WO2008153960A1

WO2008153960A1 - Method and circuit for arc suppression

Info

Publication number: WO2008153960A1
Application number: PCT/US2008/007148
Authority: WO
Inventors: Mohamed Maharsi; Mark Giacobbe; Douglas Wood; Deia Bayoumi; Douglas Voda
Original assignee: Abb Technology Ag
Priority date: 2007-06-07
Filing date: 2008-06-06
Publication date: 2008-12-18

Abstract

A method and circuit are provided for suppression of arcing across trip contacts in a protection relay of an AC power distribution system. The trip contacts are connected into a power circuit for providing DC current to a solenoid of an AC power circuit breaker. The arc suppression circuit includes a storage capacitor and a field effect transistor having a gate, a source, a drain, and a variable resistance. The source and the drain are coupled across the trip contacts. A voltage control circuit that includes a capacitor and a voltage divider controls the voltage at the gate of the transistor. When the trip contacts move apart, DC current energy is stored in the storage capacitor and DC current energy is dissipated in the transistor. The voltage across the trip contacts is controlled during and after the period in which the contacts are moving apart.

Description

METHOD AND CIRCUIT FOR ARC SUPPRESSION

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to circuits in AC power distribution switching systems used to control AC power circuit breakers. More specifically the invention relates to circuits for protecting the mechanical trip contacts used to switch off an inductive DC current load, such as the inductive load presented by the "opening solenoid" associated with an AC power circuit breaker. [0002] Arcing is a well known problem in AC power switching. Arcing is the creation of an electrical arc between mechanical contacts as the contacts begin to open from a closed position. If, as the contacts open, the voltage across the contacts reaches a sufficient level, an arc will form between the contacts. Furthermore, if an arc does form, the arc may persist even as the contacts continue to open. Arcing is well known to be undesirable because of the wear arcing inflicts on the contacts, and because of undesirable circuit effects caused by arcing. [0003] Protection relays contain circuits with mechanical trip contacts for switching AC power circuit breakers on and off. The mechanical contacts of protection relays are coupled to switch an "opening solenoid" of a circuit breaker on and off. These contacts are subjected to the inductive DC current load presented by the "opening solenoid". So the mechanical contacts of protection relays need protection from the wear and other effects caused by arcing. Increasingly, arc suppression circuits are being used to protect the mechanical contacts of protection relays. Such arc suppression circuits are typically mounted in a protection relay, and located proximate to the mechanical contacts that they are to protect. [0004] US patents 5,652,688, issued July 29, 1997, and 5,703,743, issued December 30, 1997, both assigned to Schweitzer Engineering Laboratories, Inc., disclose circuits for protecting the trip contacts that are operated by an "opening solenoid". Each of these patents discloses a circuit having a normally-off power transistor that is turned on by the increase in voltage across the trip contacts as the contacts open. Turning on the normally-off power transistor, momentarily shunts the load current around the contacts while the contacts are opening. Then, before the voltage across the trip contacts increases sufficiently to trigger an arc, the circuit for arc suppression is turned off.

Summary of Invention

[0005] In accordance with the present invention, a method is provided for suppression of arcing across trip contacts in a protection relay of an AC power distribution system. The trip contacts are connected into a circuit for providing DC current to a solenoid of an AC power circuit breaker. In accordance with the method, a field effect transistor is provided and includes a gate, a source, a drain, and a variable resistance. The source and the drain are coupled across the trip contacts. The transistor is switched when the trip contacts begin to open, thereby enabling DC current to flow through the transistor. DC current energy is stored while the trip contacts are moving apart. DC current energy is also dissipated in the transistor while the trip contacts are moving apart and after the trip contacts are done moving apart.

[0006] Also provided in accordance with the present invention is a protection relay for use with an AC power circuit breaker in an AC power distribution system. The protection relay includes a power circuit for providing DC current to a solenoid of the AC power circuit breaker and trip contacts connected into the power circuit. An arc suppression circuit suppresses arcing across the trip contacts and includes a field effect transistor having a gate, a source, a drain, and a variable resistance. The source and the drain are coupled across the trip contacts. A voltage control circuit controls voltage at the gate of the transistor. The voltage control circuit has an input coupled across the trip contacts and an output coupled to the gate.

