05 SEP-603B Diff Protection RET 670

RET 670 Transformer Protection Differential

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ABB Power Technologies AB, 2007

Substation Automation and Protection Training

2008-05-23


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RET 670 – Transformer Differential Protection

Transformer differential protection (PDIF, 87T)

Two-winding or three-winding

Two-winding variant up to 4 three-phase CT inputs

Three-winding variant up to 6 three-phase CT inputs T2D1T2WPDIF

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I3PW1CT1 I3PW1CT2 I3PW2CT1 I3PW2CT2

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T3D1T3WPDIF I3PW1CT1 I3PW1CT2 I3PW2CT1 I3PW2CT2 I3PW3CT1 I3PW3CT2



Two-winding connections

Single CT on both sides T2D1T2WPDIF

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Two-winding connections

2 CTs, on one or both sides T2D1T2WPDIF

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Up to two instances

Can protect two objects with one RET 670 IED, or one object that changes status

Two transformers

Generator / transformer separately

Generating status / pumping status for pump storage unit

Internal adaptation to power transformer vector group, turns ratio and CT ratios

Tap changer position monitoring for increased sensitivity

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Unbalance currents due to factors other than faults

Currents that flow on only one side of the power transformer

Magnetizing currents that flow on only the power source side

Normal magnetizing currents

Inrush magnetizing currents

Overexcitation magnetizing currents

Currents that cannot be transformed to the other windings

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Zero sequence currents

Error in the power transformer turns ratio due to OLTC

Inequality of the instrument current transformers

Different ratings of current transformers

Different types of current transformers




Unbalance currents due to factors other than faults (cont.)

Different relative loads on instrument transformers

Different relative currents on CT primaries

Different relative burdens on CT secondaries

Different DC time constants of the fault currents Different time of occurrence, and degree, of CT saturation

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Functionality

Fundamental frequency differential currents (per phase) – calculated as the vector sum of the fundamental frequency current contributions from all sides of the transformer – all current contributions are referred to a common reference first

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Phase reference: first star-connected winding (HV→MV → LV), otherwise if no star winding, first delta-connected winding (HV → MV → LV)

For example, if the power transformer is Yd1, the HV winding (Y) is taken as the phase reference winding; if the power transformer is Dy1, then the LV winding (y) is taken as the phase reference; if there is no star-connected winding, such as in Dd0, then the HV delta-connected winding (D) is automatically chosen as the phase reference

All current contributions are phase shifted with reference to the phase reference side




Functionality Magnitude reference: first winding (usually HV)

The magnitudes of all current contributions are referred to the magnitude reference side (i.e. W1), i.e. the magnitudes of all current contributions from all other sides are always transferred to the W1 side

It is for this reason that all differential currents are normally expressed in HV-side primary Amperes

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Transformer differential protection (PDIF, 87T) Functionality

In numerical differential protections, conversion of all current contributions to the phase and magnitude reference sides is performed mathematically by pre-programmed coefficient matrices, which depend on the protected power transformer transformation ratio and vector group connection

Once the power transformer vector group, rated currents and rated voltages have been entered by the user, the differential protection calculates off-line the matrix coefficients, which are then used in the on-line calculations

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Functionality

Differential currents (in W1-side primary amperes)


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Two-winding transformer

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Contribution from W1 side to differential currents


DCCL1_W1

DCCL1_W2

DCCL2_W1

DCCL2_W2

DCCL3_W1

DCCL3_W2


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Functionality

Differential currents (in W1-side primary amperes)


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Three-winding transformer

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DCCL1_W1

DCCL1_W2

DCCL1_W3

DCCL2_W1

DCCL2_W2

DCCL2_W3

DCCL3_W1

DCCL3_W2

DCCL3_W3



Functionality Values for the A, B, C matrix coefficients depend on

Winding connection type, i.e. star (Y/y) or delta (D/d)

Transformer vector group, i.e. Yd1, Dy11 (which introduces a phase shift between winding currents in multiples of 30°)

Zero sequence current elimination set On / Off

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Functionality On-line compensation for on-load tap-changer (OLTC) movement

The OLTC is a mechanical device that is used to stepwise change the number of turns within one power transformer winding – consequently the overall turns ratio of the transformer is changed

Typically the OLTC is located on the HV winding (i.e. W1) – by stepwise increasing or decreasing the number of HV winding turns, it is possible to stepwise regulate the LV-side voltage

