Example aspects herein generally relate to a transformer system for use in an electrical grid, and more specifically to a transformer system for converting a grid voltage to a regulated voltage which is output to a power line.
In order to increase the power transfer capability of an electrical grid for distributing electricity to consumers, flexible alternating current transmission system (FACTS) controllers are used to improve power factor voltage profiles in distribution grids. In recent years, the importance of effectively incorporating FACTS controllers into distribution grids has grown due to the unconventional power flow and the voltage profiles in distribution grids that are caused by the increased use of distributed energy resources (DERs). Generally, FACTS controllers can be classified as being of a variable impedance type, such as a Static VAR Compensator (SVC) or a Thyristor Controlled Series Compensator (TCSC), or a voltage source converter type such as a Static Synchronous Compensator (STATCOM), a Static Synchronous Series Compensator (SSSC) or a Unified Power Flow Controller (UPFC).
Conventional distribution transformers are not manufactured with reactive power control capability. Instead, FACTS controllers are typically retrofitted to the distribution grid in order to allow reactive power control on a power line connected to an output of the distribution transformer. This retrofitting process typically involves extensive installation work, which typically requires cutting into the power line to connect the FACTS controller.
In addition, due the size and weight of FACTS controllers, it is often difficult to install these devices at a site of a distribution transformer, which is usually not provided with enough space to accommodate the extra installation footprint that these devices would require. The same problem exists for pole-mounted transformers, as the supporting poles have limited installation space and FACTS controllers cannot be easily integrated without modifying the underlying supporting structure.
Furthermore, distribution transformers at grid edge, used to step supply voltage down to consumer levels, are not typically equipped with any form of voltage regulating capability. The higher voltage transformers that supply these often have ‘On Load Tap Changers’ that are able to adjust voltage in discrete steps, with only a limited number of changes available per day. This inherently limits the flexibility of the distribution network in dealing with issues emerging at low voltage.
In light of the aforementioned problems, the present inventors have devised a transformer system that integrates a step-down transformer with power electronics and coupling transformers for providing both voltage regulation and reactive power control.
More specifically, there is provided, in accordance with a first example aspect herein, a transformer system for use in an electrical grid, the transformer system configured to convert a grid voltage received from the electrical grid to a regulated voltage and output the regulated voltage to a power line. The transformer system comprises a first transformer configured to step down the grid voltage to an unregulated voltage and provide the unregulated voltage at an output of the first transformer. The transformer system further comprises a shunt coupling transformer connected in parallel with the output of the first transformer and further connected to a power electronics circuitry. The transformer system also comprises a series coupling transformer connected in series with the output of the first transformer and further connected to the power electronics circuitry. The power electronics circuitry is configured to add via the series coupling transformer a conditioning voltage in series to the unregulated voltage to generate the regulated voltage. The first transformer, the series coupling transformer and the shunt coupling transformer are housed in a single transformer tank. The power electronics circuitry is housed in a power electronics enclosure separate from the transformer tank. In addition, each of the transformer tank and the power electronics enclosure comprises one or more openings through which electrical connections between the shunt coupling transformer, the series coupling transformer and the power electronics circuitry pass.
Example embodiments will now be explained in detail, by way of non-limiting example only, with reference to the accompanying figures described below. Like reference numerals appearing in different ones of the figures can denote identical or functionally similar elements, unless indicated otherwise.
The transformer system 10 is configured to convert a grid voltage Vgrid from an electrical grid to a regulated voltage Vregulated ulated and output the regulated voltage Vregulated to a power line 30. The transformer system 10 may, as in the present example embodiment, be configured to step down a distribution grid voltage Vgrid of one or more distribution grid voltages in a distribution grid but may alternatively be configured to step down a transmission grid voltage Vgrid of one or more transmission grid voltages in a transmission grid.
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The power electronics circuitry 60 is configured to add, via the series transformer 70, a conditioning voltage Vconditioning in series to the unregulated voltage Vunregulated to generate the regulated voltage Vregulated. The power electronics circuitry 60 may, as in the present example embodiment, comprise one or more switching elements whose switching is controllable by a controller (not shown in
In some example embodiments, the power electronics circuitry 60 and the series coupling transformer 70 may be configured to provide the conditioning voltage Vconditioning either substantially in-phase or substantially in antiphase with the unregulated voltage Vunregulated so as to control an active power flow of the power line 30. In particular, adding a voltage of a controllable magnitude either in phase or in anti-phase with respect to the unregulated voltage Vunregulated allows the transformer system 10 to regulate its output voltage to the power line 30 in order to compensate voltage deviations from a voltage level required by the consumer. These voltage deviations may be voltage drops caused by an increased load or by line reactance, or voltage rises caused by high penetration of Distributed Energy Resources (DERs).
