Fault Location on Land and Submarine Links (AC & DC)

Transcription

1 Fault Location on Land and Submarine Links (AC & DC) Technical Brochure CIGRE Working Group B October 2017 Robert Donaghy (Convenor)

2 Tutorial Contents 1. Introduction 2. Terms of Reference 3. Cable Fault Types 4. Cable Fault Location Techniques 5. Design Factors Affecting Fault Location Land Cables 6. Design Factors Affecting Fault Location Submarine Cables 7. Emergency Planning 8. Innovation and Future Developments 9. Safety and Training Considerations 10. Accuracy and Suitability of Fault Location Methods 2

3 Introduction In 2014 CIGRE Study Committee B1 established Working Group B1.52 to develop a Technical Brochure as a guide on Fault Location on Land and Submarine Links (AC & DC) This presentation provides a summary of the guide which is due to be published in

4 Introduction The increasing number of land and submarine cable assets globally has created a focus on cable fault location capability There are many well established cable fault location techniques, particularly for buried underground cables Successful cable fault location depends to a great extent on applying the appropriate technique or combination of techniques Methods for locating cable faults require competent engineers and service providers Guidance is required for engineers on the correct application of the various techniques available 4

5 Terms of Reference Fault location on the following installed cable types: AC & DC (MV/HV/EHV) land and submarine cable systems single core, 3-core and pipe type cables Focus on main insulation & sheath faults Existing fault location techniques, underlying principles Guidance and strategies for effective fault location for various installations: Direct buried & ducted land cable systems Cables between GIS bays Cables installed in HDD, tunnels, large burial depths Cable systems with different bonding types Very long cables 5

6 Terms of Reference Existing fault location techniques, underlying principles Pre-location & pinpointing methods - accuracy / suitability Fault Location Flowchart Design factors affecting fault location capability Marine vessel & support for submarine cable faults Case studies of fault location experiences Applicability of on-line methods of fault location Safety and training considerations New and innovative techniques & future developments 6

7 Fault Location Steps Fault Fault Identification Prelocation Cable Tracing Pinpointing Cable Identification Repair 7

8 Cable Fault Types Open Circuit Fault Also known as a series fault or conductor continuity fault Current path is broken; current flow prevented completely or hindered In conductor and metallic sheath Uncommon in submarine cables and land cables. Shunt Fault Also known as insulation fault or short circuit fault Two or more main conductors come into contact with each other or with earth Fault resistance 0 - MOhm range. Intermittent faults - nonlinear voltage dependent faults. Behaves as high R fault until insulation breaks down and as low R fault during arcing. XLPE land cables faults often high resistance or intermittent. Submarine cables - arc often penetrates all watertight layers resulting in sea water ingress, resulting in low fault resistance, making the fault a persistent shunt fault. 8

9 Cable Fault Types Sheath Fault In cable oversheath & metallic sheath: Damage usually opens a current path from metallic sheath to earth due to water ingress. Often causes metallic sheath corrosion, leading to further damage in the metallic sheath or eventually in the main insulation. In a metallic sheath fault, the metallic sheath or screen comes into contact with earth in an unplanned manner. May be either low or high R. 9

10 Cable Fault Location Techniques Prelocation Testing the circuit from the cable terminations to estimate distance to the fault. Pre-location can determine the fault position to within a few percent of the cable length. On very long cables, the margin of error can be significant. Sometimes the fault has to be conditioned to make it detectable e.g. burned to a low resistance fault. Pinpointing A test to confirm the exact position of the cable fault following prelocation. Pinpointing is carried out directly over the cable. Before applying sophisticated pinpointing methods, don t overlook information available from visual inspections and reports from observers and the general public. 10

11 Prelocation Techniques Time Domain Reflectometry (TDR) Burn Down Techniques Arc Reflection Methods (ARM/SIM/MIM) Decay Method and Differential Decay Method Impulse Current Method (including Comparison and Differential Modes) Frequency Domain Reflectometry Bridge Methods 11

