Impressed Current Station

In this case, impressed current protection with several anodes was chosen on the one hand to achieve uniform current distribution with the relatively high protection current density, and on the other hand to avoid large anode voltage cones. A transformer-rectifier with a capacity of 10 V/1 A was chosen. 

In total, three high-silicon iron anodes of 3 kg each were installed at points a, and a3 as shown in Fig. 11-3. The anodes were bedded vertically in fine-grained coke in boreholes about 2.3 m deep and d = 0.2 m so that the length of the coke backfill was about 1 m. Each anode was connected by a separate cable to the anode bus bar of the transformer-rectifier to allow the current of individual anodes to be monitored. Three cathode cables 2x4 mm were installed for the return path of the protection current and attached on the tank end to the connecting clamps of the dome support. 

The grounding resistance of the three anodes with the stated dimensions of the coke backfill, a soil resistivity of 75 Q m and an interference factor, F = 1.2, was calculated from Eq. (24-35) as about 14 Q. After the anode installation was in operation, measurements of the grounding resistance gave a value of about 12 2. 

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With impressed current stations on privately owned sites, it is advisable to conclude contracts with the property owner in order to guarantee access to the installation at all times, to prevent damage to the cables and to exclude construction or modifications in the area of the anode beds. 

The units and ancillary equipment must be protected from mechanical damage and the effects of weather to ensure the reliable operation of an impressed current station. This is achieved by installing it in a weatherproof plastic housing (see Fig. 8-2). Sufficient ventilation must be provided to disperse heat. The ventilation holes should be protected with brass gauze to keep out animals. Transformer-rectifier units must be connected to a circuit that is continuously energized, especially if they are in a building where the current is turned off at night, e.g., gas stations that are not open for 24 h. 

Impressed current stations must be inspected regularly [16,17]. This should take place generally every month because ... 

Metal anodes using platinum and precious metal oxide coatings are also incorporated into a variety of designs of impressed current protection for pipeline and deep weU appHcations, as weU as for protection of condenser water boxes in power generating stations (see Pipelines Power generation). 

The current needed for cathodic protection by impressed current is supplied from rectifier units. In Germany, the public electricity supply grid is so extensive that the CP transformer-rectifier (T-R) can be connected to it in most cases. Solar cells, thermogenerators or, for low protection currents, batteries, are only used as a source of current in exceptional cases (e.g., in sparsely populated areas) where there is no public electricity supply. Figure 8-1 shows the construction of a cathodic impressed current protection station for a pipeline. Housing, design and circuitry of the rectifier are described in this chapter. Chapter 7 gives information on impressed current anodes. 

Fig. 8-1 Construction of an impressed current cathodic protection station.

Impressed Current Equipment and Transformer-Rectifiers 239 Table 8-2 Troubleshooting at cathodic protection stations... 

With impressed current installations supervised by technical telecommunications, it is sufficient to carry out operational control annually [17]. With gas pipeline grids, it is advisable to build in inspection instruments in the control installations or connecting stations since connection with the telecommunication system is possible. 

According to Ref. 32, the functioning of impressed current cathodic protection stations should be monitored every 2 months, and the stray current protection station every 1 month. If protection installations are provided with measuring instruments for current and potential, this supervision can be carried out by operating staff, so that the readings are recorded and sent to the technical department for... 

On the other hand, the costs for an average cathodic protection station for 6 A come to = 40,000 DM according to Table 22-2. For very small installations as, for example, the external cathodic protection of a tank, the costs of an impressed current system where a current supply is already available without cost, with lower current output, can be reduced to about 4000 DM. With larger tanks and greater soil resistivity, the following considerations point to the increased suitability of an impressed current system. 

The structural costs of impressed current protection of harbor and coastal structures are about 1.5 to 2.5% of the total cost of the object to be protected. As an example, for the installation of a cathodic protection station in a tanker discharge jetty, the construction costs amounted to 2.2% of the total costs. The annual cost of current, maintenance, testing, and repairs amounted to 5% of the construction costs of the cathodic protection [22]. 

Impressed-current systems for power stations are somewhat more sophisticated than those required for pipelines or marine structures inasmuch that a large number of items of plant, with a wide range of current requirements, are protected by one transformer-rectifier. Each section of every water box in order to provide even current distribution requires one or more anodes. In the case of a large circulating water pump as many as 30 anodes may be required to provide the current distribution necessary. Three types of system should be considered as follows ... 

Cathodic protection has many applications, e.g. in refineries, power stations, gas, water, and oil utilities on marine structures, e.g. jetties, piers, locks, offshore platforms, pipelines, ships hulls, etc. and on land structures, e.g. buried pipeline, storage tanks, cables, etc. For each use, the cathodic protection system requires careful design, either impressed current, sacrificial anodes, or a combination of both may be chosen. There may also be other protection systems, e.g. paint, the nature of which will affect the design parameters and must be taken into consideration. 

In stray current forced drainage against rails, current peaks up to 100 A can occur, particularly in the protection of old pipe networks with poor coating, although the necessary output voltage remains quite low. This current depends on the voltage between rail and pipe as well as on the resistance of the circuit. With rails laid in the street and thus earthbound tracks, this is mostly between 5 and 10 V. The dc supply for such installations is therefore considerably less than for normal impressed current anode stations. 

In soils with high resistivity, it is advisable to locate the impressed current anodes immediately next to the pipeline [12]. The pipelines then lie within the voltage cone of the anodes. Figure 12-6 shows the arrangement of the anodes for local cathodic protection of a pumping station. The distance of the anodes from the protected objects should be chosen according to Rgs. 9-5 and 9-6 so that the pipe/soil potential is reduced by the protection current to t/ = -1.2 V. The voltage cones of the individual anodes will thus overlap. 

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