In other example embodiments, the power electronics circuitry 60 and the series coupling transformer 70 may be configured to provide the conditioning voltage Vconditioning substantially in quadrature phase with respect to an output current of the first transformer 40, so as to control a reactive power flow on the power line 30. In particular, by inserting a conditioning voltage Vconditioning that lags the output current of the first transformer 40 by quadrature phase, a capacitive compensation effect is achieved and the power electronics circuitry 60 provides reactive power to the power line 30. On the other hand, inserting a conditioning voltage Vconditioning that leads the output current of the first transformer 40 by quadrature phase consumes reactive power from the power line 30 by providing an inductive compensation effect. Furthermore, in some example embodiments, the conditioning voltage Vconditioning may have any phase such that the conditioning voltage Vconditioning causes both active power and reactive power exchange with the power line 30.
In addition, in some example embodiments, the power electronics circuitry 60 is configured to exchange reactive power with the power line 30 via the shunt coupling transformer 50 to provide shunt reactive compensation of the power line 30. In particular, the power electronics circuitry 60 may be operable as a current source to inject a controllable current via the shunt coupling transformer 50 into the power line 30. When the injected current is in phase quadrature with respect to the unregulated voltage Vunregulated, the power electronics circuitry 60 controls reactive power flow on the power line 30. When the injected current is in phase or in antiphase with the unregulated voltage Vunregulated, the real power flow on the power line 30 is controlled. When the injected current has both in-phase and quadrature components with respect to the unregulated voltage Vunregulated, the power electronics circuitry 60 controls both real and reactive power of the power line 30. The control of reactive power via transformer system 10 allows for power factor correction to improve the efficiency of electricity transportation over power line 30.
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The transformer system 10 may, as in the present example embodiment, further comprising a frame supporting the first transformer 40, the frame being configured to distribute a weight of the first transformer 40 over a base of the frame having a footprint substantially the same as a footprint of the first transformer 40. The frame further supports the series coupling transformer 70 and the shunt coupling transformer 50 so as to distribute a weight of the series coupling transformer 70 and the shunt coupling transformer 50 over the base of the frame. The transformer system 10 of the present example embodiment takes the form of a ground-mounted transformer system. However, a transformer system according to another example embodiment may be pole-mounted instead.
In the present example embodiment, the use of a supporting frame having a footprint substantially the same as the footprint of the first transformer 40 allows the installation footprint of the transformer system 10 to be significantly reduced compared to installing a FACTS controller (having equivalent functionality to the coupling transformers and the power electronics circuitry) as an add-on component at the site of the first transformer 40. In particular, due the size and weight of typical FACTS controllers, it can be difficult to integrate these additional devices into substations or other sites that are not provided with the ground space require to accommodate devices of that size. In this regard, the transformer system 10 of
It should be noted that in
In addition, coupling transformers used with FACTS controllers are typically dry-cooled (air-cooled) while distribution transformers are typically liquid-cooled. By housing the series coupling transformer 70 and shunt coupling transformer 50 in the same transformer tank 80 as the first transformer 40, the coupling transformers can also be liquid-cooled, allowing a more effective heat dissipation, an increased capacity to withstand electrical breakdown, also providing a higher flashing point and aging resistivity, whilst making maintenance easier, as only one cooling system needs to be maintained.
Moreover, FACTS controllers are typically air-cooled and therefore the coupling transformers and their power electronics circuitry are typically placed in the same enclosure. However, in the example embodiment of
The transformer system 200 is configured to down-convert a three-phase grid voltage Vgrid and therefore, each of the first transformer 40, the series coupling transformer 70 and the shunt coupling transformer 50 has three sets of primary and secondary windings, each set corresponding to a respective phase. However, in some embodiments, the transformer system 200 may instead be configured to convert a single-phase grid voltage Vgrid and therefore, each of the three transformers may have a single set of primary and secondary windings.
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The shunt coupling transformer 50 is connected in parallel with the output of the first transformer 40 such that a winding 45 of the first transformer 40 is connected in parallel with a winding 55 of the shunt coupling transformer 50. Furthermore, the series coupling transformer 70 is connected in series with the output of the first transformer 40 such that the winding 45 of the first transformer 40 is further connected in series with a winding 75 of the series coupling transformer 70.
The power electronics enclosure 90 is attached to a side of the transformer tank 80. However, the power electronics enclosure 90 may alternatively be mounted on top of the transformer tank 80 or placed in any part of the transformer system 10 that is conveniently accessible for maintenance.
In addition, the transformer system 200 may, as in the present example embodiment, comprise a plurality of transformer bushings 250 and 260, a radiator element 280 and a conservator tank 220.
