FIELD

[0001] Embodiments of present disclosure relate to an inverter and an associated method for controlling the inverter, in order to reduce and eliminate the inrush current in a disconnector.

BACKGROUND

[0002] Inverters, such as alternate current (AC) sine -wave inverters, typically include a disconnector. Via the disconnector, when the inverter is off, the inverter along with the filter components (including filter inductor and filter capacitor) can be disconnected from the AC power source.

[0003] If the filter capacitor voltage does not match the AC connection voltage, then if the disconnector is closed, there will be an inrush current flowing through the disconnector due to the voltage mismatch. Such inrush current is undesirable, and sometimes could be sufficiently high, which may cause damage of the disconnector.

[0004] Currently, there are two common solutions to prevent the inrush current. One approach is to temporarily connect resistors around the disconnector. This solution requires extra hardware (i.e., both additional resistors and an additional switch to connect the resistors), which is not preferable as it directly results in additional cost.

[0005] Another approach is to use the inverter to create a reference voltage that matches the voltage on the AC connection. When the disconnector is closed, if the created voltage perfectly matches the AC connection voltage, then the inrush current is prevented. However, this solution currently relies on the Park Transform, which assumes that the three-phase voltage is perfectly balanced (that is, all three phases are of the same magnitude and uniform phase) and no harmonics of different frequencies are present. As such, the problem is that if there is distortion present on the AC connection, but the inverter still produces a perfectly balanced voltage, then the inrush current in the disconnector (when it is closed) may be dangerously large.

SUMMARY

[0006] The present disclosure proposes a solution for reducing and eliminating the inrush current in a disconnector, even when there is distortion present on the AC connection. With the proposed solution, the generated matching voltage by the inverter can always match the AC connection voltage, regardless of distortion present on the AC connection, such that when the disconnector is closed the inrush current is negligible. In addition, the method implementation complexity is similar or lower than conventional approaches, whereby lowering the requirements on the inverter controller.

[0007] In a first aspect, an inverter is provided. The inverter comprises a switching circuit operable to convert DC power to alternative current (AC) power; a means for shifting the inverter between a state of disconnecting from an AC power source and a state of connecting to the AC power source; and a controller configured to, during disclosing of the means for shifting the inverter, operate the switching circuit to generate a matching voltage (V _ref ) at output terminals of the switching circuit based upon an instantaneous voltage (V _AC ) measured at an individual phase of the AC power source, so as to eliminate voltage difference between the matching voltage (V _ref ) and the instantaneous voltage (V _AC ) when the inverter is in the state of connecting to the AC power source.

[0008] In some embodiments, the means for shifting the inverter is a disconnector, and the controller is further configured to adjust the matching voltage (V _ref ) based upon an instantaneous current (IAC) flowing through the disconnector, when the voltage difference across the disconnector is beyond a threshold voltage.

[0009] In some embodiments, the controller is further configured to adjust the matching voltage (V _ref ) further based upon a high frequency component above a predefined cut-off frequency of an instantaneous voltage (V _capacitor , Mghfreq) across a capacitor that is coupled between the output terminals of the switching circuit.

[0010] In some embodiments, the controller is further configured to adjust the matching voltage further based upon the instantaneous voltage across the capacitor (V _capacitor ) and an instantaneous power flowing in the inverter (P _Inverter ).

[0011] In some embodiments, adjusting the matching voltage further based upon the instantaneous voltage across the capacitor (V _capacitor ) and an instantaneous power flowing in the inverter (P _Inverter ) comprises adjusting the matching voltage further based upon multiplication of the instantaneous voltage across the capacitor (V _capacitor ) and an instantaneous power flowing in the inverter (P _Inverter ).

[0012] In some embodiments, the inverter is a pulse width modulation (PWM) inverter.

[0013] In a second aspect, a method for use in an inverter is provided. The method comprises detecting opening and closing of a means for shifting the inverter between a state of disconnecting from an AC power source and a state of connecting to the AC power source; and in response to detecting that the means for shifting the inverter is being closed, operating a switching circuit of the inverter to generate a matching voltage (V _ref ) at output terminals of the switching circuit, based upon an instantaneous voltage (V _AC ) measured at an individual phase of the AC power source, so as to eliminate voltage difference between the matching voltage (V _ref ) and the instantaneous voltage (V _AC ) when the inverter is in the state of connecting to the AC power source.

[0014] In some embodiments, the means for shifting the inverter is a disconnector, and the method further comprises adjusting the matching voltage (V _ref ) based upon an instantaneous current (IAC) flowing through the disconnector, when the instantaneous current (IAC) is beyond a threshold current.

