Communication Systems for Long Haul Links on 500kV HVAC Systems

D2-01-A-02COMMUNICATION SYSTEMS FOR LONG HAUL LINKS ON 500KV HVAC SYSTEMS ARIEL CAMPOS TRANSENER S.A. ARGENTINA CARLOS ALBERTO DI PALMA TRANELSA CONSULTORA S.A. ARGENTINA 0 KEYWORDS Wavelength-dispersion-DWDM-spectral width-EDF-transponder-attenuation factor 1 INTRODUCTION The 500 kV Extra High Voltage Transmission System used in Argentina, has long distance Lines between the involved substations. It means communication link lengths from 200-300km to 500km. In order to minimize the probability of malfunctions due to natural disasters and/or vandalism acts, on the communication system, it is necessary to eliminate (or minimize) isolated repeater stations, and consequently, to avoid those Non-Availability (NAi) nodes. Additionally, so as to allow the operation and control of those long HVAC Lines, as well as to carry the necessary information for the High Voltage Main Transporter (Transener SA), the communication systems must have big capacity for transmitting the whole information such as above mentioned and briefly described later. The both, high bit transmission rate and long distance links are the main waited goal that will be described afterwards. 2 2.1 • • • • • • • • • 2.2 • • • • • • • INFORMATION TO BE TRANSMITTED Current information to be transmitted along 500kV HVAC Lines: Data transmission for SCADA system (control function) Data transmission for Differential Protection (main protection function) Trip transmission from Teleprotection systems (backup protection function) Data transmission between PLC devices of the Generating/Demand automatic disconnecting system (stabilizing resources) LAN networks links for Generating/Demand automatic disconnection system Digital trunk links between PABXs telephony exchanges (operation function) LAN networks links for corporative functions (administrative function) Protection System’s remote management Communication System’s remote management Future information to be transmitted: SCADA system for transmission over IP or Ethernet Data transmission for SCADA system between SS and Control Centers according to new IEC 61850 standard Trip and data transmission for Teleprotection system between SS according to new IEC 61850 standard VoIP and videoconference functions Video monitoring of electrical equipment located in remote substations Video surveillance of both buildings and shelters where communication equipment are allocated Optical cables` remote management 3 BANDWIDTH a) The main parameters taken into account in optical fiber performance are: Comm sys for long haul links on 500kV sys-rel v.5 1 • • the attenuation factor (α) the bandwidth (B), as well as the factor B.L (MHz.km) The light source will not emit a unique wavelength, but several wavelengths located into the source spectral width Δλ. Due to this condition some light rays will propagate with different speeds and consequently will have different time delays A measure of the variation of the group refraction index (ng) at different wavelengths is represented by the material dispersion Mo (λ) that is shown in units of ps/nm.km. The value of Mo (λ) is negligible at λo: 1302-1322 nm, but it is not at λ: 1550 nm. Another important factor that affect the dispersion in single mode fibers, is the waveguide dispersion M1 (λ), that is caused by the dependence of wavelength with the light distribution in the fiber core and cladding, as well as due to the difference between their refraction index Then, the chromatic dispersion M (λ) is formed by the above mentioned two kinds of dispersion: M (λ) = Mo (λ) + M1 (λ) For the wavelength with zero dispersion (λo: 1302-1322 nm; with dispersion slope of 0,087 ps/nm.kn), the chromatic dispersion will disappear. But, at that λo the attenuation coefficient will be high enough (0,40 dB/km @1550nm) and consequently will be not acceptable for long distance links. b) As before mentioned, the light pulse will be launched from the optical emitter to the optical fiber, with a spectral width Δλ. Consequently, the full root mean square (FRMS) of the spectral width will be: When it is used Δλ: 0,4 nm (like shown in item #5), the spectral width will be Δλ(FRMS)= 0,34nm c) If the input pulse width at the injection point into the optical fiber is called t1 , and the final value of the pulse at the end of the link (when the light pulse is extracted) is called t2 , consequently the broadening of the pulse will be: The process of pulse broadening at the receiver end will limit the bit transmission rate; because in order to discriminate one output pulse from another, it is necessary to separate the input pulses between them (less bit rate). In our projects: The output pulse broadening due to dispersion