AC-AC conversion operation method of modular multilevel converter

Abstract

The invention provides an AC-AC conversion operation method of a modular multilevel converter, and belongs to the technical field of power electronics and flexible low-frequency power transmission. The method comprises the following steps: step 1, designing a connection mode with an alternating current power grid: selecting a modular multilevel converter unit connected with the alternating current power grid according to the phase number of the alternating current power grid; 2, designing a thyristor alternating current waveform: for a thyristor valve group of each phase of bridge arm at the low-frequency side or the power-frequency side of the modular multilevel converter unit, designing the thyristor valve group to be alternately conducted in a sine period, and conducting the thyristor valve group for the same and even number of times; 3, designing the alternating current waveform of the transformer; and 4, designing voltage and current waveforms of the bridge arms. According to the invention, conversion of the modular multilevel converter from low-frequency alternating current to power-frequency alternating current can be realized, and charging and discharging balance of the sub-module bridge arms can be realized by designing reasonable transformer operation frequency and trapezoidal wave current amplitude.

Description

Technical Field

[0001] This invention relates to an AC-AC conversion operation method for a modular multilevel converter, belonging to the fields of power electronics and flexible low-frequency power transmission technology. Background Technology

[0002] With the large-scale development of offshore wind power, flexible low-frequency power transmission technology has attracted widespread attention, and the frequency converter station is the core device of the entire flexible low-frequency power transmission system. The power conversion form of the frequency converter station is to convert the low-frequency AC power generated by the offshore wind farm into 50Hz industrial frequency AC power.

[0003] Patent application CN202411728005.6 discloses a bidirectional hybrid DC-DC transformer and its fault current-limiting design method, and proposes a modular multilevel converter and its operation method. The high-voltage and low-voltage sides each employ three-phase sub-module arms connected in parallel, utilizing the strong control capability of the sub-module arms to achieve DC voltage conversion. However, this operation mode does not fully release the control freedom of the sub-module arms, and cannot achieve AC-to-AC power conversion.

[0004] Therefore, in order to use modular multilevel converters in flexible low-frequency power transmission systems, it is urgent to invent a new operating method to achieve the conversion from low-frequency AC to power frequency AC. Summary of the Invention

[0005] The purpose of this invention is to solve the problems existing in the prior art and to provide an AC-AC conversion operation method for a modular multilevel converter.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] An AC-AC conversion operation method for a modular multilevel converter includes the following steps:

[0008] Step 1: Design the connection method with the AC power grid:

[0009] Select the modular multilevel converter unit connected to the AC power grid based on the number of phases of the AC power grid;

[0010] Step 2: Design the thyristor AC current waveform:

[0011] For each phase arm of the modular multilevel converter unit on the low-frequency or power-frequency side in step one, the thyristor valve group is designed to alternately conduct within one sinusoidal cycle, with the same number of conductions and an even number of conductions; the thyristor current... and Together they synthesize a low-frequency side sinusoidal alternating current and a thyristor current. and Together they synthesize a sinusoidal alternating current on the power frequency side;

[0012] Step 3: Design the AC current waveform of the transformer:

[0013] Thyristor current in step two , In submodule arm A of the modular multilevel converter unit u A w With the cooperation of the two, the synthesized amplitude is Trapezoidal transformer current ;

[0014] Step 4: Design the bridge arm voltage and current waveforms:

[0015] Submodule arm A in a modular multilevel converter unit u thyristors T in series u When the circuit is turned on, submodule bridge arm A u Used to support sinusoidal AC voltage Submodule bridge arm A u With another phase submodule bridge arm A w The voltage difference is used to provide transformer voltage. Bridge arm current Equal to the thyristor current in step two Subtract the transformer current in step three .

[0016] Preferably, in step one, if the AC power grid has a single phase, the modular multilevel converter unit is one modular multilevel converter; if the AC power grid has three phases, the modular multilevel converter unit is two modular multilevel converters connected to line voltages on the input and output sides, or three modular multilevel converters connected to phase voltages on the input and output sides.

