WO1992003866A1

WO1992003866A1 - Solid state circuit interrupter and circuit breaker

Info

Publication number: WO1992003866A1
Application number: PCT/US1991/005861
Authority: WO
Inventors: Adrian Ionescu
Original assignee: Power Management International
Priority date: 1990-08-27
Filing date: 1991-08-17
Publication date: 1992-03-05
Also published as: AU8529591A

Abstract

This is a solid state interrupter for an alternating current supply circuit for instantly cutting off alternating current supplied via the circuit to an electrical load (150) upon occurrence of a circuit fault. Transmission lines (200, 201) in the circuit convey alternating current to the load (150) via a solid state switch (100) having a power transistor (110) which can be triggered to cut off the current to the load (150) when a circuit fault occurs. A differential current sensing device (40) detects occurrence of an unbalanced current condition (202). A ground fault detector (170) generates a fault signal (206) when the unbalanced current condition (202) is sensed. An overcurrent detector (180) generates an overcurrent signal (209) when a load overcurrent (207) is sensed. A protection bloc (190) triggers the transistor (110) to cut off the load current in a matter of microseconds after a circuit fault occurs.

Description

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1 SOLID STATE CIRCUIT INTERRUPTER AND CIRCUIT BREAKER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to AC voltage protection 5 devices, and more particularly concerns circuit interrupter apparatus adapted to shut off instantly AC voltage applied to a load if currents in "hot" or "neutral" lines of an AC power supply circuit become unequal due to a ground fault, or if the load draws an 10 overcurrent due to circuit fault.

2. Description of the Prior Art

If a small current path is opened from the "hot" terminal of an AC line outlet to earth ground or to a neutral terminal of the AC outlet via a person's body, or

15 a wet wall, floor or other conductive element, the normally equal currents in the hot and neutral wires are no longer equal. If the AC voltage is not immediately shut off, injury, death, fire, explosion, or the serious, hazardous condition can occur. At present ground fault

20 circuit interrupters are known to shut off output voltage in approximately 25 milliseconds when a short circuit occurs. When a 115 volts AC power supply is operating at

60 herz, the applied voltage continues for one and a half cycles and the voltage goes from zero to +170 volts to

25 -170 volts, again to +170 volts and back to zero, before the voltage is removed. If the fault voltage is applied to a person's body, a significant shock is experienced.

If the person's electrical resistance to ground is low due to a wet floor or other reason, or if the line voltage is

30 220 volts AC, the electric shock can be quite severe and harmful. Another danger which may occur in such a situation is the possibility of physical injury caused by a person's falling upon a hard floor or against a hard object, when being electrically shocked. SUMMARY OF THE INVENTION It is a principal object of the present invention to provide a circuit interrupter which operates faster than prior circuit interrupters to prevent harm or damage to a person or object which may otherwise occur during a prolonged ground fault.

It is another object of the present invention to provide a circuit interrupter when the current becomes excessive due to a circuit fault. According to the invention there is provided a solid state circuit interrupter which includes a differential current sensing device which senses when normally equal currents in hot and neutral lines of an AC power supply to a load become unequal. A signal indicating the existence of a ground fault is sent to a solid state electronic ground fault detector which actuates a protection block circuit. The protection block circuit opens a solid state electronic switch to which the load is connected to cut off the AC voltage applied to the load. If the load should draw an overcurrent, a resistor in circuit with the switch sends a signal to an overcurrent detector circuit which actuates the protection block circuit to open the switch and cut off the power supplied to the load. The system includes manual reset means for resetting the system when the circuit fault or cause of overcurrent is cleared. As a safety feature, the system does not reset automatically. The system operates almost instantaneously, in less than § cycle of the AC voltage and in an order of magnitude of micro-seconds to cut off power to the load and thus insure maximum safety in operation.