will be: In our projects: It means that, the output pulse broadening will depend of, both the link length and the spectral width of the emission optical source d) Taking into account the above mentioned, the SM fiber optic transmission bandwidth must be: e) Resuming the before comments, as well as the issues mentioned in item #3, it is necessary to take into account the following parameters: • attenuation factor α, that will give the loss of optical power along the fiber link • the bandwidth B, in order to be used like value of fiber dispersion Comm sys for long haul links on 500kV sys-rel v.5 2 f) Another topic to consider is the differential group delay (DGD) that takes into account the maximum value that the digital system can tolerate (*). DGD (max) = 0,1 t1 = 0,1 x 6,45 nseg = 0,645 nseg In such case, the consequent spectral bandwidth to be used must be: Δλ: 0, 11 nm (*) note: this rule is applied for NRZ-formatted on-off keyed signals g) The whole pulse broadening will be affected by the operation time (top) that are formed by: • operation time of the optical emitter, as well as the additional amplification devices to be used in the transmission end • fiber optic dispersion • operation time of the optical detector, as well as the additional amplification devices to be used in the reception end Taking into account to check that: top < 0,70 TNRZ (where T is the interval of one bit) That is applied for non-return-to-zero data that is used like transmission code S-NRZ (ITU-T G.957, G.797-Y.1322), as well is shown in the design guide of ENRE558/2003 4 OPTICAL DEVICES The projected communication systems for 500kV networks include, additionally to the SDH node, several devices that formed an amplification chain whose features will permit to obtain greater link lengths. In the following item will be described the main topics of them. 4.1 Optical emitters For high speed transmission it is desirable to use distributed feedback laser (DFB) in order to generate a unique λi wavelength with low bit error rate. With this kind of Laser, the dispersion in the fiber will be lower. That is obtained due to the DFB Laser has a precise control of the longitudinal modes of the emitter oscillation, as well as the scattering of the wavelength generated. It is used emitters of InGaAsP/InP-DFB Laser: • for operation up to 2,5Gbps • with spectral width between 1 to 0,25 nm • for single mode fibers • emitter high answer time (rise time <= 1nseg) Additionally, they have an optimal optical signal to noise ratio (OSNR), as well as good linearity. On the other hand, due to the features of non-linear transfer of the Laser emitter, it must includede stabilization circuits in order to make up for: • temperature variations • ageing of the optical emitter Resuming, the DFB Laser emitter will be used in dense wavelength division multiplexing (DWDM, later described) due to: • great bandwidth • precise generation of the wavelengths to be emitted • wavelengths emitted between 1520 y 1565 nm • wavelength stability during the Laser useful life • possibility to be tuned between the bandwidth of the EDFA amplifiers (later described) 4.2 Optical converters a1) in order to detect a lower optical power with low bit error rate (BER) is necessary that the optical converters has high sensitivity in the spectral area to be used, as well as to get that with very low noise level The sensitivity of an optical converter implies the optical received power that will guarantee figures of BER: 10exp10 a2) in order to allow the transmission speed to be used, it is necessary to have a very high reaction speed of the device to be used for the optical-to-electric conversion It is possible to obtain using avalanche photo diode (APD) of maximum sensitivity. By other way, the APD devices require polarization voltage of 100V, reason why they are used mainly in high bandwidth systems like our projects. The IngaAs/InP-APD converters with mesa/planar stucture allow recovering the transmitted signals at different wavelengths between 1000 to 1600 nm. Due to that wide bandwidth feature of the APD device, it is necessary to do the de-multiplexing action before to arrive to the APD device (condition when the DWDM technique is used) The conversion efficiency of APD devices is represented by its responsivity (spectral sensitivity): { R: output current of the photodetector / input optical power } {Amp/Watt} Comm sys for long haul links on 500kV sys-rel v.5 3 A main design project issue is to consider that the responsivity (R) parameter is applied for each wavelength to be used. That subject is important to take into account in case of transmission of several wavelengths guided to a unique APD device. Consequently, it is necessary to take the worst condition