CN103795080B - A kind of combination method of MMC type HVDC light system - Google Patents
A kind of combination method of MMC type HVDC light system Download PDFInfo
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- 230000001105 regulatory effect Effects 0.000 description 6
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- 238000013016 damping Methods 0.000 description 5
- 238000004804 winding Methods 0.000 description 5
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Abstract
The invention discloses a kind of combination method of MMC type HVDC light system, by virtual synchronous control method is combined with MMC the operational mode of MMC type HVDC Light switched, without the need to special synchronization control circuit, before grid-connected can with electrical network automatic synchronization, the presynchronization time is very short, accurately can follow mains frequency after grid-connected, realize friendly grid-connected.
Description
Technical field
The present invention relates to field of power electronics, particularly relate to a kind of MMC(modularization multi-level converter) combination method of type HVDC light system (HVDC-Light).
Background technology
In recent years, along with the aggravation of the energy and environmental problem, the regenerative resource such as solar energy and wind energy constantly promotes in the permeability of electric power system.Along with the raising of distributed power source capacity, electric pressure, the especially development of offshore wind farm, HVDC Light arises at the historic moment.MMC, as the main circuit of HVDC light system, can effectively reduce harmonic wave of output voltage content, realizes high-power and runs.Distributed power source has dispersion, the nearly feature by load, if realize taking over seamlessly in grid-connected and isolated island two kinds of operational modes, not only contribute to the energy supply quality improving distributed power generation, the potential of distributed power generation can also be played, improve the power supply reliability of electric power system.Therefore, the seamless switching technique research carrying out MMC type HVDC Light operational mode is particularly important.But general operational mode switch controller has the following disadvantages:
1, operational mode switching controls adopts two-mode field technology, PQ current control method is adopted when being incorporated into the power networks, islet operation adopts V/f control method, two-mode field technology meets the needs that microgrid runs, but it is large to switch difficulty between voltage control and Current Control, there is the risk of handoff failure, but also isolated island information will be detected determine whether and need switching control strategy.
2, adopt general droop control, to be incorporated into the power networks and under islet operation pattern, its control strategy remains unchanged, to meet the feature of plug and play, but do not consider that droop control is to the adaptability be incorporated into the power networks, and may exist larger impulse current in grid-connected moment.
Summary of the invention
In order to make MMC type HVDC light system without under lock unit fast and synchronized, realize MMC to be connected to the grid fast, smoothly, be critical loads reliable power supply incessantly, the present invention proposes a kind of combination method of MMC type HVDC light system, within the system, MMC is connected to AC network by circuit breaker, and the controlling unit of this system comprises a virtual synchronous control unit and a capacitor voltage equalizing grading control unit, and this combination method comprises the steps:
Detection of grid three-phase voltage, MMC three-phase output current and MMC three-phase output voltage;
MMC three-phase output voltage and the electrical network three-phase voltage detected are compared, in conjunction with the virtual impedance between practical impedance setting MMC and electrical network, in virtual synchronous control unit, calculates the virtual current for cutting-in control;
Be the model of synchronous generator by MMC equivalent model, calculate the reference voltage signal for control MMC;
The signal of all pressing grading control unit to obtain with reference to voltage signal, voltage combines the drive singal of the switch obtaining each submodule in MMC compared with the triangular carrier of known phase shift, controls MMC output voltage;
MMC output voltage moves closer to line voltage, and virtual current is reduced gradually, and when virtual current is zero, namely MMC output voltage and line voltage are substantially identical, close circuit breaker, realizes MMC and electrical network is grid-connected.
Effect of the present invention is:
(1) MMC type HVDC light system is grid-connected without the need to complexity, special synchronization control circuit, utilizes line voltage to compare generating virtual electric current with MMC output voltage, realize MMC and export and synchronized, and the presynchronization time is very short.
(2) virtual synchronous controls the droop characteristic with similar synchronous generator, the output of own power can be regulated in conjunction with the frequency of external network, amplitude when grid-connected, while burning voltage frequency, amplitude are near rated value, ensure the reliable power supply to load.