As the turns ratio (transformation ratio) changes from the nominal, the actual primary currents flowing will automatically adjust in accordance with the actual turns ratio

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Turns ratio = NLV/NHV and │IHVNHV│ = │ILVNLV│ 2008-05-23 14




Functionality

On-line compensation for on-load tap-changer (OLTC) movement

However, as the transformation ratio changes from the nominal NLV/NHV = Ur_W2 / Ur_W1 the differential function will calculate a resulting differential current if the ratio Ur_W2 / Ur_W1 is fixed in the calculation

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The differential function in IED 670 has a built-in feature to continuously monitor the tap position and to dynamically compensate on-line for changes in the power transformer turns ratio


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Setting parameters define on which winding the OLTC is located, and what the no-load voltage change is for a tap step

By knowing the actual tap position, the differential function can then calculate the correct no-load voltage for the winding on which the OLTC is located

For example, if the OLTC is located on the HV winding (W1), the no-load voltage Ur_W1 will be treated as a function of the actual tap position, and therefore for every tap position the corresponding value for Ur_W1 will be calculated and used in the differential current calculation

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By doing this, complete on-line compensation for OLTC movement is achieved – the differential protection will be ideally balanced for every tap position and no false differential current will appear irrespective of the actual tap position

Typically, the minimum differential protection pickup for power transformers with OLTC is set between 30% to 40% - however, with the OLTC compensation feature it is possible to set the differential protection in IED 670 to more sensitive pickup values of 15% to 20%

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Two-winding differential protection in IED 670 can compensate on-line for one OLTC on the protected power transformer

Three-winding differential protection in IED 670 can compensate on-line for up to two OLTCs on the protected power transformer

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OLTC position is determined within IEC 670 by function block YLTC

Within this function block, the OLTC position value is continuously monitored to ensure its integrity – if any error with the OLTC position value is detected an alarm will be given which should be connected to the applicable OLTCxAL input of the differential protection function block

Whenever an OLTCxAL input has a logical value 1, the differential protection minimum pickup, originally defined by setting parameter IdMin, will be increased by the set range of the OLTC

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Functionality

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Differential current alarm

The fundamental frequency differential current level is monitored all the time within the differential function – as soon as all three fundamental frequency differential currents are above a set threshold, defined by setting parameter IDiffAlarm, a delay on pickup timer is started; when the pre-set time, defined by setting parameter tAlarmDelay, expires, the differential current alarm will be generated and the output signal IDALARM will be set to logical value 1

This feature can be effectively used to provide an alarm when OLTC position compensation is used and something in the whole compensation chain goes wrong

This alarm can also be used to desensitize the differential function (with some additional IED configuration logic that connects the IDALARM output to an OLTCxAL input)




Functionality

Bias current

Single circuit breaker applications

Calculated as the highest fundamental frequency current amongst all the current contributions to the differential current calculation

This highest individual current contribution is taken as the single common bias current for all three phases DCCL1_W1 DCCL2_W1 +

DCCL2_W2

DCCL3_W1

DCCL3_W2

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=

DCCL1_W2

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Functionality

Bias current

Single circuit breaker applications

The current contributions from all windings are already referred to the magnitude reference (W1) side (for this reason, the bias current is usually expressed in HV-side primary Amperes) and can therefore be directly compared regarding their magnitudes

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i.e. Ibias = MAX [DCCLx_W1; DCCLx_W2]

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Functionality

Through-fault stability for multi-breaker arrangements

Ideal CTs, external current summation

+A

+B

-A -B

Idiff

To Diff Prot +A -A -B → -B IDIFF = +B -B = 0 IBIAS = MAX(-B; +B)

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Ibias

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Functionality


Real CTs, external current summation

+A

-A -B

+B

Idiff

To Diff Prot +A -A -B ±error → -B ±error IDIFF = +B -B ±error = ±error

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IBIAS = MAX(-B ±error; +B)

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Ibias


Transformer differential protection (PDIF, 87T) Functionality


Real CTs, internal current summation

+A

Idiff

+B

-A -B

To RET 670 +A -A -B ±error

T2D1

Ibias

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to IBIAS calculation

Σ

to IDIFF & IBIAS calculation IDIFF = +A -A -B ±error +B = ±error IBIAS = MAX(-B ±error; +B; +A; -A -B ±error)

