The L2G series of fault location systems
Table of contents
The L2G is a series of automated fault locations systems for telecommunication submarine cable systems. The L2G series includes the L2G/CR and the L2G/RL systems. The L2G series of fault location systems is based on DC measurements, an essential fault location technique for modern submarine cable systems.
L2G/CR stands for "Location system of 2nd Generation for Cables with Repeaters" : this name was given because the L2G/CR are complete systems able to implement resistive or capacitive location procedures with a full software process support. The L2G/CR are in fact able to deliver accurate fault location estimates from DC measurements, without manual intervention from the operator, as shown in figure 1 below. In contrast to L2G/CR systems, the L2G/RL system is dedicated to fault location on submarine cable systems without repeater.
Fig. 1: A typical fault location result
L2G/CR fault location systems implement many innovative features such as a transportable high performance power supply and measurement unit, and advanced data processing capable of handling the effects of repeaters/regenerators and of the fault's non-linear I/V characteristics (including for instance adaptations of the well-known Schaefer and Kennelly methods).
Figure 2 below shows an L2G/CR system power feeding the Atlantis cable system in Dakar. The monitor on top of the rack shows measurement results. The power supply and measurement unit (called the TAM unit) can be seen behind the keyboard. The rack also contains the PC running the LocalDef software, a printer, etc. This L2G/CR/6250A version of the L2G/CR can deliver up to 6250 V and 0.8 A into the cable system.
Fig. 2: an L2G/CR/6250A fault location system
The L2G/RL fault location system is functionally identical to the L2G/CR. However, being dedicated to repeaterless cable systems, its power feeding capability is lower, and the data processing does not take repeaters into account. Figure 3 below shows the power supply and measurement unit (TAM unit) of an L2G/RL system. It may deliver up to 350 V and 1.4 A into the cable system under measurements.
Fig. 3: the TAM of an L2G/RL fault location system
Explanations on the implementation of fault location systems belonging to the L2G series are given in the next paragraphs. They strictly apply to the L2G/CR systems. However, they are also generally applicable to the L2G/RL with the exception of the limitations explained above. Also, more information on the L2G/CR hardware is available below.
The main features of L2G/CR fault location systems are:
The implementation of the L2G/CR is compatible with ISO 9000 quality requirements (accuracy, calibration, safety...) for instrumentation dedicated to cable maintenance.
L2G/CR systems are appropriate for the accurate location of cable faults on multi-section submarine cables, including the case of in-service repair on networked transmission systems.
The operator uses a user-friendly Windows software on a familiar PC.
L2G/CR systems can provide accurate location of virtually any cable fault within 20 minutes, from limited damage of the insulating layer to a complete break in the cable (with the exception of type 4 faults), thereby reducing time-to-repair, cable-ship expenses, and the usually much larger cost of cable unavailability.
L2G/CR systems include an advanced power supply and measurement sub-system (TAM). The TAM provides a stimulus signal during a measurement sequence (the different L2G/CR systems offer three combinations of maximum voltage and maximum current, up to 1600 mA and 6250 V), and accurately measures the appropriate voltages and currents.
The TAM of an L2G/CR can be used as a back-up powerfeeding equipment, whenever its maximum voltage and current are appropriate. The TAM was designed as a light-weight apparatus for continuous reliable operation with high performance.
L2G/CR systems were designed for safety, even when in-service repair is necessary. The operator is always safe, even if overvoltages are produced by unexpected power switching by a branching unit.
The cable is also protected against positive or negative currents exceeding the maximum rated values for the cable system.
L2G/CR systems use a linear analog filter and several DSP filters so that optimum results are obtained in the presence of high noise levels. A location typically results from the measurement of thousands of individual current and voltage values.
The LocalDef software of L2G/CR systems makes use of a cable data base containing the basic characteristics of each section of a submarine cable system, including the per-unit-length resistance temperature dependence, and sea-water temperature typical for the time of year. The operator needs only to enter the name of the cable station where he works, and select the name the cable system under test.
In order to deliver the required fault location accuracy, the LocalDef software of an L2G/CR accounts for electrically non-linear items of the cable system (repeaters, regenerators, branching units,...). The LocalDef uses a data base of the repeater/regenerator I/V characteristics to accomplish this task.
All cable system parameters stored in data base are easily edited, so that modifications (e.g. due to repair) can be taken into account.
L2G/CR systems are designed so that the non-linear I/V characteristic of the fault does not reduce the measurement accuracy. In the case of a localization following the direct method, this is achieved by the implementation of an appropriate correction method in the LocalDef software.