(3) all can run under grid-connected, island mode because virtual synchronous controls MMC type HVDC light system, when operational mode changes, without the need to detecting the information of external network change, without the need to switching control strategy, avoid overcurrent in handoff procedure, superpotential generation.
Accompanying drawing explanation
In order to be illustrated more clearly in the embodiment of the present invention or technical scheme of the prior art, be briefly described to the accompanying drawing used required in embodiment or description of the prior art below, apparently, accompanying drawing in the following describes is only some embodiments of the present invention, for those of ordinary skill in the art, under the prerequisite not paying creative work, other accompanying drawing can also be obtained according to these accompanying drawings.
Fig. 1 is Systematical control block diagram according to an embodiment of the invention;
Fig. 2 is s operation control algorithm block diagram according to an embodiment of the invention;
Fig. 3 is MMC basic structure according to an embodiment of the invention;
Fig. 4 is grid-connected single-phase MMC model according to an embodiment of the invention;
Fig. 5 is the sagging regulating characteristics of MMC output voltage frequency according to an embodiment of the invention and amplitude.
Embodiment
Below in conjunction with drawings and Examples, embodiments of the present invention are described in further detail.Following examples only for illustration of the present invention, but can not be used for limiting the scope of the invention.
See Fig. 1 and Fig. 2, respectively illustrate the s operation control algorithm block diagram in the structured flowchart of MMC type HVDC light system according to an embodiment of the invention and operational mode control method thereof.This MMC type HVDC light system mainly comprises MMC, LC filter and circuit breaker, and LC filter is connected in parallel between MMC and electrical network, and circuit breaker is used for realizing the grid-connected of MMC and off-grid.The controlling unit of this system mainly comprises a virtual synchronous control unit and a capacitor voltage equalizing grading control unit.Real-time detection of grid three-phase voltage u
ga, u
gb, u
gc(use u below
grepresent), MMC three-phase output current i
ga, i
gb, i
gc(use i below
grepresent) and MMC three-phase output voltage u
a, u
b, u
c(representing with u below).Carry out and net operation time, the line voltage detected and MMC output voltage (being approximately system-level modulation signal e) are compared, by the s operation control algorithm in virtual synchronous control unit, generate the virtual current of cutting-in control, enter virtual synchronous control unit, generating reference voltage signal, controls MMC, as virtual current i
swhen being decreased to 0 gradually, now can close circuit breaker, realize MMC and be incorporated into the power networks.After MMC is grid-connected, by the MMC output current i detected
greplace virtual current i
senter controller, and set meritorious, the idle reference value of MMC output, change the on off state of controller, make MMC have droop characteristic, complete MMC grid-connected, and stable operation under grid-connected conditions, power output is basic and reference value is identical.MMC off-grid is run, when electrical network needs to detect or when breaking down, require that MMC can run at island mode, can cut-off breaker, MMC off-grid is run, because introduce droop control in virtual synchronous control unit, MMC is made to have droop characteristic, under island mode, can by output voltage frequency, amplitude stability near rated value, and make power output available power, realize MMC islet operation, to the continuous reliable power supply of critical loads.
The s operation control algorithm in virtual synchronous control unit of the present invention is described in detail below in conjunction with Fig. 2.MMC equivalent model is the model of synchronous generator by s operation control algorithm of the present invention, and the Power Exchange according to synchronous generator and electrical network realizes the grid-connected of MMC.
The number of pole-pairs of assumes synchronization generator is 1, and stator winding self-induction is L, and mutual inductance is-M, and excitation reluctance inductance value is R
f, L
f, the mutual inductance M between excitation winding and stator winding
fchange with the change of electrical degree θ, i
ffor exciting current, be inner product, T
mfor machine torque, T
efor electromagnetic torque, D
pfor damping coefficient.
So, the expression formula of synchronous generator active power and reactive power is:
Wherein, P is the active power sent, and Q is the reactive power sent,
be the angular frequency of MMC modulation signal, θ is the phase place of A phase modulating wave; T
efor electromagnetic torque, M
ffor the mutual inductance between the excitation winding of synchronous generator and stator winding, i
ffor exciting current, <, > are inner product operation, and i is control electric current.
