CN112653203A - AC/DC hybrid power grid based on solid-state transformer and coordination control method thereof - Google Patents

AC/DC hybrid power grid based on solid-state transformer and coordination control method thereof Download PDF

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CN112653203A
CN112653203A CN202011523266.6A CN202011523266A CN112653203A CN 112653203 A CN112653203 A CN 112653203A CN 202011523266 A CN202011523266 A CN 202011523266A CN 112653203 A CN112653203 A CN 112653203A
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CN112653203B (en
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朱介北
李峰
刘迎澍
赵军
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Tianjin University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J5/00—Circuit arrangements for transfer of electric power between ac networks and dc networks
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Abstract

The invention discloses an alternating current-direct current hybrid power grid based on a solid-state transformer, which comprises a medium-voltage subsystem, a low-voltage subsystem and the solid-state transformer; the medium-voltage subsystem and the low-voltage subsystem are respectively connected with the solid-state transformer through a medium-voltage direct-current bus and a low-voltage direct-current bus; the solid-state transformer is formed by cascading a medium-voltage converter, a double-active full bridge and a low-voltage converter; the double-active full bridge realizes interconnection of the four subnets by interconnecting a medium-voltage direct-current bus and a low-voltage direct-current bus. The invention also discloses a coordination control method of the alternating current-direct current hybrid power grid based on the solid-state transformer, which comprises the following steps: a first autonomous power control, a second autonomous power control, and a third autonomous power control. According to the invention, through autonomous power coordination control of the solid-state transformer, global power balance and sharing of the AC/DC hybrid power grid are realized, and the risk of instability caused by serious reduction of voltage or frequency due to heavier load of a certain sub-network is reduced.

Description

AC/DC hybrid power grid based on solid-state transformer and coordination control method thereof
Technical Field
The invention relates to the field of alternating current and direct current hybrid power grids, in particular to an alternating current and direct current hybrid power grid based on a solid-state transformer and a coordination control method thereof.
Background
With the continuous development of photovoltaic power generation, data centers, electric vehicles and the like, direct-current power supplies and loads have become important components of power grids. The traditional method for converting alternating current into direct current needs to pass through a multi-stage AC/DC and DC/DC converter, and the problems of complex structure, large loss and low reliability are gradually highlighted. The alternating current-direct current hybrid power grid can simultaneously accommodate alternating current type and direct current type power supplies and loads, saves multistage AC/DC and DC/DC converters, and becomes an advantageous framework for the construction and development of the future power grid.
The existing research aiming at the alternating current-direct current hybrid power grid focuses on a low-voltage alternating current-direct current hybrid micro-power grid based on an interface converter, and the characteristic of low voltage level restricts the expansion of the scale and the capacity of the hybrid power grid. In order to solve the problem, related researchers propose to realize interconnection of a medium-voltage alternating-current power distribution network and a low-voltage alternating-current and direct-current micro-grid by utilizing the multiport characteristic of a solid-state transformer, or connect a low-voltage alternating-current and direct-current new energy power supply and a load into the medium-voltage alternating-current power distribution network by utilizing the solid-state transformer. In the alternating current-direct current hybrid power grid schemes, a grid-connected mode and an island mode are designed according to whether a solid-state transformer is interconnected with a medium-voltage alternating current distribution network or not. In a grid-connected mode, a medium-voltage alternating-current power distribution network is generally assumed to be an infinite system, however, a situation that a medium-voltage alternating-current side is connected with a weak network may exist in actual operation; in an island mode, only a limited low-voltage direct-current energy storage device is used for providing support for a low-voltage alternating-current and direct-current micro-grid, the support capability is limited, and stable operation is difficult to ensure. In addition, no matter in a grid-connected mode or an island mode, the medium-voltage direct-current port of the solid-state transformer is not fully utilized. With the further development of dc power supply and load, medium voltage dc will become an important port in the grid. In summary, the existing technology of the solid-state transformer-based ac/dc hybrid power grid still has many defects, which restrict and limit the further development thereof.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and solve the problems of low utilization rate of a medium-voltage direct-current port, poor adaptability of a weak grid in a grid-connected mode and limited support capability in an island mode of the conventional alternating-current and direct-current hybrid grid based on a solid-state transformer, and provides the alternating-current and direct-current hybrid grid based on the solid-state transformer and a coordination control method thereof.