of all transmitted wavelengths. It is necessary to analyze the curve R= f (λ) that the manufacturer will provide Additionally, it is convenient to mention some inconvenient of the APD devices, which ones must be taken into account: • big transit time • additional noise generated due to avalanche effect that will affect the possible maximum frequency of bits for each APD (especially due to transit time) The sensitivity to light signals (S: minimum optical power that can receive the APD device in order to produce a useful output electrical signal), will depend of the project parameters: • wavelength to be used • bit error rate BER Typically, the parameters will be specified for a design objective of BER: 10exp-10 (according ITU-T G.957) for maximum attenuation and dispersion conditions. Depending of the particular conditions of the information to be transmitted, it could be necessary to use figures of BER:10exp-12 (according ITU-T G.826). Consequently, it must increase the sensitivity of the optical receiver, and/or reduce the link attenuation. 5 LIMITATIONS TO BIT TRANSMISSION RATE a) The intermodal dispersion (that produces the pulse broadening) must not appear in a single mode fiber due to the energy of the light pulse will be transmitted in a unique mode. Anyhow, the broadening of pulse do not disappear due to the group velocity related with the mode depends of the frequency due to the chromatic dispersion (like were described at the beginning). It is generated the group velocity dispersion (GVD) or fibre dispersion. The GVD/fiber dispersion has the contribution of two components (as it was mentioned above): • material dispersion • waveguide dispersion The fiber optic dispersion effect will limit the bit transmission rate of the communication system: For standard optical fibers, the value of M dispersion is negligible for wavelength λ=1300 nm, but it is so important for other wavelengths like λ= 1550 nm. It means that the spectral width of the wavelength will strongly affect the bit transmission rate b) Taking into account a conventional link with Laser emitter (not a wavelength division multiplexing link) with a spectral width of λ = 2 a 4 nm, the bit transmission rate will be limited to: For long distance link (i.e. 400km), it means a bit transmission rate of: (not acceptable for STM-1 hierarchy) c1) Taking into account a DWDM link with a spectral width of λ = 0, 80 nm (or less), the bit transmission rate will be: (acceptable for STM-1 hierarchy) c2) Taking into account a DWDM link with a spectral width of λ = 0, 40 nm (or less), the bit transmission rate will be: (acceptable for STM-1 hierarchy) See Figure AA and BB 6 WAVELENGHT MULTIPLEXING 4 6.1 Criterion Comm sys for long haul links on 500kV sys-rel v.5 The main design concepts in order to decide when to use the WDM technique are: a) when the optical cable capacity is full used, it is more convenient to implement the wavelength division multiplexing instead to replace the existing optical cable by a new one. It means to multiply virtually the capacity of installed fibers of the cable b) when the amount and diversity of services that will be transmitted are very large and it will be necessary to use a particular wavelength for each function/service. In such case, it is necessary to take into account the partition of the total optical power in several different wavelengths to be used c) when the transmission rate of each service is so high that it is not enough the STM-1 hierarchy for transmitting the whole information (as well as when it is not enough the upgrading steps to STM-4, etc) d) when distances involved in the communication system are long haul links, where the optical fiber dispersion can restrict the bit transmission rate preliminary use with 1550 nm wavelength with wide source spectral width For the Transener`s project conditions it was applied the concepts detailed in item d) 6.2 WDM technique Since the existence of fiber amplifiers and Laser multiple wavelengths optical emitters, it was possible to increase the capacity of the transmission systems by using the wavelength division multiplexing WDM technique, without modifying the existent network architecture. Through WDM is possible to couple the output of several light emitter sources (each one at different λi) to a pair of optical fibers in the emission end. After being transmitted along the optical fiber link, each λi can be separated by the detectors in the reception end. The WDM technique allows using all the bandwidth that the fiber has, without to change equipment and/or existing links. It is necessary to take into account: • the dispersion characteristics of the optical fibers • the gap between different λi The de-multiplexing process must be done