As shown in Figure 2, for above-mentioned control current i, different current values is chosen respectively in grid-connected front and back.First, MMC three-phase output voltage and the electrical network three-phase voltage detected are compared, introduce the virtual impedance Ls+R between MMC and electrical network, calculate the virtual current for cutting-in control
in the presynchronization stage before grid-connected, control current i chooses this virtual current i
s, after grid-connected, control current i chooses the MMC three-phase output current i detected
g.
In order to carry out droop control, introduce the first damping coefficient D
qwith the second damping coefficient D
p.From accompanying drawing 2, M can be obtained
fi
fcomputing formula as follows:
Wherein Q
setfor the reactive power of setting, D
qbe the first damping coefficient, V is the amplitude of MMC output line voltage, starts, M in operation
fi
fwith Q first first making, be 0, at calculating M
fi
fduring with Q, the value of the other side's previous moment of respectively asking for, thus the state reaching a balance; V
gfor the amplitude of grid line voltage,
amplitude regulating time constant τ
v, D
qnamely K is calculated after selected.
Switch S
cbeat to position 2 time, MMC enter grid-connected after operating state.As switch S
pclosed, MMC enters setting real power control, makes the active power of the output of MMC inverter and the active-power P of setting
setequal; If switch S
pdisconnect, then MMC inverter is operated in sagging pattern.Work as switch S
qtime closed, voltage droop control works, and the reactive power Q now exported is not equal to the reactive power Q of setting
set.
Following formula can also be drawn from Fig. 2:
To in formula (4)
namely obtain after carrying out integral operation
right
namely θ is obtained, wherein P after carrying out integral operation
setfor the active power of setting,
the specified angular frequency of electrical network,
d
pbe the second damping coefficient,
reference angle frequency, be the value that obtains after PI controller of Δ T with
sum, J is the moment of inertia of following all parts that synchronous generator rotor rotates,
frequency adjustment timeconstantτ
f, D
pcan J be calculated after selected,
Finally, according to above formula (1), formula (2), formula (3), formula (4) and formula (5), the reference voltage signal e for control MMC can be calculated.
According to capacitor voltage equalizing grading control strategy, gather the electric current of each capacitance voltage in MMC inverter, upper and lower bridge arm, in conjunction with reference voltage signal e, the final modulating wave of each submodule of MMC can be obtained, compare with the triangular carrier of phase shift, namely obtain the drive singal of the electronic switch in each submodule, realize the stable of the controlled of MMC power output and capacitance voltage.
Fig. 3 gives the basic structure of MMC converter, and MMC is a kind of converter of novel modular multilevel voltage-source type, and MMC converter adopts modular structure, and often mutually upper and lower brachium pontis is made up of a reactor and n sub-block coupled in series.Each submodule is made up of two IGBT, two anti-paralleled diodes and an electric capacity.
Composition graphs 2 illustrates the cutting-in control of MMC type HVDC-Light system again below.1) presynchronization state: work as switch S
cbeat to position 1, and S
pclosed, S
qdisconnect, and P
set, Q
setall set to 0, invertor operation at presynchronization state, detection of grid three-phase voltage, MMC output voltage, electric current, according to sampling electrical network three-phase voltage, the voltage signal exported with MMC is made comparisons, through a transfer function, generating virtual current i
s, enter controller, because the set point of MMC power output is 0, the power that system cloud gray model makes MMC export moves closer to its set point, i.e. i
sgradually reduce until 0.The voltage signal of now MMC output and line voltage u
gsynchronously, i.e. voltage-phase, the amplitude of MMC output are identical with electrical network, can close circuit breaker make MMC grid-connected.In the process, also introduce pi regulator to regulate Δ T, make D
phanging portion is tending towards 0, when namely Δ T is 0, and the reference frequency of generation
equal with mains frequency.2) after process: the MMC that is incorporated into the power networks is grid-connected, MMC output current i
gvirtual current is is replaced to enter controller, meritorious, the reactive power reference qref P that setting MMC exports
set, Q
set, make meritorious, the reactive power that MMC output is specified.Now, switch S is made
pdisconnection, S
qclosed, make MMC have the droop characteristic of frequency, amplitude and power output.In the synchronization control circuit situation that nothing is complicated, special, the presynchronization time is very short, and level and smooth, the little impact of MMC is connected to the grid, and achieves friendly grid-connected.
