GB2528909A

GB2528909A - An AC to AC converter and a control system therefor

Info

Publication number: GB2528909A
Application number: GB1413786.3A
Authority: GB
Inventors: Thomas George Logan; Richard Anthony Mcmahon
Original assignee: HTIP Ltd
Current assignee: HTIP Ltd
Priority date: 2014-08-04
Filing date: 2014-08-04
Publication date: 2016-02-10
Anticipated expiration: 2034-08-04
Also published as: WO2016020285A2; GB201413786D0; WO2016020285A3; GB2528909B

Abstract

An AC-to-AC converter 10 has a half-bridge circuit 20 with first 22 and second 24 arms. Each arm has a pair of switches 26a, 26b, 28a, 28b. The converter has a control system 30 that includes a zero crossing detector 52, and/or a phase estimator 54. The zero-crossing detector is arranged to determine when an input AC voltage approaches zero volts. When the input AC voltage approaches zero volts the drive signal generator produces drive signals to turn off both switches in one of the first or second arms and to turn on both switches in the other of the first or second arms. The phase estimator estimates a phase of an input AC voltage, and a drive signal generator producing drive signals for each switch in the first and second arms according to a duty cycle ratio. In an embodiment, when an excess current condition is detected the drive signal generator reduces the duty cycle ratio. The converter may be used for regulating the mains voltage supplied to a residential property.

Description

TITLE

AN AC TO AC CONVERTER AND A CONTROL SYSTEM THEREFOR

Field of the Invention

[1] The present invention relates to AC to AC converters and in particular to control systems for such converters.

Background and Prior Art

[2] This invention relates to alternating current (Ac) voltage converters and in particular to control systems for such converters.

[3] AC to AC converters typically operate by taking an input AC voltage into a full bridge or half bridge chopping circuit. The chopping circuit is controlled under high-frequency pulse-width modulation or pulse-amplitude modulation to produce a chopped version of the AC input voltage. This chopped AC voltage is then filtered to produce an output voltage having the desired characteristics.

[4] One example of an AC converter is described in US patent 8,354,829 in which an AC input voltage is chopped at high frequency to produce an AC output voltage. The control system described in US patent 8,354829 utilises pulse-width modulation (PWM) to control the chopping circuit. A reference signal is produced by a reference oscillator that is phase-locked to the AC input voltage. An error between the reference signal and the A/c output voltage is used as a control signal to determine the PWM characteristics.

[5] One application for an AC converter operating at line voltage is regulating the AC voltage supplied to a premises such as a residential property. Regulating the AC voltage to a property provides energy savings to the power distributor by supplying an acceptable voltage to the property which is lower than the distributed voltage. For example many countries have a line voltage requirement of 220-250V AC rms. Appliances are therefore designed to operate anywhere within this region and power distributors typically distribute 230-240V. By providing an AC converter which supplies a property with 220V the power distributor can derive power savings related to the voltage difference between the distributed voltage and the supplied voltage.

[6] One difficulty of using chopping circuits directly at line voltages is the high voltage and rn current present. This is particularly challenging during zero crossings of the line voltage when the polarity of the voltage switches from positive to negative or vice versa. The operating state (on or off) of semiconductor devices used as switches in the chopping circuit must be carefully controlled to avoid damage to the semiconductor devices when the voltage polarity switches.

[7] A further difficulty with semiconductor-based chopping circuits operating directly at line voltage is the requirement to provide safety characteristics within the device in the event of a short circuit of a downstream appliance such as a fused motor. Such a short circuit can damage the semiconductor devices due to excessive current passing through the device. Providing a failsafe cut-out in the device is not necessarily desirable as the device is supplying the entire property and would lead to a loss of power throughout the io property. Nevertheless, it is required to provide some safety features within the device.

[8] It is an object of this invention to provide AC converter which provides a solution to the above problems. Other objects and benefits of the invention will be apparent to the

reader from the description below.

Summary of the Invention

[9] In accordance with a first aspect of the invention there is provided a control system for an AC-to-AC converter having a half-bridge circuit with first and second arms, each arm having a pair of switches, the control system comprising: a zero-crossing detector arranged to determine when an input AC voltage approaches zero volts; a drive signal generator producing drive signals for each switch in the first and second arms, the drive signal generator being responsive to the zero-crossing detector whereby when the zero-crossing detector determines an input AC voltage approaches zero volts) the drive signal generator produces drive signals to turn off both switches in one of the first or second arms and to turn on both switches in the other of the first or second arms.

[1O]Preferably, the control system further comprises a phase estimator for estimating a phase of the input AC voltage, the zero-crossing detector determining the input AC voltage is approaching zero volts when the estimated phase is within a threshold value of 180° or 360°.

[11]Preferably, the zero-crossing detector is further arranged to determine when the input AC voltage crosses zero volts, wherein the phase estimator adjusts the estimated phase when the zero-crossing detector determines the input AC voltage crosses zero volts.

[12]Preferably, the phase estimator increments the estimated phase periodically.

s [13]ln one arrangement, the zero-crossing detector comprises a comparator responsive to the input AC voltage and a threshold voltage) whereby when the magnitude of the input AC voltage is less than the threshold voltage the comparator determines the input AC voltage is approaching zero volts.

