1. Pasiuna: Ang Kamahinungdanon sa Cable Fault Diagnosis
Sa modernong katilingban, Ang mga kable nagsilbing mga core carryer sa gahum, telekomunikasyon, ug mga kapatagan sa industriya, Sa ilang kasaligan nga direkta nga nakaapekto sa kaluwasan sa sistema ug lig-on nga operasyon. Hinuon, Ang mga sayup sa kable dili malikayan tungod sa mga hinungdan sa kalikopan, Mekanikal nga Stress, Pag-antos sa Pagkasabig, ug uban pang mga impluwensya. Ang mga outage o pagkabalda sa komunikasyon tungod niining mga kasaypanan moresulta sa dakong pagkawala sa ekonomiya kada tuig. Busa, Ang paghanas sa sistematiko ug episyente nga pag-ila sa sayup sa cable ug mga pamaagi sa pagdayagnos hinungdanon kaayo.
Ang Cable System Expert Team nag-compile niini nga giya base sa mga sumbanan gikan sa International Electrotechnical Commission (IEC) ug ang Institute of Electrical and Electronics Engineers (IEEE), inubanan sa halapad nga kasinatian sa uma. Kini nagtumong sa paghatag og usa ka bug-os nga proseso nga teknikal nga gambalay, gikan sa sayop nga pre-assessment ngadto sa tukma nga pag-ayo, pagtabang sa mga teknikal nga personahe sa dali nga pagpangita sa mga tipo ug posisyon sa sayup, epektibo nga pagpamubo sa mga oras sa pag-ayo, pagpamenos sa mga kapildihan, ug komprehensibo nga pagpausbaw sa pagkakasaligan sa cable system.
2. Klasipikasyon sa Pagkadaot sa Cable, Mga kinaiya, ug Nagpahiping mga Hinungdan
Aron epektibo nga mahibal-an ang mga sayup sa cable, hinungdanon nga masabtan una ang mga klase sa mga sayup ug ang mga hinungdan niini. Ang lainlaing mga tipo sa sayup nagpakita sa lainlaing mga kinaiya sa elektrisidad ug nanginahanglan lainlaing mga pamaagi sa pag-ila.
2.1 Kasagarang Mga Matang sa Kasaypanan ug Ang Ilang Mga Kinaiya sa Elektrisidad
Ang mga sayup sa cable kasagarang giklasipikar base sa mga kinaiya sa pagsukol ug estado sa koneksyon sa fault point:
Short Circuit Fault:
Kinaiya: Ang abnormal nga koneksyon mahitabo tali sa mga hugna, o tali sa usa ka hugna ug yuta (o neyutral). Ang pagsukol sa fault point kasagaran ubos kaayo, hapit sa zero (nailhan nga usa ka Low Resistance Short Circuit).
Kinaiya sa Elektrisidad: Ang pagsukol sa insulasyon hapit sa zero, and loop resistance is abnormally low.
Manifestation: May lead to tripping, fuse blowing, or equipment damage.
Open Circuit Fault:
Kinaiya: The cable conductor is interrupted, preventing current flow. This can be a complete or partial break in one, two, or three phases.
Kinaiya sa Elektrisidad: Conductor resistance is abnormally high, or even infinite; insulation resistance may be normal or damaged.
Manifestation: The Equipment fails to receive power, or the communication signal is interrupted.
Ground Fault:
Kinaiya: The cable conductor (or the insulation layer after breakdown) connects to the earth. This is one of the most common types of cable faults. Based on the contact resistance at the fault point to the ground, it can be classified as a Low Resistance Ground Fault or a High Resistance Ground Fault.
Kinaiya sa Elektrisidad: Insulation resistance drops significantly, potentially from hundreds of MΩ or even infinity down to tens or a few MΩ, or even below 1kΩ (low resistance) or above 1kΩ (high resistance), sometimes reaching hundreds of MΩ (high resistance).
Manifestation: Ground fault protection device operates, system ground current increases abnormally, and may cause a voltage shift.
High Resistance Fault:
Kinaiya: The fault point resistance is high, possibly ranging from several kΩ to several MΩ. This usually results from insulation degradation, carbonization, or partial breakdown, but has not yet formed a complete low-resistance path. High-resistance faults are often an early stage of many low-resistance and breakdown faults.
Kinaiya sa Elektrisidad: Insulation resistance drops, but still has a certain value. Under high voltage, the fault point may experience flashover or discharge, leading to unstable resistance values.