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Functionality

Bias current

Multiple circuit breaker applications, on any or all sides

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From any side, the current contributions from the two T-side CTs are included in the bias current calculation, as well as the resultant current contribution for that transformer winding (which is the calculated sum of the two separate CT contributions)

to IBIAS calculation

Σ 2008-05-23 26


to IDIFF & IBIAS calculation



Functionality

Bias current


A*

From W1 CT1:

IL1_W1CT1 IL2_W1CT1

DCCL1_W1CT1

=

IL3_W1CT1

A*

From W1 CT2:

DCCL3_W1CT1

IL1_W1CT2 IL2_W1CT2

DCCL1_W1CT2

=

IL3_W1CT2

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From W2 CT1:

(Ur_W2 / Ur_W1) *

B

*

DCCL1_W2CT1

=

IL3_W2CT1

From W2 CT2:

B

*

IL2_W2CT2 IL3_W2CT2

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DCCL2_W2CT1 DCCL3_W2CT1

IL1_W2CT2 (Ur_W2 / Ur_W1) *


IL1_W2CT1 IL2_W2CT1

DCCL2_W1CT1

DCCL1_W2CT2

=



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Functionality

Bias current


Therefore, in principle, want to include the following contributions in the bias current calculation DCCLx_W1CT1/2; DCCLx_W2CT1/2; DCCLx_W1CT1+DCCLxW1CT2; DCCLxW2CT1+DCCLxW2CT2

However, for applications with two restraint CT inputs on one side (such as for breaker-and-a-half applications), the primary ratings of the CTs can be much higher than the rating of the protected power transformer

In order to determine the bias current for such Tconfigurations, the currents flowing on the T-side can be separately scaled by an additional setting – this is done in order to prevent unwanted de-sensitizing (over-biassing) of the overall differential protection

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Functionality

Bias current


When 1pu primary current flows through the T-side CT, want this to contribute a 1pu component to the bias calculation – e.g. Irated pri_CT = 1000A Irated_W1 = 500A Irated_W1 = 500A

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1000/1

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Functionality

Bias current


If no other action were taken, the contribution to the bias calculation from the 1000A (= 1pu of the CT primary rating) flowing through the T-side CT would be = 1000A (= 2pu!!!)

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Ibias (pu) = Ibias (pri A) / IBase, where IBase = Irated_W

Therefore, a scaling factor needs to be introduced (setting parameters CT1RatingW1, CT2RatingW1, CT1RatingW2, CT2RatingW2)

By applying the scaling factor, the contribution to the bias calculation from the 1000A flowing through the T-side CT is then determined as follows: (I / CT1RatingW1)*Irated_W1 = (1000/1000)*500 = 500A (=1pu!!!)

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Functionality

Bias current

Multiple circuit breaker applications, on any or all sides curr(1) = MAX [DCCLx_W1CT1 + DCCLx_W1CT2] curr(2) = MAX [DCCLx_W2CT1 + DCCLx_W2CT2] curr(3) = MAX [DCCLx_W1CT1] * (1 / CT1RatingW1) * RatedCurrentW1 curr(4) = MAX [DCCLx_W1CT2] * (1 / CT2RatingW1) * RatedCurrentW1 curr(5) = MAX [DCCLx_W2CT1] * (Ur_W1 / Ur_W2) * (1 / CT1RatingW2) * RatedCurrentW1 curr(6) = MAX [DCCLx_W2CT2] * (Ur_W1 / Ur_W2) * (1 / CT2RatingW2) * RatedCurrentW1

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i.e. Ibias = MAX [curr(1);…….curr(6)]

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Functionality

Zero sequence current elimination

Selectable (per winding) – Setting: On / Off per winding

Elimination of the zero sequence currents is necessary to avoid unwanted trips for external earth faults whenever The protected power transformer cannot transform the zero sequence currents to the other side

The zero sequence currents can only flow on one side of the protected power transformer

The zero sequence currents should be subtracted from the side of the power transformer where the zero sequence currents can flow for external earth faults

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Functionality Zero sequence current elimination

In most cases power transformers do not properly transform the zero sequence currents to the other side – a typical example is a power transformer of the star-delta type (e.g. YNd1)

Transformers of this type do not transform the zero sequence currents, but zero sequence currents can flow in the earthed starconnected winding (e.g. an external earth fault on the star-side causes zero sequence currents to flow on the star-side, but not on the delta-side)

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Functionality

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Zero sequence current elimination

