Earlier, We had discussed about the theory behind the use of Impressed current cathodic protection. To refer the aforesaid post, visit http://electricalfundablog.com/blog/?p=731 . Lets discuss how the operation of Impressed current cathodic protection is carried out practically with equipments.
The power to the unit is fed from the 440V AC, 60 Hz, 3 phase supply. However, it can be modified to 220 V also (as per requirement). This input AC is fed through three transductors to the input of the step down transformer which has a star connected primary and a delta connected low voltage secondary. The secondary feeds through Control Transformer, CTs to a three phase full wave bridge rectifier assembly (the DC output which is available upto 100 Amps at 8V). The transductors are controlled with a small direct current in their control windings. Control winding controls the drop across the transductors and hence the AC input to the mains transformers. The current in the control winding is in turn controlled from a transistorized pre amplifier. The pre amplifier compares the zinc reference voltage with a built in zener stabilized voltage and controls the transductor drop in such a way that a change of 20 mV, in reference electrode voltage, changes the impressed current from 0 to 100 Amps.
The transductors (TD 1 to TD 3) are parallel fed. Each of these saturable core reactors are enclosed in a sheet metal casing and are introduced in series with each phase of the input supply. When saturated they allow the full DC output to be obtained, and when they are not in saturation, they limit the DC output to less than 5% of the full rated output. Each transductor has three windings :
- One control winding.
- One bias winding.
- A standby winding.
The main rectifier which supplies DC voltage to the anodes is a single unit comprising of 6 silicon diodes mounted on 6 separate heat sinks and interconnected to give full wave 3 phase rectification. The 3 phase full wave rectification has ripple less than needed for the nature of application and hence further smoothing is unnecessary. The Transistorized pre amplifier – The preamplifier has mainly two parts:
- The stabilized power supplies
- The DC pre amplifier.
The stabilized power supply is conventional series regulated power supply with one zener diode of 6V as reference. The –10V supply is used as a reference for the +10V supply. Hence the –10V supply should be adjusted before adjusting the +10V supply. These two supplies have a current limit to protect them from surges during the switching ON of the unit. The unregulated supply to the stabilizers is derived by rectifying 14-0-14V secondaries of the transformers. The circuit uses only silicon semiconductors and hence is extremely reliable.
When the Impressed current cathodic protection control mode is automatic, control current flowing through the control winding of the transductors is set automatically to keep zinc reference electrode voltage at the preset internal reference. This is achieved automatically by sending the difference in preset potential and the reference electrode potential and sending a proportional current through the control winding.
The differential DC amplifier accomplishes this. The input to the differential amplifier are – the reference signal, preset reference signal and the feedback signal. The reference signal comes from the reference electrode, the preset reference is derived from the stabilized supply of –10V (this is adjustable from 0 to –500mV with the help of a potentiometer) and the feedback signal proportional to the output current is derived by sensing the current flowing through the lines connecting the secondary of the transformer to the rectifier stack. The feedback is added to the preset signal and fed as the second input of the differential amplifier.
The error signal needed to vary the current through the control winding being small the input to output current law is virtually linear irrespective of the load variations. The protection to the unit is provided by the current limit circuit provided in the feedback path. The transistor in the protection circuit does not conduct so long as the output current does not exceed 100Amps at the maximum rated load resistance. As soon as the output exceeds this limit the transistor conducts and transfers max feedback signal to the input. Hence the limiting action of the output current takes place. The current limit operates whenever the output current exceeds the rated limit current.