[14]Preferably, the drive signal generator determines a desired output AC voltage from the in phase estimate and a desired output voltage magnitude, the drive signal generator determining an output error from the desired output AC voltage and a measured output AC voltage, the drive signal generator determining a normalized output error normalized to the magnitude of sin(estimated phase)to the magnitude of sin(estimated phase), the drive signal generator including a proportional-integral control system responsive to the normalized error to determine a duty cycle ratio, the drive signal generator produces drive signals for the switches using the duty cycle ratio.

[15]Preferably, the control system further comprises a current sensor, the control system detecting whether an excess current condition exists from the current sensor, wherein when an excess current condition is detected the drive signal generator reduces the duty cycle ratio.

[16]Preferably, the second arm is connected to a neutral line of the input AC voltage, the control system further comprises a bypass switch connecting an input of the AC-to-AC converter to an output of the AC-to-AC converter) wherein when an excess current condition is detected and the input AC voltage approaches zero, the drive signal generator produces drive signals to turn off both switches in the first arm and to turn on both switches in the second arm, the drive signal generator determining from the current sensor when the current approaches zero and produces drive signals to turn off all switches in both arms, wherein the control system closes the bypass switch once the drive signal generator has turned off all switches.

[17]Preferably, the control system further comprises a cut-out switch provided at the input to the converter, wherein when a timer reaches a preset limit, the control system opens the cut-out switch to disconnect the input AC voltage from the converter.

[18]Preferably, the preset limit is chosen to provide enough time to allow a downstream circuit breaker or fuse to activate before the cut-out switch is opened.

[19]ln accordance with a further aspect of the invention there is provided a control system for an AC-to-AC converter having a half-bridge circuit with first and second arms, each arm having a pair of switches, the control system comprising: a phase estimator for estimating a phase of an input AC voltage; Jo a drive signal generator determining a desired output AC voltage from the phase estimate and a desired output voltage magnitude, the drive signal generator determining an output error from the desired output AC voltage and a measured output AC voltage, the drive signal generator determining a normalized output error normalized to the magnitude of sin(estimated phase)to the magnitude of sin(estimated phase), the drive signal generator including a proportional-integral control system responsive to the normalized error to determine a duty cycle ratio, the drive signal generator produces drive signals for the switches using the duty cycle ratio.

[20]Preferably, the control system further comprises a current sensor, the control system detecting whether an excess current condition exists from the current sensor, wherein when an excess current condition is detected the drive signal generator reduces the duty cycle ratio.

[21]Preferably, the second arm is connected to a neutral line of the input AC voltage, the control system further comprises a bypass switch connecting an input of the AC-to-AC converter to an output of the AC-to-AC converter, wherein when an excess current condition is detected and the input AC voltage approaches zero, the drive signal generator produces drive signals to turn off both switches in the first arm and to turn on both switches in the second arm, the drive signal generator determining from the current sensor when the current approaches zero and produces drive signals to turn off all switches in both arms, wherein the control system closes the bypass switch once the drive signal generator has turned off all switches.

[22]Preferably, the control system further comprises a cut-out switch provided at the input to the converter, wherein when a timer reaches a preset limit, the control system opens s the cut-out switch to disconnect the input AC voltage from the converter.

[23]Preferably, the preset limit is chosen to provide enough time to allow a downstream circuit breaker or fuse to activate before the cut-out switch is opened.

[24]ln accordance with a further aspect of the invention there is provided a control system for an AC-to-AC converter having a half-bridge circuit with first and second arms, each in arm having a pair of switches, the control system comprising: a drive signal generator producing drive signals for the switches using a duty cycle ratio; a current sensor, the control system detecting whether an excess current condition exists from the current sensor, wherein when an excess current condition is is detected the drive signal generator reduces the duty cycle ratio.

[25]Preferably, the second arm is connected to a neutral line of the input AC voltage, the control system further comprises a bypass switch connecting an input of the AC-to-AC converter to an output of the AC-to-AC converter and a zero-crossing detector arranged to determine when an input AC voltage approaches zero volts, wherein when an excess current condition is detected and the input AC voltage approaches zero, the drive signal generator produces drive signals to turn off both switches in the first arm and to turn on both switches in the second arm, the drive signal generator determining from the current sensor when the current approaches zero and produces drive signals to turn off all switches in both arms, wherein the control system closes the bypass switch once the drive signal generator has turned off all switches.

[26]Preferably, the control system further comprises a cut-out switch provided at the input to the converter, wherein when a timer reaches a preset limit, the control system opens the cut-out switch to disconnect the input AC voltage from the converter.

[27]Preferably, the preset limit is chosen to provide enough time to allow a downstream circuit breaker or fuse to activate before the cut-out switch is opened.

[28]ln accordance with a further aspect of the invention there is provided a method for controlling an AC-to-AC converter having a half-bridge circuit with first and second arms, s each arm having a pair of switches) comprising the steps of: determining when an input AC voltage approaches zero volts; turning off both switches in one of the first or second arms and turning on both switches in the other of the first or second arms when the input AC voltage approaches zero volts.