The invention is realized by the following technical scheme:
an alternating current-direct current hybrid power grid based on a solid-state transformer comprises a medium-voltage subsystem, a low-voltage subsystem and a solid-state transformer (SST);
the medium-voltage subsystem consists of a medium-voltage alternating-current sub-network and a medium-voltage direct-current sub-network, and a centralized high-capacity power supply and a load are connected into the medium-voltage subsystem; the low-voltage subsystem consists of a low-voltage alternating-current sub-network and a low-voltage direct-current sub-network; the distributed small-capacity power supply and the load are connected into a low-voltage subsystem; the medium-voltage subsystem and the low-voltage subsystem are connected with the solid-state transformer through a medium-voltage direct-current bus and a low-voltage direct-current bus.
The Solid State Transformer (SST) is formed by cascading a medium-voltage converter, a double-active full bridge (DAB) and a low-voltage converter; the low-voltage converter is connected with the low-voltage AC sub-network and the low-voltage DC sub-network; the double-active full bridge realizes interconnection of the four subnets by interconnecting a medium-voltage direct-current bus and a low-voltage direct-current bus.
Further, the medium voltage converter further comprises a first autonomous power control unit for coordinating unbalanced power in the medium voltage ac sub-network and the medium voltage dc sub-network; the double-active full bridge also comprises a second autonomous power control unit which is used for coordinating the unbalanced power of the medium-voltage subsystem and the low-voltage subsystem; the low-voltage converter further comprises a third autonomous power control unit for coordinating unbalanced power in the low-voltage alternating-current sub-network and the low-voltage direct-current sub-network.
Further, the ac ports of the medium voltage ac sub-network and the medium voltage converter are connected through a medium voltage ac bus, the dc ports of the medium voltage dc sub-network and the medium voltage converter are connected through a medium voltage dc bus, the dc ports of the low voltage dc sub-network and the low voltage converter are connected through a low voltage dc bus, and the ac ports of the low voltage ac sub-network and the low voltage converter are connected through a low voltage ac bus; the direct current port of the medium voltage converter is connected with the medium voltage side of the DAB, and the low voltage side of the DAB is connected with the direct current port of the low voltage converter.
Further, the medium-voltage converter and the low-voltage converter are both three-phase three-bridge arm type converters.
Further, the medium-voltage AC sub-network comprises a diesel generator and an AC/AC converter thereof, a fan and an AC/AC converter thereof, and an AC load;
further, the medium-voltage direct-current sub-network comprises an energy storage system and a DC/DC converter thereof, a data center and a DC/DC converter thereof, and other direct-current loads;
further, the low-voltage AC sub-network comprises a gas turbine and an AC/AC converter thereof, an energy storage system and a DC/AC converter thereof, and an AC load;
furthermore, the low-voltage direct current sub-network comprises a photovoltaic power supply and a DC/DC converter thereof, an electric automobile power supply system and a DC/DC converter thereof, and other direct current loads.
Further, the medium-voltage converter and the low-voltage converter respectively comprise a power outer ring and a current ring, the power outer ring is used for autonomous power control, and the inner ring is used for accurately and quickly tracking a power reference value.
Further, the dual active full bridge comprises a power loop for inter-autonomous power control therein.
