CN112653203B - 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 PDFInfo
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- CN112653203B CN112653203B CN202011523266.6A CN202011523266A CN112653203B CN 112653203 B CN112653203 B CN 112653203B CN 202011523266 A CN202011523266 A CN 202011523266A CN 112653203 B CN112653203 B CN 112653203B
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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 four sub-networks 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 voltage or frequency reduction of a certain sub-network due to heavy load is reduced.
Description
Technical Field
The invention relates to the field of alternating current-direct current hybrid power grids, in particular to an alternating current-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 sources and loads have become important components of power grids. The problems of complex structure, large loss and low reliability of the traditional alternating current to direct current are gradually highlighted by a plurality of stages of AC/DC converters and DC/DC converters. The AC/DC hybrid power grid can simultaneously accommodate AC-type and DC-type power supplies and loads, saves multi-stage AC/DC and DC/DC converters, and becomes an advantageous framework for the construction and development of future power grids.
The existing research on the AC/DC hybrid power grid focuses on the low-voltage AC/DC hybrid micro-power grid based on the interface converter, and the characteristic of low voltage level restricts the expansion of the scale and capacity of the hybrid power grid. In order to solve the problem, related scholars propose to use the multiport characteristic of the solid-state transformer to realize interconnection between the medium-voltage alternating-current power distribution network and the low-voltage alternating-current/direct-current micro-grid, or use the solid-state transformer to connect the low-voltage alternating-current/direct-current new energy source and the load into the medium-voltage alternating-current power distribution network. In the AC/DC 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 AC power distribution network or not. In the grid-connected mode, it is generally assumed that the medium-voltage alternating-current power distribution network is an infinite system, however, in actual operation, a situation that a medium-voltage alternating-current side is connected with a weak network may exist; in the island mode, only a limited low-voltage direct current energy storage device is used for providing support for a low-voltage alternating current-direct current micro-grid, the support capacity is limited, and stable operation is difficult to ensure. In addition, neither grid-connected mode nor island mode, the medium voltage dc port of the solid state transformer is fully utilized. With further development of direct current power sources and loads, medium voltage direct current will become an important port in the power grid. In summary, the existing ac/dc hybrid power grid technology based on the solid-state transformer still has a plurality of defects, which restrict and limit the further development of the technology.
Disclosure of Invention
The invention aims to overcome the defects of the prior art, solve the problems of low medium-voltage direct-current port utilization rate, poor weak network adaptability in a grid-connected mode and limited supporting capacity in an island mode of the existing alternating-current and direct-current hybrid power grid based on a solid-state transformer, and provides an alternating-current and direct-current hybrid power grid based on the solid-state transformer and a coordination control method thereof.
The invention is realized by the following technical scheme:
an AC/DC 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; 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 connected with the solid-state transformer through a medium-voltage direct-current bus and a low-voltage direct-current bus.
Wherein the Solid State Transformer (SST) is composed of a cascade of a medium voltage converter, a double active full bridge (DAB) and a low voltage converter; the medium-voltage converter interconnects a medium-voltage alternating-current sub-network and a medium-voltage direct-current sub-network, and the low-voltage converter interconnects a low-voltage alternating-current sub-network and a low-voltage direct-current sub-network; the double active full bridge realizes interconnection of four sub-networks 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 alternating current sub-network and the medium voltage direct current sub-network; the double-active full bridge further comprises a second autonomous power control unit for coordinating 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 medium-voltage alternating current sub-network is connected with the alternating current port of the medium-voltage converter through a medium-voltage alternating current bus, the medium-voltage direct current sub-network is connected with the direct current port of the medium-voltage converter through a medium-voltage direct current bus, the low-voltage direct current sub-network is connected with the direct current port of the low-voltage converter through a low-voltage direct current bus, and the low-voltage alternating current sub-network is connected with the alternating current port of the low-voltage converter through a low-voltage alternating current 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.
Furthermore, the medium-voltage converter and the low-voltage converter are both three-phase three-bridge arm converters.
Further, the medium-voltage alternating-current sub-network comprises a diesel generator and an AC/AC converter thereof, a blower and an AC/AC converter thereof, and an alternating-current 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 alternating current sub-network comprises a gas turbine and an AC/AC converter thereof, an energy storage system and a DC/AC converter thereof, and an alternating current load;
further, 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 loop and a current loop, the power outer loop is used for autonomous power control, and the inner flow loop is used for accurately and rapidly tracking the power reference value.
