Phase-controlled static switch for a power circuit having a variable power factor

Claim 2

A gating control for an a-c phase-controlled static switch comprising a pair of thyristors connected in inverse parallel relationship in a power circuit that is operable with a power factor substantially different from unity, said gating control comprising: a. means for developing an off-time indicating signal that varies in magnitude as a direct function of the time interval that said thyristors both remain in their nonconducting state preceding the gating of one of said thyristors, b. means for supplying an effective input control signal to said gating control that has a magnitude indicative of the conduction angles desired for said thyristors, c. means for comparinG the conduction angles of said inverse parallel thyristors on successive conducting periods and for deriving a balance-indicating signal having a magnitude indicative of the difference between said conduction angles, d. summing means effective while said thyristors are in their nonconducting state preceding the firing of said one thyristor for adding said effective input control signal, said off-time indicating signal, and said balance-indicating signal together to provide a gating control signal that has a magnitude dependent upon the sum of said signals, and e. means for developing a gating pulse for delivery to said one thyristor when said gating control signal reaches a predetermined level, whereby said one thyristor is gated into conduction at an instant dependent on the magnitudes of said effective input control signal and said balance-indicating signal.

Claim 3

A gating control as defined in claim 1 in which: a. said effective input control signal for desired conduction angles less than 180* has a polarity opposite to that of said off-time indicating signal, and b. said balance-indicating signal has a polarity the same as said off-time indicating signal when the conduction angle of said one thyristor is longer than the conduction angle of the other of said thyristors.

Claim 4

A gating control as defined in claim 1 in which: on the next nonconducting period following firing of said one thyristor, a gating pulse is developed for delivery to said other thyristor at a time following the start of said next nonconducting period that: a. is dependent upon the sum of said effective input control signal and an off-time indicating signal representative of the duration of said next nonconducting period and b. is independent of any balance-indicating signal.

Claim 5

A gating control as defined in claim 1 in which the gating of said other thyristor is effected at a time following the start of the nonconducting period preceding said latter gating that is dependent on said effective input control signal but is independent of the difference in said conduction angles.

Claim 6

A gating control for an a-c phase-controlled, single-phase static switch comprising a pair of thyristors connected in inverse parallel relationship in a power circuit that has a variable power factor, said gating control comprising: a. off-time indicating means for developing an off-time indicating signal that varies in magnitude as a direct function of the time interval that said thyristors both remain in their nonconducting state preceding the gating of either of said thyristors, b. means for supplying an effective input control signal to said gating control that has a magnitude indicative of the conduction angles desired for said thyristors, c. summing means effective while said thyristors are in their nonconducting state preceding the gating of a thyristor for adding said effective input control signal and said off-time indicating signal together to provide a gating control signal that has a magnitude dependent upon the sum of said signals, d. means for developing a gating pulse for delivery to one of said thyristors when said gating control signal reaches a predetermined level, e. resetting means for returning said off-time indicating signal to zero when the forward voltage across said one thyristor disappears, irrespective of whether or not gating of said one thyristor has occurred during the period that forward voltage has appeared across said one thyristor, f. and means for developing during the next forward voltage period across the other of said thyristors another gating pulse for delivery to said other thyristor when said gating control signal reaches a predetermined level.

Claim 7

A gating control for an a-c phase-controlled, single-phase static switch comprising a pair of thyristors connected in inverse parallel relationship in a power circuit that has a variable power factor, said gating control comprising: a. off-time indicating means for developing an off-time indicating signal that varies in magnitude as a direct function of the time interval that said thyristors both remain in their nonconducting state preceding the gating of either of said thyristors, b. means for supplying an effective input control signal to said gating control that has a magnitude indicative of the conduction angles desired for said thyristors, c. summing means effective while said thyristors are in their nonconducting state preceding the gating of a thyristor for adding said effective input control signal and said off-time indicating signal together to provide a gating control signal that has a magnitude dependent upon the sum of said signals, d. means for developing a gating pulse for delivery to one of said thyristors when said gating control signal reaches a predetermined level, e. resetting means for returning said off-time indicating signal to zero when the forward voltage across said one thyristor disappears, irrespective of whether or not gating of said one thyristor has occurred during the period that forward voltage has appeared across said one thyristor, f. and means for developing during the next forward voltage period across the other of said thyristors another gating pulse for delivery to said other thyristor when said gating control signal reaches a predetermined level.

