Patent Description:
A transformer is equipment used in an electric grid of a power network. Transformers transform voltage and current in order to transport and distribute electric energy. A Power Electronic (PE) converter uses power electronic components such as Insulated Gate Bipolar Transistors (IGBTs), to control and convert the electric power. The PE converter does not produce power, but rather transform/convert power, i.e., convert the power from a source, e.g., Alternating Current (AC) of one frequency, to the form of power required from a load, e.g., AC of another frequency or Direct current (DC).

Different topologies based on PE converters have been proposed to replace the mechanical On-Load-Tap-Changer (OLTC) of a conventional Voltage-Regulating Transformer (VRT). A simplified single-phase representation of the conventional solution that employs a mechanical OLTC for adding voltage-regulation functionality to a transformer is depicted in <FIG>. PE converters can be used to achieve the voltage-regulation functionality, as in the conventional solution, but also to achieve additional functionalities not possible with the conventional solution, e.g. power flow control and phase-unbalance mitigation. Therefore, circuits that combine PE converters with transformers are termed PEETs. PEETs are thus circuits that utilize power electronics to add functionality, such as voltage regulation, power-flow control, phase-unbalance mitigation, etc, to a transformer.

<CIT>, <CIT> and <CIT> describe circuits that combine AC-AC PE converters with a transformer. The circuits are aimed primarily at achieving voltage regulation (stepped or stepless) and/or power flow control and employ an AC-AC PE converter that is rated for a fraction of the power and the voltage of the transformer. The AC-AC PE converter may be of two main types:.

Moreover, the AC-AC PE converter may be connected to the transformer in three different ways:.

The circuits proposed in the prior art described above feature many advantages compared to the conventional VRTs, like the one shown in <FIG>. Some of these advantages are fast and stepless voltage regulation, possibility to mitigate voltage flickers, and possibility to achieve additional functionalities such as power-flow control, harmonic filtering, reactive-power compensation and phase-unbalance mitigation. However, the PE converters, typically constructed by IGBTs, feature higher cost and significantly higher losses than the mechanical OLTCs.

It is an object of embodiments herein to reduce the cost and losses of the PE converters proposed in the prior-art.

This object is solved by the features of claim <NUM>. The dependent claims recite advantageous embodiments of the invention.

According to some embodiments the thyristor-based AC-AC PE converter comprises one or more parallel connected and/or series connected thyristors.

The current solution is based on the realisation that a part of the AC-AC PE converter can be replaced with a thyristor-based AC-AC converter as thyristors are cheaper and more efficient compared to IGBTs and other devices with turn-off functionality. Consequently, a transformer arrangement that reduces the cost and losses of the AC-AC PE converters, is achieved.

Further technical features of the invention will become apparent through the following description of one or several exemplary embodiments given with reference to the appended figures, where:.

The inventive concept related to the coupled impedance is reflected by <FIG>. It is understood, that embodiments presented in the following and illustrated in the related figures, only fall under the scope of protection of claim <NUM>, as far as they reflect the features of claim <NUM> and are equipped with a coupled impedance as shown in <FIG> and with a bypass thyristor as shown in <FIG>, <FIG>, <FIG>, <FIG>.

It should be noted that the drawings have not necessarily been drawn to scale and that the dimensions of certain elements may have been exaggerated for the sake of clarity.

As described above, embodiments herein aim at replacing part of the AC-AC PE converters with thyristor-based AC-AC converters. Compared to IGBTs and other devices with turn-off functionality, thyristors are cheaper and more efficient. Therefore, the object of the solution according to embodiments herein is to reduce the cost and losses of the PE converters proposed in the prior-art.

