Overview
The “Transformer differential protection” is provided with internal CT ratio matching, vector group compensation and settable zero sequence current elimination.
For more detailed information on “Transformer differential protection”, refer to ABB, Relion 670 Series manuals.
To see other supported functions, click here.
Function Identification
| Function description | IEC 61850 identification |
IEC 60617 identification |
ANSI/IEEE C37.2 device number |
| Transformer differential protection, two-winding | T2WPDIF | 3Id/I | 87T |
| Transformer differential protection, three-winding | T3WPDIF | 3Id/I | 87T |
Signals & Setting Parameters
T2WPDIF function block
T2WPDIF Input signals
| T2WPDIF Input signals | |||
| Name | Type | Default | Description |
| I3PW1CT1 | GROUP SIGNAL | – | Three phase winding primary CT1 |
| I3PW1CT2 | GROUP SIGNAL | – | Three phase winding primary CT2 |
| I3PW2CT1 | GROUP SIGNAL | – | Three phase winding secondary CT1 |
| I3PW2CT2 | GROUP SIGNAL | – | Three phase winding secondary CT2 |
| TAPOLTC1 | INTEGER | 1 | Most recent tap position reading on OLTC 1 |
| OLTC1AL | BOOLEAN | 0 | OLTC1 alarm |
| BLOCK | BOOLEAN | 0 | Block of function |
| BLKRES | BOOLEAN | 0 | Block of trip for restrained differential feature |
| BLKUNRES | BOOLEAN | 0 | Block of trip for unrestrained differential feature |
| BLKNSUNR | BOOLEAN | 0 | Block of trip for unrestr. neg. seq. differential feature |
| BLKNSSEN | BOOLEAN | 0 | Block of trip for sensitive neg. seq. differential feature |
T2WPDIF Output signals
| T2WPDIF Output signals | |||
| Name | Type | Description | |
| TRIP | BOOLEAN | General, common trip signal | |
| TRIPRES | BOOLEAN | Trip signal from restrained differential protection | |
| TRIPUNRE | BOOLEAN | Trip signal from unrestrained differential protection | |
| TRNSUNR | BOOLEAN | Trip signal from unrestr. neg. seq. diff. protection | |
| TRNSSENS | BOOLEAN | Trip signal from sensitive neg. seq. diff. protection | |
| START | BOOLEAN | Common start signal from any phase | |
| STL1 | BOOLEAN | Start signal from phase L1 | |
| STL2 | BOOLEAN | Start signal from phase L2 | |
| STL3 | BOOLEAN | Start signal from phase L3 | |
| BLK2H | BOOLEAN | Common second harmonic block signal from any phase | |
| BLK2HL1 | BOOLEAN | Second harmonic block signal, phase L1 | |
| BLK2HL2 | BOOLEAN | Second harmonic block signal, phase L2 | |
| BLK2HL3 | BOOLEAN | Second harmonic block signal, phase L3 | |
| BLK5H | BOOLEAN | Common fifth harmonic block signal from any phase | |
| BLK5HL1 | BOOLEAN | Fifth harmonic block signal, phase L1 | |
| BLK5HL2 | BOOLEAN | Fifth harmonic block signal, phase L2 | |
| BLK5HL3 | BOOLEAN | Fifth harmonic block signal, phase L3 | |
| BLKWAV | BOOLEAN | Common block signal, waveform criterion, from any phase | |
| BLKWAVL1 | BOOLEAN | Block signal, waveform criterion, phase L1 | |
| BLKWAVL2 | BOOLEAN | Block signal, waveform criterion, phase L2 | |
| BLKWAVL3 | BOOLEAN | Block signal, waveform criterion, phase L3 | |
| IDALARM | BOOLEAN | Alarm for sustained diff currents in all three phases | |
| OPENCT | BOOLEAN | An open CT was detected | |
| OPENCTAL | BOOLEAN | Open CT Alarm output signal. Issued after a delay … | |
| IDL1 | REAL | Value of the instantaneous differential current, phase L1 | |