Claim 8

The gating control of claim 5 in which: a. control-power supply means is provided for supplying control power to said off-time indicating means independently of said power circuit, and b. means is provided for electrically insulating from said power circUit said input control signal-supplying means of (b) , claim 5.

Claim 9

The gating control of claim 5 in which: a. said means for developing said gating pulses comprises a level detector that receives said gating control signal and is operable in response to said gating control signal reaching said predetermined level, b. and permissive means is provided for preventing said level detector from effectively receiving said gating control signal so long as said switch is in a conducting state, whereby said level detector is prepared to properly respond on the next nonconducting period of said switch to a change in the gating control signal effectively received by said level detector.

Claim 10

The gating control of claim 5 in which: a. said means for developing said gating pulses comprises a crossing detector that receives said gating control signal and detects when said gating control signal passes through a threshold level slightly on one side of ., 0, b. said effective input control signal has a magnitude on the 0 side of said threshold level when said effective input control signal is calling for conduction angles less than 180*, c. and said gating control signal which results from the addition of said effective input control signal and said off-time indicating signal crosses said threshold level when said addition occurs if said effective input control signal is calling for conduction angles greater than 0* .

Claim 11

A gating control for an a-c phase-controlled single phase static switch comprising a pair of thyristors connected in inverse parallel relationship in a power circuit that has a variable power factor, said gating control comprising: a. means responsive to the voltage appearing across said switch for developing a first indicating signal of substantially constant magnitude whenever said two thyristors enter their nonconducting state, said first indicating signal beginning within a few electrical degrees after said thyristors enter their nonconducting state, even when voltage thereacross rises at the same rate as the source voltage in building up from a natural voltage 0, b. a frequency attenuator to which said first indicating signal is supplied for producing an off-time indicating signal that progressively increases in magnitude while said first indicating signal is received by said attenuator, c. summing means to which are supplied said off-time indicating signal, a constant magnitude biasing signal of a polarity opposite to that of said off-time indicating signal, and an input control signal of the same polarity as said off-time indicating signal; said summing means having an output which is representative of the sum of said biasing signal, said off-time indicating signal, and said input control signal, d. means responsive to the output of said summing means for supplying a gating pulse to the gate of one of said thyristors when said summing means output reaches a predetermined threshold level, e. said summing means output reaching said predetermined threshold level in a time commencing with entry of said thyristors into their nonconducting state that remains substantially constant for a given magnitude input control signal despite changes in the power factor of said power circuit.

Claim 12

A gating control for an a-c phase-controlled single phase static switch comprising a pair of thyristors connected in inverse parallel relationship in a power circuit that has a variable power factor, said gating control comprising: a. means responsive to the voltage appearing across said switch for developing a first indicating signal of substantially constant magnitude whenever said two thyristors enter their nonconducting state, said first indicating signal beginning within a few electrical degrees after said thyristors enter their nonconducting state, even when voltage thereacross rises at the same rate as the source voltage in building up from a natural voltage 0, b. a frequency attenuator to which said first indicating signal is supplied for producing an off-time indicating signal that progressively increases in magnitude while said first indicating signal is received by said attenuator, c. summing means to which are supplied said off-time indicating signal, a constant magnitude biasing signal of a polarity opposite to that of said off-time indicating signal, and an input control signal of the same polarity as said off-time indicating signal; said summing means having an output which is representative of the sum of said biasing signal, said off-time indicating signal, and said input control signal, d. means responsive to the output of said summing means for supplying a gating pulse to the gate of one of said thyristors when said summing means output reaches a predetermined threshold level, e. said summing means output reaching said predetermined threshold level in a time commencing with entry of said thyristors into their nonconducting state that remains substantially constant for a given magnitude input control signal despite changes in the power factor of said power circuit.

Claim 13

The gating control of claim 9 in which: a. said means responsive to the output of said summing means comprises a level detector that is adapted to receive said output, and b. permissive means is provided for preventing said level detector from effectively receiving said output so long as said switch is in a conducting state, whereby said level detector is prepared to properly respond on the next nonconducting period of said switch to a change in said output.

Claim 14

The gating control of claim 9 in combination with: a. means for comparing the conduction angles of said thyristors on successive conducting periods and for deriving a balance-indicating signal having a magnitude indicative of the difference in said conduction angles, and b. means for controlling the time required for said summing means output to reach said threshold level as a function of said balance-indicating signal.

