Trapezoidal wave operation method of T-type direct-current transformer

Abstract

The invention discloses a trapezoidal wave operation method of a T-type direct-current transformer. The method comprises the following steps: step 1, designing waveform constraint conditions of a multi-phase T-type direct-current transformer; 2, designing an alternating current waveform of the three-phase T-type direct current transformer; and 3, designing the optimal alternating current waveform of the three-phase T-type direct current transformer. The problems of low voltage and current utilization rate and high current stress caused by the fact that the T-type direct-current transformer adopts a sine operation mode are solved, the power capacity of the T-type direct-current transformer is remarkably improved, and the T-type direct-current transformer topology can be applied to high-voltage and high-capacity occasions. On the premise that the topological structure and the number of devices of the T-type direct-current transformer are not changed, the transmission power capacity of the T-type direct-current transformer is remarkably improved, the power loss of the devices is reduced, and the T-type direct-current transformer has great significance in topological application of an existing T-type direct-current transformer.

Description

Technical Field

[0001] The invention belongs to the field of power electronics and high-voltage direct current transmission, and relates to a trapezoidal wave operation method of a T-type direct current transformer. Background Art

[0002] In recent years, modular multilevel converter technology has made rapid progress in flexible DC transmission technology due to its advantages of modularity, high efficiency, low harmonics and easy maintenance, further promoting the wider application of DC grids. As the core equipment of DC grids, DC transformers are the key hub for interconnecting DC transmission lines of different voltage levels.

[0003] CN105576982B directly connects the output of the traditional modular multi-level converter in parallel to form a DC transformer. Since the bridge arm continuously bears the role of the DC voltage and DC current components, it is necessary to generate AC voltage and AC current inside to generate AC power to achieve energy balance. Therefore, the low-voltage side of the DC transformer needs to install expensive and bulky filter inductors to avoid the internal AC voltage and AC current components appearing on the DC side. In order to reduce the volume and weight of the filter inductor, CN117318474A replaces the filter inductor with the bridge arm on this basis to form a T-type DC transformer to eliminate voltage harmonics in an active filtering manner. In the above scheme, the internal AC voltage and AC current of the DC transformer are both sinusoidal waves, and the voltage and current utilization rate of the waveform is low and the current stress is large, resulting in the power capacity of the T-type DC transformer being limited by the current stress, and the device loss and bridge arm energy storage are also very significant.

[0004] Therefore, in order to improve the power capacity of the T-type DC transformer, it is urgent to invent an operating method that can reduce the current stress of the T-type DC transformer. Summary of the invention

[0005] The present invention provides a trapezoidal wave operation method for a T-type DC transformer. By designing the waveform constraints that the T-type DC transformer needs to meet for stable operation, a variety of three-phase AC waveforms that can meet the constraints are proposed, and based on this, a three-phase optimal AC waveform is invented, which solves the problems of low voltage and current utilization and high current stress caused by the sinusoidal operation of the T-type DC transformer, significantly improves the power capacity of the T-type DC transformer, and enables the T-type DC transformer topology to be applied in high-voltage and large-capacity occasions. The present invention can significantly improve its transmission power capacity and reduce the power loss of the device without changing the topological structure and number of devices of the T-type DC transformer, which is of great significance to the existing T-type DC transformer topology application.

[0006] The objective of the present invention is achieved through the following technical solutions:

[0007] A trapezoidal wave operation method of a T-type DC transformer comprises the following steps:

[0008] Step 1: Design the waveform constraints of the multi-phase T-type DC transformer

[0009] The T-type DC transformer is composed of k-phase identical circuits. Each phase circuit contains a high-voltage bridge arm, a reused bridge arm, and a low-voltage bridge arm. To ensure the stable operation of the T-type DC transformer, it is necessary to ensure that the bridge arm voltage and current waveforms in each phase circuit meet the following constraints: DC voltage constraint, DC current constraint, and energy balance constraint, where:

[0010] In view of the DC voltage constraint, in order to avoid the AC voltage component in the bridge arm being reflected on the DC side, it is necessary to ensure that the AC voltage component amplitudes of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm in each phase circuit are equal, and the AC voltages between the two bridge arms should offset each other, that is, the phase difference is 180°. Among them, the voltages of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm in each phase circuit all satisfy the following formula:

[0011]

[0012] In the formula, u Hj 、u Mj and u Lj They represent the voltages of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of the jth phase, respectively. H is the high voltage DC side voltage, U L is the low voltage DC side voltage, u acj is the AC voltage component of the bridge arm of the jth phase;

[0013] In view of the DC current constraint, in order to prevent the AC current component in the bridge arm from being reflected on the DC side, it is necessary to ensure that the AC current of the high-voltage bridge arm in the k-phase circuit can offset each other on the high-voltage DC side, and the AC current of the low-voltage bridge arm in the k-phase circuit can offset each other on the low-voltage DC side, that is, the sum of the AC current of the high-voltage bridge arm in the k-phase circuit of the T-type DC transformer is zero, and the sum of the AC current of the low-voltage bridge arm is zero:

[0014]

[0015] For the energy balance constraint, it is necessary to ensure that the average power absorbed or released by each bridge arm in an operation cycle T is zero to maintain the energy balance of the bridge arm. The average power of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm in each phase circuit satisfies the following formula:

[0016]

[0017] In the formula, u Hj 、u Mj and u Lj They represent the voltages of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of the jth phase, respectively. His the high voltage DC side voltage, U L is the low voltage DC side voltage, U dc is the DC component of the multiplexed bridge arm voltage, u acj is the AC voltage component of the bridge arm of the jth phase;

[0018] For a T-type DC transformer with any number of phases, as long as the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm meet the above three waveform constraints, the stable operation of the T-type DC transformer can be guaranteed and the smooth transmission of power can be achieved;

[0019] Step 2: Design the AC waveform of the three-phase T-type DC transformer

[0020] For the T-type DC transformer with three-phase structure, the AC waveform of the three-phase T-type DC transformer is designed by combining the waveform constraints of the multi-phase T-type DC transformer:

[0021] In view of the DC voltage constraint, the AC voltage components of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of each phase in the three-phase circuit are equal in amplitude and opposite in phase to ensure that the AC voltage will not be reflected on the DC side. In order to improve the voltage utilization rate, the AC voltage waveform of each bridge arm is designed as a square wave AC voltage, in which the voltages of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of each phase satisfy the following formula:

[0022]

[0023] For the energy balance constraint, the average power absorbed or released by the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of each phase in the three-phase circuit in one operation cycle is zero, wherein the average power of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of each phase satisfies the following formula:

[0024]

[0025] In view of the DC current constraint, the AC current of the high-voltage bridge arm in the three-phase circuit is staggered by 120° and the AC current amplitude of the low-voltage bridge arm is zero, ensuring that the AC current will not be reflected on the DC side. In order to improve the current utilization rate, the waveforms of the AC current of the high-voltage bridge arm and the AC current of the low-voltage bridge arm are designed to be trapezoidal AC currents, where the bridge arm current is expressed as:

[0026]

[0027] In the formula, i Hj 、i Mj and i Lj They represent the currents of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of the jth phase, respectively. H is the high voltage DC side current, I L is the low voltage DC side current, i Hacj and i Lacjare the AC components of the high-voltage and low-voltage bridge arm currents of the j-th phase respectively;

[0028] For a three-phase T-type DC transformer, any square wave AC voltage or trapezoidal wave AC current that meets the above three waveform constraints can ensure the stable operation of the T-type DC transformer;

[0029] Combined with the characteristics of the AC current waveform in an operation cycle T, the amplitude of the first half cycle and the second half cycle waveform are equal and the direction is opposite, it is further obtained that the AC current waveform of the high-voltage bridge arm should meet the following requirements: in the first half cycle, the sum of the AC current in the two periods [-T / 6,0] and [T / 6,T / 3] is equal to the current in the period [0,T / 6]; in the second half cycle, the sum of the AC current in the two periods [T / 3,T / 2] and [2T / 3,5T / 6] is equal to the current in the period [T / 2,2T / 3];

