Generation of high voltages at power frequency is essential for testing the efficacy of insulation of High Voltage Apparatus used in Power Systems. The generation and testing is carried out in a laboratory, using High Voltage testing transformers. These testing transformers can either be located indoor or outdoor.
Testing transformers are different from power transformers in a sense that they are not rated for high power handling. They are designed to withstand frequent short circuits when the test object fails or experiences flashover. Therefore, special methods are used, which are not applicable when generating high voltage in high power applications.
The optimum rating of a single testing transformer unit is 300 kV. Beyond this level, the cost of insulation rises rapidly and transportation becomes difficult. Cost of insulation for a single unit is proportional to square of operating voltage. At higher test voltages a cascade arrangement of several single unit transformers is used to generate high voltages for testing. Each of the units are enclosed by large size metal rings to prevent corona
Cascade Arrangements of Transformers
Figure-1 --- High Voltage Cascade Transformer
The above figure shows a typical cascade arrangement of transformers used to obtain up to 300 kV from three units each rated at 100 kV. The low voltage winding is connected to the primary of the first transformer, and this is connected to the transformer tank which is earthed. One end of the high voltage winding is also earthed through the tank. The high voltage end and a tapping near this end is taken out at the top of the transformer through a bushing, and excites the primary of the second transformer. One end of this winding is connected to the tank of the second transformer to maintain the tank at high voltage. The secondary of this transformer too has one end connected to the tank and at the other end the next stage transformer is fed. This cascade arrangement can be continued further, if a still higher voltage is required but not more than four stages.
In the cascade arrangement shown, each transformer needs only to be insulated for 100 kV, and hence the Transformer can be relatively small. If a 300 kV transformer had to be used instead, the size would be massive. High voltage transformers for testing purposes are designed purposely to have a poor regulation. This is to ensure that when the secondary of the transformer is short circuited (as commonly it happens in flash-over tests of insulation), the current does not increase to too high a value and to reduce the cost. The impedance of the transformer should be generally less than 5% and must be capable of giving the short circuit current for one minute or more depending on the design in practice, an additional series resistance (commonly a water resistance) is also used in such cases to limit the current and prevent possible damage to the transformer. What is shown in the cascade transformer arrangement is the basic principle involved. The actual arrangement could be different for practical reasons. The number of stages in this type of arrangement are usually two to four, but very often, three stages are adapted to facilitate a three-phase operation so that can be obtained between the lines.
In addition to the normal windings, namely, the low voltage winding, high voltage winding and a third winding known as meter windings is provided to measure the output voltage. Supply to the units can be obtained from a motor-generator set or through an induction regulator for variation of the output voltage. The rating of the primary or the low voltage windings is usually 230 or 400 V for small units up to 100 kVA. For larger outputs the rating of the low voltage winding may be 3.3 kV, 6.6kV or 11 kV.
Figure 1 shows the cascade transformer units in which the first transformer is at the ground potential along with its tank. The second transformer is kept on insulators and maintained at a potential of V2, the output voltage of the first unit above the ground. The high voltage winding of the first unit is connected to the tank of the second unit. The low voltage winding of this unit is supplied from the excitation winding of the first transformer, which is in series with the high voltage winding of the first transformer at its high voltage end. The rating of the excitation windings is almost identical to that of the primary or the low or the low voltage winding. The high voltage connection from the first transformer winding and the excitation winding terminal are taken through a bushing to the second transformer. In a similar manner, the third transformer is kept on insulators above the ground at a potential of 2V2 and is supplied likewise from the second transformer.
Typical Short circuit current for the High Voltage test is around 0.1 A for dry tests on solid and liquid insulation. About 1 A may be necessary for wet test on large specimens.
Figure-2--Cascade Transformer unit with isolating transformer for excitation
T1,T2,T3- Cascade Transformer Units
lS1,lS2,lS3-Isolation Transformer units
C1,C2,C3-Capacitance Voltage dividers for h.v. measurement after first second and third stages
In figure Fig. 2, a second scheme for providing the excitation to the second and the third stages is shown. Isolating transformers lS1,lS2,lS3 are 1:1 ratio transformers and are meant for supplying the excitation for the second and the third stages at their tank potentials. Power supply to the isolating transformers is also fed from the same a.c. input. This scheme is expensive and requires more space. The advantage of this scheme is that the natural cooling is sufficient and the transformers are light and compact. Transportation and assembly is easy. Also the construction is identical for isolating transformers and the high voltage cascade units. Three phase connection in delta or star is possible for the three units. Testing transformers of ratings up to 10 MVA are cascade connected to give high voltages up to 2.25 MV are available for both indoor and outdoor applications.
In order to reduce the size and cost of the insulation, sometimes transformers with a centre tap on high voltage windings earthed or connected to the tank are used. This connection results in a cheaper construction, and the high voltage insulation now needs to be designed for V2/2 , that of second transformer at 5V2/2 , and that of the third transformer at 5V2/2
All the cascade transformer units which are meant for the supply of excitation to the next stage have large leakage between the primary (or the low voltage winding) and the excitation windings. Hence, they are invariably provided with compensating windings.