Cmc Ans 11013 Enu

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Handling Zero Sequence currents when testing diff relays

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  • OMICRON Page 1 of 27

    Application Note

    How to handle Zero Sequence Elimination in Test Universe

    Author Ren Mathis | [email protected]

    Date Sept 25th, 2012

    Related OMICRON Products Test Universe, CMC Test Sets

    Application Area Protection Testing (Differential Protection)

    Keywords Zero Sequence Elimination, Calculation, Reference Winding

    Version v1.2

    Document ID ANS_11013_ENU

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    Content

    Introduction to zero sequence elimination ........................................................................................ 3 1 Why is zero sequence elimination necessary?.............................................................................. 3 1.1 YD interposing transformers .......................................................................................................... 5 1.2

    Calculating the corrected output currents ......................................................................................... 6 1.2.1 Example calculations of YD1 interposing transformers ..................................................................... 9 1.2.2

    YDY interposing transformers ....................................................................................................... 9 1.3 General ............................................................................................................................................. 9 1.3.1 Example calculation of YDY0 interposing transformers: ................................................................. 10 1.3.2

    IL-I0 Numerical Zero Sequence Elimination ................................................................................ 11 1.4 Formulas of commonly used interposing transformers ............................................................... 12 1.5

    Effect of different zero sequence elimination settings on the calculation ................................... 14 2 Importance of zero sequence elimination setting for testing the IDIFF/IBIAS characteristic ............ 14 2.1 Zero sequence elimination setting in Diff Operating Characteristic module (example) .............. 15 2.2 Zero sequence elimination setting in the Diff Configuration module ........................................... 17 2.3

    Choosing the correct Reference Winding ........................................................................................ 19 3

    Configuring the Test Object with interposing transformers .......................................................... 21 4

    Working with Ground Current Measurement inputs (CT)............................................................... 21 5 General ........................................................................................................................................ 21 5.1 Simulating the additional CT with the CMC ................................................................................. 22 5.2

    Appendix ............................................................................................................................................. 23 6 Where the factor comes from ................................................................................................. 23 6.1 Interposing CT selection guide .................................................................................................... 24 6.2

    Please use this note only in combination with the related product manual which contains several important safety instructions. The user is responsible for every application that makes use of an OMICRON product. OMICRON electronics GmbH including all international branch offices is henceforth referred to as OMICRON. OMICRON 2011. All rights reserved. This application note is a publication of OMICRON.

    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 in part, is not permitted.

    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.

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    Introduction to zero sequence elimination 1

    Why is zero sequence elimination necessary? 1.1

    An example where zero sequence elimination is necessary can be seen in Figure 1. There is an outside earth fault which causes the relay to trip because of a measured differential current. Please note that a differential protection relay must not trip in case of outside faults! In this example, the current magnitudes in Figure 1 are displayed by the number of arrows. Also, assume that the ratios between HV side and LV side of the transformer and as well the CT ratios are one. If the currents on the HV Side are compared with the currents on the LV side, it can be seen that the currents flowing into the relay are not equal. This unbalanced sum of currents can only occur when one side of the transformer is grounded. The reason for that can be explained with the following example. Think about a fault on the LV side like in Figure 1, the fault current from the LV side will be transferred to the HV side. Now the sum of currents in the star point has to be zero after Kirchhoff's 1st law. Therefore the half of the transformed fault current will flow back through the healthy phases. This will lead to differential currents, because the currents in the healthy phases of the LV side are 0. As shown in Figure 1, the reference arrows indicate the direction towards the protected object on both sides. This definition is valid for the whole document and as well for all settings in the Test Universe Advanced Differential Modules.

    Figure 1: YYN0 transformer with a fault on the LV-side

    In the event of a line-neutral fault, the circuit for the zero sequence component of the fault current closes via the grounded star point of the transformer on the LV side, which lies within the transformer differential protection zone. Therefore, this zero sequence component appears as a differential current in the measuring system of the differential protection relay. For this reason, the zero sequence component of the three-phase system must be eliminated from the phase currents on the LV side. The following calculation examples show how the relay behaves if the zero sequence component is eliminated, and what happens if the zero sequence component is not eliminated. This elimination is done either by the relay or by an interposing transformer. [1] The amount of current is again displayed by the number of arrows.

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    Current on HV side ( ); Current on LV side (

    );

    Calculation without elimination of the zero sequence component:

    Bias current: | | | | ( ) (

    ) (

    )

    Differential current: | | |( ) (

    )| (

    )

    The differential relay would trip

    The result of the differential current calculation would lead to a trip of the differential protection relay, although the fault is located outside of the protected area. In case of outside faults the relay should not determine a differential current.

