Optical Cable Management System for 500kV HVAC Networks

7/29/2019 Optical Cable Management System for 500kV HVAC Networks

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OPTICAL CABLES MANAGEMENT SYSTEM FOR 500KV HVAC NETWORKS

CARLOS ALBERTO DI PALMA ARIEL CAMPOS GUILLERMO GALARZA

TRANELSA CONSULTORA SA TRANSENER SA ABB SA

ARGENTINA ARGENTINA ARGENTINA

KEYWORDSFiber optic; wavelength; macrobending; microbending; reflection; refractive index; dark fibers

SUMMARYTransener Company is responsible of the Argentinas 500kV High Voltage Transmission System, and consequentlyit has a vast optical cables network that is installed along the whole country. The network has 5300 km of opticalcables installed, as well as an important amount of other optical cables under project development. More than 95%of their optical cables are OPGW type, and the rest are ADSS ones. See drawings of whole existing network

Additionally, it is necessary to take into account that the services of Transeners HV optical network are connectedwith services of other HV optical networks of:

Other 500, 330, 220 kV HV Transmission Operators

Generation Plants

HV trunk distributors (sub-transmission)in order to do:

Remote supervision of HV networks

Data and trip transmission for stabilizing resources like generation and load shedding (DAG/DAC system)

Connection of digital telephony trunks for HV operating actions

Data interchange with other countries where HV interconnections are installedAll these features are vital in order to have a National Interconnected HV System with high Reliability figure.

Nowadays, taking into account the importance of the Transeners 500kV HV System, there are installed severalremote management systems (NMS) as follows:

NMS for SDH digital communication systems by optical fibers

NMS for digital teleprotection system

NMS for SHF digital radiolinks NMS for private automatic exchanges (PABX) networks

NMS for optical amplifier chain (when it is applicable)

Overall NMS

The only component that is not currently being supervised is the fiber optic cable. And that is the motivation to plana remote monitoring system that will supervise the complete cable installed base. We will name this system likeOptical Cables Management System-OCMS

BENEFITSThe optical cables management system will represent a benefit to the whole optical network due to the followingfeatures:

Permanent and continue management of the whole optical cables network

Reduction of failure detection time (minutes)

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Reduction of failure location time (minutes), as well as the failure restoration time (hours), taking intoaccount the accurate and precise information provided by the system

Increase of the Total Annual Availability (Ai) of the whole optical cables network because the irreversiblefailure rate (TIF) will be minimal due to the early detection of the performance degradation and itsconsequent preventive actions

Reduction of costs of the preventive maintenance tasks (currently done by periodical measurements with

OTDR portable instruments) Warranty of the right in-service condition of the optical cables and their included fibers

Maximization of the level of utilization of the communication systems and/or the services transported bythem

To allow the interrelation between Transeners geographical documentation (GIS) and the performance ofthe optical cable network

Reduction of critical failures due to the permanent and continue monitoring, and consequently improvingthe quality of service (QoS) performance of the HV Network

Online checking of new optical cables installation quality during their start-up process

Auditing repair quality (that are done by Transeners subcontractors)

FAILURESThe failures can be classified as follows:a) Non-critical failuresThese failures are characterized by:

The attenuation factor is greater than the originally measured during the start-up process

Thresholds set for attenuation factor, and/or other parameters, are overtaken

It is modified the emitted optical power level experiences variation over the time

Cable deformation due to excessive strain (i.e. terminal hanging off)

Cable deformation due to excessive flexion (i.e. galloping effect due to ice fall down)b) Critical failuresCharacterized by:

An optical power reduction of 3dB under the determined threshold for any fiber of each optical cable