Further, the dual active full bridge includes a power loop for internal autonomous power control therein.
Further, the medium-voltage converter and the low-voltage converter are provided with phase-locked loops for detecting real-time frequency of the power grid;
a coordination control method for an AC/DC 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 specific steps are as follows:
in a 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 the 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 obtain the value of the medium-voltage AC sub-network and the voltage value of the medium-voltage DC bus within [ -1,0 [ -1 ]]Values within the interval; the normalized AC frequency value of the AC sub-network and the voltage value of the medium-voltage DC bus are subjected to difference, and the power imbalance state difference of the medium-voltage AC sub-network and the medium-voltage DC sub-network is obtained; inputting the difference value into a power outer ring of the medium-voltage converter to obtain an active power reference value to be transmitted by the medium-voltage converter; inputting the obtained active power reference value into a current inner loop of the medium-voltage converter, and generating a switching signal S of the medium-voltage converter through Sinusoidal Pulse Width Modulation (SPWM) 1 ～S 6 . A second autonomous power control for coordinating unbalanced power of the medium voltage subsystem and the low voltage subsystem; the specific steps are as follows:
the voltage value of the medium-voltage direct current bus and the voltage value of the low-voltage direct current bus are collected through a voltage transformer and normalizedObtaining both of them at [ -1,0]Values within the interval; the normalized voltage value of the medium-voltage direct-current bus and the normalized voltage value of the low-voltage direct-current bus are subjected to difference to obtain the power imbalance state difference of the medium-voltage subsystem and the low-voltage subsystem, and then the power loop of DAB is input to obtain the phase-shifting duty ratio reference value of DAB; converting the obtained phase shift duty ratio reference value into corresponding phase shift time, generating a switch signal T of DAB through Single Phase Shift (SPS) modulation 1 ～T 8 。
A third autonomous power control for coordinating unbalanced power in the low voltage ac sub-network and the low voltage dc sub-network; the specific steps are as follows:
in a 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 the low-voltage converter; normalizing the values of the AC frequency value of the low-voltage AC sub-network and the voltage value of the low-voltage DC bus within the [ -1,0] interval; the normalized AC frequency value of the low-voltage AC sub-network and the voltage value of the low-voltage DC bus are subjected to difference, and the power imbalance state difference of the low-voltage AC sub-network and the low-voltage DC sub-network is obtained; inputting the difference value into a power outer ring of the low-voltage converter to obtain an active power reference value to be transmitted by the low-voltage converter; and inputting the obtained active power reference value into a current inner loop 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 the DAB and the third autonomous power control of the low-voltage converter, 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 unbalanced power of each subnet is distributed in an equalizing mode.
Compared with the prior art, the alternating current-direct current 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 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 is realized by utilizing the solid-state transformer, and the four sub-networks are fully utilized to access power supplies and loads of different scales, different capacities, different voltage levels and different types, so that the access and integration of renewable energy power supplies are promoted, and the utilization rate of a medium-voltage direct-current port is improved;
(2) By the autonomous power coordination control method of the solid-state transformer, four subnets in the AC/DC hybrid power grid can be mutually supported, global power balance and sharing of the AC/DC hybrid power grid are realized, and the risk of instability caused by serious voltage or frequency reduction of a certain subnet due to heavy load is reduced.
(3) The method enables the AC/DC hybrid power grid based on the solid-state transformer to simultaneously consider a grid-connected mode and an island mode 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 adaptability of a grid-connected mode in a weak network environment is good; when the medium-voltage alternating-current sub-network only comprises a new energy power supply and a load, the alternating-current and direct-current hybrid power grid is equivalent to a large power grid which is separated from the large power grid and runs in an island mode, the four sub-networks can still support each other, 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;
FIG. 2 is a topology diagram of the solid state transformer of FIG. 1;
FIG. 3 is a schematic diagram of an autonomous power coordination control method of an AC/DC hybrid power grid based on a solid-state transformer;
FIG. 4 is a schematic diagram illustrating 8 different operation modes and switching modes of the solid-state transformer;
fig. 5 is a diagram of experimental waveforms for comparison of the front and rear of an ac/dc hybrid grid connected by a solid state transformer, for example, in operation mode 1;
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 shows waveforms of unbalanced power in four subnets over time; fig. 6b shows a power waveform diagram through the medium voltage converter, DAB and low voltage converter of the solid state transformer under the variation of fig. 6 a.
