WO2019179621A1

WO2019179621A1 - Voltage compensator for ac-ac converter and converter using the same

Info

Publication number: WO2019179621A1
Application number: PCT/EP2018/057198
Authority: WO
Inventors: Mao Jun HE
Original assignee: Robert Bosch Gmbh
Priority date: 2018-03-21
Filing date: 2018-03-21
Publication date: 2019-09-26

Abstract

The present disclosure relates to a compensator for an AC-AC power converter and an AC-AC power converter. The AC-AC power converter comprises a DC-link coupling an input rectifier module to an inverter. The compensator may include a filter coupled to an input of the inverter. The compensator may include a compensator capacitor configured to be coupled in series with the DC-link, between the output of the rectifier module and the filter. The compensator may include a compensator rectifier configured to provide a voltage onto the compensator capacitor. The compensator may include a capacitive bank, coupled to output of the rectifier module, wherein the capacitive bank and the compensator rectifier are coupled by an AC-link. The compensator may include a controller configured to control the voltage of the compensator capacitor. The operation frequency of the AC-link is adjustable by the controller.

Description

VOLTAGE COMPENSATOR FOR AC-AC CONVERTER AND CONVERTER

USING THE SAME

FIELD OF THE TECHNOLOGY

[001] The present disclosure relates to a voltage compensator for an AC-AC power converter and to a power converter using such a voltage compensator.

BACKGROUND ART

[002] Front-end diode rectifier AC-DC-AC converters (also named as AC-AC converters) are widely used in industry applications, such as fan and pump drive systems, because of their lower cost and higher reliability. A converter system may be subdivided in three function blocks: diode rectifier, DC-link filter and PWM inverter. The DC voltage, of a diode rectifier output with resistive load, contains significant low frequency ripple which is unwanted because it leads to small inverter modulation index and degrades the overall system performance, e.g. low inverter conversion efficiency and slow motor torque response. Therefore, a DC-link filter is necessary to maintain the DC-link voltage and minimize its ripple by absorbing instantaneous power mismatch between the diode rectifier and the pulse width modulation (PWM) inverter. The DC-link filter may be a capacitor bank which can be designed based on just one capacitor or a number of capacitors connected in parallel then in series. Among different types of capacitor, aluminum electrolytic capacitors are the most popular choice because of their high volumetric efficiency and low cost. However, they suffer from the drawbacks of high equivalent series resistance, low ripple current capability, comparably short lifetime, and may require a compromise between low voltage rating and volume.

[003] Moreover, with the DC-link capacitor bank, a pre-charge circuit is always required to avoid high inrush current when the converter is just powered up. In order to handle high DC-link current and achieve high reliability, multiple mechanical relays and multiple power resistors may be used in parallel, which results in costly and bulky design.

[004] Therefore there is a need to provide an improved AC-AC converter without the drawbacks of the prior art. SUMMARY

[005] The present disclosure relates to a voltage compensator for an AC-AC power converter, the AC-AC power converter comprising a DC-link coupling an input rectifier module to an inverter. The voltage compensator may include a filter electrically coupled to, e.g. in parallel with, an input of the inverter. The voltage compensator may include a compensator capacitor e.g. electrically coupled in series with the DC-link, between the output of the rectifier module and the filter. The voltage compensator may include a compensator rectifier configured to provide a voltage onto the compensator capacitor. The voltage compensator may include a capacitive bank, electrically coupled to, e.g. in parallel with, an output of the rectifier module, wherein the capacitive bank and the compensator rectifier are electrically coupled by an AC-link. The voltage compensator may include a controller configured to control the voltage of the compensator capacitor. The operation frequency of the AC-link is adjustable by the controller.

[006] The present disclosure also relates to an AC-AC power converter comprising a voltage compensator. The AC-AC power converter may include a DC-link, wherein the DC-link includes a voltage compensator connected in series.

BRIEF DESCRIPTION OF THE DRAWINGS

[007] In the following description, various embodiments of the present disclosure are described with reference to the following drawings, in which:

[008] Figure 1 illustrates a block diagram of a general drive system comprising a grid 1 , a rectifier 2, a DC-link filter 3, an inverter 4 and a motor 5. An AC-AC converter is formed by the rectifier 2, the DC-link filter 3, and the inverter 4.

[009] Figure 2 shows a plot 6 of voltage (shown in axis 6.1 ) versus time (shown in axis 6.2) of the DC-Link voltage of a conventional AC-AC converter with a resistive load.