Further, the medium-voltage converter and the low-voltage converter are provided with phase-locked loops for detecting the real-time frequency of the power grid;
a coordination control method for the alternating current-direct current hybrid power grid based on the solid-state transformer comprises the following steps: a first autonomous power control, a second autonomous power control, and a third autonomous power control;
a first autonomous power control for coordinating unbalanced power in the medium voltage ac sub-network and the medium voltage dc sub-network; the method comprises the following specific steps:
in the medium-voltage subsystem, acquiring a medium-voltage alternating-current bus voltage value and a medium-voltage direct-current bus voltage value through a voltage transformer, and obtaining an alternating-current frequency value of a medium-voltage alternating-current sub-network by passing the medium-voltage alternating-current bus voltage value through a phase-locked loop in a medium-voltage converter; normalizing the AC frequency value of the medium-voltage AC sub-network and the voltage value of the medium-voltage DC bus to be [ -1, 0 [ -1]Within a sectionA numerical value; the normalized alternating current sub-network alternating current frequency value and the medium-voltage direct current bus voltage value are subjected to difference to obtain the power unbalance state difference of the medium-voltage alternating current sub-network and the medium-voltage direct current sub-network; then inputting the difference value into a power outer ring of the medium-voltage converter to obtain an active power reference value which needs to be transmitted by the medium-voltage converter; inputting the obtained active power reference value into a current inner ring of the medium-voltage converter, and generating a switching signal S of the medium-voltage converter through Sinusoidal Pulse Width Modulation (SPWM)1～S6. A second autonomous power control for coordinating unbalanced power of the medium voltage subsystem and the low voltage subsystem; the method comprises the following specific steps:
the voltage transformer is used for acquiring the voltage value of the medium-voltage direct-current bus and the voltage value of the low-voltage direct-current bus, and the voltage value of the medium-voltage direct-current bus and the voltage value of the low-voltage direct-current bus are normalized to obtain the voltage value of the medium-voltage direct-current bus and the voltage value of the low]A value within the interval; the normalized medium-voltage direct-current bus voltage value and the normalized low-voltage direct-current bus voltage value are subjected to difference to obtain the power unbalance state difference of a medium-voltage subsystem and a low-voltage subsystem, and then the power unbalance state difference is input into a power ring of DAB to obtain a phase-shifting duty ratio reference value of the DAB; converting the obtained phase-shift duty ratio reference value into corresponding phase-shift time, and generating a switching signal T of DAB through single phase-shift (SPS) modulation1～T8。
A third autonomous power control for coordinating unbalanced power in the low voltage ac sub-network and the low voltage dc sub-network; the method comprises the following specific steps:
in the low-voltage subsystem, acquiring a low-voltage alternating current bus voltage value and a low-voltage direct current bus voltage value through a voltage transformer, and obtaining the alternating current frequency of a low-voltage alternating current sub-network by passing the low-voltage alternating current bus voltage value through a phase-locked loop in a low-voltage converter; normalizing the AC frequency value of the low-voltage AC sub-network and the voltage value of the low-voltage DC bus to obtain the values of the AC frequency value and the voltage value of the low-voltage DC bus within the range of [ -1, 0 ]; the normalized alternating-current frequency value of the low-voltage alternating-current sub-network and the voltage value of the low-voltage direct-current bus are subjected to subtraction to obtain the power unbalance state difference of the low-voltage alternating-current sub-network and the low-voltage direct-current sub-network; then inputting the difference value into a power outer ring of the low-voltage converter to obtain an active power reference value which needs to be transmitted by the low-voltage converter; and inputting the obtained active power reference value into a current inner ring of the low-voltage converter, and generating switching signals Q1-Q6 of the low-voltage converter through Sinusoidal Pulse Width Modulation (SPWM).
Through the first autonomous power control of the medium-voltage converter, the second autonomous power control of DAB and the third autonomous power control of the low-voltage converter, the global power balance and coordination control of four subnets of medium-voltage alternating current, medium-voltage direct current, low-voltage alternating current and low-voltage direct current can be realized, and the unbalanced power of each subnet is distributed in a balanced manner.
Compared with the prior art, the AC/DC hybrid power grid based on the solid-state transformer and the coordination control method thereof have the advantages that:
(1) the interconnection of the medium-voltage AC sub-network, the low-voltage AC sub-network, the medium-voltage DC sub-network and the low-voltage DC sub-network is realized by utilizing the solid-state transformer, the four sub-networks are fully utilized to be accessed into power supplies and loads with different scales, different capacities, different voltage grades and different types, the access and integration of renewable energy power supplies are promoted, and the utilization rate of a medium-voltage DC port is improved;
(2) through the autonomous power coordination control method of the solid-state transformer, four sub-networks in the AC/DC hybrid power grid can be mutually supported, the global power balance and sharing of the AC/DC hybrid power grid are realized, and the risk of instability caused by serious reduction of voltage or frequency due to heavier load of a certain sub-network is reduced.
(3) The method enables the AC/DC hybrid power grid based on the solid-state transformer to simultaneously take a grid-connected mode and an island mode into consideration without a complex mode switching strategy. The medium-voltage alternating-current sub-network, the low-voltage alternating-current sub-network, the medium-voltage direct-current sub-network and the low-voltage direct-current sub-network have the same status, the assumption that a certain sub-network is an infinite system does not exist, and the grid-connected mode under the weak network environment has good adaptability; when the medium-voltage alternating-current sub-network only contains a new energy power supply and a load, the alternating-current and direct-current hybrid power grid is equivalent to be separated from a large power grid and operates in an island mode, the four sub-networks can still be mutually supported, and the supporting capacity of the island mode is improved.
