GB1594313A - Equipment for power line surge eliminator - Google Patents

Description

(54) EQUIPMENT FOR POWER LINE SURGE ELIMINATOR (71) We, COMMUNICATIONS & EQUIP MENT CONSULTANTS LIMITED, a British Com- pany, of "Martlets", Chipperfield, Hertfordshire, WD4 9BS, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state ment :- This invention relates to equipment for limiting the magnitude of unwanted electrical disturbances and for the conditioning of the waveform on electrical power mains. The equipment can be used to protect electrical circuits from surges (e.g. any form of lightning related surges or transient such as those related to the switching on and off of nearby electrical loads).

Various forms of surge protectors are known which operate on one of basically three concepts of operation. These include: devices such as spark gaps and gas tubes (i.e. devices which present a short circuit to the supply line); solid state devices such as zener diodes and avalanche diodes and varistors made from silicon carbide or zinc oxide, known as metal oxide varistors.

However known surge protection equipment based on one of the above concepts only suffers from one or more weaknesses that the present invention seeks to overcome.

Among the shortcomings of the known protection equipment may be mentioned the following undesirable characteristics: Gas breakdown devices such as spark gaps and gas discharge tubes are rugged, but require considerable electrical energy to initiate a voltage clamping action. They are generally limited to voltages above 90 V.

In the case of a fast-rise surge the impulse may well have risen to over 700 V before conduction through such a gas breakdown device will be initiated. Also, once in the conducting state, a large voltage reduction is required to shut them off. However, one advantage of such gas breakdown devices is that they can handle very high currents (in excess of 40,000 amperes or more for very short periods of time).

Zener and avalanche diodes will pass current in the forward direction at a predictable overvoltage level. The clamp level is fairly independent of current flow or impedance of the transient. Both can tolerate voltage breakdown in the reverse direction without harm. However, because of their small limited physical volume, the energy in a transient can overheat and destroy the diode junction, leaving the protected circuits vulnerable. Further, the faster the rise time of a transient, the less energy they can safely handle.

Selenium diodes and silicon carbide bidirectional-breakdown cells also are used as surge protectors. They have a considerable thermal mass, and are usually intimately attached to metal plates for good heat transfer. The over-voltage necessary to effectively operate such breakdown cells are not as precise as that for zeners or avalanche diodes and they do not provide an effective clamp for some types of sensitive electrical equipment.

Thermistors have also been used as voltage surge suppressors. The resistance of a negative temperature coefficient thermistor decreases in an inverse proportion with the temperature. This characteristic is used in a circuit where the temperature rises with a voltage rise. The heated thermistor then allows the excess current to bypass the load.

However, they are both current and voltage sensitive as well as being nonlinear in their performance.

Metal oxide varistors (MOVS) are a twoelectrode symmetrical and reversible semiconductor with a voltage-dependent nonlinear resistance that is high, almost insulating, at the rated voltage, which drops markedly as the voltage is increased.

If a varistor is placed across the terminals of an electrical device to be protected, it normally will not pass a current or interfere with the operation of the device. When an overvoltage condition appears, the varistor will decrease in resistance greatly and bypass the surge current. Since it is a semiconductor, the response period is rapid (in the nanosecond range), but these varistors are limited in the amount of power they can handle and in the clamp voltages they can maintain.

All of the foregoing devices are designed to be used in parallel with the electrical equipment to be protected and are of limited capability. None of them covers the complete spectrum of possible lightning-related power surges or switching transients. Either they respond immediately but with lower than required energy handling capability, or they handle large energy levels but do not clamp the voltage fast enough or accurately enough to prevent consequential damage to sensitive equipment. Further, the surge impedance of their connecting wires is a significant factor in limiting their ability to clamp the voltage at safe levels.

