CN112271912B - An active damping method for suppressing harmonics in the low-voltage side dead zone of power electronic transformers - Google Patents

Abstract

The invention discloses an active damping method for suppressing the harmonics of the low-voltage side dead zone of a power electronic transformer. The invention simplifies the LCL filter model into a first-order system by simplifying the LCL filter model, then introduces an active resistance R _AD and selects The active resistance value R _AD , after realizing the current proportional feedforward of the active damping technology, corrects the current control, and connects the correction link in series after the controller to complete the suppression of the harmonics in the low-voltage side dead zone of the power electronic transformer. The method of the invention uses the active damping technology to improve the low-frequency impedance of the LCL filter, the realization principle is simple, and the low-frequency impedance introduced by the dead zone in the LCL filter-based DC/AC converter on the low-voltage side of the power electronic transformer is improved. The effect of harmonic components.

Description

An active damping method for suppressing harmonics in the low-voltage side dead zone of power electronic transformers

technical field

The invention relates to the technical field of power electronic transformers, in particular to an active damping method for suppressing harmonics in the low-voltage side dead zone of a power electronic transformer.

Background technique

Power electronic transformer (PET) is also known as energy router. Compared with traditional power transformer, PET can not only realize functions such as voltage level conversion, electrical isolation and energy transfer, but also realize additional functions such as power flow control and power quality control. system development trend. While the DC/AC converter outputs the fundamental wave (power frequency 50Hz) component voltage, its switching mode will introduce switching sub-high-frequency harmonics into the output voltage of the three-phase inverter bridge, and the large high-frequency harmonics of the LCL filter The impedance can significantly filter out high frequency harmonics; the dead zone added to prevent the direct connection of the inverter bridge will generate a large number of low frequency harmonic components, and the low frequency impedance of the LCL filter is small, and its suppression ability is limited.

Traditional dead-band harmonic suppression methods are mainly divided into two categories, dead-band compensation and optimal control. Among them, the method of dead zone compensation starts from the modulation process of the switch, analyzes the magnitude of the harmonic voltage caused by the dead zone, and adjusts the magnitude of the modulated wave based on this, so as to realize the suppression of the dead zone harmonics. However, the harmonic voltage caused by the dead zone is related to the direction of the current, the nonlinearity of the switching device, and the fluctuation of the DC bus voltage. It is difficult to implement accurate dead zone compensation. The optimal control method starts with the design and optimization of the current controller, such as adopting and designing a reasonable resonant controller, repetitive controller, etc. Its essence is to increase the equivalent impedance of the inverter, so that the harmonic voltage in the same dead zone can be achieved. produce smaller harmonic currents. These optimal control methods can achieve good results, but the principle is complicated and the implementation is troublesome, and the development cycle is long.

SUMMARY OF THE INVENTION

Aiming at the deficiencies and defects of the prior art, the present invention provides an active damping method for suppressing harmonics in the low-voltage side dead zone of a power electronic transformer. The method uses the active damping technology to improve the low-frequency impedance of the LCL filter, thereby Suppress the low-frequency harmonic components introduced by the dead zone in the LCL filter-based DC/AC converter on the low-voltage side of the power electronic transformer.

The object of the present invention can be realized through the following technical solutions:

An active damping method for suppressing low-voltage side dead zone harmonics of a power electronic transformer, comprising the following steps:

Step 1: Simplify the LCL filter model. The low-voltage side DC/AC converter of the power electronic transformer has an LCL filter. The transfer function from the output voltage V of the three-phase inverter bridge to the grid-connected current I _g is:

The LCL filter is simplified to a first-order system, and its transfer function is:

Step 2: Select the active resistance value R _AD . After introducing the active resistance R _AD , the transfer function is approximately equivalent to:

The value of R _AD significantly affects the frequency range of harmonic suppression, and the value of R _AD is:

R _AD ≥2πnf _{1_max} (L ₁ +L ₂ )

Among them, f _{1_max} is the maximum value taken when considering the grid voltage frequency fluctuation; n is the highest order to suppress low-frequency harmonics.

