CN105978370A - Sine power transmission method for improving electric power electronic transformer power density - Google Patents

Abstract

The invention discloses a sinusoidal power transmission method for improving the power density of a power electronic transformer, which is respectively transmitted to the three-phase public low-voltage DC side through the three-phase circuit of the power electronic transformer. The sinusoidal power transmission method includes A-phase power fluctuation transmission, B-phase Phase power fluctuation transfer and C-phase power fluctuation transfer, the power electronic transformer is composed of a cascaded AC-DC converter, an intermediate DC capacitor and a DC-DC converter according to a three-phase system structure. The three-phase power electronic transformer provided by the present invention facilitates the interconnection of AC-DC hybrid power grids of different voltage levels, and the sinusoidal power transmission method provided realizes the coordinated conversion of active power between AC-DC buses of different rated voltage levels, breaking through the volume of traditional power electronic transformers and manufacturing costs, which meet the development needs of future AC-DC hybrid intelligent distribution network interconnection.

Description

A Sinusoidal Power Transfer Method for Improving the Power Density of Power Electronic Transformers

technical field

The invention relates to flexible VSC conversion and DC voltage transformation technology, in particular to a sinusoidal power transmission method for improving the power density of a power electronic transformer.

Background technique

The future distribution network will be the main carrier of the widely interconnected, highly intelligent, open and interactive "Energy Internet", which can realize the optimization of energy production, transmission, distribution, conversion, and consumption in a wide area. In addition to undertaking the distribution task of electric energy, the future distribution network also needs to realize the exchange and distribution of energy in the region, and meet the needs of distributed power access and consumption.

As a key component of the AC distribution network, traditional power frequency transformers use power frequency iron cores to achieve basic functions such as voltage transformation, isolation, and energy transfer. The requirements of the power grid for the level of intelligence of electrical equipment. Power Electronics Transformers (PETs), also known as Solid-State Transformers (SST), are the core components of the new distribution network. They combine power electronic power conversion technology with electromagnetic induction high-frequency isolation technology. To realize the flexible control of electric energy AC and DC hybrid, it is the energy router of the future energy Internet terminal.

At present, power electronic transformers generally adopt a cascaded multi-level system structure, and a three-phase system structure is built through a "Y" or "Δ" type. In order to make the power electronic transformer truly engineering, in addition to taking into account high efficiency and high output performance, its overall power density should be improved, the space volume should be reduced, and the cost should be reduced. However, for the cascaded system, in order to reduce the influence of the secondary power fluctuation of the AC grid on the low-voltage DC side power, the traditional practice is to connect a large capacitor in parallel on the DC side of the cascaded power electronic transformer to limit the transmission of power fluctuations. At the same time, in order to increase the reliability of the system, the film capacitor is used, which increases the volume of the power electronic transformer, reduces the power density, and increases the cost.

Contents of the invention

In order to solve the above problems, the present invention provides a sinusoidal power transmission method that improves the power density of power electronic transformers. With the support of traditional three-phase AC busbars, the coordination of active power between AC and DC busbars of different rated voltage levels is realized through three-phase power electronic transformers. It is an important breakthrough for the interconnected power transmission of the multi-voltage level AC-DC hybrid distribution network in the future. It can greatly reduce the size and cost of power electronic transformers, and at the same time improve power density.

In order to achieve the above object, the technical scheme that the present invention takes is:

A sinusoidal power transmission method for increasing the power density of a power electronic transformer, which is transmitted to the three-phase public low-voltage DC side through the three-phase circuit of the power electronic transformer respectively. When power fluctuations are transmitted to the low-voltage public DC side, due to the three-phase current and The voltage is a power frequency sine wave, and its three-phase power is

The three-phase power fluctuation is twice the fundamental frequency fluctuation and the three phases are symmetrical. When the power fluctuation is transmitted to the low-voltage public DC side, since the intermediate DC side capacitor does not bear the three-phase power fluctuation, the three-phase power fluctuation P _dca on the low-voltage DC side, P _{dcb and P dcc are also} two times the frequency fluctuation of the fundamental frequency and the three phases are symmetrical. According to the three-phase symmetrical system power theory, the power fluctuation of the three-phase symmetrical system is zero, so the common DC side power P _dc (t) = P _dca + P _{dcb +P dcc will} have small fluctuations.

