18 TRANSFORMERS FOR ELECTROSTATIC PRECIPITATORS

 TRANSFORMERS FOR ELECTROSTATIC PRECIPITATORS

Electrostatic precipitators are used to remove fi ne dust particles from gases,

the most common example being the removal of minute particles 

of  pulverised fuel ash from power station flue gases.

To do this the flue gas is passed horizontally through a strong electric 

field established between negatively charged wire mesh grids suspended 

centrally between vertical sets of positively charged metal plates. 

Plate spacing may be around 300 mm and the mesh opening about 150 mm. 

The positive plates are usually connected solidly to earth. 

The corona discharge created by the electric fi eld causes the ash particles to 

become negatively charged as they enter the precipitator so that they 

migrate towards and are collected by the positively charged plates. 

These positive electrodes are periodically given a mechanical rap 

causing the collected particles to fall under gravity into hoppers placed
beneath the electrode array.

Up to three precipitation stages in series are commonly employed and in a

good precipitator collection effi ciency can be better than 99.9 per cent.

To establish the necessary fi eld for the electrode spacing identifi ed above,

a voltage of around 60 kV DC would be necessary between mesh and plates.

This is derived from a small transformer-rectifier unit designed specifically for this purpose. 

To obtain maximum extraction efficiency, the maximum possible

voltage consistent with avoiding continuous fl ashover must be applied 

to the electrodes and this is generally achieved by means of an automatic 

voltage control system which gradually increases the LV supply to the 

transformer rectifier until fl ashover is detected. 

When this occurs the control system winds back the input voltage to extinguish the arc and then repeats the process once more.

Precipitator transformers are single phase and produce an output voltage

which is rectified and connected via a length of cable to the electrode array.

The primary supply is usually taken from two phases of a 415 V three-phase   system via a voltage regulator giving a 0–415 V output. 

This enables the HV output voltage to be continuously varied from 0 V up to the maximum rated value. 

Current into the load is normally about 1 A maximum so the transformer rating is no more than 50–60 kVA. 

The unusual feature of the transformer
is that the load presented by the 

electrode array plus rectifier is capacitative so that the transformer operates at near to zero power factor 

lead and thus experiences negative regulation. 

To provide a terminal voltage of about 60 kV at full load requires a transformer open-circuit voltage of about 55 kV.

Because the normal operating mode of the electrostatic precipitator involves

frequent short-circuiting due to electrode fl ashover, it is desirable that 

the supply system should have a high impedance in order to limit the magnitude of the short-circuit current. 

Small transformers with ratings of the order of a few tens of kVA, will however, normally have very low 

impedances, probably no more than 3 or 4 per cent, and to raise this to the 

order of magnitude required, around 50 per cent on rating, can be somewhat uneconomic.

One way of doing this is to use a form of ‘sandwich’ construction, similar to

that used in a shell-type transformer, whereby alternate sections of LV and

HV winding are assembled axially onto the core with large axial ‘gaps’between

sections of the winding to create the required loose coupling. 

This leads to a fairly complex insulation structure in order to handle the 

relatively high voltage however, and it is probably more economic to design 

for the highest practicable value of impedance which can be obtained, say around 10–15 per cent, utilising 

conventional concentric LV/HV construction and then increase the overall supply impedance by means of a series connected external choke.

This type of ‘conventional’ construction will involve a helically wound LV using 

paper covered rectangular section copper conductor with a HV winding

consisting of crossover coils wound using enamel-covered circular cross-section wire.

The transformers are usually immersed in mineral oil in a common tank with 

the rectifier and are frequently located at a high level within the precipitator 

structure in order to minimise the length of HV connection between transformer and electrodes. 

Although precipitators are not
housed in structures where fi re hazard is likely to give rise to concern, it will
be necessary to make provision for oil containment in the event of a serious
leakage.