previously that the light arrives to the APD photo detector due to the fact they are wide-band devices where it is not possible to select a particular and independent wavelength (as mentioned above) 7 WDM MULTIPLEXOR The main features that must have the λ-Mux as well as the λ-Demux are: • to minimize the interference between λi channels • to maximize the gap between λi channels (discrimination of each λi) There are two kinds: a) passive units, formed by optical filters that can joint or separate the optical channels. Each subrack has its own power supply and network connection, in order to do the management of its modules For DWDM systems, those optical filters are array wave guide (AWG), which one has high uniformity between channels, as well as low insertion loss. b) active units, based on passive devices with tuned filters that permit to select a particular λi from all the transmitted wavelengths. 8 WDM FAMILY There are several kinds of DWDM, depending of the link length involved: • DWDM for extra-long-haul links (more than 1000km) • DWDM for long-haul links (order of 800km) • DWDM for medium haul links (order of 300km) It is possible to use for all of them, the range from 1260 to 1625 nm (and up to 1675 nm) In the first case, the gaps between wavelengths are between 0,1 and 0,8 nm. It is possible to locate up to 160 wavelengths, with a total capacity of 40Gbps In the second case, the gaps between wavelengths are between 0,8 and 1,6 nm. It is possible to locate up to 40 wavelengths, with a total capacity of 10Gbps 9 WAVELENGHTS BANDS According to ITU-T G.694.1 y Figure A.1/G.957, there are six bands in the 1260-1675 nm range: • Original band (O-band): 1260 to 1360 nm • Extended band (E-band): 1360 to 1460 nm Comm sys for long haul links on 500kV sys-rel v.5 5 • • • • Short band (S-band): 1460 to 1530 nm Conventional band (C-band): 1530 to 1565 nm Large band (L-band): 1565 to 1625 nm Ultra large band (U-band): 1635 to 1675 nm Additionally, the ITU-T G.694.1 specifies a grid with the wavelength that is possible to use in DWDM technique. The grid to be used is shown for each band (L-band, C-band, etc), according of the gap between channels. • 0,1 nm (12,5 GHz) • 0,2 nm (25 GHz) • 0,4 nm (50 GHz) • 0,8 nm (100 GHz) • 1,6 nm (200 GHz) See Figure CC 10 DIGITAL TRANSMISSION 10.1 Links evolution At the beginning, the communication projects for 500kV systems have used SDH systems operating in STM-1 hierarchy that were installed on HVAC lines: • Up to 200km without optical repeaters • Greater distances with intermediate optical repeaters In following stages, for long haul links, the migration to DWDM was done in order to enlarge the digital transmission to long distances, avoiding and/or minimizing the use of optical repeaters located in isolated locations. As it was described above, due to optical fiber attenuation and dispersion, it is limited the maximum distance that the optical signal can arrive to the reception end, with enough power for its right detection. It means to use optical devices that allow amplifying all the signals simultaneously, without optical-electrical-optical conversions 10.2 Devices involved For long haul links that operate up to 10Gbps, it is necessary to add an amplification chain between both ends (Tx, Rx) in order to avoid intermediate regeneration optical repeaters. b1) The optical devices involved in the amplification chain are shown in Figure AA and its main features will be described in item #10.3. It includes: • Emitter side: with EDFA amplifiers • Receiver side: with Raman amplifiers and EDFA preamplifiers As an alternative to EDFA amplifiers, can be used a semiconductor optical amplifiers (SOA) that function like a Laser emitter. When electrical current flows through the SOA, the electrons of the active layer (third material intermediate between two semiconductor materials) will be excited and return to the original state in a similar procedure like EDF. Due to this process, the input light (coming from the optical signal) will excite the SOA`s electrons, consequently it will generate additional photons that will be aggregated to the those ones that caused the original emission. Finally, the result is that the output optical signal will be increased In Figure DD has been shown a generic evaluation of the involved levels in the amplification chain. But, in the particular projects must be considered that the amplification process must be done in the linear region of functioning of the devices, such as the output amplified signal will be of the same characteristic like the input optical signal. It is necessary to maintain linearity through the control of input power levels and avoid that the devices can arrive to saturation b2) Optical amplification