The sinusoidal power transmission method includes A-phase power fluctuation transfer, B-phase power fluctuation transfer and C-phase power fluctuation transfer, and the power electronic transformer is composed of a cascaded AC-DC converter, an intermediate DC capacitor and a DC-DC converter According to the combination of the three-phase system structure, the AC side of the AC-DC converter is connected to the high-voltage AC busbar, and the DC side is connected in series to the intermediate DC side capacitor C2 _{; the two ends} of the intermediate DC side capacitor C2 are respectively connected to the AC-DC converter and DC -DC converter connection; the DC-DC converter includes a DC-AC converter, a high-frequency transformer and an AC-DC converter, and the DC side of the DC-AC converter is connected in series with the intermediate DC side capacitor C2 _; one side of the high-frequency transformer is connected to The AC side of the DC-AC converter is connected, and the other side is connected to the AC of the AC-DC converter; the AC side of the AC-DC converter is connected to the high-frequency transformer, and the DC side is connected to the low-voltage DC bus. Phase C has the same structure, and the three-phase circuits are all connected in parallel on the low-voltage DC bus side.

Among them, the current and voltage on the AC side of the A-phase power fluctuation transmission are both power frequency sine waves, respectively i _ac = I _m sinω ₀ t and v _ac = V _m sinω ₀ t; its power P _a (t) = V _m I _m sin ² ω ₀ t is twice the frequency fluctuation of the fundamental frequency. The power fluctuation of phase A flows to the low-voltage public DC side through the AC-DC converter. The capacitance value of the intermediate DC side capacitor C ₂ , the intermediate DC side capacitor does not need to bear the power fluctuation transmitted from the AC grid A phase, so the fluctuation amplitude of P _ripa on the intermediate DC side capacitor is very small; the power fluctuation will continue to pass through the DC -The DC converter transfers to the low-voltage DC side, and the power fluctuation on the low-voltage DC side is P _dca

Among them, the current and voltage on the AC side of the B-phase power fluctuation transmission are both power frequency sine waves, respectively i _ac =I _m sin(ω ₀ t-120°) and v _ac =V _m sin(ω ₀ t-120°) ; Its power P _b (t)=V _m I _m sin ² (ω ₀ t-120°) is twice the fundamental frequency fluctuation, and the B-phase power fluctuation flows to the low-voltage public DC side through the AC-DC converter. When reaching the intermediate DC side, according to the sinusoidal power transmission method, the capacitance value of the intermediate DC side capacitor _C2 of the B phase is reduced, and the intermediate DC side capacitor does not need to bear the power fluctuations transmitted from the B phase of the AC grid, so the intermediate DC side The fluctuation amplitude of P _ripb on the capacitor is very small, and the power fluctuation will continue to be transmitted to the low-voltage DC side through the DC-DC converter, and the power fluctuation on the low-voltage DC side is P _dcb .

Wherein, the C-phase power fluctuation transmission AC side current and voltage are both power frequency sine waves, respectively i _ac =I _m sin(ω ₀ t+120°) and v _ac =V _m sin(ω ₀ t+120° °); its power P _c (t)=V _m I _m sin ² (ω ₀ t+120°) is twice the fundamental frequency fluctuation, and the C-phase power fluctuation flows to the low-voltage public DC side through the AC-DC converter, When it is transmitted to the intermediate DC side, according to the sinusoidal power transmission method, the capacitance value of the intermediate DC side capacitor _C2 of the C phase is reduced, and the intermediate DC side capacitor does not need to bear the power fluctuations transmitted from the C phase of the AC grid, so the intermediate The fluctuation amplitude of P _ripc on the DC side is very small, and the power fluctuation will continue to be transmitted to the low-voltage DC side through the DC-DC converter, and the power fluctuation on the low-voltage DC side is P _dcc .

Wherein, the AC-DC converter is constructed based on cascaded sub-modules, and the cascaded sub-modules include a full-bridge VSC converter.

Wherein, the intermediate DC capacitor _C2 is composed of three-phase capacitors with small capacitance.

Wherein, the DC-DC converter is constructed based on parallel sub-modules, and the cascaded sub-modules include resonant converters.