[0017] Preferably, the modular multilevel converter includes: submodule bridge arm A u A w A U A W Thyristor valve group T u T w T U T W Bridge arm inductors L1, L2, L3, L4 and a large-capacity intermediate frequency transformer;

[0018] The thyristor valve group T u Bridge arm inductor L1 and submodule bridge arm A u The first phase bridge arm constitutes the low-frequency side of the converter; thyristor valve group T u One end is connected to the positive terminal of the low-frequency AC power supply, thyristor valve group T uThe other end is connected to one end of the bridge arm inductor L1, and the other end of the bridge arm inductor L1 is connected to the submodule bridge arm A. u The positive terminals are connected, and the submodule bridge arm A is connected. u The negative terminal is connected to the negative terminal of the low-frequency AC power supply;

[0019] The thyristor valve group T w Bridge arm inductor L2 and submodule bridge arm A w The second phase bridge arm constitutes the low-frequency side of the converter; thyristor valve group T w One end is connected to the positive terminal of the low-frequency AC power supply, thyristor valve group T w The other end is connected to one end of bridge arm inductor L2, and the other end of bridge arm inductor L2 is connected to submodule bridge arm A. w The positive terminals are connected, and the submodule bridge arm A is connected. w The negative terminal is connected to the negative terminal of the low-frequency AC power supply;

[0020] One end of the primary winding of the large-capacity intermediate frequency transformer is connected to the thyristor valve group T. u Between the bridge arm inductor L1 and the other end of the primary side of the large-capacity intermediate frequency transformer, the thyristor valve group T is connected. w Between and bridge arm inductor L2;

[0021] Submodule bridge arm A U The positive terminal is connected to the positive terminal of the AC power supply on the mains frequency side, submodule bridge arm A U The negative terminal is connected to one end of the bridge arm inductor L3, and the other end of the bridge arm inductor L3 is connected to the thyristor valve group T. U One end is connected to the anti-parallel thyristor T. U The other end is connected to the negative terminal of the AC power supply on the power frequency side;

[0022] Submodule bridge arm A W The positive terminal is connected to the positive terminal of the AC power supply on the mains frequency side, submodule bridge arm A W The negative terminal is connected to one end of the bridge arm inductor L4, and the other end of the bridge arm inductor L4 is connected to the thyristor valve group T. W One end is connected to the anti-parallel thyristor T. W The other end is connected to the negative terminal of the AC power supply on the power frequency side;

[0023] One end of the secondary side of the large-capacity intermediate frequency transformer is connected to the thyristor valve group T. U Between the bridge arm inductor L3 and the other end of the secondary side of the large-capacity intermediate frequency transformer, the thyristor valve group T is connected. W Between and the bridge arm inductor L4.

[0024] Preferably, when the modular multilevel converter unit is a single modular multilevel converter, the input voltage is a low-frequency AC voltage. The input current is a low-frequency alternating current. The output voltage is the mains frequency AC voltage. The output current is the power frequency AC current. .

[0025] Preferably, when the modular multilevel converter unit consists of two modular multilevel converters connected to line voltages on the input and output sides, the first modular multilevel converter receives a low-frequency line voltage. Input low-frequency phase current , Lag 30° electrical angle, input low-frequency line voltage of the second modular multilevel converter Input low-frequency phase current , Advanced 30° electrical angle, of which , and The following relationships are satisfied:

[0026]

[0027] The first modular multilevel converter outputs power frequency line voltage. Output power frequency phase current , Lag 30° electrical angle, the second modular multilevel converter outputs power frequency line voltage. Output power frequency phase current , Advanced 30° electrical angle, of which , and The following relationships are satisfied:

[0028]

[0029] in, and These are the phase currents on the low-frequency side and the power frequency side, respectively.