Brief Description of the Drawings

[0007] FIG. 1 is a schematic diagram of an AC power distribution system including a circuit for suppression of arcing across trip contacts according to the present invention;

[0008] FIG. 2 is a schematic diagram of the circuit for suppression of arcing across trip contacts according to the present invention; [0009] FIG. 3 is a flow chart that illustrates a method for suppression of arcing across trip contacts according to the present invention;

[0010] FIG. 4 is a flow chart providing additional description of the flow chart of

FIG. 3 relating to suppressing the development of a nascent arc;

[0011] FIG. 5 is a flow chart providing additional description of the flow charts of

FIG. 3 and FIG. 4 relating to preventing a new arc from forming;

[0012] FIG. 6 is a flow chart providing additional description of the flow chart of

FIG. 5;

[0013] FIG. 7 is a graph illustrating the separation of the mechanical trip contacts;

[0014] FIG. 8 is a graph representing a simulated oscilloscope trace showing transient electrical voltage across the mechanical contacts of FIG. 1 , during initialization and operation of the circuit, covering the period from before the contacts open until after all energy is dissipated and the circuit has ceased to be active;

[0015] FIG. 9 is a graph representing a simulated oscilloscope trace showing solenoid DC current flowing through the solenoid of FIG. 1 during initialization and operation of the circuit, corresponding to the time period of FIG. 8;

[0016] FIG. 10 is a graph representing a simulated electric field gradient between the mechanical contacts of FIG. 1 , and a threshold electric field gradient needed to trigger dielectric breakdown associated with an arcing event, covering the period from when the contacts begin to move apart until the contacts are fully open; and

[0017] FIG. 1 1 is a graph representing a simulated electric field gradient between the mechanical contacts of FIG. 1 , and a threshold electric field gradient needed to trigger dielectric breakdown associated with an arcing event, covering the period from when the contacts begin to move apart until all energy is dissipated and the circuit has shut down.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 1 ) General

[0018] The present invention provides a method and circuit for suppression of arcing across trip contacts in a protection relay of an AC power distribution system. The system includes an AC power circuit breaker with a battery-powered "opening solenoid", and a substation containing a battery and a protection relay. The protection relay contains mechanical trip contacts and a circuit for suppression of arcing.

[0019] It is customary to refer to the solenoid associated with an AC power circuit breaker as an "opening solenoid". Herein below, for brevity, the "opening solenoid" will be referred to as simply "the solenoid" or "solenoid 15".

[0020] The trip contacts are coupled to operate the solenoid. The solenoid is mechanically coupled to trip the circuit breaker. Normally, the DC current passing through the solenoid holds the AC power circuit breaker in the "on" condition to maintain the flow of AC current along an AC power line. Opening the trip contacts cuts off the DC current that holds the solenoid on.

[0021] The arc suppression circuit includes a transistor that is adapted to function as a voltage-controlled variable resistor. When the contacts begin to open, the transistor switches on quickly as a very low resistance, enabling the DC current to transfer from flowing through the contacts to flowing through the transistor. During a first phase of contacts moving apart, the transistor begins to dissipate DC current energy while a storage capacitor stores a portion of the DC current energy for dissipation later.

[0022] During the first phase of contacts moving apart, storing a portion of the DC current during this first phase, and dissipating DC current energy during and after this first phase, suppresses the development of an arc by limiting the duration of the time in which the electric field gradient between the contacts exceeds a dielectric breakdown threshold.

[0023] During a second phase of contacts moving apart, the transistor continues to dissipate DC current energy through an increasing resistance. The transistor dissipates DC current energy during and after the period in which the contacts are moving apart. Also, a voltage control circuit controls voltage across the contacts during and after the period in which the contacts are moving apart.