Detailed Description
In order to make the objects, technical solutions, advantageous effects and significant improvements of the embodiments of the present invention more clear, the technical solutions of 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 apparent that all of the described embodiments are only some embodiments of the present invention, but not all embodiments of the present invention; all other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
It should be noted that the terms "first," "second," "third," and the like in the description and claims of the present invention and in the drawings of the embodiments of the present invention are used for distinguishing between different objects and not for describing a particular sequential order.
It should also be noted that the following embodiments may be combined with each other, and the same or similar concepts or processes may not be described in detail in some embodiments.
As shown in fig. 1, a solid state transformer based ac-dc hybrid power grid 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) subnet and a Medium Voltage Direct Current (MVDC) subnet; the low-voltage subsystem consists of a low-voltage alternating current (LVAC) sub-network 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 subnetwork are connected to an MVAC bus through an AC/AC converter, and the AC load is directly connected to the MVAC bus; the energy storage system and the data center in the MVDC subnetwork are connected to the MVDC bus through the DC/DC converter, and the DC load is directly connected to the MVDC bus; the gas turbines in the LVAC subnetworks are connected into an LVAC bus through an AC/AC converter, the energy storage system is connected into the LVAC bus through a DC/AC converter, and the AC load is directly connected into the LVAC bus; photovoltaic and electric vehicles in the LVDC subnetwork are connected to the LVDC bus through a DC/DC converter, and direct current loads are directly connected to the LVDC bus.
As shown in fig. 2, the solid-state transformer is formed by cascading three parts of a Medium Voltage (MV) converter, a double active full bridge (DAB) converter and a Low Voltage (LV) converter, wherein the MV converter interconnects two subnets of MVAC and MVDC; the LV converter interconnects two subnets of LVAC and LVDC; the DAB realizes interconnection of four subnets by interconnecting MVDC and LVDC buses. The MV current converter and the LV current converter are three-phase three-bridge arm current converter with L filtering, and the DAB consists of two H-bridge current converters which are isolated and interconnected by adopting a high-frequency transformer. Alternating current port of MVAC bus and MV converter (alternating current filtering L mv Output port) connection; direct current port of MVDC bus and MV converter (direct current capacitor C mv ) Connecting; alternating current port of LVAC subnet and LV converter (alternating current filter L lv Output port) connection; direct current port of LVDC bus and LV converter (direct current capacitor C lv ) And (5) connection. DAB side is connected with the DC port (DC capacitor C) of the MV converter mv ) The other side is connected with a direct current port (direct current capacitor C) of the LV converter lv ) Thereby achieving interconnection of the MVDC bus and the LVDC bus. The MV converter, DAB and LV converter are controlled by respective autonomous power control units, respectively.