    Calculation with elimination of the zero sequence component:

    Zero sequence current:

    ( ) (

    );

    ( ) (

    )

    Corrected HV current: ( ) (

    ) (

    )

    Corrected LV current: ( ) (

    ) (

    )

    Bias current: | | | | ( ) (

    ) (

    )

    Differential current: | | |( ) (

    )| (

    )

    The differential relay would not trip Therefore, the zero sequence elimination function is necessary in order to distinguish a real fault inside the transformer from an outside fault. It is recommended to use the Advanced Differential modules for testing three-phase differential protection systems, because these modules are able to automatically calculate the adequate output currents. They take into consideration the vector group, type of zero sequence elimination, IBIAS calculation, and others. Please note that it is also possible to use the State Sequencer or QuickCMC modules for these tests, but then all calculations must be done manually. The goal of this application note is to provide assistance with choosing the correct zero sequence elimination settings in Test Universe.

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    In general, there are two possibilities for how this zero sequence elimination can be done. The relay can do it with an implemented numerical algorithm, or it can be done physically by using an interposing transformer in the current path before the relay. As we already had a small example (Figure 1) of how the mathematical zero sequence elimination can be done with help of the symmetrical components (IL-I0), we want to have a more detailed look at interposing transformers. As the name suggests, an interposing transformer is installed between the secondary winding of the main CT and the relay in order to correct the current flowing to the relay. The interposing transformer can be used on the high voltage side and/or the low voltage side of the power transformer being protected. If it is used on the grounded side, it provides a convenient method for establishing a delta connection for the elimination of zero sequence currents. It is important to find out which zero sequence elimination method is used in the protection system in order to be able to choose the correct settings in Test Universe as seen in Figure 2. In the following chapters, there is further information on how to deal with interposing transformers.

    Figure 2: Zero Sequence Elimination settings in the Test Object

    YD interposing transformers 1.2

    Before the invention of digital relays, interposing transformers were mainly used together with electromechanical relays for phase-matching, amplitude correction and zero sequence elimination purposes as seen in Figure 3. However, they can also be found in transformer protection systems with newer relays. The reason for this is often that an older relay has been replaced with a digital one. When this occurs, the YD interposing transformer is still in place, and the zero sequence elimination done internally by the relay is not necessary. Additionally, they are often used when the relay simulates this YD interposing transformers internally by software in order to perform the phase matching and zero sequence elimination.

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    Figure 3: YD interposing transformers with protection relay

    There are also some relays which can simulate YD interposing transformers internally:

    SEL relays Toshiba GRT100 (beta compensation method) Reyrolle DUOBIAS GE Multilin SR745 and T60 AREVA KBCH

    Calculating the corrected output currents 1.2.1

    In Figures 4-5, the equivalent circuit diagram of a YD1 interposing transformer is displayed. In this chapter, a description is provided of how the transmission formulas from a YD1 transformer can be derived.

    Figure 4: Equivalent circuit diagram of YD1 interposing transformer on HV-side

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    Figure 5: Secondary Winding of YD1 interposing transformer on HV-side

    Here are the equations after Kirchhoffs 1st law:

    I:

    II:

    III:

    I:

    ( )

    II:

    ( )

    III:

    ( )

    (

    )

    [

    ] (

    )

    n... ratio IHVLx ... currents on the main CT secondary side (before interposing-transformer) IHVLx... corrected currents on the relay side (after interposing-transformer) ... Chapter 6.1 explains this factor

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    Figure 6 shows the corresponding vector diagram of the YD1 interposing transformer on the HV side.

    Figure 6: Vector diagram (primary and secondary values) of YD1 interposing transformer on HV-side

    With the aid of the vector diagram in Figure 6, it is possible to determine the corrected currents (IHVLx) graphically.

    Figure 7: Graphical determination of

    The graphical calculation above leads to the following formula:

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    Example calculation of YD1 interposing transformer on HV-side 1.2.2

    Figure 8: YD1 interposing transformer (single-phase fault)

    YDY interposing transformers 1.3

    General 1.3.1

    YDY interposing transformers have the same function as YD interposing transformers. They are also used in the secondary current path of the main CT. An example can be seen in Figure 9.

    Figure 9: YDY interposing transformers + protection relay

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    Because YDY0 interposing transformers are very commonly used for zero sequence elimination, the corresponding transformation formulas for this interposing transformer type are below. In this case these formulas are valid for the HV-side.

    ( )

    ( )

    ( )

    The following example calculation is based on these formulas.