This results in false differential currents that consist exclusively of the zero sequence currents – if high enough, these false differential currents can result in the unwanted operation of the differential function (and unwanted disconnection of the healthy power transformer)

The star-winding zero sequence currents must therefore be subtracted from the fundamental frequency differential currents if an unwanted trip is to be avoided

For delta-windings, this feature should be enabled only if an earthing transformer exists within the differential zone on the delta-side of the protected power transformer




Functionality Zero sequence current elimination

If the zero sequence currents are subtracted from the current contribution of any side to the differential currents, they are automatically eliminated from the bias current contribution as well

Removing the zero sequence current from the differential currents decreases to some extent the sensitivity of the differential protection for internal earth faults

However, the automatic subtraction of the zero sequence currents from the bias current as well counteracts this effect to some degree

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=

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DCCL1_W1

DCCL1_W2

DCCL2_W1 +

DCCL2_W2

DCCL3_W1

DCCL3_W2


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Functionality

Three-section operate/restrain characteristic

Highset unrestrained limit

Reset ratio 0.95

Idiff

Section 1

Section 2

Section 3 Settings:

Idunre

IdMin IdUnre

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Slope Section 2

Slope Section 3

EndSection1 EndSection2 SlopeSection2

Idmin

SlopeSection3 End of Section 1 2008-05-23 36


End of Section 2

Ibias



Functionality Unrestrained (i.e. non-stabilized) limit

This limit determines the pickup (operate) threshold for big differential currents for which there should be no doubt that the fault is internal

This limit is constant

For measured differential currents greater than this limit, instantaneous tripping is allowed, i.e. no supervision by blocking criteria (waveform or harmonic restrain) is applied

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Functionality

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Restrained (i.e. stabilized) characteristic

This characteristic determines the pickup (operate) threshold for differential currents with magnitude less than the unrestrained limit – it is possible for these differential currents to occur and cause pickup for reasons other than internal faults

The pickup (operate) threshold is mostly not constant, as after Section 1 it increases with increasing bias current magnitude – in this way the magnitudes of the individual fundamental frequency differential currents are compared to an adaptive limit that is dependent on the bias (i.e. restrain) current magnitude

Following a start (pickup), blocking criteria (waveform and harmonic restrain) are applied before tripping is permitted, i.e. a trip will only be released if not prevented by one of the blocking criteria




Functionality


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Section 1

Most sensitive part

Characteristic a straight line

Current flow normal load current

Typical reason for existence of false differential currents in this section is non compensation for tap position

Section 2

First slope (low percentage)

Caters for false differential currents when higher than normal currents flow through the current transformers


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Functionality


Second slope (higher percentage)

Provides higher tolerance to substantial current transformer saturation for high through fault currents, which can be expected in this section

The characteristic settings should be made such that

for internal faults, the differential currents are always safely above the characteristic

for external faults, any unwanted (false) differential currents are always safely below the characteristic

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Section 3

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Functionality Blocking criteria (phase segregated)

The two blocking criteria are the waveform restrain and the harmonic restrain – these two criteria have the power to block (i.e. to prevent) a trip command being given by the differential protection whose operate-restrain characteristic has measured an operate condition (registered a start)

Instantaneous differential currents are calculated on which the waveform and harmonic analyses are performed – the same matrix equations are used as for the fundamental frequency currents, except now instantaneous values (i.e. sampled values) are used instead

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Functionality

Blocking criteria

Waveform restrain

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Based on waveform recognition – looks for the intervals within each power system cycle with low rate-of-change in the instantaneous differential currents that are typical to power transformer inrush currents



Functionality

Blocking criteria Waveform restrain

The waveform restrain function will activate a restrain signal in those phases where this wave shape is detected

If the operate-restrain characteristic registers a start in a particular phase, then the status of the waveform restrain output for the same phase will be assessed, and if activated, a block signal for that phase will be established

The BLKWAVLx output from the function block will only become logic 1 if the waveform restrain output in phase Lx is activated, and the operate-restrain characteristic has registered a start in the same phase

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Functionality

Blocking criteria Harmonic restrain

Required to prevent unwanted tripping due to magnetizing inrush currents, or due to magnetizing currents caused by overvoltages – magnetizing currents flow only on one side of a power transformer, and are therefore always a cause of false differential currents

Harmonic analysis (2nd and 5th harmonic) is performed on the instantaneous differential currents, but only after the operaterestrain characteristic has registered a start – i.e. if the operate-restrain characteristic registers a start in a particular phase, then the harmonic analysis on the instantaneous differential current in that phase will be started (phase segregated)