process includes a spontaneous emission of photons whose phase and polarization is variable and they are added to the useful signal. The characteristic of noise of an optical amplifier is a measure of degradation in the signal to noise ratio, motivated by the photon emission process. The noise figure has presence in boosters as well as preamplifiers, but it is more critical in the second ones. In the long haul links, the amplifiers form part of a chain of devices that act simultaneously with the optical fiber losses (from Tx to Rx ends). The induced noise is the more critical factor of the system, due to: • The spontaneous emission (ASE) will be accumulated on several amplifiers and, consequently will degrade the optical signal to noise ratio (OSNR) when it is used amount of devices that were necessary to use in order to extend the reach of the link Comm sys for long haul links on 500kV sys-rel v.5 6 • As much increase the level of ASE, it begins to saturate the optical amplifiers and to reduce the gain of the amplifiers located below in the amplification chain It means that if the amount of amplifiers is important, the signal to noise ratio will be degraded so much up to the situation that the APD converter can have a non-acceptable bit error rate BER It means to take into account the noise figures along the project development, in order to avoid the performance degradation of the whole system. b3) It is important that all devices involved in the amplification chain will be manageable, through a NMS system with Ethernet interface and SNMP protocol. Minimally, it will be necessary to verify: input optical power, output power, wavelengths channels By this way it will be done the whole control of the communication system, due to the implementation of: {NMS for the SDH nodes + NMS for the chain amplification + NMS of the optical cables} 10.3 Features of the devices c1) Erbium doped fiber amplifier (EDFA) This is the first element of the amplification chain that can receive a multiplexed optical signal or a single optical signal, in order to amplifier it afterwards based on EDF process. It will not detailed the functioning of the EDF physic process, where the Erbium ions are excited by a Laser pump, and continuing afterwards with the amplified spontaneous emission process (ASE). It will only consider the result, where the photons in the range of 1550 nm will be amplified as long as they go ahead in the doped fiber. It produces that the irradiated photons will have the same wavelength that the input photons, but with a output signal totally amplified This amplification process occurs in: • Wavelength between 1530-1565 nm (C-band) • Wavelength between 1565-1625 nm (L-band) It is desirable to use EDFA devices of double stage of amplification (first amplifier unit; second amplifier unit), in order to both amplifiers in a cascade arrange can provide a flat gain in a wide bandwidth, but additionally having a low noise level (no more than 5dB; see item # 10.2b) In that way it is possible to obtain: • Input power from -10 dBm to +10 dBm • Output power up to +24 dBm (functioning like a booster) • Gains up to +34dB • Operation range of 1529 -1565 nm It must consider that the available output optical power available in an EDFA can vary depending of if it is used like: • Single channel • DWDM multichannel It means that the gain per channel varies, and consequently must be revised the calculations of the original Project in the subsequent adaptations with more λj wavelengths (always considering C-band and L-band) c2) Raman Preamplifiers It is based in doing the amplification of the optical signal by means of Raman Effect. A Laser pump of short wavelength that travels along the fiber together with the useful signal will scatter atoms in the optical fiber. It means that the original optical signal will have aggregated additional photons, and consequently that optical signal will be amplified in the last meters of the transmission fiber. The main difference with the previously described EDFA amplifiers, is that the Raman amplifiers do not need to dope the optical fibre They are used in the reception end, doing the amplification of the optical signal previously that the signal goes into the EDFA preamplifier. They operate with two main Lasers and two backup Lasers (reserve). Those ones will be activated when some of the main Laser fails. They have gain between 5 dB to 10 dB, with levels in the order of: • Input optical power between -50 dBm and -24 dBm • Output optical power between -45 dBm and -19 dBm • Operation range of 1529 - 1565 nm Comm sys for long haul links on 500kV sys-rel v.5 7 c3) Preamplifiers It is the last device in the amplification chain of the link, and it is located in the reception end