PT Fuse Slow Blow: Sababi, Inganci da Kainganci

I. Turutan na Fus da Karamin Sababu

Fus ta Yawo Daga Bisa:
Daga hukumomin fus, idan yawan abubuwa mai kaiya ya gama shi, saboda adadin kayayyakin (kayayyaki masu tufafi a kan addinin kayayyakin) an yi fus a cikin rukuni mai tai. An yi fus a cikin rukuni mai tai. An yi fus a cikin rukuni mai tai. An yi fus a cikin rukuni mai tai. An yi fus a cikin rukuni mai tai.

Amma, saboda yanayin aiki mai kuli, zai iya lafiya wani fus a cikin rukuni mai tai. Wannan zai iya haifar da fus a cikin rukuni mai tai. Idan fus ta yawo daga baya, an yi fus a cikin rukuni mai tai. Idan fus ta yawo daga baya, an yi fus a cikin rukuni mai tai. Idan fus ta yawo daga baya, an yi fus a cikin rukuni mai tai. Idan fus ta yawo daga baya, an yi fus a cikin rukuni mai tai.

Tushen PT Fus Mai Yawo Daga Bisa:
Idan fus na PT na mafi yawa ba ta kula a ranar da aka sani, zai iya haifar da amfani na TV a cikin rukuni mai tai.

II. Tushen PT Fus Mai Yawo Daga Bisa

• Sistem na excitation ya fara forcing, wanda yake haifar da over-excitation da overvoltage protection.

• Maloperation na stator ground fault protection.

• Overloading na generator da turbine, wanda zai iya haifar da kafara a cikin yanayin.

III. Karamin Sababu

• Yanayin da ake amfani a cikin primary plug-in contacts na output voltage transformer sun haifar da oxidation layers da poor contact; bolta na connection sun haifar da temperature rise a cikin rukuni mai tai.

• Yanayin da ake amfani a cikin primary plug-in contacts na output voltage transformer sun haifar da oxidation layers da poor contact; bolta na connection sun haifar da temperature rise a cikin rukuni mai tai.

• PT fuses na batu ba su da kyau, suna iya haifar da degradation ko premature failure a cikin yanayin.

• Transient overvoltages na sudden breaker closing ko intermittent arc grounding sun haifar da ferroresonance, wanda yake haifar da primary da secondary fuse blowing a cikin voltage transformers.

• Low-frequency saturation current sun haifar da primary da secondary fuse blowing a cikin voltage transformers.

• Reduced insulation ko short circuits a cikin primary/secondary windings na voltage transformer, ko degraded insulation a cikin harmonic suppressor, sun haifar da fuse blowing.

• Single-phase-to-ground faults sun haifar da voltage transformer burnout.

• Generators suna haifar da grounded via an arc suppression coil a cikin neutral point. Amma, wannan configuration sun haifar da neutral point displacement voltage, wanda yake haifar da one ko two phases suka samu voltages mai kuli a cikin lokacin da ke da ita, wanda yake haifar da PT fuse blowing.

IV. Tashin Da Ya Haifar Da Tsakasa

• Don oxidation da poor contact a cikin primary plug contacts saboda material mismatch, yi polishing na contact surface a cikin maintenance da apply conductive grease.

• Don unstable fuse quality, replace high-voltage primary fuses periodically according to the equipment maintenance schedule. Contact surfaces must be de-oxidized and coated with conductive grease.

• Don systems na batu: bayan an push the PT trolley to the service position, verify all conductive connections are secure and free of looseness. If necessary, withdraw the trolley and tighten bolts. During unit outages with no work on the generator primary or generator outlet PT circuits, keep the generator outlet PT in standby (do not disconnect it). Only open the secondary circuit breaker. This minimizes frequent insertion/removal, preventing fuse drop, mechanical damage, or poor contact with socket spring clips—reducing the likelihood of high-voltage fuse failure. (Before placing the generator in hot standby, operating personnel must verify the integrity of the primary PT fuse.)

• During single-phase-to-ground faults, if the generator operates at rated frequency, transient overvoltage on healthy phases can reach up to 2.6 times the rated phase voltage. Therefore, generator outlet voltage transformers must be selected to withstand these overvoltages:

• Steady-state overvoltage withstand ≥ line voltage

• Transient overvoltage withstand ≥ 2.6 × rated phase voltage
  PT fuse selection must not only isolate internal transformer short circuits but also protect against overvoltage conditions such as voltage rise and ferroresonance.

Primary harmonic suppression: Install a grounding voltage transformer between the primary neutral point of the VT and ground. This effectively suppresses or eliminates overvoltage in the primary winding and prevents ferroresonance and transformer burnout.

Secondary harmonic suppression: Install a damping device (secondary harmonic suppressor) across the open delta of the VT’s residual winding. Modern microprocessor-based harmonic suppressors detect incipient resonance and instantly connect a damping resistor to eliminate ferroresonance. When the generator neutral is grounded via an arc suppression coil (whose inductance is much smaller than the VT’s magnetizing inductance), ferroresonance overvoltage is effectively prevented. Therefore, ferroresonance need not be considered in PT fuse blow analysis.

Coordinate with the excitation system manufacturer to ensure the excitation regulator includes logic to detect slow blowing of PT primary fuses (considering single-phase, two-phase, and three-phase fuse failure scenarios) and secondary circuit breaks. Upon detecting a PT break, the main excitation channel should automatically switch from AVR mode to FCR mode, or switch to the backup channel. Adjust the threshold settings in the PT break detection logic to reduce false triggering of field forcing due to poor PT circuit contact, thereby improving system sensitivity and reliability.

V. Hukumar Don Tabbatar PT Slow Fuse Blow

Criterion 1: Introduction of Zero-Sequence and Negative-Sequence Voltage

a) Zero-Sequence Voltage Method
Monitor the open-delta voltage on the PT secondary side. Compare the generator terminal zero-sequence voltage with the neutral point zero-sequence voltage. If the absolute difference exceeds a preset threshold, a PT slow fuse blow is indicated. In this case, the stator negative-sequence current criterion must be blocked.

b) Negative-Sequence Voltage Method
The excitation system only measures generator terminal voltage, not neutral point voltage, making the zero-sequence method inapplicable. Instead, decompose the PT secondary voltage to extract the negative-sequence component. If the negative-sequence voltage exceeds a set threshold, a PT primary fuse slow blow is detected. The stator negative-sequence current criterion must also be blocked.

Criterion 2:
UAB – Uab > 5V
UBC – Ubc > 5V
UCA – Uca > 5V

Key Point: Use zero-sequence, negative-sequence, and voltage comparison methods. Never use positive-sequence voltage (used by protection relays) to detect primary PT fuse failure, because the broken phase still induces voltage (not zero), which may not satisfy positive-sequence criteria.

A primary PT fuse break causes induced imbalance in the secondary EMF, resulting in voltage at the open delta and triggering a zero-sequence alarm. This phenomenon does not occur with a secondary fuse blow—this is the primary distinguishing criterion between primary and secondary fuse failures.

A primary PT fuse break reduces the secondary induced voltage (because the other two phases still produce flux in the core), so the corresponding secondary phase voltage decreases. In contrast, a secondary fuse break removes the winding from the circuit, causing the phase voltage to drop to zero.