    Example calculation of YDY0 interposing transformer on HV-side 1.3.2

    Figure 10: YDY0 interposing transformer (single-phase fault)

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    IL-I0 Numerical Zero Sequence Elimination 1.4

    The IL-I0 zero sequence elimination method is a numerical method which is very often used in digital relays. This method requires the relay to measure the 3 phase currents and calculate the zero sequence component with the aid of the following formula:

    ( )

    Next, the relay subtracts the zero sequence component from the phase currents in order to do the necessary zero sequence elimination. Ultimately, the relay uses the following current values for the IBIAS calculation and IDIFF calculation.

    ......... Corrected phase currents without zero sequence component The IL-I0 method and YDY0 interposing transformers provide two different ways to eliminate zero sequence currents. They both provide corrected output currents that are equal and will not cause a differential relay to trip. The following relays work with the IL-I0 zero sequence elimination method:

    AREVA 63x and 64x relay series SEL relays ABB RET relays Toshiba GRT100 (alpha method) SIEMENS 7UT6xx

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    Formulas of commonly used interposing transformers 1.5

    Table 1 shows further interposing transformer types. The equations in this table always result in the corrected output current without zero sequence component for the YDx and YYx vector groups.

    Table 1: Further interposing transformers [2]

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    Table 2: Further interposing transformers part 2 [2]

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    Effect of different zero sequence elimination settings on the calculation 2 If the zero sequence elimination method is changed from "none" to e.g. "IL-I0" in the Test Object, the output currents visible in the vector view (Figure 11) of the CMC may change as well. This chapter explains why the output currents of the CMC change if the zero sequence elimination setting in the Test Object is changed.

    Figure 11: Vector View (Output currents)

    In order to explain this behavior, let's have a closer look at the Diff Operating Characteristic module: The Diff Operating Characteristic module is used to test the operating characteristic of differential relays. The basic principle is that the operating characteristic represents the ratio between the stabilization current, IBIAS, and differential current, IDIFF. Therefore, the CMC has to output current values on the primary side and secondary side which are related to the IBIAS / IDIFF values in the operating characteristic.

    Importance of zero sequence elimination setting for testing the IDIFF/IBIAS 2.1 characteristic

    The operating characteristic of a differential protection relay is tested with specific IDIFF/ IBIAS values. The test set cannot directly output these IDIFF and IBIAS values to the relay; it can only output phase currents on the secondary side of the HV- and LV-winding of the transformer. By using the zero sequence elimination functionality in the relay, or an interposing transformer to correct the phase currents, the relay would not calculate the same IDIFF and IBIAS values that were chosen based on the characteristic alone. This is due to the correction, which is done by the relay or the interposing transformer. This will probably lead to failed test results because the IDIFF and IBIAS values in TU are not equal to the calculated IDIFF and IBIAS values from the relay. Therefore, it is very important to know the interposing device of the differential protection system so that the software correctly calculates the output currents. Additionally, it is important to note that the software does not simulate these interposing transformers; it just considers the correction system, so that the characteristic can be tested properly. As a consequence, if the relay is connected to interposing transformers and interposing transformers for zero-sequence elimination are selected in the Test Object, then the test currents need to be injected into these interposing transformers, and not directly into the relay (see chapter 4). Ultimately, the IDIFF and IBIAS values calculated from the relay and the values visible in Test Universe have to be equal.

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    Zero sequence elimination setting in Diff Operating Characteristic module (example) 2.2

    For further explanations, please follow the next example in the Diff Operating Characteristic module: First, select Generator as the Protected Object, because when the Protected Object is a YY0 transformer, a shorted delta winding is assumed between the HV- and the LV-winding. The transformer simulated is shown in Figure 12. Nominal values are set according to Figure 12. The CT ratios in the CT tab have to be set to one on both sides of the transformer. The "Reference Winding" in the Protection Device tab has to be set to the "High Voltage" side as seen in Figure 16. Additionally, use the following formula in the Protection Device tab:

    | | | |

    Figure 12: Protected Object-tab (Differential RIO-block)

    Next, set the test shot for IDIFF= 1A and IBIAS= 2A (Fault type = L1-E) in the Test View of this module. Then, open the Vector View shown in Figure 13 and look at the output currents:

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    Figure 13: Vector View in Diff Operating Characteristic module

    If the output currents of the CMC were calculated according to the selected test point (IDIFF and IBIAS value), we should be able to calculate this expected point of the characteristic such that IDIFF= 1A and IBIAS= 2A.