Manifestation: May cause local heating, increased dielectric loss, partial discharge, ug uban pa. Early on, there might be no obvious external signs, but it is easily revealed during withstand tests.
Flashover Fault:
Kinaiya: Under high voltage, discharge occurs on the surface or within the insulator, forming a transient or intermittent conduction. Insulation performance may temporarily recover after the voltage is removed.
Kinaiya sa Elektrisidad: Fault point resistance drops sharply with increasing voltage and increases when the voltage is lowered or removed.
Manifestation: The system may experience an instantaneous ground fault or short circuit, causing protection actions, but reclosing may be successful. Diagnosis is challenging.
Intermittent Fault:
Kinaiya: Fault symptoms appear and disappear intermittently, possibly related to factors such as temperature, kaumog, voltage level, or mechanical vibration. Pananglitan, a tiny crack may expand with temperature rise, causing contact, and separate when the temperature drops.
Kinaiya sa Elektrisidad: The resistance and connection state of the fault point are unstable and change with external conditions.
Manifestation: System protection devices operate intermittently, making fault capture difficult and posing a significant challenge for diagnosis.
2.2 Analysis of Internal and External Factors Leading to Cable Faults
Cable faults are not random; their causes are complex and diverse, usually resulting from the long-term or transient action of multiple factors:
Mechanical Damage:
External Causes: Accidental damage by excavators, pipe jacking equipment, ug uban pa, during construction; damage from road construction or third-party activities; tensile or compressive stress from foundation settlement or soil movement; animal (e.g., rats, termites) gnawing on the sheath.
Internal Causes: Excessive bending or pulling tension during installation; poor installation quality or external force impact on cable accessories (e.g., joints, terminations).
Chemical Corrosion:
Corrosive substances in the soil, such as acids, alkalis, and salt,s erode the cable sheath and armor layers; industrial waste liquids, oil stains, ug uban pa, penetrate the cable structure; electrolytic corrosion (especially in stray current areas).
Thermal Aging:
Long-term overload operation or high ambient temperature during laying causes accelerated aging, hardening, embrittlement, or even carbonization of cable insulation and sheath materials, leading to loss of insulation performance. Poor heat dissipation (e.g., densely packed cables, insufficient ventilation) exacerbates thermal aging.
Moisture Ingress and Humidity:
Damage to the cable sheath, poor sealing of joints, or moisture ingress into terminations allows water to enter the cable interior. Under the action of the electric field, moisture forms Water Trees, microscopic deterioration channels in the insulation material, which significantly reduce dielectric strength and eventually lead to breakdown (Electrical Trees).
Electrical Stress:
Overvoltage: Overvoltage impulses caused by lightning strikes, switching operations, resonance, ug uban pa, may exceed the cable insulation’s withstand capability, leading to insulation breakdown.
Electric Field Concentration: Design or installation defects in mga aksesorya sa kable (joints, terminations) lead to uneven electric field distribution, creating excessively high electric field strength in local areas, accelerating insulation degradation, and partial discharge.
Partial Discharge (PD): When tiny voids, impurities, umog, or other defects exist within, on the surface, or at interfaces of the insulation material, partial discharge may occur under operating voltage, releasing energy, gradually eroding the insulation material, forming discharge channels, and ultimately leading to insulation breakdown.
Design and Manufacturing Defects:
Impurities, voids, or foreign matter in the insulation material during cable body manufacturing; improper extrusion process leading to uneven insulation thickness or microcracks; rough surface or protrusions on metal shields or semi-conductive layers.
Quality issues with materials for cable accessories (joints, terminations) or unreasonable structural design.
Installation and Construction Defects:
Improper cable laying (too small bending radius, excessive pulling tension, proximity to heat or corrosive sources); non-standard cable termination fabrication processes (inaccurate stripping dimensions, improper semi-conductive layer treatment, poor sealing, incorrect stress cone installation); use of unqualified backfill material.
Understanding these fault types and causes is fundamental to effective fault diagnosis and the formulation of preventive strategies.
3. Cable Fault Diagnosis Core Techniques and Equipment
Cable fault diagnosis is a step-by-step process, typically including fault assessment, pre-location, precise fault location, and pinpointing the fault location on the ground. Different tools and techniques are needed for each stage.
3.1 Basic Testing and Preliminary Assessment
After confirming a potential cable fault, the initial step is to perform basic electrical parameter measurements to make a preliminary assessment of the fault nature.