[0030] Preferably, the specific steps for the thyristor valve group to alternately conduct within one sinusoidal cycle in step two are as follows:

[0031] Low-frequency side thyristor current It changes in a sinusoidal form, from the commutation time... The current in one phase decreases linearly to 0, while the current in the other phase thyristor increases linearly from 0 to... The switching time is The back pressure time is At any time and Together they synthesize a low-frequency side sinusoidal alternating current;

[0032] Power frequency side thyristor current The current changes in a sinusoidal form, and the commutation time is not synchronized with the low-frequency side, resulting in a commutation time delay. At the time of conversion from The current in one phase decreases linearly to 0, while the current in the other phase thyristor increases linearly from 0 to... The converter time is still at any time and Together they form a sinusoidal alternating current on the power frequency side.

[0033] Preferably, the specific steps for designing the trapezoidal current waveform of the transformer in step three are as follows:

[0034] Transformer operating frequency It must simultaneously satisfy the condition that it is twice the least common multiple of the power / low frequency, i.e.

[0035]

[0036] Among them, the AC frequency on the power frequency side The AC frequency on the low-frequency side is 50Hz or 60Hz. The frequency can be 20Hz or 50 / 3Hz, and the input and output frequencies can also be equal, meaning that the modular multilevel converter is used for interconnecting two power grids with the same frequency.

[0037] Transformer current amplitude The formula is as follows:

[0038]

[0039] in, For converter power, For the transformer operating cycle, This refers to the transformer voltage amplitude. For thyristor commutation time, This is the thyristor reverse voltage turn-off time. The bridge arm voltage adjustment time is set to achieve charge-discharge balance of the submodule bridge arm under the above frequency and amplitude settings.

[0040] Preferably, the specific steps of step four are as follows:

[0041] Bridge arm voltage As it rises in a sinusoidal manner, the bridge arm voltage... Similarly, it rises in a sinusoidal form, maintaining the following formula:

[0042]

[0043] in, The transformer voltage amplitude is adjusted before the thyristor commutation. With bridge arm voltage The difference is linearly adjusted to 0, and the adjustment time is set to T. u Then, low-frequency side thyristor commutation, low-frequency side thyristor reverse voltage turn-off, and power frequency side thyristor commutation are performed sequentially, with durations of respectively... , and .

[0044] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0045] This invention can realize the conversion of modular multilevel converters from low-frequency AC to power frequency AC, and can also realize the interconnection of two power grids with the same frequency.

[0046] This invention achieves balanced charging and discharging of submodule bridge arms by designing a reasonable transformer operating frequency and trapezoidal wave current amplitude.

[0047] Compared to the AC-AC conversion scheme using three modular multilevel converters, the scheme using two modular multilevel converters can save one large-capacity intermediate frequency transformer. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the AC-AC conversion operation method of the modular multilevel converter under single-phase conditions according to the present invention.

[0049] Figure 2 This is a schematic diagram of the first modular multilevel converter three-phase AC-AC conversion operation method of the present invention.

[0050] Figure 3 This is a schematic diagram of the second modular multilevel converter three-phase AC-AC conversion operation method of the present invention.

[0051] Figure 4 This is a schematic diagram of the input side voltage and current vectors for the second modular multilevel converter three-phase AC-AC conversion operation method of the present invention.

[0052] Figure 5 This is a schematic diagram of the output voltage and current vector of the second modular multilevel converter three-phase AC-AC conversion operation method of the present invention.

[0053] Figure 6 This is a schematic diagram of the main voltage and current waveforms of the modular multilevel converter of the present invention during AC-AC conversion operation. Detailed Implementation

[0054] The present invention will be further described in detail below with reference to the accompanying drawings: This embodiment is implemented under the premise of the technical solution of the present invention, and detailed implementation methods are given, but the protection scope of the present invention is not limited to the following embodiments.