for doing the amplification of the optical signal to the suitable level of the reception equipment (APD optical converter) The optical signal received in the preamplifier can come from: • Directly from the optical fibre of the OPGW cable • After to pass through a Raman amplifier The preamplifier is based on devices EDFA that operates with one Laser pump. It has a gain of 35 dB, with levels in the order of: • sensitivity (STM-1; BER: 10exp-10): up to -45 dBm (see note *) • output power of +14 dBm • operation range of 1529 - 1565 nm (Note *) It is convenient to mention that the acceptable maximum power (i.e. input saturation -10dBm) that is normally verified in conventional projects of medium/short link distances, it is not necessary to check in case of long haul links c4) Transponders The wavelength transponder is a bidirectional device that permits to convert the transmitted wavelength (λi) from the SDH node, to a wavelength (λj) located in the DWDM grid of ITU-T G.694.1 (the C-band is used in our projects) The transponder main features are: • input wavelength range: 1260-1600 nm • output wavelength: according ITU-T G.694.1 grid • output optical power: up to + 10dB The transponder is transparent to STM-1 transmission rate that is used in the projects, as well as to the transmission protocols that are used by the HV Transporter Agent. Additionally, it will permit future upgrading process to STM-4 transmission hierarchy, Gigabit Ethernet, etc Similarly that it was shown for optical amplifiers, it is necessary that the transponder can allow the remote configuration and supervision, by using SNMP protocol, monitoring al least: input optical power, output optical power, λi and λj wavelengths. Particularly: • emission side: to allow the conversion of input optical signals (1260-1600 nm) to output optical signals of the C-band of the DWDM wavelength grid of ITU-T • reception side: to allow the conversion of DWDM wavelength (1260-1600 nm) to an output optical signal of 1310 nm c) Amplifier with remote pump In case of optical long haul links it is possible to use Laser pump remotely located: • the firs Laser pump is an EDFA amplifier totally passive and based on the Erbium doped fiber. It will be installed in an aerial box in the OPGW trace, in order to obtain maximal gain for the optical long haul link • the second Laser pump (1480 nm) will be located in the Substation building The amplifier with a remote Laser pump, will receive: >>through an optical fiber will be received an optical signal to be amplified (useful signal) >>through another optical fiber will be received the 1480 nm Laser pump (feeder) By this way it is possible to obtain: • gain of 25 dB • noise figures of 5 dB • input optical power of -40dBm • wavelength operation range between 1529-1565 nm 11 OPTICAL CABLES 11.1 Fibers overview a) As it was described in item #3, one limitation in the operation wavelength range will be determined by the attenuation features of the optical fibres. Comm sys for long haul links on 500kV sys-rel v.5 8 Additionally, another determining factor to the operation wavelength range is the fibre dispersion, as well as the spectral width of the optical emitter. Consequently, the real useful operation wavelength range of the system will be determined by the interaction of the two limiting ranges before mentioned. b) The dispersion features of several types of optical fibres (in the zone of 1550 nm) will be shown generically in Figure FF, where the shadow area is used by EDFA amplifiers, and it represents the DWDM wavelengths range before described. • The dispersion shifted fibers (DSF) has a good performance for a unique channel • The (+D) NZ-DSF fibers will locate the zero dispersion wavelength out of the 1550 nm zone, and they have positive slope • The (-D) NZ-DSF fibers are similar to the before mentioned fibers, but they have negative slope c) It must consider that the modal field diameter of single mode fibres are an important parameter in their attenuation features, due to: • The bigger the fiber modal field diameter is, the worse the light conduction in fibre bend will be • By the contrast, the bigger the modal field diameter is, the lowest loss in fibre splices and connectors will be Resuming: c1) for single mode fibres with simple step index, the modal field diameter (2.a) is 10,4 um @1550nm c2) for single mode fibers optimized for 1550 nm (1550-DS-SM, dispersion shifted, segmented profile with triangular core), the modal field diameter (2.a) is 7,5 um @1550nm d) The fiber optic has an important absorption due to peak of water located at 1383 nm. In order to operate in the full wavelength range (1260-1625 nm), must be necessary to use zero peak water single mode fibers (ZPW-SM) according