Jo [29]Preferably, the step of determining when an input AC voltage approaches zero volts comprises estimating a phase of the input AC voltage and determining the input AC voltage is approaching zero volts when the estimated phase is within a threshold value of 18O or 36O.

[30]Preferably, the method further comprises the step of determining when the input AC is voltage crosses zero volts and adjusting the estimated phase.

[31]Preferably, the method further comprises the steps of: determining a desired output AC voltage from the phase estimate and a desired output voltage magnitude; determining an output error from the desired output AC voltage and a measured output AC voltage; determining a normalized output error normalized to the magnitude of sin(estimated phase)to the magnitude of sin(estimated phase); determining a duty cycle ratio from the normalized error; and controlling the switches using the duty cycle ratio.

[32]Preferably, the method further comprises the step of reducing the duty cycle ratio when an excess current condition is detected.

[33]Preferably, the second arm of the converter is connected to a neutral line of the input AC voltage, the method further comprises the steps of: turning off both switches in the first arm and turning on both switches in the second arm when an excess current condition is detected and the input AC voltage approaches zero; turning off all switches in both arms when the current approaches zero; and closing a bypass switch connecting an input of the AC-to-AC converter to an output Jo of the AC-to-AC converter.

[34]Preferably, the method further comprises the step of disconnecting the AC to AC converter from the input AC voltage if the excess current condition persists longer than a preset interval.

[35]Preferably, the preset limit is chosen to provide enough time to allow a downstream is circuit breaker or fuse to activate before the cut-out switch is opened.

[36]ln accordance with a further aspect of the invention there is provided a method for controlling an AC-to-AC converter having a half-bridge circuit with first and second arms, each arm having a pair of switches, comprising the steps of: determining a desired output AC voltage from the phase estimate and a desired output voltage magnitude; determining an output error from the desired output AC voltage and a measured output AC voltage; determining a normalized output error normalized to the magnitude of sin(estimated phase); determining a duty cycle ratio from the normalized error; and controlling the switches using the duty cycle ratio.

[37]Preferably, the method further comprises the step of reducing the duty cycle ratio when an excess current condition is detected.

[38]Preferably, the second arm of the converter is connected to a neutral line of the input s AC voltage) the method further comprises the steps of: turning off both switches in the first arm and turning on both switches in the second arm when an excess current condition is detected and the input AC voltage approaches zero; turning off all switches in both arms when the current approaches zero; and in closing a bypass switch connecting an input of the AC-to-AC converter to an output of the AC-to-AC converter.

[39]Preferably, the method further comprises the step of disconnecting the AC to AC converter from the input AC voltage if the excess current condition persists longer than a preset interval.

is [40]Preferably, the preset limit is chosen to provide enough time to allow a downstream circuit breaker or fuse to activate before the cut-out switch is opened.

[41]ln accordance with a further aspect of the invention there is provided a method for controlling an AC-to-AC converter having a half-bridge circuit with first and second arms, each arm having a pair of switches, the comprising: controlling the switches using a duty cycle ratio; reducing the duty cycle ratio when an excess current condition is detected.

[42]Preferably, the method further comprises the steps of: turning off both switches in one of the first or second arms and turning on both switches in the other of the first or second arms when an excess current condition is detected and the input AC voltage approaches zero; turning off all switches in both arms when the current approaches zero; and closing a bypass switch connecting an input of the AC-to-AC converter to an output of the AC-to-AC converter.

[43]Preferably, the method further comprises the step of disconnecting the AC to AC s converter from the input AC voltage if the excess current condition persists longer than a preset interval.

[44]Preferably, the preset limit is chosen to provide enough time to allow a downstream circuit breaker or fuse to activate before the cut-out switch is opened.

Brief Description of the Figures

io [45]The invention will now be described, by way of example, with reference to the accompanying drawings, in which: [46]Figure 1 shows an AC to AC converter according to one embodiment of the invention; [47]Figure 2 shows operation of the switches in the AC to AC converter as the input AC voltage varies; is [48]Figure 3 is a block diagram of a portion of the drive signal generator used to determine a duty cycle ratio from the AC to AC converter of Figure 1; [49]Figure 4 is a flowchart of the operating procedure of the AC to AC converter of Figure 1; [50]Figure S is a flowchart of the enter bypass mode procedure of the AC to AC converter of Figure 1.

Description of Preferred Embodiments

[51]Figure 1 shows an AC to AC converter 10 according to one embodiment of the invention.

The converter 10 is intended for use in controlling the AC voltage supplied to a property such as a residential house. The converter 10 shown in Figure 1 typically operates between a neutral input line 12 and a live input line 14 from a power distributor and a neutral output line 16 and a live output line 18 which connect to a property. It should be -10 -appreciated that in other embodiments, the converter 10 may be adapted for use in more than one phase of an AC supply system.

[52]The converter 10 comprises a half-bridge chopping circuit 20 having a first arm 22 and a second arm 24. The first arm 22 has a pair of semiconductor switches 26a and 2b s provided therein. Similarly, the second arm 24 has a pair of semiconductor switches 28a and 28b provided therein. In the embodiment, the semiconductor switches are IGBT devices although other suitable semiconductor devices may be used. The semiconductor switches 26a, 2Gb, 28a and 28b are N-type devices. Each semiconductor switch 26a, 2Gb, 28a and 28b has a reverse conduction diode provided across its terminals as shown in io figure 1. The semiconductor switches 26a, 2Gb, 28a and 28b operate under the control of a controller 30 via a gate drive circuit 32.