[0030] Step 3: Design the optimal AC current waveform of the three-phase T-type DC transformer

[0031] For the three-phase T-type DC transformer, in order to further reduce the device current stress and device loss, the optimal AC current waveform of the three-phase T-type DC transformer is designed and analyzed from the two perspectives of minimum bridge arm current stress and minimum bridge arm current RMS:

[0032] First half cycle [-T / 6, T / 3]: First, to ensure the minimum current stress in the bridge arm, the AC current needs to be kept constant in the period [0, T / 6], with an amplitude of I Hac ; Secondly, to ensure the minimum RMS current of the bridge arm, the AC current needs to be kept constant in the two periods [-T / 6,0] and [T / 6,T / 3], with an amplitude of I Hac / 2;

[0033] Second half cycle [T / 3, 5T / 6]: The waveform amplitude of the second half cycle is equal to that of the first half cycle, but the direction is opposite;

[0034] Considering the bridge arm inductance and line inductance, the AC current cannot change suddenly in the form of a square wave, but should change in the form of a trapezoidal wave with a certain slope. If the slope of the current change is too small, the AC current amplitude will be too large. If the slope of the current change is too large, it will easily cause the bridge arm overmodulation problem, and the final three-phase optimal AC current waveform is obtained.

[0035] Compared with the prior art, the present invention has the following advantages:

[0036] 1. Compared with the sinusoidal operation mode of the existing T-type DC transformer, the trapezoidal wave operation mode proposed in the present invention can significantly reduce the bridge arm current stress and improve the maximum power capacity of the T-type DC transformer.

[0037] 2. Compared with the sinusoidal operation mode of the existing T-type DC transformer, the trapezoidal wave operation mode proposed in the present invention can reduce the RMS of the bridge arm current and reduce the device power loss of the T-type DC transformer.

[0038] 3. Compared with the sinusoidal operation mode of the existing T-type DC transformer, the trapezoidal wave operation mode proposed in the present invention can allow the T-type DC transformer to operate at a higher voltage ratio, thereby expanding the scope of topological applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is the circuit diagram of a multi-phase T-type DC transformer;

[0040] Figure 2 is the equivalent circuit diagram of a multi-phase T-type DC transformer;

[0041] Figure 3 It is the circuit diagram of three-phase T-type DC transformer;

[0042] Figure 4 The AC waveform example 1 of the three-phase T-type DC transformer is shown in FIG.

[0043] Figure 5 The example of AC waveform of three-phase T-type DC transformer is shown in Figure 2;

[0044] Figure 6 This is the optimal AC current waveform of the three-phase T-type DC transformer. DETAILED DESCRIPTION

[0045] The technical solution of the present invention is further described below in conjunction with the accompanying drawings, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be included in the protection scope of the present invention.

[0046] The present invention provides a trapezoidal wave operation method of a T-type DC transformer, such as Figure 1 As shown, the T-type DC transformer is composed of k-phase identical circuits, each phase circuit includes a high-voltage bridge arm, a reused bridge arm and a low-voltage bridge arm, wherein the high-voltage bridge arm and the reused bridge arm are composed of a cascade of half-bridge sub-modules, and the low-voltage bridge arm is composed of a cascade of full-bridge sub-modules. H and I H is the voltage and current on the high-voltage DC side, U L and I L is the voltage and current on the low voltage DC side, u Hj 、u Mj and u Lj Respectively represent the voltages of the high-voltage bridge arm, the reused bridge arm, and the low-voltage bridge arm of the jth phase, i Hj 、i Mj and i LjThey respectively represent the high voltage bridge arm, multiplexing bridge arm and low voltage bridge arm currents of the j-th phase.