[010] Figure 3 illustrates a schematic of a general circuit for a DC-link filter 3 including a DC-link capacitor bank with series connection of in parallel connected capacitors. [011] Figure 4 shows a plot 7 of voltage (shown in axis 7.1 ) versus time (shown in axis 7.2) of the DC-Link voltage of a conventional AC-AC converter, including a capacitor bank.

[012] Figure 5 illustrates a schematic of an AC-AC converter 10 according to various embodiments of the invention. The AC-AC converter includes a DC-link 20 coupling an input rectifier module 30 to an inverter 40, wherein the DC-link 20 includes a voltage compensator 100.

[013] Figure 6 illustrates a schematic of an AC-AC converter 10 according to various embodiments of the invention. Fig .6 illustrates further details of the voltage compensator 100.

[014] Figure 7 illustrates an exemplary circuit for an AC-AC converter according to various embodiments of the invention.

[015] Figure 8 illustrates an exemplary circuit for an AC-AC converter not according to the invention.

DETAILED DESCRIPTION

[016] The following detailed description describes specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[017] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[018] The invention illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms “comprising”, “including,” containing”, etc. shall be read expansively and without limitation. The word "comprise" or variations such as "comprises" or "comprising" will accordingly be understood to imply the inclusion of a stated integer or groups of integers but not the exclusion of any other integer or group of integers. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by exemplary embodiments and optional features, modification and variation of the inventions embodied herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[019] The reference signs included in parenthesis in the claims are for ease of understanding of the invention and have no limiting effect on the scope of the claims.

[020] An AC-AC converter according to various embodiments of the invention is configured to convert an AC input into an AC output, which AC output may be different than the AC input. For example, the AC output may be different from the AC input in number of phases, in frequency, in voltage (for example in root mean square voltage), and/or in waveform.

[021] In various embodiments the expression “to control the voltage of the compensator capacitor” may refer to adjust, (or in other words to set), the voltage of the compensator capacitor, for example for reducing ripple on the input voltage of the inverter. The term“setting” may be used herein as one form of “controlling”. The controller may be configured to that means, for example to control the voltage of the compensator capacitor for reducing ripple on the input voltage of the inverter.

[022] In various embodiments the expression“switches coupled to a controller” or“switches coupled to the controller”, may mean that the controller is able to electronically control the switches. For example, the switches may be signal connected to the controller and/or the switches may be electrically connected to the controller.

[023] In various embodiments the term“controller” or“controllers”, may mean a control circuit comprising one or more group of micro-controllers and/or integrated circuit (1C) chips. The control circuit may be configured to generate PWM signals to control the switches.

[024] In various embodiments the expression“low resistance state” may also be referred to as“ON” or vice-versa.

[025] In various embodiments the expression“high resistance state” may also be referred to as“OFF” or vice-versa.

[026] The further reference signs included in the parenthesis in the claims and description are for ease of understanding of the invention and have no limiting effect on the scope of the invention, in particular no limiting effect on the scope of the claims. These further reference signs have the same effect as reference signs. The further reference signs can be deleted from the claims without affecting the scope of the claims. Examples of such further reference signs are

VDR, VDC-link, Vc5.

[027] As shown in Figure 1 , a converter system can be illustrated by three function blocks: a diode rectifier, a DC-link filter and an inverter, for example, a PWM inverter. Figure 2 shows a diode rectifier output with a resistive load. Apparently, the DC voltage contains significant low frequency ripple which is unwanted because it leads to a small inverter modulation index and degrades the overall system performance, e.g. low inverter conversion efficiency and slow motor torque response. Therefore, a DC-link filter is necessary to maintain the DC-link voltage and minimize its ripple by absorbing instantaneous power mismatch between the diode rectifier and the PWM inverter. The DC-link filter may be a capacitor bank which can be designed based on just one capacitor or a number of capacitors connected in parallel then in series. The schematics of general design for DC-link capacitor bank is presented in Figure 3. With a DC-link capacitor bank, the DC-link voltage will be filtered as shown in Figure 4 depending on the capacitance applied.

[028] According to various embodiments, the AC-AC power converter includes a rectifier configured to accept AC-voltage as input and an inverter configured to generate an AC output. The rectifier is electrically coupled to the inverter via a DC-link.

