FREQUENCY-DEPENDENT OVERVOLTAGE PROTECTIVE DEVICE FOR HIGH VOLTAGE APPLIANCES

Abstract: A frequency-dependent overvoltage protective device for high voltage appliances comprising in principle two varistors 10, 11 connected in series, whereby at least a capacitor 9 is connected in parallel to varistor 11, said varistors 10 and 11 ensuring protection at the normal network frequencies, while in the case of higher frequencies, as a consequence of capacitor 9, the voltage over the varistor 10 increases so that then this element alone taken over the protection.

Description: The current invention relates to a device for protecting an electrical apparatus against overvoltage. Such devices are generally known. Such a known device has a virtually fixed safety voltage level so that the protective device, which is generally connected in parallel to the appliance to be protected, will respond when the supplied voltage exceeds the safety voltage level of the protective device. When the safety voltage level of the protective device is selected so as to be marginally lower than the maximum permissible voltages of the appliance being protected during operating conditions the apparatus to be protected is sufficiently protected against the high voltages occurring most often in networks. These result from the use of switches or the occurrence of lightning strikes. The test voltages of the appliance for protection (for example a transformer) generally satisfy national and international norms, which are higher than the maximum permissible voltage of the network in which the appliance is used. It has been found in practice that, in spite of this at first sight apparently sufficient protection, appliances protected in this way can nevertheless be affected by internal overvoltage phenomena which can result in malfunctioning. Research has shown that as a result of resonance (such an appliance is often furnished with reactive and capacitative components), locally the tested voltage level can be exceeded while the voltage level at the terminals of the appliance and at the terminals of the protective device still lie under the safety level. It has also been found from this research that such resonance voltages usually have high frequencies. To protect electrical appliances against such phenomena US-A-4.095.163 shows an apparatus for protecting an electrical appliance against overvoltage, in which the protection voltage level thereof is frequency dependent by virtue of the series connection of a capacitor and an element with a voltage dependent impedance. As a result of the application of such frequency-dependent protection, whereby the safety voltage level of the protective device is lowered in the frequency areas, whereby resonance occurs, it is possible to prevent internal voltages developing in the appliance as a result of resonance being able to reach values which may damage the appliance. It is hereby noted that the resonant frequencies of such phenomena, even of the same appliance, are not constant; they depend among other factors upon the magnetic saturation of the iron circuits which form part of the self-inductions present inside the appliance, upon the circuit configuration of the elements located inside the appliance and upon the existing network configuration. Generally speaking the frequency of such phenomena is greater than the network frequency so that, in order to easily be able to intercept such phenomena, it is recommended in the case of frequencies which deviate from the network frequency that a lower safety level is selected. In this circuit, known from US-A-4.095.163 no account is taken of the influence of the network configuration (and neither with the impedance of the appliance to be protected) which impedances have mainly an inductive character. The present invention aims to reduce the influence of the impedance of the appliance and of the network. Consequently, the present invention provides an apparatus for protection of an electric appliance, comprising a series circuit of two elements with a voltage dependent impedance and a capacitor connected in parallel to one of said elements as specified in Claim 1. By offering more degrees of freedom to the dimensioning, the impedance (inductance) of the network and of the appliance to be protected can be taken into account, reducing the influence thereof. FR-A-2097418 discloses a circuit for protecting an electrical appliance against overvoltage, comprising a device with two main electrodes and an auxiliary electrode, to which the voltage is fed through a resonance circuit. This configuration, in which the auxiliary electrode initiates the ignition, is used to decrease the distortion between the electrodes. Further the resonance in the resonance circuit takes place at the network frequency to protect against overvoltage in the network frequency. The present invention will be further explained hereinafter with reference to the appended drawings, in which: Figure 1 shows a diagram of a known, classic protective circuit; Figure 2 shows a diagram of another known protective circuit; Figure 3 shows a diagram of a first protective circuit according to the current invention; Figure 4 shows a diagram of a second protective circuit according to the current invention; Figure 5 is a graph by way of elucidating the invention; and Figure 6 shows an embodiment of the present invention. Figure 1 shows in schematic form a network 1 represented by a voltage source 2 and an impedance 3. This network is linked by means of a voltage supply wire to a transformer 4. In order to protect transformer 4 against overvoltage originating from network 1, an overvoltage protective circuit 6 is arranged between network 1 and transformer 4. This known overvoltage protective circuit 6 is formed by a voltage-dependent resistance, in this case a varistor 7. This varistor 7 is dimensioned such that when the safety voltage level of varistor 7 is exceeded it will begin to conduct, so that a voltage peak exceeding the safety voltage level of transformer 4 is diverted to earth through varistor 7. The limitations of this circuit have been discussed in the introduction. In order to prevent these problems, a frequency-dependent overvoltage protective device has been provided in the prior art, an embodiment of which is shown in figure 2. The embodiment shown here of the overvoltage protective circuit 8 is in this case again connected between a network 1, represented by the combination of a voltage source 2 and an impedance 3, and a transformer 4. This overvoltage protective circuit 8 is formed by a series connection of a capacitor 9 and a varistor 10 circuited between the voltage supply wire 5 and earth. Capacitor 9 and varistor 10 are