In contrast to the foregoing, this invention relates to surge protection equipment of a series-connected hybrid design which is composed of several electrical parts, each working in concert to eliminate surge or transient manifestations that modify the normal 50/60 Hertz waveform of an a.c.

supply such as is produced by a power station. A surge eliminator in accordance with this invention can react within nanoseconds to intercept, dissipate and/or divert, any surge or transient energy, clean the waveform by eliminating transients below the clamp level but at variance with the normal waveform thus preventing damage to the protected equipment. A safe operating voltage supply is maintained to the protected equipment throughout the duration of a surge or transient. A "slow blow" fuse having a suitably tapered time/energy characteristic can be used to extend the protection provided by a surge eliminator in accordance with the invention, the fuse being employed to protect the electrical equipment in the case of the 0 01% of potential disturbances which are beyond the range of protection afforded by the surge eliminator.

A surge eliminator in accordance with this invention can be used for protecting electrical equipment, operating from a power main, against unwanted electrical disturbances introduced on that main by some external influence. The biggest advantage is that the surge eliminator prevents the loss of, or damage to, the protected devices regardless of the energy in the disturbance or its rate of current rise and consequent high frequency components. Another advantage is that a surge eliminator in accordance with this invention passes a clean waveform to the protected devices, regardless of the amplitude of the disturbance.

Further advantages of a surge eliminator in accordance with the invention are that because it is in series with the power main to the protected device, it intercepts and provides a high series impedance to the disturbance rather than a conventional parallel short, thus the length of the electrical leads to the surge eliminator are not important.

Additional advantages of this invention will become apparent from the following description given by way of example and with reference to the accompanying drawing, in which Figure 1 is a functional block diagram of a surge eliminator in accordance with the invention, Figure 2 is a representative schematic diagram of a specific surge eliminator in accordance with the invention, Figure 3 shows a typical input a.c. voltage waveform distorted by electrical disturbances; Figure 4 illustrates the waveform of Figure 3 after processing by the surge eliminator of Figure 2, and Figure 5 shows a typical time vs current characteristic curve of the fuse employed in the surge eliminator of Figure 2.

Referring now to the drawing, wherein like legends and referenced numerals, respectively, designate like or corresponding parts, and more particularly, to Figure 1, which represents, in the form of a block diagram, a surge eliminator in accordance with the invention for protecting electrical equipment against electrical disturbances such as power surges and transients on a supply power line. A solid state voltage controller 15 is connected between each phase output 19 and a ground line 4, a tankfilter assembly 14 is connected between that phase output 19 and a series time-controlled fuse 10, terminal 18 of which is in turn connected to the phase input line 8. An internal dissipator 11 is connected to the terminal 18 and to a gas switch 12 the switch 12 also being connected to the ground line 4.

Typical component parts employed in the block diagram of Figure 1 are illustrated in the schematic of Figure 2 which shows a representative circuit for a single phase or for each leg of a multi-phase system. The solid state voltage controller 15 consists of one or of a plurality of bipolar back-biassed avalanche diodes and/or metal oxide varistors 28 and one or more current limiting resistors 29 which (or one of which) may be a fuse having the required resistance. The tankfilter assembly 14 consists of a choke 26 of approximately 100 microhenries and at least one capacitor 27, between the lines 19 and 4, having a value based on the intended operating voltage. The internal dissipator 11 consists of at least one varistor 23 (e.g. of silicon carbide or of the metal oxide type).

The gas switch assembly 12 consists of at least one gas tube 24, each placed in series with a choke 25 of approximately 2 microhenries inductance. The time controlled fuse 10 is a slow-blow fuse with an exponentially decaying current -time blow curve.

Figure 3 shows a typical waveform 31 having two types of possible electrical disturbance 32 and 33 superimposed thereon.

32 represents a high energy surge on the positive half cycle which exceeds a clamp level potential 34, normally selected as 1-2 times the peak operating voltage. 33 represents a low level positive transient superimposed on the negative half cycle which is shown rising to just below the clamp level 34.

Figure 4 shows a waveform 35, which appears across the lines 19 and 4 when the waveform 31 of Figure 3 is applied across the lines 8 and 4 in the circuit of Figure 1.

