Australian Energy Research Laboratories P/L
The AERL Cathodic Controller is the only CP control specifically designed for use with solar electric power systems. All other controls and rectifier units rely on resistance or series power transistor control. Both of those methods waste significant power heating up components rather than supplying current to the ground bed.
Very low RFI and EMI emissions.
Modes of Operation
NOTE: To obtain correct polarity when measuring the structure potential with respect to a half cell, the (+) meter connection must be made to the structure (cathode) and the (-) meter connection to the half-cell reference on the Surge Buster's buffered metering terminals.
If, when delivering the power into a Cathodic Load, the load resistance is such that a pre-selected output current limit is reached before the pre-selected output voltage limit or structure/reference potential is reached, then the Cathodic Converter is operating in Constant Output Current Mode 1.
If the load circuit resistance is such that the pre-selected output voltage limit is reached first, the unit is operating in Constant Voltage Mode 2. This would typically occur if a ground bed dried out, causing a significant resistance increase. The constant output voltage limit (VLIMIT) is generally set to limit the total power flow (for a given constant current) to the Cathodic Load in circumstances where the circuit (ground bed) resistance may rise higher than the expected average due to reductions in the local ground bed moisture content.
If used in the Constant Structure Reference Potential Mode 3 with a permanently connected reference electrode, then the output voltage and current limiting levels are generally set some margin above those output voltage and current levels typically necessary to maintain the desired pre-selected constant structure reference potential level. In the event of non-typical conditions, these other limits may temporarily over-ride the constant potential limit. In effect, the action of the 3 modes of operation is an "OR" function, where Mode 1 "OR" Mode 2 "OR" Mode 3 can be the controlling limit for a given ground-bed condition. The Cathodic Converter will automatically move from one mode to another mode depending on the position of the adjustable set-point levels and the prevailing environmental conditions.
Disabling the Constant Potential Mode: On the units fitted with the Constant Potential option (reference cell), the Constant Structure Potential mode can be disabled by linking the "CATH RETURN" & "REF.CELL" input terminals on the Surge Buster, and by bridging the "REF.CELL" input terminals 7 & 8 on the Cathodic Converter. Alternatively, the two wire connection between 7 & 8 on the Surge Buster and 7 & 8 on the main board can be disconnected and 7 & 8 on the main board linked.
Generally, the Cathodic Converter is set to operate in the constant current mode, impressing a fixed current into the metal structure to be protected. The voltage adjusts automatically as ground resistance fluctuates with changing environmental conditions. This impressed constant current creates a reduced structure potential with respect to the surrounding medium that opposes (cancels out) the natural electro-chemical potential that would otherwise lead to a net loss of electrons from the structure to the surrounding medium and consequent oxidation of the metal structure.
A Copper/Copper Sulfate Half cell is generally used to perform a potential survey in the immediate area around (or along) the metal structure to be protected by the Cathodic Converter. This provides a means of determining (locally) the magnitude of the natural electro- chemical potential that must be canceled out by impressing current from the Cathodic Converter. Sufficient current must be injected into the metal structure (cathode) from the surrounding earth medium via an anode to achieve an errorless ("instantaneous off") metal structure potential measured with respect to the Cu/CuSO4 half cell of typically between -850mV and -950mV. These levels correspond to 000mV and -l00mV absolute corrosion potential levels respectively for the structure. The more (absolute) negative the structure potential is driven with respect to the surrounding medium, the higher the corrosion inhibiting action. Other considerations such as hydrogen liberation and consequent metal embrittlement limit just how negative this potential can safely be in a practical application. If instead of a Copper/Copper Sulfate reference half cell, a pure zinc electrode reference is used, the correct Structure Potential protection level is typically + 250rnV to + 150mV with respect to the zinc electrode. Again corresponding to absolute structure corrosion potential levels of 000mV and -l00mV respectively.
THE IxR STRUCTURE/REFERENCE "ON" POTENTIAL ERROR AND RESULTING UNDER-PROTECTION
To measure a structure potential level that accurately reflects the actual protection potential impressed upon the metal structure, it is necessary to avoid the IxR "on" potential error between the structure and the reference cell due to the impressed current flow from the (buried) anode to the structure being protected. This IxR error raises the potential of the reference cell and thus appears to add an extra negative potential to the metal structure potential with respect to this reference cell. If not eliminated from the measurement, using high Speed Output Current Interruption measurement methods, it may result in an "under-protected" current setting and the consequent risk of continued corrosion activity of an unacceptable level. - For example; if the measured "on potential" was 850mV, the "off potential" might actually be only 750mV, caused by a +l00mV IxR voltage error in the ground medium between the half cell and the structure when the current is "on", resulting in slight "under- protection" of the structure.