Megohmmeter (Insulation Resistance Tester):
Purpose: Measures the insulation resistance between cable conductors and between conductors and the shield (o yuta). This is the most common and basic method for assessing cable insulation condition.
Operation: Apply a DC test voltage (typically 500V, 1000V, 2500V, 5000V, selected according to the cable voltage rating), and record the insulation resistance value after a specified time (e.g., 1 minute or 10 minuto).
Assessment: Insulation resistance significantly lower than normal values or specification requirements (e.g., recommended standards: low voltage cables ≥ 100 MΩ/km, 10kV cables ≥ 1000 MΩ/km) indicates potential insulation degradation or a ground fault. If the resistance value is close to zero, it indicates a low resistance ground fault or short circuit.
Multimeter:
Purpose: Measures conductor DC resistance, checks continuity (bukas nga sirkito), and measures inter-phase or phase-to-ground resistance (suitable for low voltage or situations with low fault point resistance).
Operation: Use the resistance range to measure the resistance across the conductor ends to determine if it’s an open circuit; measure inter-phase or phase-to-ground resistance to determine if it’s a short circuit or low resistance ground fault.
Assessment: Infinite conductor resistance indicates an open circuit; inter-phase or phase-to-ground resistance close to zero indicates a short circuit or low resistance ground fault.
Cable Route Tracer:
Purpose: Used to determine the precise route of cables in invisible laying scenarios like underground direct burial. Particularly important in the fault pinpointing stage.
Prinsipyo: A signal of a specific frequency is applied to the cable, and a receiver detects the induced electromagnetic field to track the cable path.
Models: Common models include RD8000, vLocPro, ug uban pa.
3.2 Precise Fault Location Techniques
Basic tests can only determine the fault type, not the exact location. Precise fault location techniques aim to measure the distance between the test end and the fault point.
3.2.1 Time Domain Reflectometry (TDR)
Prinsipyo: A fast-rising voltage pulse is injected into the cable and propagates along it. When the pulse encounters an impedance mismatch (such as a fault point, hiniusang, termination, or open end), part or all of the pulse is reflected back. By measuring the time interval between the transmitted and reflected pulses, and knowing the propagation speed of the signal in the cable (velocity of propagation, Vp), the fault distance can be calculated: Distance = (Time Difference / 2) * Vp.
Applicable Scenarios: Excellent for locating open circuits and low-resistance short circuits. Reflected signals are clear and easy to interpret.
Mga Limitasyon: For high resistance faults (especially very high resistance), the pulse energy may be attenuated or absorbed at the fault point, resulting in weak or distorted reflected signals, reducing location accuracy or even making location impossible.
Accuracy: Generally high, can reach ±0.5% or even higher (depending on equipment performance, accuracy of known Vp, and operator experience). VP needs to be calibrated by testing a known length of a healthy cable section.
3.2.2 High Voltage Bridge Method (Murray Loop, Bridge Method)
Prinsipyo: Utilizes the principle of the classical Wheatstone bridge. A healthy cable segment or a healthy phase from the faulty cable is used to construct a bridge circuit. When the bridge is balanced, the fault point distance is calculated based on the resistance ratio of the cable conductors. Ang sagad nga gigamit nga taytayan sa Murray Loop angay alang sa mga single-phase ground fault o phase-to-phase short circuit..
Bentaha: Ilabi na nga angay alang sa taas nga pagsukol sa mga sayup sa yuta (bisan hangtod sa daghang MΩ), nga usa ka kahuyang alang sa TDR. Ang prinsipyo gibase sa pagsukod sa resistensya sa DC, dili maapektuhan sa gipakita nga pagpahinay sa signal.
Mga Punto sa Operasyon: Nagkinahanglan ug labing menos usa ka himsog nga konduktor isip agianan sa pagbalik; nagkinahanglan ug tukma nga pagsukod sa kinatibuk-an gitas-on sa kable ug resistensya sa konduktor; nagkinahanglan sa paggamit sa usa ka High Voltage Generator (sama sa DC makasukol sa mga ekipo sa pagsulay) ngadto “kahimtang” o “paso” ang insulasyon duol sa taas nga pagsukol fault point aron ipaubos ang fault point resistance, pagpadali sa pagsukod sa tulay o sunod nga acoustic-magnetic nga lokasyon. Ang nagdilaab nga boltahe kasagaran taas, ingon sa 8kv, 15kv, o mas taas pa, ug ang operasyon kinahanglan nga labi ka mabinantayon ug sundon ang mga regulasyon sa kaluwasan.