| IDL2 | REAL | Value of the instantaneous differential current, phase L2 | |
| IDL3 | REAL | Value of the instantaneous differential current, phase L3 | |
| IDL1MAG | REAL | Magnitude of fundamental freq. diff. current, phase L1 | |
| IDL2MAG | REAL | Magnitude of fundamental freq. diff. current, phase L2 | |
| IDL3MAG | REAL | Magnitude of fundamental freq. diff. current, phase L3 | |
| IBIAS | REAL | Magnitude of the bias current, which is common to all phases | |
| IDNSMAG | REAL | Magnitude of the negative sequence differential current | |
T2WPDIF Group settings (basic)
| T2WPDIF Group settings (basic) | |||||
| Name | Values (Range) | Unit | Step | Default | Description |
| Operation | Off On |
– | – | Off | Operation Off / On |
| SOTFMode | Off On |
– | – | On | Operation mode for switch onto fault |
| tAlarmDelay | 0.000 – 60.000 | s | 0.001 | 10.000 | Time delay for diff currents alarm level |
| IDiffAlarm | 0.05 – 1.00 | IB | 0.01 | 0.20 | Dif. cur. alarm, multiple of base curr, usually W1 curr. |
| IdMin | 0.10 – 0.60 | IB | 0.01 | 0.30 | Section1 sensitivity, multi. of base curr, usually W1 curr. |
| IdUnre | 1.00 – 100.00 | IB | 0.01 | 10.00 | Unrestr. prot. limit, multiple of Winding 1 rated current |
| CrossBlockEn | Off On |
– | – | On | Operation Off/On for cross-block logic between phases |
| NegSeqDiffEn | Off On |
– | – | On | Operation Off/On for neg. seq. differential protections |
| IMinNegSeq | 0.02 – 0.20 | IB | 0.01 | 0.04 | Neg. seq. curr. must be higher than this level to be used |
| NegSeqROA | 30.0 – 120.0 | Deg | 0.1 | 60.0 | Operate Angle for int. / ext. neg. seq. fault discriminator |
T2WPDIF Group settings (advanced)
| T2WPDIF Group settings (advanced) | |||||
| Name | Values (Range) | Unit | Step | Default | Description |
| EndSection1 | 0.20 – 1.50 | IB | 0.01 | 1.25 | End of section 1, multiple of Winding 1 rated current |
| EndSection2 | 1.00 – 10.00 | IB | 0.01 | 3.00 | End of section 2, multiple of Winding 1 rated current |
| SlopeSection2 | 10.0 – 50.0 | % | 0.1 | 40.0 | Slope in section 2 of operate-restrain characteristic, in % |
| SlopeSection3 | 30.0 – 100.0 | % | 0.1 | 80.0 | Slope in section 3 of operate-restrain characteristic, in % |
| I2/I1Ratio | 5.0 – 100.0 | % | 0.1 | 15.0 | Max. ratio of 2nd harm. to fundamental harm dif. curr. in % |
| I5/I1Ratio | 5.0 – 100.0 | % | 0.1 | 25.0 | Max. ratio of 5th harm. to fundamental harm dif. curr. in % |
| OpenCTEnable | Off On |
– | – | Off | Open CT detection feature. Open CTEnable Off/On |
| tOCTAlarmDelay | 0.100 – 10.000 | s | 0.001 | 3.000 | Open CT: time in s to alarm after an open CT is detected |
| tOCTResetDelay | 0.100 – 10.000 | s | 0.001 | 0.250 | Reset delay in s. After delay, diff. function is activated |
| tOCTUnrstDelay | 0.10 – 6000.00 | s | 0.01 | 10.00 | Unrestrained diff. protection blocked after this delay, in s |
T2WPDIF Non group settings (basic)
| T2WPDIF Non group settings (basic) | |||||
| Name | Values (Range) | Unit | Step | Default | Description |
| GlobalBaseSelW1 | 1 – 12 | – | 1 | 1 | Global base selector for winding 1 |
| GlobalBaseSelW2 | 1 – 12 | – | 1 | 1 | Global base selector for winding 2 |
| ConnectTypeW1 | WYE (Y) Delta (D) |
– | – | WYE (Y) | Connection type of winding 1: Y-wye or D-delta |
| ConnectTypeW2 | WYE (Y) Delta (D) |
– | – | WYE (Y) | Connection type of winding 2: Y-wye or D-delta |