[0024] Controlling voltage across the contacts, and dissipating DC current energy in the transistor, prevents a new arc from forming, during and after the period in which the contacts are moving apart, by holding an electric field gradient between the contacts below a dielectric breakdown threshold until substantially all DC load current energy is dissipated.

2) Circuit for Suppression of Arcing

[0025] Referring to FIG. 1 , there is shown a schematic diagram of an AC power distribution system, including a circuit 10 embodied in accordance with the present invention, which is operable to suppress arcing across each of trip contacts 21 (manual) and trip contacts 22 (automatic).

[0026] The AC power distribution system includes a substation 20 and an AC power circuit breaker 18. Substation 20 contains protection relay 14 and substation battery 16. Protection relay 14 contains manual trip switch 1 1with mechanical trip contacts 21 , automatic trip switch 12 with mechanical trip contacts 22. Protection relay 14 also contains circuit 10. AC power circuit breaker 18 contains solenoid 15 and associated AC break contacts 17 in AC power line 13 that carries AC line current l(ac). Circuit breaker 18 contains solenoid 15 which normally carries the DC current ISDC that holds the solenoid on. The coil of solenoid 15 has an inductance L1 and a resistance R6.

[0027] Trip contacts 21 and 22 are shown connected across the two terminals T1 and T2 of circuit 10. Trip contacts 21 provide for manual operation of solenoid 15. Trip contacts 22 provide for automatic operation of solenoid 15. Solenoid 15 normally carries solenoid DC current I_SDC- When both of trip contacts 21 and trip contacts 22 are open, solenoid 15 imposes an inductive load on circuit 10. [0028] Referring now to Fig. 2, circuit 10 is shown to include power transistor Q1 having a gate G1 , a source, a drain, and a variable resistance R5. Circuit 10 has two terminals: T1 and T2. Transistor Q1 is coupled across the same two terminals. Solenoid 15 and substation battery 16 are connected in series across the same two terminals. Circuit 10 includes storage capacitor C1 for storing load current energy, and resistor R1. Capacitor C1 and resistor R1 are each coupled across the same two terminals.

[0029] Circuit 10 also includes voltage control circuit 25 which is shown in FIG.1 as having an output coupled to control power transistor Q1 via transistor gate G1. Voltage control circuit 25 includes capacitor C2 and voltage divider 26. Capacitor C2 and voltage divider 26 define a time constant for controlling the rate of dissipation when dissipating the DC current energy slowly. The output of voltage divider 26 is also the output of the voltage control circuit 25. [0030] Voltage control circuit 25 helps control the voltage across the contacts 21 , 22 after the contacts 21 and/or 22 begin to open such that an electric field gradient between the contacts is held below a dielectric breakdown threshold. Capacitor C1 and the sum of resistors R1 , R2 and R3 together define a time constant that controls the rate of discharge of capacitors C1 and C2 during initialization of circuit 10 to establish solenoid load current.

[0031] Transistor Q1 provides means for dissipating DC current energy. Capacitor C1 provides means for storing and releasing DC current energy. [0032] Resistor R4 is a bias resistor for biasing transistor Q1. R5 represents the variable resistance of transistor Q1 , when Q1 is operated to function as a voltage- controlled variable resistor. Reverse polarity protection diode D1 protects Q1 against reverse polarity. Oscillation damping diode D2 damps oscillation. Diode D3 is a gate clamping diode. Capacitors C3 and C4 are transient voltage grounding capacitors. Metal oxide varistor MOV protects Q1 from being damaged by excessive overvoltage applied across its current-carrying terminals.

3) Method for Suppression of Arcing

[0033] FIGS. 3-6 are flowcharts illustrating a method according to the invention.

FIG. 3 provides an overview of the method.

[0034] The method for suppression of arcing across trip contacts, as shown in FIG.

3, includes two important aspects: 1 ) automatically suppressing the development of a nascent arc that may form in a first phase of arc suppression when the contacts first begin to open, and 2) automatically preventing the formation of a new arc that may form in a second phase of arc suppression during or after the period in which the contacts are moving apart.