Drawings
Fig. 1 is a topological structure diagram of an ac/dc hybrid power grid based on a solid-state transformer according to the present invention;
FIG. 2 is a block diagram of the topology of the solid state transformer of FIG. 1;
FIG. 3 is a schematic diagram of an autonomous power coordination control method for an AC/DC hybrid power grid based on a solid-state transformer according to the present invention;
fig. 4 schematically shows different operation modes of the solid-state transformer 8 and the switching thereof;
fig. 5 is a waveform diagram of a comparison experiment before and after connecting an ac-dc hybrid power grid with a solid-state transformer in an operation mode 1 as an example;
FIG. 6 is a waveform of an experiment for flexibly switching different operation modes of an AC/DC hybrid power grid based on a solid-state transformer; wherein FIG. 6a is a waveform diagram showing the time variation of unbalanced power in four subnets; fig. 6b shows the waveform of the power flowing through the solid state transformer medium voltage converter, DAB and low voltage converter in the variant of fig. 6 a.
Detailed Description
In order to make the objects, technical solutions, advantages and significant progress of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings provided in the embodiments of the present invention, and it is obvious that all of the described embodiments are only some embodiments of the present invention, but not all embodiments; all other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
It should be noted that the terms "first", "second", "third", etc. in the description and claims of the present invention and the accompanying drawings of the embodiments of the present invention are used for distinguishing different objects and are not used for describing a specific order.
It should be further noted that the following embodiments may be combined with each other, and the same or similar concepts or processes may not be repeated in some embodiments.
As shown in fig. 1, an ac/dc hybrid power grid based on a solid-state transformer includes a medium-voltage subsystem, a low-voltage subsystem and a solid-state transformer (SST); the medium voltage subsystem consists of a Medium Voltage Alternating Current (MVAC) sub-network and a Medium Voltage Direct Current (MVDC) sub-network; the low-voltage subsystem consists of a low-voltage alternating current sub-network (LVAC) and a low-voltage direct current (LVDC) sub-network; the medium-voltage subsystem and the low-voltage subsystem are connected with the solid-state transformer through a medium-voltage direct-current bus and a low-voltage direct-current bus. The diesel generator and the fan in the MVAC sub-network are both connected into the MVAC bus through the AC/AC converter, and the alternating current load is directly connected into the MVAC bus; the energy storage system and the data center in the MVDC sub-network are both connected into the MVDC bus through the DC/DC converter, and the direct current load is directly connected into the MVDC bus; the gas turbine in the LVAC sub-network is connected into the LVAC bus through the AC/AC converter, the energy storage system is connected into the LVAC bus through the DC/AC converter, and the alternating current load is directly connected into the LVAC bus; photovoltaic and electric vehicles in the LVDC sub-network are connected into the LVDC bus through the DC/DC converter, and the direct current load is directly connected into the LVDC bus.
As shown in fig. 2, the solid-state transformer is composed of a medium-voltage (MV) converter, a dual-active full-bridge (DAB) converter and a low-voltage (LV) converter, which are cascaded, and the MV converter interconnects two sub-networks of MVAC and MVDC; the LV current converter interconnects two sub-networks of LVAC and LVDC; and the DAB realizes interconnection of the four subnets by interconnecting the MVDC bus and the LVDC bus. The MV current converter and the LV current converter are three-phase three-bridge-arm current converters with L filtering, and the DAB is composed of two H-bridge current converters which are isolated and interconnected by adopting a high-frequency transformer. Alternating current port (alternating current filter L) of MVAC bus and MV convertermvOutput port) connection; direct current port (direct current capacitor C) of MVDC bus and MV convertermv) Connecting; alternating current port (alternating current filter L) of LVAC sub-network and LV current converterlvOutput port) connection; LVDC bus and DC port (DC capacitor C) of LV current converterlv) And (4) connecting. One side of DAB is connected with a direct current port (direct current capacitor C) of the MV convertermv) The other side is connected with a DC port (DC capacitor C) of the LV current converterlv) Therefore, the MVDC bus and the LVDC bus are interconnected. And the MV converter, the DAB converter and the LV converter are respectively controlled by respective autonomous power control units.