| ClockNumberW2 | 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] |
– | – | 0 [0 deg] | Phase displacement between W2 & W1=HV winding, hour notation |
| ZSCurrSubtrW1 | Off On |
– | – | On | Enable zer. seq. current subtraction for W1 side, On / Off |
| ZSCurrSubtrW2 | Off On |
– | – | On | Enable zer. seq. current subtraction for W2 side, On / Off |
| TconfigForW1 | No Yes |
– | – | No | Two CT inputs (T-config.) for winding 1, YES / NO |
| CT1RatingW1 | 1 – 99999 | A | 1 | 3000 | CT primary rating in A, T-branch 1, on transf. W1 side |
| CT2RatingW1 | 1 – 99999 | A | 1 | 3000 | CT primary in A, T-branch 2, on transf. W1 side |
| TconfigForW2 | No Yes |
– | – | No | Two CT inputs (T-config.) for winding 2, YES / NO |
| CT1RatingW2 | 1 – 99999 | A | 1 | 3000 | CT primary rating in A, T-branch 1, on transf. W2 side |
| CT2RatingW2 | 1 – 99999 | A | 1 | 3000 | CT primary rating in A, T-branch 2, on transf. W2 side |
| LocationOLTC1 | Not Used Winding 1 (W1) Winding 2 (W2) |
– | – | Not Used | Transformer winding where OLTC1 is located |
| LowTapPosOLTC1 | 0 – 10 | – | 1 | 1 | OLTC1 lowest tap position designation (e.g. 1) |
| RatedTapOLTC1 | 1 – 100 | – | 1 | 6 | OLTC1 rated tap/mid-tap position designation (e.g. 6) |
| HighTapPsOLTC1 | 1 – 100 | – | 1 | 11 | OLTC1 highest tap position designation (e.g. 11) |
| TapHighVoltTC1 | 1 – 100 | – | 1 | 1 | OLTC1 end-tap position with winding highest no-load voltage |
| StepSizeOLTC1 | 0.01 – 30.00 | % | 0.01 | 1.00 | Voltage change per OLTC1 step in percent of rated voltage |
T2WPDIF Monitored data
| T2WPDIF Monitored data | ||||
| Name | Type | Values (Range) | Unit | Description |
| IDL1MAG | REAL | – | A | Magnitude of fundamental freq. diff. current, phase L1 |
| IDL2MAG | REAL | – | A | Magnitude of fundamental freq. diff. current, phase L2 |
| IDL3MAG | REAL | – | A | Magnitude of fundamental freq. diff. current, phase L3 |
| IBIAS | REAL | – | A | Magnitude of the bias current, which is common to all phases |
| IDNSMAG | REAL | – | A | Magnitude of the negative sequence differential current |
T3WPDIF function block
T3WPDIF Input signals
| T3WPDIF Input signals | |||
| Name | Type | Default | Description |
| I3PW1CT1 | GROUP SIGNAL | – | Three phase winding primary CT1 |
| I3PW1CT2 | GROUP SIGNAL | – | Three phase winding primary CT2 |
| I3PW2CT1 | GROUP SIGNAL | – | Three phase winding secondary CT1 |
| I3PW2CT2 | GROUP SIGNAL | – | Three phase winding secondary CT2 |
| I3PW3CT1 | GROUP SIGNAL | – | Three phase winding tertiary CT1 |
| I3PW3CT2 | GROUP SIGNAL | – | Three phase winding tertiary CT2 |
| TAPOLTC1 | INTEGER | 1 | Most recent tap position reading on OLTC 1 |
| TAPOLTC2 | INTEGER | 1 | Most recent tap position reading on OLTC 2 |
| OLTC1AL | BOOLEAN | 0 | OLTC1 alarm |
| OLTC2AL | BOOLEAN | 0 | OLTC2 alarm |
| BLOCK | BOOLEAN | 0 | Block of function |
| BLKRES | BOOLEAN | 0 | Block of trip for restrained differential feature |
| BLKUNRES | BOOLEAN | 0 | Block of trip for unrestrained differential feature |
| BLKNSUNR | BOOLEAN | 0 | Block of trip for unrestr. neg. seq. differential feature |
| BLKNSSEN | BOOLEAN | 0 | Block of trip for sensitive neg. seq. differential feature |
T3WPDIF Output signals
| T3WPDIF Output signals | |||
| Name | Type | Description | |