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Functionality

Blocking criteria

Harmonic restrain For example, if the operate-restrain characteristic has registered a start in phase L1, then the harmonic analysis on the instantaneous differential current in phase L1 will be started – if the content of the 2nd harmonic in the instantaneous differential current of this phase is above the setting I2/I1Ratio, then a block signal for this phase will be established

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Functionality

Blocking criteria

Cross-blocking between phases

If a blocking condition is established in any phase, this phase can ‘cross’ this blocking condition to the other phases, i.e. a phase may only cross-block the other phases if it itself is blocked (and it can only be blocked if its operate-restrain characteristic first registered a start)

IdiffL2/3

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IdiffL1 Phase L1 blocked

cross-block

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Ibias



Functionality

Blocking criteria

Waveform – inrush

2nd harmonic – inrush, CT saturation

Settings: settable level in % of fundamental

5th harmonic – overexcitation

Settings: None

Settings: settable level in % of fundamental

Cross-blocking between phases Setting: On / Off

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Overview of traditional transformer differential protection function

Blocking condition: 2nd harmonic, 5th harmonic waveblock, cross block

restrained

&

≥1

TRIP

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unrestrained

START

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Functionality Switch on to fault feature

The transformer differential function in IED 670 has a built-in switch on to fault feature that ensures quick differential protection tripping in cases where a transformer is energized with an internal fault

This feature can be enabled or disabled by a setting parameter SOTFMode

The principle of operation is based on the fact that a current gap will exist within the first power system cycle when a healthy power transformer is energized, and conversely will not exist if the power transformer is unhealthy (is switched on to a fault)

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Functionality

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Switch on to fault feature

If this gap does not exist, the waveblock criterion will reset quickly, i.e. within a time window that started when the transformer was switched in

This quick resetting (within the time window) of the waveblock criterion will temporarily disable the second harmonic blocking feature, which then ensures fast operation of the transformer differential function for a switch on to fault condition

This feature is only active during initial power transformer energizing

When the switch on to fault feature is disabled, or once the time window following switch-in has elapsed, the waveblock and second harmonic blocking features work in parallel and are completely independent of each other



Transformer differential protection (PDIF, 87T) Internal / External fault discriminator

Fault position (internal / external) can be determined by comparing the direction of flow of the negative sequence currents on all sides of the transformer

This is done by simply determining the position of the source of the negative sequence currents with respect to the zone of protection – the source of the negative sequence currents is at the point of fault

External fault: the negative sequence currents will have a relative phase displacement of 180°

Internal fault: the negative sequence currents will have a relative phase displacement of about 0°

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Transformer differential protection (PDIF, 87T) Internal / External fault discriminator

What about transformation ratio and phase shift?

Before comparison, the negative sequence current contributions must first be referred to the same phase reference, and put to the same magnitude reference

Modern numerical transformer differential relays use matrix equations to compensate for vector group and turns ratio

Negative sequence differential current contributions can be calculated using the same matrix equations – in this way all negative sequence current contributions are automatically compensated for power transformer vector group and turns ratio

The 0° / 180° criterion is then still valid

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RET 670 – Transformer Differential Protection Transformer differential protection (PDIF, 87T)

Internal / External fault discriminator

Example YNd5 transformer

⎡ IDL1 _ NS ⎤ ⎡ 2 −1 −1⎤ ⎡ INS _ W 1 ⎤ ⎡ −1 0 1 ⎤ ⎡ INS _ W 2 ⎤ ⎢ IDL 2 _ NS ⎥ = 1 ⋅ ⎢ −1 2 −1⎥ ⋅ ⎢ a ⋅ INS _ W 1 ⎥ + Ur _ W 2 ⋅ 1 ⋅ ⎢ 1 −1 0 ⎥ ⋅ ⎢ a ⋅ INS _ W 2 ⎥ ⎢ ⎥ 3 ⎢ ⎥ ⎢ ⎥ Ur _ W 1 3 ⎢ ⎥ ⎢ ⎥ ⎢⎣ IDL3 _ NS ⎥⎦ ⎢⎣ −1 −1 2 ⎥⎦ ⎢⎣ a 2 ⋅ INS _ W 1⎥⎦ ⎢⎣ 0 1 −1⎥⎦ ⎢⎣ a 2 ⋅ INS _ W 2 ⎥⎦ NS contribution from W1 side to NS differential currents

