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87T 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.
ANSI:87/87T/50
Communication Mode
Optional: RS-485, CAN bus, Ethernet, IEC 60870-5-103 (IEC-103), IEC 61850
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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 of Three winding transformer protection
| 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:
Calculate secondary rated current of each side:
HV side CT ratio: 400/5
LV side CT ratio: 2000/5
Balance coefficient and phase compensation:
The microprocessor-based protection multiplies HV side current by balance coefficient
The microprocessor-based protection multiplies HV side current by balance coefficient
and implements Y-Δ phase shift to eliminate normal differential current.
Setting parameters:Minimum operating current: 0.4 times rated current;Percentage restraint coefficient: 0.5;2nd harmonic restraint coefficient: 0.15 (to block inrush current);Differential instantaneous trip setting: 6 times rated current
Q: What is a three winding transformer protection?
A: It is a complete protection scheme designed for transformers with high-voltage, medium-voltage and low-voltage three windings, covering main internal faults, external backup faults and abnormal operating conditions to prevent equipment damage.
Q: What are the core main protections?
A: Three-winding differential protection is the primary main protection, responding to winding short circuits, inter-turn faults and bushing faults inside the transformer.
Q: What standard protection functions are generally equipped?
- Main protection: Three-winding differential protection, gas (Buchholz) protection
- Backup protection: Overcurrent protection, zero-sequence overcurrent protection, negative sequence overcurrent protection
- Abnormal condition protection: Overload protection, overvoltage protection, winding over-temperature protection, pressure relief alarm
Q: Why cannot use two-winding differential protection directly?
A: Two-winding differential only calculates current between two sides. The third winding will generate circulating current leading to protection misoperation, so dedicated three-winding differential algorithm is required to collect currents from all three ports for vector compensation.
Q: Key CT wiring requirements for three-winding differential
A: CT polarity and transformation ratio must be consistent for all three sides; perform phase angle compensation according to transformer connection group (such as Yd11); eliminate unbalanced current caused by no-load switching inrush current.
Q: What causes large unbalanced current during commissioning?
A: Wrong CT polarity, mismatched CT ratio, incorrect connection group setting, poor secondary wiring contact or CT saturation.
Q: How does differential protection avoid misoperation during transformer energization?
A: Adopt second harmonic restraint, fifth harmonic restraint or waveform breaking logic to block differential action when detecting magnetizing inrush current.
Q: Which side will the backup overcurrent protection trip?
A: Configured with directional locking and time grading. It selectively trips the faulty winding side first; if the fault persists, trip the high voltage incoming breaker.
Q: Differential protection trips without internal fault
A: Check CT secondary open circuit / multi-point grounding, wiring loosening, parameter setting error, external short-circuit CT saturation, or harmonic interference.
Q: Protection refuses to act on internal winding fault
A: CT circuit disconnection, differential pickup value set too high, restraint threshold improperly configured, or main protection exit manually.
Q: What scenarios apply to three winding transformer protection?
A: Substation main transformers with three voltage classes, power plant auxiliary transformers, photovoltaic/wind booster station main transformers with two outgoing feeders plus incoming power supply.
Q: How to coordinate with upper and lower level protection?
A: Set stepped time delays stage by stage. Transformer backup protection action time is longer than outlet line protection to ensure fault isolation selectivity.
Q: Routine inspection items
A: Verify CT secondary loop integrity, check differential unbalance current regularly, back up protection fixed values, test protection trip logic annually, and link with breaker control circuit.
Q: What is the function of Buchholz protection?
A: Acts on gas accumulation and oil flow speed inside the transformer tank. Light gas sends alarm; heavy gas directly trips the transformer to respond to internal slow faults and severe short-circuit faults.
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xiao zhang –
The three-winding transformer differential protection adopts advanced multi-branch braking algorithm to restrain magnetizing inrush and CT saturation effectively. It precisely identifies internal winding faults of three-winding transformers and trips rapidly, perfectly matching substation main transformer protection configuration.
Jack –
Accurate protection, safe to operate.
Jack –
High-quality product, top choice for relay protection.
Jack –
Fast response, ensures safety — very good.