When to Use rectifier preventive maintenance？

Sustainability seems to be the latest catch-phrase, and cathodic protection (CP) is an important component for the sustainability of many metal structures. What better way to preserve and maintain infrastructure than to mitigate corrosion? Some CP systems are comprised of sacrificial anodes that naturally corrode to protect less active metals such as steel. Others require power sources to drive protective current in the right direction. The most common impressed current voltage sources are rectifiers, which can break down. Well-maintained rectifiers can provide uninterrupted CP, which reduces repair costs and labor/technician time. This article discusses the fundamentals of rectifier operation and maintenance along with basic recommendations.

Link to kelingyizhi

Sustainability is the capacity to endure. The root purpose of any cathodic protection (CP) system is to mitigate corrosion. Preserving a pipe or other metallic structure by preventing corrosion damage allows it to endure. Therefore, corrosion mitigation leads to sustainability.

CP is most commonly achieved by means of a galvanic (sacrificial) or impressed current system. A galvanic CP system is comprised of sacrificial anodes typically made of active metals (aluminum, magnesium, or zinc) that corrode in order to provide protective currents for a less active metal, such as pipeline steel. An impressed current CP (ICCP) system utilizes external power in the form of a rectifier or other voltage source that drives impressed current anodes (e.g., cast iron, graphite, and mixed metal oxide) to corrode in order to distribute protective current to the structure (cathode).

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. It is imperative that a rectifier remain in a state of constant operation. Because a rectifier is an electrical device, it is vulnerable to power surges. A nearby lightning strike can cause the circuit breaker to trip or a diode to short. Therefore, routine inspections and monitoring are necessary to maintain a properly functioning, long-lasting rectifier.

Safety is the most important aspect of all inspections. The goal of any task related to rectifier operation is to perform the work safely, which includes wearing the proper protective equipment.

Operation

There are three major components in a rectifier: transformer, stack, and cabinet. The purpose of the transformer is to safely separate the incoming AC voltage (primary side) from the secondary side, which is adjusted to control the output voltage of the rectifier. Typically, these adjustments are made with tap bars connected to the secondary side windings at intervals that offer several setting options. The stack is the actual rectifier and is comprised of a set of silicon diodes or selenium plates that function as unidirectional current valves. The diodes or plates are configured so that cycling AC flows in one direction and is blocked in the other, resulting in both directions of the AC wave flowing in the same direction. The cabinet, which includes the test panel, safely houses these components, and allows for monitoring and other advanced operations.

Additional items that may be found in a typical rectifier include a circuit breaker, voltage and current output meters, lightning arrestors, surge suppressors, transformer tap bars, and fuses.

Table 1 lists general rectifier do&#;s and don&#;ts.1 This information helps to ensure personnel safety and reliable, long-term operation of the rectifier.

Monitoring

Routine monitoring is recommended for all rectifier installations. The main purpose of monitoring is to ensure the rectifier is still operating and that a power surge hasn&#;t tripped the breaker. Some facilities require certain inspections at particular intervals. For example, natural gas and petroleum pipeline operators are required to inspect their rectifiers six times per year in intervals that don&#;t exceed 21 months. Also, company policy may dictate an even stricter inspection interval.

Monitoring is typically comprised of both a visual inspection and electrical testing. The visual inspection might include looking for physical damage of the installation/ cabinet/components, evidence of overheating, and signs of insect/rodent nests, along with recording the specifics of the rectifier unit and meter readings/tap settings. Testing often includes taking manual measurements of the rectifier output voltage and current to verify meter accuracy and structure-to-electrolyte potentials. Equipment to remotely monitor rectifiers that are difficult to access is also available; however, these devices are best used as an adjunct to on-site monitoring rather than a replacement of it.

Prior to performing visual inspection and testing, it is important to put on the appropriate personal protective equipment (PPE). As a minimum, safety glasses, leather work boots (with moisture barriers where appropriate), and leather or rubber gloves should be used. Company policy may identify additional PPE requirements.