[0015] In some embodiments, the method further comprises adjusting the matching voltage (V _ref ) further based upon a high frequency component above a predefined cut-off frequency of an instantaneous voltage across the capacitor (V _{capacitor, highfreq} ).

[0016] In some embodiments, the method further comprises adjusting the matching voltage further based upon the instantaneous voltage across the capacitor (V _capacitor ) and an instantaneous power flowing in the inverter (Pinverter).

[0017] In some embodiments, adjusting the matching voltage further based upon the instantaneous voltage across the capacitor (V _capacitor ) and an instantaneous power flowing in the inverter (P _Inverter ) comprises adjusting the matching voltage further based upon multiplication of the instantaneous voltage across the capacitor (V _capacitor ) and an instantaneous power flowing in the inverter (P _Inverter )·

[0018] In some embodiments, the inverter is a pulse width modulation (PWM) inverter.

DESCRIPTION OF DRAWINGS

[0019] Through the following detailed descriptions with reference to the accompanying drawings, the above and other objectives, features and advantages of the example embodiments disclosed herein will become more comprehensible. In the drawings, several example embodiments disclosed herein will be illustrated in an example and in a non-limiting manner, wherein:

[0020] Fig. 1 schematically illustrates an inverter according to an embodiment of the present disclosure; and

[0021] Fig. 2 is a flow chart of a method for operating an inverter during the closing operation of the disconnector, according to an embodiment of the present disclosure.

[0022] Throughout the drawings, the same or similar reference symbols are used to indicate the same or similar elements.

DETAILED DESCRIPTION OF EMBODIEMTNS

[0023] Principles of the present disclosure will now be described with reference to several example embodiments shown in the drawings. Though example embodiments of the present disclosure are illustrated in the drawings, it is to be understood that the embodiments are described only to facilitate those skilled in the art in better understanding and thereby achieving the present disclosure, rather than to limit the scope of the disclosure in any manner.

[0024] As used herein, the term“includes” and its variants are to be read as open terms that mean“includes, but is not limited to.” The term“based on” is to be read as “based at least in part on.” The term“one embodiment” and“an embodiment” are to be read as“at least one embodiment.” The term“another embodiment” is to be read as“at least one other embodiment.” The terms“first,”“second,” and the like may refer to different or same objects. Other definitions, explicit and implicit, may be included below. A definition of a term is consistent throughout the description unless the context clearly indicates otherwise.

[0025] Fig. 1 schematically illustrates an inverter 100 according to an example embodiment. It shall be appreciated that inverter 100 may be implemented in variety of applications, such as electric vehicles, power distribution system, and solar panels. In some embodiments, the inverter 100 can be a pulse width modulation PWM inverter, which produces a true sinusoidal wave.

[0026] As shown, the inverter 100 comprises a direct current (DC) bus 101 (having a positive rail 111 and a negative rail 121, for example) adapted to couple to a DC power source. It is to be appreciated that the present disclosure does not seek to limit the number of DC buses. In some other embodiments, inverters may have multiple rails corresponding to multiple voltage levels. The inverter 100 further comprises a switching circuit 102 coupled to the DC bus 101. The switching circuit 102 is operable to convert DC power to alternative current (AC) power.

[0027] In this example of Fig. 1, a smoothing capacitor 106 is coupled between the rail 111 and the rail 121. The smoothing capacitor 106 is adapted to reduce transients on the

DC bus 101 caused by fluctuations in the output power of the DC power source and switching operations of the switching circuit 102. Moreover, a capacitor 105 (also referred to as“filter capacitor”) is coupled between the output terminals of the switching circuit 102, and an inductor 107 (also referred to as“filter inductor”) is coupled between the switching circuit 102 and the capacitor 105. In this example, the capacitor 105 and the inductor 107 form a filtering circuit.

[0028] As illustrated in Fig. 1, the inverter 100 further comprises a controller 104. The controller 104 is configured to operate the switching circuit 102 to convert the DC power to the AC power. In some embodiments, the switching circuit 102 may comprise a plurality of switching devices. The type of switching device include, but not limited to, insulated-gate bipolar transistor (IGBT) having a parallel coupled freewheeling diode (as illustrated in Fig. 1 only for illustration), metal-oxide-semiconductor filed-effect transistor (MOSFET), or a silicon carbide (SiC) MOSFET.

