Specification

This invention relates to transient overvoltage and lightning protection of power connected equipment, and is particularly concerned with providing an improved circuit for providing such protection.

In particular, this invention relates to an overvoltage protection circuit comprising including at least one energy-absorbing module, said energy-absorbing module including at least one metal-oxide varistor (MOV), a gas arrestor connected in series with said at least one MOV, said overvoltage protection circuit further including a transient discriminative module connected in parallel with the energy absorbing module, said transient discriminative module including an avalanche diode. Power line transients are caused by line faults, switching of inductive circuits and power factor correction capacitors, lightning surges etc. Lightning surge peaks are rated in the low microsecond range. Other failures in the utility power supply to the protected equipment can include long -duration overvoltages, called transient over voltage (TOV), rated in the milliseconds range. There are a few techniques and devices known to clamp such overvoltages, and there are a few key parameters which are dominant in describing their performance. These are the energy absorption ability of the device, the voltage clamping level for a given pulse current, the pulse polarity and wave shape, and the speed of response. Devices that have previously been used to achieve overvoltage protection from power line transients include Sili- con Avalanche Diodes used as a transient voltage suppressor (TVS), Metal Oxide Varis- tors (MOVs), and Gas Arrestors in the form of a gas discharge tube (GDT). There have previously been proposed various combinations of the devices mentioned above for overvoltage and transient surge protection. For instance, it is possible to connect high voltage diodes in parallel across a power line to clamp transients on the power line and to provide a higher current rating than a single high voltage device. However, this is not necessarily good practice owing to the relatively wide production spread in voltage tolerance.

EP 0 974 181 B1 shows an overvoltage protection circuit comprising at least one energy- absorbing module; said energy absorbing module including at least one metal oxide varis- tor (MOV), a switching device connected in series with said at least one MOV, and a resistance or impedance connected in parallel with said switching device, said resistance or impedance forming a high pass filter with capacitance of the at least one MOV, including a plurality of series-connected avalanche diodes (TVSs) connected in parallel with the at least one energy-absorbing module. With the high pass filter, the majority of the applied mains voltage appears across the MOVs. This ensures that the Gas Arrester does not fire, but does cause a side effect. The MOV capacitance charges and discharges with the mains voltage frequency. Thus, if an impulse occurs when the MOV capacitance is fully charged, the breakover voltage of the gas arrester adds to the MOV capacitance voltage producing a higher letthrough voltage. Additionally, failures in the utility supply applying long duration voltages, the above mentioned TOVs in the millisecond range, could damage the TVSs.

It is therefore the object of the present invention to provide an improved overvoltage and lightning protection circuit and method for protecting electrical equipment from transients in which at least some of the disadvantages of the prior art are alleviated. According to the invention the above object is achieved by an overvoltage protection circuit comprising including at least one energy-absorbing module, said energy-absorbing module including at least one metal-oxide varistor (MOV), a gas arrestor connected in series with said at least one MOV and a transient discriminative module connected in parallel with the energy absorbing module, said transient discriminative module including an ava- lanche diode, wherein the said at least one energy-absorbing module includes a resistance in parallel with said gas arrestor to form a potential divider circuit with the MOV which shares the voltage about equally across the MOV and the gas arrestor, and the said transient discriminative module including a series connection of the avalanche diode and a capacitor. The resistor added in parallel to the gas arrestor creates a potential divider circuit with the MOV, which shares the voltage roughly equally across the MOV and the gas arrestor components, which allows to choose a gas arrestor with a lower DC sparkover and lower impulse sparkover than without the resistor added in parallel to the gas arrestor to create a potential divider circuit with the MOV. The capacitor in series with the avalanche diode effectively filters out transient overvoltages (TOVs) in the long-duration, milliseconds range. 50 Hz gives a high impedance of the capacitor and no operation of the avalanche diode. But at surges, having high frequency components, giving low impedance of the capacitor, makes the avalanche diode operate and clip the peaks from the gas arrestor operation of the parallel switched gas arrestor.

In a preferred embodiment of the invention, said energy-absorbing module and said transient discriminative module are connected across an AC power supply. In a preferred embodiment of the invention, said energy-absorbing module and said transient discriminative module are connected across two power lines of an AC power supply or across a power line and the neutral line of an AC power supply.

In a preferred embodiment of the invention, the circuit including a back-up module connected in parallel with said energy absorbing module, said backup module including at least one metal-oxide varistor (MOV).

In a preferred embodiment of the invention, said energy absorbing module including at least one further fuse element connected in series with said at least one MOV.

In a preferred embodiment of the invention, said back-up module including at least on further fuse element connected in series with said at least one MOV. In a preferred embodiment of the invention, said the circuit including a galvanic isolation module connected in series with said energy absorbing module, connected between said energy absorbing module and earth.

In a preferred embodiment of the invention, said galvanic isolation module including at least one further switching device. In a preferred embodiment of the invention, the further switching device comprises a gas arrester (GDT).

The invention will be described in greater detail by description of an embodiment with reference to the accompanying drawings, wherein

Fig. 1 shows a schematic and exemplary circuit scheme of an overvoltage protection circuit according to the invention, Fig. 2 shows a graph showing the effect of the circuit of figure 1 when a transient surge pulse is applied.