Step 3: Realize the current proportional feedforward of the active damping technology, detect the inverter side current I _L1 in the control system, and subtract the product of I _L1 and R _AD from the given inverter bridge output voltage V _ref .

Step 4: Correct the current control. After using the active damping technology, the transfer function is approximately equivalent to:

为数字控制系统的延时环节，T_s为控制系统的采样周期。对比G_{AD_d}(s)与G(s)，可知采用有源阻尼方法之后，被控对象的传递函数与之前相比发生了变化。in,

is the delay link of the digital control system, and T _s is the sampling period of the control system. Comparing G _{AD_d} (s) and G(s), it can be seen that after the active damping method is adopted, the transfer function of the controlled object has changed compared with before.

In order to achieve the same current control effect, the correction link is connected in series after the controller, and the suppression of the dead zone harmonics on the low-voltage side of the power electronic transformer is completed at the same time. The correction link process is as follows:

Further, step 2 uses the active damping technology to improve the low-frequency impedance of the LCL filter, thereby suppressing the low-frequency harmonic components introduced by the dead zone in the LCL filter-based DC/AC converter on the low-voltage side of the power electronic transformer.

Further, in step 2, n can be based on the demand value, considering the requirement of the 50th fundamental frequency in the relevant power quality standards, and it is recommended that the value of n satisfies n≥50.

Beneficial technical effects of the present invention:

1. The series resistance in the LCL filter increases its low-frequency impedance, which significantly improves the LCL filter's ability to suppress harmonics in the dead zone.

2. The control effect achieved by using the active damping technology in the control is equivalent to the series resistance in the actual circuit, and will not introduce additional power loss.

3. Introducing active damping technology and selecting the appropriate active resistance value R _AD , can significantly improve the low-frequency impedance of the LCL filter, thereby suppressing the dead zone in the low-voltage side of the power electronic transformer in the LCL filter-based DC/AC converter. Introduced low frequency harmonic components.

Description of drawings

FIG. 1 is a topology diagram of a power electronic transformer of the active damping method for suppressing the harmonics in the low-voltage side dead zone of the power electronic transformer according to the present invention.

2 is a schematic diagram of a DC/AC converter based on an LCL filter on the low-voltage side of a power electronic transformer using the active damping method for suppressing the dead-zone harmonics on the low-voltage side of the power electronic transformer according to the present invention

FIG. 3 is a flow chart of the active damping method for suppressing the harmonics in the low-voltage side dead zone of the power electronic transformer according to the present invention.

4 is a Bode diagram of the LCL filter transfer function G _LCL (s) and its simplified form G(s) of the active damping method for suppressing the low-voltage side dead zone harmonics of a power electronic transformer according to the present invention.

5 is a Bode diagram of the simplified form G _AD (s) of the LCL filter after adding the active resistance R _AD in the active damping method for suppressing the harmonics in the low voltage side dead zone of the power electronic transformer according to the present invention.

Figure 6 is a topology diagram of grid-connected current control without active resistor technology.

FIG. 7 is a topological diagram of grid-connected current control using the active resistance technology of the active damping method for suppressing the harmonics in the low-voltage side dead zone of the power electronic transformer according to the present invention.

Figure 8 is the simulation result of the grid-connected current for grid-connected current control without active resistance technology.

9 is the simulation result of the grid-connected current of the grid-connected current control using the active resistance technology of the active damping method for suppressing the harmonics in the low-voltage side dead zone of the power electronic transformer according to the present invention.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to illustrate the present invention, but not to limit the present invention.

The topology diagram of the power electronic transformer applied in the present invention is shown in FIG. 1 . The power electronic transformer has medium and high voltage AC ports and low voltage AC ports. The medium and high voltage AC port side is a modular structure. The A, B, and C three-phase modules are respectively composed of N modules. The H bridges of the N modules of each phase are connected in series to form the input stage of the cascaded H bridge (CHB) structure. , to achieve AC/DC conversion; the DC bus of each H-bridge is connected to the low-voltage DC bus through a series resonant dual-active H-bridge isolation converter, and realizes DC/DC conversion and isolation. On the low-voltage AC port side, the AC/DC converter connects the low-voltage DC bus and the low-voltage AC power grid to realize DC/AC conversion.