A conjugate method for fault localization, implementing two L2G/CR systems, one on each side of the fault, can in some cases be used with very good results thanks to the cancellation of the currents through the fault.
L2G/CR systems are effective on virtually all electrical faults, including a break with an open fault, which can be localized with a capacitive method.
L2G/CR system takes advantage of proprietary research work and the latest hardware and software technologies. None the less, it is also based on the experience obtained using DC and capacitive location techniques since the old telegraph cable days.
The L2G/CR hardware is fully transportable.
In fact the L2G/CR system cannot be compared to anything else available today! Note that the L2G is now widely implemented, and many implementations of the L2G have been documented for controlling its real-world accuracy.
The next paragraphs will describe the hardware and the localisation methods of the L2G. Other important characteristics of the LocalDef software package and the specifications of the L2G/CR hardware are available on another web page.
The L2G/CR hardware consists of three or four units, depending on the power-feeding capability of the power supply and measurement sub-system TAM :
An L2G/CR system is controlled by a PC, this acronym standing here for any IBM PC-compatible micro-computer equipped with an IEEE 488 bus controller board and a modem, running our LocalDef software under Windows.
The BI is an intelligent interface between the IEEE 488 bus connector of the PC and the fiber-optic control link of the TAM, providing perfect isolation. The BI includes both an IEEE bus microcontroller and a CPU micro-controller for accurately triggering the measurements, for preliminaly data reduction, and for storing results in RAM. Thanks to the pre-treatment and storage capabilities of the BI, the PC does not have to idly wait for measurement results nor does its operation have to be interrupted to accept incoming data flow : full Windows performance is obtained from the L2G/CR software and other applications, without data bottlenecks or losses.
The TAM is a power supply and measurement sub-system, with a fiber-optic input/output interface for remote control. The TAM includes several specific safety related features, for both operator safety and cable protection. Once the microcontroller controlled TAM is switched on in the remote mode, normal measurements are completely remote-controlled by the PC through the BI.
The next paragraph will explain the localisation methods offered by the L2G. The specifications of the L2G/CR hardware are available on another web page.
DC fault localization is a key fault localization technique needed for all optical cable systems. The L2G/CR performs DC fault localizations using one of three methods:
The measurement set-up for the single-end measurements is shown in fig. 4 below.
Fig. 4: Schematic of single-end measurements
During fault localization with the direct method, a current injected in the cable by the TAM according to figure 4 flows through the cable system, and through the resistive fault. Usually, several current values are used in a sequence (positive or negative currents in the range of 1 mA to 1.6 A). Typically, many hundreds of accurately measured voltage and current values are stored and reduced. After digital filtering, and rejection of anomalous sampling sequences, the data processing for location takes into account the non-linearities in the individual active items of the cable system (repeaters...), and in the fault itself. The treatment of the latter can be made using the well known Schaefer's model, also known as Kennelly's model, or with a more up-to-date treatment based on modern electrochemistry and invented by Excem, the exponential model. Using the values of the temperature-compensated resistance of each section of the cable system stored in the section data base, the distance to the fault is obtained automatically.
During a fault localization with the capacitive method, a small current (e.g. 2 mA) is injected in the cable by the TAM according to figure 4. The voltage rises more or less rapidly according to the length of cable between the L2G system and the type 2 fault, and the total capacitance is deduced from the time variation of the voltage. The distance to the fault is derived using the capacitance values of the sections of the cable system, stored in the section database.
The measurement set-up for the current-balance measurements is shown on fig. 5 below.
Fig. 5: Schematic of current-balance measurements
Fault locations with the conjugate method implement current-balance measurements as shown above. The method involves two separate L2G/CR systems, one on each side of the fault. The modem included in the L2G/CR is implemented, and allows a dialog between the two systems to be established. The PC in the station A runs the L2G/CR Master LocalDef software whereas the PC in the station B runs the L2G/CR Remote LocalDef software (or the opposite). This allows accurate current matching, adequate synchronization of the measurement sequence, and eliminates human errors in the measurement.
The two TAM source currents of opposite values, so that almost no current flows in the fault. This measurement can be performed using large currents for ultimate accuracy, but requires good synchronization and relative accuracy (otherwise a current imbalance may cause excessive electrochemical degradation of the fault). In this case, data processing involves digital filtering, rejection of anomalous sampling sequences, and treatment of the non-linearities of the active items of the cable system. The distance to the fault is obtained from the resistance of the cable system sections.
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