[029] According to various embodiments, the DC-link includes a voltage compensator connected in series. The voltage compensator may be connected in series with the DC-link. The voltage compensator is e.g. a voltage compensator as described in the present disclosure. In series may mean that the connection is such, that the compensator voltage (e.g. the voltage of the compensator capacitor) may be added to the otherwise DC-link voltage, thus added to the voltage used for the input to the inverter

[030] According to various embodiments, the voltage compensator includes an input, configured to receive DC voltage from the rectifier (e.g. a diode rectifier) of the AC-AC power converter. According to various embodiments, the voltage compensator includes an output configured to supply DC voltage to the inverter of the AC-AC power converter.

[031] Fig. 5 shows a voltage compensator 100 for an AC-AC power converter

10. Fig. 5 also shows an AC-AC converter according to various embodiments. The AC-AC power converter 10 includes a DC-link 20 coupling an input rectifier module 30 to an inverter 40.

[032] The voltage compensator 100 includes a compensator capacitor 102 configured to be electrically coupled in series with the DC-link 20, between the output of the rectifier module 30 and the inverter 40. By controlling the voltage over the compensator capacitor 102, a ripple of the VDc-unk may be decreased. Further details of the working principle will be explained below.

[033] Fig. 6 shows a voltage compensator 100 for an AC-AC power converter 10, and a power converter 10, according to various embodiments of the invention. The AC-AC power converter 10 including a DC-link 20 coupling an input rectifier module 30 to an inverter 40. The voltage compensator 100 includes a filter 104. The voltage compensator 100 includes a compensator capacitor 102 configured to be electrically coupled in series with the DC-link 20, between the output of the rectifier module 30 and the filter 104.

[034] The voltage compensator 100 may include a compensator rectifier 108 configured to provide a voltage (Vcs) onto the compensator capacitor 102. An inverter circuit 106, electrically coupled to the output of the rectifier module 30, wherein the inverter circuit 106 and the compensator rectifier 108 are electrically coupled by an AC-link 110, and a controller 112 configured to control the voltage (Vos) of the compensator capacitor 102 for example, for reducing the ripple on an input voltage of the inverter (VDc-unk), wherein an operation frequency of the AC-link 110 is controllable by the controller 112.

[035] According to various embodiments, the filter 104 may be configured to decouple harmonics between the voltage compensator 100 and the inverter 40. The filter 104 may be a capacitor.

[036] In one alternative variation of the voltage compensator of Fig.6, the filter 104 is not include in the voltage compensator 100, but may also be otherwise electrically coupled to the DC-link 20 between the voltage compensator 100 and the inverter 40. In another alternative, the filter 104 may, for example, be included in the inverter 40.

[037] According to various embodiments, the AC-link may be galvanically isolated by a transformer comprising a primary side electrically coupled to the capacitive bank, and a secondary side electrically coupled to the compensator rectifier.

[038] According to various embodiments, the capacitive bank may include a first half bridge, the first half bridge comprising a first pair of capacitors electrically coupled in series and a first pair of switches electrically coupled in series. The first pair of capacitors electrically coupled in series may be electrically coupled in parallel to the first pair of switches electrically coupled in series. The first coil of the primary side of the transformer may be electrically coupled to a common electrical connection of the first pair of capacitors on one end and to a common electrical connection to the first pair of switches on the other end. According to various embodiments, the first pair of the switches may be electrically coupled to the controller.

[039] According to various embodiments, the capacitive bank may include a second half bridge, the second half bridge comprising a second pair of capacitors electrically coupled in series and a second pair of switches electrically coupled in series. The second pair of capacitors electrically coupled in series may be electrically coupled in parallel to the second pair of switches coupled in series. The second coil of the primary side of the transformer may be electrically coupled to a common electrical connection of the second pair of capacitors on one end and to a common electrical connection to the second pair of switches on the other end. According to various embodiments, the second pair of the switches may be coupled to the controller.

[040] According to various embodiments, each switch, of the first pair of switches and the second pair of switches, may each be an electronic switch, for example including a semiconducting component, such as, e.g. a bipolar transistor, a FET, a MOSFET. Thus, no electromechanical components are needed for the switches.

[041] According to various embodiments, the compensator rectifier may be a synchronous rectifier comprising a set of switches, and the set of switches may be coupled to the controller. According to various embodiments, each switch of the set of switches may each be an electronic switch, for example including a semiconducting component, such as, e.g. a bipolar transistor, a FET, a MOSFET. Thus, no electromechanical components are needed for the switches.