[0055] like Figure 1 As shown in this embodiment, an AC-AC conversion operation method for a modular multilevel converter includes:

[0056] An AC-AC conversion operation method for a modular multilevel converter includes the following steps:

[0057] Step 1: Connection method with AC power grid:

[0058] For AC-AC conversion in a single-phase AC power grid, a modular multilevel converter is used. The input voltage is a low-frequency AC voltage. The input current is a low-frequency alternating current. The output voltage is the mains frequency AC voltage. The output current is the power frequency AC current. .

[0059] For transformation scenarios under a three-phase AC power grid, such as Figure 2 As shown, three single-phase modular multilevel converters can be used to achieve three-phase AC-AC conversion by connecting phase voltages on the input and output sides, such as... Figure 3 As shown, two modular multilevel converters can also be used to complete the three-phase AC-AC conversion.

[0060] The first modular multilevel converter input low-frequency line voltage Input low-frequency phase current , Lag 30° electrical angle, input low-frequency line voltage of the second modular multilevel converter Input low-frequency phase current , Advanced 30° electrical angle, of which , and The following relationships are satisfied:

[0061]

[0062] The first modular multilevel converter outputs power frequency line voltage. Output power frequency phase current , Lag 30° electrical angle, the second modular multilevel converter outputs power frequency line voltage. Output power frequency phase current , Advanced 30° electrical angle, of which , and The following relationships are satisfied:

[0063]

[0064] in, and These are the phase currents on the low-frequency side and the power frequency side, respectively.

[0065] like Figure 4 As shown, u, v, and w represent the three-phase voltages on the input side, as follows: Figure 5 As shown, a, b, and c represent the three-phase voltages on the output side, respectively.

[0066] Due to the bidirectional conductivity of the anti-parallel thyristors and the submodule, the input and output sides can be interchanged. Since the bridge arm needs to output negative voltage, the submodule bridge arm should be a full-bridge submodule.

[0067] The modular multilevel converter includes: sub-module bridge arm A u A w A U A W Thyristor valve group T u T w T U T W Bridge arm inductors L1, L2, L3, L4 and a large-capacity intermediate frequency transformer;

[0068] The thyristor valve group T u Bridge arm inductor L1 and submodule bridge arm A u The first phase bridge arm constitutes the low-frequency side of the converter; thyristor valve group T u One end is connected to the positive terminal of the low-frequency AC power supply, thyristor valve group T u The other end is connected to one end of the bridge arm inductor L1, and the other end of the bridge arm inductor L1 is connected to the submodule bridge arm A. u The positive terminals are connected, and the submodule bridge arm A is connected. u The negative terminal is connected to the negative terminal of the low-frequency AC power supply;

[0069] The thyristor valve group T w Bridge arm inductor L2 and submodule bridge arm A w The second phase bridge arm constitutes the low-frequency side of the converter; thyristor valve group T w One end is connected to the positive terminal of the low-frequency AC power supply, thyristor valve group T w The other end is connected to one end of bridge arm inductor L2, and the other end of bridge arm inductor L2 is connected to submodule bridge arm A. wThe positive terminals are connected, and the submodule bridge arm A is connected. w The negative terminal is connected to the negative terminal of the low-frequency AC power supply;

[0070] One end of the primary winding of the large-capacity intermediate frequency transformer is connected to the thyristor valve group T. u Between the bridge arm inductor L1 and the other end of the primary side of the large-capacity intermediate frequency transformer, the thyristor valve group T is connected. w Between and bridge arm inductor L2;

[0071] Submodule bridge arm A U The positive terminal is connected to the positive terminal of the AC power supply on the mains frequency side, submodule bridge arm A U The negative terminal is connected to one end of the bridge arm inductor L3, and the other end of the bridge arm inductor L3 is connected to the thyristor valve group T. U One end is connected to the anti-parallel thyristor T. U The other end is connected to the negative terminal of the AC power supply on the power frequency side;

