This invention relates to improvements in the protection of equipment, in particular electrically-powered and/or electronic equipment, against lightning surges on power lines. Lightning is capable of causing both physical damage to electronic equipment, and/or corruption of data, either by virtue of the equipment's direct attachment to a power line, or by induction due to a near stroke, that is, a strike in proximity to the equipment.

Currently, basic protection is provided by using over-voltage limiters across the phase, neutral and earth wires of a power system. These surge diverters can take the form of an air gap with a series non-linear resistor to prevent power follow-on after the surge. More recently the metal oxide varistor (MOV) has gained favour. This gapless solid state device conducts above a certain voltage threshold and after a surge, will go out of conduction when the voltage returns to the threshold.

In Figure 1 there is shown a graph of electric current I against time t which illustrates the waveshape of a typical lightning surge. Normally, voltages rise to kilovolts in approximately 1 microsecond. A set of standard waveforms has been developed from statistical averaging of discharges. These waveforms now appear in many International Standards for lightning protection throughout the world.

Damage to equipment occurs through two processes. First, there is the problem of overvoltage, for which surge diverters such as MOVs are provided. A 275V AC rated device will start clamping at around 400V and may rise to 1000V dependent on the severity of the surge current. This let-through voltage may be sufficient to cause breakdown, even though it is well below the tens of kilovolts expected on unprotected circuits.

The second process relates to the rate of rise of voltage (dV/dt). Many equipment power supplies operate at 50Hz and have components rated for low frequency. Application of voltages rising at rates of 1000V per microsecond can cause transients to enter equipment, resulting either in electrical breakdown, or in data corruption in the case of digital equipment.

Many manufacturers use both MOVs and a low pass filter to clamp and modify the let-through voltage to produce a lower dV/dt. The filter action can actually reduce the peak residual voltage and simultaneously bring the dV/dt into a range compatible with power supply components. Figures 2a to 2e shows five typical arrangements for such low pass filters which normally include inductors and capacitors as shown.

In Figure 2a there is shown a simple low pass L/C filter comprising a single inductor L and a capacitor C. Figure 2b shows an L/C filter having two inductors h- connected in parallel with a single capacitor C connected between the inductors. Figure 2c is similar to Figure 2b, but differs in that the two inductors 1^ share a single core. Figure 2d shows an L/C filter arrangement with a pair of separate inductors L connected in parallel and another pair of inductors L_-, connected in parallel but sharing a single core. Figures 2e shows an arrangement with two inductors I_g connected in series and a capacitor C. It will, however, be appreciated that various other arrangements according to conventional filter design may be used.

In summary, it can be said that current practice uses an energy absorption device in the form of a surge diverter such as a MOV and a subsequent filter to modify the residual waveform. However, it is important to note that a filter uses reactive components comprising inductors and capacitors. These components have no ability to dissipate energy. Instead, they ultimately feed forward their stored energy to the protected equipment in a different form.

Filters can also have an undesirable effect when an impulse voltage is applied. Ringing effects can occur which according to load impedance, may cause oscillatory voltages to be applied to equipment of a greater voltage than would be the case with a simple shunt connected MOV. For this reason, there is usually installed a second MOV on the output of the protection device to act as a voltage limiter. In such case the dV/dt is reduced but the residual peak voltage can remain high.

It is an object of this invention to provide for improved protection for equipment, against lightning-induced surges in power lines.

It is also desirable to provide a protective device in which at least some of

the aforementioned disadvantages of known types of protective devices are ameliorated.

The invention may provide, in one broad aspect, a device for protection of equipment from lightning strike-induced surges on power lines, including means to dissipate magnetic stored energy in at least the form of heat.

According to one aspect of the present invention there is provided a device for protecting electrical equipment from the effects of lightning strike-induced surges on power lines comprising a filter incorporating an inductor and a protective device connected across the inductor of the filter which, upon the completion of an electrical impulse or transient applied to the filter, acts to short circuit said inductor.

The protective device is conveniently arranged to act in response to a voltage polarity reversal on the inductor upon completion of the transient, so that when field collapse and voltage reversal occur the inductor is short-circuited and the stored magnetic energy in the inductor is dissipated in the form of heat instead of being passed forward as current to a capacitor of the filter circuit and subsequently to the electrical equipment requiring protection. The device may also have the further advantageous effect wherein, upon short circuiting of an inductor, the protective device is connected to the power line causing at least part of the energy stored in the capacitor to return substantially to the power line instead of being passed forward into the protected equipment.