Wherein, the AC-DC converter and the DC-DC converter have power transferability.

The present invention has the following beneficial effects:

1) The sinusoidal power transmission method provided by the present invention can realize the mixed and coordinated conversion of active power between AC and DC buses of different rated voltage levels;

2) The sinusoidal power transmission method provided by the present invention can effectively prolong the service life of the intermediate DC side capacitor of the power electronic transformer and improve the safety of the capacitor;

3) The sinusoidal power transmission method provided by the present invention can effectively increase the power density of the three-phase cascaded power electronic transformer and reduce the manufacturing cost of the three-phase cascaded system;

4) The sinusoidal power transmission method provided by the present invention can effectively reduce the power fluctuation of the DC side capacitance in the middle of the power electronic transformer, so that the dynamic response of the three-phase cascaded power electronic transformer becomes faster;

5) The sinusoidal power transmission method provided by the present invention can realize combined modular design.

Description of drawings

Fig. 1 is a basic structural diagram of a three-phase cascaded two-stage power electronic transformer capable of power transmission in an embodiment of the present invention;

2 is a system diagram of a sinusoidal power transmission method for improving the power density of a power electronic transformer according to an embodiment of the present invention;

3 is a structural diagram of an AC/DC converter submodule in a three-phase cascaded power electronic transformer in an embodiment of the present invention;

Fig. 4 is a sub-module structure diagram of a DC/DC converter in a three-phase cascaded power electronic transformer in an embodiment of the present invention.

detailed description

In order to make the objects and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the examples. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

As shown in Figure 1, the embodiment of the present invention provides a three-phase cascaded two-stage power electronic transformer capable of power transmission, the electronic transformer is composed of a cascade-based AC-DC converter 1, an intermediate DC capacitor 2 and DC-DC converter 3 are combined according to the three-phase system structure. The AC side of AC-DC converter 1 is connected to the high-voltage AC busbar, and the DC side is connected in series to the intermediate DC side capacitor C ₂ 2; the intermediate DC side capacitor C ₂ 2 The two ends are respectively connected to AC-DC converter 1 and DC-DC converter 3; DC-DC converter 3 includes a DC-AC converter, a high-frequency transformer and an AC-DC converter, and the DC side of the DC-AC converter is connected to The intermediate DC side capacitor C ₂ 2 is connected in series; one side of the high-frequency transformer is connected to the AC side of the DC-AC converter, and the other side is connected to the AC side of the AC-DC converter; the AC side of the AC-DC converter is connected to the high-frequency transformer , the DC side is connected to the low-voltage DC busbar, the A-phase, B-phase and C-phase have the same structure, and the three-phase circuits are connected in parallel on the low-voltage DC busbar side.

For the traditional three-phase AC-DC conversion system, because the current and voltage of the three-phase AC side of the Y-type or Δ-type connection are power frequency sine waves, the power is transmitted to the DC side through the power electronic transformer at twice the fundamental frequency fluctuation. In order to prevent this power fluctuation from being transmitted to the low-voltage DC side and ensure the power stability of the low-voltage DC side connected in parallel, the traditional method is to install a large-volume, large-capacity capacitor C on the intermediate DC side of the three-phase cascaded power electronic transformer. ₂ to suppress power fluctuations.

The sinusoidal power transmission method based on the three-phase cascaded power electronic transformer provided by the present invention is to reduce the intermediate DC side capacitance C ₂ to ensure that the three-phase power fluctuations on the AC side can be transmitted to the public low-voltage DC through the three-phase circuits of the power electronic transformer respectively. side, according to the three-phase symmetrical system power theory, the three-phase instantaneous power fluctuation is zero, so the power of the common low-voltage DC side of the three-phase cascaded power electronic transformer has no effect. Therefore, the sinusoidal power transmission method can effectively reduce the volume of the intermediate DC side capacitor without affecting the power of the high-voltage AC side and the public low-voltage DC side, thereby reducing the volume and manufacturing cost of the three-phase cascaded power electronic transformer and increasing the power. Power density of electronic transformers.

