Modular multilevel brachium pontis capacitance voltage balance control method based on ac-side current compensation
Technical field
The invention belongs to the brachium pontis capacitance voltage balance control field of converter.
Background technology
Modular multilevel converter superposes by submodule output voltage, can realize high voltage output.Its DC side parallel, can realize energy exchange simultaneously, is specially adapted to high voltage direct current transmission (HVDC) field, controls also application to some extent in middle pressure SVG at medium voltage frequency converter.It is that its capacitance voltage balance is controlled that the method exists one of difficult problem on controlling.Its capacitance voltage balance is controlled and is comprised between alternate, brachium pontis and brachium pontis inner equilibrium is controlled.
For the capacitive coupling balance of voltage, control, can be by power being controlled from DC side or AC; For capacitance voltage balance in brachium pontis, can be by sequence, the methods such as superimposed voltage solve.Yet for capacitance voltage balance between brachium pontis, the method that document is recorded is less, mainly by the Alternating Component that superposes on DC side circulation, realizes.
For modular multilevel converter (MMC) as shown in Figure 4, its equivalent electric circuit as shown in Figure 5, according to this figure, can be listed as and write upper and lower bridge arm voltage expression formula and be,
Loop current i
zjwith ac-side current i
ojby formula (2) statement, and be considered as common and different mode electric current by two:
Ignore voltage on inductance, the instantaneous power of upper and lower brachium pontis is:
In one-period, carrying out integration, can obtain the exchange of upper and lower bridge arm energy, is also the energy of upper and lower bridge arm storage, and its difference is
If this is not equal to 0, represent in one-period that upper and lower bridge arm capacitance voltage stored energy not etc., will not cause that the variable quantity of upper and lower bridge arm capacitance voltage is different, therefore can realize brachium pontis capacitance voltage balance and control by adjusting each component of this formula.In prior art scheme, because AC electric current and voltage is controlled difficulty, mainly adopt at i
zitem stack of ac, makes u
oj* i
zjafter integration, be not that 0 method realizes brachium pontis capacitance voltage balance.
But this method is at i
zupper generation of ac, easily causes U
dcupheaval, this is not wish to occur in direct current transportation.While U
dcof ac and i
ojproduct term can weaken counterbalance effect, U
dcof ac also can with each phase energy control hazard.For solving this difficult problem, herein from adjusting i
ojset out, by increasing i
ojdC quantity, causing after formula (3) is not 0 after a product term integration, reaches the effect that balance is controlled.
Summary of the invention
The present invention controls difficult problem in order to solve existing current transformer brachium pontis capacitance voltage balance, the invention provides a kind of modular multilevel brachium pontis capacitance voltage balance control method based on ac-side current compensation.
Modular multilevel brachium pontis capacitance voltage balance control method based on ac-side current compensation, it comprises the steps:
Step 1: the set-point Δ i that obtains AC three-phase current side-play amount
a, Δ i
b, Δ i
c;
Step 2: the set-point Δ i of the AC three-phase current side-play amount that step 1 is obtained
a, Δ i
b, Δ i
c, be superimposed upon respectively the desired value i of AC three-phase current side-play amount
* oA, i
* oB, i
* oCafter upper, obtain AC three-phase current reference value i
refA, i
refB, i
refC, this AC three-phase current reference value i
refA, i
refB, i
refCafter three phase transformation two-phase coordinate transforms, send into pi regulator,
Meanwhile, detect the three-phase current actual value i of converter output
oA, i
oB, i
oC, and after three phase transformation two-phase coordinate transforms, send into pi regulator,
Pi regulator carries out PI control to received signal, obtains two phase voltage directive u
d, u
q, and to two phase voltage directive u
d, u
qcarry out after two phase transformation three-phase coordinate transforms, obtain PI and control the three-phase voltage u obtaining
a, u
b, u
c, u wherein
d, u
qrepresent respectively d shaft voltage component and q shaft voltage component under dq coordinate system;
Step 3: gather alternating current net side three-phase voltage u
sA, u
sB, u
sC;
Step 4: the set-point Δ i of the AC three-phase current side-play amount obtaining according to step 1
a, Δ i
b, Δ i
cand the AC three-phase current reference value i of step 2 acquisition
refA, i
refB, i
refC, obtain residual voltage u
0;
Step 5: PI step 2 being obtained by adder controls the three-phase voltage u obtaining
a, u
b, u
c, the alternating current net side three-phase voltage u that step 3 obtains
sA, u
sB, u
sCand the residual voltage u of step 4 acquisition
0summation, obtains the final AC three-phase voltage set-point u of converter respectively
oA, u
oB, u
oC, by the final AC three-phase voltage set-point u of this converter
oA, u
oB, u
oCconverter is controlled, realized modular multilevel brachium pontis capacitance voltage balance.
