CN103795080A - Grid connection method of MMC-type light direct-current power transmission system - Google Patents

Abstract

The invention discloses a grid connection method of an MMC-type light direct-current power transmission system. A virtual synchronization control method is combined with an MMC so that switching of operation modes of the MMC-type light direct-current power transmission system can be carried out, a special synchronization control circuit is needless, the system can be automatically synchronized with a power grid before grid connection, pre-synchronization time is short, the system can accurately follow frequency of the power grid after grid connection, and friendly grid connection is achieved.

Description

A kind of combination method of MMC type HVDC light system

Technical field

The present invention relates to field of power electronics, relate in particular 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 resources such as solar energy and wind energy constantly promote 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 operation.Distributed power source has dispersion, closely leans on the feature of load, if realize and taking over seamlessly in grid-connected and two kinds of operational modes of isolated island, not only contribute to improve the energy supply quality of distributed power generation, can also bring into play the potential of distributed power generation, improve the power supply reliability of electric power system.Therefore the seamless switching technique of, carrying out MMC type HVDC Light operational mode is studied particularly important.But general operational mode switch controller has the following disadvantages:

1, operational mode switching controls adopts double mode control technology, while being incorporated into the power networks, adopt PQ current control method, islet operation adopts V/f control method, double mode control technology meets the needs of microgrid operation, but it is large between voltage control and Current Control, to switch difficulty, there is the risk of handoff failure, determine whether to need switching control strategy but also will detect isolated island information.

2, adopt general droop control, be incorporated into the power networks and islet operation pattern under its control strategy remain unchanged, meet the feature of plug and play, but do not consider that droop control, to the adaptability being incorporated into the power networks, may exist larger impulse current in grid-connected moment.

Summary of the invention

For make MMC type HVDC light system without under lock unit fast and synchronized, realize MMC fast, be smoothly connected to the grid, be critical loads reliable power supply incessantly, the present invention proposes a kind of combination method of MMC type HVDC light system, in this 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 detecting are compared, set the virtual impedance between MMC and electrical network in conjunction with practical impedance, in virtual synchronous control unit, calculate the virtual current for grid-connected control;

The model that is synchronous generator by MMC equivalent model, calculates the reference voltage signal for controlling MMC;

The signal of all pressing grading control unit to obtain with reference to voltage signal, the voltage driving signal of the switch that obtains each submodule in MMC of comparing with the triangular carrier of known phase shift that combines, controls MMC output voltage;

MMC output voltage moves closer to line voltage, and virtual current is reduced gradually, and in the time that virtual current is zero, MMC output voltage and line voltage are basic identical, and 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 complexity, special synchronization control circuit, utilizes relatively generating virtual electric current of line voltage and MMC output voltage, realize MMC output and synchronized, and the presynchronization time is very short.

(2) virtual synchronous control has the droop characteristic of similar synchronous generator, when grid-connected, can regulate in conjunction with the frequency of external network, amplitude the output of self power, when burning voltage frequency, amplitude are near rated value, guarantee the reliable power supply to load.

(3) because virtual synchronous control MMC type HVDC light system all can move under grid-connected, island mode, in the time that operational mode changes, without the information that detects external network variation, without control strategy is switched, 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, to the accompanying drawing of required use in embodiment or description of the Prior Art be briefly described below, apparently, accompanying drawing in the following describes is only some embodiments of the present invention, for those of ordinary skills, do not paying under the prerequisite of creative work, can also obtain according to these accompanying drawings other accompanying drawing.

Fig. 1 is system 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 are only for the present invention is described, but can not be used for limiting the scope of the invention.

Referring to Fig. 1 and Fig. 2, show respectively the s operation control algorithm block diagram in structured flowchart and the operational mode control method thereof of MMC type HVDC light system according to an embodiment of the invention.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 and off-grid of MMC.The controlling unit of this system mainly comprises a virtual synchronous control unit and a capacitor voltage equalizing grading control unit.Detection of grid three-phase voltage u in real time _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).Carrying out and when net operation, by the line voltage detecting and MMC output voltage, (being approximately system-level modulation signal e) compares, by the s operation control algorithm in virtual synchronous control unit, generate the virtual current of grid-connected control use, enter virtual synchronous control unit, generating reference voltage signal, controls MMC, as virtual current i _sbe decreased to gradually at 0 o'clock, 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 detecting _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 there is droop characteristic, complete MMC grid-connected, and stable operation under grid-connected condition, power output is basic and reference value is identical.Move for MMC off-grid, in the time that electrical network need to detect or break down, require MMC to move at island mode, can cut-off breaker, make the operation of MMC off-grid, because introduced droop control in virtual synchronous control unit, make MMC there is droop characteristic, under island mode, can be 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.

Describe the s operation control algorithm in virtual synchronous control unit of the present invention in detail below in conjunction with Fig. 2.The model that s operation control algorithm of the present invention is synchronous generator by MMC equivalent model, realizes the grid-connected of MMC according to the Power Exchange of synchronous generator and electrical network.