[042] According to various embodiments, the voltage compensator may further comprise a diode electrically coupled in parallel with the compensator capacitor, to enable charging of the filter. The diode may be a Zener diode. The Zener diode may be configured to protect the compensator capacitor in case of voltage overshoot from the inverter.

[043] According to various embodiments, the controller may include an initialization mode. In an initialization mode of the controller, the controlling the voltage (Vcs) of the compensator capacitor may include setting the compensation rectifier in a high resistance state, thereby allowing the compensator capacitor to be charged via the diode.

[044] According to various embodiments, the controller may include a first controller more, wherein the controller is configured to operate in the first controller mode when the input voltage of the inverter (VDc-unk) is insufficiently for the input of the inverter. In the first controller mode the controlling the voltage (Vcs) of the compensator capacitor may include setting the voltage of the compensator capacitor until the input voltage of the inverter (VDc-unk) is sufficient high. The term“insufficiently” may mean lower than a pre-determ ined VDc-unk, min. [045] According to various embodiments, the controller may include a second controller mode, wherein the controller is configured to operate in the second controller mode when the input voltage of the inverter (VDc-unk) is sufficiently high for the input of the inverter. In the second controller mode, the controlling the voltage (Vcs) of the compensator capacitor comprises setting the compensation rectifier in a low resistance state for maintaining zero voltage (Vcs) across the compensator capacitor. The term“sufficiently” may mean, equal or higher than a pre-determ ined Voc-unk, min.

[046] According to various embodiments, the controller is configured to switch between the first mode and the second mode.

[047] Fig. 6 shows an AC-link 110, which may be include a transformer. A transformer has the advantage of galvanically isolate the inverter circuit 106 from the compensator rectifier 108.

[048] According to various embodiments, the inverter circuit 106 may be controlled, by the controller 112, to generate an AC current on the primary of the transformer. The secondary of the transformer may apply secondary AC current to the compensator rectifier 108, which compensator rectifier 108 may be controlled to control the voltage over the capacitor 102. With this configuration, the voltage over the capacitor 102 may be controlled thereby adjusting the Voc-unk voltage.

[049] Fig. 6 shows a voltage compensator 100 for an AC-AC power converter 10, and an AC-AC power converter, according to various embodiments of the invention. The AC-AC power converter 10 including a DC-link 20 coupling an input rectifier module 30 to an inverter 40. The voltage compensator 100 includes a filter 104. The voltage compensator 100 includes a compensator capacitor 102 configured to be electrically coupled in series with the DC-link 20, between the output of the rectifier module 30 and the filter 104.

[050] According to various embodiments and as illustrated in Fig. 7 as example, the input rectifier module 30 may be a diode rectifier module.

[051] According to various embodiments and as illustrated in Fig. 7 as example, the input rectifier module 30 includes 3 phases (Va, Vb, Vc), however it may be configured to work with any number of input phases, for example 1 phase or 2 phases. [052] According to various embodiments and as illustrated in Fig. 7 as example, the inverter 40 may be PWM inverter.

[053] According to various embodiments and as illustrated in Fig. 7 as example, the inverter 40 includes 3 phases, however it may be configured to work to output any number of phases, for example 1 phase or 2 phases. Thus, any kind of power device may be powered by the output of the inverter, for example, a three-phase motor.

[054] The operation of the in series voltage compensator will be explained in more detail as follows.

[055] In order to maintain a certain level of drive performance, the DC-link voltage is desired not to be below a minimum level VDc-unk,min. Therefore, the voltage compensator operates in two modes depending on the output voltage level of diode rectifier. The system control is illustrated in Figure 7. Steps of the process are explained as below:

[056] - In an initialization mode, when the frequency converter is just powered up, the voltage compensator is initialized with all switches including S1 , S2, S3, S4, Q1 , Q2, Q3 and Q4 set to OFF. The rectified DC voltage charges the input capacitors C1 , C2, C3 and C4 directly and charges the capacitor C6 via Zener diode ZD. Capacitor C5 is empty. This initialization may be carried out by a controller configured to execute the initialization mode.

[057] - If VDR<VDC -Link, min, operate the voltage compensator in a first mode:

Regulate the voltage across capacitor C5, i.e. VC5, to achieve VDc-Link=VDR+Vc5>VDc-unk,min. In this operation mode, all switches including S1 , S2, S3, S4, 01 , 02, 03 and Q4 are operating with PWM control. Moreover, group of switches S1 , S3, Q1 and Q4 and group of switches S2, S4, Q2 and Q3 may be controlled in a complementary manner, i.e. when switches S1 , S3, Q1 and Q4 are set to ON, then meanwhile switches S2, S4, Q2 and Q3 are set to OFF, vice versa. This first mode may be carried out by a controller configured to execute the first mode.