[53]Where the first and second arms meet at 34, an output filter 36 is provided. The output of the filter 36 is the output AC voltage provided at the live output line 18. The other end of the arm 24 is connected to the neutral line 12. The other end of the arm 22 is is connected to the live AC input line 14 via an input filter 38.

[54]An input voltage sensor 40 is provided between the live input line 14 in the neutral input line 12, after the input filter 38. An output voltage sensor 42 is provided between the live output line 18 and the neutral input line 16, after the output filter 36, so that the sensed output voltage has high-frequency components produced from the chopping circuit 20 removed by the output filter 36. Further, any phase shift caused by the output filter 36 and chopping circuit 20 will be measured by the output voltage sensor 42 and is then reduced by the control system 30. An input current sensor 44 is provided in the live A/C input line 14 and an output current sensor 46 is provided in the neutral output line 16.

[55]The input voltage sensor 40, the output voltage sensor 42, the input current sensor 44 and the output current sensor 46 each provide signals to the control system 30 indicative of the input voltage, output voltage, input current and output current, respectively.

-11 - [56]A pair of cut-out switches 48 are provided, one in the live input line 14 on the other in the neutral input line 12. A bypass switch 50 is provided which connects the live input line 14 to the live output line 18 when closed. The cut-out switches 48 and the bypass switch 50 are relays in the embodiment operating under control of the control system 30 as will be described in more detail below.

[57]The controller 30 includes a zero crossing detector 52, a phase estimator 54, an excess current detector 56 and a drive signal generator 58, the operation of which will be described in detail below. In one embodiment the controller 30 is implemented as a microcontroller and the zero crossing detector 52, phase estimator 54, excess current in detector 56 and drive signal generator 58 are implemented as embedded code on the microcontroller. As would be apparent to a person skilled in the art, other implementations are possible such as equivalent circuits.

[58]ln one embodiment, the controller 10 includes at least one temperature sensor (not shown) for determining the operating temperature of the switches 26a, 26b, 28a, 28b.

is [59]The typical operating mode of the converter 10 will be referred to as regulate mode, in which the converter 10 is regulating the voltage supplied to the live output line 18 as described in detail below.

[60]ln regulate mode, the cut-out switches 48 are closed and the bypass switch 50 is open.

The input AC voltage at the live input line 14 passes through the input filter 38 to the chopping circuit 20. The switches 26a, 2Gb, 28a and 28b, operate under control of the drive signal generator 58 to produce a chopped AC signal at 34. The output filter 36 removes the high-frequency components of the chopped AC signal to produce an AC output voltage at the live output line 18.

[61]The drive signal generator 58 produces drive signals for the switches 26a, 26b, 28a and 28b according to whether the AC voltage is in one of three states: positive half-cycle, negative half-cycle, or zero-crossing, as shown in Figure 2.

[62]During the positive half-cycle, when the AC input voltage is positive, the drive signal generator 58 produces drive signals such that the switches 26b and 28b are on. The drive signal generator 58 produces drive signals such that the switches 26a and 28a are -12 -alternately switched using PWM according to a duty cycle p. The switch 26a is on and the switch 28a is off with a duty cycle p, and the switch 28a is on and the switch 26a is off with a duty cycle 1-p.

[63]During the negative half-cycle) when the AC input voltage is negative) the drive signal generator 58 produces drive signals such that the switches 26a and 28a are on. The drive signal generator 58 produces drive signals such that the switches 26b and 28b are alternately switched using PWM according to a duty cycle p. The switch 2Gb is on and the switch 28b is off with a duty cycle p. and the switch 28b is on and the switch 2Gb is off with a duty cycle 1-p.

[64]During the zero crossing portion of a cycle, the drive signal generator 58 produces drive signals such that the switches 28a and 28b are off and the switches 26a and 2Gb on.

[65]The zero crossing detector 52 determines when a zero crossing portion of a cycle is present with reference to the phase estimator 54. The zero crossing detector 52 checks whether an instantaneous phase estimate from the phase estimator 54 is close to a zero crossing, such as when the phase estimate is within a threshold value of 180 or 360 degrees of phase. If the phase estimate is within the threshold value) a zero crossing is determined by the zero crossing detector 52.

[66]The drive signal generator 58 responds to the zero crossing detector 52's determination of a zero crossing by producing drive signals such that the switches 28a and 28b are off and the switches 26a and 26b are on, as described above. During the zero crossing portion of a cycle, the switches 26a, 2Gb in the upper arm allow voltage and current to pass from the input to the output of the converter 10 without regulation. Whilst operating in the zero crossing state reduces the regulation effectiveness of the converter 10, the magnitude of the voltage close to the zero crossing is small and so the reduction in regulation performance is minimal. The switch configuration of the zero crossing portion of a cycle prevents the switches 26a, 26b, 28a and 28b from damage that would occur if PWM were continued during a reversing of the voltage polarity.

