Industrial Power Transformers -- Special features of transformers for particular purposes [part 14]

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TRANSFORMERS FOR ELECTROSTATIC PRECIPITATORS

Electrostatic precipitators are used to remove fine dust particles from gases, the most common example being the removal of minute particles of pulverized 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 ground. The corona discharge created by the electric field 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 efficiency can be better than 99.9 percent.

To establish the necessary field for the electrode spacing identified 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 flashover 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 flashover 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 1A 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 flashover, 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 percent, and to raise this to the order of magnitude required, around 50 percent 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 percent, utilizing 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 minimize the length of HV connection between transformer and electrodes. Although precipitators are not housed in structures where fire 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.

Testing

Transformers for electrostatic precipitators are of a very specialized nature and have tended to be developed in isolation from 'mainstream' transformers as defined in EN 60076. As a result, dielectric tests, for example, are not normally carried out in the manner that would be appropriate for a transformer falling into the category of highest voltage for equipment of 72.5 kV of EN 60076. Testing is normally as agreed between the transformer manufacturer and the designer of the precipitator equipment in conjunction with his customer. An induced over voltage test is usually carried out at 1.5 times rated voltage for 1 minute rather than the figure of twice rated voltage for transformers with uniformly insulated HV windings specified in Clause 12.2 of EN 60076-3. Lightning impulse withstand tests are rarely carried out, partly because the electrical location of the installation is not exposed to lightning surges so that there is not considered to be a need to simulate any operational condition, and partly because the specialist manufacturers who produce these transformers will probably not have the necessary impulse testing equipment. If an impulse test is specified it will probably be carried out at 250 kV for a transformer providing a 60 kV precipitator supply.

Since the transformer and rectifier are housed in a common tank, it is necessary to ensure that the top oil temperature rise due to the transformer does not exceed the value which can be tolerated by the rectifiers. It is necessary, therefore, to measure the precipitator transformer losses, not because efficiency guarantees are of great importance, but because a temperature rise test must be carried out. It is likely that a complete testing schedule will include a short-circuit temperature rise test on the transformer alone and also a further temperature rise test on the complete transformer and rectifier under simulated operating conditions.

The other important feature of the transformer is its ability to withstand the repeated short-circuits which occur in operation. It is usual, therefore to include in the test program a series of short-circuits at full output voltage. These will be carried out on the complete equipment including any external choke, in order to ensure that the current on short-circuit is a true representation of that experienced under service conditions.

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