Figure 2 shows the schematic diagram of the DC/AC converter based on the LCL filter on the low-voltage side of the power electronic transformer. While the DC/AC converter outputs the fundamental wave (power frequency 50Hz) component voltage, its switching mode will be in three-phase The switching sub-high-frequency harmonics are introduced into the output voltage of the inverter bridge, and the large high-frequency impedance of the LCL filter can significantly filter out the high-frequency harmonics. In addition, the dead zone added to prevent the direct connection of the inverter bridge will generate a large number of low-frequency harmonic components, and the low-frequency impedance of the LCL filter is small, and its suppression capability is limited.

Figure 3 shows the implementation steps of the present invention. The introduction of active damping technology and selection of appropriate active resistance value R _AD significantly improves the low-frequency impedance of the LCL filter, thereby suppressing the low-voltage side of the power electronic transformer. The low frequency harmonic components introduced by the dead zone in the /AC converter.

An active damping method for suppressing harmonics in the low-voltage side dead zone of a power electronic transformer of the present invention includes the following steps:

Step 1: Simplify the LCL filter model. The DC/AC converter on the low-voltage side of the power electronic transformer is equipped with an LCL filter, and the transfer function from the output voltage V of the three-phase inverter bridge to the grid-connected current I _g is:

The above equation shows that the LCL filter is a third-order system, and the Bode plot of its transfer function G _LCL (s) is shown in Figure 4. It can be seen from Figure 4 that the high-frequency impedance of the LCL filter is large, so it has a good high-frequency filtering effect; but its low-frequency impedance is small, so the ability to suppress low-frequency harmonic components is limited. The harmonic components introduced by the dead zone in the DC/AC converter are mainly low frequency, and the LCL filter has a limited ability to suppress them.

In order to simplify the analysis of the low frequency band, the LCL filter is simplified to a first-order system, and its transfer function is:

A Bode plot of the simplified form G(s) of the LCL filter transfer function is shown in FIG. 4 .

Step 2: Select the active resistance value R _AD . The series resistance in the LCL filter can significantly increase its low-frequency impedance, which can significantly improve the LCL filter's ability to suppress harmonics in the dead zone. After introducing the active resistor R _AD , the transfer function is approximately equivalent to:

The Bode plot of the simplified form G _AD (s) of the LCL filter after adding the active resistor R _AD is shown in Figure 5. It can be seen that the Bode plot of G _AD (s) contains two asymptotes, and the intersection is called as The turning point, whose angular frequency is:

To the left of the inflection point (in the direction of decreasing frequency), the Bode plot of G _AD (s) is mainly affected by the active resistance R _AD , indicating that R _AD can significantly increase the low-frequency impedance of the LCL filter; to the right of the inflection point, R AD _AD is less affected. This shows that the value of R _AD significantly affects the frequency range of harmonic suppression, and an appropriate active resistance value R _AD should be selected.

The value of _RAD is:

R _AD ≥2πnf _{1_max} (L ₁ +L ₂ )

Among them, f _{1_max} is the maximum value taken when considering the grid voltage frequency fluctuation; n is the highest order to suppress low-frequency harmonics. n can be based on the demand value, considering the requirements for the 50th fundamental frequency in the power quality related standards, the selected active resistance value R _AD should at least have a suppressing effect on the 50th fundamental frequency, and it is recommended that the value of n satisfies n≥50 .

Step 3: Implement current proportional feedforward with active damping technology. After the current in the actual circuit flows through the resistor, a voltage drop will be generated across the resistor. In order to achieve the same effect, the inverter side current I _L1 is detected in the control system and subtracted from the given inverter bridge output voltage V _ref The product of I _L1 and R _AD .