[0072] Submodule bridge arm A W The positive terminal is connected to the positive terminal of the AC power supply on the mains frequency side, submodule bridge arm A W The negative terminal is connected to one end of the bridge arm inductor L4, and the other end of the bridge arm inductor L4 is connected to the thyristor valve group T. W One end is connected to the anti-parallel thyristor T. W The other end is connected to the negative terminal of the AC power supply on the power frequency side;

[0073] One end of the secondary side of the large-capacity intermediate frequency transformer is connected to the thyristor valve group T. U Between the bridge arm inductor L3 and the other end of the secondary side of the large-capacity intermediate frequency transformer, the thyristor valve group T is connected. W Between and the bridge arm inductor L4.

[0074] Step 2: Design the thyristor AC current waveform:

[0075] For each phase arm of the bridge on the low-frequency or power-frequency side, the thyristor valve group alternates conduction within one sinusoidal cycle, with the number of conductions being the same and even. Figure 6 As shown. Specifically:

[0076] Low-frequency side thyristor current It changes in a sinusoidal form, from the commutation time... The current in one phase decreases linearly to 0, while the current in the other phase thyristor increases linearly from 0 to... The switching time is The back pressure time is at any time and Together they synthesize a low-frequency side sinusoidal alternating current;

[0077] Power frequency side thyristor current The current changes in a sinusoidal form, and the commutation time is not synchronized with the low-frequency side, resulting in a commutation time delay. At the time of conversion from The current in one phase decreases linearly to 0, while the current in the other phase thyristor increases linearly from 0 to... The converter time is still at any time and Together they form a sinusoidal alternating current on the power frequency side.

[0078] Step 3: Design the AC current waveform of the transformer:

[0079] Thyristor current flowing through the thyristor , In submodule bridge arm A u A w With the cooperation of the two, the synthesized amplitude is trapezoidal current ,like Figure 6 As shown. To ensure the charging and discharging balance of the submodule bridge arms, the trapezoidal current waveform of the transformer is designed as follows:

[0080] Transformer operating frequency It must simultaneously satisfy the condition that it is twice the least common multiple of the power / low frequency, i.e.

[0081]

[0082] Among them, the AC frequency on the power frequency side The low-frequency side AC frequency is 50Hz (or 60Hz). Typically 20Hz (or 50 / 3Hz). The input and output frequencies can also be equal, meaning that modular multilevel converters are used to interconnect two power grids with the same frequency;

[0083] The transformer current amplitude satisfies:

[0084]

[0085] in, For converter power, For the transformer operating cycle, This refers to the transformer voltage amplitude. For thyristor commutation time, This is the thyristor reverse voltage turn-off time. The bridge arm voltage adjustment time is set to achieve charge-discharge balance of the submodule bridge arm under the above frequency and amplitude settings.

[0086] Step 4: Design the bridge arm voltage and current waveforms:

[0087] Submodule bridge arm A u thyristors T in seriesu When the circuit is turned on, submodule bridge arm A u Used to support sinusoidal AC voltage Submodule bridge arm A u With another phase arm A w The voltage difference is used to provide transformer voltage. The details are as follows:

[0088] Bridge arm voltage As it rises in a sinusoidal manner, the bridge arm voltage... Similarly, it rises in a sinusoidal form, maintaining the following formula:

[0089]

[0090] in, The transformer voltage amplitude is adjusted before the thyristor commutation. With bridge arm voltage The difference is linearly adjusted to 0, and the adjustment time is set to... Then, low-frequency side thyristor commutation, low-frequency side thyristor reverse voltage turn-off, and power frequency side thyristor commutation are performed sequentially, with durations of respectively... , and Bridge arm current Thyristor current Subtract transformer current .

