A commutation circuit

Abstract

The present invention discloses a commutation circuit, which relates to the technical field of current conversion circuits. The circuit includes a filter capacitor, a commutation reactor, and a commutator; the filter capacitor is connected in parallel with the AC port; the commutation reactor is connected in series with the AC port; the commutator includes an arm connected to the commutation reactor, and the arm is composed of one or more series-connected unit circuits, and each unit circuit includes a current on-off control component and an overvoltage suppression component connected in parallel. The commutation circuit proposed by the present invention not only has high reliability, but also is beneficial to the design of circuit parameters.

Description

Technical Field

[0001] The present invention belongs to the technical field of current conversion circuits, and particularly relates to a commutation circuit. Background Art

[0002] The DC side of a current source converter has a step-down characteristic. The maximum voltage borne by each bridge arm is the peak line voltage, which is less than twice the peak phase voltage in a voltage source device, so the number of series devices can be reduced. During each commutation process, the change in the bridge arm voltage is low, reducing the equalizing RC loss. Current source converters can be used for high-voltage DC power transmission to solve the commutation failure problem, and can also be used in medium- and low-voltage applications such as AC grid closing, DC power distribution, and grid de-icing. However, the drawback of a current source converter is that there is always a bridge arm that needs to carry DC current on one side. When using devices such as IGBTs, the on-state loss ratio in the converter is too high and the efficiency is low. Summary of the Invention

[0003] To solve the above problems, the present invention proposes a commutation circuit.

[0004] The circuit includes a filter capacitor, a commutation reactor, and a converter; the filter capacitor is connected in parallel with the AC port; the commutation reactor is connected in series with the AC port; the converter includes a bridge arm connected to the commutation reactor, and the bridge arm is composed of one or more series-connected unit circuits, and each unit circuit includes a current on-off control component and an overvoltage suppression component connected in parallel.

[0005] The current on-off control component includes an asymmetric IGCT and a fast recovery diode connected in series.

[0006] The current on-off control component is a reverse blocking IGCT.

[0007] The overvoltage suppression component includes an absorption resistor and an absorption capacitor connected in series.

[0008] Each unit circuit further includes a static voltage equalizing resistor, and the static voltage equalizing resistor is connected in parallel with the current on-off control component and the overvoltage suppression component.

[0009] The commutation circuit further includes a smoothing reactor, one end of the smoothing reactor is connected to the bridge arm, and the other end is connected to the DC load.

[0010] When the AC port is an AC three-phase grid port, the main topology of the converter is a three-phase fully controlled bridge.

[0011] The IGCT has a P-N-P-N structure.

[0012] When the bridge arm is composed of multiple series-connected unit circuits, the calculation formula for the number n of the unit circuits is:

[0013] The U ac is the effective value of the line voltage on the AC side of the converter, and U N is the operating voltage designed for the device.

[0014] The inductance value L of the commutation reactor C has the following calculation formula:

[0015] The U ac is the effective value of the line voltage on the AC side of the converter, and (di / dt) max is the lower value of the current change rate tolerance values of the IGCT and the diode.

[0016] The selection of the static voltage-sharing resistor R a should comply with

[0017] The U is the blocking voltage of the device or the maximum voltage borne by each unit in the blocking state of the bridge arm during the normal operation of the converter;

[0018] The I off is the higher one of the forward and reverse leakage currents of the turn-off state current on-off control component.

[0019] The calculation formula of the U is

[0020] The U ac is the effective value of the line voltage on the AC side of the converter;

[0021] The n is the number of unit circuits connected in series on each bridge arm.

[0022] Compared with the prior art, the technical solution of the present invention has the following advantages:

[0023] 1. Using IGCT as a new type of current-controlled fully-controlled device, due to the bidirectional conductivity modulation effect, it has an extremely low on-state voltage drop, making the converter have a very high efficiency; the IGCT device has reliable fault short-circuit and explosion-proof characteristics, which is conducive to setting redundancy in series applications.