These and other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by 3 reference to the following detailed description when considered in connection with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a solid state circuit interrupter and circuit breaker system embodying the invention;

Fig. 2 is a diagram of a differential current sensing device or current employed in the system of Fig.l; Fig. 3 is a circuit diagram of a ground fault detector and overcurrent detector employed in the system of Fig. 1; and

Fig. 4 is a diagram of a protection block circuit employed in the system of Fig. 1. DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein like reference characters designate like or corresponding parts throughout, there is illustrated in Fig. 1 a solid state ground fault circuit interrupter and circuit breaker system. The AC terminal H of an AC line voltage which may be nominally 123 VAC or 220 VAC is connected to a hot line

10, and a terminal N is connected to neutral line 20. A capacitor 30 connected across the lines 10 and 20 provides filtration of undesired high frequency noise fed to the lines 10, 20 by the AC line voltage. The lines 10 and 20 are connected to a differential current sensing device 40 which sends a DC signal to a solid state electronic ground fault detector 17C ^ria a line 202 if the norn>__lly equal rents in a pair of ines 211 and 204 connected to a load 150 become unequal due to a ground fault. A DC regulated power supply 160 supplies DC voltage to the ground fault detector 170 via lines 205 and 205a. The DC power supply

160 (DCRPS) is energized via a pair of AC input lines 200 and 201 connected to the AC lines 20 and 10 respectively. A solid state electronic switch generally designated as reference nurriDer 100 has four diodes 60, 70, 80 and 90 arranged in a conventional full wave rectifier array. An AC control terminal 101 which is the junction of the diodes 60, 70 is connected via a line 208 and an inductor 50 to one side of the differential current sensing device 40. A DC terminal 102 at the junction of oppositely polarized diodes 70 and 90 is grounded. An AC terminal 103 at the junction of the diodes 80 and 90 is connected via a line 211 to a load 150. A DC terminal 104 at the junction of the oppositely polarized diodes 60 and 80 is connected via a power mosfet 110 and a current sensing resistor 120 to the terminal 102 which serves as the ground reference for the differential current sensing device 40, the DC regulated power supply 160 , the ground fault detector 170 an overcurrent detector 180 and a protection block 190. The power mosfet transistor 110 is a field effect transistor (FET) and acts as a solid state switching element in the switch 100. The power mosfet transistor 110 operates according to a voltage applied to a gate 114 to open a little or to open wide depending on the magnitude of the applied voltage. The power mosfet transistor 110 has a source 112 connected to a line 207 which carries a signal representing the current sensed by the resistor 120 to the overcurrent detector 180 where it is compared with a predetermined limit. A gate 114 of the power mosfet transistor 110 is connected via a line 210 to the output of the protection block 190. The source 112 and the gate 114 are connected to ground respectively via the current sensing resistor 120 and a resistor 130 which prevents excessive gate charge.

An inductor 50 in a line 203, 208 prevents abrupt increases in the load. current to allow enough time for the load current to be turned off by the switch 100 in the event of a load overcurrent or short circuit condition.

The inductor 50 also prevents overcurrent which would otherwise damage the components of the switch 100.

The power mosfet 110 is connected across the DC terminals 102, 104 of the rectifier bridge 60, 70, 80, 90.

The AC terminals 101, 103 of the rectifier bridge are connected in series with the AC line 203 and the load 150 by the lines 208 and 212. A metal oxide varistor 99 is connected across the power mosfet 110 to protect the power mosfet 110 against excessive voltage surges.

The resistor 120 senses the load current and passes a DC ignal to an overcurrent detector 180 via a line 207. This DC signal is proportional in aosolute value to the load current. To avoid excessive power dissipation and high voltage drop across the resistor 120, its resistance value should be low, for example, in a range of 5.0 to 20.0 milliohms. The resistor 130 prevents excessive charge on the gate 114 of the power mosfet 110.

The solid state switch 100 has a inherent leakage current when the load is off, of a few microamperes. To limit the output voltage, at a very low value such as under 0.5 volts, when the solid state switch 100 is of and the load impedance is high or infinite (as a result of this current l^eakage), and as a safety feature, to protect circuit operators, a resistor 140 which may have a resistance value of 10,000 to 100,000 ohms, may be connected in parallel with or across the load 150 and across the output lines 211 and 204. When the output is off, the residual voltage across the resistor 140 will not exceed 0.5 volts. Although the load 150 is shown in Fig.l as a resistor, it will be understood that the load 150 can have a resistive, capacitive and inductive components.