Increase of Hydrogen absorption in the optical cable and/or splice boxes

Loss of end-to-end signals

Maximum reflection figures are exceeded Fiber break

c) Extrinsic factorsIt means external mechanisms that interact with the optical cable and can affect their fibers.c1) macro-bendingOptical cable curves can affect the refraction index of the fibers, as well as the critical angle of the light ray, andconsequently producing diffraction of the light in the core (towards the cladding).Moreover, the installation procedure could not rightly control the installation parameters like pulling, mechanicalstrain, bending radius of pulley, etc, as well as not to have a permanent supervision during the installation process.The entire mentioned item can increase the probability of failures.c2) micro-bendingThe pressure over the optical fibers can produce non viewable effects, but they can affect the performance of fibersdue to non perceptible deterioration by micro-bending effect.

c3) temperature effectThe temperature range where the optical cable will act, as well as the projected over length of fibers into the cable,could produce contractions and/orstraightening of fibers, that will produce located micro-bending effects. These effects canreturn (or not) to the original right conditions (hysteresis effect). Nevertheless, that situation has not happened in theTranseners optical network

d) Threshold valuesThe threshold levels for alarm generation are shown in a preliminary way in Table I.1 of ITU-T L.40 standard, andthey can be acceptable for the project stage. It may be adapted later in accordance to the mode of operation of theUtility Company


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For long haul optical links the optical reserve margin is small. Consequently, it is mandatory to control any smallvariation in the fiber optic attenuation because it could represent a critical situation.

QUALITY OF SERVICE (QoS)

a) Main aspectsUp to now, in order to assure the quality of service of the Utility Company, the optical cables and their fibers werecarefully taken into account during all the project and work stages, as follows:

Performing type tests in well known and recognized laboratories

Doing factory acceptance tests (FAT) to 100% of the optical cable spools, as well as to 100% of the fibers

Doing site acceptance tests (SAT) to 100% of the fibers, with a complete set of detailed protocols

Currently, Transener does not have a monitoring system of their optical fibers/cables. Therefore the failuredetection is indirect:

Alarm detection through SDH system

Switch to other fiber route (section protection) of the same optical cable

Switch to other independent route (path protection) that will be different of the optical way

The HV Network of Transener is very critical mainly due to: Extra long haul distances

Extra high voltage level of 500kV

Trunk HV network for the whole countryFor those reasons the proposed optical cable monitoring and supervision system can obtain the main followingadvantages:

Reduction of troubleshooting time (early and precise location)

Reduction of out of service period of cables and systems (perception of degradation)

Reduction of catastrophic failures rate (preventive recognition of degradation)

Reduction of maintenance costs (preventive actions before than corrective ones)

After the development and explosive growth of the optical network, it is decided to shift the focus into the quality of

service (QoS) and the maintenance costs.

The main issues are the following:

To reduce the maintenance costs

To increase the knowledge of the optical network in real time

To increase the Availability (Ai) of the whole communication systems

To increase the Reliability (Ri) of the services (protection, stabilizing resources, etc) that are transmitted bythe communication systems

Consequently, it is necessary to go ahead with the next stage that must include:

Monitoring of normal behavior of the optical cable network

Preventive maintenance in case of an eventual degradation

Monitoring of the maintenance tasks

In case of failure, to do a fast and precise localization of itIt is necessary to control the following events:

Optical fiber degradation: measurement of total attenuation exceeding limit values

Breaking situation: measurement of fiber length and comparison with the original length

Splice damage: measurement of splice losses exceeding limit values

Water admission: measurement of H2 absorption (splice boxes, etc)

b) Integration requirementIt is essential to point out that the integration concept of the whole work must be mandatory in order to assure anoptimized solution between:

Optical cable

Cable accessories

Right engineering process


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Installation experienced task groups

Fiber splicing by specialized peopleThe monitoring system is a control of the well done both, implemented solution and installation tasks. Themonitoring system has not come to replace the right implementation to be obtained by Main Contractors.c) Preventive maintenanceThe process of failure in a galvanized steel cable of a conventional HV guard cable is different that what happen in

an OPGW cable, due to: In metallic cable the failure will be evidenced for a cut and fall down effects (critical failure)

In an optical cable the failure will begin with evidences like::*increasing of fiber optic losses (step by step)*straightening and/or deformation of fibers (tendency)*humidity admission into the optical cable (modification)

Consequently, there is an essential advantage in case of having a NMS optical cables monitoring system.

