Cathodic Protection Rectifier Tips
What Typeof Rectifier Cooling is best? Air or Oil?
Cathodic protection (CP) rectifiers generate heat due to the loss of energy as a result of converting AC power to DC. This is caused by the relative inefficiency of the internal components, such as the transformer and diodes. Therefore, this heat must be moved away from the rectifier components.
Air-Cooled Cabinets
Air-cooled cabinets use the movement of cooler air across the rectifier components to cool the components. Called convection, warm air rises and escapes through upper cabinet vents and then pulls cool air into the cabinet via the lower cabinet vents. This removes the heat from inside the rectifier cabinet to the outside environment. This is the most common type of cooling used for CP rectifiers. However, as convection draws in outside air, it also brings with it dust and possibly damp, corrosive air that can cause corrosion, thereby aging rectifier components.
Oil-Cooled Cabinets
Oil-cooled cabinets use the movement of transformer oil to cool rectifier components. Similar to air-cooled units, as the oil is heated, it rises and moves across the components. The heat in the oil is then dissipated through the cabinet walls and cooling fins into the atmosphere. The oil also protects the rectifier components from dust and corrosion, so oil-cooled cabinets may be more practical in dusty or humid environments. Oil-cooled rectifiers can be used in certain hazardous locations because any electrical arcing due to abnormal conditions in the rectifier is suppressed by the oil and is prevented from reaching the atmosphere.
Common Air-Cooled Rectifier Mounting Techniques:
Most air-cooled rectifier cabinets will mount on a steel pole or wood post, which is common in rural areas. Standard, air-cooled rectifier cabinets typically include a mounting channel that can be bolted to the pole or post, and the channel fits the contour of the pole or post for mounting ease.
This same cabinet configuration can be mounted to a wall or backboard instead of a pole or post. This is less common but is done in locations such as production facilities or similar.
Air-cooled rectifier cabinets can also be fitted with a mounting pedestal. This pedestal frame converts a pole or post mount cabinet to a free-standing cabinet, which is typically mounted to a concrete pad. 
Ornamental Mounting:
In more populated areas, CP rectifiers can be mounted inside a free-standing cabinet referred to as an “ornamental” or “decorative” cabinet. The standard factory cabinet is no longer required because the rectifier sub-frame mounts inside the ornamental cabinet. In addition, other components, such as a shunt panel or remote monitor can be included inside the cabinet, making for a nicer project appearance. 
Farwest designed and offers the POWERHOUSEplus line of ornamental, air-cooled cabinets, which is a multifunction cabinet solution providing an AC input breaker, a power company watt-hour meter location, and an AC convenience outlet. The cabinet provides room for a CP rectifier, shunt board and a remote monitor. This configuration provides an orderly rectifier installation.
Oil-Cooled Cabinet Mounting:
Oil-cooled cabinets are usually mounted to the ground in some fashion, typically on a concrete pad. Depending on the size of the cabinet, rectifier pads can be quite large and may require the use of a reinforcing bar or other structural support within the concrete. In some locations, a containment system may be required in the case of an unlikely transformer oil leak. 
Rectifier Cabinet Materials:
Steel is most often used for air-cooled rectifier cabinets due to its relative durability and cost. Aluminum and stainless steel are also available but at a greater cost. Oil-cooled cabinets are almost always (if not always) heavy-gauge steel for durability purposes.
Cabinet Finish
Many standard, air-cooled, steel CP rectifier cabinets utilize a hot-dipped galvanized (zinc) finish, that is very durable, but not extremely attractive. There are more attractive paint or FBE powder coating options that allow for color variations, which may be a need in city settings.
Aluminum cabinets are available in raw aluminum (not recommended), painted or FBE powder coated, and/or anodized.
Stainless steel cabinets are typically provided in raw metal due to the protective nature of stainless. 
SELECTING THE PROPER RECTIFIER OUTPUT RATING
Output Voltage Ratings
The DC output “ratings” for Cathodic Protection (CP) rectifiers refer to the DC output voltage and current capacity of the rectifier. Output voltage ratings are typically between 5 and 125 volts, while current ratings range between 5 and 400 amperes.
Choosing the Correct Rating
All CP rectifiers must have adjustable output ranges to provide the needed DC current to the structure in need of protection. For instance, a rectifier with an output rating of 50 volts and 50 amps can be adjusted to operate at a much lower output, such as 5 volts and 2 amps. While possible, this is not ideal and is like having a car designed to travel at 200 MPH when you need to only travel at 5 MPH—obviously not a good match and certainly a waste of money.
While technicians may be required to specify the voltage and current output rating of a rectifier, many have no knowledge of the actual, site-specific requirements for the CP system. To properly specify a rectifier’s output rating, the following information is required:
- The actual DC current requirement for the pipeline or structure that is to be protected
- The resistance-to-earth of the impressed anode system (bed) that is to be powered by the rectifier
- The length and size of the DC cables that will be used to connect the rectifier to the structure and anodes
- Note: The size and length of the cable will play a large part in determining voltage needs
- The available AC input power at the project site
- Is the input voltage 120, 240, 480 volts, or something different?
- Is the provided power single or three phases?
With this information, the CP technician can calculate the appropriate rectifier output range. When selecting this output range, it should be somewhat conservative and allow a higher range in case of field changes. If it is determined that 20 volts, 10 amp output is required for the job, upsizing this by 20% may be prudent in case operating parameters change over time. This could include changes in anode bed resistance, coating performance, or adding more steel to the protected structure.
Most CP rectifiers are manually adjustable, and there is a finite number of available output adjustments. Depending on the manufacturer, the standard number of rectifier tap adjustments includes 3 coarse and 6 fine settings. This provides a total of only 18 steps of adjustment over the rated voltage range. If the rectifier output range is too high for the application, control is limited, and you may not be able to adjust the rectifier to the exact output setting needed. Therefore, when ordering a manually controlled rectifier, it is wise to order a greater number of tap settings to allow more precise adjustment options in the field.
To combat the adjustment issue, there is always the option of an automatic, current-controlled rectifier, but that’s a topic for a future tip.
FOR MORE INFORMATION ABOUT CP RECTIFIERS OR QUESTIONS ON THE INFORMATION PRESENTED ABOVE, PLEASE CONTACT US AT: [email protected]