When first approaching the rectifier, be aware of its surroundings, such as uneven footing, poisonous plants, or standing water. Use all senses to detect evidence of malfunction, including visual (e.g., scorching) and audible (e.g., crackling). Test the cabinet for the presence of AC with an approved AC detector. The old-fashioned way of determining whether the cabinet is electrified (or hot) was to brush it with the back of the hand. With the advent of the AC detector, this is no longer necessary or advisable. Knock on the cabinet to notify any inhabitants (wasps, mice, spiders, and even snakes) that you are coming in. Be sure to have insect spray handy.

Maintenance

The major causes of rectifier failure are neglect, age, and lightning. Before performing any troubleshooting of a non-functioning rectifier, be sure to turn it off, both at the circuit breaker and at the panel disconnect. The most common rectifier problems include faulty meters, loose terminals, blown fuses, open structure/groundbed leads, and lightning damage (even where lightning arresters are present). The purpose of troubleshooting is to systematically isolate the rectifier components until the defective part is found, and following the rectifier manufacturer&#;s recommendations for maintenance and troubleshooting is recommended.

Test the circuit breaker, transformer, rectifier stack, meters, fuses, choke, capacitors, and lightning arrestors separately. Keep an eye out for loose connections, signs of arcing, and strange odors. Additional testing may be required to verify the integrity of the structure and groundbed lead wires.

Table 2 contains a troubleshooting chart2 designed to help diagnose rectifier problems quickly.

Common Scenarios and Tricks of the Trade

Often a rectifier is found with voltage output and no current output. Since output voltage suggests that the rectification circuits are intact, one or both of the output cables might be broken, or the anode groundbed might be completely depleted. To begin troubleshooting, identify a suitable temporary ground that is electrically isolated, such as a culvert, fence, power pole guy wire anchor, or street sign. Turn the rectifier off, then disconnect the structure lead wire, and connect the temporary ground to the negative lug. Adjust the tap bars to one of the lowest settings, and energize the rectifier. If the rectifier now produces both volts and amps, then the structure lead wire is broken. If there are still no amps, then turn the rectifier off, return the structure lead wire to the negative lug, disconnect the anode lead wire, and connect the temporary ground to the positive lug. Energize the rectifier. If the rectifier now produces both volts and amps, then the anode lead wire is broken or the existing groundbed is depleted. If there are still no amps, then additional testing is required to evaluate the effectiveness of the structure and anode lead wires to determine if the problem involves both wires.

Another common occurrence is to find the rectifier with a blown fuse. This may be the result of a power surge and simply requires the installation of a replacement fuse. However, rectifier fuses can be quite expensive. Temporarily installing a circuit breaker across the fuse clips permits testing the rectifier&#;s operation without consuming several fuses. A typical home-style circuit breaker, appropriately sized for the application, may be used for this test. Simply attach test lead wires to each end of the circuit breaker, and attach the lead wires to each of the fuse&#;s existing mounting clips. Be sure to prevent the circuit breaker and lead wires from contacting the rectifier cabinet or any other metallic object. Energize the rectifier. If the breaker doesn&#;t trip, then simply replace the fuse. If the circuit breaker trips, then other problems exist and additional troubleshooting should be performed.

Sometimes a rectifier can be found with a tripped circuit breaker. This might be the result of a power surge and simply requires the circuit breaker to be reset. However, surges are not desirable as the rectifier might remain off for long durations. Be sure to test the effectiveness of the rectifier&#;s electrical grounding and follow the National Electric Code (NEC) guidelines. Install supplemental grounding as required. In addition, there are available surge suppressors that may be installed to help mitigate power surges. Be sure to follow the manufacturer&#;s sizing recommendations.

