EXPERT TIP #4: OVERCOMING RECTIFIER ADJUSTMENT ISSUES Improper Rectifier Sizing
Situation
A new, deep well anode CP system and a standard, air-cooled CP rectifier were installed on a pipeline. Prior to installation, current requirement tests were conducted. Test results indicated that rectifier output of 36 to 40 DC amperes would be required to achieve proper protection levels.
Hardware Description
- 450’ deep anode well with 20 cast iron anodes in coke breeze
- New CP rectifier with a DC output rating of 50 volt and 50 amperes with a “standard” voltage adjustment tap configuration of 3 Coarse and 6 Fine settings
- AC input rating of the rectifier is 115/230 volts, 1 phase, 60 Hz
System Start Up/Energizing the Rectifier
- Installation - Checked the entire CP system to ensure proper installation techniques.
- Power - Confirmed that AC power is available to the rectifier.
- Tap Settings – Confirmed that the tap setting was set to Coarse 1 and Fine 1.
Initial Rectifier Start Up Results
When attempting to achieve the target range of 36 to 40 amps, the following occurred:
- Tap setting C1-F3 provided current output of 33 amps (not enough current)
- The next setting, C1-F4 provided current output of 45 amps (too high)
Diagnosis
The anode well and cabling had a very low total resistance (less than 0.25 ohms), which is common. This is good for CP efficiency but can be a problem.
A 50V-50A rated rectifier with the standard 3 Coarse and 6 Fine tap settings (total of 18 steps) yields approximately 2.8 volts differential between each tap change. With a 0.25-ohm circuit resistance, the current output change between tap settings is approximately 11 amps.
Options
Option 1 (Not Recommended)
Install a resistor to the DC output to increase the system circuit resistance.
Likely Result: With a relatively high current requirement, installing a series resister is inefficient and will consume 50 to 60 watts of (wasted) power. The resistor would require a separate enclosure. With materials and labor, this modification would be a costly (multi-thousand dollar) ‘Band-Aid’.
Option 2
Remove and replace the rectifier with one that has the proper output voltage rating. In addition, it would be advisable to increase the number of Coarse and Fine taps.
Likely Result: In this case, a 12V-50A rectifier with 5 Coarse and 5 Fine taps would provide 25 steps of adjustment. This would provide approximately 0.5 volts and 2.0 amps between tap settings.
Option 3
Reconfigure the existing rectifier to a 25V-50A rectifier in less than 5 minutes.
Likely Result: If the existing rectifier is manufactured with a dual AC input voltage rating of 115/230, you are in luck. Simply reconfigure the primary AC input voltage links to operate the rectifier on 230 VAC AND reconfigure (if required) the actual AC input power to provide 115 VAC.
The rectifier is now only receiving half the input voltage than before. Therefore, the total DC output voltage would also be half of the rated output of the rectifier.
Explanation: With this option, the rectifier will operate like a 25V–50A unit. This change now provides approximately 1.4 volts between and 5.6 amps per tap setting change. With the taps set at C2 & F1, the output would be approximately 39 amps.
Even with the reconfigured 25-volt output rating, the rectifier still has a large current change between taps. If the pipeline had a low current requirement, even with the reconfiguration, it still may have been a problem. As mentioned in Option 2, a 12-volt output rating with additional taps would have been a better selection and application.
Option 3 will not damage the rectifier. Unlike a motor that would be damaged with the low input voltage, a CP rectifier transformer is simply a voltage conversion device. Because the reconfigured rectifier is only capable of putting out half of its rated power, the maximum rated current of the transformer primary will not be exceeded.
Preventing Rectifier Adjustment Issues
The issue discussed here occurs when the CP designer does not know the final system circuit resistance required. Therefore, to be safe, the designer will order a rectifier with a conservatively high DC voltage output rating.
The best way to ensure that the proper rectifier output rating is specified is to conduct onsite field tests after the anode system is installed. A qualified CP Technician is needed to conduct the tests and after the deep well anode system is installed. The downside of this method is that the completion of the project is now on hold until the proper rectifier can be built, delivered and installed, which might take up to 12 weeks. This is often considered unacceptable because in extends the project and ultimately increases the project costs.
To avoid project delays waiting on a rectifier to be built, many CP operators will purchase and store rectifiers for future CP installation projects. To “standardize” their inventory, they purchase a single output rating, typically 50V-50A. In this case, the “one size fits all” approach did not work.
Conclusion
A proper CP design requires many considerations. Guessing can be costly and frustrating. As illustrated above, proper testing is recommended. Conferring with a CP expert can also help. In the case of the rectifier, ordering a model with a high number of tap adjustments is always a good idea and the 25-step example is a good start.
There are new CP power supplies now available that can overcome these output issues. The Farwest DCPro is an advanced CP power supply that can provide as many as 500 ‘steps’ of current adjustment. With the DCPro, you can order a 50V-50A output rating, have it shipped in a matter of days and eliminate all the adjustment issues discussed.