Expert Tip #22: 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 that has 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 colts or something different?
- Is the Provided power single or three phase?
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 for higher range in case of field changes. If it is determined that 20 volt, 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 the addition of 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.