In case of considering a monitoring system with periodic preventive maintenance cycles, it must develop anautomatic sequential interrogation process (polling) looking for eventual degradations, and consequently generatingstate reports that will be later analyzed by O&M area of the HV Transporter

d) Corrective maintenance

As soon as a malfunction is detected in the optical cable, it will be generated: An alarm

A malfunction report (event)

The geographical location of the event, according to GIS systemAccording with that information will begin an immediate lot of actions, with the consequent increase in the quality ofservice, as follows:

It is decided the most convenient maintenance organization and the beginning of the repair process

It is re-routed the data traffic through other acceptable communication way, with a minimum bit error rate(BER), like other optical cable, radiolink alternative, etc

e) Work maintenance auditingIt is possible to verify the quality level of optical cables maintenance work that is made by subcontractors. It ismade by comparing the original values saved on the start-up process with the values that optical cables will haveduring their life period. It means to monitor performance changes in real time.

OTDR AND BACKSCATTERING TECHNIQUEThe Optical Time Domain Reflectometer is designed to detect the Rayleigh backscattering level along the fiber link.A little amount of this scattering that is determined by the acceptance angle will return to the coupling point and bemeasured.The OTDR measures backscattered signals which are much smaller than the signal previously sent to the fiber.Taking into account the scattered power and the propagation time in the fiber, the instrument draws a diagram thatshows the attenuation factor along the whole fiber link.

It is necessary to take into account that fiber length could not fully characterized during the recovery timeimmediately after a saturated period that is called the dead zone (IEC 61746)

OTDR module is capable of:

fault location and analysis of fiber system

attenuation, rate of attenuation, distance, etc, measurements

reflectance (as ratio of reflected power to incident power of an event/connector/splice) measurement

additional features like automatic event detection, table of events, optical return loss, trace overlay/overlap

data storage capability

additional functionalities like light source, power meterSee Figure BB

Large mode fields are less sensitive to lateral offset during splicing, but they are more sensitive to losses producedby bending during either installation or cabling processes.

a) : 1310 nm will be generally used for measurement of splice and connector losses

b) : 1550 nm is more sensitive to bends (called as macro-bending that will be considered later)


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See Figure CC

Considering that OTDR determines the distance to the event on time, the refractive index serves as a correlationfactor between time and distance, allowing the OTDR to display distances.But, if the refractive index (from fiber manufacturer data sheet) is known, it can be entered to the OTDR in order toimprove the accuracy of the optical distance displayed.

Additionally, it is normal that the length of the fiber is greater than the length of the optical cable. Then, the physicaldistance can vary from the optical distance.It is desirable that the OTDR to displays cable distance instead optical distance. Then, it is necessary to enter tothe OTDR data base, the effective refractive index (RIeff) that was adjusted for fiberover length.Consequently:

Effective refractive index >> RIeff = [ Lopt * RIopt ] / Leffwhere:

Leff: optical cable distance (by GPS measurement between two well known points)Lopt: optical fiber distance between two events (by OTDR measurement)RIopt: refractive index (provided by fiber manufacturer)

Main features of OTDR must be as follows:

dead zone of events: 5 m @1625/1650 nm

dead zone of attenuation: 10 m @1625/1650 nm dynamic range: 50 dB

wavelength alternatives: 1310, 1490, 1550, 1625/1650 nm

measurement distances: about 250km

precision of distance: order of metersDuring the detailed project stage the full description must be done

MONITORING PROCEDURES

1) In order to monitor the quality of service of the optical cable it is possible to adopt the criterion of monitoring allfibers of the whole cable.

a) That means that the monitoring process must co-exist with the transmission of information in the same fibers.