    | | | |

    | | | |

    | | | |

    Now, if the Protected Object is changed to Transformer and the Zero Sequence Elimination-type to YDY-interposing transformer, the output values change to those shown in Figure 14. If the IDIFF and IBIAS calculations are done with the values in Figure 14, the calculated IDIFF and IBIAS values from the relay and the values in TU are different. This is because these values are not the actual values that the relay will use to calculate IDIFF and IBIAS. This is shown more clearly in the following calculation.

    | | | |

    | | | |

    | | | |

    When the calculated values from the relay are compared to the test point in Test Universe, it can be seen that they are different, which should not be the case. Explanation of why these values are different: In reality the relay measures the 3 phase currents and afterwards it does all the necessary corrections including amplitude correction, phase matching, and if necessary, zero sequence elimination. This means that the relay always takes the corrected phase currents for the calculation of IDIFF and IBIAS. In this case, we want to test a certain point on the characteristic. In order to get a positive assessment, the relay has to determine exactly the same point as we wanted to test. Because the ratios of the transformer and the CTs are the same, no amplitude correction is necessary. Furthermore, the vector group index of the transformer model (YY0) is 0, which means that no phase matching has to be done. However, the zero sequence component of the HV side phase currents has to be eliminated before the calculation of IDIFF and

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    IBIAS is done. In order to get the same values for IDIFF and IBIAS in the relay and in the Test Universe, this zero sequence component correction has to be considered by Test Universe.

    Figure 14: YY0 Transformer (Zero Sequence Elimination type: YDY interposing transformer)

    Now, take the output values of the test set shown in Figure 14 and make use of the formulas mentioned in chapter 1.3 YDY interposing transformers to calculate the output currents of the YDY0 interposing transformer, which are similar to the input currents of the relay. For this calculation, note that a phase shift of 180 is equivalent to changing the sign of the value. High Voltage Side correction with interposing transformer:

    ( )

    ( ( ) )

    ( )

    ( ( ) )

    ( )

    ( ( ) )

    Low Voltage Side correction with interposing transformer:

    ( )

    ( ( ) ( ))

    ( )

    ( ( ) ( ))

    ( )

    ( ( ) ( ))

    By using the corrected phase current values to calculate the differential current IDIFF and the stabilization current IBIAS, it can be seen that the relay would correctly calculate the values that were originally selected in the Diff Operating Characteristic module of Test Universe.

    | | | |

    | | | |

    | | | |

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    Zero sequence elimination setting in the Diff Configuration module 2.3

    The Diff Configuration module is not used to test the relay with specified IDIFF and IBIAS values like it is done in the Diff Operating Characteristic module. In this module, it is only necessary to enter a test current on the fault side, and the module automatically calculates the current values of the other side according to the settings in the Adv. Differential RIO-block (Vector group, CT ratio, etc.). These values do not change if the Zero Sequence Elimination-type in the Adv. Differential RIO-block is changed, because the aim of the module is to verify proper (non-tripping) operation at through-fault conditions and not to test specified points of the operating characteristic. In these tests, the IDIFF calculated from the relay has always to be zero.

    Figure 15: Diff Configuration Module

    Example: We want to output a test current of 1A on the LV side as seen in Figure 15 and the Zero Sequence

    Elimination-type in the Adv. Differential RIO-Block is set to "none." The red zig-zag flash in the diagram indicates that the LV side is the side where the L1-E fault

    occurs. The currents on the HV side (Supply side) are automatically calculated according to the settings in

    the Adv. Differential RIO-block. Especially the transformer model and current transformer settings are important for this calculation. The Zero Sequence Elimination-type in the Adv. Differential RIO-block is not important for this calculation. In this test, the relay must not detect a differential current because an outside fault is simulated. Outside faults should not lead to a trip of the relay because the fault is not within the protected area.

    If the Zero Sequence Elimination-type is changed from "none" to "e.g. YD transformer" in the Adv. Differential RIO-block, no changes regarding the output currents should be noticeable. If an outside fault leads to a trip of the relay, the settings in the Adv. Differential RIO-block are either incorrectly entered or the zero sequence elimination function of the relay is not activated.

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    Choosing the correct Reference Winding 3 Figure 16 shows the setting "Reference Winding" in the Test Object: (Diff RIO block)

    Figure 16: Reference Winding (Protection Device-tab)

    This information can be found in the manual of the relay, and it is very important to enter this setting in accordance with the relay manual. In most cases, this information is located in the chapters which explain how the relay does phase matching and zero sequence elimination. The reference winding in the Test Universe software is always the side where the fault is simulated. Therefore, it is not possible to create a test case with a zero sequence component if the reference side is not grounded. For these cases, it is necessary to use the Diff Configuration module in order to verify if the relay eliminates the zero sequence current on the star point grounded sides correctly.

