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MANUAL RECTIFIERS FOR CATHODIC PROTECTION

AUTOMATIC RECTIFIERS FOR CATHODIC PROTECTION

 WITH STABILIZED OUTPUT SERIES RPCC

From 30 To 100 Vcc, Of 10 Amp To 100 Amp

TO VOLTAGE, TO CURRENT OR CONSTANT POTENTIAL

The Cathodic Protection equipments of series RPCC are specially designed to provide protection to the exposed metallic structures to possible corrosion.

All metallic structure as gas lines, pipeline, aqueducts, bridges, wharves, antenna towers, etc, are subject to the corrosive action of the elements that surrounds them as air, sea water or soil.

This condition is determined by the presence of corrosion currents throughout the metallic structure which behaves like an anode, producing a loss of material in points where the current passes by the structure to the environment that surrounds it. 

The best way to protect a structure is to vary its potential in regards to the elements that surround it making it cathodic, in this way we eliminate the flow of the damaging corrosion current.

The practical way  to carry this out, is by means of a source of CC connected between the metallic structure and an electrode embedded in the element that surrounds it, introducing in this way a current until we obtain a potential so that the structure is left protected.

Due to that corrosion currents vary in accordance to the magnitude in conductivity of the elements that surround the structure, the protecting currents will also be variable.

From this necessity, ADI has developed a variety of cathode protection equipments based on a rectifier of CC controlled by thiristors in such a way that enable it to modify the electrical current introduced in the structure in a totally automatic manner, protecting it according to the variations in the corrosion current.

The rectifying equipment continuously compares the real value of the variable to control (output voltage, output tension or potential on the structure to protect), with one adjustable desired value of this variable, modifying the output voltage of the rectifying bridge until both values, real and the desired are balanced.

In this manner, the operator for example, can adjust the output voltage of the equipment in a value that allows this structure to remain protected in a totally automatic way, the rectifier will independently maintain that value of constant output voltage in variations of the conductivity of the terrain or the supply voltage and frequency of the equipment.

ADI rectifying equipment base their operation on the control phase of the thiristors that compose the rectifying bridge, for which it is necessary to modify the conduction angle of those thiristors.

This is obtained by means of digital control which have three proportional controllers: one for the control of the output voltage,  a second controller for the total output current of the rectifier, and a third controller for  the potential of the structure to protect.

The output of any of these three controllers is applied in cascade to one last controller of type proportional + integral (P + I), whose value of output corresponds to the angle of conduction of the thiristors.

This digital control system is implemented by means of employing a microprocessor and associated digital analogical circuits.  

 THE USE OF A DIGITAL CONTROL  SYSTEM HAS THE FOLLOWING ADVANTAGES:

• Divides the conduction angle of the thiristors in 11000 parts, assuring the stability in output voltage in + / - 1 %, considering variations in load of 100 %.

• Limits the total output current to the structure, maintaining the equipment within the safe zone of operation.

• Selects different operation modes, whose characteristics are stored in the memory of the control system (EPROM).

• Modifies the different values of adjustment of the rectifier (output voltage, maximum output current, desired potential, desired output current, etc) and the control system constants without the need to stop its operation, visualizing them in the front panel display.

• Uses a digital display to enable it  to make up to 10 measurements in sequential form such as output voltage, the output current, structure potential, etc.

• Accomplishes the remote supervision of one or several equipments, through a serial communications port type RS-232, that allows to send by means of a modem through a communication  system (telephony, radio, satellite, etc), to a PC, the values of different measurements that the equipment generates, the different states of operation and the status of alarms.

The use of this technique allows to conceive an equipment of excellent benefits and high reliability for its use in heavy industry, and in places of difficult access.

GENERAL DESCRIPTION  

The cathodic protection rectifying equipment of series RPCC are basically composed of a power transformer whose output supplies a complete oscillation rectifying bridge with three phase or mono phase according to what corresponds.

When exiting the rectifying bridge there is an inducer filter to reduce the ripple effect of the output voltage.

At the front of the equipment is a Control Panel in where are located the digital displays, which allow to make measurements of the different variables of the equipment, a set of buttons for rectifier operation and a group of LEDS that signal alarms and state of operation.

All the electronic components of the control system are mounted in a curved-slide tray to allow an easy access to these components.

The ADI Rectifying equipment series RPCC are manufactured in standard form for mono phase input voltages of 220 V - 50 Hertz and three phase of 3 x 380 V - 50 Hertz.

All the equipment components are mounted on a metallic frame, which fit within a metallic cabinet apt for inclemency, in this way, in case of being necessary to remove the equipment to make maintenance, it is only necessary to remove this frame.

In conformity with the type of structure to protect and the environment that surrounds the equipment, they have three ways of operation: to Constant Voltage, to Constant Current or Constant Potential.

CONNECTION SCHEME OF AN AUTOMATIC CATHODIC PROTECTION EQUIPMENT 

Constant Voltage Output Rectifiers

These equipment provide voltage with a constant output, being independent from the electrical variations of voltage and frequency output of the supply line.

Its most frequent use is in zones where the conductivity of the surroundings of the structure remain relatively constant throughout the year, in such a way that the corrosion currents also stay relatively constant.

Therefore applying to a stabilized voltage on the structure, it will remain protected as long as the conductivity does not change too much.

Constant Electrical Output Rectifiers

These equipments provide a constant electrical output independent from variations in conductivity of the surroundings of the structure to protect and the voltage / frequency of the supply line.

Its most frequent use is in zones where the structure to protect is near other metallic structures with their corresponding protection rectifiers, interacting between themselves.

This produces abrupt changes in the corrosion currents due to circulation of currents between structures with different degrees of protection.

Once the equipment is installed and the required voltage output is set to protect the structure, the rectifier will maintain this constant current independent from variations in conductivity of surroundings or abrupt changes in currents that circulate around the structure to protect due to other near metallic structures.

Constant Potential Rectifiers

When the environment that surrounds the structure to protect change its conductivity throughout the year in gradual form due, for example, to the elevation of the water tables due to seasonal rains and this structure is not put under abrupt changes in currents originating from other metallic structures, rectifiers are used whose function is to maintain a constant potential of protection between the structure and the environment that surrounds it within a certain value.

This model of rectifying equipment measures by means of a sensor (for example an SO₄Cu sensor), the structures potential in regards to the soil and modifies the output voltage of the equipment until the value of the measured potential is balanced with a value of required potential (previously adjusted).

TWO WAY OPERATION EQUIPMENT

Since between the three ways of operation of the equipment: to constant voltage, to constant current and to constant potential, the difference is only in the control system and this has been stored in the EPROM memory of the control board, it is possible to implement in a single equipment two ways of operation, for example to constant current and constant potential, modifying the program memory and being able to select any of the types of operation by means of a key in the Front Panel of the equipment.

REMOTE  SUPERVISION

Due to that this equipments generally protect structures in hard to reach places, for example pipelines, power towers, etc. the ADI  RPCC series rectifiers are provided with the possibility of a working status monitor using the serial connector type RS232 located on the control board.

Through this connection we can access data as voltage and current output, measured potentials, alarms, etc.

This information can be sent through a modem and transmission equipment to the receiving center for its processing.

Optionally the retrieval of information sent by each equipment can be done in a quick way from a vehicle as a truck or a small plane that recovers this information as it passes by the structures without having to go to each one individually.