[0091] The above description is merely a preferred embodiment of the present invention. These specific embodiments are different implementations based on the overall concept of the present invention, and the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for AC-AC conversion operation of a modular multilevel converter, characterized in that, Includes the following steps: Step 1: Design the connection method with the AC power grid: Select the modular multilevel converter unit connected to the AC power grid based on the number of phases of the AC power grid; Step 2: Design the thyristor AC current waveform: For each phase arm of the modular multilevel converter unit on the low-frequency or power-frequency side in step one, the thyristor valve group is designed to alternately conduct within one sinusoidal cycle, with the same number of conductions and an even number of conductions; the thyristor current... and Together they synthesize a low-frequency side sinusoidal alternating current and a thyristor current. and Together they synthesize a sinusoidal alternating current on the power frequency side; Step 3: Design the AC current waveform of the transformer: Thyristor current in step two , In submodule arm A of the modular multilevel converter unit u A w With the cooperation of the two, the synthesized amplitude is Trapezoidal transformer current ; Step 4: Design the bridge arm voltage and current waveforms: Submodule arm A in a modular multilevel converter unit u thyristors T in series u When the circuit is turned on, submodule bridge arm A u Used to support sinusoidal AC voltage Submodule bridge arm A u With another phase submodule bridge arm A w The voltage difference is used to provide transformer voltage. Bridge arm current Equal to the thyristor current in step two Subtract the transformer current in step three .

2. The method for AC-AC conversion operation of a modular multilevel converter according to claim 1, characterized in that, In step one, if the AC power grid has a single phase, the modular multilevel converter unit is one modular multilevel converter; if the AC power grid has three phases, the modular multilevel converter unit is two modular multilevel converters connected to line voltages on the input and output sides, or three modular multilevel converters connected to phase voltages on the input and output sides.

3. The method for AC-AC conversion operation of a modular multilevel converter according to claim 2, characterized in that, The modular multilevel converter comprises a sub-module bridge arm A u , A w , A U , A W , a thyristor valve group T u , T w , T U , T W , a bridge arm inductance L1, L2, L3, L4 and a large-capacity medium-frequency transformer; The thyristor valve group T u , the bridge arm inductor L1 and the submodule bridge arm A u The first phase bridge arm constituting the low frequency side of the converter; the thyristor valve group T u One end is connected with the positive pole of the low frequency side alternating current power supply, the thyristor valve group T u The other end is connected with one end of the bridge arm inductor L1, the other end of the bridge arm inductor L1 is connected with the positive pole of the submodule bridge arm A u The negative pole of the submodule bridge arm A u Is connected with the negative pole of the low frequency side alternating current power supply; The thyristor valve group T w , the bridge arm inductor L2 and the submodule bridge arm A w The second phase bridge arm constituting the low frequency side of the converter; the thyristor valve group T w One end is connected with the positive pole of the low frequency side alternating current power supply, the thyristor valve group T w The other end is connected with one end of the bridge arm inductor L2, the other end of the bridge arm inductor L2 is connected with the positive pole of the submodule bridge arm A w The negative pole of the submodule bridge arm A w Is connected with the negative pole of the low frequency side alternating current power supply; One end of the primary side of the large-capacity intermediate-frequency transformer is connected to the thyristor valve group T u The other end of the primary side of the large-capacity intermediate-frequency transformer is connected to the thyristor valve group T between the bridge arm inductors L1 and L2 w The other end of the primary side of the large-capacity intermediate-frequency transformer is connected to the thyristor valve group T between the bridge arm inductors L1 and L2 Submodule bridge arm A U The positive terminal is connected to the positive terminal of the AC power supply on the mains frequency side, submodule bridge arm A U The negative terminal is connected to one end of the bridge arm inductor L3, and the other end of the bridge arm inductor L3 is connected to the thyristor valve group T. U One end is connected to the anti-parallel thyristor T. U The other end is connected to the negative terminal of the AC power supply on the power frequency side; Submodule bridge arm A W The positive terminal is connected to the positive terminal of the AC power supply on the mains frequency side, submodule bridge arm A W The negative terminal is connected to one end of the bridge arm inductor L4, and the other end of the bridge arm inductor L4 is connected to the thyristor valve group T. W One end is connected to the anti-parallel thyristor T. W The other end is connected to the negative terminal of the AC power supply on the power frequency side; One end of the secondary side of the large-capacity intermediate frequency transformer is connected to the thyristor valve group T. U Between the bridge arm inductor L3 and the other end of the secondary side of the large-capacity intermediate frequency transformer, the thyristor valve group T is connected. W Between and the bridge arm inductor L4.