Rectifier housekeeping is also very important to prevent insects, rodents, and other animals from building nests. Insect and rodent nests can be dangerous inside a rectifier cabinet. Insect stings or even snake bites are definitely not desirable. However, the nests themselves can cause problems too. Apart from being a possible fire hazard, a nest can impede air flow through the rectifier cabinet and lead to overheating (and eventual failure) of the components. Ensure that insects and rodents do not move into a rectifier. Some of the ways to keep pests out are to seal all penetrations into the cabinet other than those intended for ventilation, or use a chemical pesticide to dissuade their interest in moving in. For sealing the penetrations and conduits, a duct seal or a viscous-elastic amorphous apolar polyolefin (e.g., VISCOTAQ&#;) may be used to close any of the cabinet openings. A simple and effective chemical pesticide that is ideal for use within a rectifier is a small, open cup of mothballs. They are easy to acquire and work very well.

Summary

A key to the sustainability of structures is effective CP as a means of corrosion control/mitigation. Rectifiers are great tools that help to provide effective ICCP. They require routine monitoring and, at times, minor repair. Rectifier monitoring and maintenance is a necessity, but it can be performed safely, which helps ensure reliable, long-term rectifier operation.

Acknowledgments

The author acknowledges the support of Integrated Rectifier Technologies, Inc., &#;116 Ave., Edmonton, AB, Canada, T5M 3Z6; Universal Rectifiers, Inc., Cottonwood School Rd., Rosenberg, TX ; ERICO International, Solon Rd., Solon, OH ; Amcorr Products & Services, IH 10 W. #600, San Antonio, TX ; Tim Jenkins; and Don Olson.

References

1 &#;General Rectifier Do&#;s and Don&#;ts,&#; Integrated Rectifier Technologies, Inc., http://irtrectifier.com/technical-info/rectifier-safety/ (July 15, ).

2 &#;Rectifier Troubleshooting,&#; Universal Rectifiers, Inc., http://www.universalrectifiers.com/PDF%20Files/Troubleshooting.pdf (July 15, ).

This article is based on CORROSION paper no. , presented in Dallas, Texas.

&#;Trade name.

Anyone who has sat through one of my training classes has had to chant the phrase &#;Effective Rectifier Maintenance is Good Heat Management&#; at least once. My new hire is still locked in the training room because he is trying to call my bluff &#; I told him he isn&#;t leaving unless he says it out loud. His family will call the police soon if he doesn&#;t crack. It&#;ll happen any minute now&#;

But in all seriousness, about every breakdown in electronics can be attributed to the raw power of electricity causing the mechanism that is holding it back to melt, releasing it into the wild. If we can find a way to manage that heating, we no longer have a situation where the electricity is not doing what it was designed to do. If we can keep an electrical conductor cool, it can&#;t melt, and can work forever. Over simplifying, sure, and it may even be a clunky awkward metaphor for the true science of electricity, but this simple thought experiment causes epiphanies all over the room and improves the potency of a maintenance technician&#;s tactics. So I would like to share it with you.

The state-of-the-art power supply will have three major parts: Control mechanisms, power components, and semiconductors. Each one is affected by, and can cause heat, in a different way. Knowing how to prevent unnecessary heat from generating is paramount to keeping your rectifier running.

Semiconductors

In the semiconductor game, none are created perfectly. What I mean by that is electrically perfect. There is always some form of losses through the device as it does its job manipulating the electricity for your purpose. This is where the need for cooling comes in. The smallest of these devices can have the surrounding air cool them with no worry. The largest of the devices needs water cooling on both sides and a low ambient air temperature to work at full capacity. The consequences of not respecting the cooling need? Pictured below are two semiconductors: One SCR (Figure 1) and one Diode (Figure 2). The red arrows depict where the current was so high, and the heat was immense, and the SCR began welding to the copper bus bar it was mounted to. The weld was uneven and high intensity. The blue arrows point to where the arc flash blasted onto the surrounding device. Really not good.

