Ferroresonant Pulse Voltage Generator

V. S. Dmitrevsky

(Presented by the scientific seminar of the Department of Electrical Insulation and Cable Engineering)

Pulse voltage generators are used for testing electrical insulation and in electro-impulse technology. In order to study the multi-pulse strength of electrical insulation, it is desirable to have generators with a large number of pulses per unit time. The commonly used voltage-pulse generators are made according to the Arkad’ev–Marx circuit, with the capacitors charged by means of rectifier devices.

To obtain a high pulse repetition rate, it is necessary to have a rectifier unit of high power. At present, insulation testing by long-duration pulses reproducing switching overvoltages is becoming increasingly important.

We propose for insulation testing a circuit in which the phenomenon of ferroresonance is used. Figure 1 gives the circuit of the pulse voltage generator. The generator consists of transformer 1, a choke with an iron core 2, the test object 3, a switching element (spark gap) 4, and an active resistance 5. Instead of the active resistance, an inductance may be inserted, and an oscillatory voltage drop may be obtained.

The operation of the ferroresonant generator proceeds as follows. The voltage drop across the choke and the capacitor is equal to

\[ U = U_g + U_c \]

where \(U_g\) and \(U_c\) are the voltage drops across the choke and the capacitor, respectively.

Figure 2 shows the dependence of \(U\), \(U_g\), and \(U_c\) on the current flowing through the circuit. Before phase reversal, at \(U < U_1\), a current with an inductive phase shift flows through the circuit. At \(U > U_1\), the current will have a capacitive phase shift. The voltage across the capacitance, as is seen from Fig. 2, rises sharply and will be equal to the sum of the supply voltage and the voltage drop across the choke, i.e.,

\[ U'_c = U + U'_g . \tag{2} \]

Taking into account that the voltage across the choke after phase reversal increases to \((1.3 \div 1.6)\,U\), the voltage across the capacitor will be

\[ U'_c = (2.3 \div 2.6)\,U . \tag{3} \]

Thus, the voltage obtained from the test transformer can be approximately 2 times less than the voltage required for testing the insulation.

The distance between the electrodes of the sphere gap is chosen so that its breakdown occurs at a voltage \(\sqrt{2}\,U_c\). Then, in each half-period of the alternating current, the capacitance will discharge into the resistor. The discharge of the capacitance will occur at the moment when the alternating current in the choke–capacitor circuit passes through zero. If the time of discharge of the capacitance into the resistor is small in comparison with one quarter of the alternating-current period, then it may be assumed that there will be no recharge of the discharge from the alternating-current circuit through the choke. As a result, after the capacitor has discharged, the arc in the sphere gap will go out.

Fig. 1. Circuit of a ferroresonant pulse-voltage generator

Fig. 2. Dependence of voltage on load current

In the next half-period of the alternating current, the capacitor will again be charged through the choke, and the capacitance will again discharge into the resistor.

Thus, the capacitor under test will be subjected to pulses whose duration is determined by the charging time, equal to \(\frac{T}{4}\) (\(T\) is the alternating-current period), and by the discharge time—

on the order of approximately \(3RC\) (\(R\) is the value of the active resistance 5, and \(C\) is the capacitance of the object).

The polarity of the pulses acting on the insulation will alternate. At an alternating-current frequency of 50 Hz, the pulse duration will be 5000 μsec, which approximately corresponds to the duration of the wave of switching voltages.

The proposed circuit of the pulse-voltage generator has high operating reliability and is very simple to operate. A certain disadvantage of the circuit is poor voltage regulation. Voltage regulation is carried out by changing the inductance of the choke. For this purpose it is advisable to use a choke with sectionalized windings, which makes it possible to perform stepwise voltage regulation. In those cases where regulation of the pulse voltage is not required, the ferroresonant generator has substantial advantages.