[0024] 2. The described circuit parameter design scheme has good feasibility, can fully protect the IGCT device during the turn-on and turn-off processes, and improve the life and reliability of the device and equipment. At the same time, the parameter design does not have too much margin, and it also has advantages in terms of cost, volume and efficiency. Brief Description of the Drawings

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0026] Figure 1 Schematic diagram of the first embodiment of the present invention.

[0027] Figure 2 Schematic diagram of the second embodiment of the present invention.

[0028] Figure 3 Overvoltage diagram generated at the circuit end of the bridge arm unit. Detailed implementation manners

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0030] It should be noted that: Unless otherwise specifically stated, the relative arrangements, numerical expressions, and values of the components, units, circuits, and steps described in these embodiments do not limit the scope of the present invention.

[0031] The following description of at least one exemplary embodiment is actually only illustrative and in no way limits the present invention or its application or use.

[0032] For technologies, methods, and devices known to those of ordinary skill in the relevant fields, detailed discussions may not be made, but where appropriate, the said technologies, methods, and devices should be regarded as part of the authorization specification.

[0033] In all the examples shown and discussed here, any specific value should be interpreted as merely exemplary and not as a limitation. Therefore, other examples of the exemplary embodiments may have different values.

[0034] It should be noted that: Similar reference numerals and letters denote similar items in the following drawings. Therefore, once an item is defined in one drawing, it does not need to be further discussed in subsequent drawings.

[0035] To describe the embodiments of the present invention in detail, the commutation circuit involved in the AC three-phase power grid will be mainly described here.

[0036] The commutation circuit involved in the embodiments of the present invention includes a filter capacitor, a commutation reactor, and a commutator; the filter capacitor is connected in parallel with the AC port; the commutation reactor is connected in series with the AC port; the commutator includes a bridge arm connected to the commutation reactor, and the bridge arm is composed of one or more series-connected unit circuits, and each unit circuit includes a current on-off control component and an overvoltage suppression component connected in parallel.

[0037] As Figure 1 shown, it is a schematic diagram of the first embodiment of the present invention. The filter capacitor C AC is used for filtering and absorbing the overvoltage generated on the commutator bridge arm by the equivalent series inductance of the AC power grid at the moment of turn-off. The commutation reactor L C functions to limit the current change rate when the current on-off control component is turned on, thereby protecting the device.

[0038] The main topology of the commutator is a three-phase fully controlled bridge, and each bridge arm is composed of a unit circuit, and each unit circuit includes a current on-off control component and an overvoltage suppression component connected in parallel.

[0039] Among them, the current on-off control component includes an asymmetric IGCT device Q2 and a fast recovery diode Q1 connected in series; or the current on-off control component includes a reverse blocking type IGCT (not shown in the figure). The IGCT is an integrated gate-commutated thyristor. The asymmetric IGCT device Q2 is an IGCT device that can control whether current passes through in the forward direction but cannot withstand a relatively high voltage in the forward or reverse direction, and the reverse blocking type IGCT is an IGCT device that can control whether current passes through in the forward direction and can also withstand a relatively high forward voltage.

[0040] The overvoltage suppression component includes an absorption resistor Rb and an absorption capacitor C connected in series, which are used to suppress the overvoltage caused by the commutation reactor when the bridge arm is turned off, and to achieve voltage balance during the switching instant when there are multiple series-connected units in the bridge arm.

[0041] The other end of the commutator is connected to a DC load or a DC power grid.

[0042] Further, the commutation circuit further includes a smoothing reactor L DC , one end of the smoothing reactor is connected to the bridge arm, and the other end is connected to a DC load or a DC power grid.

[0043] As Figure 2 shown, it is a schematic diagram of the second embodiment of the present invention. The filter capacitor C AC is used for filtering and absorbing the overvoltage generated on the commutator bridge arm by the equivalent series inductance of the AC power grid at the moment of turn-off. The commutation reactor L CIts function is to limit the current change rate when the current on-off control component is turned on, thereby protecting the device.

[0044] The main topology of the converter is a three-phase fully controlled bridge. Each bridge arm is composed of multiple unit circuits, which include a current on-off control component, an overvoltage suppression component and a static voltage balancing resistor R connected in parallel. a .

