A voltage transformer

Abstract

This invention provides a hybrid variable voltage transformer for industrial load applications comprising a line frequency transformer and an integrated power electronics convertor block. The line frequency transformer has first and second winding sides, the second winding side has a first winding section and a second winding section. A first side of the convertor block is operatively connected to the second winding section of the second winding side of the line frequency transformer, and the second side of the convertor block is operatively connected in series with the first winding section of the second winding side of the line frequency transformer. The output voltage across the second winding side is dependent on the voltage change from the first side of the convertor block to the second side of the convertor block.

Description

[0001] A VOLTAGE TRANS FORMER

[0002] Field of the Invention

[0003] This invention relates to a voltage transformer . In particular , thi s invention relates to a hybrid variable voltage transformer for industrial load applications .

[0004] Background of the invention

[0005] Line frequency transformers are used to step-up or step-down a supply voltage to a load voltage so that the voltage applied to the load is at an appropriate level . For some applications , for example in some industrial applications , it is neces sary to vary the voltage applied to a load in order to vary the power supplied to the load .

[0006] It is pos sible to vary the voltage change acros s a trans former by changing the turns ratio of the trans former . Such transformers are known as multi-tap trans formers . Although multi-tap transformers are able to vary the voltage change acros s the transformer, they are limited to set voltage change steps and so are not suited to fine control applications . The sudden change in voltage which occurs when the turns ratio i s changed can also cause inrush currents and undesirable harmonics .

[0007] It is an aim of the current invention to overcome at least some of the disadvantages of the known system .

[0008] Summary of the Invention

[0009] According to an aspect of the invention , there is provided a hybrid variable voltage transformer for industrial load applications , comprising : a line frequency trans former having a first winding side and a second winding side , wherein the second winding side compri ses a first winding section and a second winding section ; and an integrated power electronics convertor block having a first s ide and a second side , wherein the first side of the convertor block i s operatively connected to the second winding section of the second winding side , and wherein the second side of the convertor block is operatively connected in series with the first winding section of the second winding side such that the output voltage across the second winding side is dependent on the voltage change from the f irst side of the convertor block to the second side of the convertor block .

[0010] The hybrid trans former of the invention is advantageous as a portion of the voltage change acros s the trans former is enabled by the integrated power electronics convertor block . This provides flexible design options allowing the hybrid transformer to be optimised for a particular application through the design and control of the convertor block .

[0011] Optionally the voltage change from the f irst side of the convertor block to the second s ide of the convertor block i s variable from a pre-specif ied minimum to a prespecified maximum - which may be the full rated output voltage of the convertor block . Optionally the prespecified minimum is zero volts . Optionally the prespecified minimum is the negative value of the prespecified maximum .

[0012] Providing a convertor block with variable voltage change capability and / or voltage phase change capability is advantageous as the set point of the voltage change acros s the convertor block can be adj usted in a continuous , rather than a stepwise , manner allowing for fine control of the voltage applied to the load by the hybrid transformer .

[0013] The convertor block optionally comprises : a back-to- back convertor for applications in which no electrical isolation is required ; or a solid-state trans former with isolation for applications requiring electrical isolation . The convertor block may be bidirectional so that power may flow through the hybrid transformer in either direction . Alternatively, the convertor block may be unidirectional .

[0014] In one example , the first winding section of the second winding side comprise s a multi-tap winding having a plurality of taps and a tap changing switch . This is benef icial when the ability to provide stepwi se voltage changes is desirable .

[0015] An example hybrid trans former having : a second winding side comprising a multi-tap winding with a plurality of taps and a tap changing switch ; and a convertor block with variable voltage change capability, is advantageous as the voltage change acros s the convertor block can be varied during tap changes in the multi-tap winding section to help compensate for the step changes in voltage . The maximum and minimum voltage change s across the convertor block can be selected so that the difference between them matches the step voltage change between taps of the multi-tap section . Optionally the number of winding turns between the taps in the multi-tap winding are equal . Alternatively, the number of winding turns between the taps in the multi-tap winding are unequal . The first winding section of the second winding side optionally comprises a first portion and a second portion , wherein the first portion comprises the multi-tap winding , and wherein the second portion is connectable to the first portion via a switch , wherein the switch is configured to : connect a first end of the f irst portion to a f irst end of the second portion ; or connect the first end of the second portion to a second end of the first portion . This arrangement maximises the number of voltage set points of the multi-tap section .

