Currents from rectified current sources

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. 

Direct Current (DC). This current is transmitted for industrial uses only in exceptional situations. The most common sources of direct current are storage batteries and industrial devices called rectifiers, in which alternating current is changed (rectified) to direct current, as is used in electrolytic cells for the manufacture of chlorine gas, magnesium, aluminum, and a few other chemicals. The direct current is flowing from the source through the user application and back to the source, in one direction. The motor is primarily used for speed control of selected equipment. 

The ED process is a reliable, membrane-based, electrically driven demineralization technology that is more than 50 years old. It is suitable for the production of basic pure water from lower TDS supply sources. Commercial ED units consist of hundreds of individual cells in a modular membrane sandwich arrangement, bounded by electrodes and linked to a direct current (DC) rectifier-controller. The total treated water flow is a function of the number of cells in the ED unit. 

The principle of an impressed current system involves the supply of a protective current from some direct current power source, e.g. storage batteries, rectifiers or d.c. generators, through an auxiliary anode. Fig. 15.4 illustrates the arrangement. 

As shown in Fig. 15.6, external DC current is supplied from a power source such as a rectifier. The external DC current is used to cathodicaUy polarize the pipeHne. Impressed Current System (ICS) can be used to protect bare and poorly coated pipeHnes because of high current capacity. The anodes are made of durable materials that resist wear or dissolution. Iron with 14% silicon, carbon, and graphite are some commonly used anodes for pipeline protection [17,18]. All impressed current CPs require routine maintenance because they involve a power supply and more electrical connections than sacrificial systems. 

By means of an external current source, usually a rectifier. A reference electrode may be used to control the current from the rectifier. 

Rectifiers are used more than any other source of impressed-current power. Areas discussed include rectifier types, rectifier selection, specification requirements, and typical installation details. Various types of impressed-current anodes and components that make up an impressed-current system are also presented. Impressed-current-type cathodic protection systems provide cathodic current from an external power source. A direct current (DC) power source forces current to discharge from expendable anodes through the electrolyte and onto the structure to be protected. Although the current is not generated by the corrosion of a sacrificial metal/alloy, the energized materials used for the auxiliary anodes do corrode. 

Frequently dc generators have been used as sources of dc energy however, ac sources may be used and rectified by vacuum tube rectifier circuits or by use of high-current solid state diodes. A rectifier circuit for producing dc energy from an ac source is shown in Figure 5-7. The inductance in the primary circuit is a variable core reactor for current control. Numerous variations of this basic circuit are commercially available. A dc source such... 

The m-g UPS also can be built around a synchronous AC motor, as illustrated in Fig. 10.189. Utility AC energy is rectified and used to drive an inverter, which provides a regulated frequency source to power the synchronous motor. The output from the DC-to-AC inverter need not be a well-formed sine wave, nor a well-regulated source. The output from the generator will provide a well-regulated sine wave for the load. The m-g set also can be operated in a bypass mode that eliminates the rectifier, batteries, and inverter from the current path, operating the synchronous motor directly from the AC line. 

If a delta/wye connected power transformer is installed between the power source and the load, the power factor at the transformer input generally will reflect the average PF of the loads on the secondary. This conclusion works on the assumption that the low PF is caused by inductive and capacitive reactances in the loads. However, if the load current is rich in harmonics from rectifiers and switching regulators, some of the harmonic currents will flow no farther toward the power source than the transformer delta winding. The third harmonic and multiples of three will flow in the delta winding and will be significantly reduced in amplitude. By this means, the transformer will provide some improvement in the PF of the total load. 

In contrast to the galvanic anode system, the flow of current from the anode to the cathode is forced from a DC source in the impressed current system. Thus, whereas the current is provided by the corrosion of the electrode in the anodic galvanic system, the electrode acts as a conductor and hardly corrodes in the impressed system and the AC input is transformed and rectified to a varying DC voltage. A transformer rectifier is the most important component of the system. 

The modern procedure to minimise corrosion losses on underground structures is to use protective coatings between the metal and soil and to apply cathodic protection to the metal structure (see Chapter 11). In this situation, soils influence the operation in a somewhat different manner than is the case with unprotected bare metal. A soil with moderately high salts content (low resistivity) is desirable for the location of the anodes. If the impressed potential is from a sacrificial metal, the effective potential and current available will depend upon soil properties such as pH, soluble salts and moisture present. When rectifiers are used as the source of the cathodic potential, soils of low electrical resistance are desirable for the location of the anode beds. A protective coating free from holidays and of uniformly high insulation value causes the electrical conducting properties of the soil to become of less significance in relation to corrosion rates (Section 15.8). 

DC (direct current) electricity, which powers the electrolyzer, can come from a variety of renewable sources, such as wind generators, photovoltaic panels and small hydro and geothermal systems. Batteries are a common non-renewable source of DC current, as well as AC power supplies with rectifiers that change the AC to DC. 

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