[0035] The method shown in FIG. 3 includes steps 2A and 2B using the circuit 10.

The method further includes steps 2C-2F involving automatic operation of the circuit 10. Step 2C includes automatically switching the transistor Q1 on quickly with its resistance set initially to a small resistance, step 2D includes automatically transferring solenoid DC current from flowing through the contacts 21 , 22 to a first portion flowing into a storage capacitor, and a second portion flowing through the transistor Q1 , step 2E includes automatically suppressing the development of a nascent arc that may form when the contacts first begin to open in a first phase of automatic arc suppression, and step 2F includes automatically preventing the formation of a new arc that may form during or after the period in which the contacts are moving apart in a second phase of automatic arc suppression.

3.1) Initial Step

[0036] As indicated in step 2A of FIG. 3, in an initial step, power is applied to the circuit 10 with the trip contacts 21 and 22 open. This charges capacitors C1 and C2. When power is first applied to circuit 10, trip contacts 21 and 22 will be open. So current will start to flow through solenoid 15 (including coil inductance L1 and coil resistance R6), diodes Dl and D2, and resistors R1 , R2 and R3; and some of this current will begin to charge capacitors C1 and C2. Power transistor Q1 will turn on briefly but the current through solenoid 15 will not create a sufficient magnetic field to trip the breaker 18. When capacitors C1 and C2 become fully charged, the inductor current will be limited by resistor R1. Also, capacitors C1 and C2 will be charged to the same voltage (battery voltage) so no voltage will appear across either of resistors R2 and R3. With no voltage at the gate of power transistor Q1 from voltage divider R2/R3, transistor Q1 will be off.

[0037] Also in step 2A, the operator closes the trip contacts 21 or 22 with power on and capacitors C1 and C2 fully charged. When either of trip contacts 21 or 22 closes, two things happen:

(i) Circuit 10 connects battery voltage across solenoid 15. So solenoid 15, accepting full DC current limited by its own inherent resistance R6, operates to close AC break contacts 17; and

(ii) capacitors C1 and C2 both discharge through whichever of trip contacts 21 or 22 is closed. [0038] Therefore, closing the trip contacts 21 or 22 (with the power on and capacitors C1 and C2 fully charged) activates the solenoid 15 (by establishing solenoid DC current ISCD) and discharges capacitors C1 and C2.

3.2) Using the Circuit

[0039] Referring again to FIG. 3, using the circuit requires that the trip contacts 21 or 22 be opened manually or automatically to initiate automatic operation of steps 2C-2F.

3.3) Opening the Trip Contacts and Switching the Power Transistor On

[0040] Step 2C of FIG. 3 provides opening of the trip contacts 21 or 22. As soon as the contacts begin to move apart, the power transistor Q1 switches on quickly with the resistance R5 set initially to a small resistance. FIG. 7 is a graph illustrating the opening of the mechanical trip contacts. The graph shows separation of the trip contacts as a function of time Sc(t), as the contacts open during the period of contact opening, 43 in FIG. 7, which is approximately 1OmS.

3.4) Transferring Solenoid Current to Storage and Dissipation

[0041] Step 2D of FIG. 3 provides automatic transfer of solenoid current. Step 2D provides that when the contacts 21 or 22 have begun to move apart and the transistor Q1 is on, solenoid current transfers from flowing through the contacts, to a first portion flowing into a storage capacitor for storage and later release, and a second portion flowing through the transistor for dissipation.

3.5) Suppressing a Nascent Arc

[0042] Steps 2A - 2F of FIG 2 and steps 3A - 3E of FIG. 4 together provide an overview of the method of automatically suppressing the development of a nascent arc in a first phase of automatic arc suppression when the contacts first begin to open.

