GB2535306B

GB2535306B - Thyristor battery chargers with improved control system

Info

Publication number: GB2535306B
Application number: GB1522684.8A
Authority: GB
Inventors: Malcilm Pike John
Original assignee: Norco Group Ltd
Current assignee: Norco Group Ltd
Priority date: 2014-12-23
Filing date: 2015-12-22
Publication date: 2019-09-25
Anticipated expiration: 2035-12-22
Also published as: GB201522684D0; GB2533599A; GB2535306A

Description

THYRISTOR BATTERY CHARGERS WITH IMPROVED CONTROL SYSTEM

The present invention relates to battery chargers, in particular battery chargers that employ thyristors to convert an AC mains supply to a DC source used to charge batteries.

Thyristor battery chargers use a controlled thyristor bridge rectifier to convert an AC mains supply to a DC source that is used to charge batteries. The AC mains voltage source is a sinusoidal waveform, which means that the output of the thyristor bridge rectifier varies non-linearly with respect to the firing angle of the thyristors. This leads to a variation in gain, reducing the overall performance of the rectifier. The system therefore needs to be optimised to maximise gain, and thus the control loop for all firing angles must also be optimised as this is dependent on the maximum gain. One issue arises in optimising for too high a gain as this produces oscillation in the control loop. Therefore it is necessary to optimise to a low value of maximum gain, resulting in a control loop that cannot be optimised for all firing angles of the thyristors.

The present invention aims to address this issue by providing a thyristor-controlled battery charger comprising: a thyristor-controlled rectifier comprising one or more thyristors; and a control system in connection with the thyristor-controlled rectifier and adapted to control the output of the battery charger; wherein when a non-linear demand control signal is applied to the thyristor-controlled rectifier the control system is adapted to apply a correction function to the signal to compensate for the nonlinearity.

The use of a correction function ensures a constant gain in the control system for all firing angles of the one or more thyristors, meaning that a higher gain can by used in the control system. The charger will operate over a wider range of input and output voltages, have a larger bandwidth and a faster response to step loads.

Preferably, the correction function is a linearization function.

Most preferably, the linearization function takes the form of:

where x is a firing angle of the one or more thyristors, Dem is the non-linear demand control signal, and Vmax is a maximum input voltage to the thyristor-controlled rectifier.

Preferably, the thyristor-controlled rectifier is a thyristor bridge.

The control system may comprise a microprocessor.

Preferably, the battery charger is powered by a single or three phase input.

In another aspect, the present invention provides a method of correcting the output of a thyristor-controlled battery charger comprising a thyristor-controlled rectifier, the method comprising the steps of: taking a non-linear demand control signal; generating a correction function; and applying the correction function to the nonlinear demand control signal to compensate for the non-linearity.

Preferably, the correction function is a linearization function.

Most preferably, the linearization function takes the form of:

The invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a battery charger system in accordance with an embodiment of the present invention;

Figure 2 is a chart of normalised voltage against time showing the firing angle;

Figure 3 is a chart of normalised voltage against angle in radians showing the firing angle; and

Figure 4 is a chart of voltage against angle in radians showing the correction function.

The present invention provides a battery charger with a control system comprising a linearization function to correct for the non-linearity of the thyristor-controlled rectifier. This improves the bandwidth of the battery charger and therefore the response to step changes in input voltages and output loads. The same approach may be used on all types of two and three phase battery chargers, and with single or three phase AC mains input voltage. Figure 1 is a schematic representation of a battery charger system in accordance with an embodiment of the present invention. The amplitude of the incoming AC mains supply 1 is reduced by a transformer 2. The voltage is then rectified by a thyristor-controlled rectifier 3, which comprises two thyristors and two diodes arranged as a thyristor bridge. The rectifier voltage is smoothed by a filter comprising an inductor 4 and a capacitor 5 to produce the charger output 6. The output voltage and output current are measured by a microprocessor 7 and fed into a control system 8 hosted by the microprocessor 7. The control system 8 produces a demand control signal Dem that is passed through a linearization function, producing a corrected demand control signal 9 for the thyristor-controlled rectifier 3. The correction of the control signal allows the gain of the battery charger system to be maximised without oscillation occurring, such that stable DC voltages and be produced at the output of the battery charger system.

Figure 2 is a chart of normalised voltage against time showing the firing angle. The firing angle x is shown as the time between the rectifier input voltage 13 (the output of the rectifier 3) crossing through zero and the point when the thyristor is turned on in the thyristor-controlled rectifier 3. The output of the thyristor-controlled rectifier 3 is shown as a stepped positive waveform 15. The relationship between the firing angle of the thyristors and the DC output of the thyristor-controlled rectifier 3 is given by: VDC = Vmax(1+cosx)/m

Equation 1 where VDC is the DC output of the thyristor rectifier 3, Vmax is the maximum input voltage to the thyristor rectifier 3, and x is the firing angle.

