Fundamentals and Practice of Electrical Measurements
3.6 Location of Faults
3.7.1 Quantities to be Measured and Objectives of Intensive Measurement Technique
3.7.1 Quantities to be Measured and Objectives of Intensive Measurement
Fig. 3-30 Results of intensive measurement on a long-distance pipeline in the region of a holiday with insufficient polarization (U0 = calculated true potential).
Modern pipelines with plastic coating usually have only a few defects, which can be clearly recognized from differences in the pipe/soil potential (see Fig. 3-30) [47,52]. In support it can be assumed approximately in Fig. 3-24 that:
Um(a) + U$(a) = £U~) (3-59)
The terms Uon(a) and U%(a) apply to the region of a defect at position a. £/on(°°) is the measured switching on potential against a remote reference electrode; it corre- sponds also to the values that can be measured over sections of pipeline with defect- free coating. Since this value can be considered constant, the two measured values have a similar change for variations in a, as in Eq. (3-59) and mentioned in Fig. 3-25.
This is also confirmed by the values given in Fig. 3-30 [51,52]. Local changes in on potential indicate voltage cones of defects which can be described in more de- tail by AU values.
In group (b), AC/ values are determined according to the data in Section 3.6.2.2 for circular defects. Examples are shown in Figs. 3-28 and 3-29. Figure 3-31 shows that in fact practically symmetrical voltage cones can occur, as predicted in Eq. (3-48) [47]. Errors in measurement must take into account the information in
Fig. 3-31 Equipotential lines on the soil surface above a defect in the pipe coating (DN 700, thickness of soil covering 1.5 m).
Eq. (3-5 Ib) and Fig. 3-27 because of the unknown depth, t, of the damage, but not the length, x, in the measurement of U^. The difference for t = 1 m at x - 10 m compared with x = °° only amounts to 10%. Further sources of error which must be heeded are electrode faults and foreign fields. These can be best eliminated by substituting the difference (At/on - At/off) for AUX in Eq. (3-5la).
The evaluation of AU values together with off potentials are aids in deciding whether coating damage should be repaired. In addition, by comparison with pre- vious results, intensive measurements indicate whether new coating defects have arisen. These could be the result of external foreign forces on the pipeline.
A further objective is the evaluation in group (c) of the local polarization state by taking account of IR errors due to direct currents. Here Eq. (3-28) and the fur- ther explanations in the second half of Section 3.3.1 are relevant. In practical ap- plication, the error effect of A(/off must be estimated [2]. When foreign fields are present, it is necessary to substitute for the At/ value the average of the measure- ments made on both sides of the pipeline [2,52]. Figure 3-30 gives an example of
where
measured values which show quite clearly that at the position a = 105 m the calcu- lated potential U0 does not fulfill the protection criterion.
3.7.2 Carrying Out an Intensive Measurement
The precondition of an intensive measurement is the exact location of the course of the pipeline. Methods described in Section 3.6.1.2 using the pipe locator are applicable. The harmonics of the protection current can be used. The protection current is interrupted to obtain the pipe/soil potentials and AU value for both switch- ing phases. Although the AU value can be determined independently of the mea- suring point, measuring the pipe/soil potentials requires a measuring cable about 1 km in length as the connection to the pipeline. On practical grounds, simplifica- tions can be introduced which certainly in most cases are applicable and low in error [1]. These concern AU values for both on and off switching phases as well as the pipe/soil potential.
(a) Determination of the pipe/soil potential
The pipe/soil potential is conventionally measured at a test point with a refer- ence electrode BQ:
The reference electrode B} is set up at a distance, a, over the pipeline. The voltage between the two reference electrodes
gives the pipe/soil potential at this point:
The determination of U} only requires the measured value U0 and no cable connec- tion to the pipeline. This technique can be repeated at equal intervals Aa = av - av_{:
A further possibility of error is the addition of electrode errors. For this reason examination of the Un value according to Eq. (3-62) at the next measuring point is necessary.
(b) Determination of AU values
According to Fig. 3-24 the reference electrode B^ is remotely positioned, out- side the voltage cones of the pipeline holidays. Thus it may also be positioned above a section of the pipeline where there are no defects in the coating, which can be tested by a conventional U% measurement. If now the electrode B0 lies on such a section (0g0 = 0^), but electrode Bj already lies in the voltage cone of a defect in the pipe coating, Eq. (3-61) applies for the position, a}:
and for the general position an from Eq. (3-64):
It is recommended that further evaluation be carried out by direct measurement according to the data in Section 3.7.1 because of possible errors caused by foreign sources in the case of large readings.
The determination of At/ values as well as the potential is effected by simple measurements of the difference in potential between two reference electrodes over the pipeline. To reduce possible electrode errors, partial sums can also be mea- sured for a larger span, e.g., (ak - at) ~ 300 m:
Here the voltage Um is measured directly between reference electrodes at the posi- tions ak and at.
The use of microelectronics for obtaining measurements and evaluating them is rational for intensive measurements with about 1000 values per kilometer [53].
The additions according to Eqs. (3-63) and (3-66) are performed with a pocket calculator using the correct sign, and read off by the engineer.
Equipment has been developed that records measured values in steps of 2.5 mV through a digital converter connected to it, analyzes them and transmits them to a hard disk. The measured values can be fed to a central computer and further analyzed and plotted. This system can be used for intensive measurements and also for monitoring measurements, and offers the following possibilities: listing
measured data, preparing potential plots, tracking trends over a timespan of sev- eral years, searching for larger coating defects, correcting potential values as well as listing information and data on defects in a protection installation database. At present there are no suggestions for a general need to repeat intensive measure- ments. It is, however, recommended that in particular local situations (e.g., the construction of parallel lines), repeat intensive measurements be made [54].