Can you guess what could have caused this? The maintenance technician mounted the SCRs unevenly, which caused excess heat to build on one side. Once the SCR failed to turn back off during the AC cycle, the current it &#;let out&#; tripled and caused a catastrophic weld to occur. Even distribution of pressure means even distribution of heat.

Figure 1: SCR damage due to uneven distribution of heat.
Photo Credit: All photos courtesy of Dynapower

Figure 2 shows a black arrow pointing to a zone of pitting on the Diode. This area began welding onto the copper heatsink and the SCR was being depleted. The white zone on the picture indicates where the copper was welding onto the diode, and the copper was being depleted. Bizarre, right? What could have caused this slow weld? As a hint, the unit was fully operational when this was pulled off. Or rather, it was producing good product and all SCRs and Diodes were working &#; just until the machine overheated and turned off, then once it cooled down, it was able to resume operation. 

The answer: The maintenance tech used improper mounting paste. The maintenance tech used heat sink paste that is thermally conductive, but does not pass electricity very well. Once the paste set and dried, there was a layer of resistance built between the conducting surfaces, and electricity began punching small welds through to make the contact more efficient. This further heated the entire device, gradually causing system overtemp faults.

Figure 2: Pitting on diode due to overheating.

Because the semiconductors do a majority of your work in the rectifier, you really need to take expert care of them. Keep them firing when they are supposed to, and keep them off at all other times. Make sure they receive adequate cooling, and that they are mounted evenly with even uniform pressure. And finally, most importantly, make sure the clamping pressure is exactly right. Each SCR has an engineered clamping force and size to have it run properly. Old clamps, especially those exposed to the metal finishing environment for too long, will lose tolerance and fall out of calibration. When the clamps are full of chemical and corroded, replace the clamps. You will be glad you did.

For more information, please visit rectifier preventive maintenance.

Power Components

Transformers, capacitors, and resistors can be some of the most interesting and scarily spectacular things to see fail. Hopefully, if you ever witness it, you are a safe distance away, already wearing sunglasses, and you don&#;t own the expensive asset that just went poof! We have had a rash of transformers and filters go bad in the last few years, mainly due to power fluctuations in the incoming power, and maintenance guys not knowing how to prevent excess heat being the other. These components need to be given a highly disciplined cleaning regiment. Just like the SCRs you should be focusing on making sure these stay cooled adequately, and you prioritize keeping these dry, clean, and free from foreign objects or contamination. Transformers make up a large portion of purchasing a new rectifier, and certainly if you are repairing one.

Rectifier Repair

Read quick tips for improving the efficiency of rectifier maintenance.

Too much heat on the windings can overcure the varnish and make it lose all of its insulating processes. Wet environments provide a path for transformer voltage leak to ground or phase to phase, bad news.

Dirty environments where that dirt, plating chemistry, and chip bags (this never happens, right?) are all stuffed into the transformer windings and need to be cleared. This can restrict airflow and cause the transformer to overheat, let alone be an arc path for electricity to go where it wants to. We want to make sure the transformer is not exposed to these bad things, or it can permanently compromise the integrity of the machine, causing you to need replacement years before its natural design life.

Figure 3 shows a transformer that witnessed a high-power fluctuation on the incoming side of the rectifier. Notice the carbon gets everywhere, and the replacement option may be the only option &#; repair may not be possible. In an arc flash, the carbon gets into the small, microscopic pores of whatever material it slams into (arc flashes can generate as much force as a 50 mph car crash) and permanently change the electrical properties of insulators the smoke hits. This is dangerous because something you previously depended on to hold electricity in, now is a probable path for the electricity to escape through. This means we need to be removing all the carbon and smoke from the material mechanically, which in turn means moving surface material from whatever the smoke hit. It&#;s a very destructive process. In arc flashes as large as these, this means enough material is compromised that the functional design on the windings, insulation and core of the transformer would be altered enough that they would no longer work as designed. Total loss.

Figure 3: This transformer witnessed a high-power fluctuation on the incoming side of the rectifier.