The protective device preferably remains inactive and presents a high impedance to the electrical impulse or transient, and is adapted to recognise the voltage polarity reversal on the inductor at the completion of the transient and thereupon to act to short circuit the inductor. Preferably, the device should also include a preceding element, such as a metal oxide varistor, to absorb some of the applied energy.

The protective device may comprise a bridge rectifier in which the energy is at least partly dissipated as current through diodes of the bridge rectifier following voltage polarity reversal at the completion of the transient. In one preferred embodiment of the invention the inductor of the filter may be located within a rectifier bridge such that when field collapse and voltage

reversal occurs on the inductor, the inductor is short-circuited and the magnetic energy thereof dissipates as current. Alternatively, the device may be an solid-state arrangement including energy dissipation means in the form of bridge rectifier, means to sense voltage polarity reversal across the inductor, and a switching device within the bridge rectifier responsive to the voltage polarity reversal sensing means.

According to another aspect of the invention there is provided a two- terminal protective device for connection across the inductor of a filter wherein the protective device remains inactive and presents a high impedance to a transient applied to the filter, the device being adapted to short circuit the filter when voltage polarity reversal on the inductor occurs at the completion of the transient so that energy stored by the filter during said transient is dissipated.

The protective device preferably includes energy dissipation means responsive to a voltage polarity reversal on the inductor to dissipate energy stored by the filter during the transient. The protective device preferably includes voltage reversal sensing means arranged to sense the voltage polarity reversal on the inductor and may include switching means responsive to a voltage polarity reversal on said inductor to operate the energy dissipation means.

In another preferred arrangement, the protective device includes at least one capacitor means adapted to be charged as a result of an electrical impulse or transient applied to the filter, and the arrangement is such that the voltage in said capacitor means is added to the post-impulse post-transient voltage in the inductor of the filter, the combined voltage being adapted to cause the energy dissipation means to operate.

As a result, low impedance is applied across the inductor at a time when its magnetic energy is about to be returned to the circuit.

In one preferred embodiment, the energy dissipation means comprises a gas arrestor, but it will be appreciated that a protective device in accordance with the invention may incorporate a triac, thyristor, triggertron or any device capable of switching rapidly to operate on high pulse current. The protective device is preferably adapted to reset itself after the stored energy of the filter has been substantially dissipated. Preferably, it is adapted to

reset itself within a period falling substantially within the range from 8 to 200 milliseconds.

A protective device in accordance with the invention may be capable of responding to voltage polarity reversal on the inductor of a filter within a matter of microseconds, preferably within one microsecond. The protective device may not operate upon application of high initial dV/dt during a transient, but will preferably respond to a voltage polarity reversal on the inductor of the filter, irrespective of dV/dt at the time of the voltage polarity reversal.

The protective device may be powered from an external power source so that it can operate when a lightning surge or the like has previously disrupted the normal power supply. Alternatively and preferably, the operating energy of the protective device may be derived from the applied transient so that the device acts independently of other external energy sources.

Various embodiments of the invention will be described in detail, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 is a graph of electric current against time showing a typical lightning surge waveform;

Figures 2a to 2e show various filter types used in equipment protection;

Figure 3 is a graph of voltage against time for a lightning surge waveform across a series inductor of a filter;

Figure 4 is an embodiment of a device in accordance with this invention;

Figures 5a and 5b show a second embodiment of a device in accordance with the invention;

Figure 6 is a third embodiment of a device in accordance with the invention; Figure 7 is a fourth embodiment of a device in accordance with the invention;

Figure 8 is a graph similar to that of Figure 3, showing the waveforms obtained using the device of Figure 4; and

Figure 9 is a graph showing the improvement of let-through voltage using a device such as the one of Figure 4.

In Figure 3 there is shown the voltage waveform across a series inductor of

a filter, due to the impressed impulse current I representing the lightning surge.

A MOV acts as a primary clamp while the inductor passes residual current, albeit in a retarded manner to the capacitor. In so doing the current magnetises the inductor. When the applied impulse ceases, the magnetic field of the inductor is no longer sustainable and starts to collapse. This is seen in Figure 3 as a voltage reversal 3J commencing at the conclusion of the applied impulse. The energy in the collapsing magnetic field is converted to current which re enters the filter circuit.

