Description
Overview of Three winding transformer protection
- The AST-442H transformer differential protection device is mainly applied to three winding transformer protection , realizing differential protection for high, medium and low voltage sides. It fits voltage levels up to 133 kV and adopts panel mounting installation style.
Transformer protection methods
- Differential protection ANSI 87
- 2nd & 3rd harmonic restraint
- Differential instantaneous trip protection ANSI 87
- Differential current overload alarm
- TA disconnection detection ANSI 50
- Non-electrical protection signal acquisition
- B-code & NTP time synchronization
Wiring Diagram
Setting Table
| No. | Setting Name | Unit | Range | Remark |
|---|---|---|---|---|
| 1 | Secondary Rated Current(Ie) | A | 0.05～30 | Calculated based on HV side rated current of transformer |
| 2 | Differential Protection Setting | Ie | 0.1～2 | |
| 3 | Differential Instantaneous Trip Setting | Ie | 0.1～20 | |
| 4 | Percentage Restraint Coefficient | – | 0.3～1 | |
| 5 | 2nd Harmonic Restraint Coefficient | – | 0.1～0.3 | |
| 6 | 3rd Harmonic Restraint Coefficient | – | 0.1～0.3 | |
| 7 | MV Side Balance Coefficient | – | 0～10 | |
| 8 | LV Side Balance Coefficient | – | 0～10 | |
| 9 | Differential Current Overlimit Setting | Ie | 0.1～1 | Generally 50% of differential protection setting |
| 10 | Differential Current Overlimit Delay | s | 0.1～20.00 |
Switch Status
| No. | Control Word | Option | Remark |
|---|---|---|---|
| 1 | yyy | □ Disable □ Enable | |
| 2 | ydd1 | □ Disable □ Enable | |
| 3 | ydy1 | □ Disable □ Enable | |
| 4 | yyd1 | □ Disable □ Enable | |
| 5 | ydd11 | □ Disable □ Enable | |
| 6 | ydy11 | □ Disable □ Enable | |
| 7 | yyd11 | □ Disable □ Enable | |
| 8 | dyy11 | □ Disable □ Enable | |
| 9 | dyy1 | □ Disable □ Enable | |
| 10 | Differential Instantaneous Trip | □ Disable □ Enable | |
| 11 | 2nd Harmonic Restraint | □ Disable □ Enable | |
| 12 | 3rd Harmonic Restraint | □ Disable □ Enable | |
| 13 | Phase A Differential Protection | □ Disable □ Enable | |
| 14 | Phase B Differential Protection | □ Disable □ Enable | |
| 15 | Phase C Differential Protection | □ Disable □ Enable | |
| 16 | Differential Current Overlimit Alarm | □ Disable □ Enable | |
| 17 | CT Disconnection Signal Output | □ Disable □ Enable | |
| 18 | Differential Blocking by CT Disconnection | □ Disable □ Enable |
Outline and Installation Dimensions
FAQ
Q：What is the protection scope of transformer differential protection?
A:The scope of differential protection in transformer covers internal windings, insulating bushings and connecting wires on all sides.
Q：How to Select Differential Protection Current Transformers?
A:When selecting current transformers (CTs) for differential protection, choose Type P or TPY CTs of the same model to ensure that the turns ratio matches, the accurate limit factor (ALF) is sufficiently high (at least 20 times), and the actual load is less than the rated load, in order to prevent false tripping due to saturation caused by faults outside the protection zone.
Q：Core Issues and Solutions of Differential Protection of Star Delta Transformers Caused by Two-side Current Phase Deviation?
A:The core issue with Y/d transformer differential protection is that there is a 30° phase difference between the currents on the two sides; directly incorporating this would result in a significant unbalanced current. Solution: Connect the secondary windings of the CTs on the star side in a delta configuration, and the secondary windings of the CTs on the delta side in a star configuration. This compensates for the phase difference through the wiring, thereby canceling out the differential current.
Q：What functions are generally enabled for medium voltage transformer protection?
A:Generally speaking, medium voltage covers 6 kV to 35 kV. Main transformers shall be equipped with differential protection, backup protection and non-electrical protection. Distribution transformers require three-stage current protection, overload protection and zero-sequence current protection. Specific requirements vary with actual operating conditions.
Q：What is the calculation method for transformer differential protection?
A:
Transformer differential protection calculation:
The core of transformer differential protection calculations is the elimination of unbalanced currents. Take, for example, a 10/0.4 kV transformer with a Y/d-11 connection:
HV side CT ratio: 400/5
LV side CT ratio: 2000/5
The microprocessor-based protection multiplies HV side current by balance coefficient
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