of the distance protection even if the measured impedance is within a distance protection tripping zone. >
For test case 1 (TC 1) a slip angle of -180° is used. This angle is the theoretical limit for a stable power swing. It ensures that the impedance ZLoad resulting from the test voltages and currents is close to the line impedance. Hence it is probably seen in Zone 1 (Z1) of the distance protection function. Figure 16 shows the impedance pane while Figure 17 illustrates the power swing. Both figures were created with the Synchronize to TransView feature.
Figure 16: Test Case 1 - Impedance Pane
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Figure 17: Test Case 1 – Power Swing
>
For test case 2 a slip angle of +180° is used. While in TC 1 infeed A leads infeed B during the power swing, in TC 2 the situation is reversed. Therefore in TC 1 the impedance ZLoad moves from the right to the left site of the impedance plane and in TC 2 it moves from the left to the right site.
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Figure 18: Test Case 2
→ During both test cases the impedance ZLoad moves into the distance protection tripping zones. Hence the distance protection may pickup, if the pickup criteria are fulfilled. The relay should detect the stable power swing. Therefore it should not trip. 4.1.4 Test Case 3 and 4 – Stable Power Swing with Load These test cases are similar to test case 1 and 2. But in contrast to them now a load current exists due to different phase angles of the infeeds. > If there is no phase angle difference between the infeeds, the impedance ZLoad is infinity and no load current is flowing. For test case 3 the parameter phase angle of infeed A is 20° while the phase angle of infeed B stays at 0°. This causes a load current and changes the impedance ZLoad. That means while in all previous test cases at the beginning of a power swing period ZLoad was infinity (phase angle of infeed A and B are 0°), now it differs from infinity. It has a certain finite value that depends on the phase angle difference of the infeeds. It is useful to take an angle difference that fits to the system under test. Figure 19 shows a power swing with a finite starting point. > For test case 3 a slip angle of -160° is used. That means the maximum phase angle difference between the infeeds is -180° (-20° phase angle and –160° slip angle).
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Figure 19: Finite Starting Point
> For test case 4 the phase angle of infeed A is -20°, while the phase angle of infeed B stays at 0°. Furthermore a slip angle of +160° is used. → In the beginning of both test cases the impedance ZLoad has a finite value due to the phase angle difference of the infeeds. During the power swing this impedance changes and moves into the distance protection tripping zones. Hence the distance protection may pickup, if the pickup criteria are fulfilled. The relay should detect the stable power swing. Therefore it should not trip. 4.1.5 Test Case 5 and 6 – Stable Power Swing with Fault These test cases should show, that the relay detects a fault during a power swing and that it reacts to it in an appropriate manner. > The fault location defines the tripping zone the relay has to use. For the first test steps the fault is located at 50% of the line (Zone 1 of the distance protection). For the last test steps it is located at 100% (Zone 2). > The moment when the fault occurs defines the impedance jump, from the “power swing” to the “fault” impedance. Figure 20 illustrates this. It is useful to create different test cases with different times for the fault occurrence. To keep the test template simple it uses only one where the fault occurs 0.25 s after the beginning of the power swing (slip time period is 1 s). > For test case 5 a slip angle of -180° is used. > For test case 6 a slip angle of 180° is used.
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Figure 20: Test Case 5
→ In the beginning of the power swing the impedance ZLoad moves relatively slowly. The relay should detect the power swing. The moment the fault occurs ZLoad jumps into the distance protection tripping zones. The distance protection has to trip using the corresponding tripping zone. 4.1.6 Test Case 7 and 8 – Unstable Power Swing These test cases should show, that the relay detects an unstable power swing. Furthermore it should show that its power swing blocking function prevents the tripping of the distance protection even if the measured impedance is within a distance protection tripping zone. (According to the application example, the relay should not trip even if there is an unstable power swing, s. chapter 2.3 .) > For test case 7 the parameter actual frequency of infeed B is set to 49 Hz. > For test case 8 this parameter is set to 51 Hz. > For both test cases the frequency of infeed A is 50 Hz. > Figure 21 illustrates the unstable power swing of test case 7
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Figure 21: Test Case 7 – Unstable Power Swing
→ The absolute value of the phase angle difference between the infeeds increases in both test cases from 0° to 360°. That means both test cases show an unstable power swing. In TC 7 the impedance ZLoad moves from the right to the left site of the impedance plane, in TC 8 the situation is reversed. In both © OMICRON 2017
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cases the impedance crosses the distance protection tripping zones. The relay has to detect the unstable power swing and the power swing blocking function has to prevent the distance protection from tripping. 4.1.7 Test Case 9 and 10 – Unstable Power Swing with Load These test cases are similar to test case 3 and 4. But in contrast to them now an unstable power swing is used instead of a stable one. > The unstable power swing is realized in the same way as in test cases 7 and 8. → Due to the unstable power swing, the impedance ZLoad crosses the distance protection tripping zones in the impedance plane. The relay has to detect the unstable power swing and the power swing blocking function has to prevent the distance protection from tripping. 4.1.8 Test Case 11 and 12 – Unstable Power Swing with Fault These test cases should show, that the relay detects a fault during an unstable power swing and that it reacts to it in an appropriate manner. The test cases are similar to the test cases 5 and 6. But now an unstable power swing is used instead of a stable one. > The unstable power swing is realized in the same way as in test cases 7 and 8. → In the beginning of the power swing the impedance ZLoad moves relatively slowly. The relay should detect the power swing. The moment the fault occurs ZLoad jumps into the distance protection tripping zones. The distance protection has to trip using the corresponding tripping zone.
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5
List of Literature [1] “Getting started with RelaySimTest”; OMICRON electronics GmbH; 2017 [2] “Numerical Distance Protection: Principles and Applications”; 4th edition; Gerhard Ziegler; PUBLICIS; 2011 [3] “SIPROTEC Distance Protection 7SA6 V4.70 Manual”, SIEMENS [4] “TransmissionCode2007 – Netz und Systemregeln der deutschen Übertragungsnetzbetreiber“, Verband der Netzbetreiber – VDN – e.V. beim VDEW, Version 1.1, August 2007
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