The set-point Δ i of the acquisition AC three-phase current side-play amount described in step 1
a, Δ i
b, Δ i
cprocess be,
Step is one by one: detects respectively the level voltage of n submodule of A, B, C phase, obtains n the upper and lower brachium pontis capacitance voltage of submodule value of A phase, and n the upper and lower brachium pontis capacitance voltage of submodule value of n submodule upper and lower brachium pontis capacitance voltage value of B phase and C phase,
Step 1 two: each mutually upper and lower brachium pontis capacitance voltage mean value of statistics, and by low pass filter filtering high-frequency fluctuation composition, each the mutually upper and lower brachium pontis capacitance voltage difference obtaining is sent into pi regulator and is regulated, and obtains the set-point Δ i of ac-side current side-play amount
a, Δ i
b, Δ i
c.
The set-point Δ i of the AC three-phase current side-play amount obtaining according to step 1 described in step 4
a, Δ i
b, Δ i
cand the AC three-phase current reference value i of step 2 acquisition
refA, i
refB, i
refC, obtain residual voltage u
0process be,
By the set-point Δ i of AC three-phase current side-play amount
a, Δ i
b, Δ i
cand the AC three-phase current reference value i of step 2 acquisition
refA, i
refB, i
refC, obtain feedback current zero-sequence component (Δ i
a+ Δ i
b+ Δ i
c)/3 and reference current zero-sequence component (i
refA+ i
refB+ i
refC)/3,
And by feedback current zero-sequence component (Δ i
a+ Δ i
b+ Δ i
c)/3 and reference current zero-sequence component (i
refA+ i
refB+ i
refCpI control is carried out in)/3, obtains residual voltage u
0.
Described low pass filter adopts sliding window averaging method to realize.
Concrete methods of realizing of the present invention is as follows:
Preliminary analysis: the brachium pontis stored energy difference of describing according to formula (4), the DC offset of increase output current.The A of take is example mutually, supposes that electric current adjustment amount is Δ i
a,
Visible, adjust alternating current side-play amount, can realize the adjustment of upper and lower bridge arm stored energy, reach the object of balance brachium pontis capacitance voltage, U
dcrepresent current brachium pontis DC bus-bar voltage, i
ojrepresent to exchange transient current, T represents power frequency period, and t represents the time.
In order to realize astatic brachium pontis capacitance voltage balance, first the present invention adopts pi regulator to carry out closed-loop control to side-play amount, and the difference of upper and lower bridge arm capacitance voltage is inputted as pi regulator, is output as side-play amount set-point, referring to Fig. 2.Secondly, in order to make converter ac-side current can obtain direct current offset, the present invention adopts the synchronously rotating reference frame decomposed control with zero sequence, as shown in Figure 3.After the DC offset that the stack of converter predetermined current of ac is controlled for balance, in rotating coordinate system, calculate respectively positive sequence, zero-sequence component as set-point, extract positive sequence in converter AC actual current, zero sequence as value of feedback simultaneously, by positive sequence, zero sequence controller, obtain corresponding control voltage, and obtain three phase control voltages by anti-change, modular multilevel brachium pontis capacitance voltage balance control method based on ac-side current compensation of the present invention, control procedure is simple, and can not cause current brachium pontis DC bus-bar voltage upheaval.
In addition, because one-period inner module capacitance voltage exists fluctuation, and upper and lower bridge arm voltage fluctuation of capacitor opposite direction, this wave component is introduced controller can be affected the stability of controller and excessively regulate, for avoiding this phenomenon, need to, to the filtering of upper and lower bridge arm capacitance voltage, only extract the low frequency composition of capacitance voltage.The inventive method does not need to increase new circuit, is applicable to various modular multilevel converter applications occasions, as high voltage direct current transmission, and reactive power compensation etc.