4. The method for AC-AC conversion operation of a modular multilevel converter according to claim 2, characterized in that, When the modular multilevel converter unit is a single modular multilevel converter, the input voltage is a low-frequency AC voltage. The input current is a low-frequency alternating current. ; The output voltage is the mains frequency AC voltage. The output current is the power frequency AC current. .

5. The AC-AC conversion operation method of a modular multilevel converter according to claim 3, characterized in that, When the modular multilevel converter unit consists of two modular multilevel converters connected to line voltages on the input and output sides, the first modular multilevel converter receives a low-frequency line voltage. Input low-frequency phase current , Lag 30° electrical angle, input low-frequency line voltage of the second modular multilevel converter Input low-frequency phase current , Advanced 30° electrical angle, of which , and The following relationships are satisfied: The first modular multilevel converter outputs power frequency line voltage. Output power frequency phase current , Lag 30° electrical angle, the second modular multilevel converter outputs power frequency line voltage. Output power frequency phase current , Advanced 30° electrical angle, of which , and The following relationships are satisfied: in, and These are the phase currents on the low-frequency side and the power frequency side, respectively.

6. The AC-AC conversion operation method of a modular multilevel converter according to claim 5, characterized in that, The specific steps for the thyristor valve group to alternately conduct within one sinusoidal cycle in step two are as follows: Low-frequency side thyristor current It changes in a sinusoidal form, from the commutation time... The current in one phase decreases linearly to 0, while the current in the other phase thyristor increases linearly from 0 to... The switching time is The back pressure time is at any time and Together they synthesize a low-frequency side sinusoidal alternating current; Power frequency side thyristor current The current changes in a sinusoidal form, and the commutation time is not synchronized with the low-frequency side, resulting in a commutation time delay. At the time of conversion from The current in one phase decreases linearly to 0, while the current in the other phase thyristor increases linearly from 0 to... The converter time is still at any time and Together they form a sinusoidal alternating current on the power frequency side.

7. The AC-AC conversion operation method of a modular multilevel converter according to claim 6, characterized in that, The specific steps for designing the trapezoidal current waveform of the transformer in step three are as follows: Transformer operating frequency It must simultaneously satisfy the condition that it is twice the least common multiple of the power / low frequency, i.e. Among them, the AC frequency on the power frequency side The AC frequency on the low-frequency side is 50Hz or 60Hz. The frequency can be 20Hz or 50 / 3Hz, and the input and output frequencies can also be equal, meaning that the modular multilevel converter is used for interconnecting two power grids with the same frequency. Transformer current amplitude The formula is as follows: in, For converter power, For the transformer operating cycle, This refers to the transformer voltage amplitude. For thyristor commutation time, This is the thyristor reverse voltage turn-off time. The bridge arm voltage adjustment time is set to achieve charge-discharge balance of the submodule bridge arm under the above frequency and amplitude settings.

8. The AC-AC conversion operation method of a modular multilevel converter according to claim 7, characterized in that, The specific steps of step four are as follows: Bridge arm voltage As it rises in a sinusoidal manner, the bridge arm voltage... Similarly, it rises in a sinusoidal form, maintaining the following formula: in, The transformer voltage amplitude is adjusted before the thyristor commutation. With bridge arm voltage The difference is linearly adjusted to 0, and the adjustment time is set to T. u Then, low-frequency side thyristor commutation, low-frequency side thyristor reverse voltage turn-off, and power frequency side thyristor commutation are performed sequentially, with durations of respectively... , and .

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