Carrying out application control method of the present invention below in conjunction with Fig. 4 is how to realize off-grid to run.As previously mentioned, be the model of synchronous generator by MMC equivalent model.Can obtain grid-connected after MMC model, as shown in Figure 4.What it produced gains merit, reactive power is as follows:
Wherein, U
gbe the amplitude of ac bus, E is the induced electromotive force amplitude of synchronous generator, by exciting current or voltage control E, and θ, θ
gbe respectively the voltage phase angle of alternating current net side, synchronous generator, X
sfor the synchronous impedance of synchronous generator.
Reactive power Q is regulated by control E, when
Time, there is no meritorious, idle exchange between grid side and synchronous generator.If P, Q are 0, then meet above formula, the voltage e of generation and grid side voltage u
gequal, during this time, the grid-connected or off-grid of synchronous generator, can not cause larger transient process, and it is synchronous that this can be used at grid-connected front MMC and electrical network.
When electrical grid failure or regularly maintenance, need cut-off breaker to make MMC separately to load supplying, this just requires that MMC can run at island mode.When MMC stable operation, switch S
pdisconnection, S
qclosed.Now MMC type HVDC-Light system has the droop characteristic of similar synchronous generator, MMC by mode conversion of being incorporated into the power networks to islet operation pattern time, without the need to detecting the information of operational mode, also without the need to switching control strategy.
When grid maintenance or chance failure, the circuit breaker of grid side disconnects, and causes MMC to operate in island mode, needs stablizing of ME for maintenance frequency and amplitude.Here, electrical network can be taken as an infinitely-great power supply, and when bearing power is not mated with the set point of MMC power output, then MMC coordinates electrical network to meet bearing power.When bearing power is less than MMC power output, under the prerequisite meeting bearing power, unnecessary power will flow to electrical network.Otherwise electrical network is by powering load.
See Fig. 5, show the sagging regulating characteristics of MMC output voltage frequency according to an embodiment of the invention and amplitude.Switch S
pclosed, S
qduring disconnection, the power that MMC exports has same similar characteristic with frequency, voltage, namely
F
0, v
0be respectively output voltage rated frequency and amplitude, P
0, Q
0be respectively specified meritorious, reactive power, k
p, k
qbe respectively the sagging system of electric voltage frequency and amplitude, be all less than 0.When the active power that synchronous generator exports increases (reduction), the frequency of its output voltage can reduce (increase) accordingly; When the reactive power that synchronous generator exports increases (reduction), the amplitude of its output voltage can reduce (increase) accordingly.
Suppose the position 1 of MMC system starts at Fig. 5, the meritorious, idle of output is respectively P
1, Q
1, when circuit breaker disconnects, the state after MMC system is finally stable is position 2, and the meritorious, idle of output is respectively P
2, Q
2.When MMC power output is not mated with bearing power, MMC will adjust according to the direction of arrow, imbalance power is carried out Reasonable adjustment betwixt, running status is transitioned into position 2 from position 1 gradually along curve, finally make MMC power output and bearing power balance, make MMC stable output in new electric voltage frequency and amplitude.Therefore, in this process, because output voltage frequency and amplitude regulating characteristics, system there will not be the sudden change of voltage, electric current.
According to the combination method of MMC type HVDC light system of the present invention, when without lock unit, the presynchronization time is very short, realizes level and smooth, little impact and is connected to the grid, and realizes friendly grid-connected.