Consequently, optical filters as well as WDM multiplexers must be used in order to handle the monitoringwavelength m with the signal wavelength j of the communication system, without interfering each other.See Figure I.2

Summarizing, the following equipment must be included:

optical filters in the communication systems, in order to avoid the interference of m with the useful j

optical filters in the OTDR circuit in order to avoid the interference of j in the measurement process

b) Taking into account that the existing SDH systems are in-service, the before mentioned adaptations will interruptthe service in involved fibers. The proposed procedure will be the following:

re-route the traffic by other way

test of the system after its modifications

start-up process of the modified system in order to return to the original traffic and/or route

c) In order to avoid interferences between j y m wavelengths, the optical power of the signal to be used by themeasurement instrument OTDR must be at least -60dB under the optical signal used by the communication systemwithin the overlapping spectrum.

d) It is important to mention that the insertion of filters and multiplexers adds attenuation to the link and therefore itwill be total undesirable in the case of long haul existing links.

2) In order to monitor the quality of service of optical cables, it is possible to adopt the criterion of monitoring darkfibers that are not used by useful transmission.In such case it is not necessary (exceptions that will be seen later) neither to install optical switches in order toverify all fibers of the optical cable, nor install optical filters in the enlighten fibers.

Consequently, the monitoring system will not interact with the communication system that is transmitting in theenlighten fibers. See Figure I.1


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Due to this monitoring style, these main advantages are obtained:

low cost of implementation

no existence of interferences with the useful wavelengths

no necessity to do adaptations in systems that are in-service

possibility to use any m monitoring wavelength

On the other hand, this option shortens the delay time for detecting degradation or failures events, due to the factthat it is not necessary to have a measurement cycle of every fibers in the cable.

The Server located in O&M area will do the analysis of the OTDRs graph/trace together with additional informationcoming from the NMS system of the SDH communication system. Consequently, it is possible to do optimal actionsin order to do the best repair as well as to recover rapidly the degraded system

It must be taken into account that the dark fiber monitoring style will detect 85% of possible failures in the opticalcable, due to the fact that most of the failures will happen in the whole optical cable, and they will not happen in aparticular fiber alone.Of course monitoring of all fibers will detect 100% of failures and performance degradations (but with the abovementioned problems)

3) The monitoring options can be:

permanent and continue monitoring

periodic monitoring (monthly, daily, etc)By using dark fibers monitoring it is easy to implement a permanent and continue monitoring real time process.

4) it is suitable to design the optical cable monitoring system like a great management system in order to allowfuture addition of new monitoring stations, and/or new optical cables, without any changes in the installedmonitoring system (to assure scalability performance)

REMOTE FIBER MONITORING SYSTEMThe monitoring system will be as it is visualized in Figure DD. It will be mainly composed as follow:

a) Remote measurement unit (RMU)Devices that are part of the remote measurement unit:

optical coupler that permit the injection and extraction of light signal to/from the dark fiber

optical switch automatically controlled that can select between monitoring dark fiber # 1 (main) and darkfiber # 2 (backup monitoring fiber) on all supervised cables in the Node

in case of monitoring of enlighten fibers, it will be used an optical switch for 12, 24, 50 fibers

optical measurement unit that includes OTDR multitest module, as well as light source emitter, opticalpower meter

the OTDR module is remotely controlled and provides the optical signals for measurement, as well as todraw a reflection graphs by bidirectional tests. Several wavelengths will be used within the OTDR dynamicrange, as detailed before

the controller and data processor (CPU) will do, among others, the following functions:* control of the whole measurement process* storage of the measured data* analysis of the obtained results* communication with the remote Server through the existing communication systems (SDH fiber optic

System; SHF digital radiolink)

Additionally, a low reflection termination unit will be located at the final end of the dark fiber in order to allow theright OTDR reflection measurements, as well as the sensors to be decided (that it will be analyzed later)The mentioned 5300 km of the Transeners optical cable network must be divided in several Management RemoteNodes (MRN), each of one will do the monitoring of several optical cables, depending on the Substation where theNode will be located. The specific arrangement must be analyzed during the project stage. An example could be:

Node located at Choele Choel site: monitoring of three optical cables (ChChoel-BBlanca; ChChoel-PAguila;ChChoel-PMadryn)


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Node located at Cobos site: monitoring of three optical cables (Cobos-Bracho; Cobos-Sanjuancito; Cobos-MQuemado)

Node located at Colonia Elia site: monitoring of two optical cables (CElia-Belgrano; CElia-Mercedes)The installation of new MRN nodes, as well as some new optical cables that can be incorporated in the existingnodes, will be easily implemented. The immediate automatic beginning of monitoring tasks of the new node and/orthe new optical cable will be warranted (assured scalability).

The controller/data processor (CPU) will include data of dark fibers to be used, as well as the original and rightreflection graph that corresponds to the specific monitoring optical fiber. Each later measurement will be comparewith the original one (as reference), and it will be analyzed through an algorithm of failure automatic tracking

In case of more than one optical cable (to different directions) in the same MRN node, the controller/data processorwill do the simultaneous control of several RMU units (assigned each one to a particular/specific optical cable)

The controller/data processor (CPU) will include a dual LAN Gigabit Ethernet (10/100/1000 Base-T) interface inorder to be linked with the SDH and SHF radiolink multiplexers located in the Substation where the MRN node willbe installed. By the mentioned communication links, the MRN node will send the information to the remotemanagement center (RMC below described) by using a TCP/IP protocol, through a dedicated service channel ofeach multiplexer (duplicated sending).

b) Remote management center (RMC)It will be formed by a Server that will centralize the information of several MRN nodes, as well as storage the wholeinformation of all optical cables in its Data Base. The Server will manage all the collected information and will dothe consequent maintenance actions, giving alerts, etc, to the O&M area.

The measurement information will be included in the geographical system (GIS) that Transener normally use for hiswhole HV network. Consequently, the event of optical cable failures as well as their main features will be preciselylocated.The RMC Center will be suitable for processing the OTDR`s measurements and traces, as well as the rest ofinformation of each MRN node, like optical cable, dark fiber, date, etc. Consequently, the failed optical cable aswell as the failure location, place, etc, will be shown immediately, in spite of the network location where the eventhave happened

The range of the measurement must be as follows:

Optimal coverage: up to 100% of the whole cable length

Acceptable coverage: between 7080% of the whole cable length

Additionally, it would be necessary that the MRN remote nodes can use internet protocol for managing the remoteinstalled devices through a simple network management protocol SNMP. It means to use the same managementservers nowadays used by the SDH system. It will allow to do the management from anywhere by means ofspecific level access by each user.

c) Process

failure detection: in case of an optical fiber/cable performance degradation, the Remote Unit (RMU) willsend an alarm to the Server (RMC), together with its related information (date, time, failure characteristic,

location, etc) processing: the Server will register the failure/degradation event received and send a notification to the

O&M office with all details collected about it

information: additionally to the GIS failure location, it will be possible to open several lapels with informationlike:

* splice details* specific span (between two splice boxes)* end-to-end optical link* monitoring dark fiber (main dark fiber; backup one)

d) SensorsThey are necessary in order to verify the optical fibers/cables performance.

d1) humidity detection


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These sensors are based on the Hydrogen absorption, and they include a hydro-absorbent material that it willexpand causing a fiber loss due to a consequent macrobending of the fiberd2) stress detectionThese are fibre optic distributed strain and temperature sensors for monitoring mechanical stress. They permit todo high resolution measurement, as well as to do the measurement for long distance links.These sensors are based on Brillouin effect, and can measure simultaneously, both temperature and mechanical

stress on optical standard fibres according to ITU-T G.652. See Figure EE

e) Measurement tracesThe MNR node, as it was detailed above, uses an OTDR module multitest in order to measure the features of theoptical fibers/cables that undertake to the node.Each event of the link that could cause a reflection will be drawn on an OTDR trace, giving the information relatedto that event:

attenuation measurement

reflections produced

discontinuities

etcThe failures will be represented by reflections, due to several situations like:

misalignment between optical cables and light emitter

failures due to microbending located in splicing boxes presence of dust in optical connectors

cut in fibers

macrobending of the optical cable (*)