| TRIP | BOOLEAN | General, common trip signal | |
| TRIPRES | BOOLEAN | Trip signal from restrained differential protection | |
| TRIPUNRE | BOOLEAN | Trip signal from unrestrained differential protection | |
| TRNSUNR | BOOLEAN | Trip signal from unrestr. neg. seq. diff. protection | |
| TRNSSENS | BOOLEAN | Trip signal from sensitive neg. seq. diff. protection | |
| START | BOOLEAN | Common start signal from any phase | |
| STL1 | BOOLEAN | Start signal from phase L1 | |
| STL2 | BOOLEAN | Start signal from phase L2 | |
| STL3 | BOOLEAN | Start signal from phase L3 | |
| BLK2H | BOOLEAN | Common second harmonic block signal from any phase | |
| BLK2HL1 | BOOLEAN | Second harmonic block signal, phase L1 | |
| BLK2HL2 | BOOLEAN | Second harmonic block signal, phase L2 | |
| BLK2HL3 | BOOLEAN | Second harmonic block signal, phase L3 | |
| BLK5H | BOOLEAN | Common fifth harmonic block signal from any phase | |
| BLK5HL1 | BOOLEAN | Fifth harmonic block signal, phase L1 | |
| BLK5HL2 | BOOLEAN | Fifth harmonic block signal, phase L2 | |
| BLK5HL3 | BOOLEAN | Fifth harmonic block signal, phase L3 | |
| BLKWAV | BOOLEAN | Common block signal, waveform criterion, from any phase | |
| BLKWAVL1 | BOOLEAN | Block signal, waveform criterion, phase L1 | |
| BLKWAVL2 | BOOLEAN | Block signal, waveform criterion, phase L2 | |
| BLKWAVL3 | BOOLEAN | Block signal, waveform criterion, phase L3 | |
| IDALARM | BOOLEAN | Alarm for sustained diff currents in all three phases | |
| OPENCT | BOOLEAN | An open CT was detected | |
| OPENCTAL | BOOLEAN | Open CT Alarm output signal. Issued after a delay … | |
| IDL1 | REAL | Value of the instantaneous differential current, phase L1 | |
| IDL2 | REAL | Value of the instantaneous differential current, phase L2 | |
| IDL3 | REAL | Value of the instantaneous differential current, phase L3 | |
| IDL1MAG | REAL | Magnitude of fundamental freq. diff. current, phase L1 | |
| IDL2MAG | REAL | Magnitude of fundamental freq. diff. current, phase L2 | |
| IDL3MAG | REAL | Magnitude of fundamental freq. diff. current, phase L3 | |
| IBIAS | REAL | Magnitude of the bias current, which is common to all phases | |
| IDNSMAG | REAL | Magnitude of the negative sequence differential current | |
T3WPDIF Group settings (basic)
| T3WPDIF Group settings (basic) | |||||
| Name | Values (Range) | Unit | Step | Default | Description |
| Operation | Off On |
– | – | Off | Operation Off / On |
| SOTFMode | Off On |
– | – | On | Operation mode for switch onto fault feature |
| tAlarmDelay | 0.000 – 60.000 | s | 0.001 | 10.000 | Time delay for diff currents alarm level |
| IDiffAlarm | 0.05 – 1.00 | IB | 0.01 | 0.20 | Dif. cur. alarm, multiple of base curr, usually W1 curr. |
| IdMin | 0.10 – 0.60 | IB | 0.01 | 0.30 | Section1 sensitivity, multi. of base curr, usually W1 curr. |
| IdUnre | 1.00 – 100.00 | IB | 0.01 | 10.00 | Unrestr. prot. limit, multi. of base curr. usually W1 curr. |
| CrossBlockEn | Off On |
– | – | On | Operation Off/On for cross-block logic between phases |
| NegSeqDiffEn | Off On |
– | – | On | Operation Off/On for neg. seq. differential protections |
| IMinNegSeq | 0.02 – 0.20 | IB | 0.01 | 0.04 | Neg. seq. curr. limit, mult. of base curr, usually W1 curr. |
| NegSeqROA | 30.0 – 120.0 | Deg | 0.1 | 60.0 | Operate Angle for int. / ext. neg. seq. fault discriminator |