The intention of this embodiment of the invention is to provide a "crowbar" type of device which prepares itself during magnetising phase and actuates on the voltage reversal which occurs across the inductor on the cessation of the applied impulse. Such a crowbar device effectively short-circuits the inductor and causes the stored magnetic energy to dissipate in the form of heat. In this manner the energy cannot be returned to the circuit. Furthermore, the short circuit on the inductor is preferably such that it directly connects the partially charged shunt capacitor to the power line of the filter. The stored energy of the capacitor now substantially returns to the low impedance power line or grid instead of being forced forward to be absorbed in the protected equipment. In Figure 4, there is shown a protective "crowbar" device 10 which may typically serve the purpose of this invention. The protective device 10 comprises a two-terminal device which has a diode Dl connected in series with first and second capacitors CI and C2, first and second resistors Rl and R2 connected in parallel with the diode Dl and the capacitors CI and C2, and a gas arrestor 14 with two of its terminals connected in parallel with the resistor Rl and R2 and its centre pin connected to the line between the capacitors CI and C2 and between the resistors Rl and R2. The device is connected across the inductor L of an L/C filter and operates in this manner. When the impulse is applied, the capacitors CI and C2 charge with the diode Dl in conduction. The values of CI and C2 are preferably identical as are the values of Rl and R2. Thus, the centre pin of the three-element gas arrestor 14 is maintained at half the voltage applied to the outer

plates. Provided the voltage across these plates does not exceed their conduction threshold, the arrestor will not operate. Typical operating voltages across the plates could range from 700V to 1110V according to device geometry.

At the cessation of the applied impulse the voltage across the inductor reverses as shown in Figure 3. However, the capacitors CI and C2 have been prior charged with a positive voltage and now find a negative voltage applied to the plates of the arrestor. The sum of the applied negative voltage and the half- positive voltage prior developed, now exceeds the device breakdown. The result is that the arrestor 14 strikes and goes immediately into the arc mode. This places a short-circuit across the inductor 12 coil and cancels the pending return of energy. The stored energy is dissipated in the resistance of the inductor 12 and in the arc of the arrestor. In this arc mode the arrestor may typically develop 15V to 20V across the plates, with currents in the order of 1000 A.

Had such a crowbar device 10 not operated, the main filter capacitor would have continued charging. The collapsing magnetic field would try to continue the previously applied impulse current. That is, while the voltage across the inductor reverses, the current direction is sustained in a forward direction. This is due to the fact that under applied impulse the inductor is a "load" while on collapsing magnetic field it is a "generator". In the case of filters for lower load current levels, say 10A to 20A, a more simple arrangement shown in Figures 5a and 5b is to place the inductor L of the L/C filter inside a rectifier bridge formed from diodes Da, Db, Dc and Dd.

In Figure 5a, the magnetic charging is unipolar during a transient, but when the field collapses and voltage reversal occurs - see the right-hand figure in Figure 5 - the rectifiers of the bridge are in their conducting direction and act to short- circuit the inductor. In the right hand diagram, arrows show the current direction on the reversal of inductor voltage.

The magnetic energy is then dissipated as current in the diodes and the resistance of the inductor, and any stored energy of the capacitor is substantially returned to the power line instead of being forced forward to be absorbed in the protected equipment.

The 'crowbar' may alternatively comprise a solid state "trigger" arrangement.

One such arrangement is shown in Figure 6, which has a switching device 18 in a bridge rectifier 20 with coupling to a voltage reversal sensing means 16 across the inductor L of an L/C filter having a MOV 22 connected as a primary clamp. The power supply line is shown as 24 and the equipment connected thereto as 26.

The "trigger" arrangement of Figure 6 is connected in parallel across the inductor L and is operative to carry the current only when the solid state sensing means 16 senses a voltage reversal across the inductor due to collapsing magnetic energy. In the case of an inductor connected within a bridge rectifier as shown in Figure 5, the inductor and the diodes of the rectifier carry the full load current and may create excess heat. However, whilst a parallel triggered "crowbar" device is a preferred form of this invention, it will be appreciated that this invention also covers removal of inductor magnetic energy by an inductor connection within a bridge rectifier. An alternative solid state "trigger" arrangement is shown in Figure 7 which illustrates a two-terminal protective device 30 connected across the inductor L of the filter. The protective device 30 includes a triac 31 connected to an inductor 32 of an L/R snubber circuit, a bipolar Zener diode 29, a pair of Zener diodes 27 and 28 in parallel, various resistors 33, 34, 35, 36 and 37, a pair of capacitors 38 and 39 and a further pair of diodes 41 and 42.