Claims (1)
1. the combination method of a MMC type HVDC light system, within the system, MMC is connected to AC network by circuit breaker, the controlling unit of this system comprises virtual synchronous control unit and the capacitor voltage equalizing grading control unit based on phase shift carrier modulation, it is characterized in that, this combination method comprises the steps:
Detection of grid three-phase voltage, MMC three-phase output current and MMC three-phase output voltage;
MMC three-phase output voltage and the electrical network three-phase voltage detected are compared, in conjunction with the virtual impedance between practical impedance setting MMC and electrical network, in virtual synchronous control unit, calculates the virtual current for cutting-in control;
Be the model of synchronous generator by MMC equivalent model, calculate the reference voltage signal for control MMC;
The signal obtained with reference to voltage signal, capacitor voltage equalizing grading control unit combines the drive singal of the switch obtaining each submodule in MMC compared with the triangular carrier of known phase shift, controls MMC output voltage;
MMC output voltage moves closer to line voltage, and virtual current is reduced gradually, and when virtual current is zero, namely MMC output voltage and line voltage are substantially identical, close circuit breaker, realizes MMC and electrical network is grid-connected;
After MMC is grid-connected, virtual current is replaced to enter virtual synchronous control unit the MMC output current detected, and set meritorious, the idle reference value of MMC output, change the on off state of the switch of virtual synchronous control unit, make MMC have the droop characteristic of similar synchronous generator.
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| CN104701886A (en) * | 2015-04-02 | 2015-06-10 | 国家电网公司 | Presynchronization control method based on virtual synchronous generator |
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| CN106451536A (en) * | 2016-09-30 | 2017-02-22 | 中国电力科学研究院 | Integrated system of virtual synchronous machine and control method thereof |
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| CN108493966B (en) * | 2018-04-24 | 2020-03-24 | 东北大学 | Virtual synchronization technology-based microgrid unbalanced load control method and device |
| CN109004814B (en) * | 2018-08-13 | 2020-03-24 | 南昌工程学院 | Submodule capacitor voltage balance control system for MMC |
| CN109256971A (en) * | 2018-10-14 | 2019-01-22 | 浙江大学 | A kind of modular multilevel submodule pressure equalizing control method |
| CN110034586A (en) * | 2019-05-10 | 2019-07-19 | 中国石油大学(华东) | A kind of MMC-HVDC island-grid active/passive method for handover control |
| CN112769322B (en) * | 2019-11-05 | 2022-05-24 | 台达电子企业管理（上海）有限公司 | Inverter and soft start method thereof |
| CN111490558A (en) * | 2020-03-31 | 2020-08-04 | 杭州鸿晟电力设计咨询有限公司 | Microgrid grid-connected and off-grid safety control method for improved virtual synchronous generator |
| CN111856311B (en) * | 2020-07-06 | 2021-08-03 | 上海交通大学 | Island detection method and system for distributed power supply of medium and low voltage direct current system |
| CN112701732B (en) * | 2021-01-25 | 2023-06-16 | 南方电网科学研究院有限责任公司 | Control method and device for virtual synchronous generator and storage medium |
| CN113346531B (en) * | 2021-06-23 | 2023-10-24 | 新风光电子科技股份有限公司 | Active parallel-to-off-grid switching method for cascade energy storage system |
| CN114024294B (en) * | 2021-10-22 | 2023-08-15 | 武汉大学 | Virtual-physical integrated current limiting system and method suitable for half-bridge MMC |
| CN114336770B (en) * | 2022-01-10 | 2023-07-25 | 湖南工业大学 | Virtual current-based VSG presynchronization control method and grid-connected method thereof |
| CN114374206B (en) * | 2022-03-22 | 2022-06-17 | 西安热工研究院有限公司 | Grid-connected point voltage adjusting method and system of fused salt energy storage and conversion device for thermal power |
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| CN102074967B (en) * | 2011-01-10 | 2013-05-22 | 清华大学 | Method for controlling energy storage type wind power station with synchronization property |
| JP2013046503A (en) * | 2011-08-25 | 2013-03-04 | Waseda Univ | Power storage system and control method thereof |
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