[0043] At the instant when the contacts 21 or 22 first begin to open, a voltage will exist between the contacts, but initially the separation of the contacts will be very small. FIG. 7 is a graph representing the separation of the trip contacts. In FIG. 7, S_c(t) represents the separation of the trip contacts as a function of time. The graph shows contacts closed (separation = 0cm), contacts opening during period 43 of approximately 10 milliseconds, and contacts fully open at the end of period 43. Contact separation when the contacts are fully open is approximately 1cm. [0044] While the separation of the contacts 21 or 22 is very small, i.e. very much less than 1cm, the electric field gradient will be very large, possibly exceeding the dielectric breakdown voltage, at least for a very short period of time. So it is reasonable to suppose that a nascent arc may form at this time. [0045] The method of automatically suppressing the development of a nascent arc that is formed when the contacts 21 or 22 first begin to open, in a first phase of automatic arc suppression, is illustrated steps 3A-3E of FIG. 4. [0046] The method of automatically suppressing the development of a nascent arc proceeds as follows:

[0047] Solenoid DC current is diverted primarily through diode D1 (a reverse polarity protection diode) to storage capacitor C1 and resistor R1. Current ceases to flow through the contacts 21 or 22. The contacts continue to move apart. A first portion of the solenoid DC current flows to charge storage capacitor C1 and timing capacitor C2. Storage capacitor C1 charges quickly. Capacitor C2 charges more slowly, the charge rate of C2 being limited by resistors R2 and R3. Since the voltage across capacitor C2 changes slowly, resistors R2 and R3 form a voltage divider which turns on power transistor Q1 quickly. Transistor Q1 turns on with its resistance R5 set at a small resistance. A second portion of the solenoid DC current flows through Q1 which dissipates DC current energy in its resistance R5. [0048] As C2 continues to charge, it slowly reduces the voltage at the output of voltage divider 26. The output of the voltage divider is connected to the gate of power transistor Q1 , so slowly reducing the voltage at the output of the voltage divider has the effect of slowly increasing the resistance of the transistor's resistance R5. The rate at which the resistance of R5 is increased is determined by the resistance/capacitance time constant RτC2, where "Rr is the sum of the resistances of resistors R2 and R3, and "C2" is the capacitance of capacitor C2. Because the resistance of R5 is increased slowly, the current flowing through solenoid 15 decreases slowly. This reduces the intensity of the inductive kickback. [0049] Solenoid DC current flowing into storage capacitor C1 automatically limits the voltage appearing across the contacts 21 or 22 while the contacts are continuing to move apart. Solenoid DC current flowing through transistor Q1 automatically dissipates DC current energy slowly, during and after the period in which the contacts are moving apart. Stored DC current energy automatically releases after the first phase of contacts moving apart, and the released energy automatically dissipates in transistor Q1.

[0050] Thus, in the first phase of automatic arc suppression, the development of a nascent arc, that may form immediately after the contacts 21 or 22 begin to open, is suppressed by storing the first portion of DC current energy as soon as the contacts begin to move apart, by dissipating dc current energy slowly while the contacts are continuing to move apart, and by limiting the time period during which the electric field gradient exceeds a dielectric breakdown threshold.

3.6) Preventing Formation of a New Arc

[0051] Steps 2A - 2D and 2F of FIG 2, steps 3C - 3D of FIG. 3, and steps 4A - 4D of FIG. 5 together provide an overview of the method of automatically preventing the formation of a new arc that may form in the second phase of arc suppression, during or after the period in which the contacts 21 or 22 are moving apart. The method is more simply illustrated in steps 4A-4D of FIG. 5. The method proceeds as described in the paragraphs that follow.

[0052] The voltage across the contacts 21 or 22 is controlled during and after the period in which the contacts are moving apart such that the electric field gradient is held below a dielectric breakdown threshold until all DC current energy is dissipated.

[0053] DC current energy that is transferred to circuit 10 during and after the period in which the contacts are moving apart, including all DC current energy that is stored and later released, is dissipated, until all DC current energy is dissipated.

[0054] These steps prevent the formation of a new arc during the second phase of arc suppression, after the suppression of a nascent arc in the first phase. 3.7) Controlling Voltage and Electric Field Gradient

[0055] Control of voltage across the contacts 21 or 22 is illustrated in the graphs of FIGS. 6-8. Control of electric field gradient between the contacts is illustrated in the graphs of FIGS. 10-11.