The protective device operates in the following manner. An electrical impulse or transient is applied to the filter inductor L will create very high dV/dt due to the retarding effect of the inductor and the low impedance of the uncharged filter capacitor C. Triac 31 and L/R snubber circuit 32 and 36 reduce the dV/dt observed by the triac to prevent false triggering. The inductor 32 in the snubber is designed to saturate on operation of triac 31. As the voltage across the inductor L builds, resistor 33 and the bipolar Zener diode 29 act to produce a clamped voltage, say about 18 volts. This clamped voltage will cause either capacitor 38 or 39 to charge via its associated diode 25 or 26 depending on the applied impulse polarity. However, forward conduction to traic 31 is retarded by Zener diodes 27 and 28 which may typically be 24 Volt rating.

Upon observance of voltage reversal on the filter inductor L, the clamped voltage of the bipolar Zener 29 reverses and this new voltage adds to that of capacitor 38 or 39 to double the total voltage. Some 36 volts is now applied to either Zener diode 27 or 28 which now exceeds the operation threshold and conduct current to the gate of the triac 31, causing it to conduct. In the conduction mode the triac will latch to the "on" state until a residual threshold of about 50mA is reached. Thereafter, reset will be automatic when the magnetic energy of the inductor is substantially dissipated. Resistors 34 and 35 also act to bleed capacitors 38 and 39 and ensure the system resets in a time interval suitable for the recurring strokes in multiple stroke lightning discharges. These are typically 8-200 millisecond intervals.

In a conventional L/C filter used to protect electrical equipment from a lightning induced surge on a power line, the voltage on the capacitor C of the filter increases until the surge ceases. Its value then becomes higher after the cessation of the surge because it absorbs current produced by the collapsing field of the inductor L of the filter.

When a protective "crowbar" device in accordance with the invention, such as one of the various devices described above, is applied to an L/C filter two beneficial effects occur. The first is the removal of stored magnetic energy, after the inductor "switch" has correctly functioned to retard the forward flow of energy into the capacitor. Secondly, the capacitor C, which has typically charged to half its normal voltage, prior to operation of the "crowbar", will now find a small discharge paths forward to the relatively high impedance of the equipment and a larger backward path to the low impedance power grid. The return path is no longer impeded by the inductor which now is in the short circuit mode. The result is a significant reduction in stored energy, a reduction in peak let-through voltage, a reduction in dV/dt and the elimination of the risk of ringing voltages being generated.

Figure 8 shows a graph, generally in accordance with that of Figure 3, but showing the difference between the use of a conventional filter, and a device in accordance with the invention.

The graph of Figure 8 shows the inductor voltage V, the voltage across the inductor, against time t. Curve 8J shows the voltage without a device in accordance with the invention, and 8.2 shows the voltage with the device of one of

Figures 4 to 7 in use. The transferred current T to the whole module is also shown.

The arrestor or "crowbar" device such as described with reference to Figure 4, 6 or 7 is preferably designed to self extinguish and to reset itself at the end of the magnetic discharge of the inductor. The filter inductor L being in series with the power supply will have been designed for an approximate 3V drop at full load current due to its inductive reactance at 50Hz. Higher voltages are not applicable as these would affect voltage regulation as seen by the protected equipment. This lack of sufficient and sustained voltage means that the device will automatically reset after the passage of the surge, irrespective of the 50Hz normal load current.

Devices to achieve the purposes of this invention not necessarily limited to gas arrestors in the embodiment of Figure 4. They may be triacs, thyristors, triggertrons or any device capable of rapid turn-on, high pulse current and automatic (by circuit design) reset.

Also a plurality of devices may be used wherein two parallel devices may be reverse polarised to cater for either positive or negative applied surges. These devices, requiring no power, offer a simplicity of connection to an inductor. A modular format is possible with two wires simply connected across the inductor. Such devices may be incorporated in new equipment or retrofitted into existing protection devices to improve performance. In Figure 9 there is shown the reduction of residual or let-through voltage which appears across the protected equipment which results from use of devices in accordance with the invention. The voltage-time curve is shown without the device in use (9J) and with the device in use (9.2).

In one aspect, the present invention uses the applied voltage across the inductor of a lightning protection filter to charge a capacitor and store a triggering voltage. This voltage only becomes effective on voltage reversal due to the collapse of the forward surge. Such concept may be used to fire an avalanche

device or a low impedance switch to absorb the residual magnetic energy. By this means a significant improvement occurs in the complete lightning protection device by reduction in the residual let-through pulse, and in the elimination of the potential for ringing voltages to be generated. It can be seen that the present invention causes additional energy absorption in the filter and simultaneously completely eliminates any risk of ringing voltages. The result will be a low dV/dt and a low residual (typically about 50%) peak let- through voltage.

Since modifications within the spirit and scope of the invention may be readily effected by persons skilled in the art, it is to be understood that the invention is not limited to the particular embodiment described, by way of example, hereinabove.