[0016] The total number of winding turns in the f irst portion may be equal or different to the number of winding turns in the second portion .

[0017] In one example the first winding s ide may comprise a multi-tap winding having a plurality of taps and a tap changing switch to provide an additional way of adj usting the turns ratio acros s the hybrid variable voltage trans former .

[0018] According to another aspect of the invention , there is provided a method of controlling a hybrid variable voltage transformer comprising : setting the voltage pha se of the convertor block output to 0 degree s or 180 degrees phase difference to the voltage pha se of the first winding section of the second winding side .

[0019] Optionally, the method may comprise varying the voltage phase of the convertor block away from 0 degree s or 180 degrees to provide reactive power compensation .

[0020] Brief Description of the Drawings

[0021] Figure 1 schematically illustrates a hybrid variable voltage transformer for industrial load applications ; Figure 2a schematically illustrate s an integrated power electronics convertor block compris ing a back-to- back convertor ;

[0022] Figure 2b schematically illustrate s an integrated power electronics convertor block compris ing a solid-state trans former with isolation ;

[0023] Figure 3 schematically illustrates a hybrid variable voltage transformer installed between a power supply and a load; and

[0024] Figure 4 schematically illustrates a control system for controlling the hybrid variable voltage transformer .

[0025] These drawings depict one or more implementations in accordance with the present teachings , by way of example only, not by way of limitation . In the figure s , like reference numerals refer to the same or s imilar element s .

[0026] Detailed Description of the Drawings

[0027] Figure 1 schematically illustrates a hybrid variable voltage transformer 10 for industrial load applications . The hybrid variable voltage transformer 10 comprises a line frequency transformer 12 having a first winding side 14 and a second winding side 16 . The second winding side 16 comprises a first winding section 18 and a second winding section 20 . A first side 22 of an integrated power electronics convertor block 24 is connected to the second winding section 20 of the second winding side 16 , and a second side 26 of the convertor block 24 is connected in serie s with the first winding section 18 of the second winding side 16 via a connection 28 . Consequently, the output voltage Vout acros s the second winding s ide 16 i s dependent on the voltage change from the first side 22 to the second side 26 of the convertor block 24 . The ratio of the input voltage Vin to the output voltage Vout of the hybrid variable voltage transformer 10 is therefore dependent on the turns ratio between the first winding side 14 and the second winding side 16 , and the voltage change from the first side 22 to the second s ide 26 of the convertor block 24 .

[0028] Figure 2a shows a convertor block 24a which is suitable for use as the convertor block 24 in the hybrid variable voltage transformer 10 . The convertor block 24 a comprises a back-to-back convertor compri sing an AC to DC convertor 30 and a DC to AC inverter 32 . The back-to-back convertor 24a i s a non-isolated convertor block .

[0029] Figure 2b shows an alternative convertor block 24b which i s suitable for use as the convertor block 24 in the hybrid variable voltage transformer 10 . The convertor block 24b comprises solid-state transformer with isolation compris ing an AC to DC convertor 34 , a DC to AC inverter 36 , and a dual active bridge 38 . I solation is provided by a medium or high frequency transformer 40 of the dual active bridge 38 . Both the back-to-back convertor 24 a and the solid- state trans former with isolation 24b may be bidirectional so that power may pas s through the convertor 24a , 24b in either direction . Alternatively, the back-to- back convertor 24a and the solid-state transformer with isolation 24b may be unidirectional so that power may only pas s through the convertor 24a , 24b in one direction .

[0030] Figure 3 schematically illustrates the hybrid variable voltage transformer 10 installed between a power supply 42 and a load 44 . In this example , the load 44 comprises an industrial heater , but the load 44 may comprise any electrical load requiring variable voltage input . As above , the hybrid variable voltage transformer 10 comprises a line frequency transformer 12 having a f irst winding side 14 and a second winding side 16 . The second winding side 16 comprises a first winding section 18 and a second winding section 20 . A first side 22 of an integrated power electronics convertor block compris ing a solid-state transformer with isolation 24b is connected to the second winding section 20 of the second winding side 16 . The second side 2 6 of the convertor block 24b is connected in series with the first winding section 18 of the second winding side 16 via a connection 28 .

[0031] In this example , the first winding section 18 of the second winding side 16 comprises a first portion 46 and a second portion 48 . The first portion 46 comprises a multitap winding 50 having a plurality of taps 52 (only one labelled for clarity) and a tap changing switch 54 . The number of winding turns between the taps 52 in the multitap winding 50 are equal so that the step change in voltage change between taps 52 is equal . However , this is not e s sential , and in another example , the number of winding turns between the taps 52 may vary to provide different step change in voltage change between taps 52 .

[0032] The first portion 46 of the f irst winding section 18 is connected to the second portion 48 via a switch 56 . When in the setting shown in Figure 3 , the switch 56 connect s a first end 58 of the first portion 46 to a first end 60 of the second portion 48 . When in the opposite setting , the switch 56 connects the first end 60 of the second portion 48 to a second end 62 of the f irst portion 46 . The switch 56 therefore control s the polarity of the first portion 46 to minimize total tap 52 quantity . In another example (not shown) the first portion 46 of the first winding section 18 is directly connected in serie s to the second portion 48 without a switch 56.

[0033] In this example , the voltage change from the first side 22 to the second side 2 6 of the convertor block 24b is variable from zero volts to 300 volts , and the voltage change between taps 52 is 300 volts . The convertor block 24b may be used to compensate for inrush currents and undes irable harmonics generated as a result of the tap changing switch 54 moving between taps 52 . Once the current in the first winding section 18 has reached a steady state , the voltage change from the first side 22 to the second side 26 of the convertor block 24b may return to zero volts .

[0034] It is not neces sary that the voltage change from the first s ide 22 to the second side 26 of the convertor block 24b be variable from zero volts to some pre-specified maximum ( such a s 300 volts ) , and the voltage change acros s the convertor block 24b may go from a pre-specified minimum ( for example 300 volts ) to a pre- specif ied maximum ( for example 600 volt s ) . In another example , the voltage change from the first side 22 to the second s ide 26 of the convertor block 24b may vary from -V volt s to +V volts , where V is the maximum rated output voltage of the convertor block 24b .

[0035] In this example , the total number of winding turns in the first portion 46 of the first winding section 18 is equal to the number of winding turns in the second portion 48 . However , this is not es sential and the number of turns in the first portion 46 may differ from the number of winding turns in the second portion 48 . The voltage phase of the convertor block 24b output is usually locked to the voltage phase of the f irst winding section 18 of the second winding side 16 with a 0 degree or 180 degree phase difference . However, the voltage phase of the convertor block 24b can be shifted away from 0 or 180 degrees phase difference to provide reactive power compensation . This i s also true for the convertor block 24a which may be used in place of the convertor block 24b in Figure 3 .

[0036] When the voltage phase of the convertor block 24a / 24b output is set with respect to the voltage phase of the f irst winding section 18 at 0 degrees pha se angle difference , the magnitude of the converter block output is added to the magnitude of the first winding section 18 . When the voltage phase of the convertor block 24a / 24b output is set with re spect to the voltage pha se of the first winding section 18 to 180 degrees phase angle difference , the magnitude of the converter block output is subtracted from the magnitude of the first winding section 18 . The phase difference between the first winding section 18 and the convertor block 24a / 24b thereby controls the magnitude of the voltage output Vout of the hybrid variable voltage transformer 10 . By varying the phase difference away from 0 degrees or 180 degrees , the magnitude of the voltage output Vout can be controlled to provide reactive power compensation , provide reactive power to the grid connection .