The DC regulated power supply 160 converts the AC line voltage on the lines 200, 201 into a DC voltage which US91/05861

6 appears on the lines 205, 205a, 205b and 205c. The magnitude of this voltage must be sufficiently large to supply the gate 114 of the power mosfet transistor 110 with enough voltage to allow a complete turn of the power mosfet and to render the transistor 110 fully conducting. The ground fault detector 170 which is powered by the DC power supply 160 sends a fault signal to the protection block circuit 190 via a line 206 when the difference between the currents in the lines 203 and 204 exceeds a present magnitude.

The solid state electronic overcurrent detector 180 sends a DC fault signal to a protection block 190 via the line 209 when the current drawn by the load 150 exceeds a predetermined magnitude. A DC voltage is applied to the detector 180 via the lines 205 and 205b.

The protection block 190 which is powered by the

DC power supply 160, removes the voltage applied to the gate 114 via the line 210 when a fault signal received via the line 206 from the ground fault detector 170 or via the line 209 from the overcurrent detector 180, or both, or when the DC regulated power supply 160 does not produce enough + DC voltage, so that gate 114 sees insufficient DC voltage on the line 210 to turn the power mosfet transistor 110 completely on. The DC regulated power supply 160 may not produce enough +_ DC voltage due to its output settling time when first turned on, or for other reasons.

Fig. 2 shows details of the electronic differential current sensing device or circuit 40 employed in the system of Fig. 1. The device 40 has a differential current sense transformer 41 with a core 41' , two primary windings 42 and 43 and a secondary winding 44. When no fault current exists, the sum of the magnetic flux induced in both of the primary windings 42 and 43 is zero; so the 7 voltage across the secondary 44 is also zero. If the currents in the primary windings 42, 43 connected to the hot line 10 and the neutral line 20 respectively, are unequal, an AC voltage will be rectified by the bridge 45 and will appear as a DC voltage across a differential current sensing resistor 46 connected across terminals

45c, 45d. The bridge 45 may have four Schotkey type diodes 49 which are characterized by their fast response time and low forward voltage drop. The resistance value of the resistor 46 is determined by the voltage magnitude desired on the line 202 which is connected to the ground fault detector 170. A suitable magnitude may be between

110 millivolts and 200 millivolts. A fast acting silicon diode 47 is connected across the resistor 46 and prevents any excess voltage on the line 202 if an excessively large voltage appears across the secondary 44.

Fig. 3 shows circuitry of the ground fault detector 170 and the overcurrent detector 180 which are both solid state electronic circuits employed in the system of Fig. 1. In ground fault detector 170, a time delay network comprising a resistor 171 and a capacitor 172 introduce a desired amount of time delay in tne event of a ground fault. The ground fault threshold is set by a voltage divider 173, 174. Then this threshold is exceeded by a ground fault signal on the line 202 a comparator 175 sends a "high" signal on the line 206 to the protection block 190. A resistor 177 connected between the comparator 175 and the line 205 via the line 205a limits the current in the line 206 applied to the protection block 190. The capacitor 176 bypasses the DC power supply supplied by t: DC regulated power supply (DCRPS) 160 via the lines 205, 205a to the comparator 175.

In overcurrent detector 180, a time delay network, comprising a resistor 181 and a capacitor 182 introduces the desired amount of time delay in the event of an overcurrent appearing on the line 207 from the current sensing resistor 120. The overcurrent threshold is set by a voltage divider 183, 184. When this threshold is exceeded by a fault signal current on the line 207 ' a comparator 185 will send a "high" signal to the line 209 to the protection block circuit 190. A resistor 187 limits the current on the line 209. A capacitor 186 bypasses the DC power supply supplied via lines 205, 205b to the comparator 185.