<FIG> schematically illustrates a transformer arrangement <NUM> according to embodiments herein. The transformer arrangement <NUM> comprises a transformer <NUM>. The transformer <NUM> has a primary side <NUM> for receiving input voltage and current from a source and a secondary side <NUM> for providing output voltage and current to a load. The transformer arrangement <NUM> further comprises an AC-AC PE converter <NUM> connected to a thyristor <NUM>. The thyristor is used for bypassing the AC-AC PE converter <NUM> in case of a short-circuit fault in a terminal of the primary side <NUM> and/or the secondary side <NUM>. The transformer arrangement <NUM> further comprises a thyristor-based AC-AC PE converter <NUM> connected to a thyristor-tapped winding 13a. The AC-AC PE converter <NUM> is augmented, i.e. connected, with the thyristor-tapped winding 13a via the thyristor-based AC-AC PE converter <NUM>. The thyristor-tapped winding 13a may be connected, e.g. in series, at the input (left-hand-side terminal) of the AC-AC PE converter <NUM>. The thyristor-based AC-AC PE converter <NUM> is connected to an impedance <NUM> to protect the thyristor-tapped winding 13a from short-circuit faults of the thyristor-based AC-AC PE converter <NUM>. The impedance <NUM> may also be connected to the thyristor-tapped winding 13a. The impedance <NUM> is a coupled impedance, wherein the coupled impedance is associated to high impedance along the path of a short-circuit fault of the thyristor-based AC-AC PE converter <NUM> and associated to low impedance for commutation between the thyristors <NUM>. The impedance <NUM> connected to the thyristor-based AC-AC PE converter <NUM> will be described in further detail below with respect to <FIG>.

According to some embodiments the thyristor-based AC-AC PE converter <NUM> comprises one or more parallel connected and/or series connected thyristors <NUM>.

According to some embodiments the AC-AC PE converter <NUM> is connected to an auxiliary winding 13b. The auxiliary winding 13b may be tapped, as shown in <FIG>. That the auxiliary winding 13b is tapped means that it is connected in series to the secondary main winding <NUM> and, depending on the thyristor switching state, also to the thyristor-tapped winding 13a. Thus, the current that flows through the tapped auxiliary winding is of the same magnitude as that of the secondary main winding and the thyristor tapped winding.

According to some embodiments the AC-AC PE converter <NUM> is connected to an impedance <NUM> to protect the auxiliary winding 13b from a short-circuit fault of the AC-AC PE converter <NUM>. The impedance <NUM> may also be connected to the auxiliary winding 13b. The impedance <NUM> connected to the AC-AC PE converter <NUM> and the impedance <NUM> connected to the thyristor-based AC-AC PE converter <NUM> do not necessarily need to have the same impedance value.

<FIG> schematically illustrates a transformer arrangement <NUM> according to embodiments herein. The transformer arrangement <NUM> comprises a transformer <NUM>. The transformer <NUM> has a primary side <NUM> for receiving input voltage and current from a source and a secondary side <NUM> for providing output voltage and current to a load. The transformer arrangement <NUM> further comprises an AC-AC PE converter <NUM> connected to a thyristor <NUM>. The thyristor is used for bypassing the AC-AC PE converter <NUM> in case of a short-circuit fault in a terminal of the primary side <NUM> and/or the secondary side <NUM>. The transformer arrangement <NUM> further comprises a thyristor-based AC-AC PE converter <NUM> connected to a thyristor-tapped winding 13a. The AC-AC PE converter <NUM> is augmented, i.e. connected, with the thyristor-tapped winding 13a via the thyristor-based AC-AC PE converter <NUM>. The thyristor-tapped winding 13a may be connected, e.g. in series, at the output (right-hand-side terminals) of the AC-AC PE converter. The thyristor-based AC-AC PE converter <NUM> is connected to an impedance <NUM> to protect the thyristor-tapped winding 13a from short-circuit faults of the thyristor-based AC-AC PE converter <NUM>. The impedance <NUM> may also be connected to the thyristor-tapped winding 13a. The impedance <NUM> is a coupled impedance, wherein the coupled impedance is associated to high impedance along the path of a short-circuit fault of the thyristor-based AC-AC PE converter <NUM> and associated to low impedance for commutation between the thyristors <NUM>. The impedance <NUM> connected to the thyristor-based AC-AC PE converter <NUM> will be described in further detail below with respect to <FIG>.

According to some embodiments the AC-AC PE converter <NUM> is connected to an auxiliary winding 13b. The auxiliary winding 13b may be separate, as shown in <FIG>. That the auxiliary winding 13b is separate means that it is not directly connected to the secondary main winding <NUM> or to the thyristor-tapped winding 13a, but through the AC-AC PE converter <NUM>. Thus, the current of the auxiliary winding 13b does not need to be of the same magnitude as that of the main winding <NUM> and the thyristor-tapped winding 13a. This means that the design of the separate auxiliary winding 13b is more flexible than that of the thyristor-tapped auxiliary winding 13a.