to ITU-T G.652 C/D Then, the absorption due to the peak of water will be reduced in an important manner, and consequently it is possible of obtaining an attenuation factor lower than the value for 1310 nm. It permits a lower and stable attenuation figures along the whole 1310-1625 nm range (even after installation process) e) The standard fibers according to G.652 have bending radius in the order of 30 mm. Then, it is convenient to use bend-insensitive fibers with bending radius of 7,5 mm. This kind of fibers will allow having low losses due to bending processes when the 1550 nm wavelength operation is used. Additionally, the fusion losses, due to the splicing process of two bend-insensitive fibers, will allow to obtain splicing losses as low as 0,01 dB/splice (comparing with the typical 0,2 dB/splice obtained with standard fibers) 11.2 Reducing dispersion effect a) DCM modules As at the beginning mentioned, it is essential to reduce the fiber dispersion effects in order to have a better communication system performance. In order to find it, it must use an additional element in the link with an opposite dispersion from that of the installed optical cable. It is a dispersion compensating module DCM. It is formed by spools of DCM fibers with opposite dispersion features and guaranteed values. By contrast, it must take into account the attenuation values that will be introduced in the link. Additionally, it is necessary to take into account that this alternative, when is used in DWDM systems, can be used for correcting an unique channel (not the whole band) b) fiber combination Another solution so as to compensate dispersion effects, is to alternate lengths of different kind of fibers (see Figure FF) as follows: • Lengths of fibers whose features are (+D) NZ-DSF, with • Lengths of fibers whose features are (-D) NZ-DSF The special feature is that this solution must be specifically calculated for the length of the link, and it is not applicable for several lengths (that situation is not essential for our projects) 11.3 Factors to be taken into account It is necessary to mention that the OPGW optical cables that are used in the 500kV Lines, as well as the optical fibers included into them, are fully detailed in the technical specifications included in the project. The main aspects taken into account are: Comm sys for long haul links on 500kV sys-rel v.5 9 a) Intrinsic factors (scattering and absorption) The fiber attenuation implies the loss of optical power and consequently producing adverse effects on the communication system, as was detailed at the beginning: • Band with reduction • Bit transmission rate reduction • Lower efficiency of the whole system Consequently, it is necessary to take into account some design criterion as follow: a1) to purchase optical cables manufactured with high quality fiber optics, so as to assure low dispersion features (see Figure GG) a2) to purchase optical cables with fiber optic whose attenuation factor (dB/km) will be as low as 0,20 dB/km or less (in spite of the attenuation range accepted by G.652) a3) to do the optical budget considering 0,30 dB/km (following the criterion of ITU-T G.957 Annex A-Fig A.1), in order to take into account the features of the installed fiber (including splice losses, repair tasks, temperature range, etc) a3) to use zero peak water fibers (ZWP-SM) according to ITU-T G.652C/D so as to increase the range of useful wavelengths (1280-1625 nm; free of OH ions) It means greater range into the spectrum so as to locate more wavelengths in all zones, and consequently to allow higher information transmission at the present and future greater benefits. Those criteria imply: • to utilize fiber optics in conventional applications that implies to operate in wavelengths of 1310 & 1550 nm • to utilize fiber optics in new applications by using the E-band (1360-1460 nm) • to increase the allowable bandwidth of each fiber optic up to 60% a4) it is necessary to take into account the polarization mode dispersion (PMD) of the fibers included in the optical cable. Typically, PMD = 0,07 ps/km½ is measured during the factory acceptance tests. The causes of birefringence are: • due to design factors (i.e. core stress, cladding eccentricity, elliptical fiber design) • due to external factors (i.e. fiber twist, fiber stress, fiber bend) Consequently, it is necessary to measure the PMD values during the FAT process, as well as to check it during the commissioning tests. This last recommendation is typically rejected by Main Contractors arguing that they need special instruments, etc. b) Extrinsic factors It means to pay attention about precautions that are necessary to take into account in some external actions that act over the OPGW cable and