Project Description
After becoming the Wolf Creek Cathodic Protection (CP) System Engineer, I realized there was major work that needed accomplished on the system. The standard for CP systems is to take Instant-OFF readings at test stations. Instant-OFF, also known as interrupted or polarized readings, are a measure of the buildup of charge on the structures being protected measured as a voltage difference between the structure and a CSE reference cell. During normal operation, rectifiers provide protective current to the structures which keeps them at a steady-state level of polarization. This current introduces error known as IR-drop, which is a potential added to the measurement due to resistance in the circuit. To remove this current, interruption relays are added to the rectifier circuit to open the circuit on a cycle. This allows a measurement to be taken at the earliest moment after the circuit is interrupted, which is the maximum polarization of the structure. Additionally, all of the rectifiers must be interrupted simultaneously as the system is electrically continuous on the structure side. The modification design is for permanent interrupter relays on all of the rectifiers. The design includes remote monitoring via Elecsys Watchdog Scout RMUs for each rectifier, which can be used to monitor the current and voltage of the rectifier and set interruption cycles. This project is still in the development phase.
Background Information
CP is a method of reducing or eliminating corrosion for buried piping and structures. Background details on how CP works is given on the Cathodic Protection CIS page. The system at Wolf Creek consists of 11 rectifiers, 10 of which are currently energized. Two of the rectifiers provide power to a remote anode bed which provides protection to 95% of the site (Figure 1). In addition to these rectifiers, eight others provide spot protection. The anodes associated with spot protection are
known as close-proximity anodes. As the entirety of the plant's buried metal piping and structures are connected to a lead-lined copper ground mat, all of these structures are electrically continuous. This indicates that any rectifier in the system is capable of protecting any metal attached to the ground mat (including the ground mat) no matter how minutely. This is a vital piece of information in regards to designing the remote monitoring system. As discussed in the Project Description, Instant-OFF readings are the desired readings to determine the protection levels of our piping and structures. Instant-OFF readings are accomplished by interrupting the rectifier DC output circuit, a circuit between rectifier, anode(s), structure, and electrolyte (Figure 2). Interrupting a single rectifier in a multi-rectifier circuit would not remove all of the protective current
Fig. 1. Rectifier internals for large anode bed rectifier responsible for much of the plant's protection.
Fig. 2. Representation of rectifier circuit.
being applied by the power sources in the system. Therefore, it is necessary to interrupt the protective current of all of the rectifiers. Additionally, polarized pipe is similar to a capacitor in the way it cannot hold charge. To measure the largest buildup of charge, and the most accurate polarization, the measurement must be taken instantly after interrupting the circuits (hence "Instant"-OFF readings). This poses a problem when interrupting the rectifier circuits. If all of the rectifiers must be interrupted at once, it is not sufficient to manually signal each rectifier to interrupt. Indeed, each millisecond counts when it comes to interruption, as the piping depolarizes exponentially when current is removed. These facts indicate that a timing mechanism
for interruption must be utilized to synchronize the interruption of the rectifiers to allow Instant-OFF measurements to be taken at a test station. This is accomplished using GPS synchronization. GPS is known for its ability to provide pinpoint locations anywhere on Earth by utilizing a system of satellites that overlap their service area at all times during orbit. The satellites utilize a precise timing mechanism to ensure this location providing capability. By utilizing an antenna that can tap into a GPS frequency band, this timing mechanism can be utilized to synchronize the interruption of the rectifiers. The antenna would connect to a Remote Monitoring Unit (RMU), which is dedicated to a rectifier, and can set an interruption cycle. When the RMUs for every rectifier are synchronized to the GPS frequency band, each will signal interruption at precisely the same time, solving the issue of multi-rectifier interruption.
RMUs being used are Watchdog Scouts by Elecsys. In addition to sending interruption signals, the RMUs can also be used to monitor vital rectifier data such as DC output voltage and current. The Scouts are also used for monitoring other inputs, such as the on/off status of the rectifier using an AC detection probe.
Change Package Development
In the nuclear industry, new designs and existing design modifications are performed via the change package process. This is a process that has been standardized across the industry to foster increased understanding and sharing across the industry. Understanding the change package process allows design modifications to be streamlined and industry OE to be easily interpreted. Cathodic Protection, however, can be a unique system for every plant, which makes interpreting OE difficult. Wolf Creek has a sister plant, Callaway, owned by Ameren in Missouri. To understand how CP is done by other plants, I spent two days at Callaway discussing the plant's CP system with the CP engineer at Callaway. I quickly learned that, while nearly identical in the nuclear reactor side of the plant, the commercial side, and especially the buried piping, was quite different than at Wolf Creek. Callaway's cathodic protection system was set up to protect the buried piping using a series of close-proximity rectifiers protecting different areas of the plant. By overlapping anode protective current, the entirety of the plant's buried piping and structures are protected within the desired range of polarization. Wolf Creek utilizes a remote anode bed, primarily, to provide protective current. Callaway contracted Corrpro to redesign and install the entire CP system, from brand new rectifier and anode installations to the RMU system. Conversely, I am designing the RMU system
for our plant. Even so, I learned that Callaway uses the Scout units as well, and are very happy with how these units work. The differences with our systems, and indeed our system with most every CP installation in the industry, however, requires customization for installing the interruption side of the system.