dimensioned such that the operation of this overvoltage protective circuit 8 is the same in the case of network frequency as that of the known overvoltage protective circuit 6 in accordance with figure 1. At higher frequencies the impedance of the capacitor 9 will however diminish, so that the voltage over the varistor 10 becomes greater. This has the consequence that the safety voltage level of varistor 10 is already reached at a lower working voltage than is the case at the network frequency. No account is taken in this circuit of the impedance 3 of the network (and the appliance to be protected), this impedance generally having an inductive character. This impedance will tend to disturb the required frequency-dependent nature of the circuit. The circuit according to the present invention shown in figure 3 aims to lessen the influence of the network impedance on the operation of the circuit. This circuit comprises two varistors 10, 11 connected in series, whereby a capacitor 9 is connected in parallel to varister 11. Account is taken in the dimensioning of this circuit of the impedance (inductance) of the network so that the desired characteristic can be approached. Varistors 10 and 11 will ensure protection at the normal network frequencies, while in the case of higher frequencies, as a consequence of capacitor 9, the voltage over the varistor 10 increases so that then this element alone taken over the protection. The operation of this circuit is still however to some extent dependent on the (variable) impedance of the network and the appliance to be protected. Figure 4 shows a second overvoltage protective circuit according to the present invention. This overvoltage protective circuit 12 is again connected between a network 1 and a transformer 4. Overvoltage protective circuit 12 is formed by a series connection of a capacitor 13 and an inductance 14. Connected in parallel to capacitor 13 as well as to inductance 14 is a varistor 15 and 16 respectively. In this circuit the influence of the impedance of the network and of the appliance to be protected is virtually eliminated, since these impedances are small relative to the inductance 14. The operation of this circuit is likewise based on the fact that the voltage over the capacitor decreases with the frequency and also on the fact that the voltage over the inductance increases with the frequency. By making a suitable choice of the relevant varistors the safety voltage level of the protective device is fixed. Account must of course be taken here of the fact that the safety voltage level in the network frequency corresponds with the safety voltage level of the circuit shown in figure 1. It is also possible through use of for instance a switch to made the safety voltage level adjustable. In this way, when the protective circuit is installed, the safety voltage level can be set to the value most appropriate in this situation. This circuit has the further advantage over the circuits shown in figure 3 that because of the greater number of components the number of degrees of freedom in the choice of the desired frequency characteristic is greater. Account must of course be taken here of the fact that there is not an entirely free choice of the values of the capacitor and the inductance, since the reactive power input and output of this circuit has to be taken into account. The lowest possible resonant frequency of the series connection of the capacitor and the inductance may be selected so that the safety voltage level falls steeply above the network frequency. Figure 5 shows a graph of the admittance of the circuit as in figure 4, whereby Y = admittance, f = frequency and f o = resonant frequency of L-C series circuit. Figure 6 shows an embodiment of the overvoltage protective circuit shown in figure 4. This overvoltage protective circuit is formed by an insulating housing placed on a metal foot 17, the housing being closed off at the top by a metal cover 19. Arranged in housing are two column-shaped spaces 21, 22 each surrounded by an insulating casing 20. Mounted beneath each of these columns 21, 22 is a spiral spring 23 provided with a casing. Fitted onto one of the spiral springs 23 in the column 21 is a coil 24 which represents the inductance 14. A succession of capacitors 25 connected in series are arranged above coil 24. These capacitors represent the capacitance 13. Fitted in the other column 22 above the other spiral spring 23 is a stack of varistors 26. This stack of varistors 26 represents the varistor 16. A second stack of varistors 27 is arranged above the stack of varistors 26. This second stack of varistors represents the varistor 15. Arranged between coil 24 and the stack of capacitors 25 and between the two stacks of varistors 26, 27 is a conducting plate 30 which represents the cross connection 31 in figure 4. Mounted above the second stack of varistors 27 as well as above the stack of capacitors 25 is again a spiral spring 28. Spiral springs 28 and 23 retain the stacks of varistors, capacitors and the coil and fix them in their correct position. The spiral springs also serve as conductor so that the underside of the relevant stack is connected to the foot 17 and the upper end to the cover 19. Arranged on top of cover 19 is a connection 29. The circuit shown in figure 4 need not of course be embodied in this manner, many other embodiments being possible.

Claims: Apparatus for protecting an electrical appliance (4) against overvoltage, in which a capacitor (9, 13) is connected in parallel with an element with a voltage dependent impedance (11, 15), characterised by a series connection of two elements with a voltage dependent impedance (10, 11; 15, 16), the capacitor (9, 13) being connected in parallel to one (11; 15) of these elements, the dimensioning of the circuits being such that both elements ensure protection at the normal network frequencies and that for higher frequencies the element not being connected in parallel to the capacitor (9, 13) takes over the protection, thus effectively lowering the safety voltage of the protection apparatus. Apparatus as claimed in claim 1, characterized in that an inductance (14) is connected in parallel to the other element (16). Apparatus as claimed in claim 1 or 2, characterized in that the safety voltage level is adjustable. Apparatus as claimed in any of the preceding claims, characterized in that the elements with a voltage-dependent impedance (10, 11; 15, 16) are formed by varistors. Apparatus as claimed in any of the preceding claims, characterized in that all elements are placed in one housing.

Inventors: BOSE, AJIT KUMAR
Applicants: SMIT TRANSFORMATOREN B.V.
Application year: 1988
Main classification: H02H9
IPC codes: H02H9