[0047] The following will explain in detail the trapezoidal wave operation mode of the T-type DC transformer. The specific steps are as follows:

[0048] Step 1: Waveform constraints of multi-phase T-type DC transformer

[0049] For the convenience of analysis, each bridge arm of the T-type DC transformer can be equivalent to a voltage source, and the following can be obtained: Figure 2 The equivalent circuit diagram of the T-type DC transformer is shown in Figure 1, where U H -U dc is the DC component of the high voltage bridge arm voltage, U dc is the DC component of the multiplexed bridge arm voltage, U dc -U L is the DC component of the low voltage bridge arm voltage, u acj is the AC voltage component of the bridge arm of phase j, with amplitude and frequency respectively being U ac and f ac ,i Hacj and i Lacj are the AC components of the high-voltage and low-voltage bridge arm currents of the jth phase, with amplitudes of I Hac and I Lac .

[0050] For the DC voltage constraint, according to the voltage reference direction in the T-type DC transformer equivalent circuit and Kirchhoff's voltage law, the sum of the high-voltage bridge arm voltage and the reused bridge arm voltage is the high-voltage side DC voltage U H The difference between the reuse bridge arm voltage and the low voltage bridge arm voltage is the low voltage side DC voltage U L , and the AC voltages of the high-voltage bridge arm and the reused bridge arm are in phase, while the AC voltages of the reused bridge arm and the low-voltage bridge arm are in phase, so as to avoid the AC voltage component in the bridge arm being reflected on the DC side. Therefore, the voltages of each bridge arm in the j-th phase circuit can be expressed as:

[0051]

[0052] In view of the DC current constraint, in order to ensure the smooth and continuous current on the DC side of the T-type DC transformer, the AC current component in the bridge arm should not be reflected on the DC side, that is, the AC currents in the multi-phase circuit can offset each other on the DC side. Therefore, the sum of the AC currents in the high-voltage bridge arm in the k-phase circuit of the T-type DC transformer is zero, and the sum of the AC currents in the low-voltage bridge arm is zero:

[0053]

[0054] According to the energy balance constraint, the average power absorbed / released by the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of each phase of the T-type DC transformer in one operating cycle should be zero, that is, the DC power and AC power of each bridge arm in one operating cycle are equal.

[0055]

[0056] For a T-type DC transformer with any number of phases, as long as the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm meet the above-mentioned DC voltage constraint, DC current constraint and energy balance constraint conditions, the stable operation of the T-type DC transformer can be guaranteed and smooth power transmission can be achieved.

[0057] Step 2: Design of AC waveform for three-phase T-type DC transformer

[0058] For example Figure 3 For the three-phase T-type DC transformer shown in the figure, any three-phase non-sinusoidal AC waveform that meets the above three waveform constraints can be applied to the T-type DC transformer. Among the non-sinusoidal AC waveforms, the square wave AC voltage and square wave AC current have the highest waveform utilization rate, which can effectively reduce the bridge arm current stress of the T-type DC transformer, thereby increasing the power capacity and reducing the device loss. However, considering the bridge arm inductance and line inductance, the AC current cannot change suddenly in the form of a square wave, and a short rise and fall time is required. By combining the waveform constraints of the multi-phase T-type DC transformer, the AC current waveform of the three-phase T-type DC transformer is designed.

[0059] For the DC voltage constraint, the voltages of the high-voltage bridge arm, the reused bridge arm, and the low-voltage bridge arm of each phase in the three-phase circuit should satisfy formula (1). In order to improve the voltage utilization rate, the AC voltage is designed to have an amplitude of U L The square wave AC voltage, so the bridge arm voltage in the j-phase circuit can be expressed as:

[0060]

[0061] For the energy balance constraint, the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of each phase in the three-phase circuit should satisfy formula (3), that is, the DC power and AC power of the bridge arm in one operation cycle should be equal:

[0062]

[0063] For the DC current constraint, the AC current components of the high-voltage bridge arms in the three-phase circuit should be equal in amplitude and staggered by 120°, and the DC current components should all be equal to I H / 3 to ensure smooth and continuous DC current on the high voltage side. To improve current utilization, the AC current is designed to have an amplitude of I HacHowever, due to the limitation of bridge arm inductance and line inductance, AC current cannot change suddenly in the form of square wave, and requires a certain rise and fall time. Therefore, the actual high-voltage bridge arm AC current should be a trapezoidal wave AC current. Similarly, the low-voltage bridge arm AC current component in the three-phase circuit should have zero amplitude, and the DC current component should be equal to I L / 3 to ensure smooth and continuous DC current on the low voltage side. Therefore, the bridge arm current in the j-phase circuit can be expressed as:

[0064]

[0065] For a three-phase T-type DC transformer, any square wave AC voltage or trapezoidal wave AC current that satisfies the waveform constraints shown in equations (4), (5) and (6) can ensure the stable operation of the T-type DC transformer.