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 aim is to coordinate two subnets of MVAC and MVDC, so that the MVAC and MVDC subnets can bear unbalanced power in the two subnets in a balanced way; the method comprises the following specific steps:
step 101: collecting MVAC bus voltage value u through a voltage transformer mvac And MVDC bus voltage value V mvdc The MVAC bus voltage u mvac Through a phase-locked loop PLL in said medium voltage converter mvac Obtaining MVAC sub-network alternating frequency f mvac ；
Step 102: according to MVAC subnet AC frequency value f mvac And MVDC bus voltage value V mvdc Calculating power non-of MVAC and MVDC subnetsEquilibrium normalized metric PIS, i.e., f mvac And V mvdc Unified quantization to [ -1,0]Values within the interval:
wherein f mvac-max 、f mvac-min Respectively f mvac Maximum limit and minimum limit of (2); v (V) mvdc-max 、V mvdc-min V respectively mvdc Maximum limit and minimum limit of (2);
step 103: and carrying out difference on the normalized measurement values of the power unbalance states of the MVAC sub-network and the MVDC sub-network:
e 1 ＝PIS mvac -PIS mvdc (3)
step 104: will be the difference e 1 Power loop PI for input MV converter mv The controller obtains an active power reference value P which needs to be transmitted by the MV converter mv-ref ：
Wherein k is mv-p 、k mv-i Respectively MV converter power loops PI mv Proportional and integral coefficients of the controller; s is the Laplace operator;
step 105: reference value P of active power mv-ref The current is input into the current inner loop of the MV converter, and then the current is modulated by Sinusoidal Pulse Width Modulation (SPWM) to generate a switching signal S of the MV converter 1 ～S 6 。
Second autonomous power control: the control aim 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 transformer mvdc And LVDC bus voltage value V lvdc ；
Step 202: since the medium voltage subsystem and the low voltage subsystem are connected by the MVDC bus and the LVDC bus, the MVDC bus voltage value V mvdc And LVDC bus voltage value V lvdc Normalized to [ -1,0]Values in the interval measure the power imbalance states of the medium voltage subsystem and the low voltage subsystem:
wherein V is lvdc-max 、V lvdc-min V respectively lvdc Maximum limit and minimum limit of (2);
step 203: and comparing the difference of the normalized measurement values of the power unbalance states of the medium-voltage subsystem and the low-voltage subsystem with the difference of the power unbalance states of the two subsystems:
e 2 ＝PIS mvdc -PIS lvdc (7)
step 204: will be the difference e 2 Power loop PI for DAB input dab The controller obtains a phase shift duty ratio reference value of DAB:
wherein k is dab-p And k dab-i DAB power loops PI respectively dab Proportional and integral coefficients of the controller;
by controlling the phase-shift duty cycle D ref Active power P for DAB transmission can be realized dab-ref Is controlled by:
wherein f S A switching frequency of DAB; l (L) dab Equivalent leakage inductance of the DAB medium-high frequency transformer; n is the primary side voltage transformation ratio and the secondary side voltage transformation ratio of the high-frequency transformer;
step 205: phase-shifting duty cycle reference value D of DAB ref Converted into corresponding phase shift time T Dref ＝D ref T S /2，T S ＝1/f S . Will T Dref Input into Single Phase Shift (SPS) modulation to generate DAB switch signal T 1 ～T 8 。
Third autonomous power control: the control aim is to coordinate two subnets of LVAC and LVDC, so that the subnets of LVAC and LVDC can bear unbalanced power in the two subnets in a balanced way; the method comprises the following specific steps:
step 301: collecting LVAC bus voltage value u through voltage transformer lvac And LVDC bus voltage value V lvdc The LVAC bus voltage value u lvac Through a phase-locked loop PLL lvac Obtaining an alternating current frequency value f of the LVAC sub-network lvac ；
Step 302: according to the obtained LVAC sub-network alternating current frequency value f lvac And LVDC bus voltage value V lvdc Calculating normalized measurement value PIS of power unbalance states of LVAC sub-network and LVDC sub-network, namely f lvac And V lvdc Unified quantization to [ -1,0]Values within the interval:
wherein f lvac-max 、f lvac-min Respectively f lvac Maximum limit and minimum limit of (2);
step 303: and comparing the normalized measurement values of the power unbalance states of the LVAC sub-network and the LVDC sub-network with the power unbalance state difference of the two sub-networks:
e 3 ＝PIS lvac -PIS lvdc (12)
step (3-4): will be the difference e 3 Power loop PI of input LV converter lv The controller obtains an active power reference value P which needs to be transmitted by the LV converter lv-ref ：
Wherein k is lv-p And k lv-i LV converter power loops PI, respectively lv Proportional and integral coefficients of the controller;
step 305: reference value P of active power lv-ref The current is input into the inner loop of the LV converter, and then the switching signal Q of the LV converter is generated through SPWM modulation 1 ～Q 6 。
Through the MV converter autonomous power control, the DAB autonomous power control and the LV converter autonomous power control, global power balance and coordination control of four subnets of MVAC, LVAC, MVDC and LVDC can be realized, unbalanced power of each subnet is distributed in an equalizing mode, namely, normalized measurement values of the four subnets in an equilibrium state are equal:
PIS mvac ＝PIS lvac ＝PIS mvdc ＝PIS lvdc (14)
fig. 4 shows 8 different operation modes of the solid state transformer and its switching diagram according to the actual power P in the MV, DAB and LV converters mv 、P dab 、P lv The different flow directions (the regulated positive directions are shown in figure 1), the alternating current-direct current hybrid power grid based on the solid-state transformer has 8 running modes, and the different modes can be flexibly switched, so 56 mode switching processes are total. S is S 1-2 Indicating that mode 1 switches to mode 2 and so on.