[67]The zero crossing detector 52 determines when the zero crossing portion of a cycle is completed in a similar manner. The zero crossing detector 52 checks whether the phase -13 -estimate from the phase estimator 54 has exceeded a threshold value beyond 180 or 360 degrees of phase, in which case a zero crossing portion of a cycle is complete. The drive signal generator 58 responds to the zero crossing detector 52's determination of a zero crossing portion being complete by entering either positive half-cycle or negative half-cycle operation according to the polarity of the input voltage determined from the input voltage sensor 40.

[68]Example threshold values for zero crossing detection are 1-2 degrees of phase either side of 180 or 360 degrees. It would be appreciated by those skilled in the art that larger threshold values increase the region when regulation performance is reduced but io present a simpler design requirement on the accuracy of the phase estimator 52.

Conversely, smaller threshold values decrease the region when regulation performance is reduced but present a more stringent design requirement on the accuracy of the phase estimator 52. Other values than the examples given above can be used according to the desired design criteria.

is [69]ln one embodiment, an external quartz crystal (not shown) is used to provide a clock signal to the controller 10. The phase estimator 54 periodically updates its instantaneous phase estimate based on the clock signal. For instance, a main loop may execute 20,000 times each second, and during each loop the phase estimator 54 updates its phase estimate. The amount of each phase update is 360 degrees * / where is the frequency of the AC voltage and is the frequency of the phase updates, 20,000 Hz in this case. For an AC voltage frequency of 50 Hz, this gives a phase update increment of 0.9 degrees of phase.

[70]The zero crossing detector 52 determines a zero crossing has occurred when the polarity of the input voltage sensor 40 changes during a zero crossing portion of a cycle. The first such polarity change is used to prevent multiple triggering in the presence of noise on the input AC voltage. When a zero crossing is determined, the phase estimator 54 updates its phase estimate. The zero crossing corresponds to a phase of either 180 or 360 degrees depending on whether the previous cycle was a positive or negative half-cycle, respectively. The phase estimator 54 calculates the difference between its phase estimate and either 180 or 360 degrees phase according to the respective part of the cycle the AC voltage is in. This difference represents a phase error, and the phase -14 -estimator 54 updates its phase estimate by an amount proportional to the phase error, such as by 80-95% of the phase error to provide some dampening of the phase correction. This keeps the phase estimator 54's phase estimate synchronised with the input AC voltage.

s [71]The phase estimator 54 may be configured to update its phase estimate during positive to negative zero crossings, negative to positive crossings, or both. Updating the phase estimator 54's phase estimate at zero crossings keeps phase synchronisation with the input AC voltage without using an analog phase-locked loop (PLL) which would pass noise within the PLL filter bandwidth through to the output.

o [72]Applying alternate PWM to switches in both arms 22 and 24 during each half-cycle allows current to flow from the input to the output or vice versa. This enables the converter 10 to provide a regulated AC voltage across a wide range of power factors.

Similarly, by turning both switches 26a, 26b in the arm 22 on during the zero crossing portion of a cycle, current can still flow in either direction.

i5 [73]The drive signal generator 58 determines the duty cycle as follows. The phase estimator 54 provides a phase estimate, shown in Figure 3 at 102. At 104, the drive signal generator 58 determines an amplitude of a sinusoid of a phase equal to the phase estimate -i.e. the amplitude of sin(estimated phase) -from the phase estimator 54.

The drive signal generator 58 then multiplies this amplitude with a target output voltage magnitude stored in the controller 30 at 106 to produce a target output voltage.

[74]The drive signal generator 58 subtracts the target output voltage from the output voltage determined from output voltage sensor 42 to produce an error value at 108.

Next, the drive signal generator 58 determines a normalised output error at 110 by dividing the error value by a normalising value. The normalising value is produced from the sinusoid amplitude determined at 104, with a normalising minimum value applied at 112. The normalising minimum value prevents the normalising value from approaching zero whereby the division at 110 would overflow. Normalising the error value improves the performance of the controller 10 during parts of the voltage supply when the magnitude of the voltage is small. A given absolute error value represents a greater $0 percentage error during regions where the desired output voltage is small than at the -15 -peak output voltage: normalising the error value by the magnitude of the sinusoid improves voltage regulation during regions where the magnitude of the output voltage is small.

[75]The normalised output error determined at 110 is used by the drive signal generator 58 as a control signal in a proportional-integral (P1) controller shown generally at 114.

Proportional-integral controllers are one form of PID control systems which as well known in the art, and it would be appreciated by the skilled addressee that other forms of PID controllers may be utilised. The P1 controller 114 includes an integral windup limit in the embodiment.

[76]The proportional and integral outputs of the P1 controller 114 are summed by the drive signal generator 58 at 116 and the resulting value limited at 118 to produce a duty cycle ratio at 120. The duty cycle ratio is used by the drive signal generator 58 to produce drive signals as described above.

[77]Referring now to Figure 4, there is shown a flowchart of the operating sequence of the controller 30. Regulate mode, described above, is shown at 310.

[78]During operation in regulate mode, the converter 10 checks whether conditions are correct for operation in bypass mode at 312. These conditions include: a. The excess current detector 56 indicating the input current measured by the input current sensor 44 has been above a predetermined threshold value for a predetermined time.

b. In embodiments where at least one temperature sensor is provided, the sensed temperature of the switches is above a predetermined temperature for a predetermined time.