[0066] Combined with the characteristics of the three-phase T-type DC transformer AC current phase staggered and half-cycle negative symmetry, the waveform of the high-voltage bridge arm AC current is further designed, and the following relationship is obtained:

[0067]

[0068] Where i Hac1 is the AC current component of the first phase high voltage bridge arm, so in one operation cycle, i Hac1 (t) represents the AC current in the period [0, T / 6], i Hac1 (tT / 6) and i Hac1 (t+T / 6) represents the AC current in the periods [-T / 6,0] and [T / 6,T / 3], that is, the AC current should satisfy the requirement that the sum of the currents in the periods [-T / 6,0] and [T / 6,T / 3] is equal to the current in the period [0,T / 6], and the sum of the currents in the periods [T / 3,T / 2] and [2T / 3,5T / 6] is equal to the current in the period [T / 2,2T / 3]. The AC currents of the remaining two phases are i Hac2 、i Hac3 Similarly, the relationship shown in equation (7) should also be satisfied. Through the analysis of the AC waveform design of the three-phase T-type DC transformer, there are many AC waveforms that can meet the constraint conditions. Figure 4 , Figure 5 Two square wave AC voltage and trapezoidal wave AC current waveforms that can meet the constraint conditions are listed respectively, both of which can ensure the stable operation of the T-type DC transformer.

[0069] Step 3: Design of optimal AC current waveform for three-phase T-type DC transformer

[0070] For the three-phase T-type DC transformer, in order to further reduce the device current stress and device loss, the optimal three-phase AC current waveform is designed from the two perspectives of minimum bridge arm current stress and minimum bridge arm current RMS. First, the bridge arm current stress of the T-type DC transformer appears in the [0, T / 6] period. The integral of the product of the AC voltage and the AC current in this period can be calculated through the energy balance constraint condition, which can be expressed as:

[0071]

[0072] When the transmission power of the three-phase T-type DC transformer is determined, the right side of equation (8) (U H -U L )I H / 2 and U ac are all fixed quantities, and the left side of the equation is the average value of the AC current in [0, T / 6]. Under certain operating conditions, the AC current i Hac1 The average value in the period [0, T / 6] is constant. According to the inequality theory, the AC current i Hac1 In the period [0, T / 6], it is always equal to I Hac When the AC current amplitude is the smallest, that is, the bridge arm current stress is the smallest, and the AC current i Hac1 Should be equal to -I during the period [T / 2, 2T / 3] Hac The AC current waveforms of the other two phases are Hac2 、i Hac3 Same reason.

[0073] Secondly, the alternating current i Hac1 The RMS current calculation formula is:

[0074]

[0075] At the AC current amplitude I Hac Under certain conditions, when the AC current amplitude in this period is always equal to I Hac / 2, the AC current RMS is minimum, that is, the bridge arm current RMS is minimum. When the AC current in the period [-T / 6,0] is constant equal to I Hac / 2, then the AC current in the period [T / 6, T / 3] is also always equal to I Hac / 2.

[0076] In summary, from the perspective of minimum current stress and minimum current RMS, the optimal AC current waveform of the three-phase T-type DC transformer is determined. According to the positive and negative symmetry of the waveform, the complete cycle waveform of the three-phase optimal AC current can be obtained, which is a step wave shape. However, considering the effects of the bridge arm inductance and line inductance, the AC current should have a certain slope when changing, and the bridge arm provides the driving voltage for the current change in this process. The actual three-phase optimal AC waveform is as follows Figure 6 The optimal three-phase AC current waveform takes into account the advantages of minimum bridge arm current stress and minimum bridge arm current RMS. Without changing the topological structure and number of components of the T-type DC transformer, the power capacity of the T-type DC transformer is significantly improved and the component loss is reduced compared with the three-phase sinusoidal AC current waveform.