NS contribution from W2 side to NS differential currents

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NS differential current

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Two-winding transformers

Three-winding transformers

Operation of the internal / external fault discriminator is based on similar principles

Three directional comparisons are made

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Operation of the internal / external fault discriminator is based on the relative position of the two phasors representing the HV/LV (W1/W2) negative sequence current contributions, i.e. on a directional comparison between these two phasors

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Internal / External fault discriminator The Internal / External fault discriminator works equally well for symmetrical 3-phase faults

When a symmetrical 3-phase fault occurs, negative sequence currents (the negative sequence current source) will be present until the dc component in the fault currents die out

This interval of time is long enough for the internal / external fault discriminator to declare either an internal or an external fault

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Characteristic

Settings: IMinNegSeq

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NegSeqROA

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Operation (two-winding transformer)

The LV-side phasor is positioned along the zero degree line – then the relative position of the HV-side phasor in the complex plain is determined

To perform the directional comparison, the magnitudes of both phasors must be high enough to ensure that they are due to a fault, i.e. both must be greater than the settable limit IminNegSeq

To guarantee good sensitivity, this IminNegSeq limit must not be set too high

If the magnitude of any one of the phasors is below the limit, no directional comparison will be made – the internal / external fault discriminator will not operate during transformer energization (inrush)

The settable relay operate angle NegSeqROA determines the boundary between the internal and external fault regions


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Internal / External fault discriminator CT saturation

During heavy faults, CT saturation might cause the measured phase angle to differ from 180° for external faults, and from about 0° for internal faults

Effective means to counteract the negative effects of main CT saturation have been integrated into the algorithm

At heavy faults, approximately 5ms time-to-saturation of the main CT is sufficient in order to produce a correct discrimination between internal and external faults

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Discriminates between internal and external faults with very high dependability

Detects even minor faults with high sensitivity and high speed

High performance differential protection is achieved by combining the good properties of traditional differential protection with advanced features of the internal / external fault discriminator

Unrestrained negative sequence differential protection

Sensitive negative sequence protection

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Features of Internal / External fault discriminator

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Unrestrained negative sequence differential protection

A start from the ordinary differential protection is required to activate the unrestrained negative sequence differential protection

If the internal / external fault discriminator categorizes this same fault as internal, the blocking supervision is bypassed, and the trip output is generated without any further delay

This logic guarantees a fast operating time, even for heavy internal faults with severely saturated current transformers – harmonic distortions do not slow down the differential protection operation

If the fault is categorized as external, the traditional differential protection is NOT blocked, but additional trip criteria are posed to ensure high external fault stability




Overview of unrestrained negative sequence differential protection in conjunction with traditional transformer differential protection

Blocking condition: 2nd harmonic, 5th harmonic waveblock, cross block Neg Seq Int / Ext Fault Discriminator

internal

&

&

restrained

≥1

TRIP

©


unrestrained

START

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Sensitive negative sequence protection

Gives sensitive turn-to-turn fault protection

Independent from the traditional differential protection

No start from the ordinary differential protection is required to activate the sensitive negative sequence differential protection

If the internal / external fault discriminator categorizes the disturbance as internal, a separate trip request is placed – this trip request must be confirmed several times in succession before the final trip request is placed – this security feature results in an increase in the operating time

Nevertheless, an operating time of about 30-50ms is achieved for very low-level turn-to-turn faults




Overview of sensitive negative sequence protection

Blocking condition: 2nd harmonic, 5th harmonic waveblock, cross block Neg Seq Int / internal Ext Fault I- start & Discriminator internal

&

&

restrained

≥1

TRIP

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unrestrained

START

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Functionality

Open CT detection feature

The purpose of the open CT detection feature is to prevent a maloperation of the transformer differential function if a loaded main CT connected to the differential protection is by mistake open-circuited in just one phase on the secondary side

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Note: This feature is able to detect an open CT condition (interruption) in one phase only – it is not able to detect the simultaneous interruption in two or three phases, for which the differential function will operate and give a trip output if the false differential current is high enough

This feature can be enabled or disabled by the setting parameter OpenCTEnable




Functionality Open CT detection feature

In order to prevent an unwanted operation of the differential function for an open-circuit condition, the open CT detection feature must operate within 10ms, to ensure in-time blocking of the differential function

It is required to work only during normal load conditions, and is automatically disabled during external faults, big overloads and inrush conditions