3.2.3 Impulse Current nga Pamaagi (Yelo) ug Secondary Impulse Method (OO/AKO)
Prinsipyo: Kini nga mga pamaagi mga pagpaayo sa TDR alang sa pagpangita sa mga sayup nga adunay taas nga resistensya. Gipadapat nila ang taas nga boltahe nga pulso sa sayup nga kable, hinungdan sa pagkaguba o flashover sa high-resistance fault point, paghimo sa usa ka kasamtangan nga pulso. Gikuha dayon sa mga sensor ang kasamtangang pulse waveform nga nagpakaylap sa cable, ug ang pagtuki nga susama sa TDR gigamit sa pagpangita sa sayup pinaagi sa pag-analisar sa gipakita nga balud.
Yelo: Direkta nga pag-analisar sa gipakita nga kasamtangan nga pulso nga nahimo sa sayup nga punto.
OO/AKO (nailhan usab nga Arc Reflection Method): Gigamit ang arko nga naporma sa panahon sa pagkaguba sa fault point aron makamugna og ubos nga impedance “mubo nga circuit” alang sa TDR pulse sa fault point, paghimo sa usa ka tin-aw nga reflected waveform. This overcomes the issue of weak TDR reflections in high-resistance faults and is currently a very effective method for dealing with them.
Applicable Scenarios: Precise pre-location of high-resistance ground faults and flashover faults.
Equipment: Usually integrated into professional cable fault locators, requiring coordination with a surge high-voltage generator (high-voltage equipment in a cable fault test van).
3.2.4 Fault Point Pinpointing
Pre-location techniques provide the fault distance, but the actual fault point may be within a small area. Fault point pinpointing uses external methods based on the pre-location result to accurately determine the fault location on the ground.
Acoustic-Magnetic Method:
Prinsipyo: A high-voltage surge (using a surge high-voltage generator) is applied to the faulty cable. When the fault point breaks down and discharges, kini nagpatunghag tingog (balud sa presyur) ug electromagnetic signal. Ang operator naggamit ug Acoustic-Magnetic Synchronized Receiver aron maminaw sa tingog pinaagi sa mga headphone ug makadawat sa electromagnetic signal pinaagi sa induction coil.. Tungod sa mahinungdanong kalainan sa katulin sa pagpasanay tali sa tingog ug electromagnetic waves, madeterminar sa mga ekipo kung ang tunog ug electromagnetic nga signal naggikan sa parehas nga lokasyon ug kung ang tunog nalay sa electromagnetic signal (Ang gikusgon sa electromagnetic wave duol sa gikusgon sa kahayag, mas hinay ang sound wave speed), sa ingon nagpakita sa direksyon ug lokasyon sa fault point. Ang sound signal mao ang pinakalig-on direkta sa ibabaw sa fault point.
Applicable Scenarios: Nagkalainlain nga mga lahi sa mga sayup sa pagtangtang sa pagkaguba (yuta, mubo nga circuit, flashover), labi ka epektibo alang sa ilawom sa yuta nga direkta nga gilubong nga mga kable.
Mga Punto sa Operasyon: Ang ambient nga kasaba sa background mahimong makaapekto sa pagpaminaw; ang kusog sa pagdagsang kinahanglan nga ipasibo aron mahimong hinungdan sa padayon nga pag-discharge sa fault point nga dili makadaot sa himsog nga mga bahin sa kable; nanginahanglan ang operator og kasinatian aron mailhan ang mga tunog nga gipagawas sa sayup gikan sa ubang mga kasaba.
Pamaagi sa Boltahe sa Lakang:
Prinsipyo: Ang DC o low-frequency AC boltahe gipadapat sa usa ka ground-faulted cable, hinungdan sa pag-agas sa kasamtangan sa yuta sa fault point. Naghimo kini og boltahe nga gradient field sa palibot sa fault point. Duha ka probe ang gisal-ot sa yuta ug konektado sa usa ka high-sensitivity voltmeter, ug mibalhin sa dalan sa cable. Direkta sa ibabaw sa fault point, ang kalainan sa boltahe makabalik sa polarity.
Applicable Scenarios: Ubos o medium nga resistensya sa mga sayup sa yuta, ilabinang mapuslanon alang sa mga fault point nga dili makahatag ug klaro nga discharge sound.
Mga Punto sa Operasyon: Significantly affected by soil moisture and uniformity; requires sufficient test voltage and current; probe insertion depth and spacing affect accuracy.