12 Prelocation Time Domain Reflectometry (TDR) Also known as pulse echo or echometer High frequency pulse applied between core and sheath. Pulse propagates at an assumed constant velocity until it meets a discontinuity in the surge impedance. Positive reflection at the cable end or at open circuit fault Negative reflection at shunt fault The velocity of propagation (v/2) depends on the cable dielectric Dielectric v/2 (m/microsecond) MI 78 m/µs range 64-80m/µs PE/XLPE 84 m/µs range 80-90m/µs Oil Filled 78 m/µs range 75-80m/µs EPR 80 m/µs range 75-84m/µs Open Circuit TDR October

13 Prelocation Burn Down Techniques TDR works for low R shunt faults or for high R open circuit faults where +ve pulse of sufficient height is generated. High resistance shunt faults or low resistance open circuit faults cannot be detected using TDR as there is little or no detectable impedance change at the fault. Conditioned Fault on AC submarine cable (PILC) L1 & L2 are fault conditioned, L3 healthy core Fault conditioning: DC generator burns the high resistance shunt fault into a low resistance fault which then allows location by TDR. Can make root cause of the fault difficult to determine due to burning Conditioned Fault 13

14 Prelocation Arc Reflection Methods Arc Reflection Method (ARM), Secondary Impulse Method (SIM) and Multiple Impulse Method (MIM). A single HV impulse is transmitted via a pulse shaper into the faulty cable. HV pulse is applied to arc the high resistance or intermittent fault. During the arc, the fault turns into a low impedance fault. This short arcing time is sufficient to superimpose a TDR signal that is time synchronized to allow the arced fault to be captured. Red curve Reference Trace; Blue curve Arc Reflection Trace 14

15 Prelocation Arc Reflection Methods Multiple Impulse Method (MIM) Wet faults or oil reflow in oil filled cables can complicate fault location as it affects the fault arcing by delaying the fault ignition time In MIM, only one HV pulse ignites the fault. Multiple TDR records are captured during the fault arc and compared to the healthy TDR trace Arc burning time must be for long fault distances > 5 ms which requires high energy output from the surge generator and is fault condition dependent Multiple Impulse Arc Reflection Traces on a high resistant wet fault 15

16 Prelocation Decay & Differential Decay To locate breakdown faults and intermittent faults with a high breakdown voltage in the range of a few kv to 150 kv The decay method is useful to detect breakdown faults on long XLPE cables or radial flow type oil filled cables Not applicable on wet faults For measurement on long cables, safety precautions required for discharge due to the high stored energy Decay Method on intermittent fault 16

17 Prelocation Impulse Current Method (ICM) ICM (incl. Comparison and Differential Modes) used for high-resistance faults, especially in long cables. Surge wave generator creates impulse (>30kV): The impulse creates arc at fault, causing an impulse current there. Two current waves travel to the cable ends. Current wave is reflected at the surge capacitor and travels back and forth to the burning arc at the fault. HF coupler detects the component generated by the fault arc and transfers the signal to a transient recorder in TDR. Fault position is defined by the time from the fault ignition pulse and its reflection. Display shows the first measuring period with ionisation time delay, the second period offers accurate fault distance 17

18 Prelocation Frequency Domain Reflectometry FDR detects and locates local degradations in the cable caused by mechanical stress and damages, heatinduced oxidation and radiation. It will also detect global degradation caused by general aging. FDR detects capacitance variations through its impact on the measured and evaluated impedance spectrum. A measurement cycle in FDR takes from a few seconds to a few minutes and the analysis can be performed immediately after the measurement. More detailed analysis of the results can be made offline later. Two traces of the same cable: the trace indicates e.g. a joint at 102km and some minor variations in the trace at 150km 18

19 Prelocation Bridge Methods Murray Bridge Applicable to: Resistive faults on multicore cable systems. DC bipolar links with one healthy pole Monopolar links with a healthy return Accuracy 0.5% - 1% Allow for: Change of conductor cross section or material along cable Connection leads and jumper at the far end 19

20 Prelocation Bridge Methods Glaser Bridge Fault Location Sheath Fault Location Variant of Murray Bridge Applicable to: resistive shunt faults and multicore cables Requires 2 good return lines Often applied for fault distance measurement on sheath faults where the return path is built up with the centre conductor. Can be applied for circuits with unequal conductor cross sections for the return lines. 20