[0029] As further illustrated in Fig.l, the inverter 100 further comprises means for shifting the inverter. In this example, the mean is a disconnector 103. The disconnector 103 is adapted to disconnect the inverter 100 from the AC power source when the disconnector 103 is open, and to connect the inverter 100 to the AC power source when the disconnector 103 is closed.

[0030] As discussed above, when the inverter 100 is disconnected from the AC power source, and if the output voltage of the inverter does not match the AC connection voltage, then if the disconnector 103 is closed, there will be an inrush current flowing through the disconnector 103 due to the voltage mismatch, which may cause damage of the disconnector 103. [0031] Therefore, in accordance with various embodiments of the present disclosure, the controller 104 is further configured to, during disclosing of the disconnector 103, operate the switching circuit 102 to generate a matching voltage V _ref for each individual phase at output terminals of the switching circuit 102, based upon an instantaneous voltage V _AC measured at the corresponding individual phase of the AC power source, such that a voltage difference across the disconnector 103 is substantially zero. In this way, the inrush current flowing through the disconnector 103 can be reduced or even eliminated.

[0032] It is to be noted that, in some embodiments,“during” the disclosing of the disconnector may be understood as a time period including a short time interval before commanding the disconnector to close, during the closing operation and another short time interval after. This is because the exact switching instant of the disconnector closing is unknowable due to mechanical delays. Optionally, the control algorithm may remain active until the disconnector is definitively closed, confirmed either by an auxiliary feedback switch or a timeout representative of the mechanical closing time. [0033] In some embodiments, the substantially zero voltage difference is zero volts.

In some other embodiments, the substantially zero voltage difference can be a small percentage of the instantaneous voltage V _AC , for example, less than 20% of V _AC , because in some scenarios, the matching voltage created by the inverter does not have to perfectly match the AC connection voltage as a small difference will still result in greatly reduced inrush current.

[0034] In the conventional matching voltage generation solution based on the Park Transform, a perfectly balanced (that is, all three phases are of the same magnitude and uniform phase), single-frequency (for example, 50 or 60Hz), three-phase reference is produced. However, in many situations, the AC connection may be distorted such that the three-phase voltage is not perfectly balanced and/or may have harmonics of different frequencies. In this case, if the inverter produces a perfectly balanced voltage but the AC connection voltage is distorted, then the inrush current in the disconnector (when it is closed) may be dangerously large. Additionally, the Park Transform requires calculating and synchronizing an angle reference which is computationally expensive for the inverter controller.

[0035] Unlike the conventional Park Transform based matching voltage generation algorithm, the matching voltage V _ref in the present disclosure is generated independently for each individual phase of the three phases or four phases. In this way, the generated matching voltage V _ref can always track the instantaneous voltage V _AC for each individual phase, even when there is unbalance and/or distortion existing in the AC grid during the process of closing the disconnector. In other words, if detecting that there is unbalance and/or distortion exiting in the AC grid, the matching voltage V _ref generated for each phase (by controlling opening and closing of the switching devices of the switching circuit) will also consider such unbalance and/or distortion effect.

[0036] The matching voltage generation solution as proposed in the present disclosure can be simply represented by Eq. (1) as below:

V _ref = V _AC (1), where V _ref is the generated matching voltage, V _AC is the instantaneous voltage for each phase, and the measurement of the instantaneous voltage V _AC may be relative to neutral (4-wire) or a line-to-line representation, such 3 -wire or the alpha-beta (aka Clarke) transformation.

[0037] In this way, the generated matching voltage V _ref by the inverter according to various embodiments of the present disclosure can perfectly match the AC connection voltage such that when the disconnector is closed, the inrush current is negligible, regardless of distortion present on the AC connection.

[0038] In addition, the method implementation complexity is similar or lower than conventional approaches, whereby lowering the requirements on the inverter controller.

[0039] Although the instantaneous AC voltage V _AC is the main component to be taken into account when creating the matching voltage V _ref for each phase, some additional components may also be required to minimize the current in the disconnector when closed due to any errors in the voltage measurement and/or the inverter voltage production. Some of those components are described in the following embodiments.

[0040] For example, in some embodiments, the controller 104 may be further configured to adjust the matching voltage V _ref based upon an instantaneous current I _AC flowing through the disconnector 103, when the voltage difference across the disconnector 103 is beyond a predetermined threshold voltage.

[0041] In an example embodiment, such adjustment can be represented by Eq. (2) as below

V _re = V _AC -k ₁ * I _AC (2), where ki is a constant whose absolute value can be determined empirically. For example, ki can be determined through experiments, simulations, or machine learnings based on historical measurement data. IAC is the instantaneous current flowing through the disconnector. The component k ₁ *I _AC is the dominant component for minimizing undesired current flowing in the disconnector when it is closed.