Having discovered the key differences between our systems, I was ready to approach the change package. The change package consists of key sections that are vital to the change and work to keep the disposition concise. These sections include a problem statement, a suggested solution (a brief overview of the change), a description of the change, descriptions of key design characteristics, and any post modification testing that will be necessary. As this change is a commercial change, various nuclear "hold points" were not included in the package.
Problem and Solution Statements
You may be wondering why we are bothering with developing such an extensive modification to the CP system. The change will likely cost over $100k and will not change protection levels of the plant, which were fine on last inspection in 2020. The reason this change is necessary is that Wolf Creek is undergoing the extensive process of renewing its license for an additional 20 years. A small part of the license renewal process is having a CP system with the capability of taking accurate readings annually. This gives us two options. Either we install an interruption system permanently to allow us to take polarized readings, or we hire a vendor to conduct a survey once a year. The second option still requires our electricians to install the interrupters each year, then uninstall them. By doing a net present value calculation on the differences between the two options, it was quickly determined that the permanent installation would be the better economic option in as little as two years. This does not include the resources saved by the monitoring part of the system, which keeps us from requiring a preventative maintenance check on the rectifiers once every two months.
Modification Description/Design Attributes
This modification to the CP system consists of several key components. These components are the Scout unit, the interruption module, and the components to support interruption, primarily to cool the relays. The Scout acts as part of the interruption module, but also as a device for monitoring rectifier attributes. These attributes include DC amperage, voltage, and AC power. Amperage and voltage are two attributes that are monitored on a regular basis. The AC power, monitored by an AC voltage probe, is used to identify if the rectifier is receiving power at the primary coils. As part of the interruption module, the Scout interrupts a 12 VDC source that powers a relay circuit. Each Scout also includes a GPS receiver for synchronizing the interruption of the rectifiers.
In addition to the Scout, the interruption module consists primarily of the interruption relays. The interruption relays are connected in either one or two circuits, depending on the rating of the rectifier. An electromechanical, mercury relay included with the Scout by the vendor is used for every rectifier. The mercury relay may be used to interrupt rectifiers' DC output circuits within the mercury relay rating. The mercury relay is normally closed, indicating that the Scout will not normally energize the relay coil. This uses less energy and removes the need to cool the mercury relay as its duty cycle is 0% during normal operation. For rectifiers beyond the mercury relay rating, the mercury relay shall act as a control relay for
Fig. 3. Picture of Scout taken at Callaway.
Fig. 4. Corrpro rectifier with Scout indication wire connections.
a second set of relays. The second set of relays are solid-state relays, which can be used in parallel to split the current of the rectifiers' output circuits. These relays are normally open; by connecting these relays in series with the mercury relay and a 12 VDC power source, the solid-state relay coils shall be energized, operating at 100% duty cycle normally. The relay contacts shall close when the coil(s) are energized, allowing the rectifier to deliver voltage to the anodes. When an interruption cycle is activated, the mercury relay coil energizes and deenergizes on a cycle, opening and closing the control relay circuit, respectively. The coils of the solid-state relays would deenergize each time the control circuit opens, opening the contacts of the solid-state relays. Unlike the mercury relay, these contacts open in a matter of microseconds, which allows them to be used in parallel without damaging them from overloading.
The components supporting interruption consist primarily of those used to cool the solid-state relays. These components include heat sinks, fans, and enclosures for the relays, as seen in the slideshow above. The mercury relay will be held in the Scout, which is no problem as it will produce negligible heat, but the solid-state relays, running constantly, require constant cooling. The heat sinks used to cool the relays directly are rated based on an amperage-temperature curve. The solid-state relays require heat sinks to operate at all temperatures and amperages due to the high energy application of the relays, but the rectifiers must be derated when they reach certain amperages and temperatures. While the heat sinks pull heat away from the relays directly, they are not the limiting factor in cooling the relays. The relays' enclosure must have the capability of dissipating the heat the relays release. An improperly sized enclosure can cause the enclosure environment to become much hotter than ambient. Radiation is the primary heat transfer mode while the fans are turned off, which is the normal state of the fans. Enclosure surface area drives the heat dissipation, indicating the size of the enclosure is the critical characteristic of the enclosure. Using thermostat relays, the fans will kick on when the enclosure environment becomes hot enough, which could happen in the hottest part of the year. One fan will energize when hitting the first temperature setpoint. A second fan will energize upon reaching the second temperature setpoint. The Scout will also be used to monitor the on/off status of the fans using two dry contact channels. The fans operate at 120 VAC, but monitoring the fans using dry contacts must be done under 50 VAC. The Scout also operates at low AC voltage, and will be stepped down to 12 VAC from 120 VAC. This 12 VAC will be used in conjunction with single pole relays to energize and deenergize the dry contact channels when the fans are energized and deenergized. The circuit for this installation is in the slideshow below along with scale drawings of the relay enclosure.
The size of the enclosure and installation circuit will change based on the rectifier. For instance, four of the ten rectifiers are rated within the mercury relay rating. This indicates the installation circuit will not include the solid-state relays and fan circuitry. The mercury relay will be installed directly into the rectifier output circuit in those installations. The enclosure size will also change based on the heat released by the solid-state relays.
Conclusions
This project is still being developed, however initial development of the design is complete and the testing phase is in progress. The above circuit and enclosure models will be implemented in a testing capacity on one of the anode bed rectifiers. If the installation proves reliable, the installation will be applied to the rest of the rectifiers. With the completion of this modification, reliable testing and surveys will be able to be accomplished at will on the CP system. This is an important step for the health of buried piping as it ages at the plant and the completion of the License Renewal process.