21 Pinpointing Techniques Fault pinpointing techniques considered: Acoustic Method Step Voltage Method Magnetic Field Methods Impulse magnetometry (primary faults) DC magnetometry (sheath faults) Audio Frequency Methods Sectionalising Methods 21

22 Pinpointing Acoustic Method To pinpoint high resistance or intermittent faults in direct buried cables. A surge generator in repetitive pulsing mode causes a flashover at the fault. This creates a high acoustic signal that is locally audible. Acoustic signals are detected on the ground surface by a ground microphone, receiver and headphone. Acoustic Method At the fault position the highest level of flashover noise can be detected. 22

23 Pinpointing Step Voltage Method Also known as pool of potential For pinpointing faults on direct buried cables (main insulation, sheath faults). A pulsed DC voltage or step sequence voltage is applied. The HV pulses are discharged via the resistive fault to the surrounding soil without a flashover. At the fault location, a voltage gradient in the soil is measured by 2 earth probes. The receiver indicates the positive or negative voltage. Only applicable where there is a path from the fault to the outside soil. Step Voltage Method 23

24 Pinpointing Sectionalising If prelocation does not work, split the cable into sections to narrow down the fault also gives additional access point to conductor or screen. (e.g. at sea land transition joint/grounded joints in X-bonding section). Trade-off between the outage time costs and additional repair costs, but also the time that is invested for further fault location efforts and the time for preparing and executing the cable cut. 24

25 Design Factors Affecting Fault Location Land Cables Fault location capability is not always a primary concern in system design. Actions can be taken at the design stage to help fault location: Multi-Section Cable Systems and access via link boxes Circuits with More than One Cable Type & Multi-core Cables Cables Terminated into GIS at Both Ends Cables Installed in ducts Cables installed in tunnels Secondary Insulation system materials Cable systems provided with instrumentation HVDC System Design 25

26 Design Factors - Multi-Section Cable Systems Hard sectionalizing using link boxes enables more localized testing Use link boxes at joints and terminations; Ensure they are accessible (e.g. avoid placing in busy roads where possible) Hybrid Circuits: provide capability to disconnect cable sections from OHL sections. Cable systems with multiple cable types: Time-domain methods and ratiometric methods are more complex 26

27 Design Factors - GIS Terminations at Both Ends Poses challenges for fault location: GIS chamber needs de-gassing; access to HV terminals difficult Most fault prelocation methods require: Disconnection of voltage transformers SF6 gas works & certified personnel to work with SF6 Installation of HV test bushings Possible Solutions: Provide AIS or GIS test bushing on GIS (instead of VT) Unplug cable from GIS, connect to SF6 GIS connector Provide a HV test bay in the GIS design Provide a double cable end unit (one with dummy plug) for cables between 2 x GIS at one side 27

28 Design Factors - Fibre Optic Cables Fibre optic cables enable wider range of fault locations to be applied: Distributed temperature sensing (DTS) at the fault position Distributed vibration sensing (DVS) at the fault position during surging OTDR - to prelocate if optical fibre is completely severed Integrated or Separate fibre optic cable Install as close as possible to power cable core. 28

29 Design Factors - Cables Installed In Ducts Poses particular challenge for fault location: Plastic duct introduces additional insulating layer Detecting and locating faults requires current flow from the metallic screen or sheath through the fault to earth Difficult to apply step potential and acoustic methods Avoid using cables without outer semicon Check that cable sections are clear of fault before jointing Consider using conductive couplers / duct coating Techniques to apply for faults in ducts: Sectionalise if possible Use DC bridge prelocation before excavation Do step potential survey. Check bonding leads at joint bays; check link boxes for water ingress. Check direction of voltage drop on exposed section of cable at excavated joint bay. Isolate the semiconducting layer at end of duct. Dig into duct at the indicated position (if no indicated position, then dig into the duct at mid point). Expose cable surface. Apply step potential technique again. 29