As shown in Figure 2, the embodiment of the present invention provides a sinusoidal power transmission method for increasing the power density of the power electronic transformer. For the A-phase circuit in the three-phase cascaded power electronic transformer adopting the sinusoidal power transmission method, the AC side current and The voltages are all power frequency sine waves, respectively

$\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; ac</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}\mathrm{<span\; class="notranslate">\; t</span>}\\ {\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; ac</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}\mathrm{<span\; class="notranslate">\; t</span>}\end{array}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Its power P _a (t)=V _m I _m sin ² ω ₀ t is twice the frequency fluctuation of the fundamental frequency. The power fluctuation transmission process is shown in (1). The A-phase power fluctuation flows to the DC side through the AC-DC converter , when it is transferred to the intermediate DC side, according to the sinusoidal power transmission method, the capacitance value of the intermediate DC side capacitor C ₂ of the A-phase circuit is reduced, and the intermediate DC side capacitor does not need to bear the power fluctuation transmitted from the A-phase of the AC grid. Therefore, the fluctuation amplitude of P _ripa on the capacitor on the intermediate DC side is very small; since the DC-DC converter has power transmission capability, the power fluctuation will continue to be transmitted to the low-voltage DC side through the DC-DC converter, and the power fluctuation on the low-voltage DC side is P _dca .

For the B-phase circuit in the three-phase cascaded power electronic transformer using the sinusoidal power transmission method, the AC side current and voltage are both power frequency sine waves, respectively

Its power P _b (t) = V _m I _m sin ² (ω ₀ t-120°) is twice the frequency fluctuation of the fundamental frequency. The power fluctuation transmission process is shown in (2). The B-phase power fluctuation is converted by AC-DC When the converter flows to the DC side, when it is transferred to the intermediate DC side, according to the method of sinusoidal power transmission, the capacitance value of the intermediate DC side capacitor C ₂ of the B-phase circuit is reduced, and the intermediate DC side capacitor does not need to bear the load from the AC grid B-phase The transmitted power fluctuates, so the fluctuation amplitude of P _ripb on the intermediate DC side capacitor is very small; because the DC-DC converter has power transmission capability, the power fluctuation will continue to be transmitted to the low-voltage DC side through the DC-DC converter, and the low-voltage DC The power fluctuation on the side is P _dcb .

For the C-phase circuit in the three-phase cascaded power electronic transformer using the sinusoidal power transmission method, the AC side current and voltage are both power frequency sine waves, respectively

Its power P _c (t)=V _m I _m sin ² (ω ₀ t+120°) is twice the frequency fluctuation of the fundamental frequency, and the transmission process of the power fluctuation is shown in (3). The C-phase power fluctuation is converted by AC-DC When it is transferred to the intermediate DC side, according to the sinusoidal power transmission method, the capacitance value of the intermediate DC side capacitor C ₂ of the C-phase circuit is reduced, and the intermediate DC side capacitor does not need to bear the transfer from the C-phase of the AC grid The power fluctuations in the past, so the fluctuation amplitude of P _ripc on the intermediate DC side is very small; because the DC-DC converter has power transmission capability, the power fluctuation will continue to be transmitted to the low-voltage DC side through the DC-DC converter, and the power on the low-voltage DC side The fluctuation is P _dcc .

Since the three-phase current and voltage on the AC side are all power frequency sine waves, the three-phase power is

The three-phase power fluctuation is twice the fundamental frequency fluctuation and the three phases are symmetrical. According to the sinusoidal power transmission method, the three-phase power fluctuations are transmitted to the low-voltage public DC side respectively, and the three-phase power fluctuations are shown in (4) on the public low-voltage DC side. Since the capacitor on the DC side of the three-phase cascaded power electronic transformer does not bear the three-phase power fluctuations and the DC-DC converter has power transmission capability, the three-phase power fluctuations P _dca , P _{dcb and P dcc on} the low-voltage DC side are also The frequency fluctuation is twice the fundamental frequency and the three phases are symmetrical. According to the three-phase symmetrical system power theory, the power fluctuation of the three-phase symmetrical system is zero, so the common DC side power P _dc (t) = P _dca +P _{dcb +P dcc will} be There are small fluctuations.

The AC-DC converter 1 is constructed based on cascaded sub-modules, and the specific structure of the sub-modules is a full-bridge type, as shown in FIG. 3 .