[0056] Separation of the trip contacts Sc(t) is shown in FIG. 7 as a function of time. The contacts are moving apart during period 43 in FIGS. 7 and 8, approximately 10mS from closed to fully open.

[0057] The graph of FIG. 8 is a simulated oscilloscope trace showing transient electrical voltage Vc(t) across the mechanical trip contacts of FIG. 1. The graph shows Vc(t) over the same time period as FIG. 7: before the opening of the contacts, during the period in which the contacts are moving apart and the circuit is active, and during and after the period in which the contacts are fully open while the circuit remains active.

[0058] Referring to FIG. 8, voltage spike 41 in the voltage V_c(t) is produced shortly after the contacts begin to open at the start of time period 42. Time period 42 defines a first phase of contacts moving apart. Voltage spike 41 is produced when the increasing voltage at the output of voltage divider 26, being fed to gate G1 , turns power transistor Q1 on. With Q1 on, and the contacts just barely open, solenoid load current is diverted away from the trip contacts and into the charging of capacitor C1. In time period 43 the contacts go from closed to fully open. Within time period 43, following the voltage spike, is a second phase of contacts moving apart. In this second phase is the beginning of a controlled increase in voltage across the contacts resulting from a controlled decrease in voltage at the output of voltage divider 26 being fed to gate G1. The controlled increase in voltage across the contacts continues through voltage ramp-up time period 44 to produce an overvoltage 45. Overvoltage 45 is a voltage in excess of battery voltage. The decrease in voltage at the output of voltage divider 26, and the creation of overvoltage 45, may be viewed as a result of the solenoid acting as a constant current source causing solenoid current to flow into capacitors C1 and C2. During time period 46, voltage across the contacts declines as solenoid current falls to zero. The declining voltage across the contacts reflects dissipation of remaining stored energy. Controlling voltage across the contacts continues through the end of time period 46.

[0059] The graph of FIG. 9 shows solenoid DC current Is_Dc(t) flowing through solenoid 15 during the initialization and the operation of circuit 10. The graph of FIG. 9 is a simulated oscilloscope trace over the same time period as FIGS. 7 and 8. [0060] Referring again to FIG. 8, voltage Vc(t) across the contacts in time period 44 increases at a rate that is in part determined by a time constant as described above. FIG. 9 shows a corresponding decrease in IsDc(t) in time period 44. [0061] The graph of FIG. 10 represents a simulated electric field gradient between the mechanical contacts of FIG. 1 , and a threshold electric field gradient needed to trigger dielectric breakdown associated with an arcing event, covering the period from when the contacts begin to move apart until the contacts are fully open. [0062] The graph of FIG. 11 represents a simulated electric field gradient between the mechanical contacts of FIG. 1 , and a threshold electric field gradient needed to trigger dielectric breakdown associated with an arcing event, covering the period from when the contacts begin to move apart until all energy is dissipated and the circuit has shut down.

4.0) Transistor Q1 Operating as a Variable Resistance

[0063] In one embodiment, transistor Q1 operates as a variable resistance having a resistance value that is inversely proportional to small changes in the value of the gate-to-source voltage. In one embodiment, transistor Q1 is an SMPS (Switch Mode Power Supply) IRFPS43N50K HEXFET® Power MOSFET, available from International Rectifier. IRFPS43N50K is also available from other vendors. [0064] In circuit 10, Q1 is used in an enhancement mode and in a common source configuration as a voltage-controlled variable resistor. FIG. 1 illustrates Q1 as having a variable resistor R5 effectively in series with current-limiting resistor R4. [0065] The resistance value of resistor R5 is controlled primarily by a gate-to- source voltage Vgs. For small variations (Δ variations) around the threshold Vt, saturation drain current Ids is expressed as:

Ids = 0.5(Vgs - Vt)2 [1 +LAMBDA^* Vds], Equation 1 wherein Vgs is the gate-source voltage, Vds is drain-source voltage, and LAMBDA is one tenth of the reciprocal of the channel length modulation parameter, i.e. a small constant number.