30 Design Factors - Cables Installed In Air Tunnels & Galleries Cables are only in contact with anything conductive at clamping points White fire retardant paint turns black after a fault which aids fault location. Step-potential method of limited use; can give misleading results. Potentially hazardous for operator in the conductive environment in tunnel. Pinpointing using surge methods may be effective - no attenuation of breakdown sound by intervening soil. Fault pinpointing may be possible with the unaided ear. For oversheath faults, surge methods may also be used breakdown may be visible and audible. 30

31 Design Factors Submarine Cables The importance of good as laid records Submarine cable design Different insulation types Cables with multiple cores per phase Very long cables Installation design Mechanical protection and embedment Water depth Shore landings in HDD Areas of high current flow Circuit Configuration Circuits with land & submarine sections Circuits with OHL & cable sections 31

32 Emergency Planning Topics considered: Cable System Records Permits Preparatory works Marine logistics Aspects of Repair Preparedness Plans relevant to fault location 32

33 Cable System Records Maintain database of engineering information relevant for fault location Information must be readily available and up to date. As-laid record of cable route (including joint positions) Cable cross-section and accessory details Technical schedules including physical and electrical characteristics of cable Cable installation design including all trench cross sections Electrical bonding and earthing schematic drawing Distance between cable terminations Details of any cable re-routing Location of all repair, field and factory joints Oil system profile for fluid filled cables Descriptions of integrated optical fibre cable characteristics. Descriptions of any Distributed Temperature and Stress Sensing systems TDR fingerprint 33

34 Fault Location Manual Should describe the following aspects: Safety procedures and risk analyses to be performed before fault finding starts Identification of type of fault Steps to follow for fault location (flowchart) Chronological overview of tests and measurements to be performed (TDR, OTDR, HV test, Pre-location techniques, pin-pointing, etc.) Testing and measuring equipment needed for fault location which is adapted to the characteristics of the cable connection 34

35 Cable Corridor Monitoring 3rd party damage is most common type of submarine cable damage (TB398) Approximate fault location can be inferred from vessel locations. The recorded position of vessels (via Automated Information System (AIS) transponders) near the cables provides information to locate the damage area for earlier pin-pointing. Alerts can be raised as alarms in control centre. Systems now available to combine AIS information with Distributed Acoustic Sensing (DAS) to give early indication of third party interference. 35

36 Prelocation and pinpointing with AIS Data AIS estimation can be useful to pre-locate and partly pin-pointing the fault. Combined with other pre-locating techniques like TDR and pin pointing with diver or ROV the actual fault position can be establish fast. 36

37 Marine vessels and support DP Class Vessels recommended for de-burial ROV Launch and Recovery System Mass Flow Excavators 37

38 Permits for Cable Fault Location Activities Every link owner needs to generate their own permit list. National and international regulations may apply. Permits required may include: Survey Diving activities Permission from authorities responsible for planning Marine licenses for a foreign repair vessels to enter the country Performance of work in navigation channels / shipping lanes To avoid delays: advance preparation is needed 38

39 Innovation and Future Developments 1. Fibre Optic Fault Location Methods Distributed Temperature Sensing Brillouin Strain Measurements Distributed Acoustic Sensing / Distributed Vibration Sensing 2. Electrical/ Conventional Fault Location Methods Partial Discharge Measurements Advancements in conventional Fault Location Methods Online Fault Location Methods 3. Submarine Pinpointing Techniques Using ROVs Visual/ Tone tracing /Step Voltage method/ Use of Hydrophones Submersible Habitat technique Multibeam equipment 39.

40 Innovation - Fibre Optic Methods Fibre can be installed: Externally Directly attached to the cable In a separate duct Fibre optical test and specifications: IEC and IEC Distributed Temperature Sensing Brillouin Strain Measurements Distributed Acoustic Sensing / Distributed Vibration Sensing 40

41 Distributed Temperature Sensing (DTS) The fibre acts as a linear sensor to detect hot spots along the cable High accuracy over long distance DTS advantages: Accuracy independent of the laying depth Not sensitive to electromagnetic interference Can be used from the shore Raman spectroscopy - scattered magnitude is temperature dependent Brillouin spectroscopy - the Brillouin shift is temperature and strain sensitive. 41

