The Model 3015 Pulse Rectifier by Farwest Corrosion Control Company is a “new generation” cathodic protection power unit designed to protect up to four structures using one common ground bed. This feature is very attractive where multiple structures., i.e., well casings or pipelines, are in the same vicinity (within 300 meters of each other) and can all be protected from a common ground bed / power supply installation. U.S. Patent 6,224,742 B1, Canadian Patent Pending.
Components
Enclosure
The enclosure is an 11-gauge cold rolled steel welded unit. After fabrication, the components are hot-dip galvanized. An optional paint or thermally fused powder coating is available. The enclosure is divided into three compartments: 1) AC input section, 2) Main (Oil) compartment containing the Control & Pulse Modules, and 3) Control / Power Distribution Section. All compartments are gasketed to eliminate dirt and moisture intrusion. For extreme high temperature operating conditions, the unit is provided with sun shields on top and two sides.
AC Input Section
The AC supply to the Pulse Rectifier enters the AC input compartment. This compartment contains the AC circuit breaker and a state of the art RMI / EMI AC interference filter.
Main Power Transformer
The main power transformer is a single-phase toroidal transformer with four independent (isolated) secondaries (one for each individual pulse module). The transformers are Class B rated and must pass a 4 kV dielectric test.
Control Module
The control module delivers pulse information to each individual pulse module. The frequency and the pulse characteristics of the output power are adjustable at the control module
Pulse Modules
The pulse modules supply DC pulse current to the power distribution panel. Using current level information received at the Current Control Panel, the Pulse Module produces high voltage DC pulse current to the structure. Each pulse module is infinitely adjustable from 0 to 15 A by turning one knob. Once adjusted, the internal (closed loop) current monitoring circuit will automatically adjust the pulse output level to maintain a “constant current.” This feature will compensate for changes in apparent circuit resistance.
Control Panel
The Control Panel provides a means for controlling and monitoring the average current to the four pulse output circuits. The four Current Output Meters indicate the average DC pulse current flowing to each of the four structures. The DC pulse current can also be measured at the Pulse Current Output Pin Jacks. Each pair of jacks are connected to a .001 ohm shunt in series with the individual pulse module circuit. This allows for cross checking the accuracy of the panel meters. The four Pulse Current Output Adjustments provide a means for adjusting the amount of DC pulse current flowing to each structure. The Pulse Train Switch is used to interrupt the DC pulse current flowing to all four structures.
Power Distribution Panel
The Power Distribution Panel is located behind the Control Panel and provides a means for delivering the DC pulse current to the individual structures to be protected. The four Structure Connectors permit connecting to four individual structures. The Anode Connection Lug permits connecting to a common ground bed. The Meter Shunts provide a means for measuring the average DC pulse current flowing to each structure. The four Power Distribution Tap Links permit connecting the negative output of two or more Pulse Modules to a common Structure Connector. This feature allows for two or more Pulse modules to provide current to a common structure.
Serviceability Features
All normal monitoring is performed by opening one side door. The main oil chamber is opened only during initial setup and if trouble shooting is required.
The system is designed for easy maintenance. The Control Module, Pulse Modules and main transformer are “rack mounted” and can be individually removed.
The system is provided with a pulse interrupt circuit that will function with any portable current interrupter.
The system is fitted with temperature monitoring devices that monitor both the individual pulse module operating temperature as well as the oil temperature. If in the event that the operating temperature should exceed a maximum safe temperature (typically 80°C / 176°F), the pulse module output will be interrupted until the temperature reduces to a safe level at which time system will automatically restart and return to the previous operating parameters.
If a structure has an unusually high current requirement and additional pulse module positions are available, two or more pulse modules can be connected in parallel to provide additional current
Multiple pulse rectifiers may be installed on a common ground bed. Two Pulse Rectifiers, with up to six (6) pulse modules, can be connected to a common anode ground bed.
Installed Model 3015 Pulse Rectifier without sunshield.
Theory of Operation
The Pulse Rectifier (Pulser) produces very short duration, very high voltage, direct current electrical pulses for cathodically protecting buried metallic structures, such as pipelines and well casings, from external corrosion.
Pipelines, well casings, and other steel structures buried in soil, corrode. Cathodic Protection has been used to mitigate corrosion for many years. Sufficient direct electrical current (DC) is caused to flow from the soil to the surface of the structure. Typically, the negative lead from a DC rectifier is connected to the structure to be protected, and the positive lead to an anode bed. The ground bed usually consists of one or more rods, called anodes, fabricated from expendable materials such as graphite or silicon steel. The anodes are buried in deep wells, or shallow holes or trenches, in the vicinity of the structure to be protected. The process is called Cathodic Protection, because protective cathodic reactions are caused to take place on the surface of the structure being protected.
The DC current mitigates corrosion by electrochemically reducing any corrosive oxygen molecules and hydrogen ions that may be adsorbed on the surface of the structure in contact with the soil. As additional molecules and ions diffuse from the soil and become adsorbed onto the steel surface, they are also reduced by the continuously flowing direct current. There are certain drawbacks to conventional cathodic protection, however. Current flows in the ground continuously in a relatively large three dimensional envelope. If there are other foreign metallic structures buried in the soil within the current envelope, damage in terms of metal loss will very likely occur to the foreign structures. Also, the "throw", or distance down the pipeline or well casing, that can be protected, is limited by the magnitude of the current reaching the more distant corroding surfaces.
The corrosive oxygen molecules and hydrogen ions, adsorbed on the surface of the buried structure, are electrochemically reduced within microseconds after the application of cathodic current. Before the corrosion reactions can begin again, additional molecules and ions must diffuse to the surface of the structure from the surrounding soil and become adsorbed. The diffusion and adsorption processes are orders of magnitude slower than the reduction process, particularly when the ions are not being driven by a continuously applied cathodic protection voltage.
With Pulse Cathodic Protection, the corrosive molecules and ions are electro- chemically reduced with very short duration but very high current pulses. Since throw is a function of current magnitude, the very high pulsed currents result in greatly improved throw down the length of the structure. Because of the natural electrical characteristics of the structure/anode system, the greatly improved high current throw is further enhanced by the inductive and capacitive redistribution of that current.
Also, the very short duration pulses are separated by relatively long "off" intervals. The pulse voltage is applied for less than 20% of the total time. Thus, the continuous current envelope and the resulting interference on foreign structures is greatly reduced.
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