Faults and Handling of Single-phase Grounding in 10kV Distribution Lines
Field: Manufacturing
China
Characteristics and Detection Devices for Single-Phase Ground Faults
1. Characteristics of Single-Phase Ground Faults
- Central Alarm Signals:
The warning bell rings, and the indicator lamp labeled “Ground Fault on [X] kV Bus Section [Y]” illuminates. In systems with a Petersen coil (arc suppression coil) grounding the neutral point, the “Petersen Coil Operated” indicator also lights up. - Insulation Monitoring Voltmeter Indications:
- The voltage of the faulted phase decreases (in case of incomplete grounding) or drops to zero (in case of solid grounding).
- The voltages of the other two phases increase—above normal phase voltage in incomplete grounding, or rising to line voltage in solid grounding.
- In stable grounding, the voltmeter needle remains steady; if it fluctuates continuously, the fault is intermittent (arc grounding).
- In Petersen Coil-Grounded Systems:
If a neutral displacement voltmeter is installed, it shows a certain reading during incomplete grounding or reaches phase voltage during solid grounding. The Petersen coil’s ground alarm light also activates. - Arc Grounding Phenomena:
Arc grounding generates overvoltages, causing non-fault phase voltages to rise significantly. This may blow the high-voltage fuses of voltage transformers (VTs) or even damage the VTs themselves.
2. Distinguishing True Ground Faults from False Alarms
- Blown High-Voltage Fuse in VT:
A blown fuse in one phase of the VT can trigger a ground fault signal. However:- In an actual ground fault: faulted phase voltage drops, other two phases rise, but line voltage remains unchanged.
- With a blown fuse: one phase voltage drops, the other two do not rise, and line voltage decreases.
- Transformer Charging an Unloaded Bus:
During energization, if the circuit breaker closes asynchronously, unbalanced capacitive coupling to ground causes neutral displacement and asymmetric three-phase voltages, triggering a false ground signal.
→ This occurs only during switching operations. If bus and connected equipment show no abnormalities, the signal is false. Energizing a feeder line or station service transformer usually eliminates the indication. - System Asymmetry or Improper Petersen Coil Tuning:
During operational mode changes (e.g., switching configurations), asymmetry or incorrect Petersen coil compensation may cause false ground signals.
→ Coordination with dispatch is required: revert to original configuration, de-energize the Petersen coil, adjust its tap changer, then re-energize and switch modes again.
→ Ferroresonance during no-load bus energization can also produce false signals. Immediately energizing a feeder line disrupts resonance conditions and clears the alarm.
3. Detection Devices
The insulation monitoring system typically consists of a three-phase five-limb voltage transformer, voltage relays, signal relays, and monitoring instruments.
- Structure: Five magnetic limbs; one primary winding and two secondary windings, all wound on the three central limbs.
- Wiring Configuration: Ynynd (star-primary, star-secondary with neutral, and open-delta tertiary).
Advantages of this wiring:
- The first secondary winding measures both line and phase voltages.
- The second secondary winding is connected in open delta to detect zero-sequence voltage.
Operation Principle:
- Under normal conditions, the three-phase voltages are balanced; theoretically, zero voltage appears across the open delta.
- During a solid single-phase ground fault (e.g., Phase A), zero-sequence voltage appears in the system, inducing voltage across the open delta.
- Even during non-solid (high-impedance) grounding, a voltage is induced at the open ends.
- When this voltage reaches the pickup threshold of the voltage relay, both the voltage relay and signal relay operate, triggering audible and visual alarms.
Operators use these signals and voltmeter readings to identify the occurrence and phase of the ground fault, then report to the dispatcher.
⚠️ Note: The insulation monitoring device is shared across the entire bus section.
⚠️ Note: The insulation monitoring device is shared across the entire bus section.
Causes of Single-Phase Ground Faults
- Broken conductor falling to ground or resting on a crossarm;
- Loose binding or fixing of conductors on insulators, causing them to drop onto crossarms or ground;
- Excessive wind causing conductors to approach buildings too closely;
- Broken high-voltage lead wire from distribution transformer;
- Insulation failure in 10 kV surge arresters or fuses on transformer platforms;
- Insulation breakdown or grounding in one phase of the transformer high-voltage winding;
- Insulator flashover or puncture;
- Insulation failure in branch-line fuses;
- Detached guy wire from upper crossarm on multi-circuit poles contacting lower conductors;
- Lightning strikes;
- Tree contact;
- Bird-related faults;
- Foreign objects (e.g., plastic sheets, branches);
- Other accidental or unknown causes.
Hazards of Single-Phase Ground Faults
- Damage to Substation Equipment:
After a 10 kV ground fault, the bus VT detects no current but develops zero-sequence voltage and increased current in the open delta. Prolonged operation can damage the VT.