42 Distributed Acoustic Sensing (DAS) Based on backscattered light from variations in refractive index of fibre. Single mode fibre is generally preferred for DAS applications. Thumping high resistive faults with surge generator creates vibration at the fault. Localised vibrations can be captured on the DAS interrogator (onshore) and compared with the initial surge vibration detected at the fault location. Consider laying temporary external FO cable adjacent to the submarine cable near the fault to pinpoint the fault position. Prelocating with 3 kv surge generator on submarine cable Diver tapping the cable to Pinpoint the fault location 42

43 Distributed Vibration Sensing (DVS) Single mode fibre optic cable acts as vibration sensor 2 no. fibre optic interferometers in FO cable - sensitive to pressure and motion Capable of detecting vibrations transmitted through media (soil, water, etc.) Local vibrations create acoustic waves near the vibration which affect the propagating wave. The light wave is split at the start module and re-joined at the end module; the difference in phase angle is calculated using the 3rd fibre as a return from the end module. If no integrated FO element, DVS can be used by laying external fibre adjacent to the cable. 43

44 DAV versus DVS DVS DAS Spatial Resolution 10-25m for 45km sensing* 10-25m for 45km sensing Signal To Noise Ratio Signal Band Width Cable configuration Constant over the entire sensing range Transmissive wave as signal Exponentially decaying over distance Independent of sensing range khz for 50km sensing 3 optical fibres Loop configuration required Back-scattered wave as signal Inversely proportional to sensing range <1 khz for 50km sensing 1 optical fibre Single-end access required * subject to clarification 44

45 Partial Discharge Measurements Integration of fault and PD detection techniques embedded into the cable. Active sensors distributed along the cable enable identification of the PD affected section. Comparing the 2 signals acquired at different positions (i.e. before and after a joint) and discriminating pulses coming from outside the segment (noise) or PD pulses originating inside it. The defect location can be detected to 20 cm and, in case of alarm, the information transmitted to one end of the subsea link. Main challenges: how to embed sensor array into the cable reliability of power supply to sensor arrays alarm signal transmission to ground station 45

46 Online Fault Location Methods Prelocation during the initial breakdown using measurement devices at one or both cable ends. Evaluate transient wave shapes after break down. Delay between the initial voltage impulse and its first reflection is used to calculate the fault distance. Propagation speed must be known. Accuracy affected by partial reflections of travelling wave at impedance mismatches. Accuracy of TFR: ~1% Alternatively, evaluate time difference between the arrival times of the first pulses. Devices at cable ends need exact time synchronisation to calculate fault distance. Some utilities use centralised network quality/fault location system to improve prelocation accuracy and speed. Systems combine various data sources e.g. protection devices, transient recorders, sub automation system, GIS data and network topology. 46

47 Submarine Pinpointing using ROV s Visual/ Tone tracing /Step Voltage method/ Use of Hydrophones Multibeam equipment Graphical interface for acoustic data logger during fault thumping 47

48 Cable Identification Before cutting and handling: the faulty cable should be identified by means of an injected signal and tracing tools: 48

49 Safety Considerations Fault location to be performed according to the national, international and company safety regulations. Personnel performing fault location shall have an appropriate training and authorisation to carry out the works. Risk Assessment & Method Statements Particular Risks: Maximum Allowed Test Parameters and Test Voltages Stored energy in long and extra long cables Return voltage in DC cable systems Induced and Impressed Voltages Impulse voltages Touch and step voltages Re-energizing the cable 49

50 Training Considerations General training requirements and certificates Electrical safety training Basic safety training Working at height First aid training Fire awareness training Offshore training requirements where necessary 50

51 Fault finding training Training program should suit the practical conditions in the network and according conditions under which fault location personnel need to work. Safety rules Understanding the fault types and the fault behavior Connection methods Understanding fault type analysis Understanding the various pre-location and pinpointing methods Equipment specific training 51