As shown in fig. 3, a coordination control method using the ac/dc hybrid power grid includes: a first autonomous power control, a second autonomous power control, and a third autonomous power control;
first autonomous power control: the control target is to coordinate the MVAC and MVDC subnets, so that the MVAC and MVDC subnets can balance unbalanced power in the MVAC and MVDC subnets; the method comprises the following specific steps:
step 101: MVAC bus voltage value u is collected through a voltage transformermvacAnd MVDC bus voltage value VmvdcThe MVAC bus voltage umvacThrough a phase-locked loop PLL in said medium voltage convertermvacObtaining the alternating current frequency f of the MVAC sub-networkmvac；
Step 102: according to the MVAC subnet alternating current frequency value fmvacAnd MVDC bus voltage value VmvdcCalculating power unbalance state normalization measurement value PIS of MVAC sub-network and MVDC sub-network, namely fmvacAnd VmvdcUniform quantization to [ -1, 0 [)]Values within the interval:
Figure BDA0002849945390000061
Figure BDA0002849945390000062
wherein f ismvac-max、fmvac-minAre respectively fmvacMaximum and minimum limits of; vmvdc-max、Vmvdc-minAre each VmvdcMaximum and minimum limits of;
step 103: and (3) differencing the power imbalance state normalized metric values of the MVAC sub-network and the MVDC sub-network:
e1＝PISmvac-PISmvdc (3)
step 104: will be different by e1Power loop PI of input MV convertermvA controller for obtaining an active power reference value P to be transmitted by the MV convertermv-ref：
Figure BDA0002849945390000063
Wherein k ismv-p、kmv-iPower loop PI of MV current converter respectivelymvProportional and integral coefficients of the controller; s is a laplace operator;
step 105: reference value P of active powermv-refInputting the current into the current inner loop of the MV converter, and then modulating the current by Sinusoidal Pulse Width Modulation (SPWM) to generate a switching signal S of the MV converter1～S6。
Second autonomous power control: the control objective is to coordinate the unbalanced power of the medium-voltage subsystem and the low-voltage subsystem; the method comprises the following specific steps:
step 201: MVDC bus voltage value V is collected through a voltage transformermvdcAnd LVDC bus voltage value Vlvdc；
Step 202: because the medium-voltage subsystem and the low-voltage subsystem are connected through the MVDC bus and the LVDC bus, the voltage value V of the MVDC bus is usedmvdcAnd LVDC bus voltage value VlvdcNormalized to [ -1, 0]And (3) measuring the power unbalance state of the medium-voltage subsystem and the low-voltage subsystem by using the numerical value in the interval:
Figure BDA0002849945390000064
Figure BDA0002849945390000071
wherein Vlvdc-max、Vlvdc-minAre each VlvdcMaximum and minimum limits of;
step 203: and (3) making a difference between the normalized measurement values of the power unbalance states of the medium-voltage subsystem and the low-voltage subsystem, and comparing the power unbalance state difference of the two subsystems:
e2＝PISmvdc-PISlvdc (7)
step 204: will be different by e2Power loop PI for input DABdabA controller for controlling the operation of the electronic device,obtaining a phase-shifting duty ratio reference value of DAB:
Figure BDA0002849945390000072
wherein k isdab-pAnd kdab-iAre respectively DAB power ring PIdabProportional and integral coefficients of the controller;
by controlling the phase-shift duty cycle DrefCan realize active power P for DAB transmissiondab-refThe control of (2):
Figure BDA0002849945390000073
wherein f isSIs the switching frequency of DAB; l isdabThe equivalent leakage inductance of the DAB medium-high frequency transformer is obtained; n is the primary and secondary side voltage transformation ratio of the high-frequency transformer;
step 205: phase-shift duty ratio reference value D of DABrefConverted into corresponding phase shift time TDref＝DrefTS/2，TS＝1/fS. Will TDrefInputting into Single Phase Shift (SPS) modulation to generate DAB switching signal T1～T8。
Third autonomous power control: the control target is to coordinate two sub-networks of LVAC and LVDC, so that the LVAC and LVDC sub-networks can balance unbalanced power in the LVAC and LVDC sub-networks; the method comprises the following specific steps:
step 301: LVAC bus voltage value u collected by voltage transformerlvacAnd LVDC bus voltage value VlvdcSaid LVAC bus voltage value ulvacThrough a phase-locked loop PLLlvacObtaining the alternating current frequency value f of the LVAC subnetlvac；
Step 302: according to the obtained LVAC subnet alternating current frequency value flvacAnd LVDC bus voltage value VlvdcCalculating the normalized measurement value PIS of the power unbalance state of the LVAC sub-network and the LVDC sub-network, namely flvacAnd VlvdcUniform quantization to [ -1, 0 [)]Values within the interval:
Figure BDA0002849945390000074
Figure BDA0002849945390000075
wherein f islvac-max、flvac-minAre respectively flvacMaximum and minimum limits of;