So how do we protect these transformers, capacitors, and relays? The major components that handle and transfer the power from base AC power at the wall to useable voltage at our tanks? We need to protect them from overheating! If we fire a semiconductor at the right time, only as designed, and into a load (parts at the tank) that is suitable for the rectifier, we do not overdrive those circuits. Over exerting those circuits creates massive amounts of heat. That heat is generated faster than the air or water (less dense materials) can transfer and remove the heat. And the component that is overheating has the possibility to melt even before a thermal circuit can register the heat.

The solution? Make sure your semiconductors and firing boards are well maintained and babied. They are the weakest link in the process and control how the power moves through the rest of the power components in the rectifier. Without them, the machine wouldn&#;t work. It&#;s similar to how without a human, an old school car wouldn&#;t drive (think , not now with self-driving cars). But similarly, without a human, an old school car wouldn&#;t crash into a tree either. The regulator is the brain of the human, and the semiconductors are the hands and feet making the car move, working the controls and allowing the car to drive.

Oh, and obviously, don&#;t let your cooling loop or air channel get obstructed and stop heat removal from the process. It still amazes me how many times I need to remind people of that &#; the damage is permanent and the electrical devices change under that extreme heat, to where they never go back to working the same way again. Too many overheats? You have a really decorative, expensive, boat anchor.

Firing Boards

Firing boards are the brain of the operation. They are the weakest, least robust part, especially now that the circuit boards are using surface mount components and size has overall shrunk in the market over the years. Conformal coatings have changed to be more environmentally friendly and easier to apply, this can be challenging for a metal finishing environment because they can be more prone to dissolve in the presence of strong acids and bases. The one trick I like to teach my technicians is to use a blacklight. This picks up on smoke and residue that our eyes cannot see in dimly lit environments, and can show a density shift in conformal coating often caused by heat. This can be a game changer for predictive analysis.

If you are lucky enough to see the heat on the boards before something fails, you can save yourself a lot of money. Over the years the circuit boards give off substantial amounts of heat at power components like resistors and caps. This is as designed, but over seven to 10 years, can leave a pattern. Change the boards before you see discoloration affect components or the PCB fiberglass itself. Is the damage in Figure 4 due to a fault or prolonged use? 

The answer is that the circuit board damage here can be from both an event, or from prolonged use. You need to be careful when making root cause determinations because initial results can be misleading.

Figure 4: Heat damage can occur from either an event or prolonged use.

The final indicator is seeing heat rapidly develop where it wasn&#;t there before. This can occur when a semiconductor or transformer is performing at a lesser state than designed, and this puts &#;pressure&#; on other parts of the system. Then, we notice the circuit board or small component damage, change the symptom, but not the root cause of the problem. The more experience you have learning these relationships, the better and easier time you will have in making a long-term fix that &#;sticks.&#; Figure 5 shows a circuit board small resistor that burns in the presence of a larger component failure in the unit. What do you think happened here? 

This resistor on the circuit board burned due to too much current being run through it. This happened when a component on the back side of the machine experienced a short but continued to run in the circuit. The rectifier didn&#;t shut itself off as the component wasn&#;t a catastrophic failure&#;yet.

Figure 5: Damaged circuit board resistor due to too much current running through it.

In thinking about the long-term care of your critical DC power equipment, heat management is the number one root cause of failure. The bigger the amperage delivered, the more important heat management is to your process. Beyond the obvious of keeping the cooling channels free and clear, and making sure you are using the correct temperature cooling water/air across the cooling surfaces in the rectifier, small details like how components are mounted or tightened down can play a large impact to your success. Understanding the warning signs all around the machine and preventing catastrophic heat generation can save you thousands over the life cycle of your equipment. Being familiar with the warning signs can improve performance and prevent you from ultimately destroying your equipment.

 

Are you interested in learning more about huawei etp? Contact us today to secure an expert consultation!