<FIG> schematically illustrates a transformer arrangement <NUM> according to some embodiments herein. The transformer arrangement <NUM> comprises a transformer <NUM>. The transformer <NUM> has a primary side <NUM> for receiving input voltage and current from a source and a secondary side <NUM> for providing output voltage and current to a load. The transformer arrangement <NUM> further comprises an AC-AC Power Electronic (PE) converter <NUM> connected to a thyristor <NUM>. The thyristor is used for bypassing the AC-AC PE converter <NUM> in case of a short-circuit fault in a terminal of the primary side <NUM> and/or the secondary side <NUM>. The transformer arrangement <NUM> further comprises a thyristor-based AC-AC PE converter <NUM> connected to a thyristor-tapped winding 13a. The AC-AC PE converter <NUM> is augmented, i.e. connected, with the thyristor-tapped winding 13a via the thyristor-based AC-AC PE converter <NUM>. The AC-AC PE converter <NUM> may be connected in series with the thyristor-tapped winding 13a. In some embodiments the AC-AC PE converter <NUM> is connected in series to the main winding <NUM>, and not connected to the thyristor-tapped winding 13a or any auxiliary winding. The thyristor-based AC-AC PE converter <NUM> is connected to an impedance <NUM> to protect the thyristor-tapped winding 13a from short-circuit faults of the thyristor-based AC-AC PE converter <NUM>. The thyristor-based AC-AC PE converter <NUM> may comprise one or more parallel connected and/or series connected thyristors <NUM>.

<FIG>, <FIG> and <FIG> thus show circuits that have been augmented with one or more thyristor-based AC-AC PE converter <NUM>. For the thyristor-based AC-AC PE converter <NUM> the instantaneous output voltage is given by: <MAT>.

In terms of current rating, the thyristor-based AC-AC PE converter <NUM> may be rated for the same current as the thyristor <NUM>. In this way, the thyristor-based AC-AC PE converter <NUM> does not require extra overcurrent protection against short circuits in the secondary side <NUM> and/or primary side <NUM> and thyristor losses are lower compared to thyristors that are rated for the nominal current.

According to some embodiments the AC-AC PE converter <NUM> is connected to ground or close to ground. This is illustrated in <FIG>, <FIG> and <FIG> where the AC-AC PE converter <NUM> is connected close to ground in order to minimize the required insulation voltage. However, according to some embodiments, it is possible to have the AC-AC PE converter <NUM> floating with respect to ground, which is illustrated in <FIG>, <FIG> and <FIG>.

According to some embodiments the AC-AC PE converter <NUM> may be based on IGBTs. The AC-AC PE converter <NUM> may also be implemented with other semiconductor switches, e.g., Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), Integrated Gate-Commutated Thyristors (IGCTs) or Bi-mode Insulated Gate Transistors (BIGTs). Moreover, each thyristor switch of the thyristor-based AC-AC PE converter <NUM> may be implemented either with two separate Phase-Controlled Thyristors (PCT) connected in anti-parallel or implemented with a single Bidirectionally Controlled Thyristor (BCT). The thyristor-based AC-AC PE converter <NUM> implemented with the single BCT integrates two thyristors in one single package and simplifies the mechanical design, compared to the implementation with anti-parallel PCTs.

The operating principle of the thyristor-based AC-AC PE converter <NUM> is very similar and will be explained for the circuit of <FIG>. The switching states of the thyristor-based AC-AC PE converter <NUM> of <FIG> are shown in <FIG> and <FIG> are briefly described as follows:.

In other words, the thyristor-based AC-AC PE converter <NUM> offer a hybrid voltage regulation, which combines coarse voltage regulation by the thyristor-based AC-AC PE converter <NUM> and fine voltage regulation by the AC-AC PE converter <NUM>. The coarse voltage regulation should be employed less frequently than the fine voltage regulation, which is in line with the characteristics of the thyristors <NUM> that are typically optimized for conduction and not fast and frequent switching operations.

The AC-AC PE converter <NUM> and the thyristor-based AC-AC PE converter <NUM> may be rated for providing any proportion of the regulation range ±ΔV. Yet, based on the operating principle illustrated in <FIG> and <FIG>, as well as based on a) vs = vm + vo1 + vo2, and b) vs = vm + vo described earlier, the following observations can be made:.