step 303: and (3) making a difference between the normalized measurement values of the power imbalance states of the LVAC sub-network and the LVDC sub-network, and comparing the power imbalance state difference of the two sub-networks:
e3＝PISlvac-PISlvdc
(12)
step (3-4): will be different by e3Power loop PI of input LV current converterlvA controller for obtaining an active power reference value P to be transmitted by the LV current converterlv-ref：
Figure BDA0002849945390000081
Wherein k islv-pAnd klv-iPower loops PI for LV current converters respectivelylvProportional and integral coefficients of the controller;
step 305: reference value P of active powerlv-refInputting the current into the current inner ring of the LV converter, and then generating a switching signal Q of the LV converter through SPWM modulation1～Q6。
Through the MV converter autonomous power control, the DAB autonomous power control and the LV converter autonomous power control, the global power balance and coordination control of the MVAC, the LVAC, the MVDC and the LVDC sub-networks can be realized, the unbalanced power of each sub-network is distributed in a balanced manner, namely, the normalized measurement values of the four sub-networks in a balanced state are equal:
PISmvac＝PISlvac＝PISmvdc＝PISlvdc (14)
FIG. 4 shows 8 different operation modes of the solid-state transformer and the switching schematic thereof according to the actual power P in the MV converter, DAB converter and LV convertermv、Pdab、PlvThe flow direction is different (the specified positive direction is shown in fig. 1), the alternating current-direct current hybrid power grid based on the solid-state transformer has 8 operation modes, and the different modes can be flexibly switched, so 56 mode switching processes are total. S1-2Indicating that mode 1 switches to mode 2 and so on.
For example, when the unbalanced power state in the four sub-networks is low-voltage ac > medium-voltage dc > low-voltage dc > medium-voltage ac, since the unbalanced power of the medium-voltage ac is the most severe, the other three sub-networks are required to provide power support to the medium-voltage dc autonomously, and thus the power flow direction in the LV converter is from the dc side of the LV converter to the ac side thereof, and is positive; because the unbalanced power of the medium-voltage subsystem is smaller than that of the low-voltage subsystem, the medium-voltage subsystem provides power support for the low-voltage subsystem, and the power flow direction in the DAB is positive from the medium-voltage direct-current side to the low-voltage direct-current side of the DAB; since the unbalanced power in the medium voltage ac sub-network is the least and therefore the most power support it can provide, the power flow in the MV converter is positive going from the ac side to the dc side of the MV converter. When T is 1s, the unbalanced power state of the four subnets is changed into low-voltage direct current, low-voltage alternating current, medium-voltage direct current and medium-voltage alternating current, the unbalanced power of the low-voltage direct current subnetwork is the most, therefore, the other three subnets provide support for the low-voltage direct current subnetwork, and the power flow direction of the LV current converter is changed into negative from the alternating current side to the direct current side; the power flow direction of the DAB converter and the MV converter is unchanged. Thereby, the switching of mode 1 to mode 2 is completed.
FIG. 5 shows experimental waveforms (taking operation mode 1 as an example), P, of a front-to-back comparison of AC/DC hybrid power grid connected by a solid-state transformermvac、Plvac、Pmvdc、PlvdcUnbalanced power is borne by MVAC, LVAC, MVDC and LVDC respectively. No solid state transformer interconnection (t 1s ago): the MVAC, LVAC, MVDC and LVDC sub-networks all independently run and respectively bear internal unevennessThe balanced power, and the power flowing through the MV converter, the DAB converter and the LV converter of the SST are all 0 MW. Since the unbalanced powers in the four sub-networks are not equal, the ac frequency and the dc voltage drop are different. Unbalanced power in the four subnets of MVAC, LVAC, MVDC and LVDC is 1.5MW, 4.5MW, 2.5MW and 3.5MW respectively, unbalanced power of LVAC subnet is the most serious, and PIS is normalizedmvac<PISmvdc<PISlvdc<PISlvacThe ac frequency droop of the LVAC sub-network is the most severe at this time. Interconnection with solid state transformers (after t ═ 1 s): after the four subnets of MVAC, LVAC, MVDC and LVDC are interconnected through the solid-state transformer, the unbalanced power borne by each subnet is 3MW, namely, each subnet bears the unbalanced power of the four subnets in a balanced manner, at the moment, the frequency drop of the LVAC subnet is reduced to some extent, and the frequency drop level of the LVAC subnet is the same as that of the MVAC subnet, which indicates that the LVAC subnet is supported by the power of the other three subnets. At this time, the power in the MV converter, DAB and LV converter of the solid-state transformer is all positive, and the mode 1 is operated.