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The controller 210 may, as in the present example embodiment, be configured to receive measurement values indicative of at least one of an output voltage of the first transformer 40, an output current of the first transformer 40, an output voltage of the transformer system 300, an output current of the transformer system 300, and a voltage of the DC link capacitor 330. In this case, the controller 210 is further configured to control the switching of the one or more switching elements the power electronics circuitry 60 based on the measurement values. In some embodiments, the controller 210 is configured to calculate a target voltage phase and a target voltage magnitude based on the measurement values and one or more reference parameters, and to control the switching of the power electronics circuitry 60 such that the conditioning voltage Vconditioning has substantially the target voltage magnitude and the target voltage phase. The reference parameters may comprise one or more of a value indicative of a target voltage of the power line 30, a value indicative of a target real power flow of the power line, a value indicative of a target reactive power flow of the power line 30, and a target power factor. However, additional reference parameters may also be used.
Furthermore, the controller 210 may, as in the present example embodiment, further be configured to implement a control law, such as proportional, integral and derivative, PID, control, for example, and thus use a set of P, I and D values to calculate a switching control signal Scontrol for controlling the switching of the one or more switching elements in the power electronics circuitry 60. For example, the controller 210 may determine an error signal based on the one or more measurement values and the one more reference parameters, and generate the switching control signal Scontrol based on the error signal. The controller 210 may further control the switching of the power electronics circuitry 60 using the switching control signal Scontrol. It should be noted that the control law algorithm need not be PID, and another control law algorithm, such as PI, PD, P and I, can alternatively be used to generate the switching control signal Scontrol.
In some example embodiments, the transformer system 300 may comprise measurement circuitry for obtaining the measurements values at the output of the first transformer 40 and/or at the output of transformer system 300, and providing the measurements to controller 210.
Furthermore, in some example embodiments, the transformer system 300 may comprise a telemetry module (not shown) for receiving a command requesting the switching of the power electronics circuitry 60 to be adjusted. The controller 210 may further derive a modified switching control signal Scontrol based on the command and control the switching of the power electronics circuitry 60 using the modified switching control signal Scontrol. For example, transformer system 300 may receive a command requesting the transformer system 300 to change its voltage set point to regulate the voltage of the power line 30, or receive a command to adjust the reactive power flow of the power line 30 to obtain a target power factor. The controller 210 may further switch the power electronics module 60 based on this command. In such an embodiment, the controller 210 does not need to calculate the switching control signal based on the measurement values and may instead derive the switching control signal Scontrol based on the command.
The configuration of the power electronics circuitry 60 in
Furthermore, the latency of control is governed by the switching frequency of the inverter 340, rather than the speed that discrete contactors, breakers or tap changers can operate at. Therefore, response to changes in load can be almost instantaneous, allowing the output voltage to be tightly regulated.
Moreover, varying the phase relationship between the conditioning voltage Vconditioning and the output voltage of the first transformer 40 additionally allows for the transfer of reactive power into or out of power line 30 using switching control techniques on the inverter.
In the present example embodiment, the combination of the hardware components shown in
However, single-phase rectifier and single-phase inverter may alternatively be used for a single-phase implementation of the transformer system 10.
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Although a specific rectifier circuit is shown in
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It should be noted that although inverter 340 of
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As with the embodiment in
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In addition, VSC 620 is also operable to be switched by controller 210 to supply reactive power to or absorb reactive power from the power line 30, thereby providing independent control of the reactive power flow of the power line 30. In the present example embodiment, to control the exchange of real and reactive power by VSC 620, controller 210 is configured to switch VSC 620 to control the voltage VAC at the AC terminal of the VSC 620. For example, when the controller 210 employs pulse width modulation, the controller 210 may vary the magnitude of voltage VAC by changing the modulation index used to generate Scontrol_rect. Furthermore, controller 210 may vary the phase of voltage VAC by generating switching control signal Scontrol_rect to change the firing angle of each IGBT 622 of the VSC 620. When the magnitude of voltage VAC is less than the magnitude of the unregulated voltage Vunregulated at the output of the first transformer 40, reactive power is absorbed by VSC 620 from the output of the first transformer 40. On the other hand, if the magnitude of the voltage VAC is greater than the magnitude of Vunregulated, then reactive power is supplied by the VSC 620 to the power line 30. Furthermore, when the phase angle of voltage VAC at the AC terminal of VSC 620 is greater than phase angle of the voltage Vunregulated, VSC 620 supplies real power to the power line 30. When the phase angle of the voltage VAC is less than the voltage Vunregulated, then VSC 620 absorbs real power from power line 30.
Accordingly, by using two voltage source converters as shown in
Filing Document | Filing Date | Country | Kind |
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PCT/EP2020/053873 | 2/14/2020 | WO |