Additionally, ferroresonant overvoltages (several times normal voltage) may occur, breaking down insulation and causing major equipment failures. - Damage to Distribution Equipment:
Intermittent arc grounding and overvoltages can puncture insulation, leading to short circuits, burned-out transformers, and failed arresters/fuses, potentially causing electrical fires. - Threat to Regional Grid Stability:
Severe ground faults may destabilize the local power grid, triggering cascading failures. - Risk to Humans and Animals:
Downed conductors energize the ground, creating step voltage hazards. Pedestrians, linemen (especially during night patrols), and livestock near the fault site risk electric shock or electrocution. - Impact on Power Supply Reliability:
- Requires manual faulted-feeder selection.
- Non-faulted feeders may be unnecessarily de-energized during troubleshooting, interrupting supply to unaffected customers.
- Fault location and repair require line outage, especially challenging during crop-growing seasons, adverse weather (wind, rain, snow), or in mountainous/forested areas and at night, leading to prolonged, widespread outages.
- Line Energy Losses:
Ground faults cause significant earth leakage currents, representing direct energy loss. Regulations typically limit ground-fault operation to no more than 2 hours to avoid excessive waste. - Electricity Loss Quantification:
Average ground fault current ranges from 6 to 10 A. At typical 10 kV levels, this results in approximately 34,560 kWh of wasted energy per 24-hour period.
Methods and Procedures for Handling Single-Phase Ground Faults
- Small-Current Ground Fault Auto-Selection Devices:
Install automatic ground-fault line-selection devices in substations. These work with zero-sequence current transformers (ZCTs) at each feeder outlet to accurately identify the faulted line before isolation. - Single-Phase Ground Fault Detection Systems:
Modern distribution systems deploy signal injectors at the beginning, middle, and end of feeders. Fault indicators pinpoint the exact fault location, enabling rapid response. - Preventive Measures:
- Conduct regular line patrols: check conductor clearances to trees/buildings, bird nests on poles, secure conductor binding on insulators, loose bolts on insulators/crossarms/guy wires, broken or frayed guy wires, and abnormal conductor sag.
- Periodically test insulation of insulators, branch fuses, and surge arresters; replace defective units promptly.
- Perform routine tests on distribution transformers; repair or replace faulty units.
- Install branch fuses on rural feeders to limit fault scope, reduce outage area/duration, and speed up fault location.
- Use insulators rated one voltage class higher to enhance overall system insulation strength.
- Post-Fault Response Procedure:
Upon detection of a single-phase ground fault:- Substation operators must immediately log the event, report to the dispatcher and responsible personnel, and follow dispatch instructions to locate the fault.
- Sequentially open feeder breakers; when the ground signal disappears after opening a specific line, that line is identified as the faulted circuit.
Give a tip and encourage the author!
Recommended
Neutral point grounding operation mode for 110kV～220kV power grid transformers
The arrangement of neutral point grounding operation modes for 110kV～220kV power grid transformers shall meet the insulation withstand requirements of transformer neutral points, and shall also strive to keep the zero-sequence impedance of substations basically unchanged, while ensuring that the zero-sequence comprehensive impedance at any short-circuit point in the system does not exceed three times the positive-sequence comprehensive impedance.For 220kV and 110kV transformers in new constructi
01/29/2026
Why Do Substations Use Stones, Gravel, Pebbles, and Crushed Rock?
Why Do Substations Use Stones, Gravel, Pebbles, and Crushed Rock?In substations, equipment such as power and distribution transformers, transmission lines, voltage transformers, current transformers, and disconnect switches all require grounding. Beyond grounding, we will now explore in depth why gravel and crushed stone are commonly used in substations. Though they appear ordinary, these stones play a critical safety and functional role.In substation grounding design—especially when multiple gr
01/29/2026
HECI GCB for Generators – Fast SF6 Circuit Breaker
1.Definition and Function1.1 Role of the Generator Circuit BreakerThe Generator Circuit Breaker (GCB) is a controllable disconnect point located between the generator and the step-up transformer, serving as an interface between the generator and the power grid. Its primary functions include isolating generator-side faults and enabling operational control during generator synchronization and grid connection. The operating principle of a GCB is not significantly different from that of a standard c
01/06/2026
Design Principles for Pole-Mounted Distribution Transformers
Design Principles for Pole-Mounted Distribution Transformers(1) Location and Layout PrinciplesPole-mounted transformer platforms should be located near the load center or close to critical loads, following the principle of “small capacity, multiple locations” to facilitate equipment replacement and maintenance. For residential power supply, three-phase transformers may be installed nearby based on current demand and future growth projections.(2) Capacity Selection for Three-Phase Pole-Mounted Tr
12/25/2025
Related Products