Fig. 6 shows experimental waveforms for flexibly switching different operation modes of an alternating-current and direct-current hybrid power grid based on a solid-state transformer. When the initial unbalanced power states of the MVAC, LVAC, MVDC and LVDC sub-networks are different, the MV converter, the DAB converter and the LV converter of the solid-state transformer can flexibly and autonomously switch the operation modes, the power flow directions of the three converters are automatically changed, the operation requirements under different states are met, and all unbalanced power is uniformly borne by the four sub-networks all the time.
Although the present invention has been described in detail with reference to the foregoing embodiments, it should be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made on the technical solutions described in the foregoing embodiments, or some or all of the technical features of the embodiments can be replaced with equivalents, without departing from the scope of the embodiments of the present invention, and the technical solutions can not be modified or replaced by the modifications, the modifications and the substitutions in the non-essential scope of the present invention.

Claims (9)

1. An alternating current-direct current hybrid power grid based on a solid-state transformer is characterized by comprising a medium-voltage subsystem, a low-voltage subsystem and the solid-state transformer;
the medium-voltage subsystem consists of a medium-voltage alternating-current sub-network and a medium-voltage direct-current sub-network, and a centralized high-capacity power supply and a load are connected into the medium-voltage subsystem;
the low-voltage subsystem consists of a low-voltage alternating current sub-network and a low-voltage direct current sub-network, and a distributed small-capacity power supply and a load are connected into the low-voltage subsystem; the medium-voltage subsystem and the low-voltage subsystem are respectively connected with the solid-state transformer through a medium-voltage direct-current bus and a low-voltage direct-current bus;
the solid-state transformer is formed by cascading a medium-voltage converter, a double-active full bridge (DAB) and a low-voltage converter; the low-voltage converter is connected with the low-voltage AC sub-network and the low-voltage DC sub-network; the double-active full bridge realizes interconnection of the four subnets by interconnecting a medium-voltage direct-current bus and a low-voltage direct-current bus;
the medium-voltage converter also comprises a first autonomous power control unit, a second autonomous power control unit and a third autonomous power control unit, wherein the first autonomous power control unit is used for coordinating unbalanced power in the medium-voltage alternating-current sub-network and the medium-voltage direct-current sub-network; the double-active full bridge also comprises a second autonomous power control unit which is used for coordinating the unbalanced power of the medium-voltage subsystem and the low-voltage subsystem; the low-voltage converter further comprises a third autonomous power control unit for coordinating unbalanced power in the low-voltage alternating-current sub-network and the low-voltage direct-current sub-network.
2. The solid state transformer based ac/dc hybrid power grid according to claim 1, wherein the ac ports of the medium voltage ac sub-network and the medium voltage inverter are connected by a medium voltage ac bus, the dc ports of the medium voltage dc sub-network and the medium voltage inverter are connected by a medium voltage dc bus, the dc ports of the low voltage dc sub-network and the low voltage inverter are connected by a low voltage dc bus, and the ac ports of the low voltage ac sub-network and the low voltage inverter are connected by a low voltage ac bus; the direct current port of the medium voltage converter is connected with the medium voltage side of the DAB, and the low voltage side of the DAB is connected with the direct current port of the low voltage converter.
3. The solid state transformer based ac/dc hybrid power grid of claim 1, wherein the medium and low voltage converters are each three-phase three-limb type converters.
4. The solid state transformer based AC/DC hybrid power grid according to claim 1, wherein the medium voltage AC subnetwork comprises a diesel generator and its AC/AC inverter, a wind turbine and its AC/AC inverter, and an AC load.