[058] Various embodiments may use PWM control. With PWM control, the principle of power conversion and energy transmission within the in-series voltage compensator is explained as below: o When switches S1 & S3 (S2 & S4) are ON (OFF), voltages across capacitors C1 and C3 (C2 and C4) are applied to the primary coils TX-1 and TX-2 of the transformer TX via inductors L1 and L2 respectively. The energized transformer TX transfers energy from its primary side to its secondary side. With the same switch pattern as switches S1 & S3 (S2 &

S4), switches Q1 & Q4 (Q2 & Q3) synchronously rectifies the alternative voltage from the secondary coil TX-3 of transformer TX to charge capacitors C5 and C6;

o Inductors L1 and L2 may coordinate with capacitors C1 , C2, C3 and C4 to achieve zero voltage switching and/or zero current switching for switches S1 , S2, S3 and S4;

o A controllable phase lag may be added to between switches S1 (S2) and S3 (S4) for balancing voltages across capacitors C1 , C2, C3 and C4;

[059] - If VDR³VDC -Link, min, operate the voltage compensator in a second mode: Maintain zero voltage across capacitor C5, i.e. Vc5=0. In this operation mode, switches S1 , S2, S3 and S4 are set to OFF while switches Q1 , Q2, Q3, and Q4 are set to ON. This second mode may be carried out by a controller configured to execute the second mode.

[060] Once the voltage across capacitor C6 reaches its set point and becomes stable, i.e. VDc-unk³VDc-unk,min, PWM inverter may be started.

[061] As can be seen above, depending on the output voltage level of diode rectifier, the in-series voltage compensator may operate in either the first mode or the second mode alternatively.

[062] With the voltage compensator according to the invention, there is no high inrush current when the converter is powered up. Therefore, a pre-charging circuit can be avoided.

[063] Figure 8 illustrates an exemplary circuit for an AC-AC converter not according to the invention. It is composed of: a three-phased diode bridge 82 which rectifies the three-phase input AC voltage to DC voltage; a pre-charge circuit 84 which consists of a mechanical relay SW1 and a power resistor R, that limits the charging current to the DC-link capacitor bank when the converter is powered up; a DC-link capacitor bank C (86); and a three-phase PWM inverter 88 which converts the DC-link voltage to three-phase AC voltage to drive a motor.

[064] In order to have a small DC-link voltage ripple, a large capacitance is required for the capacitor bank. Furthermore, electromechanical and resistive elements are required for the pre-charge circuit 84, to avoid a current inrush and damaging of the capacitor bank. In comparison, the AC-AC converter including a voltage compensator according to various embodiments is compacter and does not require electromechanical components. Therefore, it has more reliability, prolonged lifetime and a decreased system cost

[065] The proposed voltage compensator and/or AC-AC converter has following advantages and improvements:

[066] - Decrease the system total cost;

[067] - Replacing the DC-link capacitor bank by a low power in-series voltage compensator;

[068] - Omitting pre-charging relays and power resistors as the capacitor used in the low power in-series voltage compensator is small;

[069] - Increased lifetime;

[070] - Used of capacitor(s), such as film capacitor(s) to achieve longer product lifetime;

[071] - High power density;

[072] - Smaller geometry;

[073] The power density and compact size may be optimized by increasing the switching frequency of the operation of the voltage compensator.

[074] An AC-AC converter according to various embodiments, may be used to power an electrical device by electrically coupling an output of the inverter to an input of the electrical device.

[075] It is also envisaged to provide for an electrical device including an AC-AC converter according to various embodiments, wherein an output of the inverter is electrically coupled to a load. An example of a load is a motor.

[076] Examples of electrical devices are an electrical pump, or an electrical fan.

Claims

1. A voltage compensator (100) for an AC-AC power converter (10), the AC-AC power converter (10) comprising a DC-link (20) coupling an input rectifier module (30) to an inverter (40), the voltage compensator (100) comprising:

- a filter (104) electrically coupled to an input of the inverter (40),

- a compensator capacitor (102) electrically coupled in series with the DC-link (20), between the output of the rectifier module (30) and the filter (104), and

- a compensator rectifier (108) configured to provide a voltage (Vos) onto the compensator capacitor (102),

- a capacitive bank (106), electrically coupled to an output of the rectifier module (30), wherein the capacitive bank (106) and the compensator rectifier (108) are electrically coupled by an AC-link (110),

- a controller (1 12) configured to control the voltage (Vcs) of the compensator capacitor (102) and to adjust an operation frequency of the AC-link (110).