[0077] Specific application examples are as follows:

[0078] According to the waveform design analysis of the three-phase T-type DC transformer, the maximum power capacity of the T-type DC transformer under trapezoidal wave operation can be calculated. The DC current component of the reused bridge arm of the T-type DC transformer is higher. When the reused bridge arm current stress I Stress_1 When the maximum current stress is reached, the power capacity of the T-type DC transformer will also reach its maximum. Stress_1 It can be expressed as:

[0079]

[0080] Similarly, for a three-phase T-type DC-DC transformer under sinusoidal operation, the current stress I Stress_2 It can be expressed as:

[0081]

[0082] Compared with the sinusoidal AC operation mode, the AC current amplitude under the trapezoidal wave operation mode is reduced by 25%. The trapezoidal wave operation mode of the T-type DC transformer significantly improves the power capacity of the T-type DC transformer and reduces the device loss by reducing the device current stress.

Claims

1. A trapezoidal wave operation method of a T-type DC transformer, characterized in that The method comprises the following steps: Step 1: Design the waveform constraints of the multi-phase T-type DC transformer The T-type DC transformer is composed of k-phase identical circuits. Each phase circuit contains a high-voltage bridge arm, a reused bridge arm, and a low-voltage bridge arm. To ensure the stable operation of the T-type DC transformer, it is necessary to ensure that the bridge arm voltage and current waveforms in each phase circuit meet the following constraints: DC voltage constraint, DC current constraint, and energy balance constraint, where: In view of the DC voltage constraint, in order to prevent the AC voltage component in the bridge arm from being reflected on the DC side, it is necessary to ensure that the AC voltage component amplitudes of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm in each phase circuit are equal, and the AC voltages between the two bridge arms should cancel each other out, that is, the phase difference is 180°; In view of the DC current constraint, in order to prevent the AC current component in the bridge arm from being reflected on the DC side, it is necessary to ensure that the AC current of the high-voltage bridge arm in the k-phase circuit can offset each other on the high-voltage DC side, and the AC current of the low-voltage bridge arm in the k-phase circuit can offset each other on the low-voltage DC side, that is, the sum of the AC current of the high-voltage bridge arm in the k-phase circuit of the T-type DC transformer is zero, and the sum of the AC current of the low-voltage bridge arm is zero; For the energy balance constraint, it is necessary to ensure that the average power absorbed or released by each bridge arm within an operation cycle T is zero to maintain the energy balance of the bridge arm; For a T-type DC transformer with any number of phases, as long as the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm meet the above three waveform constraints, the stable operation of the T-type DC transformer can be guaranteed and the smooth transmission of power can be achieved; Step 2: Design the AC waveform of the three-phase T-type DC transformer For the T-type DC transformer with three-phase structure, the AC waveform of the three-phase T-type DC transformer is designed by combining the waveform constraints of the multi-phase T-type DC transformer: In view of the DC voltage constraint, the AC voltage components of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of each phase in the three-phase circuit are equal in amplitude and opposite in phase to ensure that the AC voltage will not be reflected on the DC side. In order to improve the voltage utilization rate, the AC voltage waveform of each bridge arm is designed to be a square wave AC voltage. In view of the energy balance constraint, the average power absorbed or released by the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of each phase in the three-phase circuit in one operation cycle is zero; In view of the DC current constraint, the AC current of the high-voltage bridge arm in the three-phase circuit is staggered by 120°, and the AC current amplitude of the low-voltage bridge arm is zero, to ensure that the AC current will not be reflected on the DC side. In order to improve the current utilization rate, the waveforms of the AC current of the high-voltage bridge arm and the AC current of the low-voltage bridge arm are designed to be trapezoidal AC currents. For a three-phase T-type DC transformer, any square wave AC voltage or trapezoidal wave AC current that meets the above three waveform constraints can ensure the stable operation of the T-type DC transformer; Combined with the characteristics of the AC current waveform in an operation cycle T, the amplitude of the first half cycle and the second half cycle waveform are equal and the direction is opposite, it is further obtained that the AC current waveform of the high-voltage bridge arm should meet the following requirements: in the first half cycle, the sum of the AC current in the two periods [-T / 6,0] and [T / 6,T / 3] is equal to the current in the period [0,T / 6]; in the second half cycle, the sum of the AC current in the two periods [T / 3,T / 2] and [2T / 3,5T / 6] is equal to the current in the period [T / 2,2T / 3]; Step 3: Design the optimal AC current waveform of the three-phase T-type DC transformer For the three-phase T-type DC transformer, in order to further reduce the device current stress and device loss, the optimal AC current waveform of the three-phase T-type DC transformer is designed and analyzed from the two perspectives of minimum bridge arm current stress and minimum bridge arm current RMS: First half cycle [-T / 6, T / 3]: First, to ensure the minimum current stress in the bridge arm, the AC current needs to be kept constant in the period [0, T / 6], with an amplitude of I Hac ; Secondly, to ensure the minimum RMS current of the bridge arm, the AC current needs to be kept constant in the two periods [-T / 6,0] and [T / 6,T / 3], with an amplitude of I Hac / 2; Second half cycle [T / 3,5T / 6]: The waveform amplitude of the second half cycle is equal to that of the first half cycle, but the direction is opposite.