Figure 1 shows an overvoltage protection circuit 1 in AC power supply, the AC power supply having a phase line L, a neutral line N and an earth line E. The overvoltage protection circuit 1 includes an energy-absorbing module 2, a backup module 3, a transient discriminative module 4 and an isolation module 5. The energy absorbing module 2, backup module 3 and transient discriminative module 4 are connected in parallel between the phase line L and the neutral line N. The isolation module 5 is connected in series to the energy absorbing module 2, backup module 3 and transient dis- criminative module 4, between the neutral line N and the earth line E.

The energy-absorbing module 2 includes a metal-oxide varistor (MOV) 6 and a gas arres- tor 7, also described as a gas discharge tube GDT, connected in series with the MOV 6.

Considering that, under normal circumstances, the MOV 6 is low impedance and the GDT 7 is high impedance, then under normal operating conditions, i.e. no surge occurring, all the voltage between the phase line L and the neutral line N appears on the GDT 7. To make sure that this does not switch into conductive state under normal conditions, one would have to select a GDT element with a high DC sparkover. Then its impulse sparko- ver will also be high. If a transient surge pulse occurred, it would then turn on only at a relatively high impulse sparkover voltage of, for example, 1000V, so up to this high turn-on voltage the transient surge pulse would stress the electrical equipment connected to the power supply. In order to avoid that, there is a resistor 8 connected in parallel to the GDT 7. The resistor 8 creates a potential divider with the MOV 6, which shares the voltage roughly equally across the MOV 6 and the GDT 7 components under normal conditions. This allows to fit a GDT 7 component with a lower DC sparkover and lower impulse spar- kover, in the range of 700V to 800V.

This is still relatively high compared to a target clamping level of 600V to 660V in case of a transient surge.

So for clipping the peak from the GDT 7 action in case of a transient surge, there is the transient discriminative module 4 with a transient voltage suppressor 9, an avalanche di- ode 9. The TVS 9 component is selected such that it clips the peak voltage to less that 660V and thus results in a lower protection level for the equipment connected to the power supply.

The TVS 9 component has a low energy handling and so is only needed for lightning surge peaks, rated in microseconds.

However, surges are not the only high voltage or overvoltage the overvoltage protection circuit 1 can see. There are also possible failures in the AC power supply that can apply long-duration overvoltages in the range of normal operation frequency of the AC power supply, at a low utility supply frequency range of e.g. 47Hz - 63Hz, preferably at 50Hz for example in a 50Hz network, so-called transient overvoltages or TOV's, which are rated in the milliseconds range and could possibly damage the TVS 9 device. To find a remedy for this, according to the present invention there is a capacitor 10 connected in series to the TVS 9 in the transient discriminative module 4. The capacitor 10 effectively filters out the TOV's from the surges. This is because in the range of normal operation frequency of the AC power supply, at the low frequencies in the above mentioned range 47Hz - 67Hz, preferably of e.g. 50Hz, it gives a high impedance, so low volt- age occurs at the TVS 9 element and no operation of the TVS element will occur. However, if a surge occurs, this has a high frequency component resulting in a low impedance of the capacitor, high voltage occurring at the TVS 9 element, the TVS 9 element will come into operation and clip the peaks from the GDT operation according to its intended function. In addition, the circuit in the backup module 3 offers redundancy (back-up) protection, if the energy absorbing module 2 should fail or become damaged. The backup module 3 has a series connection of an additional MOV 1 1 and a fuse element 12.

The energy absorbing module 2 also includes a fuse element 13 connected in series to the MOV 6. The isolation module 5 has an additional GDT 14, connected in series to the energy absorbing module 2, between the energy absorbing module 2 and the earth line E. This circuit galvanically isolates the energy absorbing module 2 circuit from earth, so no leakage current will occur.

Figure 2 shows in line B an oscilloscope trace of the residual voltage which is applied to the equipment protected by the circuit according to figure 1 when the circuit of figure 1 is subjected to a transient surge pulse of more than 1000V. Line A in figure 2 shows for comparison reasons the trace for a circuit without the transient discriminative module 4.

Each square in figure 2 represents a time of 100ns on the x-axis indicating time t, and 200V on the y-axis representing voltage V. At point 15 the surge pulse starts. At point 1 6 the GDT 7 switches from high impedance to low impedance, having a residual voltage drop of about 20V across the GDT 7. The GDT 7 has an impulse sparkover of about 750V. So in curve A, without the transient discriminative module 4, this would be the voltage peak of 750V applied to the equipment.

The effect of the transient discriminative module 4 is seen in curve B of figure 2. The ca- pacitor 10 is in its low impedance state, and the TVS 9 clips the voltage peak to about 600V.

Immediately after ignition of the GDT, the voltage drops to the threshold voltage of the MOV 6, about 200V. With time passing, the MOV 6 voltage will rise as an increasing current flows through the MOV 6, and the rest of the clamping, from point 17 on, is done by the MOV 6 at its clamping voltage of about 480V.

List of reference signs

1 overvoltage protection circuit

2 energy absorbing module

3 backup module

4 transient discriminative module

5 isolation module

6 MOV

7 GDT

8 Resistor

9 TVS

10 Capacitor

1 1 further MOV

12 Fuse

13 Fuse

14 further GDT

15 Point in time

16 Point in time

17 Point in time