Based on the above-mentioned observations, stepless voltage regulation with minimum PE converter cost can be achieved if each type of converter is rated for providing half of ±ΔV. In this way, the voltage regulation can be realized by employing the following manner:.

The commutation of the thyristors <NUM> may be based on the principle illustrated in <FIG>. The thyristors <NUM> are in <FIG> shown as 24a and 24b. For commutating the current is between upper and lower thyristors 24a and 24b, the thyristors 24a, shown in bold, may be pulsed, while no gate pulse should be sent to the thyristors 24b. The thyristors that need to be triggered depend on the current direction and the successful commutation depends on the polarity of the commutating voltage (vcom) and the timing of the gate pulses.

<FIG> shows in the thyristors 24a that need to be pulsed for different current directions. More specifically, the current commutation between upper and lower thyristor switches should be performed by pulsing the thyristors 24a, while no gate pulse should be sent to the thyristors 24b. By triggering the thyristors 24a, the current can be transferred from one thyristor to the other (depending on the polarity of the commutation voltage vcom), without causing a short circuit current to flow between the terminals of the thyristor-tapped winding 13a, as this current is blocked by one of the two thyristors 24a. The current commutation can be performed in the following manner:.

If the polarity of the commutating voltage vcom is not suitable, a short-term short circuit, e.g. in the range of some milliseconds, can be allowed, since that the impedance <NUM>, e.g. short-circuit limiting reactance, is dimensioned for protecting the thyristor-tapped winding 13a against short-circuit faults of the thyristor-based AC-AC PE converter <NUM>.

The protection of the thyristor-based AC-AC PE converter <NUM> and the corresponding thyristor-tapped winding 13a is an important feature of the thyristor-based AC-AC PE converter <NUM>. <FIG> illustrates different ways of introducing the impedance <NUM>, e.g., the short-circuit limiting reactance, which is employed for various purposes, namely: <NUM>) to protect the thyristor-tapped winding 13a from short-circuit faults of the thyristor-based AC-AC PE converter <NUM> and <NUM>) for limiting the rate-of-change of current (di/dt) of the thyristor that is being turned on. Typically, the impedance <NUM>, e.g. reactance for limiting the di/dt of the thyristors is relatively small, thus, the size of the short-circuit limiting reactance is defined by the short-circuit withstand capability of the thyristor-tapped winding 13a. This impedance <NUM> may be introduced in the circuit of the thyristor-based AC-AC PE converter <NUM> as a single unit, connected either at the top or bottom terminal of the thyristor-tapped winding 13a, as shown in <FIG>. Alternatively, the impedance <NUM> may be split in two parts, each of which is connected to the top and bottom terminals of the thyristor-tapped winding 13a, so that the circuit of the thyristor-based AC-AC PE converter <NUM> is symmetrical, as illustrated in <FIG>. Finally, a special type of coupled impedance <NUM> may be employed, as shown in <FIG>. This coupled impedance is designed in such a way that it fulfils the following requirements:.

Moreover, for the implementation of the thyristor-based AC-AC PE converter <NUM>, additional passive components may be required for protecting the thyristors <NUM> from high rate-of-change of voltage (dv/dt) after a turn-off. <FIG> and <FIG> shows two variations of snubber networks that would limit the dv/dt of the turned-off thyristor. More specifically:.

<FIG> shows a combination of a common capacitor (C) or RC network, branch denoted by symbol Sx, with RC snubbers, branches denoted by symbol S, connected in parallel to each thyristor <NUM>.

Thus, according to some embodiments the RC snubber network S may be connected in parallel to each thyristor <NUM>, wherein the RC snubber network S comprises a resistor and a capacitor connected in series and/or in parallel. And, according to some embodiments, a further RC snubber network Sx is a common snubber network for all thyristors <NUM> which is connected in parallel to the thyristors <NUM>, wherein the RC snubber network Sx comprises a resistor and a capacitor connected in series and/or in parallel, or a capacitor. Sx may be defined as a common snubber for both thyristors and may be connected in parallel to both thyristors, i.e., a thyristor bridge, and not to each thyristor separately. Each resistor and capacitor may be constructed by series and parallel connections of multiple smaller resistors and capacitors, i.e., the resistor may be a single resistor or several smaller resistors connected in series and/or parallel and the capacitor may be a single capacitor or several smaller capacitors connected in series and/or parallel.