Fig. 4 shows circuitry of the solid state electronic protection block 190 employed in the system of Fig. 1. The line 206 from the ground fault detector (GFD) 170 and the line 09 from the overcurrent detector (OD) 180 are connected to a gate 191 of a fast turn on SCR 1903. The gate 191 turns on the SCR 1903 if either the line 206 from the ground fault detector or the line 209 from the over current detector applies a "high" signal indicitive of a fault. When the SCR 1903 conducts, the voltage at junction 195 is below the avalanche voltage of a zener diode 1906, and the current at a base of a transistor 1910 drops thereby turning off the transistor 1910, which turns off a relay 1909 whereby a switch arm engages a contact 193 as illustrated in Fig. 4. In this relay position the contact 193 is at ground potential and therefore the power mosfet gate 114 is at ground potential and the power mosfet 110 is off. Clearly, when the SCR is off or non-conducting, the voltage from the power supply 160 through the resistors 1902, 1905 and the zener diode 1906 turn on the transistor 1910 which in turn causes the relay 1909 to move the switch arm 191 to a contact 192, to thereby provide a gate voltage to the power mosfet gate 114. The purpose of the relay 1909 is to protect the power mosfet 110 from operating in a destructive linear mode whenever a voltage across the SCR 1903 is lower (for whatever reason) than the avalance voltage of the zener diode 1906. Although the response time of the relay 1909 is relatively long, the variations of the voltage across the SCR 1903 are also slow. The SCR 1903 rapidly removes the voltage from the mosfet gate 114 (long before the relay reacts) but only in case of a fault.

To preset the system, the pushbutton 1904 connected between the junction point 195 and ground is pressed momentarily to short circuit to turn off the transistor 1903. When the pushbutton 1904 is released the resistor 1905 ad the zener diode 1906 supply charge current to the capacitor 1907, to insure a small delay in turning on the relay 1909, and hence in applying gate voltage to the power mosfet transistor 110. This small delay insures that full voltage across the transistor 1903 is reached when the gate 114 of the power mosfet transistor 110 is again supplied with DC voltage via the line 210. After the capacitor 1907 is charged to approximately 0.6 volts, the transistor 1903 turns on the relay 1909 closing the contacts 191, 192 and applying gate voltage to the power mosfet transistor 110. This small delay insures that full voltage across the transistor 1903 is reached when the gate 114 of the power mosfet transistor 110 is again supplied with DC voltage via the line 210.

After the capacitor 1907 is charged to approximately 0.6 volts, the transistor 1903 turns on the relay 1909 closing the contacts 191, 192 and applying gate voltage to the power mosfet transistor 110 which turns on the load current. The diode 1908 in the collector circuit of the transistor 1910 protects the transistor 1910 against voltage spikes which may occur across the coil 196 of the relay 1909. The resistor 1911 sets the gate discharge time of the power mosfet transistor 110 in case of a circuit fault.

The response time of the system is turning off the output voltage at the load 150 in less than one half cycle of the AC voltage supplied on the lines 10, 20 and may be as low as 2 microseconds, depending on the amount of time delay selected in the ground fault detector 170 and in the overcurrent detector 180. This short period helps to reduce or prevent electric shock to an operator or other person whose body may become part of a short circuit to ground. The system makes it possible to adjust the critical residual current which exists when the load 150 is turned off from a fraction of millampere upwards. This feature contributes to the safety characteristics of the system.

Instead of the power mosfet transistor 110 other types of power transistors can . be used to open the AC power supply to the load 150 in the event of a ground fault or load overcurrent. For example, a bipolar power transistor or a power sensefet transistor can be substituted for the power mosfet transistor.

It should be understood that the foregoing relates to a limited number of preferred embodiments of the invention which have been by way of and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purpose of the disclosure, which do not constitute departures from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED:

1. An interrupter for an alternating current supply circuit for instantly cutting off alternating current supplied via said circuit to an electrical load, upon occurrence of a fault in said circuit, comprising: transmission lines in said circuit for conveying said alternating current to said load; solid state electronic switching means in said circuit having a power transistor for passing said circuit to said load when said transistor is in fully conducting condition; a differential current sensing device in said circuit arranged for detecting occurrence of an unoalanced current condition in said transmission lines. a solid state electronic ground fault detector in circuit with said device for generating a fault signal when said device senses said unbalanced current condition in said transmission lines; and a solid state electronic protection block in circuit with said detector and said switching means for triggering said power transistor to substantially nonconducting condition upon receipt of said fault signal from said detector, to cut off supply of said alternating current to said load in less than i cycle of alteration of said alternating current after said unbalanced current condition develops in said transmission lines.

2. An interrupter as claimed in Claim 1, wherein said switching means comprises a plurality of solid state diodes arranged in a full wave rectifier array for applying DC voltage to said transistor for energizing and keeping said transistor in conducting condition.

