Adjustable Transformer-Rectifiers

Usually the annual variations in the specific resistance of the soil lead only to small changes in the current circuit resistance so that adjustment of the protection 

If an adjustable T-R is connected as forced stray current drainage between pipeline and rails and its output voltage is fixed at a definite level, the protection current and the pipe/soil potential can undergo considerable fluctuation. 

Cathodic protection stations frequently operate under conditions that are continually changing. These include  

In such conditions, it is recommended that the T-R be equipped with an electrical control circuit, which primarily keeps the potential constant, and, in exceptional circumstances, also the protection current. These pieces of equipment are potentiostats (for controlling potential) and galvanostats (for controlling current) [8]. 

The adjustment of a protection station or of a complete protection system where there is stray current interference is made much easier by potential control. Potential control can be indispensable for electrochemical protection if the protection potential range is very small (see Sections 2.4 and 21.4). This saves anode material and reduces running costs. 

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Manually Controlled System A manually controlled system comprises one or more transformer-rectifiers each with its associated control panels which supply the d.c. to the various anodes installed in the water box spaces. Each transformer-rectifier is provided with its own control panel where each anode is provided with a fuse, shunt and variable resistor. These enable the current to each anode to be adjusted as required. Reference cells should be provided in order to monitor the cathodic protection system. In the case of a major power station, one transformer-rectifier and associated control panel should be provided for separate protection of screens, circulating water pumps and for each main condenser and associated equipment. 

The Transformer/Rectifier (T/R or rectifier) is the DC power supply that transforms mains AC to a lower voltage and rectifies it to DC. The positive terminal is connected to the anode and the negative to the cathode. The level of the output is controlled as described later. T/R s can be run at constant voltage, constant current or constant potential (against a half cell). They can be adjusted manually or automatically by circuitry or computer control or remotely using a telephone line and modem link or a similar remote connection as described later. 

The problem with selecting a transformer rectifier is ensuring that it is powerful enough, but not so powerful that all adjustments are made in the first 10% or less of the output. This can lead to difficulties in control and to inefficiency in output, with electrical energy lost as heat. The electrical engineering of transformer rectifiers is covered in some of the main texts on cathodic protection and will not be discussed in detail here. 

A dilatometer constructed by Sauer is shown in Fig. 45. The sensing element is a linear variable differential transformer, whose rectified output is recorded directly. A pivot arm connects the transformer (A) to the bearing arm (B) and a micrometer screw allows for calibration and adjustment of the transformer. By adjustment of the compensating screw, the pivot arm can be moved so that the contact point (B) is exactly one sample length from the fulcrum. Temperatures above and below ambient can be obtained by circulating gas round the specimen. 

On a single-phase rectifier an intermpter can, in principle, be installed at three locations the input ac to the rectifier, the transformed ac current, or the dc from the rectifier. The current output of the rectifier to the pipeline should be verified before and after this type of intermpter installation. It will need to be adjusted if the current drawn to power the interrupter unit affects the output to the pipeline. 

The power transformer provides four separate low-voltage ac sources. These sources are full-wave-rectified, capacitor-filtered, and regulated by electronic series regulator circuits, which have the capability of regulation and current limitation with foldback characteristics. Each supply voltage has its own reference source and the possibility to adjust the output voltage by a potentiometer. 

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