Due to natural disasters like thunderstorms, wind, and snow, as well as external forces and environmental pollution, 10kV lines in operation often experience trip faults. When a trip fault occurs on a 10kV line, it can cause large-scale power outages on the entire line or parts of it, leading to economic losses for power consumers. Therefore, the sooner the fault location and cause are identified, the sooner the hidden danger can be eliminated, the scope of the power outage can be reduced, and power supply can be restored, thereby minimizing economic losses for both power suppliers and consumers. To facilitate quick identification, the methods for locating 10kV line faults are summarized in the following mnemonic.
1. Quick Location of 10kV Line Faults
(1) Mnemonic for Fault Location
- Line fault causes power outage, protection action helps judge.
- Fast tripping means check front end, usually about half the total length.
- Small overcurrent means fault is further away.
- Fast tripping with overcurrent tripping together means the fault is in the middle of the line.
(2) Explanation of the Mnemonic
The type of fault protection action varies depending on the location of the fault along the line.
- Fast tripping protection action: The protection range of fast tripping is generally at maximum under the system’s largest operating condition, covering approximately 50% of the total line length. Under the minimum operating condition, the protection range is at its smallest, covering about 15% to 20% of the total line length. Therefore, if the fast tripping protection device acts and trips, it indicates that the fault is likely located in the front section of the line (near the substation side).
- Overcurrent protection device action: The protection range for overcurrent protection covers 100% of the protected line. However, overcurrent protection devices are typically equipped with time delay relays and only trip when faults occur in the rear section of the line in coordination with fast tripping protection.
- Simultaneous action of fast tripping and overcurrent protection: This generally indicates that the fault lies within the common range of both protections, typically in the middle section of the line.
Therefore, when a substation circuit breaker trips, it is necessary to promptly investigate the relay protection action. Based on the type and characteristics of the relay protection action, the nature and range of the fault can be approximately located.
2. Judgment of Ground Fault in 10kV Line
(1) Mnemonic for Ground Fault Judgment
- Ground fault judgment is skillful, with one low and two highs remaining unchanged.
- Disconnection and ground fault combined, one high and two lows are common.
- Difficult to distinguish between disconnection and ground faults, user voltage differentiates.
- Disconnected users have only two-phase voltage, grounding users’ voltage changes are not obvious.
(2) Explanation of the Mnemonic
Upon receiving a notice from the duty dispatcher about a ground fault on the line, power supply personnel or electricians in factories and mines should understand: which phase is grounded, what are the voltage values of each phase to ground? Is the voltage changing continuously or stable? This helps further analyze the grounding situation and locate the fault point as quickly as possible.
1. Metallic Grounding: One phase-to-ground voltage approaches zero, while the other two phase-to-ground voltages increase by √3 times, with no change in three-phase line voltage.
Non-Metallic Grounding: One phase-to-ground voltage decreases but is not zero, while the other two phase-to-ground voltages increase but do not reach √3 times.
2. Characteristics of Non-Metallic Grounding and High-Voltage Disconnection:
- High-voltage disconnection with the load side conductor falling on wet ground causes the two non-disconnected phases to connect to the grounding conductor, forming non-metallic grounding. This lowers the phase-to-ground voltage for the disconnected phase.
- High-voltage disconnection without grounding or falling on poorly conductive objects, or a blown fuse on the line, can lead to imbalance in three-phase capacitance current, causing two-phase voltages to imbalance and the disconnected phase voltage to rise.
- Burnt transformer windings grounding through the shell with blown fuses can also raise the grounded phase voltage, lowering the other two-phase voltages.
3. Distinguishing High-Voltage Disconnection from Non-Metallic Grounding:
After receiving a notice from the dispatcher, power supply personnel must distinguish high-voltage disconnection from non-metallic grounding by checking the voltage balance at end users to determine if the issue is due to high-voltage disconnection or non-metallic grounding. Disconnected users have only two-phase voltage, while grounding users’ voltage changes are not obvious.
3. Insulation Judgment for 10kV Line Ground Fault
(1) Mnemonic for Insulation Judgment
- Measure insulation for line faults, below 40MΩ is unhealthy.
- Transformer switches not pulled, below 30MΩ is unsafe.
- Single insulator 300MΩ, porcelain insulator 200MΩ, lower values indicate hidden danger.
(2) Explanation of the Mnemonic
The overall insulation testing method can quickly and effectively identify poor insulation as the key to finding line ground faults.
- Overall Insulation Testing Method: This is suitable for shorter lines with fewer distribution transformers and no cross-over with other 10kV or higher voltage lines. Before implementing this method, ensure safety measures are in place to prevent back-feeding on the test line, especially when no short-circuit grounding wire can be hung at both ends of the working line. Measure the insulation resistance at the maximum segment points on both sides of the line, or consider a branch line as an entire line for insulation resistance testing.
This method is useful for monitoring overall line insulation and identifying poor insulation when traditional methods fail. By comparing insulation values on both sides of the testing point, the lower value indicates the faulty segment.
- Insulation Value Comparison: In normal conditions, the insulation values of the A, B, and C phases on the same side are generally similar. Therefore, comparing the insulation values of the faulty segment, the phase with the lowest insulation value is likely the faulty phase. Follow this method to locate the fault segment until the fault point is found.
4. Insulator Values
- Single Suspension Insulator (300MΩ)
- Post Insulator (200MΩ)
For specific line segments, measure and record insulation resistance values and environmental temperatures when the line is in operation to create a complete insulation file. This forms a solid foundation for longitudinal and horizontal comparison of insulation data during preventive tests to determine line insulation quality. For ground fault location on sunny days, insulation values below 40MΩ are considered unqualified, and below 30MΩ when transformer switches are not pulled.
For specific line segments, compare insulation values with the most recent preventive test values. A significant decrease indicates insulation damage. For lines with fewer segments, disconnect the tensile rods in the middle, and test insulation on both sides to determine the fault point.