2. The voltage compensator (100) of claim 1 , wherein

the AC-link (110) is galvanically isolated by a transformer (113) comprising a primary side (113.1 ) electrically coupled to the capacitive bank (106), and a secondary side (113.2) electrically coupled to the compensator rectifier (108).

3. The voltage compensator (100) of claims 1 or 2, wherein

the capacitive bank (106) comprises a first half bridge, the first half bridge (106.1 ) comprising a first pair of capacitors (C1 , C2) electrically coupled in series and a first pair of switches (S1 , S2) electrically coupled in series, wherein a first coil (TX-1 ) of the primary side (113.1 ) of the transformer (113) is electrically coupled to the first pair of capacitors (C1 , C2) on one end and to the first pair of switches (S1 , S2) on the other end, and wherein the first pair of the switches (S1 , S2) is coupled to the controller (1 12).

4. The voltage compensator (100) of claim 3, wherein

the capacitive bank (106) comprises a second half bridge (106.2), the second half bridge (106.2) comprising a second pair of capacitors (C3, C4) electrically coupled in series and a second pair of switches (S3, S4) electrically coupled in series, wherein a second coil (TX-2) of the primary side (113.1 ) of the transformer (113) is electrically coupled to the second pair of capacitors (C3, C4) on one end and to the second pair of switches

(S3, S4) on the other end, and wherein the second pair of the switches (S3, S4) is coupled to the controller (112).

5. The voltage compensator (100) of any of the previous claims, wherein the compensator rectifier (108) is a synchronous rectifier comprising a set of switches, and wherein the set of switches are coupled to the controller (1 12).

6. The voltage compensator (100) of any of the previous claims, further comprising

a diode electrically coupled in parallel with the compensator capacitor (102), configured to allow charging of the filter.

7. The voltage compensator (100) of claim 6, wherein

in an initialization mode of the controller, the controlling the voltage (Vcs) of the compensator capacitor (102) comprises setting the compensation rectifier (108) in a high resistance state, thereby allowing the

compensator capacitor (102) to be charged via the diode.

8. The voltage compensator (100) of any of the previous claims, wherein in a first controller mode, wherein the input voltage of the inverter

(VDc-unk) is insufficiently high for the input of the inverter (40), the controlling the voltage (Vcs) of the compensator capacitor (102) comprises setting the voltage of the compensator capacitor (102) until the input voltage of the inverter (VDc-unk) is sufficient high.

9. The voltage compensator (100) of any of the previous claims, wherein in a second controller mode, wherein the input voltage of the inverter

(VDc-unk) is sufficiently high, the controlling the voltage (Vcs) of the compensator capacitor (102) comprises setting the compensation rectifier (106) in a low resistance state for maintaining zero voltage (Vcs) across the compensator capacitor (102).

10. The voltage compensator (100) of the previous claims 8 and 9, wherein the controller (112) is configured to switch between the first mode and the second mode.

11.An AC-AC power converter (10) comprising a DC-link (20), wherein the

DC-link (20) includes a voltage compensator (100) connected in series and optionally wherein the voltage compensator is according to one of the preceding claims.

12. The AC-AC power converter (10) of claim 11 , further comprising:

a DC-link (20) coupling an input rectifier module (30) to an inverter (40); wherein the input rectifier module (30) is configured to rectify AC input voltage into a DC-link (20) voltage (VDR);

wherein the inverter (40) is configured to convert an input voltage of the inverter (VDc-unk) into AC voltage for powering a load;

wherein the voltage compensator comprises a compensator capacitor (102), and wherein the compensator capacitor (102) is configured to be electrically coupled in series with the DC-link (20) such that the voltage of the compensator capacitor (Vcs) is added to the DC-link voltage (VDR), and wherein the input voltage of the inverter (VDc-unk) is proportional to a sum of the DC-link voltage (VDR) and the voltage of the compensator capacitor (Vcs), and wherein the voltage compensator (100) is configured to control the voltage of the compensator capacitor (Vcs).

13. The AC-AC power converter (10) of claims 11 or 12, wherein the voltage compensator (100) is a voltage compensator according to any of the previous claims 1 to 10.