5. The solid state transformer based AC/DC hybrid power grid according to claim 1, wherein the medium voltage DC sub-network comprises an energy storage system and its DC/DC converter, a data center and its DC/DC converter, and other DC loads.
6. The solid state transformer based AC/DC hybrid power grid according to claim 1, wherein the low voltage AC sub-network comprises a gas turbine and its AC/AC inverter, an energy storage system and its DC/AC inverter, and an AC load.
7. The solid state transformer based AC/DC hybrid power grid according to claim 1, wherein the low voltage DC sub-network comprises a photovoltaic power source and its DC/DC converter, an electric vehicle power supply system and its DC/DC converter, and other DC loads.
8. The solid state transformer based ac/dc hybrid power grid of claim 1, wherein the medium and low voltage inverters have phase locked loops for detecting real time frequency of the power grid.
9. A coordinated control method using the ac/dc hybrid grid according to claim 1, wherein the method comprises: a first autonomous power control, a second autonomous power control, and a third autonomous power control;
a first autonomous power control for coordinating unbalanced power in the medium voltage ac sub-network and the medium voltage dc sub-network; the method comprises the following specific steps:
in the medium-voltage subsystem, acquiring a medium-voltage alternating-current bus voltage value and a medium-voltage direct-current bus voltage value through a voltage transformer, and obtaining an alternating-current frequency value of a medium-voltage alternating-current sub-network by passing the medium-voltage alternating-current bus voltage value through a phase-locked loop in a medium-voltage converter; normalizing the AC frequency value of the medium-voltage AC sub-network and the voltage value of the medium-voltage DC bus to be [ -1, 0 [ -1]A value within the interval; the normalized alternating current sub-network alternating current frequency value and the medium-voltage direct current bus voltage value are subjected to difference to obtain the power unbalance state difference of the medium-voltage alternating current sub-network and the medium-voltage direct current sub-network; then inputting the difference value into a power outer ring of the medium-voltage converter to obtain an active power reference value which needs to be transmitted by the medium-voltage converter; inputting the obtained active power reference value into a current inner ring of the medium-voltage converter, and generating a switching signal S of the medium-voltage converter through sinusoidal pulse width modulation1～S6；
A second autonomous power control for coordinating unbalanced power of the medium voltage subsystem and the low voltage subsystem; the method comprises the following specific steps:
the voltage transformer is used for acquiring the voltage value of the medium-voltage direct-current bus and the voltage value of the low-voltage direct-current bus, and the voltage value of the medium-voltage direct-current bus and the voltage value of the low-voltage direct-current bus are normalized to obtain the voltage value of the medium-voltage direct-current bus and the voltage value of the low]A value within the interval; the normalized medium-voltage direct-current bus voltage value and the normalized low-voltage direct-current bus voltage value are subjected to difference to obtain the power unbalance state difference of a medium-voltage subsystem and a low-voltage subsystem, and then the power unbalance state difference is input into a power ring of DAB to obtain a phase-shifting duty ratio reference value of the DAB; converting the obtained phase-shift duty ratio reference value into corresponding phase-shift time, and generating a switch signal T of DAB through single phase-shift modulation1～T8；
A third autonomous power control for coordinating unbalanced power in the low voltage ac sub-network and the low voltage dc sub-network; the method comprises the following specific steps:
in the low-voltage subsystem, acquiring a low-voltage alternating current bus voltage value and a low-voltage direct current bus voltage value through a voltage transformer, and obtaining the alternating current frequency of a low-voltage alternating current sub-network by passing the low-voltage alternating current bus voltage value through a phase-locked loop in a low-voltage converter; normalizing the AC frequency value of the low-voltage AC sub-network and the voltage value of the low-voltage DC bus to obtain the values of the AC frequency value and the voltage value of the low-voltage DC bus within the range of [ -1, 0 ]; the normalized alternating-current frequency value of the low-voltage alternating-current sub-network and the voltage value of the low-voltage direct-current bus are subjected to subtraction to obtain the power unbalance state difference of the low-voltage alternating-current sub-network and the low-voltage direct-current sub-network; then inputting the difference value into a power outer ring of the low-voltage converter to obtain an active power reference value which needs to be transmitted by the low-voltage converter; and inputting the obtained active power reference value into a current inner ring of the low-voltage converter, and generating switching signals Q1-Q6 of the low-voltage converter through sinusoidal pulse width modulation.
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