[0066] The output resistance R5 of Q1 (incremental drain source voltage ΔVds divided by incremental drain source current Δ Ids) is expressed as:

R5 = ΔVds/Δ Ids = 1/(0.5(Vgs - Vt)2)^* LAMBDA Equation 2

[0067] Solenoid 15 acts as an approximately constant current source, outputting load current IL until almost all load current energy is dissipated. In circuit 10, ISDC = IL, and IL is an approximately constant current. So the values of ISDC, Vt, LAMBDA, and L are all approximately constant. Because these values are all approximately constant, it can be seen from Equation 2 that the variable resistance R5 of Q1 , varies inversely in response to small changes in the value of Vgs. [0068] It is to be understood that the description of the foregoing exemplary embodiment(s) is (are) intended to be only illustrative, rather than exhaustive, of the present invention. Those of ordinary skill will be able to make certain additions, deletions, and/or modifications to the embodiment(s) of the disclosed subject matter without departing from the spirit of the invention or its scope, as defined by the appended claims.

Claims

What is claimed is:

1. A method for suppression of arcing across trip contacts in a protection relay of an AC power distribution system, the trip contacts being connected into a circuit for providing DC current to a solenoid of an AC power circuit breaker, the method comprising: providing a field effect transistor having a gate, a source, a drain, and a variable resistance, the source and the drain being coupled across the trip contacts; switching the transistor on when the trip contacts begin to open, thereby enabling DC current to flow through the transistor; storing DC current energy while the trip contacts are moving apart; and dissipating DC current energy in the transistor while the trip contacts are moving apart and after the trip contacts are done moving apart.

2. A method according to claim 1 , further comprising: controlling voltage across the trip contacts during and after the period in which the contacts are moving apart.

3. A method according to claim 2, further comprising: releasing stored DC current energy; and dissipating energy in the transistor after DC current ceases to flow through the solenoid.

4. A method according to claim 2, wherein the controlling of voltage across the trip contacts includes operating the field effect transistor to function as a voltage- controlled variable resistor.

5. A method according to claim 4, wherein the controlling of voltage across the trip contacts includes increasing the resistance of the variable resistance at a controlled rate.

6. A method according to claim 5, wherein increasing the resistance of the variable resistance is performed by decreasing the voltage applied to the gate.

7. A method according to claim 5, wherein the controlled rate is determined by the time constant of a capacitor and a resistance.

8. A method according to claim 2, wherein the controlling of the voltage across the contacts is performed until all DC current energy is dissipated.

9. A protection relay for use with an AC power circuit breaker in an AC power distribution system, the protection relay comprising: a power circuit for providing DC current to a solenoid of the AC power circuit breaker; trip contacts connected into the power circuit; an arc suppression circuit for suppressing arcing across the trip contacts, the arc suppression circuit comprising: a field effect transistor having a gate, a source, a drain, and a variable resistance, the source and the drain being coupled across the trip contacts; and a voltage control circuit for controlling voltage at the gate of the transistor, the voltage control circuit having an input coupled across the trip contacts and an output coupled to the gate.

10. The protection relay of claim 9, wherein when the trip contacts begin to open, DC current flows through the transistor.

1 1. The protection relay of claim 10, wherein the voltage control circuit decreases the voltage at the gate as the trip contacts open.

12. The protection relay of claim 1 1 , wherein the transistor is configured to function as a voltage-controlled variable resistor, the variable resistance being controlled by the voltage applied to the gate.

13. The protection relay of claim 12, wherein the resistance of the transistor increases as the voltage applied to the gate decreases.

14. The protection relay of claim 11 , wherein the voltage control circuit comprises a capacitor and a voltage divider, the voltage divider having an output, the output of the voltage divider being the output of the voltage control circuit.

15. The protection relay of claim 14, further comprising a storage device for storing DC current energy, the storage device being coupled across the trip contacts.

16. The protection relay of claim 15, wherein the storage device comprises a capacitor.