Minimum Current / Maximum Magnetic Field Method:
Prinsipyo: An audio frequency or specific frequency current signal is applied to the faulty cable. If the fault is a short circuit or low resistance ground fault, the current forms a loop at the fault point; if it’s an open circuit, the current stops at the break point. A current clamp or magnetic field sensor is used to detect current or magnetic field strength along the cable path. After a short circuit or low resistance ground fault point, the current will significantly decrease or disappear (minimum current), or the magnetic field will change. Before an open circuit point, the current is normal, and after the point, the current is zero.
Applicable Scenarios: Low resistance short circuits, ground faults, or open circuit faults. Also often used in conjunction with a route tracer to confirm the path.
3.3 Insulation State Assessment and Early Warning Techniques
These techniques are primarily used to assess the overall health of the cable insulation and detect potential defects. They fall under the category of preventive maintenance or the diagnosis of high resistance/early-stage faults.
Partial Discharge (PD) Deteksiyon:
Prinsipyo: Defects in the insulation material (such as voids, impurities) cause partial discharge under the influence of the electric field, generating electrical pulses, electromagnetic waves, acoustic waves, light, and chemical byproducts. PD detectors capture these signals to assess the extent of insulation degradation and the type of defect.
Technical Parameters: Sensitivity is typically measured in picocoulombs (pC), capable of detecting very weak discharge signals (e.g., 1 pC).
Methods:
Electrical Method: Detects current pulses generated by discharge (e.g., through High Frequency Current Transformer HFCT sensors on ground leads, or by measuring capacitively coupled signals). Applicable for online or offline testing.
Acoustic Method: Detects ultrasonic waves generated by discharge (e.g., through contact or air-coupled sensors). Suitable for testing cable accessories.
Ultra-High Frequency (UHF) Method: Detects UHF electromagnetic waves (300 MHz – 3 GHz) generated by discharge. Offers strong interference immunity, commonly used for GIS, transformers, ug uban pa, and can also be used for cable terminations.
Transient Earth Voltage (TEV) Method: Detects transient voltages to ground coupled onto the metal enclosures of switchgear, ug uban pa, from internal PD.
Purpose: Detects early insulation defects in cables and their accessories (e.g., voids in joints, moisture ingress into terminations, water trees/electrical trees in the cable body). It is a key technology for predictive maintenance.
Dielectric Loss (Kaya nga Delta, tgδ) Test:
Prinsipyo: Measures the tangent of the dielectric loss angle of the cable insulation material under AC voltage. Dielectric loss represents the insulation material’s ability to convert electrical energy into heat. Healthy insulation materials have low losses, a low tanδ value, and the value changes little with increasing voltage. Moisture ingress, aging, or the presence of water trees and other defects in the insulation will cause the tanδ value to increase and increase rapidly with rising voltage.
Purpose: Assesses the overall level of moisture ingress or widespread aging in the cable insulation. Often performed in conjunction with AC or VLF withstand testing.
Makasukol sa Pagsulay:
Purpose: Gipamatud-an ang katakus sa cable nga makasukol sa usa ka piho nga lebel sa overvoltage nga wala’y pagkaguba sa insulasyon. Kini epektibo nga nagbutyag sa mga depekto nga makita lamang ubos sa taas nga boltahe.
Methods:
DC Makasukol: Usa ka tradisyonal nga pamaagi, apan ang boltahe sa DC mahimong makaipon sa bayad sa luna sa XLPE ug uban pang mga extruded insulats, posibleng makadaot sa himsog nga mga kable. Kini inanay nga gipulihan sa VLF.
AC Makasukol: Mas hugot nga gisundog ang aktuwal nga mga kondisyon sa pag-operate sa cable, apan ang mga kagamitan sa pagsulay dako ug nanginahanglan taas nga kusog.
Ubos kaayo nga Frequency (VLF) AC Makasukol (0.1 Hz): Kaylap nga gigamit karon alang sa pagsukol sa pagsulay sa XLPE ug uban pang mga extruded insulation cable. Ang mga ekipo madaladala, nagkinahanglan og ubos nga enerhiya, ug dili hinungdan sa pagtipon sa bayad sa wanang. Kanunay nga gihiusa sa tanδ ug PD nga mga pagsukod.
Sa sunod nga artikulo, we will explain cable troubleshooting in different scenarios with specific cases. Follow ZMS CABLE FR to learn more about cables.