Its principle of operation is based on the fact that, for an interruption in one phase, the current in that phase will suddenly drop to zero, while the currents in the other two phases will continue as before

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If the load currents are very low, or zero, an open CT condition cannot be detected – the open CT algorithm will only detect an open CT condition if the load on the power transformer is in the range 10 – 110% of the rated load

The search for an open CT condition starts 60 seconds (50 seconds in 60 Hz systems) after the bias current enters the 10 – 110% range

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If an open CT is detected, all the differential functions, except the unrestrained (instantaneous) level, will be blocked immediately

The unrestrained level will also be blocked after a settable time delay tOCTUnrstDelay

An alarm signal will be given after a settable delay tOCTAlarmDelay

The open CT algorithm provides detailed information about the location of the open CT secondary circuit

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Transformer side,

CT input, and

Phase

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A detected open CT condition will be reset automatically by the differential function itself once the condition has been repaired – it is not possible to externally reset the open CT detection

After reset, the open CT detection algorithm will again start to search for an open CT condition within the protected zone

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Functionality

Open CT detection feature

The automatic reset of the open CT detection requires the following conditions to be fulfilled

The bias current must be in the range 10 – 110% of the rated load for at least one minute

The current asymmetry must disappear, i.e. the open CT condition must be repaired Note: not only must the current asymmetry disappear, but current symmetry must be detected

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The above two conditions must be fulfilled for the time interval defined by the setting parameter tOCTResetDelay – the differential functions will remain blocked until this time delay has elapsed (the reason for this is to prevent a maloperation immediately following the reconnection of the open CT circuit)


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RET 670 – Transformer Differential Protection Transformer differential protection (PDIF, 87T)

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Transformer differential protection (PDIF, 87T) Simplified logic diagram for phase L1

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Simplified logic diagram for internal / external fault discriminator Internal/ External Fault discrimin ator

IDNSMAG

Constant

a

INTFAULT

AND

OpNegSeqDiff=On IBIAS

EXTFAULT

t

TRNSSENS

b>a

b

BLKNSSEN BLKNSUNR BLOCK ABB Power Technologies AB, 2007

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AND

STL1 STL2 STL3

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OR

TRNSUNR



Internal grouping of Trip signals TRIPRESL1 TRIPRESL2 TRIPRESL3

OR

TRIPRES

OR

TRIPUNRE

TRIPUNREL1 TRIPUNREL2 TRIPUNREL3

TRIP

OR

TRNSSENS

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TRNSUNR

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Internal grouping of Start and Block signals STL1 STL2 STL3

OR

START

OR

BLK2H

OR

BLK5H

OR

BLKWAV

BLK2HL1 BLK2HL2 BLK2HL3 BLK5HL1 BLK5HL2 BLK5HL3 BLKWAVL1 BLKWAVL2

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BLKWAVL3

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Transformer differential protection (PDIF, 87T) Differential current alarm

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Functions blocks for two- and three-winding functions

‘Analog’ quantity Inputs for 2-W OLTC inputs Block inputs

Trip outputs Start outputs

‘Analog’ quantity Inputs for 3-W OLTC inputs

Block outputs

Diff current alarm CT output Open CT outputs

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‘Analog’ quantity outputs

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RET 670 – Transformer Differential Protection - Settings

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Settings General settings Differential protection TransformerDiff3Wind(PDIF,87T)

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Settings General settings Differential protection TransformerDiff3Wind(PDIF,87T)

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WYE (Y); Delta (D)

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0 [0 deg]; 1 [30 deg lag]; 2 [60 deg lag]; 3 [90 deg lag]; 4 [120 deg lag]; 5 [150 deg lag]; 6 [180 deg]; 7 [150 deg lead]; 8 [120 deg lead]; 9 [90 deg lead]; 10 [60 deg lead]; 11 [30 deg lead]

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Off; On

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No; Yes

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No; Yes

No; Yes

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Not Used Winding 1 (W1) Winding 2 (W2) Winding 3 (W3)

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Not Used Winding 1 (W1) Winding 2 (W2) Winding 3 (W3)

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RET 670 – Transformer Differential Protection - Settings Settings Settings group N1 Differential protection TransformerDiff3Wind(PDIF,87T)

Off; On Off; On

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Off; On Off; On

Off; On

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Transformer differential protection (PDIF, 87T) Service values

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05 SEP-603B Diff Protection RET 670

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