Rectifier/transformer

A battery backed-up d.c. source of control supply is provided for the AMF panel and engine ignition. The control scheme, as illustrated, generally eonsists of a 220 or 240 V a.c. source of supply, with a transformer rectifier unit, to provide a 24 or 48 V d.c. control voltage, to charge the battery as required and a battery back-up of suitable capacity. 

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. 

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. 

In single-phase bridge circuits for ac connections and for very low ac output voltages below 5 V, single-phase center tap circuits are used as rectifier circuits for CP transformer-rectifiers. They have an efficiency of 60 to 15% and a residual ripple of 48% with a frequency of 100 Hz. A three-phase bridge circuit for three-phase alternating current is more economical for outputs of about 2 kW. It has an efficiency of about 80 to 90% and a residual ripple of 4% with a frequency of 300 Hz. The residual ripple is not significant in the electrochemical effect of the protection current so that both circuits are equally valid. 

Fig. 8-4 Circuit diagram of a transformer-rectifier with overvoltage protection (protected against high voltage).

Fig. 8-5 Circuit diagram illustrating the principle of a potential-controlled transformer-rectifier.

Fig. 8-6 Principle of a potentiostatic transformer-rectifier (Vq = amplification factor, S = power factor).

Fig. 8-7 Principle of a transformer-rectifier control device with set-limiting potentials.

Transformer-rectifiers should have an ammeter to indicate the current and a high-resistance voltmeter to indicate the potential at the protection station. 

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

The housing of the transformer-rectifier unit should be erected in an area with a right-of-way for the pipeline. 

The information in Section 9-1 covers the anode installation. Housing, layout and circuitry of the transformer-rectifier unit are described in Chapter 8, and type and possible choices of anode materials in Chapter 7. 

Fig. 11-3 Cathodic protection of a gas station with a mains-fed transformer-rectifier.

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, aj and as shown in Fig. 11-3. The anodes were bedded vertically in fine-grained coke in boreholes about 2.3 m deep and J = 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. 

As an example, a tank farm that is to be cathodically protected by this method is shown schematically in Fig. 11-4. As can be seen in the figure, injection of the protection current occurs with two current circuits of a total of about 9 A, via 16 vertically installed high-silicon iron anodes embedded in coke. These are distributed over several locations in the tank farm to achieve an approximately uniform potential drop. The details of the transformer-rectifier as well as the individual anode currents are included in Fig. 11-4. Anodes 4, 5 and 6 have been placed at areas where corrosion damage previously occurred. Since off potentials for 7/ -free potential measurements cannot be used, external measuring probes should be installed for accurate assessment (see Section 3.3.3.2 and Chapter 12). 

Anodes are connected to the object to be protected or to the transformer-rectifier by insulated conductors that are resistant to mineral oil (e.g., Teflon-coated cable) with a cross-section of 2.5 mm of Cu. The transformer-rectifier must meet the demands according to Ref. 6 and have the capability for monitoring and controlling its operation. The life of the anodes is in every case designed to be at least 15 years. 

A direct current flows in the installations during operation of cathodic protection stations therefore, the transformer-rectifier must be switched off when pipes are out or other work on the fuel installation is carried out, and the separated areas must be bridged with large cross-section cables before the work is started in order to avoid sparking that could come from the current network. 

With forced stray current drainage, the current is returned from the pipeline to the rails by means of a grid-fed rectifier. The transformer-rectifier is connected into the stray current return conductor, the negative pole is connected with the installation to be protected and the positive pole is connected to the rails or the negative side of the bus bar in the transformer substation. 

Six iron anodes are required for corrosion protection of each condenser, each weighing 13 kg. Every outflow chamber contains 14 titanium rod anodes, with a platinum coating 5 /tm thick and weighing 0.73 g. The mass loss rate for the anodes is 10 kg A a for Fe (see Table 7-1) and 10 mg A a for Pt (see Table 7-3). A protection current density of 0.1 A m is assumed for the coated condenser surfaces and 1 A m for the copper alloy tubes. This corresponds to a protection current of 27 A. An automatic potential-control transformer-rectifier with a capacity of 125 A/10 V is installed for each main condenser. Potential control and monitoring are provided by fixed zinc reference electrodes. Figure 21-2 shows the anode arrangement in the inlet chamber [9]. 

Figure 21 -3 shows a schematic diagram of the turbine in which the black areas are CrNi steel. The plate anodes are situated in the region of the suction manifold (A 1 to A3) in a triangular arrangement in the impeller ring (A4 to A6) and distributed in the vicinity of the segmented water inlet ring (A7 to A10) (see Fig. 21-4). These four anodes were individually connected to the transformer-rectifier with chloride-resistant pressure-resistant cable.

Six caustic soda evaporators were anodically protected against stress corrosion in the aluminum industry in Germany in 1965 [27]. Each evaporator had an internal surface area of 2400 m. The transformer-rectifier had a capacity of 300 AJ 5 V and was operated intermittently for many years. Automatic switching on of the protection current only took place in case of need when the drop in potential reached... 

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