T3WPDIF Group settings (advanced)
| T3WPDIF Group settings (advanced) | |||||
| Name | Values (Range) | Unit | Step | Default | Description |
| EndSection1 | 0.20 – 1.50 | IB | 0.01 | 1.25 | End of section 1, multi. of base current, usually W1 curr. |
| EndSection2 | 1.00 – 10.00 | IB | 0.01 | 3.00 | End of section 2, multi. of base current, usually W1 curr. |
| SlopeSection2 | 10.0 – 50.0 | % | 0.1 | 40.0 | Slope in section 2 of operate-restrain characteristic, in % |
| SlopeSection3 | 30.0 – 100.0 | % | 0.1 | 80.0 | Slope in section 3 of operate-restrain characteristic, in % |
| I2/I1Ratio | 5.0 – 100.0 | % | 0.1 | 15.0 | Max. ratio of 2nd harm. to fundamental harm dif. curr. in % |
| I5/I1Ratio | 5.0 – 100.0 | % | 0.1 | 25.0 | Max. ratio of 5th harm. to fundamental harm dif. curr. in % |
| OpenCTEnable | Off On |
– | – | Off | Open CT detection feature. Open CTEnable Off/On |
| tOCTAlarmDelay | 0.100 – 10.000 | s | 0.001 | 3.000 | Open CT: time in s to alarm after an open CT is detected |
| tOCTResetDelay | 0.100 – 10.000 | s | 0.001 | 0.250 | Reset delay in s. After delay, diff. function is activated |
| tOCTUnrstDelay | 0.10 – 6000.00 | s | 0.01 | 10.00 | Unrestrained diff. protection blocked after this delay, in s |
T3WPDIF Non group settings (basic)
| T3WPDIF Non group settings (basic) | |||||
| Name | Values (Range) | Unit | Step | Default | Description |
| GlobalBaseSelW1 | 1 – 12 | – | 1 | 1 | Global base selector for winding 1 |
| GlobalBaseSelW2 | 1 – 12 | – | 1 | 1 | Global base selector for winding 2 |
| GlobalBaseSelW3 | 1 – 12 | – | 1 | 1 | Global base selector for winding 3 |
| ConnectTypeW1 | WYE (Y) Delta (D) |
– | – | WYE (Y) | Connection type of winding 1: Y-wye or D-delta |
| ConnectTypeW2 | WYE (Y) Delta (D) |
– | – | WYE (Y) | Connection type of winding 2: Y-wye or D-delta |
| ConnectTypeW3 | WYE (Y) Delta (D) |
– | – | Delta (D) | Connection type of winding 3: Y-wye or D-delta |
| ClockNumberW2 | 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] |
– | – | 0 [0 deg] | Phase displacement between W2 & W1=HV winding, hour notation |
| ClockNumberW3 | 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] |
– | – | 5 [150 deg lag] | Phase displacement between W3 & W1=HV winding, hour notation |
| ZSCurrSubtrW1 | Off On |
– | – | On | Enable zer. seq. current subtraction for W1 side, On / Off |
| ZSCurrSubtrW2 | Off On |
– | – | On | Enable zer. seq. current subtraction for W2 side, On / Off |
| ZSCurrSubtrW3 | Off On |
– | – | On | Enable zer. seq. current subtraction for W3 side, On / Off |
| TconfigForW1 | No Yes |
– | – | No | Two CT inputs (T-config.) for winding 1, YES / NO |
| CT1RatingW1 | 1 – 99999 | A | 1 | 3000 | CT primary rating in A, T-branch 1, on transf. W1 side |
| CT2RatingW1 | 1 – 99999 | A | 1 | 3000 | CT primary in A, T-branch 2, on transf. W1 side |
| TconfigForW2 | No Yes |
– | – | No | Two CT inputs (T-config.) for winding 2, YES / NO |
| CT1RatingW2 | 1 – 99999 | A | 1 | 3000 | CT primary rating in A, T-branch 1, on transf. W2 side |
| CT2RatingW2 | 1 – 99999 | A | 1 | 3000 | CT primary rating in A, T-branch 2, on transf. W2 side |
| TconfigForW3 | No Yes |
– | – | No | Two CT inputs (T-config.) for winding 3, YES / NO |
| CT1RatingW3 | 1 – 99999 | A | 1 | 3000 | CT primary rating in A, T-branch 1, on transf. W3 side |