3. An interrupter as claimed in Claim 1, further comprising: an overcurrent detector in circuit with said switching means and said protection block and arranged to detect when an overcurrent is drawn by said load and to apply an overcurrent fault signal to said protection block, whereby said protection block triggers said transistor to substantially nonconducting condition to cut off supply of said alternating current to said load in less than one cycle of alternation of said alternating current after said overcurrent is detected by said overcurrent detector.

4. An interrupter as claimed in Claim 3, wherein said ground fault detector, overcurrent detector and protection block are energized by DC voltage; and further comprising a direct current regulated power supply in circuit with said transmission lines for generating a DC voltage and for applying the same to said ground fault and overcurrent detectors and to said protection block, for energizing the same.

5. An interrupter as claimed in Claim 4, wherein said switching means comprises a plurality of solid state diodes arranged in a full wave rectifier array for applying another DC voltage to said transistor for energizing and keeping said transistor in conducting condition.

6. An interrupter as claimed in Claim 4, further comprising a resistor connected across said switching means and said load for limiting any leakage through said switching means to negligibly low magnitude when said transistor is in nonconducting condition.

7. An interrupter as claimed in Claim 4, wherein said protection block further comprises a manually operable switch for resetting said transistor to conducting condition after said unbalanced current condition and said ground fault and overcurrent conditions no longer exists. 8. An interrupter as claimed in Claim 7, wherein said switching means comprises a plurality of solid state diodes arranged in a full wave rectifier array for applying another DC voltage to said transistor for energizing and keeping said transistor in conducting condition.

9. An interrupter as claimed in Claim 8, further comprising a resistor connected across said switching means and said load for limiting any leakage through said switching means to negligibly low magnitude when said transistor is in nonconducting condition.

10. An interrupter as claimed in Claim 1,wherein said power transistor is a power mosfet transistor having a gate which can be triggered by an applied voltage to render said transistor conducting in a variable amount depending on the magnitude of the last named voltage applied to said gate. li. An interrupter as claimed in Claim 1, wherein said power transistor is a bipolar power transistor having a base which can be triggered by ana applied voltage to render said transistor conducting in a variable amount depending on the magnitude of the last named voltage applied to the said gate.

12. An interrupter as claimed in Claim 1, wherein said power transistor is a power sensefet transistor having a gate which can be triggered by an applied voltage to render said transistor conducting in a variable amount depending on the magnitude of the last named voltage applied to said gate.

13. An interrupter for an alternating current supply circuit for instantly cutting off alternating current supplied via said circuit to an electrical load, upon occurrence of a fault in said circuit, comprising: transmission lines in said circuit for conveying said alternating current to said load; solid state electronic switching means in said circuit having a power transistor for passing said circuit to said load when said transistor is in fully conducting condition; a differential current sensing device in said circuit arranged for detecting occurrence of an unbalanced current condition in said transmission lines; an over current detector in circuit with said switching means and arranged to generate a fault signal when an over current is drawn by said load; and a solid state electronic protection block in circuit with said detector and said switching means for triggering said power transistor to substantially nonconducting condition upon receipt of said fault signal from said detector, to cut off supply of said alternating current to said load in less than i cycle of alteration of said alternating current after said unbalanced current condition develops in said transmission lines.

14. An interrupter as claimed in Claim 13, wherein said switching means comprises a plurality of solid state diodes arranged in a full wave rectifier array for applying DC voltage to said transistor for energizing and keeping said transistor in conducting condition.

15. An interrupter as claimed in Claim 13, wherein said power transistor is a power mosfet transistor having a gate which can be triggered by an applied voltage to render said transistor conducting in a variable amount depending on the magnitude of the last named voltage applied to said gate.

16. An interrupter as claimed in Claim 13, wherein said power transistor is a bipolar power transistor having a base which can be triggered by ana^' applied voltage to

15 render said transistor conducting in a variable amount depending on the magnitude of the last named voltage applied to said gate.

17. An interrupter as claimed in Claim 1, wherein said power transistor is a power sensefet transistor having a gate which can be triggered by an applied voltage to render said transistor conducting in a variable amount depending on the magnitude of the last named voltage applied to said gate.