RESONANT TRANSFORMER

Transcription

1 RESONANT TRANSFORMER Whenever the requirement of the test voltage is too much high, a single unit transformer can not produce such high voltage very economically, because for high voltage measurement, a single unit transformer construction becomes difficult and costly due to insulation problems. Moreover, transportation and erection of large transformer becomes difficult. To overcome these drawbacks, cascading of transformer is

2 But cascading of transformer has also some disadvantages such as (1) design is complicated. (2) size is bulky. (3) costly. (4) losses are more (summation of individual transformer) and hence efficiency reduces. Resonant transformer is one of the best choice for high voltage generation which operates on resonance phenomenon (X L = Xc). In resonance condition, the current through test object is very large and that is limited only by the resistance of the circuit.

3 RESONANT TRANSFORMER The equivalent circuit of a highvoltage testing transformer consist of the leakage reactances of the windings, the winding resistances, the magnetizing reactance and the shunt capacitance across the output terminals due to the bushing of the high voltage terminal and also that of the test object.

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5 It is possible to have series resonance at power frequency ω, if (L 1 +L 2 ) = 1/ ωc. With this condition, the current in the test object is very large and is limited only by the resistance of the circuit. The waveform of the voltage across the test object will be purely sinusoidal.

6 The magnitude of the voltage across the capacitance C of the test object will be Vc = [-jv Xc/{ R+j(X L- Xc)}] = (V/R) Xc = V/(ωCR) where, R is the total series resistance of the circuit.

7 The factor (Xc/R)=1/(ωCR) is the Q factor of the circuit and gives the magnitude of the voltage multiplication across the test object under resonance conditions. Therefore, the input voltage required for excitation is reduced by a factor 1/Q, and the kva required is also reduced by a factor 1/Q. The secondary power factor of the circuit is unity.

8 This principle is utilized in testing at very high voltages and on occasions requiring large current outputs such as cable testing, dielectric loss measurements, partial discharge measurements, etc. A transformer with 50 to 100 kv voltage rating and relatively large current rating is connected together with an additional choke, if necessary.

9 The test condition is set such that ω(le + L)= 1/ ωc where, L e = the total equivalent leakage inductance of the transformer including its regulating transformer.

10 Advantages of Resonant transformer: (1) Application: Resonant transformer is able to produce high voltage and high current and for extremely high voltage requirement, the cascading of resonant transformer is also possible. (2) Output waveform: It gives an output of pure sine wave.

11 (3) Power requirement: The power requirement is less ( only 5 to 10 % of total kva required ) (4) Design: Simple and compact test arrangement. (5) Safety: If the test object is failed during testing, then there is no danger of high power arcing and heavy current surges occurance because, due to failure of test object, the resonance disappears and voltage across test object collapse.

12 In case of partial failure of test object, the resonance disappears and the voltage across the test object collapse. This reduces chances of partial failure leading to complete failure i.e. no repeated flashovers occur in case of partial failures of the test object and insulation recovery. Disadvantages of Resonant transformer: (1) Choke: The requirement of additional variable chokes should be capable of withstanding the full test voltage and the full current rating.

13 A simplified diagram of the series resonance test system is given in figure (a) & that of the parallel resonant test system in figure (b). A voltage regulator of either the autotransformer type or the induction regulator type is connected to the supply mains and the secondary winding of the exciter transformer is connected across the H.V. reactor, L, and the capacitive load C.

14 The inductance of the reactor L is varied by varying its air gap. Capacitance C comprises of the capacitance of the test object, capacitance of high voltage bushing etc. The Q factor obtained in these circuits will be typically of the order of 50. In the parallel resonant mode, the high voltage reactor is connected as an autotransformer and the circuit is connected as a

15 The advantage of parallel resonant circuit is that more stable output voltage can be obtained along with a high rate of rise of test voltage, independent of the degree of tuning and the Q- factor. Single unit resonant test systems are built for output voltages upto 500 kv, while cascaded units for outputs upto 3000 kv, 50/60 Hz are available.