For example, when the unbalanced power state in the four subnets 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 serious, the other three subnets are required to autonomously provide power support to the medium-voltage dc, so that the power flow direction in the LV converter is positive from the dc side of the LV converter to the ac side thereof; 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, so that the power flow direction in DAB is from the medium voltage direct current side to the low voltage direct current side of DAB and is positive; since the unbalanced power in the medium voltage ac sub-network is the least, so the power support it can provide is the most, the power flow direction in the MV converter is positive from the ac side to the dc side of the MV converter. When t=1s, the unbalanced power state of the four subnets becomes low voltage direct current > low voltage alternating current > medium voltage direct current > medium voltage alternating current, and the unbalanced power of the low voltage direct current subnets is the most, so that the other three subnets provide support for the low voltage direct current subnets, and the power flow direction of the LV converter becomes 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 the mode 1 to the mode 2 is completed.
Fig. 5 shows experimental waveforms (taking operation mode 1 as an example) of the front-back comparison of the connection of an ac/dc hybrid grid using a solid-state transformer, P mvac 、P lvac 、P mvdc 、P lvdc Unbalanced power is borne by MVAC, LVAC, MVDC, LVDC respectively. No solid state transformer interconnections (t=1 s before): MVAC, LVAC, MVDC and LVDC are operated independently, each carrying internal unbalanced power, and the power flowing in the MV, DAB and LV converters of the SST is 0MW. Since the unbalanced power in the four sub-networks is not equal, the ac frequency and the dc voltage drop are also different. Unbalanced power in MVAC, LVAC, MVDC and LVDC sub-networks is 1.5MW, 4.5MW, 2.5MW and 3.5MW respectively, unbalanced power of LVAC sub-network is the most serious, and PIS is obtained after normalization mvac <PIS mvdc <PIS lvdc <PIS lvac At this point the ac frequency of the LVAC subnetwork drops most severely. Solid state transformer interconnections are used (after t=1 s): after MVAC, LVAC, MVDC and LVDC sub-networks are interconnected through a solid-state transformer, the unbalanced power born by each sub-network is 3MW, namely, each sub-network is balanced to bear the unbalanced power of the four sub-networks, at the moment, the frequency drop of the LVAC sub-network is reduced, and the frequency drop level is the same as that of the LVAC sub-network, which indicates that the LVAC sub-network is obtainedPower support for the other three subnets. At this time, the power in the MV, DAB and LV converters of the solid state transformer is all forward, operating in mode 1.
Fig. 6 shows experimental waveforms for flexibly switching different operation modes of an ac/dc hybrid power grid based on a solid-state transformer. When the initial unbalanced power states of MVAC, LVAC, MVDC and LVDC subnets are different, the MV converter, DAB and LV converter of the solid-state transformer can flexibly and autonomously switch the operation modes, the power flow directions in the three converters are automatically changed, the operation requirements in different states are met, and all unbalanced power is uniformly borne by the four subnets all the time.
While the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those skilled in the art that the foregoing embodiments may be modified or equivalents may be substituted for some or all of the features thereof, and that the modifications or substitutions may be made without departing from the spirit and scope of the embodiments of the present invention.