The grid-connected current control topology without active resistance technology and the grid-connected current control topology with active resistance technology are shown in Figure 6 and Figure 7, respectively, where Figure 7 shows the current proportional feedforward with active damping technology. Implementation details. The control effect achieved by using active damping technology in the control is equivalent to the series resistance in the actual circuit, and no additional power loss is introduced.

Step 4: Correct the current control. After using the active damping technique, the transfer function is approximately equivalent to:

为数字控制系统的延时环节，T_s为控制系统的采样周期。对比G_{AD_d}(s)与G(s)，可知采用有源阻尼方法之后，被控对象的传递函数与之前相比发生了变化。in,

is the delay link of the digital control system, and T _s is the sampling period of the control system. Comparing G _{AD_d} (s) and G(s), it can be seen that after the active damping method is adopted, the transfer function of the controlled object has changed compared with before.

In order to achieve the same current control effect, the correction link is connected in series after the controller, and the suppression of the dead zone harmonics on the low-voltage side of the power electronic transformer is completed at the same time. The correction link process is as follows:

Figure 7 shows the location of the correction link.

The grid-connected current simulation results of the grid-connected current control without using the active resistance technology and the grid-connected current simulation results of the grid-connected current control using the active resistance technology are shown in Figure 8 and Figure 9, respectively. In the current waveform of Figure 8 A large amount of low-frequency harmonics are included, and the current waveform of FIG. 9 using the method of the present invention is closer to a sine, and the low-frequency harmonic components are significantly reduced, indicating the effectiveness of the present invention.

The above-mentioned embodiments are descriptions of specific embodiments of the present invention, rather than limitations of the present invention. Those skilled in the art can also make various transformations and changes without departing from the spirit and scope of the present invention to obtain Corresponding and equivalent technical solutions, therefore all equivalent technical solutions should be included in the patent protection scope of the present invention.

Claims (2)

1. An active damping method for suppressing dead zone harmonics on the low-voltage side of a power electronic transformer is characterized by comprising the following steps:

step 1: the model of the LCL filter is simplified, the low-voltage side DC/AC converter of the power electronic transformer is provided with the LCL filter, and the three-phase inverter bridge outputs the voltage V to the grid-connected current I _g The transfer function of (a) is:

the LCL filter is simplified into a first-order system, and the transfer function of the LCL filter is as follows:

step 2: selecting an active resistance value R _AD Introducing an active resistor R _AD The transfer function is then approximately equivalent to:

wherein R is _AD The value of (a) significantly affects the frequency range of harmonic suppression, R _AD The values of (A) are as follows:

R _AD ≥2πnf _{1_max} (L ₁ +L ₂ )

wherein f is _{1_max} The maximum value is taken when the voltage frequency fluctuation of the power grid is considered; n is the highest number of times of low-frequency harmonic suppression, the low-frequency impedance of the LCL filter is improved by using an active damping technology, and the low-frequency harmonic component introduced by a dead zone in the DC/AC converter of the low-voltage side of the power electronic transformer based on the LCL filter is suppressed;

and step 3: implementing active damping techniquesCurrent proportional feedforward, detecting current I on inverter side in control system _L1 And a given V is output by the inverter bridge _ref Minus I _L1 And R _AD The product of (a) and (b),

and 4, step 4: correcting current control, and after adopting an active damping technology, approximating and equating a transfer function as:

wherein,

for time-delay links of digital control systems, T _s To control the sampling period of the system, contrast G _{AD_d} (s) and G(s), it is known that the transfer function of the controlled object is changed from before after the active damping method is adopted,

in order to realize the same current control effect, a correction link is connected in series behind a controller, and the suppression of the harmonic waves of the dead zone at the low-voltage side of the power electronic transformer is completed at the same time, wherein the correction link process is as follows: 0

2. The active damping method for suppressing the dead zone harmonic waves at the low-voltage side of the power electronic transformer according to claim 1, wherein n in the step 2 can consider the requirement on the frequency of 50 fundamental waves in the power quality related standard according to a required value, and the value of n is more than or equal to 50.

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