Claim 15

A gating control for an a-c phase-controlled static switch comprising a pair of thyristors connected in inverse parallel relationship in a power circuit that is operable with a power factor substantially different from unity, said gating control comprising: a. means for developing an off-time indicating signal that varies in magnitude as a direct function of the time interval that said thyristors both remain in their non-conducting state preceding the gating of one of said thyristors, b. means for supplying an effective input control signal to said gating control that has a magnitude indicative of the conduction angles desired for said thyristors, c. means for sensing any d-c component of current in said power circuit resulting from current asymmetry and for deriving a balance-indicating signal having a magnitude indicative of the magnitude of said d-c component, d. summing means effective while said thyristors are in their non-conducting state preceding the firing of said one thyristor for adding said effective input control signal, and said balance-indicating signal together to provide a gating control signal that has a magnitude dependent upon the sum of said signals, and e. means for developing a gating pulse for delivery to said one thyristor when said gating control signal reaches a predetermined level, whereby said one thyristor is gated into conduction at an instant dependent on the magnitudes of said effective input control signal and said balance-indicating signal.

Claim 16

A gating control for an a-c phase-controlled static switch comprising a pair of thyristors connected in inverse parallel relationship in a power circuit that is operable with a power factor substantially different from unity, said gating control comprising: a. means for developing an off-time indicating signal that varies in magnitude as a direct function of the time interval that said thyristors both remain in their non-conducting state preceding the gating of one of said thyristors, b. means for supplying an effective input control signal to said gating control that has a magnitude indicative of the conduction angles desired for said thyristors, c. means for sensing any d-c component of current in said power circuit resulting from current asymmetry and for deriving a balance-indicating signal having a magnitude indicative of the magnitude of said d-c component, d. summing means effective while said thyristors are in their non-conducting state preceding the firing of said one thyristor for adding said effective input control signal, and said balance-indicating signal together to provide a gating control signal that has a magnitude dependent upon the sum of said signals, and e. means for developing a gating pulse for delivery to said one thyristor when said gating control signal reaches a predetermined level, whereby said one thyristor is gated into conduction at an instant dependent on the magnitudes of said effective input control signal and said balance-indicating signal.

Claim 17

A gating control as defined in claim 12 in which the gating of said other thyristor is effected at a time following the start of the non-conducting period preceding said latter gating that is dependent on said effective input control signal but is independent of any balance-indicating signal.

Claim 18

A gating control for an a-c static switch comprising a pair of thyristors connected in inverse parallel relationship in a power circuit that is operable with a power factor substantially different from unity, said gating control comprising: a. means for developing gating pulses for delivery to alternate thyristors at predetermined gating angles during successive half cycles of voltage appearing across said switch, thus causing said alternate thyristors successively to conduct for predetermined conduction angles, b. balancing means for measuring and comparing the conduction angles of said alternate thyristors on successive conducting periods and for deriving a balance-indicating signal having a magnitude indicative of the difference between said conduction angles, c. said balancing means comprising: (c'') means for developing first and second signals of opposite polarities each having a magnitude substantially proportional to the conduction-angle of the conducting one of said thyristors on successive conduction periods, and (c'''') means for adding together said opposite polarity signals to derive a signal equal to the difference between said opposite polarity signals which serves as said balance-indicating signal, d. and means sensitive to said balance-indicating signal for varying the gating angles at which said gating pulses are delivered as a function of the magnitude of said balance-indicating signal.

Claim 19

A gating control for an a-c static switch comprising a pair of thyristors connected in inverse parallel relationship in a power circuit that is operable with a power factor substantially different from unity, said gating control comprising: a. means for developing gating pulses for delivery to alternate thyristors at predetermined gating angles during successive half cycles of voltage appearing across said switch, thus causing said alternate thyristors successively to conduct for predetermined conduction angles, b. balancing means for measuring and comparing the conduction angles of said alternate thyristors on successive conducting periods and for deriving a balance-indicating signal having a magnitude indicative of the difference between said conduction angles, c. said balancing means comprising: (c'') means for developing first and second signals of opposite polarities each having a magnitude substantially proportional to the conduction-angle of the conducting one of said thyristors on successive conduction periods, and (c'''') means for adding together said opposite polarity signals to derive a signal equal to the difference between said opposite polarity signals which serves as said balance-indicating signal, d. and means sensitive to said balance-indicating signal for varying the gating angles at which said gating pulses are delivered as a function of the magnitude of said balance-indicating signal.