| CT2RatingW3 | 1 – 99999 | A | 1 | 3000 | CT primary rating in A, T-branch 2, on transf. W3 side |
| LocationOLTC1 | Not Used Winding 1 (W1) Winding 2 (W2) Winding 3 (W3) |
– | – | Not Used | Transformer winding where OLTC1 is located |
| LowTapPosOLTC1 | 0 – 10 | – | 1 | 1 | OLTC1 lowest tap position designation (e.g. 1) |
| RatedTapOLTC1 | 1 – 100 | – | 1 | 6 | OLTC1 rated tap/mid-tap position designation (e.g. 6) |
| HighTapPsOLTC1 | 1 – 100 | – | 1 | 11 | OLTC1 highest tap position designation (e.g. 11) |
| TapHighVoltTC1 | 1 – 100 | – | 1 | 1 | OLTC1 end-tap position with winding highest no-load voltage |
| StepSizeOLTC1 | 0.01 – 30.00 | % | 0.01 | 1.00 | Voltage change per OLTC1 step in percent of rated voltage |
| LocationOLTC2 | Not Used Winding 1 (W1) Winding 2 (W2) Winding 3 (W3) |
– | – | Not Used | Transformer winding where OLTC2 is located |
| LowTapPosOLTC2 | 0 – 10 | – | 1 | 1 | OLTC2 lowest tap position designation (e.g. 1) |
| RatedTapOLTC2 | 1 – 100 | – | 1 | 6 | OLTC2 rated tap/mid-tap position designation (e.g. 6) |
| HighTapPsOLTC2 | 1 – 100 | – | 1 | 11 | OLTC2 highest tap position designation (e.g. 11) |
| TapHighVoltTC2 | 1 – 100 | – | 1 | 1 | OLTC2 end-tap position with winding highest no-load voltage |
| StepSizeOLTC2 | 0.01 – 30.00 | % | 0.01 | 1.00 | Voltage change per OLTC2 step in percent of rated voltage |
T3WPDIF Monitored data
| T3WPDIF Monitored data | ||||
| Name | Type | Values (Range) | Unit | Description |
| IDL1MAG | REAL | – | A | Magnitude of fundamental freq. diff. current, phase L1 |
| IDL2MAG | REAL | – | A | Magnitude of fundamental freq. diff. current, phase L2 |
| IDL3MAG | REAL | – | A | Magnitude of fundamental freq. diff. current, phase L3 |
| IBIAS | REAL | – | A | Magnitude of the bias current, which is common to all phases |
| IDNSMAG | REAL | – | A | Magnitude of the negative sequence differential current |
Logics & highlights
CT group arrangement for differential protection
Typical CT location and definition of positive current direction
Fundamental frequency differential currents calculation
Application Notes
Matrices for differential current calculation (Part 1/2)
Matrices for differential current calculation (Part 2/2)
Matrix equation for a sample two winding power transformer
Description of the restrained, and the unrestrained operate characteristics
Fundamental frequency negative sequence differential currents calculation
Operating characteristic of the internal/external fault discriminator
Trajectories of Negative Sequence Current Contributions from HV and LV sides of Yd5 power transformer during external fault
Operation of the internal/external fault discriminator for internal fault with CT saturation
Inrush currents to a transformer as seen by a protective IED. Typical is a high amount of the 2nd harmonic, and intervals of low current, and low rate-of change of current within each period.
Treatment of measured currents within IED for transformer differential function
Transformer differential protection simplified logic diagram for Phase L1
Transformer differential protection simplified logic diagram for external/internal fault discriminator
Transformer differential protection internal grouping of tripping signals
Transformer differential protection internal grouping of logical signals
Differential current alarm logic