The mode of operation of the circuit of Figures 1 and 2 is as follows:- The solid state voltage controller 15 monitors the peak voltage at the output terminal 19; if the voltage rises above the selected clamp level 34, the controller 15 starts to conduct. If there is only a short, low-power transient, that energy is dissipated within the controller 15; if the transient exceeds, approximately, the two joules level, a voltage drop is created across the tankfilter assembly 14 causing the voltage across the internal dissipator 11 to rise and activate the gas switch 12. The gas switch 12 closes, completing the electrical connection between lines 8 and 4 in the internal dissipator 11, causing part of the excess energy to be dissipated within the internal dissipator, and diverting the residue of this energy to ground.

The internal dissipator 11 is necessary to ensure that the line voltage remains within the required operating voltage range. If the voltage surge appearing on the line 8 is in excess of the design limits of the surge eliminator, the high current flow through the high energy internal dissipator 11 will force the fuse 10 to blow without permitting the line voltage to rise beyond the permissible operating range of the protected equipment (not shown) attached to the line 19. The tank-filter assembly 14 serves two functions; it removes high-frequency energy related to fast rise time surges, thus preventing overshoot, and it cleans the waveform of low-level high-frequency transients.

The overall mode of operation of the surge eliminator is best illustrated through reference to Figure 5. In Figure 5, the curve 61 represents a typical, exponentially decaying timecurrent blow curve for the fuse 10. The fusecurve 61 is selected to operate on exceptional power surges in excess of the unit's capability.

The probability of such occurrences is less than once in ten years in temperate zones.

It is asymptotic to the maximum permissible operating current line 62. The area 63 between curves 61 and 62, represents the combination of situations handled within the surge eliminator.

It should be appreciated that the herein described and illustrated embodiments are merely of an illustrative nature and that additional configurations, combinations and variations, particularly in the component values, are possible within the scope of this invention as defined in the following claims.

WHAT WE CLAIM IS: 1. Equipment for the protection of sensitive electrically powered devices against deleterious disturbances, such as surges, transients and impulses appearing on power lines from a primary source of power, comprising: a hybrid input circuit designed to intercept the disturbance and thereby prevent passage of said disturbance to the sensitive devices and to pass a relatively clean power line waveform to said devices, said input circuit being disposed between said power lines and said devices and comprising two branches in parallel, one branch including a solid state voltage controller and the other branch a gas-filled discharge device and power dissipating means in series therewith.

2. Equipment as claimed in claim 1, in which a tank filter assembly is disposed between said parallel connected branches.

3. Equipment as claimed in claim 2, in which the solid state voltage controller is made up of a plurality of bipolar, back biased, avalanche diodes and/or metal oxide varistors, selected and arranged to achieve a given control voltage and internal dissipating capability, all in series with a resistor and/or fuse to provide a current limiting facility for the branch.

4. Equipment as claimed in claim 2 or claim 3, in which said series tank-filter assembly includes at least one choke in series with a power line and is adapted to reduce voltage overshoot on the said power line and eliminate low energy and high frequency transients and thereby smooth out the a.c.

waveform fed to the sensitive devices.

5. Equipment as claimed in claim 2, 3 or 4, in which said power dissipating means consists of a plurality of varistors.

6. Equipment as claimed in claim 2, in which said other branch consists of a plurality of gas tubes and a series inductor to assure distribution of the energy between the gas tubes, to thereby provide enhanced energy handling capabilities while maintaining the voltage across the power lines at near normal levels.

7. A power line surge eliminator comprising: a voltage controller containing in a series circuit, connected across the line and ground, at least one current-limiting resistor, a filter-tank containing at least one choke, connected in series with the line and at least one capacitor connected across the line and the ground, and an internal dissipator connected across the line and the ground, containing, in a series circuit, at least one varistor, at least one gas tube and at least one choke, said voltage controller being adapted

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   having two types of possible electrical disturbance 32 and 33 superimposed thereon.

   32 represents a high energy surge on the positive half cycle which exceeds a clamp level potential 34, normally selected as 1-2 times the peak operating voltage. 33 represents a low level positive transient superimposed on the negative half cycle which is shown rising to just below the clamp level 34.

   Figure 4 shows a waveform 35, which appears across the lines 19 and 4 when the waveform 31 of Figure 3 is applied across the lines 8 and 4 in the circuit of Figure 1.