[0042] In some embodiments, with reference to Fig. 1, in addition to the component based on the IAC, the matching voltage V _ref may be further adjusted based on a high frequency component above a predefined cut-off frequency (for example, 50 or 60Hz) of an instantaneous voltage V _capacitor , _highfreq across the capacitor 105 that is coupled between the output terminals of the switching circuit 102. The high frequency component of the capacitor voltage is attained by filtering out the dominant low frequency component, such as 50 or 60Hz.

[0043] In an example embodiment, such adjustment can be represented by Eq. (3) as below

V _ref V _AC -(k ₁ * I _AC k ₂ * V _{capacitor, highfreq} ) (3), where k2 is a constant, whose absolute value can also be determined empirically, and V capacitor, _highfreq is the high frequency component of the instantaneous capacitor voltage.

The component k2 * V _capacitor , _highfreq aims to dampen any resonances between the filter capacitor 105 and either the filter inductor or grid inductance.

[0044] In some embodiments, in addition to the component based on the IAC and the component based on the V _capacitor , _highfreq , the matching voltage V _ref may be further adjusted based upon the instantaneous voltage V _capacitor across the capacitor 105 and an instantaneous power P _Inverter flowing in the inverter 100.

[0045] The instantaneous inverter power P _Inverter may be measured or calculated at either the DC side of the inverter 100, the AC side of the inverter 100, or at the disconnector 103. This component aims to minimize any power flowing in the inverter which may have the undesired effect of charging or discharging the DC bus.

[0046] In an example embodiment, such adjustment can be simply represented by Eq. (4) as below V _ref V AC -(k ₁ * I _AC +k ₂ * V _{capacitor, highfreq} k3 * V _Capacitor *P _lnverter ) (4), where k3 is a constant, whose absolute value can also be determined empirically, V _capadtor is the instantaneous voltage across the capacitor 105, and P _lnverter is the instantaneous power flowing in the inverter.

[0047] Fig. 2 is a flow chart of a method 200 for operating an inverter according to an embodiment of the present disclosure. The method 200 will be described with reference to the inverter 100 that has been described above with reference to Fig. 1. The method 200 can be implemented by the controller 104 as described above.

[0048] At block 210, the opening and closing of means for shifting the inverter 100 is detected, the means can shift the inverter 100 between a state of disconnecting from an AC power source and a state of connecting to the AC power source. At block 220, in response to detecting the means for shifting the inverter is being closed, a switching circuit 102 of the inverter 100 is operated to generate a matching voltage V _ref at output terminals of the switching circuit 102, based upon an instantaneous voltage V _AC measured at an individual phase of the AC power source, so as to eliminate voltage difference between the matching voltage V _ref and the instantaneous voltage V _AC when the inverter 100 is in the state of connecting to the AC power source.

[0049] In some embodiments, the means for shifting the inverter 100 is a disconnector 103, and the method 200 may further comprise adjusting the matching voltage V _ref based upon an instantaneous current IAC flowing through the disconnector 103, when the instantaneous current IAC is beyond a threshold current.

[0050] In some embodiments, the method 200 may further comprise adjusting the matching voltage further based upon a high frequency component above a predefined cut-off frequency of an instantaneous voltage across the capacitor V _{capacitor, highfreq} ·

[0051] In some embodiments, the method 200 may further comprise adjusting the matching voltage further based upon the instantaneous voltage across the capacitor V _capacitor and an instantaneous power flowing in the inverter P _lnverter ·

[0052] In some embodiments, adjusting the matching voltage further based upon the instantaneous voltage across the capacitor V _capacitor and an instantaneous power flowing in the inverter P _lnverter comprises adjusting the matching voltage further based upon multiplication of the instantaneous voltage across the capacitor V _capacitor and an instantaneous power flowing in the inverter P _lnverter ·

[0053] In some embodiments, the inverter is a pulse width modulation PWM inverter.

[0054] It should be appreciated that the above detailed embodiments of the present disclosure are only to exemplify or explain principles of the present disclosure and not to limit the present disclosure. Therefore, any modifications, equivalent alternates and improvement, etc. without departing from the spirit and scope of the present disclosure shall be included in the scope of protection of the present disclosure. Meanwhile, appended claims of the present disclosure aim to cover all the variations and modifications falling under the scope and boundary of the claims or equivalents of the scope and boundary.