The design efficacy of the insulation system is ascertained through the one minute Power Frequency Withstand test. The withstand test voltages are higher than the rated voltages. Typically, a dry type transformer of 8.7 kV class must withstand 19 kV for one minute at power frequency. IEC 60060 standard specifies the definitions and requirements for High Voltage Testing of various equipments.

High Voltage Testing Transformers are used as the test source to generate these high voltages in the laboratory. A single unit of High Voltage testing transformer can generate at best up to a voltage of 350 kV. Beyond this rating the transformer tends to become bulky and unwieldy. One of the most commonly used methods of obtaining a High AC Voltage is by combining two or more units of HV testing transformers in Cascade.

High Voltage testing transformers are essentially low in power rating and are not used continuously. The voltage ratings are high and the current ratings are low , not in excess of 1 A. The output voltage (especially for Capacitance & loss factor measurement) must be truly sinusoidal with a good form factor.

Cascade connection of one or more units of High Voltage Testing Transformers is one of the most common ways of generating High Voltages at power frequency in the laboratory.

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

Fig: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.

Fig.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.

Fig:2: Cascade Transformer unit with isolating transformer for excitation

Where T1,T2,T3- Cascade Transformer Units
l_S1,l_S2,l_S3-Isolation Transformer units
C1,C2,C3-Capacitance Voltage dividers for h.v. measurement after first second and third stages

In Fig.2, a second scheme for providing the excitation to the second and the third stages is shown. Isolating transformers l_S1,l_S2,l_S3 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 center 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.

• To understand the generation of high voltage AC for testing purposes through the use of Cascade transformers.
• To use the High Voltage Power Frequency Test Transformer and hence demonstrate the use of Cascade connection to perform breakdown studies on Sphere Gap under varying atmospheric conditions.
• How High AC Voltages are generated for testing ?

Simulation

1. When the page is loaded choose the Diameter of the Sphere value from the displayed dropdown box.
2. The sphere gap distance values are populated for selected diameter value and one value should be choosen from the dropdown.
3. The user gets displayed the value of Break-down voltage from the database for selected sphere gap distance & diameter of the sphere values.
4. The R.M.S Voltage calculated as Break-down voltage/√2 also gets displayed.
5. The Room-Temperature and Atmospheric Pressure values in the corresponding text boxes has to be mentioned in order to run the experiment.
6. The setup Experiment button is clicked to display the experimental setup of power frequency AC Test Source.
7. The number of transformers displayed(1/2/3/4) in the experimental setup is decided by the entered values of diameter of sphere,sphere gap distance,Room temperature and atmospheric pressure.
8. The Trigger setup button in clicked to know the voltage value at which break-down occurs.
9. The voltage so applied is displayed in the text box.

The main application fields of Power Frequency AC Test Source are:

• High Voltage Testing Transformers are a vital component of a High Voltage Laboratory. The are used to perform the one minute Power Frequency withstand test (also called as Hi-pot testing) for High Voltage power apparatus like Power transformers, motors, generators , switchgear and overhead insulators (both under dry and wet conditions). In many cases, to get the required voltage the transformers must be connected in Cascade.
• High Voltage Testing transformers are also used as a source for measurement of Capacitance, tanδ and partial discharges in High Voltage power equipment. In most of the cases a single high voltage testing transformer is used.
• High Voltage Power Frequency Testing transformers are extensively used in breakdown studies of various solid, liquid, gaseous insulating materials with different electrode combinations.
• They also serve as a source for DC Charging set of the Impulse Voltage Generator and in generating high DC voltages.