Claims (8)
1. A coordination control method of an alternating current/direct current hybrid power grid based on a solid-state transformer is characterized in that the alternating current/direct current hybrid power grid comprises 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 low-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 consists of a medium-voltage converter, a double-active full-bridge DAB and a low-voltage converter which are cascaded; the medium-voltage converter interconnects a medium-voltage alternating-current sub-network and a medium-voltage direct-current sub-network, and the low-voltage converter interconnects a low-voltage alternating-current sub-network and a low-voltage direct-current sub-network; the double active full bridge realizes interconnection of four sub-networks by interconnecting a medium-voltage direct current bus and a low-voltage direct current bus;
the medium voltage converter further comprises a first autonomous power control unit 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 further comprises a second autonomous power control unit for coordinating 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;
the coordination control method of the alternating current/direct current hybrid power grid 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 specific steps are as follows:
in a 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 the 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 obtain the value of the medium-voltage AC sub-network and the voltage value of the medium-voltage DC bus within [ -1,0 [ -1 ]]Values within the interval; the normalized AC frequency value of the AC sub-network and the voltage value of the medium-voltage DC bus are subjected to difference, and the power imbalance state difference of the medium-voltage AC sub-network and the medium-voltage DC sub-network is obtained; inputting the difference value into a power outer ring of the medium-voltage converter to obtain an active power reference value to be transmitted by the medium-voltage converter; inputting the obtained active power reference value into a current inner loop of the medium-voltage converter, and generating a switching signal S of the medium-voltage converter through sine pulse width modulation 1 ～S 6 ；
A second autonomous power control for coordinating unbalanced power of the medium voltage subsystem and the low voltage subsystem; the specific steps are as follows:
the voltage value of the medium-voltage direct current bus and the voltage value of the low-voltage direct current bus are collected through a voltage transformer and normalized to obtain the voltage value of the medium-voltage direct current bus and the voltage value of the low-voltage direct current bus within the range of < -1 > and 0 [ -0 []Values within the interval; the normalized voltage value of the medium-voltage direct-current bus and the normalized voltage value of the low-voltage direct-current bus are subjected to difference to obtain the power imbalance state difference of the medium-voltage subsystem and the low-voltage subsystem, and then the power loop of DAB is input to obtain the phase-shifting duty ratio reference value of 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 modulation 1 ～T 8 ；
A third autonomous power control for coordinating unbalanced power in the low voltage ac sub-network and the low voltage dc sub-network; the specific steps are as follows:
in a 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 the low-voltage converter; normalizing the values of the AC frequency value of the low-voltage AC sub-network and the voltage value of the low-voltage DC bus within the [ -1,0] interval; the normalized AC frequency value of the low-voltage AC sub-network and the voltage value of the low-voltage DC bus are subjected to difference, and the power imbalance state difference of the low-voltage AC sub-network and the low-voltage DC sub-network is obtained; inputting the difference value into a power outer ring of the low-voltage converter to obtain an active power reference value to be transmitted by the low-voltage converter; and inputting the obtained active power reference value into a current inner loop of the low-voltage converter, and generating switching signals Q1-Q6 of the low-voltage converter through sine pulse width modulation.
2. The coordination control method of an alternating current-direct current hybrid power grid based on a solid-state transformer according to claim 1, wherein the medium voltage alternating current sub-network and an alternating current port of a medium voltage converter are connected through a medium voltage alternating current bus, the medium voltage direct current sub-network and a direct current port of the medium voltage converter are connected through a medium voltage direct current bus, the low voltage direct current sub-network and a direct current port of a low voltage converter are connected through a low voltage direct current bus, and the low voltage alternating current sub-network and an alternating current port of the low voltage converter are connected through a low voltage alternating current 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 coordination control method of an ac/dc hybrid power network based on a solid-state transformer according to claim 1, wherein the medium-voltage converter and the low-voltage converter are three-phase three-leg converters.
4. The method for coordinated control of a solid state transformer based AC-dc hybrid power grid according to claim 1, wherein the medium voltage AC sub-network comprises a diesel generator and its AC/AC converter, a wind turbine and its AC/AC converter, an AC load.
5. The method for coordinated control of a solid state transformer based ac/DC hybrid power grid according to claim 1, wherein the medium voltage DC sub-grid comprises an energy storage system and its DC/DC converter, a data center and its DC/DC converter, and other DC loads.
6. The method of coordinated control of a 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 converter, an energy storage system and its DC/AC converter, an AC load.
7. The coordination control method of an alternating current-direct current hybrid power grid based on a solid-state transformer according to claim 1, wherein the low-voltage direct current sub-grid 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.
8. The method for coordinated control of a solid state transformer based ac/dc hybrid power network according to claim 1, characterized in that the medium voltage converter and the low voltage converter have a phase locked loop for detecting the real time frequency of the power network.
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