The number of pole-pairs of assumes synchronization generator is 1, and stator winding self-induction is L, and mutual inductance is-M that excitation resistance sense value is R _f, L _f, the mutual inductance M between excitation winding and stator winding _fvariation with electrical degree θ changes, 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:

$\left\{\begin{array}{c}P=\stackrel{\·}{\mathrm{\θ}}{M}_f{i}_f\⟨i,\sin \mathrm{\θ}\⟩\\ Q=-\stackrel{\·}{\mathrm{\θ}}{M}_f{i}_f\⟨i,\cos \mathrm{\θ}\⟩\end{array}\right.$ Formula (1)

$T_e=P/\stackrel{\·}{\mathrm{\θ}}=M_f{i}_f\⟨i,\sin \mathrm{\θ}\⟩$ Formula (2)

Wherein, P is the active power of sending, and Q is the reactive power of sending, 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 excitation winding and the stator winding of synchronous generator, i _ffor exciting current, <, > is inner product operation, i is control electric current.

As shown in Figure 2, for above-mentioned control current i, choose respectively different current values in grid-connected front and back.First, MMC three-phase output voltage and the electrical network three-phase voltage detecting are compared, introduce the virtual impedance Ls+R between MMC and electrical network, calculate the virtual current for grid-connected control

in the presynchronization stage before grid-connected, control and choose this virtual current i by current i _s, after grid-connected, control chooses by current i the MMC three-phase output current i detecting _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, can obtain M _fi _fcomputing formula as follows:

$M_f{i}_f=\frac{1}{K}\&Integral;[Q_{\mathrm{set}}-Q+D_q(V-V_g)]\mathrm{dt}$ Formula (3)

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 first first the making of Q, be 0, calculating M _fi _fduring with Q, the value of the other side's previous moment of respectively asking for, thus reach the state of a balance; V _gfor the amplitude of grid line voltage,

amplitude regulates timeconstantτ _v, D _qafter selected, calculate K.

Switch S _cbeat during to position 2, MMC enters the operating state after grid-connected.As switch S _pclosure, MMC enters setting real power control, makes the active power of output and the active-power P of setting of MMC inverter _setequate; If switch S _pdisconnect, MMC inverter is operated in sagging pattern.Work as switch S _qwhen closed, voltage droop control works, and now the reactive power Q of output is not equal to the reactive power Q of setting _set.

From Fig. 2, can also draw following formula:

$\stackrel{\&CenterDot;\&CenterDot;}{\mathrm{\&theta;}}=(P_{\mathrm{set}}\&CenterDot;\frac{1}{{\stackrel{\&CenterDot;}{\mathrm{\&theta;}}}_n}-\mathrm{\&Delta;T}-T_e)\&times;\frac{1}{J}$ Formula (4)

To in formula (4) carry out obtaining after integral operation

right

carry out after integral operation, obtaining θ, wherein P _setfor the active power of setting,

the specified angular frequency of electrical network,

d _pbe the second damping coefficient,

reference angle frequency, be Δ T through the value that obtains after PI controller with sum, J is the moment of inertia of following all parts of synchronous generator rotor rotation,

frequency adjustment timeconstantτ _f, D _pafter selected, can calculate J,

$e=M_f{i}_f\stackrel{\&CenterDot;}{\mathrm{\&theta;}}\sin \mathrm{\&theta;}$ Formula (5)

Finally, according to above formula (1), formula (2), formula (3), formula (4) and formula (5), can calculate the reference voltage signal e for controlling MMC.

According to capacitor voltage equalizing grading control strategy, gather the each capacitance voltage on MMC inverter, the electric current of upper and lower bridge arm, in conjunction with reference voltage signal e, can obtain the final modulating wave of each submodule of MMC, compare with the triangular carrier of phase shift, obtain the driving signal of the electronic switch in each submodule, realize controlled and capacitance voltage stable of MMC power output.

Fig. 3 has provided 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 every mutually upper and lower brachium pontis is in series by a reactor and n submodule.Each submodule is made up of two IGBT, two anti-paralleled diodes and an electric capacity.

The grid-connected control of MMC type HVDC-Light system is described in conjunction with Fig. 2 below again.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 is at presynchronization state, detection of grid three-phase voltage, and MMC output voltage, electric current, according to the electrical network three-phase voltage of sampling, make comparisons with the voltage signal of MMC output, through a transfer function, generating virtual current i _s, enter controller, because the set point of MMC power output is 0, system operation makes the power of MMC output move closer to its set point, i.e. i _sgradually reduce until 0.Voltage signal and the line voltage u of now MMC output _gsynchronous, i.e. voltage-phase, the amplitude of MMC output are identical with electrical network, can close circuit breaker make MMC grid-connected.In this process, also introduce pi regulator and regulated Δ T, make D _phanging portion is tending towards 0, and Δ T is 0 o'clock, the reference frequency of generation

equate with mains frequency.2) be incorporated into the power networks process: MMC grid-connected after, MMC output current i _greplace virtual current is to enter controller, set meritorious, the reactive power reference qref P of MMC output _set, Q _set, meritorious, reactive power that MMC output is specified.Now, make switch S _pdisconnection, S _qclosure, makes MMC have the droop characteristic of frequency, amplitude and power output.In the synchronization control circuit situation without complicated, special, the presynchronization time is very short, and level and smooth, the little impact of MMC is connected to the grid, and has realized friendly grid-connected.