52 Accuracy and Suitability of Prelocation Methods Fault Type Location Method Low Resistance High Resistance Open Circuit Intermittent Sheath Fault Land & Submarine Fault Burning Required Land & Submarine Fault Burning Required 1-3 % accuracy 1-3 % accuracy TFR/FDR Methods Fingerprint (reference) helpful Fingerprint (reference) helpful Limitations for cross bonded systems and Limitations for cross bonded systems and screen interrupted systems screen interrupted systems Arc Reflection Methods Decay Method No need as TDR will work Land & Submarine No need as TDR will work Land & Submarine 1-3 % accuracy 1-3 % accuracy Length limited Limitations for submarine cables (water ingress). Limitations for cross bonded systems and screen interrupted systems Length limited Limitations for submarine cables (water ingress). Limitations for cross bonded systems and screen interrupted systems No need as TDR will work The cable will not be able to be charged to a DC voltage due to low resistance fault. No need as TDR will work Land & Submarine 1-10 % accuracy Propagation velocity unknown, no reference points available. Limitations for cross bonded systems and screen interrupted systems applicable not applicable possibly Differential HV Methods Impulse Current Method No need as TDR will work Land & Submarine No need as TDR will work Land & Submarine 1-3 % accuracy 1-3 % accuracy Access to reference conductor required Access to reference conductor required No need if TDR works Land & Submarine No need as TDR will work Land & Submarine 5-10 % accuracy 5-10 % accuracy Ionization time to be considered. Ionization time to be considered. Limited by breakdown voltage & distance Limited by breakdown voltage & distance to fault. to fault. Limitations for cross bonded systems and Limitations for cross bonded systems and screen interrupted systems. screen interrupted systems. Bridge Methods Voltage Drop Method Fibre Optic Methods Land & Submarine Land & Submarine Land ~ 1 % accuracy ~ 1 % accuracy ~ 1 % accuracy Need at least one healthy return conductor Up to few M ohms fault resistance, Need at least one healthy return conductor Need at least one healthy return conductor Land & Submarine Land & Submarine Land ~ 1 % accuracy ~ 1 % accuracy ~ 1 % accuracy Need at least one healthy return conductor Up to few M ohms fault resistance, Need at least one healthy return conductor Need at least one healthy return conductor Land & Submarine Land & Submarine Land & Submarine Land & Submarine Land & Submarine 1-3 % accuracy 1-3 % accuracy 1-3 % accuracy 1-3 % accuracy 1-3 % accuracy Vibration from fault spot or magnetic force along cable route needed DAS & DTS Already OTDR will work Sound or heat production in fault spot needed Vibration from fault spot needed 52

53 Prelocation Process 53

54 Accuracy and Suitability of Pinpointing Methods Fault Type Location Method Low Resistance High Resistance Open Circuit Intermittent Sheath Fault Land & Submarine Land & Submarine Land & Submarine Land & Submarine 1-3 m accuracy 1-3 % accuracy 1-3 m accuracy Limited application. Cables in ducts Earthing of disconnected sections Cables in ducts Acoustic Methods A solid short circuit will not create needed noise. Can be used for submarine Cables in ducts faults with water ingress Land Land Land Land Land 1-3 m accuracy 1-3 m accuracy 1-3 m accuracy 1-3 m accuracy 0.1 m accuracy Step voltage Method Audio Frequency Method Fibre Optic Methods Fault needs soil contact Cables in ducts Fault needs soil contact Cables in ducts Fault needs soil contact Cables in ducts Fault needs soil contact Cables in ducts Fault needs soil contact Cables in ducts Galvanic contact of neutral conductor to grounding systems Land & Submarine Land & Submarine Land Land: 1-3 % accuracy Submarine: 10m - several hundered m accuracy Land: 1-3 % accuracy Submarine: 10m - several hundered metre accuracy Audio frequency measurement with capacitive probes Step voltage usually preferred Land & Submarine Land & Submarine Land 1-3 % accuracy 3-5m accuracy 3-5m accuracy 3-5m accuracy 1-3 % accuracy Vibration from fault spot or magnetic force along cable route needed Up to few M ohms fault resistance, Need at least one healthy return conductor Sound or heat production in fault spot needed Vibration from fault spot needed. applicable not applicable possibly 54

55 Conclusion There are many well established techniques available for fault location in cables, particularly for buried underground cables Successful cable fault location depends to a great extent on applying the appropriate technique or combination of techniques Methods for locating cable faults require competent engineers and service providers Technical brochure to be published in 2018 Thank you for your attention! 55