MINIMIZING THE IxR ERROR EFFECT
The closer the reference cell is placed to the metal structure (with respect to the anode current injection point), the lower will be the fraction of total resistance contained between the reference cell and the structure, and so the smaller will be the component of voltage error due to the current flow from the anode to the structure. To this end, if the reference cell is positioned directly above the structure, as close as possible but still in the surrounding medium, the IxR "on" potential error can be minimized or even reduced to a negligible level.
ELIMINATING THE IxR ERROR EFFECT
All electrical systems polarize themselves into plus (+) and minus (-) charge levels in the process of establishing electrical/chemical equilibrium. If no artificial cathodic protection is applied to the structure, this "polarization" potential is simply the background corrosion potential that is stripping the structure of electrons and causing it to corrode. It can be measured simply by placing a digital voltmeter between the structure and a suitable reference cell and subtracting the reference cell voltage from the reading. (For example: With DVM negative connected to half cell reference and positive to the structure take a reading. If the reading is -550mV for example and the half-cell is Cu/CuS04 then the polarization or corrosion potential is -550 minus -850 which equals +300mV absolute. It is this +300mV that must he driven to 000mV (absolute) or slightly lower (-100mV) to stop the loss of electrons and the consequent corrosion of the structure. when cathodic protection is first applied to a structure it has to be discharged to its new "safe"" level or simply re-polarized down to an artificial level where corrosion ceases. Depending on the structure, this may take seconds, minutes, days or weeks. It is this new polarization potential that must be measured using high-speed current interruption methods. Because the structure being protected takes a finite time to re-polarize or re-charge back to the natural background level after the cathodic protection current is switched off, it is possible to measure the true structure potential without the erroneous effects of the IxR voltage error from the Cathodic Converter. This true structure potential is termed the "instantaneous off potential", as it is the polarization potential of the structure the instant the cathodic protection current falls to zero, but before the structure begins to recharge back to the natural background (pre- protection) level. It usually measured within milliseconds of the protection current reaching zero.
CURRENT INTERRUPTION FUNCTION
All Cathodic Converters have a high speed output shutdown control function that permits the output current to be readily reduced to zero in typically less than 1 to 2 milliseconds, permitting instantaneous "off" potentials to be measured. This precise, clean shutdown function can be operated manually simply by linking the INTERRUPT (5) & GROUND (6) terminals on the main board. Alternatively, if a Surge Buster is being used, apply 8 to 30 volts DC to the terminals as marked on the Surge Buster board. At a nominal 12 volt level, the circuit will draw 5 ma during interruption. This control point is optically isolated from the Cathodic Converter, with a minimum of 5 kV of isolation. The output current shutdown is achieved internally by switching off the output power transistors within the Cathodic Converter. Linking the INRPT (5) & GND (6) terminals involves less than .5 ma of current flow at 5 volts with respect to the negative input of the Cathodic Converter and can thus be performed with a small switch or relay. Any externally timed automated current interruption sequences can be performed via these control terminals on all Cathodic Converters. (E.g. for synchronous interruption of several units in the same area.) Multiple unit interruption should be performed with isolated switches, or simply via the isolated opt-couple buffer on the Surge Buster. (See Page 8). Always make use of this optical isolation facility when using a permanently connected interrupt sequencer. The isolation provided will protect both the Cathodic Converter and the interrupt sequencer in the event of a high voltage surge on the pipeline or structure.
Parallel Operation for Larger Output Currents: If large structures, such as vertical oil and gas well casements require cathodic protection, then output currents greater than 28 Amps (the maximum current for a single CP control) may be required. In these cases more than one Cathodic Converter can be used. If this method is required, the arrays and battery systems used MUST be isolated from each other electrically. Essentially, this is the same as if two or more completely separate systems were used in tandem. This method has the advantage of providing a certain amount of "fail safe", so that even if one system goes down you will still have some protection from the other unit.
I. Lightning Protection All Cathodic Converters have 4.5kA MOV surge arrestor protection on-board to protect all inputs and outputs against moderate high voltage surges that may occur in the course of normal use, such as thunderstorms.
For all units used in pipeline and high structure applications, the "Surge Buster" lightning protection unit must be used to maintain the warranty. The 50KA/500 Joule common and differential mode protection board will stop almost all pipeline induced surges. It also provides protection for the reference cell inputs and 5kV opto-isolation for the ON/OFF interrupter.