PT Fuse Slow Blow: سەبێکەکان، شیکاردناو پاشگیری

I. Strukturê Pîvzê û Analîza Sereke

Dûr Bûyandana Pîvzê:
Ji dîrokên dizaynê ya pîvzê, heta ke çendî meheng a hata nek di tîpa pîvzekê de yê bigihîne, ji ber tevahî metal (herêmên din dergeha ku ji alîyoyekan serbest bine) pîvza lê zêde di topa tinê de. Arka li ser rast dikare guhertina herêmê ya pîvzekê. Arka wê li ser kavirê hat îndaxistin.

Lê, ji ber cihêna kirîna sivis, tîpa pîvzekê dikare tuhaf bike ji herêmê da ku ser agirî û tevahî girîngiyê ve têne. Ev dikare vebike ku pîvza li ser baranîya normala bûyande. Ji ber ku pîvza li ser baranîya normala bûyade, procesa bûyanê lê zêde ye. Heke direksîyonê ya pîvzekê bi tevahî girîng bikat, amplitûda fezikê ya fazeyê girîng bikat, ku dikare maloperasyonê ya releyên parastina hejmaran bibe.

Têkiliya Bûyandanê ya Dûr a Pîvzê PT-ya:
Heke pîvza PT-ya li ser tarîfa bala nek bi destpiyawê xwe nabe, direksîyonê ya tubekê pîvzekê bi tevahî girîng bikat, ku dikare guhertina dibareyê ya dujmêna voltage transformer (TV) bi tevahî zêde bikat.

II. Têkiliyan a Bûyandanê ya Dûr a Pîvzê PT-ya

• Sistemê ya excitation field forcing dike, ku dikare aktivkirina over-excitation û overvoltage protection bibe.

• Maloperasyonê ya parastina hata stator ground fault.

• Overloading ê ya generator û turbine, ku dikare bixwazîna perantiya bibe di virsena vekirîna.

III. Analîza Sereke

• Materîalan jî yên bikar anîn di contactên plug-in ê ya primary de dikare tabakên oxidize û poor contact bide; boltên connection yên lê zêde bikin temperature rise li ser pîvzekê.

• Temperature ambienta bilind a pîvzê PT-ya. Tîpa pîvzekê ji metalên low-melting-point û zêde ye—mechanical vibration alone dikare bexwaşîne.

• Pîvzan PT yên çêd bikar anîn di operasyonê de dikane degradation û premature failure bide.

• Transient overvoltages ji closing-a breaker sudden û intermittent arc grounding dikane ferroresonance bide, ku dikare bûyandanê ya pîvzan primary û secondary voltage transformers bide.

• Low-frequency saturation current dikane bûyandanê ya pîvzan primary û secondary voltage transformers bide.

• Reduced insulation û short circuits di windings primary/secondary de voltage transformer, û degraded insulation harmonic suppressor, dikane bûyandanê ya pîvzê bide.

• Fault-an single-phase-to-ground dikane burnout a voltage transformer bide.

• Generators genellikê bi coil-a arc suppression di neutral point de grounding an. Lekin, ev configuration dikane voltage-a displacement neutral point increase bide, ku dikane phases yan phase-an li ser voltages-a significantly above normal bide, ku dikane bûyandanê ya pîvzê PT-ya bide.

IV. Çareseran Preventive

• Ji bo oxidation û poor contact di contactên primary plug de material mismatch, polish contact surface di maintenance de û apply conductive grease.

• Bi bo unstable quality a pîvzê, replace high-voltage primary fuses periodic according to the equipment maintenance schedule. Contact surfaces must be de-oxidized and coated with conductive grease.

• Ji bo systems û high vibration: after pushing 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. Methods for Detecting 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.