Electricity: electrical systems and devices – Safety and protection of systems and devices – High voltage dissipation
Reexamination Certificate
2000-02-07
2001-05-01
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
High voltage dissipation
C361S058000, C361S111000, C361S115000
Reexamination Certificate
active
06226166
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
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.
DESCRIPTION OF THE RELATED ART
Power line transients are caused by line faults, switching of inductive circuits and power factor correction capacitors, lightning surges etc. While there are many techniques and devices used to clamp such overvoltages, there are only 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 waveshape, and the speed of response.
Various devices have previously been used to achieve overvoltage protection from power line transients. Such devices include Silicon Avalanche Diode (SADs) Metal Oxide Varistors (MOVs), Spark Gaps and Gas Arresters.
Each of these devices have their own technical advantages and disadvantages which are summarised below.
SADs
These are improved zener diodes, which may be unipolar or bipolar devices, to which heat sinking has been added to the substrate to improve their short term energy handling capability. They are noted for their very fast response time with break over into avalanche mode occurring in approx 1 nanosecond. SADs have a very good clamping ratio with a multiplier of only 1.5 from avalanche onset to peak current rating. For example, a diode with clamping onset at 36V will only rise to 54V when impressed with maximum rated impulse current
The disadvantage of SADs is that their cost versus peak current ratio is high when compared with other devices such as MOVs. A further disadvantage is that their peak impulse current rating is quite low compared with other technologies,irrespective of cost.
MOVs
These devices can handle many kiloamps of impulse current. They are low cost devices which may be batch selected for paralleling. MOVs have a non linear voltage/current clamping curve and, therefore, cannot normally be paralleled with other devices such as SADs. The relatively poor generic clamping ratio of MOV technology in a paralleling situation would mean that the SADs would absorb the most energy and will catastrophically fail before the high energy capability of the MOV can be realised.
Spark Gaps
These are high current devices that break over at normal air pressure with impulse voltages near 3 kV. Lower voltages are not normally practical due to the very small gaps required. Small gaps rise the problem of vaporised metal bridging the gap and causing premature circuit failure. However, the spark gap has the advantage of a very low arc voltage, typically in the order of 50V. This is excellent for surge diversion, but precludes their use on power circuits unless special measures are taken to restrict power follow current. Spark gaps fall into the generic category associated with crowbar devices.
Gas Arresters
These devices are also spark gaps but within a medium of low pressure rare gas. The gas allows the strike voltage to be lowered to values in the range of 90-500V. When connected to power circuits, they have the same problem as normal spark gaps. They exhibit a low arc voltage which will lead to high power follow currents.
There have previously been proposed various combinations of the devices described above for overvoltage 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. Alternatively, several strings of series-connected low voltage SADs could be connected in parallel to increase their peak current rating, but at a significantly higher cost.
It has also been proposed to place MOVs in parallel with one or more SADs to provide secondary protection. This procedure is carried out in the knowledge that the SAD primary protection will fail at relatively low energy levels. Within these prior art technologies, there is no grading of energy absorption in the transition from one technology to another. The low protective voltage afforded by the SADs will disappear upon their failure, and there will appear in place a much higher voltage protective level.
One possibility would be to lower the rated voltage of the MOVs to make them absorb more of the impulse before failure of the SADs. However, if this were to occur, the MOVs would then draw continuous current from the power supply and consequently overheat.
It is therefore desirable 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.
SUMMARY OF THE INVENTION
According to one aspect of the invention there is provided an overvoltage protection circuit comprising:
at least one energy-absorbing module;
said energy absorbing module including:
at least one metal oxide varistor (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.
According to another aspect of the invention there is provided a method of protecting electrical equipment from overvoltages caused by transients on power lines of an a.c. power supply including the steps of: connecting at least one metal oxide varistor (MOV) across said power lines to absorb energy from a transient, said method being characterised by connecting a switching device in series with said at least one MOV to isolate said at least one MOV from the a.c. power supply in the absence of a transient on the power lines, and connecting a resistance or impedance in parallel with said switching device so as to form a high pass filter with capacitance of the at least one MOV.
Preferably, in the overvoltage protection circuit and method of the invention, a plurality of series-connected avalanche diodes are connected across the a.c. power supply to provide initial, high speed clamping before the at least one high energy absorbing MOV is brought on line.
Preferably, the switching device is arranged to connect the at least one MOV to the power lines to provide secondary clamping when a triggering threshold voltage induced by a transient is reached on the power lines. The voltage characteristic of the MOV or MOVs is preferably such that the voltage does not increase above the limit for the SADs at maximum peak impulse current. The switching device is preferably also arranged to disconnect the MOV or MOVs at the end of the transient The switching device preferably has a pre-determined holding current below which the switch resets and disconnects the MOVs. This prevents MOV failure which could otherwise result from the drawing of current continuously from the power supply.
Preferably, two or more MOVs are connected in parallel in the energy absorbing module to improve lifetime characteristics, the switching device being connected in series with the MOVS.
The MOV or MOVs may have sufficient natural capacitance to form the high pass filter with the resistance or impedance. Alternatively, a capacitance may be connected in parallel with the at least one MOV to supplement the inherent capacitance of the MOV or MOVs. Such a high pass filter arrangement is required so as to ensure that mains voltage across the energy absorbing module does not appear totally across the switching device, causing the switching device to trigger and start conducting. At low frequencies, such as 50 Hz, very little voltage appears across the resistor or impedance and hence the switching device. However, in the presence of a higher frequency transient, the voltage across the resistor or impedance increases causing the switching device to trigger.
The arrangement of the circuit components of t
Gumley John Richard
Kossmann Christopher John
Erico Lighting Technologies Pty LTD
Jackson Stephen W.
Renner , Otto, Boisselle & Sklar, LLP
LandOfFree
Transient overvoltage and lightning protection of power... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Transient overvoltage and lightning protection of power..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Transient overvoltage and lightning protection of power... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2472396