[0045] The current on-off control component includes an asymmetric IGCT device Q2 and a fast recovery diode Q1 connected in series, or the current on-off control component includes a reverse resistance IGCT (not shown in the figure). The IGCT is an integrated gate commutated thyristor. The asymmetric IGCT device Q2 is an IGCT device that can control whether the current passes through the forward direction, but cannot withstand a higher voltage in the forward or reverse direction. The reverse resistance IGCT is an IGCT device that can control whether the current passes through the forward direction and can withstand a higher forward voltage.

[0046] The overvoltage suppression component includes an absorption resistor Rb and an absorption capacitor C connected in series, which are used to suppress the overvoltage caused by the commutation reactor when the bridge arm is turned off, and to balance the voltage at the switching moment when the bridge arm has multiple series units.

[0047] The static voltage balancing resistor R a It is used to achieve voltage balance among each unit when the bridge arm is blocked.

[0048] The other end of the converter is connected to a DC load or a DC grid.

[0049] Furthermore, the commutation circuit further includes a smoothing reactor L DC One end of the smoothing reactor is connected to the bridge arm, and the other end is connected to the DC load or the DC grid.

[0050] like Figure 1 and Figure 2 As shown in the figure, the design results of the parameters of the circuit devices designed in the figure should be able to provide sufficient protection for the IGCT device during the opening and closing processes. Based on this principle, we have designed the parameters of the circuit devices in detail.

[0051] When there are multiple unit circuits on each bridge arm, the calculation formula for the number of series-connected units n is:

[0052]

[0053] Where Uac is the effective value of the line voltage on the AC side of the converter, U N The designed operating voltage of the converter components must be less than their nominal DC withstand voltage with a margin. Redundancy can be set for series units.

[0054] The inductance calculation formula of the commutation reactor is as follows:

[0055] In the formula, U ac is the effective value of the line voltage on the AC side of the converter, and (didt) max is the lower value of the current change rate tolerance values of the selected IGCT device and diode device, and a margin needs to be reserved.

[0056] The selection principle of the static voltage-sharing resistor Ra is:

[0057] In the formula, the U is the blocking voltage of the converter device or the maximum voltage borne by each unit in the blocking state of the bridge arm during the normal operation of the converter:

[0058] U ac is the effective value of the line voltage on the AC side of the converter, and n is the number of unit circuits connected in series in each bridge arm;

[0059] The I off is the higher one of the forward and reverse leakage currents of the turn-off state current on-off control component. Specifically, the I off is the higher one of the forward and reverse leakage currents of the turn-off state non-symmetric IGCT device Q2 and diode Q1 connected in series (or the reverse-blocking IGCT in the turn-off state) when the forward and reverse voltages are both U. It can be the nominal value in the device manual or the measured value.

[0060] For the selection of the snubber capacitor C and the snubber resistor Rb, we perform simulation calculations through circuit simulation or programming for circuit characteristics. The input variables are the values of the snubber capacitor and the snubber resistor, and the output variable is the peak overvoltage generated at the end of each bridge arm unit when turning on and turning off the rated or maximum current under the peak value of the AC line voltage. Then, a set of parameters whose overvoltage level meets the following requirements are selected:

[0061] 1. When turning off the rated current, the generated overvoltage peak is not higher than the nominal DC withstand voltage of the device, and a certain margin is reserved.

[0062] 2. When turning off the maximum current, the generated overvoltage peak is not higher than the nominal instantaneous maximum withstand voltage of the device, and a certain margin is reserved.

[0063] The rated current refers to the DC current under the rated operation designed for the converter; the maximum current refers to the maximum current that the designed bridge arm needs to turn off due to transient overload operation or the need for control and protection redundancy. The circuit simulation software that can be used is Simulink or PSCAD, and the circuit characteristics programming software that can be used is MATLAB.

[0064] In circuit simulation or circuit characteristic modeling, the turn-on process of an IGCT device can be equivalent to an ideal switch, and the turn-off process can be equivalent to a current source with a constant rate of current decreasing from the initial value to 0. The constant rate can be given in the device data sheet or measured in experiments.

[0065] Under the condition of meeting the overvoltage requirement, the capacitance value of the capacitor should be as small as possible to reduce losses and improve the efficiency of the converter.