Carrying out application control method of the present invention below in conjunction with Fig. 4 is how to realize off-grid operation.As previously mentioned, the model that is synchronous generator by MMC equivalent model.Can obtain the MMC model after grid-connected, as shown in Figure 4.Its generation meritorious, reactive power is as follows:

$P=\frac{{3U}_{g}E}{{2X}_s}\sin (\mathrm{\&theta;}-{\mathrm{\&theta;}}_g)$

$Q=\frac{{3U}_g}{{2X}_s}[E\cos (\mathrm{\&theta;}-{\mathrm{\&theta;}}_g)-{\mathrm{\&theta;}}_g]$

Wherein, U _gbe the amplitude of ac bus, E is the induced electromotive force amplitude of synchronous generator, controls E by exciting current or voltage, θ, θ _gbe respectively the voltage phase angle of alternating current net side, synchronous generator, X _sfor the synchronous impedance of synchronous generator.

Regulate reactive power Q by controlling E, when

$\left\{\begin{array}{c}E=U_g\\ \mathrm{\&theta;}={\mathrm{\&theta;}}_g\end{array}\right.$

Time, between grid side and synchronous generator, there is no meritorious, idle exchange.If P, Q are 0, meet above formula, the voltage e of generation and grid side voltage u _gequate, during this time, the grid-connected or off-grid of synchronous generator, can not cause larger transient process, and this can be used for synchronizeing at grid-connected front MMC and electrical network.

When electrical network breaks down or when regularly maintenance, needs cut-off breaker to make MMC separately to load supplying, this just requires MMC to move at island mode.In the time of MMC stable operation, switch S _pdisconnection, S _qclosed.Now MMC type HVDC-Light system has the droop characteristic of similar synchronous generator, and MMC is by the mode conversion of being incorporated into the power networks during to islet operation pattern, without the information that detects operational mode, also without switching control strategy.

In the time of grid maintenance or chance failure, the circuit breaker of grid side disconnects, and causes MMC to operate in island mode, need to maintain the stable of electric voltage frequency and amplitude.Here, electrical network can be taken as an infinitely-great power supply, and in the time that bearing power is not mated with the set point of MMC power output, MMC coordinates electrical network to meet bearing power.In the time that bearing power is less than MMC power output, under the prerequisite that meets bearing power, unnecessary power will flow to electrical network.Otherwise electrical network is by powering load.

Referring to 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 _qwhen disconnection, the power of MMC output has same similarly characteristic with frequency, voltage,

$\left\{\begin{array}{c}{f}_0-f=k_P(P_0-P)\\ v_0-v=k_Q(Q_0-Q)\end{array}\right.$

F ₀, v ₀be respectively output voltage rated frequency and amplitude, P ₀, Q ₀be respectively specified meritorious, reactive power, k _p, k _qthe sagging system that is respectively electric voltage frequency and amplitude, is all less than 0.In the time that the active power of synchronous generator output increases (reducing), the frequency of its output voltage can reduce (increase) accordingly; In the time that the reactive power of synchronous generator output increases (reducing), the amplitude of its output voltage can reduce (increase) accordingly.

Suppose that MMC system begins operating in the position 1 of Fig. 5, meritorious, the idle P that is respectively of output ₁, Q ₁, in the time that circuit breaker disconnects, the state after MMC system is finally stablized is position 2, meritorious, the idle P that is respectively of output 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 rationally adjusted 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, without lock unit in the situation that, the presynchronization time is very short, realizes level and smooth, little impact and is connected to the grid, and realizes friendly grid-connected.

Claims (2)

1. the combination method of a MMC type HVDC light system, in this 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 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 detecting are compared, set the virtual impedance between MMC and electrical network in conjunction with practical impedance, in virtual synchronous control unit, calculate the virtual current for grid-connected control;

The model that is synchronous generator by MMC equivalent model, calculates the reference voltage signal for controlling MMC;

The signal of all pressing grading control unit to obtain with reference to voltage signal, the voltage driving signal of the switch that obtains each submodule in MMC of comparing with the triangular carrier of known phase shift that combines, controls MMC output voltage;

MMC output voltage moves closer to line voltage, and virtual current is reduced gradually, and in the time that virtual current is zero, MMC output voltage and line voltage are basic identical, and close circuit breaker, realizes MMC and electrical network is grid-connected.

2. the combination method of MMC type HVDC light system claimed in claim 1, is characterized in that, the method also comprises:

After MMC is grid-connected, replace virtual current to enter virtual synchronous control unit the MMC output current detecting, and set meritorious, the idle reference value of MMC output, change the on off state of controller, make MMC there is the droop characteristic of similar synchronous generator.

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