The main difference between the variations of <FIG> and <FIG> is that the dv/dt limitation of the thyristors <NUM> defines directly the capacitance of the S branches for the former, but defines the effective capacitance of the Sx and S branches for the latter. Thus, the variation of <FIG> allows for some freedom in designing the S branches, e.g., the bulk capacitance required for not exceeding the dv/dt limitation could be installed in the common Sx branch and smaller capacitances can be installed in the S branches. The Sx branch may be implemented in the following ways:.

The S branches can be implemented as mentioned in the last point above. Finally, special surge arresters, e.g., varistors, which are not illustrated in <FIG> or <FIG>, may be connected in parallel to the S and Sx branches in order to protect these branches and the thyristors <NUM> from overvoltage transients.

<FIG>and <FIG> illustrate single-phase circuits of the thyristor-based AC-AC PE converter <NUM>. However, the thyristor-based AC-AC PE converter <NUM> of <FIG> and <FIG>may be connected in a three-phase wye (Y) or delta (D) configuration in the secondary side <NUM>, while the primary side <NUM> may be connected in either a Y or D configuration. Thus, the possible three-phase configurations of the primary-secondary can be Y-Y, D-Y, Y-D, Delta-Y, D-D. <FIG> illustrates an example of a three-phase Y-Y connection.

Accordingly, in terms of integration of the transformer <NUM> with the AC-AC PE converter <NUM> and the thyristor-based AC-AC PE converter <NUM>, the following options are thus possible:
Fully integrated: According to some embodiments the transformer arrangement <NUM> may further comprise a transformer tank, and wherein the AC-AC PE converter <NUM>, the thyristor-based AC-AC PE converter <NUM> and the impedances <NUM>, <NUM> are installed inside the transformer tank.

Partly integrated: According to some embodiments the transformer arrangement <NUM> further comprises the transformer tank and a separate container, wherein the impedances <NUM>, <NUM> are installed inside the transformer tank and wherein the AC-AC PE converter <NUM> and the thyristor-based AC-AC PE converter <NUM> are installed in the separate container.

Decoupled: According to some embodiments the transformer arrangement <NUM> further comprises the separate container, wherein the impedances <NUM>, <NUM>, the AC-AC PE converter <NUM> and the thyristor-based AC-AC PE converter <NUM> are installed in the separate container.

Embodiments herein provide the following benefits and advantages:.

It is to be noted that any feature of any of the aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of any of the aspects may apply to any of the other aspects.

Claim 1:
A transformer arrangement (<NUM>) comprising:
a transformer (<NUM>) having a primary side (<NUM>) for receiving input voltage and current from a source and a secondary side (<NUM>) for providing output voltage and current to a load;
an AC-AC Power Electronic, PE, converter (<NUM>) connected to a thyristor (<NUM>) configured to bypass the AC-AC PE converter (<NUM>) in case of a short-circuit fault in a terminal of the primary side (<NUM>) and/or the secondary side (<NUM>);
wherein
the transformer arrangement (<NUM>) further comprises a thyristor-based AC-AC
PE converter (<NUM>), including upper thyristors (<NUM>) and lower thyristors (<NUM>) having a common terminal, connected to a thyristor-tapped winding (13a), which comprises a top terminal and a bottom terminal, and wherein
the AC-AC PE converter (<NUM>) is connected with the thyristor-tapped winding (13a) via the thyristor-based AC-AC PE converter (<NUM>), and wherein the thyristor-based AC-AC PE converter (<NUM>) is connected to an impedance (<NUM>) configured to protect the thyristor-tapped winding (13a) from short-circuit faults of the thyristor-based AC-AC PE converter (<NUM>);
wherein the impedance (<NUM>) is a coupled impedance (<NUM>) connected to the top terminal and the bottom terminal of the thyristor-tapped winding (13a),
wherein the coupled impedance (<NUM>) is associated to high impedance when a magnetic flux is added when current flows from the top terminal to the bottom terminal of the thyristor-tapped winding (13a) and vice versa during a short-circuit fault of the thyristor-based AC-AC PE converter (<NUM>), and associated to low impedance when the magnetic flux is subtracted when current flows from the top terminal and the bottom terminal of the thyristor-tapped winding (13a) to the common terminal of the thyristors (<NUM>) and vice versa during a commutation of a current between the upper and lower thyristor switches (<NUM>).