The resonant converter 3 is constructed based on parallel sub-modules, and the specific structure of the sub-modules is a resonant converter with power transmission capability, as shown in FIG. 4 .

The above is only a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, and these improvements and modifications should also be It is regarded as the protection scope of the present invention.

Claims (8)

1. A sinusoidal power transmission method that improves power electronic transformer power density, is characterized in that, is passed to public low-voltage DC side (4) respectively by the three-phase circuit of power electronic transformer, and described sinusoidal power transmission method comprises A phase power fluctuation Transfer (1), B-phase power fluctuation transfer (2) and C-phase power fluctuation transfer (3), the power electronic transformer is composed of a cascaded AC-DC converter (1), an intermediate DC capacitor (2) and a DC - The DC converter (3) is composed according to the three-phase system structure. The AC side of the AC-DC converter (1) is connected to the high-voltage AC busbar, and the DC side is connected in series to the intermediate DC side capacitor C ₂ (2); the intermediate DC side capacitor Both ends of C ₂ (2) are respectively connected to AC-DC converter (1) and DC-DC converter (3); DC-DC converter (3) includes DC-AC converter, high-frequency transformer and AC-DC Converter, the DC side of the DC-AC converter is connected in series with the intermediate DC side capacitor C ₂ (2); one side of the high-frequency transformer is connected to the AC side of the DC-AC converter, and the other side is connected to the AC side of the AC-DC converter ; The AC side of the AC-DC converter is connected to the high-frequency transformer, and the DC side is connected to the low-voltage DC bus. The A-phase, B-phase and C-phase have the same structure, and the three-phase circuits are connected in parallel on the low-voltage DC bus side. 2. a kind of sinusoidal power transmission method that improves power electronic transformer power density as claimed in claim 1, is characterized in that, A phase power fluctuation transfers (1) AC side current and voltage are power frequency sine waves, respectively i _ac ＝I _m sinω ₀ t and V _ac ＝V _m sinω ₀ t; its power P _a (t)＝V _m I _m sin ² ω ₀ t is twice the fundamental frequency fluctuation, and the power fluctuation on the low-voltage DC side is P _dca . 3. a kind of sinusoidal power transmission method that improves power electronic transformer power density as claimed in claim 1, is characterized in that, B-phase power fluctuation transfers (2) AC side current and voltage are power frequency sine waves, respectively i _ac = I _m sin (ω ₀ t-120°) and V _ac = V _m sin (ω ₀ t-120°); its power P _b (t) = V _m I _m sin ² (ω ₀ t-120° ) is twice the frequency fluctuation of the fundamental frequency, and the B-phase power fluctuation flows to the low-voltage public DC side (4) through the AC-DC converter, and the power fluctuation on the low-voltage DC side is P _dcb . 4. a kind of sinusoidal power transmission method that improves power electronic transformer power density as claimed in claim 1, is characterized in that, described C-phase power fluctuation transfers (3) AC side current and voltage are power frequency sine waves, respectively It is i _ac =I _m sin(ω ₀ t+120°) and V _ac =V _m sin(ω ₀ t+120°); its power P _c (t)=V _m I _m sin ² (ω ₀ t+ 120°) is twice the fundamental frequency fluctuation, and the C-phase power fluctuation flows to the low-voltage public DC side (4) through the AC-DC converter, and the power fluctuation on the low-voltage DC side is P _dcc . 5. A kind of sinusoidal power transmission method that improves power electronic transformer power density as claimed in claim 1, is characterized in that, described AC-DC converter (1) is based on cascade sub-module construction, and cascade sub-module comprises all Bridge type VSC converter. 6 . A sinusoidal power transmission method for increasing the power density of a power electronic transformer according to claim 1 , wherein the intermediate DC capacitor C ₂ ( 2 ) is composed of three-phase capacitors with small capacitance. 7 . 7. A kind of sinusoidal power transmission method that improves power electronic transformer power density as claimed in claim 1, is characterized in that, described DC-DC converter (3) is based on parallel sub-module construction, and cascade sub-module comprises resonant conversion device. 8. A kind of sinusoidal power transmission method that improves the power electronic transformer power density as claimed in claim 1, is characterized in that, described AC-DC converter (1) and DC-DC converter (3) have power transferability .