consequently can affect their optical fibers. b1) macro-bending A bend in the optical cable affect the fiber optic refractive index, as well as the critical angle of the light rays, producing that the light will be refracted in the core (through the cladding). In order to avoid this situation, it is necessary a very precise control about: • the installation process itself • use of qualified subcontractors (specialized on optical cables tasks) • the parameters involved in the process (pulling stress, bending pulley, etc) • utilization of special devices for automatic control of eventual wrong process (disconnection mechanical fuses, etc) • permanent supervision during the installation process • involved personnel certified by optical cable manufacturer and/or recognized authorities b2) micro-bending It is necessary to take special precautions so as to avoid pressure over the fibers themselves, because it can produce effects unable to be seen, but that can affect the development of the fiber optics by introducing nonperceptible micro-bending into them. As well as, it must consider some previsions in the temperature range where the optical cable will operate (especially for low temperature environment). It is necessary to project carefully the appropriated over length of fibers into the optical cable, so as to avoid localized micro-bending effects produced by straightening and/or contractions of the optical fibers. Those effects can return to the original conditions or not, due to hysteresis effect (and consequently can affect permanently the cable performance) b3) splices Comm sys for long haul links on 500kV sys-rel v.5 10 It is necessary to take precautions during the optical fiber splicing process, as follow: • to do the fusion splices by means of automatic instruments of well-known manufacturer • to do the splicing tasks into a vehicle/trailer with enough grade of sealing • to use certified splicing personnel (certification emitted by national authorities and/or the optical cable manufacturer) • to use a splicing procedure handbook/guide (written by the optical cable manufacturer) • to do a permanent supervision by a representative of the optical cable manufacturer • to do measurement of each spliced optical fiber, per each stretch of OPGW that is installed • to protocol each fiber optic per pair of two stretch of OPGW cables • to protocol each fiber optic between optical distribution frames (end-to-end measurement) b4) fiber optic characterization An additional topic to mention for future applications, is the following question: “which could be the measures to do to fibers optics so as to know the potential transmission capacity of the OPGW cables?” In spite of the current in-use STM-1/SDH systems it is necessary to take into account that new technologies can be used by Transener HV Main Transporter, as well as other Government Entities, in a share utilization of the optical cables. 12 CONCLUSION The objective of eliminating optical repeater stations in long haul links can be better obtained taking into account at least, the above mentioned concepts and criteria. It means to minimize the probability of malfunctions of the communication system due to several kinds of acts that normally can affect the behavior of the 500kV Transmission System. The necessary big capacity for transmitting the information specifically required by the EHV System (not for eventual other services out of the scope of the EHV Transporter) will be also achieved. The Availability of the whole communication system as well as the Reliability of the EHV System will be consequently increased. 13 BIBLIOGRAPHY (1) Electronic communications system, fundamentals through advanced, W. Tomasi, Prentice-Hall (2) Optical fiber planning guide for power utilities-Cigre SC 35-WG04 (3) Redes comunicaciones SDH en sistemas electricos de potencia, Eriac 2005, Rodriguez-Di Palma-etc (4) Guide for planning of power utility digital telecomm networks- Cigre SC35 WG 02 (5) Guias de diseño Enre 558/2003 (6) Technology of optical network for electric power utilities – ETRA (7) Emergency communication system for operation of HVDC, D2 Colloquium 2009, Galarza-Di Palma (8) New optical access technology-Cigre WGD2.15 (9) Fiber optic communication systems, G. Agrawal, John Wiley and Sons (10) ITU-T G.662 Generic characteristics of opt amp devices and subsystems Comm sys for long haul links on 500kV sys-rel v.5 11 FIGURE AA FIGURE BB Comm sys for long haul links on 500kV sys-rel v.5 12 FIGURE CC Comm sys for long haul links on 500kV sys-rel v.5 13 FIGURE DD Comm sys for long haul links on 500kV sys-rel v.5 14 FIGURE FF Comm sys for long haul links on 500kV sys-rel v.5 15 FIGURE GG FIGURE HH Comm sys for long haul links on 500kV sys-rel v.5 16