[79]ln other embodiments, more sophisticated conditions may be used. For example, in the case of the excess current detector 56, the predetermined time may be adjusted according to the amount by which the measured input current exceeds the predetermined threshold value. Thus an input current slightly above the threshold value may be permitted for a longer time before entering bypass mode, whereas a measured -16 -input current corresponding to a short-circuit condition where the measured input current exceeds the threshold value by a significant margin may enter bypass mode immediately or after a shorter predetermined time. This more sophisticated condition capability provides superior protection for the switches in the event of a short circuit by entering bypass mode rapidly.

[80]lf none of the conditions are satisfied, operation continues in regulate mode at 310.

Otherwise, the controller 30 enters bypass mode at 302.

[81]The procedure for entering bypass mode is as follows. First, the drive signal generator 58 increases the duty cycle ratio at a gradual rate until the output AC voltage is close to the input AC voltage, i.e. that regulation is no longer occurring. Typically, this gradual increase in the duty cycle rate occurs over 1-10 seconds.

[82]Next, the drive signal generator turns on the switches 26a and 26b and turns off the switches 28a and 28b. The controller 30 then closes the bypass switch 50.

[83]After entering bypass mode, the converter 10 operates in bypass mode at 304. In this is mode, the converter 10 allows voltage present at the live input line 14 to pass directly to the live output line 18 via the bypass switch 50.

[84]During operation in bypass mode, the controller 30 tests whether conditions are correct for operation in regulate mode at 306. These conditions include: i. The excess current detector 56 indicating the input current measured by the input current sensor 44 has been below a predetermined threshold value for a predetermined time.

ii. The phase corrections applied by the phase estimator 54 at zero crossing have been less than a predetermined amount for a predetermined number of zero crossings.

iii. In embodiments where at least one temperature sensor is provided, the sensed temperature of the switches is below a predetermined temperature for a predetermined time.

-17 - [85]lf none of the conditions are satisfied, operation continues in bypass mode at 306.

Otherwise, the controller 30 enters regulate mode at 308 in a similar manner to that described above for entering bypass mode.

[86]At power-on) shown in Figure 3 at 300, the controller 30 sets its operating mode to a bypass mode.

[87]Next, converter 10 enters bypass mode at 302. At initialisation, an alternative procedure for entering bypass mode is used since the controller will likely not have achieved phase synchronisation with the input AC voltage. This alternative procedure involves the drive signal generator 58 producing drive signals turn off the switches 26a, 26b, 28a, and 28b via the gate drive circuit 32. In addition, the controller 30 closes the cut-out switches 48 and the bypass switch 50.

[88]ln one embodiment, the controller 30 then measures the frequency of the input AC voltage, such as by determining the time between zero crossings, and uses this measured frequency to set the phase estimate update increment. In other embodiments, the frequency and the phase estimate update increment may be pre-set within the controller.

[89]Operation of the controller 30 then resumes as described above from operating in bypass mode at 304.

[90]The controller 30 includes an additional operating mode in the embodiment, namely a shutdown mode. During operation, the controller 30 checks whether the excess current detector 56 indicates an excess current condition corresponding to a possible short circuit according to the input current measured by the input current sensor 44. If such an excess current condition is found, the controller 30 enters shutdown mode as follows.

[91]The procedure for entering shutdown mode is shown in Figure 5. At 402 the controller 30 determines whether shutdown mode is being entered due to the excess current detector 56 indicating an excessive current condition is present which would indicate a short-circuit. If an excess current condition is present, the drive signal generator 58 of the controller 30 reduces the duty cycle ratio to limit the current at 404. For instance, the drive signal generator 58 may reduce the duty cycle ratio proportionally to the -18 -amount of excess current to reduce the current to within a safe operating range for the converter 10.

[92]Next, the controller 30 waits for the zero crossing detector 52 to indicate a zero crossing at 406. Thus, the current limiting phase at 404 lasts no longer than a half-cycle.

s [93]The drive signal generator 58 then produces drive signals such that the switches 28a and 28b are on and the switches 26a and 26b are off, at 408. By turning on the switches 28a and 28b, the live output is connected to the neutral line with the result that the current drawn by the load will reduce to zero.

[94]Next, the controller 30 waits for the output current sensed by the output current sensor in 46 to be below a threshold value corresponding to a near-zero current) at 410. Typical values of the threshold value are 50-lOOmA.

[95]When the current is below this threshold value, drive signal generator 58 then produces drive signals such that all of the switches 26a, 26b, 28a and 28b are off, at 412. Next, the controller 30 closes the bypass switch 50 at 414.

is [96]While in shutdown mode, if the controller 30 determines an excess current condition has been indicated for a predetermined time, the controller 30 opens the cut-out switches 48 to prevent damage to the converter 10. The predetermined time is chosen to be sufficiently long such that fuses, circuit breakers and residual current detectors have time to act preferentially to the converter 10. In the event that the controller 30 opens the cut-out switches 48, the controller 30 waits for a predetermined time, for example 30 seconds, then closes the cut-out switches 48. Alternatively, the controller 30 may be arranged to require manual reset once the cut-out switches 48 are opened.

