Cathodic protection (CP) rectifiers have the following major components. These typ-ically include a transformer to step down AC line voltage to low voltage AC on the secondary with a tap arrangement to permit selecting a range of voltage, a rectifying element (usually full wave silicon diodes for rectification), and a housing for outdoor mounting. These components are supplemented by an AC circuit breaker and DC output meters. Both single-phase and three-phase units are in common use.Figure 8.1, illustrates diagrammatically single-phase and three-phase units of the full-wave bridge-connected type.
Where electrical storm activity is prevalent, it is advisable to provide protection against lightning damage.
• Lightning surges may occur from the electric distribution line (the more probable) and/or
• Surges coming from the pipeline (both from lightning and AC ground fault)
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GROUND Figure 8.1 Rectifier schematic diagrams.
Specifying rectifiers having transformers with an electrical shield between primary and secondary transformer windings may provide some protection from lightning surges.
Such a shield (shown in the figure), when grounded properly, intercepts the high voltage peak surge of a lightning pulse and carries it to ground. Otherwise, it can break down a rectifying element and may burn out the element. Low voltage lightning arrestors can also be placed across rectifier terminals and may provide protection from lightning surges from the pipeline. Neither type of protection, however, is effective against a direct strike to the rectifier itself.
Rectifier manufacturers produce units for CP applications with a wide range of out-puts. Data are available in the supply catalogs of CP equipment. Three general housing types are available. Ventilated housings that provide for convection air cooling are used for most pipeline applications. Where highly corrosive atmospheres exist (marine or industrial, for example), the equipment may be oil-immersed in a tank-type housing.
For locations subject to hazards and explosives an explosion proof housing is available for oil-immersed units.Figure 8.2illustrates the appearance of typical air-cooled and oil-immersed units.
Figure 8.2 Typical rectifiers: (left) air cooled unit; (right) oil immersed unit. (Courtesy of Universal Rectifier, Inc.)
Rectifying elements typically used in units today are silicon diodes. A silicon diode has high resistance to current flow in one direction and low resistance in the other. This characteristic makes rectification possible. The diagrams ofFigure 8.1 show that for a given direction of current flow in the transformer secondary winding, the current can flow on only one route through the rectifying element (in the direction of the arrow heads). This flow is out the positive terminal to the ground bed and back from the pipeline connection to the negative terminal. When the direction of current flow in the transformer secondary reverses (this occurs 120 times at normal 60-cycle AC power frequency), the current will take a different route through the rectifying element but will still flow out at the positive terminal and back at the negative terminal. The result is direct current (DC).
A rectifying diode is rated by the manufacturer for specific maximum current flow at a given ambient temperature and for maximum inverse voltage (the voltage impressed across the element in the high resistance direction). Diodes are assembled into stacks or assemblies with series-parallel combinations to attain the over-all DC voltage and current rating needed for the rectifier produced. In older rectifiers, the rectifying elements were selenium stacks or discs, but modern rectifiers use mainly diodes as rectifying ele-ments. Rectifying elements have been continuously improved over the years in operating characteristics and efficiency. The rectifier user is advised to review developments in this field and replace older rectifying elements with improved stacks when justified by the improved characteristics and efficiency of newer ones.
Cathodic protection rectifiers may be equipped with filters in the DC output which smooth out the ripple in the rectified DC and permit higher over-all efficiency. These filters are practical where savings in power cost justify the additional investment.
Under certain circumstances, “constant potential” rectifiers are useful on pipeline systems. Such rectifiers are designed to maintain a constant protective potential on the pipeline at the rectifier location, which changes to match pipeline current requirements.
Applications include areas subject to stray current from transit systems or in mining operations where potential variations are not beyond the corrective capacity of this type of unit. Use with ground beds subject to wide seasonal variations in resistance (wet to dry) is also included.
Constant potential rectifiers (seeFigure 8.3) differ from more conventional rectifiers (which require manual adjustment of transformer taps to change output) in that a sensing circuit that maintains continual checks on the pipe-to-soil potential changes the output current automatically. Typically, this can be accomplished by burying a permanent ref-erence electrode at the point where constant pipe-to-soil potential is to be maintained.
Once the rectifier is adjusted for the desired pipe-to-soil potential, any increase in absolute value of this potential serves (through the electronic controller) to increase the reactor reactance. This cuts the output current back until a balance at the preset potential is regained. Likewise, any decrease in absolute value of this potential between pipeline and reference electrode will cause the rectifier output current to increase automatically until balance is regained.