[0066] The following takes the specific asymmetric IGCT device CAC5000-45 Plus and the series diode device D2700U as examples for detailed illustration.

[0067] Here, the rated AC voltage is set to 10 kV, the rated DC current is 1 kA, the maximum turn-off current is 1.5 kA, the nominal DC withstand voltage is 2800 V, and the nominal instantaneous maximum withstand voltage is 4500 V. The nominal current change rate withstand level is 5000 A / us, and the device operating voltage is 2.5 kV.

[0068] Then the calculation result of the number of series unit circuits n is as follows: After considering redundancy, 7 unit circuits are selected in series for each bridge arm.

[0069] Value of the commutation reactor: After leaving a margin, it can be set to 3.5 uH.

[0070] Selection of the static voltage-sharing resistor Ra: After leaving a margin, it can be selected as 100 kΩ.

[0071] We analyze the selection of different component parameters through MATLAB programming. When turning off and turning on the rated DC current of 1 kA or the maximum turn-off current of 1.5 kA under the peak line voltage of 14.14 kV, the overvoltage diagrams generated at the terminals of each unit circuit of the bridge arm are as Figure 3 shown. Considering the overvoltage levels in two cases, referring to the nominal value of the reference device and leaving a margin, and at the same time minimizing the value of the absorption capacitor, a 0.6 uF absorption capacitor and a 2.6 Ω absorption resistor are selected.

[0072] The buffer circuit parameter design scheme given in the embodiment of the present invention has good feasibility. The design results can fully protect the IGCT device during the turn-on and turn-off processes, improving the lifespan and reliability of the device and equipment. At the same time, the parameter design does not have too much margin, and it has advantages in terms of cost, volume, and efficiency.

[0073] Although some specific embodiments of the present invention have been described in detail by way of examples, those skilled in the art should understand that the above examples are for illustrative purposes only and not for limiting the scope of the present invention. Those skilled in the art should understand that modifications to the technical solutions described in the above embodiments or equivalent replacements of some of the technical features may be made without departing from the scope and spirit of the present invention, and all such modifications and replacements are within the protection scope of the present invention.

Claims

1. A commutation circuit, characterized in that, The circuit includes a filter capacitor, a commutation reactor, and a converter; The filter capacitor is connected in parallel with the AC port; The commutation reactor is connected in series with the AC port; the inductance value L of the commutation reactor C is calculated by the formula: The (di / dt) max is the lower value of the di / dt tolerance values of the IGCT and the diode. The converter includes bridge arms connected to the commutation reactor. Each of the bridge arms is composed of a plurality of series-connected unit circuits, and each of the unit circuits includes a current on-off control component and an overvoltage suppression component connected in parallel; The overvoltage suppression component only includes an absorption resistor and an absorption capacitor connected in series; Each of the unit circuits further includes a static voltage-sharing resistor, and the static voltage-sharing resistor is connected in parallel with the current on-off control component and the overvoltage suppression component; The static voltage-sharing resistor R a is selected to meet Wherein, the is the blocking voltage of the device or the maximum voltage borne by each unit in the blocking state of the bridge arm when the converter is operating normally; the I off is the higher one of the forward and reverse leakage currents of the turn-off state current on-off control component; the U ac is the effective value of the line voltage on the AC side of the converter; the n is the number of unit circuits connected in series on each bridge arm; The calculation formula for the number n of all unit circuits is as follows: The said U N is the operating voltage designed for the device.

2. The circuit according to claim 1, wherein The current on-off control component includes an asymmetric IGCT and a fast recovery diode connected in series; 3. The circuit according to claim 1, wherein The current on-off control component is a reverse-blocking IGCT; 4. The circuit according to claim 1, wherein The commutation circuit further includes a smoothing reactor. One end of the smoothing reactor is connected to the bridge arm, and the other end is connected to the DC load; 5. The circuit according to claim 1, wherein When the AC port is an AC three-phase power grid port, the main topology of the converter is a three-phase fully controlled bridge; 6. The circuit according to claim 2 or 3, wherein The IGCT has a P-N-P-N structure;

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