[97]Otherwise, the controller 30 monitors the current via the input current sensor, whereby once the input current reduces to an acceptable level, the controller 30 reverts to operating in bypass mode. Since the bypass switch 50 is already closed, transition from shutdown mode to bypass mode is straightforward.

[98]Modifications and variations such as would be apparent to a person skilled in the art are within the scope of the invention.

Claims

-19 -CLAIMS1. A control system for an AC-to-AC converter having a half-bridge circuit with first and second arms, each arm having a pair of switches, the control system comprising: a zero-crossing detector arranged to determine when an input AC voltage approaches zero volts; a drive signal generator producing drive signals for each switch in the first and second arms, the drive signal generator being responsive to the zero-crossing detector whereby when the zero-crossing detector determines an input AC voltage approaches zero volts, the drive signal generator produces drive signals to turn off both switches in one of the first or second arms and to turn on both switches in the other of the first or second arms.
2. The control system of claim 1, further comprising a phase estimator for estimating a phase of the input AC voltage, the zero-crossing detector determining the input AC voltage is approaching zero volts when the estimated phase is within a threshold value of 180° or 360°.
3. The control system of claim 2, wherein the zero-crossing detector is further arranged to determine when the input AC voltage crosses zero volts, wherein the phase estimator adjusts the estimated phase when the zero-crossing detector determines the input AC voltage crosses zero volts.
4. The control system of claim 2 or 3, wherein the phase estimator increments the estimated phase periodically.
5. The control system of claim 1, wherein the zero-crossing detector comprises a comparator responsive to the input AC voltage and a threshold voltage, whereby when the magnitude of the input AC voltage is less than the threshold voltage the comparator determines the input AC voltage is approaching zero volts.
6. The control system of any one of claims 2 to 4, wherein the drive signal generator determines a desired output AC voltage from the phase estimate and a desired output voltage magnitude, the drive signal generator determining an output error from the -20 -desired output AC voltage and a measured output AC voltage, the drive signal generator determining a normalized output error normalized to the magnitude of sin(estimated phase), the drive signal generator including a proportional-integral control system responsive to the normalized error to determine a duty cycle ratio, the drive signal generator produces drive signals for the switches using the duty cycle ratio.
7. The control system of any one of claims ito 6, further comprising a current sensor, the control system detecting whether an excess current condition exists from the current sensor, wherein when an excess current condition is detected the drive signal generator reduces the duty cycle ratio.
8. The control system of claim 7, the second arm is connected to a neutral line of the input AC voltage, the control system further comprises a bypass switch connecting an input of the AC-to-AC converter to an output of the AC-to-AC converter, wherein when an excess current condition is detected and the input AC voltage approaches zero, the drive signal generator produces drive signals to turn off both switches in the first arm and to turn on is both switches in the second arm, the drive signal generator determining from the current sensor when the current approaches zero and produces drive signals to turn off all switches in both arms, wherein the control system closes the bypass switch once the drive signal generator has turned off all switches.
9. The control system of claim 8, further comprising a cut-out switch provided at the input to the converter, wherein when a timer reaches a preset limit, the control system opens the cut-out switch to disconnect the input AC voltage from the converter.
10. The control system of claim 9, wherein preset limit is chosen to provide enough time to allow a downstream circuit breaker or fuse to activate before the cut-out switch is opened.
11. A control system for an AC-to-AC converter having a half-bridge circuit with first and second arms, each arm having a pair of switches, the control system comprising: a phase estimator for estimating a phase of an input AC voltage; -21 -a drive signal generator determining a desired output AC voltage from the phase estimate and a desired output voltage magnitude, the drive signal generator determining an output error from the desired output AC voltage and a measured output AC voltage, the drive signal generator determining a normalized output error s normalized to the magnitude of sin(estimated phase), the drive signal generator including a proportional-integral control system responsive to the normalized error to determine a duty cycle ratio, the drive signal generator produces drive signals for the switches using the duty cycle ratio.
12. The control system of claim 11, further comprising a current sensor, the control system in detecting whether an excess current condition exists from the current sensor, wherein when an excess current condition is detected the drive signal generator reduces the duty cycle ratio.
13. The control system of claim 12, the second arm is connected to a neutral line of the input AC voltage, the control system further comprises a bypass switch connecting an is input of the AC-to-AC converter to an output of the AC-to-AC converter, wherein when an excess current condition is detected and the input AC voltage approaches zero, the drive signal generator produces drive signals to turn off both switches in the first arm and to turn on both switches in the second arm, the drive signal generator determining from the current sensor when the current approaches zero and produces drive signals to turn off all switches in both arms, wherein the control system closes the bypass switch once the drive signal generator has turned off all switches.
14. The control system of claim 13, further comprising a cut-out switch provided at the input to the converter, wherein when a timer reaches a preset limit, the control system opens the cut-out switch to disconnect the input AC voltage from the converter.
15. The control system of claim 14, wherein preset limit is chosen to provide enough time to allow a downstream circuit breaker or fuse to activate before the cut-out switch is opened.
16. A control system for an AC-to-AC converter having a half-bridge circuit with first and second arms, each arm having a pair of switches, the control system comprising: -22 -a drive signal generator producing drive signals for the switches using a duty cycle ratio; a current sensor, the control system detecting whether an excess current condition exists from the current sensor, wherein when an excess current condition is detected the drive signal generator reduces the duty cycle ratio.
17. The control system of claim 16, the second arm is connected to a neutral line of the input AC voltage, the control system further comprises a bypass switch connecting an input of the AC-to-AC converter to an output of the AC-to-AC converter, wherein when an excess current condition is detected and the input AC voltage approaches zero, the drive signal generator produces drive signals to turn off both switches in the first arm and to turn on both switches in the second arm, the drive signal generator determining from the current sensor when the current approaches zero and produces drive signals to turn off all switches in both arms, wherein the control system closes the bypass switch once the drive signal generator has turned off all switches.
18. The control system of claim 17, further comprising a cut-out switch provided at the input to the converter, wherein when a timer reaches a preset limit, the control system opens the cut-out switch to disconnect the input AC voltage from the converter.
19. The control system of claim 18, wherein preset limit is chosen to provide enough time to allow a downstream circuit breaker or fuse to activate before the cut-out switch is opened.
20. An AC converter having a control system according to any one of claims ito 19.
21. A method for controlling an AC-to-AC converter having a half-bridge circuit with first and second arms, each arm having a pair of switches, comprising the steps of: determining when an input AC voltage approaches zero volts; turning off both switches in one of the first or second arms and turning on both switches in the other of the first or second arms when the input AC voltage approaches zero volts.-23 -
22. The method of claim 21, wherein the step of determining when an input AC voltage approaches zero volts comprising estimating a phase of the input AC voltage and determining the input AC voltage is approaching zero volts when the estimated phase is within a threshold value of 180° or 360°.s
23. The method of claim 22, further comprising the step of determining when the input AC voltage crosses zero volts and adjusting the estimated phase.
24. The method of claim 22 or 23, further comprising the steps of: determining a desired output AC voltage from the phase estimate and a desired output voltage magnitude; Jo determining an output error from the desired output AC voltage and a measured output AC voltage; determining a normalized output error normalized to the magnitude of sin(estimated phase); determining a duty cycle ratio from the normalized error; and is controlling the switches using the duty cycle ratio.
25. The method of any one of claim 21 to 24, further comprising the step of reducing the duty cycle ratio when an excess current condition is detected.
26. The method of claim 25, wherein the second arm of the converter is connected to a neutral line of the input AC voltage, the method further comprises the steps of: turning off both switches in the first arm and turning on both switches in the second arm when an excess current condition is detected and the input AC voltage approaches zero; turning off all switches in both arms when the current approaches zero; and closing a bypass switch connecting an input of the AC-to-AC converter to an output of the AC-to-AC converter.-24 -
27. The method of claim 26, further comprising the step of disconnecting the AC to AC converter from the input AC voltage if the excess current condition persists longer than a preset interval.
28. The method of claim 27, wherein preset limit is chosen to provide enough time to allow a downstream circuit breaker or fuse to activate before the cut-out switch is opened.
29. A method for controlling an AC-to-AC converter having a half-bridge circuit with first and second arms, each arm having a pair of switches, comprising the steps of: determining a desired output AC voltage from the phase estimate and a desired output voltage magnitude; Jo determining an output error from the desired output AC voltage and a measured output AC voltage; determining a normalized output error normalized to the magnitude of sin(estimated phase); determining a duty cycle ratio from the normalized error; and is controlling the switches using the duty cycle ratio.
30. The method of claim 29, further comprising the step of reducing the duty cycle ratio when an excess current condition is detected.
31. The method of claim 30, wherein the second arm of the converter is connected to a neutral line of the input AC voltage, the method further comprises the steps of: turning off both switches in the first arm and turning on both switches in the second arm when an excess current condition is detected and the input AC voltage approaches zero; turning off all switches in both arms when the current approaches zero; and closing a bypass switch connecting an input of the AC-to-AC converter to an output of the AC-to-AC converter.-25 -
32. The method of claim 31, further comprising the step of disconnecting the AC to AC converter from the input AC voltage if the excess current condition persists longer than a preset interval.
33. The method of claim 32, wherein preset limit is chosen to provide enough time to allow a downstream circuit breaker or fuse to activate before the cut-out switch is opened.
34. A method for controlling an AC-to-AC converter having a half-bridge circuit with first and second arms, each arm having a pair of switches, the comprising: controlling the switches using a duty cycle ratio; reducing the duty cycle ratio when an excess current condition is detected.Jo
35. The method of claim 34, further comprising the steps of: turning off both switches in one of the first or second arms and turning on both switches in the other of the first or second arms when an excess current condition is detected and the input AC voltage approaches zero; turning off all switches in both arms when the current approaches zero; and closing a bypass switch connecting an input of the AC-to-AC converter to an output of the AC-to-AC converter.
36. The method of claim 35, further comprising the step of disconnecting the AC to AC converter from the input AC voltage if the excess current condition persists longer than a preset interval.
37. The method of claim 36, wherein preset limit is chosen to provide enough time to allow a downstream circuit breaker or fuse to activate before the cut-out switch is opened.