The last mentioned situation (*) of failure that is produced by external actions, is one of the reasons for choosingm = 1625/1650 nm like measurement wavelength. The bigger the wavelength is, the better the detection ofmacrobending will be. The losses in 1625/1650 nm are grater than in 1550 nm (0,4 dB/km@1625nm; 0,20 dB/km@1550nm)

The OTDR must measure in 1310, 1490, 1550 y 1625/1650 nm wavelengths. To have the possibility of using 1490nm is very important for the use of single mode zero peak water fibers (SM-ZPW; according to G.652C/D)

In the MRN node will be saved the right graph/trace of the optical fiber during the start up period, in order to beused as reference trace.Subsequent traces that can have a failure will differ from the original one. Consequently, it will be produced:

alarm generation

distance calculation to the event

event location on GIS system

The measurement will run continuously, doing a permanent comparison between the original trace and new traces.Both traces will be overlapped showing the differences as well as doing a comparative analysis. See Figure BBThe traces will be selected by the monitoring system, as follow:

original trace (reference trace)

trace with failure (alarm trace)

overlapped traces (showing differences)

MONITORING WAVELENGHTSIt is important that the wavelength to be used for the monitoring system can not interfere with the usefulwavelengths that are used for information transmission (SDH communication systems).This condition is advisable in spite of the installation of filters in order to block the monitoring wavelength (m) tothe communication equipment. Then, it is advisable to follow with ITU-T recommendations like it would existinterference with useful wavelengths, in spite of the decision will be to use dark fibers for monitoring purposes..According to ITU-T L.40 Table II.3 the monitoring system must consider to use m = 1650/1625 nm as monitoringwavelength, for both:a) supervision function

detection of greater fiber attenuation (measurement by OTDR)

detection of reduction of optical power (optical power metering)


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detection of humidity into the cable (measurement by OTDR and sensors)b) tests

measurement of distance to the failure (event) (measurement by OTDR)

measurement of exceeded strain (measurement by OTDR-B and sensors)

measurement of water (measurement by OTDR and sensors)

ITU-T L.41 Table 1 suggests the monitoring wavelength for equivalent conditions that are used by Transener (seeCase # 3)

This suggested wavelength is applicable for monitoring optical cables that include either standard single mode(SM) fibres, or dispersion shifted (SM-DS) fibres.

Additionally, the ITU-T L.41 suggests that the gap between monitoring wavelength (m) and the useful wavelength(j ), will be 100 nm. This criterion gives validity to the election of m: 1650/1625 nm

Moreover, it is necessary to take into account the effect of greater losses for long link distances that are mentionedin before paragraphs.In spite of using dark fibres monitoring, it is advisable to follow with the recommendations like it could exist

interferences between monitoring fibres and useful fibres

CONCLUSIONIt is a solution to be implemented as a pending issue in order to get a complete and integrated networkmanagement system for the whole communication system that nowadays assists the 500kV networks.The cost reduction in the current preventive maintenance tasks, the checking process during installation tasks ofnew optical cables, the auditing the repair tasks of subcontractors, etc, as well as the increase of the Total AnnualAvailability (Ai) of the whole optical cables network, will have completely justified the decision for implementing theremote monitoring system of optical cables

BIBLIOGRAPHY*Guide to fiber optic measurement, Wavetek Saint-Ethiene*Fundamentals cable engineering system planning, Siemens

*Fiber optic communication systems, Govind Agrawal*ITU-T recommendations L.40, L.41


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FIGURE BB

FIGURE CC


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FIGURE DD


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FIGURE EE


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Optical Cable Management System for 500kV HVAC Networks

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