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    Reference side = star point grounded side:

    Figure 17: Reference side = star point grounded side

    In Figure 17, the reference side (reference winding) is the HV side (e.g. AREVA P630C), which means that the reference side is equal to the star point grounded side of the transformer. This allows us to simulate single-phase faults with a zero sequence component in the Advanced Differential modules. Reference side star point grounded side:

    Figure 18: Reference side star point grounded side

    In Figure 18, the reference side is also the HV side (e.g. AREVA P630C), which means that the reference side is not equal to the star point grounded side of the transformer. Please note that it is still possible to simulate single-phase faults, but without a zero sequence component because the earth connection is missing. As already mentioned in such a case, the Diff Configuration module must be used to verify proper zero sequence elimination.

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    Configuring the Test Object with interposing transformers 4 In most cases, interposing transformers are simulated in the relay itself, which means that the relay uses mathematical formulas to simulate the behavior of "interposing transformers" by software (e.g. AREVA P6xx, SEL787). However, physical interposing transformers can be implemented in the substation. In this case, it is very important that the current injection is done before this interposing transformer because this "correction device" is responsible for phase matching and zero sequence elimination. If the current injection is done after the interposing transformer, the relay will get wrong current values which can lead to failed test results. The "Adv. Differential RIO-block" does not know whether the relay simulates the interposing transformer internally by software or if the interposing transformer is physically implemented in the substation. It is just important that the settings in the test object are correct for the interposing transformer type.

    Working with Ground Current Measurement inputs (CT) 5

    General 5.1

    Occasionally, the Advanced Differential modules require the "Ground Current Measurement Inputs". This option can be found in the CT tab in the Test Object as shown in Figure 19. There are two cases where a "Ground Measurement CT" has to be simulated from the test set in order to do the tests successful.

    Figure 19: Ground Current Measurement Inputs (CT) in Test Object

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    1. Some relays (e.g. Siemens 7UTxx) are able to measure the star point current directly via an additional ground current CT and therefore the ground current needs also to be simulated in order to test the relay properly

    2. When a Restricted Earth Fault (REF) protection function and a stabilized differential protection scheme are working in parallel. During testing of the stabilized differential protection characteristic, the relay may trip because of the REF functionality when the ground connection is not configured. Therefore, it is important to enable the checkbox "Use Ground Current Measurement inputs" and to connect an additional CMC output to the ground measurement CT input of the relay. An example of a REF scheme can be seen in Figure 20.

    Figure 20: Restricted earth fault

    Simulating the additional CT with the CMC 5.2

    Because all 6 CMC current outputs are used for the stabilized differential protection test an additional amplifier (CMA156 or CMS156) is necessary to get the 7th current output to simulate the neutral current for the ground current measurement input. Another option is to deselect this checkbox in the Test Object and to do the wiring manually. As a first step, the current outputs of the CMC have to be connected to the current inputs of the relay. Next, the feedback has to be connected to a star and afterwards to be reentered again into the "Ground CT" measurement input of the relay. Please be aware that this only works in case that the CT ratio of the neutral phase is equal to the CT ratio of the line phases. A diagram of this can be seen in Figure 21. For proper connection of the ground current relay input regarding current polarity make sure to check the relay manual.

    Figure 21: Building current summation for grounding CT input of protective relay

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    Appendix 6

    Where the factor comes from 6.1

    Figure 22: HV-Winding (Y) Figure 23: LV- Winding (D1) The transferred voltage in a star connection is not the phase-phase voltage (e.g. UHV12) but the phase-neutral voltage (e.g. UHV1). This voltage is smaller by the factor of . The problem in this case is that the transferred voltage of the HV-winding (UHV1 = phase-neutral voltage) to the LV-Winding is equivalent to the phase-phase voltage (ULV12) of the LV-Winding. Therefore the factor of must be considered in the ratio calculation of voltages and currents.

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    Interposing CT selection guide (e.g. Reyrolle DUOBIAS-M) 6.2

    Figure 24: Interposing CT selection guide part 1 [3]

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    Figure 26: Interposing CT selection guide part 2 [3]

  • OMICRON 2012 Page 26 of 27

    List of Literature

    [1] MiCOM30 Series Transformer Differential Protection, Application Guide (Issue F, June 2003), Page 8

    [2] Reyrolle DUOBIAS-M reference manual (2 Description of Operation / Figure 3 - Vector Group Compensation)

    [3] Reyrolle DUOBIAS-M reference manual (6 Applications Guide / 7.0 DUOBIAS-M, Interposing CT Selection Guide)

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