2. The trapezoidal wave operation method of a T-type DC transformer according to claim 1, characterized in that In the DC voltage constraint of step 1, the voltages of the high-voltage bridge arm, the reused bridge arm, and the low-voltage bridge arm in each phase circuit all satisfy the following formula: In the formula, u Hj 、u Mj and u Lj They represent the voltages of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of the jth phase, respectively. H is the high voltage DC side voltage, U L is the low voltage DC side voltage, u acj is the AC voltage component of the bridge arm of the jth phase.

3. The trapezoidal wave operation method of a T-type DC transformer according to claim 1, characterized in that In the energy balance constraint of step 1, the average power of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm in each phase circuit satisfies the following formula: In the formula, u Hj 、u Mj and u Lj They represent the voltages of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of the jth phase, respectively. H is the high voltage DC side voltage, U L is the low voltage DC side voltage, U dc is the DC component of the multiplexed bridge arm voltage, u acj is the AC voltage component of the bridge arm of the jth phase.

4. The trapezoidal wave operation method of a T-type DC transformer according to claim 1, characterized in that In the DC voltage constraint of step 2, the voltages of the high-voltage bridge arm, the reused bridge arm, and the low-voltage bridge arm of each phase all satisfy the following formula: In the formula, u Hj 、u Mj and u Lj They represent the voltages of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of the jth phase, respectively. H is the high voltage DC side voltage, U L is the low voltage DC side voltage, u acj is the AC voltage component of the bridge arm of the jth phase.

5. The trapezoidal wave operation method of a T-type DC transformer according to claim 1, characterized in that In the energy balance constraint of step 2, the average power of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of each phase satisfies the following formula: In the formula, u Hj 、u Mj and u Lj They represent the voltages of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of the jth phase, respectively. H is the high voltage DC side voltage, U L is the low voltage DC side voltage, U dc is the DC component of the multiplexed bridge arm voltage, u acj is the AC voltage component of the bridge arm of the jth phase.

6. The trapezoidal wave operation method of a T-type DC transformer according to claim 1, characterized in that In the DC current constraint of step 2, the bridge arm current is expressed as: In the formula, i Hj 、i Mj and i Lj They represent the currents of the high-voltage bridge arm, the reused bridge arm and the low-voltage bridge arm of the jth phase, respectively. H is the high voltage DC side current, I L is the low voltage DC side current, i Hacj and i Lacj are the AC components of the high-voltage and low-voltage bridge arm currents of the j-th phase respectively.

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