[0037] Figure 4 schematically illustrates a control system 100 for controlling the hybrid variable voltage trans former 10 . The control system 100 compri se s a main controller 101 which communicates with a load control system 102 , a cloud based remote monitoring system 103 , a multi-tap controller 104 , and a convertor block controller 105 . The control system 100 also comprise s a controller 106 for controlling the sub-components of the convertor block 24 . The main controller 101 communicate s with the rest of the control system components via communication channel s 107 which may be wired or wirele s s communication channel s .

[0038] As illustrated in Figure 4 , the control system 100 is arranged into three level s . Level 1 compri se s the main controller 101 which communicates with systems external to the hybrid variable voltage transformer 10 such as the load control system 102 and the cloud based remote monitoring system 103 .

[0039] The level 2 systems comprise the multi-tap controller 104 and the convertor block controller 105 , and the level 3 system comprises the controller 106 for controlling the sub-components of the convertor block 24 .

[0040] In the example of Figure 3 above , the convertor block comprises a solid-state trans former with isolation 24b . In another example , the convertor block may compri se a back-to-back convertor 24a .

[0041] It is not es sential that the hybrid variable voltage trans former 10 compri se a multi-tap portion , and in one example the first winding section 18 of the second winding side 16 may comprise a fixed number of turns .

[0042] In another , example the first winding side 14 of the hybrid variable voltage transformer 10 may comprise a multi-tap winding comprising a plurality of taps and a tap changing switch ( similar to the multi-tap winding 50 described above ) . While many pos sible variations of the hybrid variable voltage transformer have been de scribed above , it will be clear to the s killed person that additional variations and modifications can be made without departing from the s cope of the invention as claimed in the appended claims .

Claims

C L A I M S1 . A hybrid variable voltage transformer for industrial load applications , comprising : a line frequency transformer having a first winding side and a second winding side , wherein the second winding side comprises a first winding section and a second winding section ; and an integrated power electronics convertor block having a f irst side and a second side , wherein the first side of the convertor block i s operatively connected to the second winding section of the second winding side , and wherein the second s ide of the convertor block is operatively connected in series with the f irst winding section of the second winding side such that the output voltage acros s the second winding side is dependent on the voltage change from the first side of the convertor block to the second side of the convertor block .2 . A hybrid variable voltage transformer as claimed in Claim 1 , wherein the voltage change from the first s ide of the convertor block to the second s ide of the convertor block i s variable from a pre-specif ied minimum to a prespecified maximum, wherein optionally the pre-specif ied minimum is zero volts .3 . A hybrid variable voltage transformer as claimed in any preceding Claim, wherein the convertor block comprises : a back-to-back convertor ; or a solid-state transformer with isolation .4 . A hybrid variable voltage transformer as claimed in any preceding Claim, wherein the convertor block is bidirectional or unidirectional .5 . A hybrid variable voltage transformer as claimed in any preceding Claim, wherein the first winding section of the second winding side comprises a multi-tap winding having a plurality of taps and a tap changing switch .6 . A hybrid variable voltage transformer a s claimed in Claim 5 , wherein the number of winding turns between the taps in the multi-tap winding are equal or unequal .7 . A hybrid variable voltage transformer as claimed in Claim 5 or 6 , wherein the first winding section of the second winding side comprise s a first portion and a second portion , wherein the first portion compri ses the multi-tap winding , and wherein the second portion i s connectable to the f irst portion via a switch , wherein the switch i s configured to : connect a first end of the first portion to a f irst end of the second portion ; or connect the first end of the second portion to a second end of the first portion .8 . A hybrid variable voltage transformer as claimed in Claim 7 , wherein the number of winding turns in the first portion is equal to the number of winding turns in the second portion .9 . A hybrid variable voltage transformer as claimed in any preceding Claim, wherein the first winding side comprises a multi-tap winding having a plurality of taps and a tap changing switch .

10. A method of controlling a hybrid variable voltage transformer as claimed in any preceding Claim, the method comprising : setting the voltage phase of the convertor block output to 0 degrees or 180 degrees phase difference to the voltage phase of the first winding section of the second winding side.

11. A method as claimed in Claim 10, comprising varying the voltage phase of the convertor block away from 0 degrees or 180 degrees.

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