Figure 3 is a chart of normalised voltage against angle in radians showing the firing angle. This illustrates the non-linearity of the DC output of the of the thyristor rectifier 3 with respect to the firing angle x. In order to make this into a linear function, the linearization function must perform the inverse of the output of the thyristor rectifier 3:

Equation 2

where x is the firing angle, Dem is the demand control signal (the uncorrected waveform), and Vmax is the maximum input voltage to the thyristor rectifier 3. Figure 4 is a chart of voltage against angle in radians showing the correction function. This shows the relationship between the firing angle x and the demand control signal Dem during the application of the linearization function.

The invention will now be illustrated by a series of examples embodying the present invention by using a linearization function in accordance with the present invention (“exemplary embodiments), and a series of comparative examples, in which the linearization function in accordance of the present invention is absent (“comparative examples”). A 24V 33A battery charger was used with a single-phase input, having a filter comprising an inductor having an inductance of 6mH, and a capacitor having a capacitance of 4.7mF. The minimum mains AC input voltage used was 207Vrms (which equates to 90% of 230Vrms) and the maximum mains AC input voltage used was 265Vrms (which equates to 110% of 240Vrms). The software used to generate the control signal was reprogrammed to include a step applying the linearization function shown in Equation 2 above in the exemplary embodiments.

The following test methodology was used for both the exemplary embodiments and the comparative examples:

1. The charging load was set to approximately 10% of the battery charger’s full load (2.5 A). The maximum gain values were obtained where the system retains a stable output at 265Vrms. This was achieved by setting the proportional and derivative gains to zero and increasing the integral gain until the battery charger’s output started to oscillate. The maximum stable integral gain value and the charger’s corresponding steady state response were then recorded. 2. The mains AC input voltage was then reduced to 207 Vrms and the battery charger’s steady state response was recorded. 3. The values of the proportional and derivative gains were then calculated based on the integral gain found in Step 1 and the output LC filter values. A unit step response of the battery charger was then recorded to analyse the settling time of the output at both 207Vrms (route mean square voltage) and 265Vrms AC mains input voltages. This was achieved by a sudden increase in output load from 10% (2.5 A) to 70% (23 A).

The following tables compare the performance of the exemplary embodiments and the comparative examples.

Table 1 Maximum integral gain

Table 2 Maximum functional integral gain

Although an integral gain of 0.05 can be used for lower AC mains input voltages without the use of the linearization function, the battery charger’s output starts to become unstable as the AC mains voltage increases. When the input voltage is at its maximum value of 265 Vrms, the output voltage oscillates. Therefore, it is

impractical to use a higher integral gain value of 0.05 without the use of the linearization function.

Table 3 Time taken for output load to settle after sudden disruption

The above exemplary embodiments and comparative examples illustrate that with the use of a linearization function in accordance with an embodiment of the present invention, a higher gain may be used at both low and high input AC mains voltages, and that oscillation of the output voltage is minimised after a change in input voltage at a faster rate when the linearization function in accordance with an embodiment of the present invention is used.

Further advantages of the present invention will be apparent from the appended claims.

Claims

1. A thyristor-controlled battery charger comprising: a thyristor-controlled rectifier comprising one or more thyristors; and a control system in connection with the thyristor-controlled rectifier and adapted to control the output of the battery charger; wherein when a non-linear demand control signal is applied to the thyristor-controlled rectifier the control system is adapted to apply a correction function to the signal to compensate for the non-linearity.

2. A thyristor-controlled battery charger according to claim 1, wherein the correction function is a linearization function.

3. A thyristor-controlled battery charger according to claim 2, wherein the linearization function takes the form of:

4. A thyristor-controlled battery charger according to any of claims 1 to 3, wherein the thyristor-controlled rectifier is a thyristor bridge.

5. A thyristor-controlled battery charger according to any preceding claim, wherein the control system comprises a microprocessor.

6. A thyristor-controlled battery charger according to any preceding claim, wherein the battery charger is powered by a single or three phase input.

7. A method of correcting the output of a thyristor-controlled battery charger comprising a thyristor-controlled rectifier, the method comprising the steps of: taking a non-linear demand control signal; generating a correction function; and

applying the correction function to the non-linear demand control signal to compensate for the non-linearity.

8. A method of correcting the output of a thyristor-controlled battery charger according to claim 7, wherein the correction function is a linearization function.

9. A method of correcting the output of a thyristor-controlled battery charger according to claim 8, wherein the linearization function takes the form of: