CN108718083A - Series connection multiport DC power flow controller based on MMC - Google Patents
Series connection multiport DC power flow controller based on MMC Download PDFInfo
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- CN108718083A CN108718083A CN201810616732.1A CN201810616732A CN108718083A CN 108718083 A CN108718083 A CN 108718083A CN 201810616732 A CN201810616732 A CN 201810616732A CN 108718083 A CN108718083 A CN 108718083A
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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
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
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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
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
- H02J1/08—Three-wire systems; Systems having more than three wires
- H02J1/082—Plural DC voltage, e.g. DC supply voltage with at least two different DC voltage levels
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Abstract
A kind of series connection multiport DC power flow controller based on MMC, belongs to technical field of electric power.The purpose of the present invention is from the angle of power flowcontrol, flow controller from simple DC grid, promote out the DC power flow controller of multiport cascaded structure, the effective solution series connection multiport DC power flow controller based on MMC of the uncontrollable problem of DC grid trend.Step of the present invention is：Select the structure for being most suitable for while interconnecting more DC voltage levels；Establish the internal model of the topological structure；Build the multi-voltage grade DC grid model containing SM-DCPFC；By simulating, verifying, the multiport flow controller can realize the flexible control to Line Flow in several cases.The exchange side of this multiport flow controller is attached by the present invention by transformer and neighbouring AC power, such as current conversion station, electric substation, marine wind electric field, public exchange side is connect with these AC powers, ac bus voltage can more be stablized.
Description
Technical field
The invention belongs to technical field of electric power.
Background technology
Traditional ac transmission structure, running technology are faced with huge challenge in terms of accessing new energy, this just makes directly
Stream transmission of electricity again obtained the concern of people, it can be good at solving the problems, such as new-energy grid-connected.In recent years, both at home and abroad
Scholar to the research of direct current transportation from simple to complexity, from the line commutation D.C. high voltage transmission at most basic both ends to current more
Terminal voltage source type D.C. high voltage transmission, from the topological structure of point-to-point type to the ring-like and net type knot comprising a large amount of DC lines
The degree of redundancy of structure, system is greatly improved, but the trend of circuit far can not meet actual requirement, trend
The unreasonable of distribution will cause circuit overload, and DC grid fallback is caused even to bring serious security risk.Trend
The introducing of controller solves the problems, such as this well, for improving the degree of freedom of DC grid power flowcontrol, ensures direct current
The safe and stable operation of net has important practical significance.
Apply four classes that the flow controller in single voltage class includes mainly following：Resistor-type flow controller, DC-
DC power flow controller between DC converter types, auxiliary voltage source type and line, their introducing solve DC line tide well
Flow uncontrollable problem.And the DC grid interconnection of multi-voltage grade has become the development trend of the following DC grid, at present
The DC network of multiple and different voltage class is interconnected mainly based on multiport DC/DC converters, is broadly divided into following
Three classes：Multiport active bridge DC/DC converters, multiport LCL resonant mode DC/DC converters and the multiport DC/ based on MMC
DC converters.There is the field of respective advantage and application, but there is also certain shortcomings, such as active bridge DC/
DC converters are more applicable for low-pressure field, and no-load voltage ratio is limited between each DC port of LCL resonant mode DC/DC converters, is based on MMC
The loss of high cost of multiport DC/DC converters it is big.
Invention content
The purpose of the present invention is from the angle of power flowcontrol, the flow controller from simple DC grid is promoted
Go out the DC power flow controller of multiport cascaded structure, the effective solution uncontrollable problem of DC grid trend based on MMC
Series connection multiport DC power flow controller.
Step of the present invention is：
Step 1：By analyzing several topological structures of multiport flow controller, selecting a kind of most suitable while interconnecting more direct currents
The structure of voltage class；
Step 2：Establish the internal model of the topological structure；
Step 3：The multi-voltage grade DC grid model containing SM-DCPFC is built, by the tidal current analysis to DC grid,
It calculates SM-DCPFC port voltages and Line Flow relationship, builds inner and outer ring Controlling model, by the control to SM-DCPFC,
Realize more DC voltage level interconnections and the control of Line Flow；
Step 4：By simulating, verifying, the multiport flow controller can be realized in several cases to circuit tide
The flexible control of stream.
Multiport flow controller of the present invention can be specifically designed as four following classes：Open net, Star network, close net and
Radiation network.
The present invention passes through the comparison to common ground and difference, from the aspect of cost and control complexity and stability, choosing
The internal model of each port of stelliform connection topology configuration is more its essence is the modularization that is made of full-bridge submodule is used
Level converter possesses voltage and current way traffic ability, disclosure satisfy that the requirement of system load flow control.
The more DC voltage levels of the present invention interconnect and the control process of Line Flow is：
Step 301：Establish the ring-like DC grid topological structure of the multi-voltage grade containing SM-DCPFC；
Step 302：According to system structure, the trend distribution situation of the straight-flow system is obtained, and then analyze each port voltage
The relationship met between Line Flow；
The trend distribution situation for obtaining circuit is shown below:
Formula (2) shows line current I14, I24, I34With Vm1~Vm3Between relationship, by adjusting being inserted into circuit
The voltage of MMC1~MMC3 modules, so that it may final to realize circuit tide with the size and Orientation of independent control DC line electric current
The control of stream；V in formula1, V3, V4, I2Essentially definite value is not influenced by SM-DCPFC；
Step 303：Equivalent circuit of the multiport flow controller under d-q rotating coordinate systems is established, by equivalent circuit
Analysis, obtains mathematical model of each port under d-q coordinate systems, when current regulator uses pi regulator, derives interior
Circular current governing equation, outer shroud then use constant DC voltage control, the purpose of load disturbance are realized by control port voltage；
Step 3031：Equivalent circuit is established, ignores the additional resistance of inductance, tandem type multiport DC power flow controller is in d-q
N MMC current changer module, L are contained in equivalent circuit under rotating coordinate system, the flow controller of port, insideiFor each module
Bridge arm reactance, LkiBy the leakage reactance for the transformer that each module connects, idiAnd iqiRespectively i-th of module flow direction exchange
The d axis and q axis components of bus current, EdiAnd EqiFor the d axis and q axis components of i-th of inside modules electromotive force, usdAnd usqRespectively
For the d axis and q axis components of ac bus voltage；
Step 3032：Current direction is positive direction, the mathematical modulo that each MMC modules can be expressed as under d-q coordinate systems
Type：
W is the angular frequency of exchange side voltage in formula；
When the ac-side current of module is using proportional integration PI controls, the expression formula of inner ring current controller can be obtained：
kpiAnd kliRespectively proportionality coefficient and integral coefficient；
Step 3033：The reference value for setting DC line electric current is respectively I14ref, I34refAnd I24ref, can be obtained according to formula (2)
Following matrix form：
V in formula1、I2、V3、V4Substantially it is not influenced, can be kept constant, I by SM-DCPFC14ref, I34refAnd I24refIt is desired
Obtained line current reference value passes through the relationship for being converted to line current and port voltage of formula (5).
The present invention can while interconnecting more DC voltage levels the trend of flexible adjusting circuit and with lower
Cost, by can to the analysis of tandem type multiport flow controller topological structure, equivalent circuit, mathematical model and control strategy
To find out, because each MMC modules are to connect in the line, the voltage that it is born is the base in both ends current conversion station difference in voltage
It is finely adjusted on plinth, used change of current device number will greatly reduce, and will be had in this operational process afterwards less
Power attenuation.It can interconnect the DC grid of different voltages grade, and be capable of the trend of flexible adjusting circuit, Neng Gouyou
Effect solves the problems, such as that the trend of DC grid is uncontrollable.By the exchange side of this multiport flow controller by transformer near
AC power be attached, such as current conversion station, electric substation, marine wind electric field, by public exchange side and these AC powers
Connection, ac bus voltage can more be stablized.
Description of the drawings
Fig. 1 is multiport DC/DC converters (a) and multiport DC power flow controller (b) of connecting；
Fig. 2 opens net (b) Star network (c) for multiport flow controller topological structure (a) and closes net (d) radiation network；
Fig. 3 is general multiport flow controller topological structure；
Fig. 4 opens net/Star network (b) for three port flow controller topological structures (a) and closes net (c) radiation network；
Fig. 5 is the internal structure of four port SM-DCPFC；
Fig. 6 is the ring-like DC grid topology containing SM-DCPFC；
Fig. 7 is SM-DCPFC equivalent circuits；
Fig. 8 is the detailed control strategies of SM-DCPFC；
Fig. 9 is electric current bidirectional modulation analogous diagram, and Fig. 9 a are the electric current of four DC lines, and Fig. 9 b are the voltage of each MMC modules
Analogous diagram；
Analogous diagram when Figure 10 is power step, Figure 10 a current conversion station DC side electric currents, Figure 10 b current conversion stations 1 and current conversion station 2 node electricity
Pressure, the electric current of each DC lines of Figure 10 c, the voltage of each MMC modules of Figure 10 d；
Figure 11 is the DC grid topology containing four port DC/DC converters；
Figure 12 is DC/DC transducer ports voltage and electric current, each port voltage of Figure 12 a DC/DC converters, the ports Figure 12 b 1, end
Mouth 2 and 4 DC current of port.
Specific implementation mode
The present invention proposes a kind of series connection multiport DC power flow controller based on MMC.By straight to four kinds of multiports
The analysis of stream flow controller topological structure is compared, different to interconnect using lower-cost stelliform connection topology configuration simple in structure
DC voltage level.In order to meet voltage and current way traffic ability, the inside of each port of stelliform connection topology configuration is full-bridge
The modularization multi-level converter that module is formed.In the ring-like DC grid topological structure of the multi-voltage grade containing SM-DCPFC
In, the relational expression for obtaining meeting between each port voltage and Line Flow by Load flow calculation, the control plan of SM-DCPFC
It slightly uses inner ring current control and the control of outer loop voltag to be combined, and then realizes the control to Line Flow, pass through simulating, verifying
The multiport flow controller can realize the flexible control to Line Flow in several cases.
The present invention step be：
Step 1：By analyzing several topological structures of multiport flow controller, selecting a kind of most suitable while interconnecting more direct currents
The structure of voltage class；
Step 2：Establish the internal model of the topological structure；
Step 3：The multi-voltage grade DC grid model containing SM-DCPFC is built, by the tidal current analysis to DC grid,
It calculates SM-DCPFC port voltages and Line Flow relationship, builds inner and outer ring Controlling model, by the control to SM-DCPFC,
Realize more DC voltage level interconnections and the control of Line Flow；
Step 4：By simulating, verifying, the multiport flow controller can be realized in several cases to circuit tide
The flexible control of stream.
Multiport flow controller of the present invention can be specifically designed as four following classes：Open net, Star network, close net and
Radiation network.
The present invention is examined by the comparison to common ground and difference in terms of cost and control complexity and stability
Consider, the internal model of each port of stelliform connection topology configuration of selection, its essence is use the mould being made of full-bridge submodule
Block multilevel converter possesses voltage and current way traffic ability, disclosure satisfy that the requirement of system load flow control.
The more DC voltage levels of the present invention interconnect and the control process of Line Flow is：
Step 301：Establish the ring-like DC grid topological structure of the multi-voltage grade containing SM-DCPFC.
Step 302：According to system structure, the trend distribution situation of the straight-flow system is obtained, and then analyze each port
The relationship met between voltage and Line Flow；
The trend distribution situation for obtaining circuit is shown below:
Formula (2) shows line current I14, I24, I34With Vm1~Vm3Between relationship, by adjusting being inserted into circuit
The voltage of MMC1~MMC3 modules, so that it may final to realize circuit tide with the size and Orientation of independent control DC line electric current
The control of stream；V in formula1, V3, V4, I2Essentially definite value is not influenced by SM-DCPFC.
Step 303：Equivalent circuit of the multiport flow controller under d-q rotating coordinate systems is established, by equivalent electricity
The analysis on road obtains mathematical model of each port under d-q coordinate systems, when current regulator uses pi regulator, derives
Go out inner ring current control equation, outer shroud then uses constant DC voltage control, load disturbance is realized by control port voltage
Purpose.
Step 3031：Equivalent circuit is established, the additional resistance of inductance is ignored, tandem type multiport DC power flow controller exists
N MMC current changer module, L are contained in equivalent circuit under d-q rotating coordinate systems, the flow controller of port, insideiIt is each
The bridge arm reactance of module, LkiBy the leakage reactance for the transformer that each module connects, idiAnd iqiRespectively i-th of module flow direction
The d axis and q axis components of ac bus electric current, EdiAnd EqiFor the d axis and q axis components of i-th of inside modules electromotive force, usdAnd usq
The respectively d axis and q axis components of ac bus voltage.
Step 3032：Current direction is positive direction, the number that each MMC modules can be expressed as under d-q coordinate systems
Learn model：
W is the angular frequency of exchange side voltage in formula；
When the ac-side current of module is using proportional integration PI controls, the expression formula of inner ring current controller can be obtained：
kpiAnd kliRespectively proportionality coefficient and integral coefficient.
Step 3033：The reference value for setting DC line electric current is respectively I14ref, I34refAnd I24ref, can be with according to formula (2)
Obtain following matrix form：
V in formula1、I2、V3、V4Substantially it is not influenced, can be kept constant, I by SM-DCPFC14ref, I34refAnd I24refIt is desired
Obtained line current reference value passes through the relationship for being converted to line current and port voltage of formula (5).
The present invention is described in further detail below：
Step 1：Analysis is compared between multiport DC/DC converters and multiport flow controller of connecting proposed by the invention
Difference, compares several topological structures of multiport flow controller, therefrom selects a kind of most suitable structure.
Step 101：Multiport DC/DC converters and multiport flow controller comparison of connecting, multiport DC/DC converters
Topological structure can be with Fig. 1 (a) come equivalent, each DC port is to be connected in parallel on the DC side of current conversion station, is from two-port
DC/DC converters grow up, it can be understood as a direct current hub (dc-hub) is by the direct current of different voltages grade
Net links together.Series connection multiport DC power flow controller proposed by the present invention can be with Fig. 1 (b) come equivalent, it is evident that it
Different from multiport DC/DC converters, it connects in the line, can be connected simultaneously with multiple DC voltage levels, institute
The voltage class born is the difference of both ends DC voltage level, this can greatly reduce cost.
Step 102：Specific topological structure can be designed as four following classes：It opens net, Star network, close net and radiation network
As shown in Fig. 2, it can be seen from the figure that they are all the flow controllers for including m port, each structure is all with similar side
Formula is run, and can summarize this operation using general MDCPFC shown in Fig. 3：1. in figure comprising m port respectively with direct current
M DC line is attached in network, according to all electric current I of KCL equations1, I2…ImAdd up to zero；2. by adjusting inserting
Enter the module voltage into circuit to control increased m-1 DC voltage (V12, V23…V(m-1)m), and then carry out control port
Electric current achievees the purpose that control circuit trend；3. all increased m voltages meet KVL laws.
Step 103：Their difference is exactly the ability of adjustment module voltage and current with three port shown in Fig. 4
For DCPFC.It can analyze to obtain from figure, in the DCPFC of three ports, opening net and Star network, there is same topology to tie
Structure can be indicated with Fig. 4 (a).(b) and (c) structure has additionally used a module more relative to other two kinds of structures,
Can more flexibly in control mode, the electric current and voltage of control module that can be respectively, the other less mould of two kinds of structures
Block can handle basic operation, and cost is relatively low, can compare in control aspect it is simple, and hub-and-spoke configuration relative to open net knot
Structure voltage can more be stablized, in the case of common point voltage stabilization, port that the voltage of each module is only connected with it
Voltage is related, therefore the present invention interconnects different DC voltage levels using the relatively simple star topology of structure.
Step 2：Establish the internal model of the stelliform connection topology configuration；Shown in fig. 5 is exactly that detailed four port is star-like
Internal structure chart, V1To V4For the voltage of four DC bus, can be the same or different, I1To I4For four DC lines
On electric current, VmlTo Vm3The voltage of DC line, L are sealed in for SM-DCPFCiFor bridge arm internal driving.As can be seen from the figure it
It is to be inserted into the form of concatenated in three connected DC lines, difference is realized by being inserted into additional voltage in the line
How electric interconnection between DC voltage level, six bridge arm moduleization of three-phase that the inside of port is made of full-bridge submodule be
Flat controller (F-MMC), full-bridge submodule can export+1,0, -1 three kind of level, can ensure pair of electric current and voltage simultaneously
To service ability, it is capable of the trend of sufficient adjusting circuit.The exchange side of MMC linked together by transformer and with it is neighbouring
AC system connects, the balance for maintaining system power.
Step 3：The multi-voltage grade DC grid model containing SM-DCPFC is built, the trend point to DC grid is passed through
Analysis, calculates SM-DCPFC port voltages and Line Flow relationship.
Step 301：By taking unipolarity DC grid as an example, Fig. 6 give the present invention it is designed study contain MMC-
The looped network formula DC grid wiring diagram of SMDCPFC.V1~V4For the voltage of DC bus, V1And V2Directly connected with current conversion station
It connects, is directly interconnected by resistance for identical DC voltage level, V3And V4Both current conversion station can be connected with direct current can also pass through
DC/DC converters are connected with other end straight-flow system.I1And I2For the electric current of current conversion station DC side, I12, I14, I24, I34It is four
The electric current of DC line.MMC1~MMC3 is the equivalent structure figure of inside modules in Fig. 4, Vm1~Vm3For multiport power flowcontrol
The DC voltage of device output, the interconnection of different DC levels and the control of Line Flow are realized by the DC voltage sealed in.
R12, R14, R24, R34For four DC line resistance, whole SM-DCPFC is mounted on DC bus V4Place, therefore corresponding thereto
The line resistance answered, which can be ignored, not to be remembered, and specific line parameter circuit value is as shown in table 1.It is female to control direct current for current conversion station 1 under stable situation
Line voltage is 160kV, and the dc power of the control output of current conversion station 2 is 120MW, DC bus V3And V4Can with current conversion station or
The network of DC bus connection is connected, and using the control mode for determining DC voltage, controlling DC bus-bar voltage respectively is
250kV and 200kV.
Different DC voltage levels is interconnected by SM-DCPFC.
1 transmission line parameters of table
| DC line | Distance/km | Resistance/Ω | Reactance H |
| L12 | 200 | 2 | 0.13 |
| L14 | 300 | 3 | 0.18 |
| L24 | 400 | 4 | 0.23 |
| L34 | 300 | 3 | 0.18 |
Step 302：The trend distribution situation that circuit can be obtained according to the structure of system is shown below：
Formula (2) shows line current I14, I24, I34With Vm1~Vm3Between relationship, by adjusting being inserted into circuit
The voltage of MMC1~MMC3 modules, so that it may final to realize circuit tide with the size and Orientation of independent control DC line electric current
The control of stream.V in formula1, V3, V4, I2Essentially definite value is not influenced by SM-DCPFC.This theory is equally applicable to N-port tide
Stream controller can either meet different DC voltages etc. when connecting N DC line using the flow controller of N number of port
The interconnection of grade can also realize the independent control to N-1 Line Flow.
Step 4：Equivalent circuit of the multiport flow controller under d-q rotating coordinate systems is established, by equivalent circuit
Analysis, obtain mathematical model of each port under d-q coordinate systems, to realize control to SM-DCPFC, reach adjusting
The purpose of Line Flow.
Step 401：Equivalent circuit is established, the additional resistance of inductance is ignored, tandem type multiport DC power flow controller exists
It can be expressed as equivalent circuit shown in Fig. 7 under d-q rotating coordinate systems.Fig. 6 is the flow controller of the ports n+1, and n is contained in inside
A MMC current changer modules, LiFor the bridge arm reactance of each module, LkiBy the leakage reactance for the transformer that each module connects, idi
And iqiRespectively i-th of module flows to the d axis and q axis components of ac bus electric current, EdiAnd EqiIt is electronic for i-th of inside modules
The d axis and q axis components of gesture, usdAnd usqThe respectively d axis and q axis components of ac bus voltage.
Step 402：Using current direction shown in figure as positive direction, each MMC modules can indicate under d-q coordinate systems
At following mathematical model：
W is the angular frequency of exchange side voltage, L in formulani=Li/2+Lki, there it can be seen that bridge-type MMC and semi-bridge type MMC
Other than bridge arm structure differs, remaining external characteristics is essentially identical, therefore can use for reference the inner ring control strategy of semi-bridge type.When
When the ac-side current of module is controlled using proportional integration (PI), the expression formula of inner ring current controller can be obtained：
kpiAnd kliRespectively proportionality coefficient and integral coefficient.It can be seen that the design of inner ring current controller includes mainly three
Part, the compensating for coupling item of current feedback part, voltage feed-forward control part and voltage, by the way that PI parameters are rationally arranged not only
Existing steady-state error can be eliminated, and the respective capabilities of system are also remarkably reinforced.
The effect of inner ring current controller is exactly that the d axis of electric current and q axis components is allowed quickly to track its reference value, and outer shroud
The effect of controller is then the reference value for calculating interior circular current.Reactive Power Control may be used for q axis, be to allow
The reactive power transmitted in system is zero, enables iqiref=0.And for d axis, then to determine according to the actual needs.Direct current tide
The target of stream controller is the electric current of control circuit, and the control of MMC transverters is using the port voltage of each module as ginseng
Examine value, it is therefore necessary to find the relationship between the port voltage of each transverter and the DC reference value of circuit.
The reference value for such as setting DC line electric current is respectively I14ref, I34refAnd I24ref, according to formula (2) can obtain as
Lower matrix form：
V in formula1、I2、V3、V4Substantially it is not influenced, can be kept constant, I by SM-DCPFC14ref, I34refAnd I24refIt is desired
Obtained line current reference value passes through the relationship for being converted to line current and port voltage of formula (5).Work as SM-DCPFC
Each MMC module port voltages VMirefAfter determination, outer shroud can take the control strategy of constant voltage and then realize to circuit tide
The control of stream, whole control block diagram are as shown in Figure 8.
The present invention is described in detail with emulation below in conjunction with the accompanying drawings：
It is soft in electromagnetic transient simulation in order to verify the validity of SM-DCPDC topological structures proposed by the invention and control strategy
Three voltage class, four end DC grid simulation model as shown in Figure 5 is built in part PSCAD/EMTDC, internal structure is also wrapped
Containing a four port flow controllers, public side ac bus voltage operates in the mains frequency of 50Hz, changes by taking 220kV as an example
Stream station VSC1Control DC bus-bar voltage V1For 160kV, current conversion station VSC2Control the dc power P of output2For 120MW, direct current net
Network V3, V4It is respectively 250kV and 200kV to keep busbar voltage.It below will be to electric current bidirectional modulation, current conversion station VSC2Changed power
In the case of keep I14And I24It does not change to carry out simulating, verifying, and by emulating come comparative analysis multiport direct current
Difference between flow controller and multiport DC/DC converters.
Emulation one：Electric current bidirectional modulation
Step 101：System is in initial operating state, by VM1~VM3It is precharged to 40kV, 40kV and -50kV respectively, at this time I14
And I34It is almost nil, do not pass power, I24=-0.25kA, I12=-0.5kA；
Step 102：The control strategy for starting SM-DCPFC in 1.5s, sets I14=-0.5kA, I24=-0.6kA, I34=
0.5kA simulation results as shown in figure 9, Fig. 9 (a) be DC line electric current, (b) be SM-DCPFC modules voltage.From figure
It can be seen that the I after 1.5s14And I34Electric current can maintain essentially in -0.5kA and 0.5kA, I24Electric current from -0.25kA
Become -0.5kA, I12Electric current -0.15kA is become from -0.5kA；
Step 103：I is set in 3.5s34=1kA, two other line current remain unchanged；
From being not difficult to find out that electric current can maintain near setting value well in figure, and the electric current of several ports meets KCL always
Law.Small change has occurred in the voltage of module after 1.5s and 3.5s, but can finally stablize, and shows SM-
DCPFC also has the ability of adjusting circuit electric current way traffic while interconnecting different voltages grade.
Emulation two：Current conversion station VSC2Power step keeps I14And I24It is constant
Step 201：Ibid；
Step 202：When t=5s, current conversion station VSC2Power 160MW is become from 120MW, keep I14And I24It is constant；
Step 203：Set I34=0.4kA.It can be obtained using formula, I12=-0.4kA, VM1=41.5kV, VM2=41.6kV, VM3
=-51.2kV.Simulation result is as shown in Figure 10, and Figure 10 (a) is current conversion station DC side electric current, is (b) current conversion station 1 and current conversion station 2
Node voltage is (c) electric current of DC line, is (d) voltage of SM-DCPFC modules；
As can be seen that in 2 power step of current conversion station, current conversion station I2Electric current becomes larger, and the voltage of current conversion station 1 is also with the increase of power
And increase, but maintain essentially near a reference value.SM-DCPFC is able to maintain that I14And I24Electric current do not change, I34Electricity
Stream is identical as setting value.Because of I2Increase and leads to I12Increase, and then so that the voltage of current conversion station 2 is also increased and be consistent with theoretical value.From
And SM-DCPFC proposed by the invention is demonstrated in power step, it can ensure that some branch currents do not change.
Emulation three：Multiport DC power flow controller and multiport DC/DC converter comparative analyses
Step 301：The DC voltage level that the present invention contains is applied to mutual by multiport DC/DC converters shown in Figure 11
In the DC grid topology of connection, Vdc1And Vdc2The DC network connected is 160kV, Vdc3And Vdc4Respectively 200kV and
250kV.Under steady state conditions, for controlling the public ac bus voltage of inverter inside, VSC4, which is then used, determines direct current for port 4
Voltage control for it provides steady dc voltage, if for other ports using constant DC voltage control, phase therewith
Current conversion station even is using constant dc power control come the trend of control circuit；
Step 302：It is consistent with the variation of 2 line currents is emulated in order to ensure under accidental conditions, current conversion station is controlled respectively
1 and 2 power sent out are 80MW and 96MW, and are remained unchanged, and it is 74MW that current conversion station 3, which controls the power sent out,；
Step 303：The power that current conversion station 3 is sent out in t=5s becomes -76MW, the theoretical value I of line current1And I2It can
Always 0.5kA and 0.6kA, the line current I after 5s are maintained40.4kA is become from 1kA, it should with the circuit corresponding to emulation 2
Curent change is equal, and the variation of power causes port 3 and 4 voltages that but small oscillations have occurred.Simulation result is as shown in figure 12, Figure 12
(a) it is each port voltage of DC/DC converters, is (b) port 1,4 DC current of port 2 and port；
Can significantly it find out from simulation architecture, multiport DC power flow controller is compared to its is excellent with multiport DC/DC converters
Gesture is that the voltage class that device is born is small, when interconnecting the DC grid of identical voltage class, the voltage of each MMC modules
It is tens kilovolts, and the voltage of each port voltage of DC/DC converters is kilovolt up to a hundred, current conversion station exports in terms of operation
Power is required for arriving the conversion of direct current again to exchange by direct current, will produce a large amount of power attenuation, it is assumed that set by this paper
Fixed line electricity flow valuve, controls I respectively1And I2For 0.5kA and 0.6kA, I4For 1kA, come using multiport DC/DC converters mutual
The transverter total capacity that connection needs is 500MW, and the transverter total capacity needed using proposed SM-DCPFC is about
100MW greatly reduces running wastage, saves cost, but two ways can flexible adjusting circuit trend.
Conclusion
From the simulation result of Fig. 9~Figure 12 the to have can be seen that proposed by the invention a kind of series connection multiport direct current tide based on MMC
Stream controller is capable of the trend of control circuit in several cases, effectively solves the problems, such as that the trend of DC grid is uncontrollable.Mutual
Cost has effectively been saved while joining different DC voltage levels, has been conducive to the development of the following DC grid.
The detailed description of attached drawing：
(a) of Fig. 1 is the isoboles of existing multiport DC/DC converters, it, can for connecting different DC voltage levels
The DC grid of different voltages grade is linked together with being interpreted as a direct current hub (dc-hub).Each DC/DC becomes
It is the structure of parallel connection that the positive and negative anodes of parallel operation port are connected with the positive and negative anodes of corresponding straight-flow system respectively, generally passes through DC-AC-
Generally there are AC transformer in the conversion of DC, centre to realize the connection of different DC voltages.(b) of Fig. 1 is multiport direct current tide
The content that the isoboles and the present invention of stream controller are mainly studied, can clearly find out it and multiport DC/DC from figure
Converter is different, and multiple straight-flow systems can link together after multiport flow controller, its each port
It is the anode of the straight-flow system of connection, connects in the line, will greatly reduce system relative to multiport DC/DC converters
Cost, for different DC voltages interconnection and Line Flow control.
What Fig. 2 was presented is exactly several topological structures of multiport flow controller, can be designed as out net, Star network,
Close net and radiation network, be equivalent to the figure that further spreads out of upper figure (b), each module M represent be inserted into positive in circuit or
Negative DC voltage, by the voltage of control module M can control circuit trend and realize that different DC voltages interconnect.
Fig. 3 is the whole isoboles of multiport flow controller, each structure can all meet certain with it come equivalent
Rule：1. it is attached respectively with m DC line in DC network comprising m port in figure, it is all according to KCL equations
Electric current I1, I2…ImAdd up to zero；2. controlling increased m-1 direct current by adjusting the module voltage being inserted into circuit
Press (V12, V23…V(m-1)m), and then carry out the electric current of control port, achieve the purpose that control circuit trend；3. all increased m
Voltage meets KVL laws.
Fig. 4 is the further explanation to these types of topological structure, by taking three port flow controllers as an example, by its difference
The analysis of point, considering cost control complexity and stability, and final choice hub-and-spoke configuration interconnects different voltage etc.
Grade.
Fig. 5 is the four detailed internal structure charts of port star topology, and each of which M modules are made of full-bridge submodule
Six bridge arm module multilevel device (F-MMC) of three-phase, compared with half-bridge submodule, full-bridge submodule can export negative electricity
Pressure, can ensure the way traffic ability of electric current and voltage, the size and Orientation of each MMC module ports voltage is by tide simultaneously
Flow control demand is determined, the trend of sufficient adjusting circuit is capable of while interconnecting voltage.The exchange side of each of which MMC is logical
It crosses transformer to link together, and energy supply part is connected to neighbouring AC system, main function is for series electrical
Potential source part provides energy support, while being converted according to the different energy realized between AC and DC side in output DC voltage direction.
Fig. 6 shows the topology of the ring-like DC grid containing SM-DCPFC, and the present invention is by the star-like DC power flow control in four ports
The model of device processed is applied in ring-like DC grid structure as shown in the figure, V1~V4For the voltage of DC bus, V1And V2Directly
It is attached, is directly interconnected by resistance for identical DC voltage level, V with current conversion station3And V4Both the change of current can be connected with direct current
It stands and can also be connected with other end straight-flow system by DC/DC converters, current conversion station 1 controls DC bus-bar voltage under stable situation
Dc power for 160kV, the control output of current conversion station 2 is 120MW, DC bus-bar voltage V3And V4It can be equivalent to ideal straight
Galvanic electricity potential source, it is that 200kV and 250kV calculates each MMC moulds for the purpose of control circuit trend to control DC voltage respectively
The reference value of block output voltage is also capable of the trend of adjusting circuit while interconnecting voltage, effectively solves the tide of DC grid
Flow uncontrollable problem.
SM-DCPFC can be expressed as equivalent circuit shown in Fig. 7 under d-q rotating coordinate systems, and upper figure is the ports n+1
N MMC current changer module, L are contained in flow controller equivalent circuit, insideiFor the bridge arm reactance of each module, LkiFor each mould
The leakage reactance for the transformer that block is connected, idiAnd iqiRespectively i-th of module flows to the d axis and q axis point of ac bus electric current
Amount, EdiAnd EqiFor the d axis and q axis components of i-th of inside modules electromotive force, usdAnd usqRespectively the d axis of ac bus voltage and
Q axis components then analyze the control of module by that can obtain the mathematical model of each MMC modules to the analysis of equivalent circuit
System strategy.
Fig. 8 is exactly the control strategy of SM-DCPFC entirety, IirefReference value for the line current actually wanted to, V1,
I2, V3, V4The system value of DC network, is regarded as fixed value under steady state conditions, is obtained by Load flow calculation formula (5)
The reference value V of each module voltageMiref, the relationship between flow controller and converter Control is also just had found, transverter
Outer shroud is controlled using constant voltage, and inner ring uses constant current control, and then is exported bridge arm and controlled required reference voltage, then is passed through
The control of valve grade carries out submodule the triggering of selectivity.
Fig. 9 (a) is the electric current of four DC lines, and I is controlled respectively in 1.5s14=-0.5kA, I24=-0.6kA, I34
=0.5kA sets I in 3.5s34=1kA, two other line current remain unchanged.It can significantly find out this by analogous diagram
Multiport flow controller of connecting effectively adjusting circuit trend, four connected DC lines can pass through four port trend controls
Device processed can control the trend of three-line, the adjusting that electric current can be two-way.
Fig. 9 (b) is the voltage of each MMC modules, and it is micro- that the variation of current reference value makes the voltage of each MMC modules have occurred
Small variation coincide substantially with the result of Load flow calculation formula, it was demonstrated that the control strategy has validity.
Figure 10 (a) is the electric current of 2 DC side of current conversion station 1 and current conversion station, and the dc power of current conversion station output is controlled in 5s
160MW is become from 120MW, it can be seen that electric current I2It is 1kA by 0.75kA saltus steps, hence it is evident that increase, I2Also respective change symbol has occurred
Close practical situation.
In Figure 10 (b) in 2 power step of current conversion station, the voltage of current conversion station 1 also increases with the increase of power, but basic
It maintains near 160kV a reference values, because of I2Increase and leads to I12Increase, and then the voltage of current conversion station 2 is made also to increase and theoretical value
It is consistent.
In Figure 10 (c) in t=5s current conversion station power steps, I is set34=0.4kA keeps I14And I24It is constant.From figure
It can significantly find out I14And I24It keeps constant, I34It is identical as setting value, to demonstrate SM-DCPFC proposed by the invention
In power step, it can ensure that some branch currents do not change, I12Numerical value with I2Increase and increase.
It can find out U from the small figure of amplification in Figure 10 (d)M3Module voltage, because of I34The variation of current reference value and occur
Small change, UM1Voltage is fluctuated, but is basically unchanged, and UM2Voltage with electric current I2Increase present reduce
The variation of trend, current reference value makes the voltage of each MMC modules that small variation have occurred, the result with Load flow calculation formula
Substantially it coincide, it was demonstrated that the validity of the control strategy.
Figure 11 is the four end DC grid topological structures containing four port DC/DC converters, Vdc1~Vdc4For each port
Corresponding DC voltage, I1~I4For the DC current corresponding to each port, AC transformer is used to connect MMC outputs
Different alternating voltage, to realize the requirement of large velocity ratio.Vdc1And Vdc2The DC network connected is 160kV, Vdc3And Vdc4Point
It Wei not 200kV and 250kV.Under steady state conditions, port 4 is for controlling the public ac bus voltage of inverter inside, VSC4
Then constant DC voltage control is used to provide steady dc voltage for it, if voltage-controlled using direct current is determined for other ports
System, then the current conversion station being attached thereto is using constant dc power control come the trend of control circuit.
Figure 12 (a) is each port DC voltage of DC/DC converters, and the voltage of each port can be significantly found out from figure
It is all identical as a reference value, V is led to due to the variation of 3 power of current conversion station in 5sdc3And Vdc4Slight fluctuation all has occurred, but
It can maintain near a reference value.From figure it can also be seen that, when interconnecting the DC grid of identical voltage class, each MMC
The voltage of module is all tens kilovolts, and the voltage of each port voltage of DC/DC converters is kilovolt up to a hundred, in terms of operation
The power of current conversion station output is required for arriving the conversion of direct current again to exchange by direct current, will produce a large amount of power attenuation, the change of current
Device total capacity is relatively large, and cost is higher.
Figure 12 (b) is port current, and 0.5kA, 0.6kA can be respectively maintained in the electric current of the front port 1,2,4 of 5s
And 1kA, the variation of current conversion station power after 5s and cause electric current that corresponding variation also has occurred, it can also be seen that multiterminal from figure
Mouth DC/DC converters are also capable of the trend of flexible control circuit.
Claims (4)
1. a kind of series connection multiport DC power flow controller based on MMC, it is characterised in that：Its step is：
Step 1：By analyzing several topological structures of multiport flow controller, selecting a kind of most suitable while interconnecting more direct currents
The structure of voltage class；
Step 2：Establish the internal model of the topological structure；
Step 3：The multi-voltage grade DC grid model containing SM-DCPFC is built, by the tidal current analysis to DC grid,
It calculates SM-DCPFC port voltages and Line Flow relationship, builds inner and outer ring Controlling model, by the control to SM-DCPFC,
Realize more DC voltage level interconnections and the control of Line Flow；
Step 4：By simulating, verifying, the multiport flow controller can be realized in several cases to circuit tide
The flexible control of stream.
2. the series connection multiport DC power flow controller according to claim 1 based on MMC, it is characterised in that：Multiport
Flow controller can be specifically designed as four following classes：It opens net, Star network, close net and radiation network.
3. the series connection multiport DC power flow controller according to claim 1 based on MMC, it is characterised in that：By right
The comparison of common ground and difference, from the aspect of cost and control complexity and stability, the stelliform connection topology configuration of selection
The internal model of each port possesses its essence is the modularization multi-level converter being made of full-bridge submodule is used
Voltage and current way traffic ability disclosure satisfy that the requirement of system load flow control.
4. the series connection multiport DC power flow controller according to claim 1 based on MMC, it is characterised in that：More direct currents
Voltage class interconnects and the control process of Line Flow is：
Step 301：Establish the ring-like DC grid topological structure of the multi-voltage grade containing SM-DCPFC；
Step 302：According to system structure, the trend distribution situation of the straight-flow system is obtained, and then analyze each port voltage
The relationship met between Line Flow；
The trend distribution situation for obtaining circuit is shown below:
Formula (2) shows line current I14, I24, I34With Vm1~Vm3Between relationship, by adjusting MMC1 in circuit is inserted into
The voltage of~MMC3 modules, so that it may with the size and Orientation of independent control DC line electric current, final realization Line Flow
Control；V in formula1, V3, V4, I2Essentially definite value is not influenced by SM-DCPFC；
Step 303：Equivalent circuit of the multiport flow controller under d-q rotating coordinate systems is established, by equivalent circuit
Analysis, obtains mathematical model of each port under d-q coordinate systems, when current regulator uses pi regulator, derives interior
Circular current governing equation, outer shroud then use constant DC voltage control, the purpose of load disturbance are realized by control port voltage；
Step 3031：Equivalent circuit is established, ignores the additional resistance of inductance, tandem type multiport DC power flow controller is in d-q
N MMC current changer module, L are contained in equivalent circuit under rotating coordinate system, the flow controller of port, insideiFor each module
Bridge arm reactance, LkiBy the leakage reactance for the transformer that each module connects, idiAnd iqiRespectively i-th of module flow direction exchange
The d axis and q axis components of bus current, EdiAnd EqiFor the d axis and q axis components of i-th of inside modules electromotive force, usdAnd usqRespectively
For the d axis and q axis components of ac bus voltage；
Step 3032：Current direction is positive direction, the mathematical modulo that each MMC modules can be expressed as under d-q coordinate systems
Type：
W is the angular frequency of exchange side voltage in formula；
When the ac-side current of module is using proportional integration PI controls, the expression formula of inner ring current controller can be obtained：
kpiAnd kliRespectively proportionality coefficient and integral coefficient；
Step 3033：The reference value for setting DC line electric current is respectively I14ref, I34refAnd I24ref, can be obtained according to formula (2)
Following matrix form：
V in formula1、I2、V3、V4Substantially it is not influenced, can be kept constant, I by SM-DCPFC14ref, I34refAnd I24refTo want
The line current reference value arrived passes through the relationship for being converted to line current and port voltage of formula (5).
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