   The mode of operation of the circuit of Figures 1 and 2 is as follows:- The solid state voltage controller 15 monitors the peak voltage at the output terminal 19; if the voltage rises above the selected clamp level 34, the controller 15 starts to conduct. If there is only a short, low-power transient, that energy is dissipated within the controller 15; if the transient exceeds, approximately, the two joules level, a voltage drop is created across the tankfilter assembly 14 causing the voltage across the internal dissipator 11 to rise and activate the gas switch 12. The gas switch 12 closes, completing the electrical connection between lines 8 and 4 in the internal dissipator 11, causing part of the excess energy to be dissipated within the internal dissipator, and diverting the residue of this energy to ground.

   The internal dissipator 11 is necessary to ensure that the line voltage remains within the required operating voltage range. If the voltage surge appearing on the line 8 is in excess of the design limits of the surge eliminator, the high current flow through the high energy internal dissipator 11 will force the fuse 10 to blow without permitting the line voltage to rise beyond the permissible operating range of the protected equipment (not shown) attached to the line 19. The tank-filter assembly 14 serves two functions; it removes high-frequency energy related to fast rise time surges, thus preventing overshoot, and it cleans the waveform of low-level high-frequency transients.

   The overall mode of operation of the surge eliminator is best illustrated through reference to Figure 5. In Figure 5, the curve 61 represents a typical, exponentially decaying timecurrent blow curve for the fuse 10. The fusecurve 61 is selected to operate on exceptional power surges in excess of the unit's capability.

   The probability of such occurrences is less than once in ten years in temperate zones.

   It is asymptotic to the maximum permissible operating current line 62. The area 63 between curves 61 and 62, represents the combination of situations handled within the surge eliminator.

   It should be appreciated that the herein described and illustrated embodiments are merely of an illustrative nature and that additional configurations, combinations and variations, particularly in the component values, are possible within the scope of this invention as defined in the following claims.

   WHAT WE CLAIM IS: 1. Equipment for the protection of sensitive electrically powered devices against deleterious disturbances, such as surges, transients and impulses appearing on power lines from a primary source of power, comprising: a hybrid input circuit designed to intercept the disturbance and thereby prevent passage of said disturbance to the sensitive devices and to pass a relatively clean power line waveform to said devices, said input circuit being disposed between said power lines and said devices and comprising two branches in parallel, one branch including a solid state voltage controller and the other branch a gas-filled discharge device and power dissipating means in series therewith.
2. 2. Equipment as claimed in claim 1, in which a tank filter assembly is disposed between said parallel connected branches.
3. 3. Equipment as claimed in claim 2, in which the solid state voltage controller is made up of a plurality of bipolar, back biased, avalanche diodes and/or metal oxide varistors, selected and arranged to achieve a given control voltage and internal dissipating capability, all in series with a resistor and/or fuse to provide a current limiting facility for the branch.
4. 4. Equipment as claimed in claim 2 or claim 3, in which said series tank-filter assembly includes at least one choke in series with a power line and is adapted to reduce voltage overshoot on the said power line and eliminate low energy and high frequency transients and thereby smooth out the a.c.

   waveform fed to the sensitive devices.
5. 5. Equipment as claimed in claim 2, 3 or 4, in which said power dissipating means consists of a plurality of varistors.
6. 6. Equipment as claimed in claim 2, in which said other branch consists of a plurality of gas tubes and a series inductor to assure distribution of the energy between the gas tubes, to thereby provide enhanced energy handling capabilities while maintaining the voltage across the power lines at near normal levels.
7. 7. A power line surge eliminator comprising: a voltage controller containing in a series circuit, connected across the line and ground, at least one current-limiting resistor, a filter-tank containing at least one choke, connected in series with the line and at least one capacitor connected across the line and the ground, and an internal dissipator connected across the line and the ground, containing, in a series circuit, at least one varistor, at least one gas tube and at least one choke, said voltage controller being adapted

   to reduce voltage surges and transients from the power lines leading to equipment to be protected.
8. 8. A power line surge eliminator substantially as hereinbefore described with reference to, Figures 1, 3, 4 and 5 of the accompanying drawing.
9. 9. A power line surge eliminator substantially as hereinbefore described with reference to Figure 2 of the accompanying drawing.