The voltage of the 10kV bus fluctuates between high and low, and the culprit is actually the PT cabinet fuse that is constantly on and off!

2025-09-05 13:56:46

A strange phenomenon occurred in a 35kV substation for several consecutive days: the 10kV bus voltage fluctuated irregularly between 6.5kV and 11.3kV, causing the reactive power compensation device to frequently switch on and off, and even triggering load alarms. After three days of investigation, the root cause of the malfunction was finally discovered - a high-voltage fuse in the PT cabinet that appeared to be normal but had poor internal contact.

Fault phenomenon: Seemingly complex systemic issues

-Monitoring system: The displayed value of the bus voltage fluctuates sharply, and the imbalance of the three-phase voltage varies greatly

-Protection device: Occasionally reports a "PT disconnection" alarm, but quickly resets automatically

-On site inspection: The protective pressure plate and air switch are in normal positions with no obvious fault points

-Load situation: There is no significant correlation between voltage fluctuations and load size, ruling out the possibility of voltage fluctuations caused by load fluctuations

Fault mechanism: How does a fuse cause system voltage fluctuations?

1. During normal operation

graph LR

A [10kV busbar] -->B [PT cabinet]

B -->C [Voltage Transformer PT]

C -->D [Secondary voltage 100V]

D -->E [protective/measuring/metering device]

```

All devices obtain stable and accurate voltage signals

When the fuse has poor contact

graph LR

A [10kV busbar] -->B [PT cabinet]

B -->C [Poor contact of A-phase fuse]

C -->D [Voltage Transformer PT]

D -->E [Secondary voltage fluctuation 60-120V]

E -->F [protective/measuring/metering device]

```

The entire system voltage sampling is distorted, resulting in:

-The monitoring system displays voltage fluctuations

-Reactive power compensation misjudging system voltage

-Measurement device records abnormal data

Step by step investigation process: How to identify the "culprit"

Step 1: Preliminary inspection of the secondary circuit

1. Measurement of secondary voltage: Three phase secondary voltage was measured at the PT Terminal box, and it was found that the A-phase voltage fluctuated between 60V-120V

2. Check the wiring Terminals: Tighten all wiring terminals, and the fault persists

3. Exclude the influence of load: Disconnect the secondary load and measure directly at the PT outlet, but the voltage still fluctuates

Step 2: Key investigation of the primary circuit

1. Power outage inspection (after completing the power outage procedures):

-Measure the insulation resistance of PT primary and secondary windings using a 2500V megohmmeter:>1000M Ω, normal

-Measure PT DC resistance: Consistent with factory value

-Key inspection: When disassembling the A-phase fuse, it was found that there were slight oxidation and erosion marks on the lower contact of the fuse

Step 3: Fault point confirmation

1. Replace the fuse: Replace the A-phase fuse and send power

2. Continuous monitoring: The three-phase secondary voltage remains stable at 100 ± 2V

3. Conclusion confirmation: The cause of the fault is poor contact between the fuse and the base, resulting in the primary side circuit being switched on and off during operation

Why does poor contact of a fuse cause voltage fluctuations?

1. Unreliable physical connection:

-Insufficient pressure or oxidation of fuse base contacts

-The tolerance between the contact cap and the base of the fuse itself is too large

2. The "micro arc" effect of current:

-Small gaps are generated at poor contact points

-Voltage breakdown gap forms tiny intermittent arcs

-Sudden changes in arc resistance lead to abnormal voltage sampling

3. System resonance impact (most dangerous situation):

-PT nonlinear inductance and line to ground capacitance form resonance

-Poor contact may trigger ferromagnetic resonance, resulting in overvoltage several times higher than the rated voltage

Prevention and solution measures

1. Selection and installation of Fuses

2. Key points of daily inspection

-Infrared temperature measurement: Regularly use an infrared thermal imager to detect the temperature of PT fuses. Attention should be paid when the temperature difference is greater than 2K

-Sound monitoring: Listen for any slight "sizzling" discharge sound from the PT cabinet

-Observing voltage: Pay attention to voltage fluctuations, especially voltage abnormalities under light loads

3. Technical transformation suggestions

-Install harmonic elimination device: Install a microcomputer harmonic elimination device on the PT secondary side to prevent resonance overvoltage

-Replace the fuse base: Replace the old base with a new one with stronger elasticity and thicker silver plating

-Consider removing fuses: For important substations, a solution of using small air switches instead of fuses can be considered

Summary: The significant impact of small components

This malfunction has brought us important insights:

1. Don't ignore the basic components: Although high-voltage fuses are only worth a few hundred yuan, they can affect the stable operation of the entire substation

2. Check PT for voltage fluctuations first: When encountering system voltage fluctuations, priority should be given to checking PT and its related circuits

3. Maintenance needs to pay attention to details: fastening terminals, measuring contact resistance, and these "small things" cannot be ignored

Finally, a reminder: When dealing with PT circuit faults, safety regulations must be strictly followed to prevent personal injury accidents caused by PT secondary side reverse power transmission. The best practice when PT related faults are discovered is to immediately report and wait for professional personnel to handle them.