Accompanying drawing explanation
Fig. 1 is the principle schematic of the modular multilevel brachium pontis capacitance voltage balance control method based on ac-side current compensation described in embodiment two; Wherein, 3/2 represents three phase transformation two-phase coordinate transforms, and 2/3 represents two phase transformation three-phase coordinate transforms;
Fig. 2 is in embodiment two, and the A of take is example mutually, obtains the set-point Δ i of ac-side current side-play amount
atheory diagram;
Fig. 3 is the acquisition residual voltage u described in embodiment three
0theory diagram; i
drefrepresent the d shaft current reference value component under dq coordinate system; i
qrefrepresent the q shaft current reference value component under dq coordinate system; i
dOrepresent the d shaft current actual value component under dq coordinate system; i
qOrepresent the q shaft current actual value component under dq coordinate system; ω represents line voltage angular frequency, and t represents the time;
Fig. 4 is the topological structure schematic diagram of modular multilevel converter in background technology;
Fig. 5 is modular multilevel converter Simplified equivalent model schematic diagram in background technology.
Embodiment
Embodiment one: the modular multilevel brachium pontis capacitance voltage balance control method based on ac-side current compensation described in present embodiment, it comprises the steps:
Step 1: the set-point Δ i that obtains AC three-phase current side-play amount
a, Δ i
b, Δ i
c;
Step 2: the set-point Δ i of the AC three-phase current side-play amount that step 1 is obtained
a, Δ i
b, Δ i
c, be superimposed upon respectively the desired value i of AC three-phase current side-play amount
* oA, i
* oB, i
* oCafter upper, obtain AC three-phase current reference value i
refA, i
refB, i
refC, this AC three-phase current reference value i
refA, i
refB, i
refCafter three phase transformation two-phase coordinate transforms, send into pi regulator,
Meanwhile, detect the three-phase current actual value i of converter output
oA, i
oB, i
oC, and after three phase transformation two-phase coordinate transforms, send into pi regulator,
Pi regulator carries out PI control to received signal, obtains two phase voltage directive u
d, u
q, and to two phase voltage directive u
d, u
qcarry out after two phase transformation three-phase coordinate transforms, obtain PI and control the three-phase voltage u obtaining
a, u
b, u
c, u wherein
d, u
qrepresent respectively d shaft voltage component and q shaft voltage component under dq coordinate system;
Step 3: gather alternating current net side three-phase voltage u
sA, u
sB, u
sC;
Step 4: the set-point Δ i of the AC three-phase current side-play amount obtaining according to step 1
a, Δ i
b, Δ i
cand the AC three-phase current reference value i of step 2 acquisition
refA, i
refB, i
refC, obtain residual voltage u
0;
Step 5: PI step 2 being obtained by adder controls the three-phase voltage u obtaining
a, u
b, u
c, the alternating current net side three-phase voltage u that step 3 obtains
sA, u
sB, u
sCand the residual voltage u of step 4 acquisition
0summation, obtains the final AC three-phase voltage set-point u of converter respectively
oA, u
oB, u
oC, by the final AC three-phase voltage set-point u of this converter
oA, u
oB, u
oCconverter is controlled, realized modular multilevel brachium pontis capacitance voltage balance.
Present embodiment, the transformation for mula of two phase transformation three-phase coordinate transforms can be realized by following general formula
The present invention mainly realizes by following steps, and one, current offset amount sets; Two, design current controller.By adder, the side-play amount of controlling for the balance of voltage is superimposed upon to current reference value, by coordinate converter, transform on rotating coordinate system and control, simultaneously, detect after converter actual current, by coordinate converter, transform to rotating coordinate system equally, at rotational coordinates, fasten by PI and control, obtain voltage instruction u
d, u
q; Three, generate converter AC voltage instruction.Introduce electric voltage feed forward link, sampling alternating current net side voltage u
sA, u
sB, u
sC, and obtain zero sequence control voltage u by zero-sequence current control
0, finally by adder, these three magnitudes of voltage are sued for peace, obtain the final AC voltage given value u of converter
oA, u
oB, u
oC, by this voltage given value, converter is controlled, can realize brachium pontis capacitance voltage balance.
Embodiment two: present embodiment is described referring to Fig. 1 and 2, the difference of the modular multilevel brachium pontis capacitance voltage balance control method based on ac-side current compensation described in present embodiment and embodiment one is, the set-point Δ i of the acquisition AC three-phase current side-play amount described in step 1
a, Δ i
b, Δ i
cprocess be,
Step is one by one: detects respectively the level voltage of n submodule of A, B, C phase, obtains n the upper and lower brachium pontis capacitance voltage of submodule value of A phase, and n the upper and lower brachium pontis capacitance voltage of submodule value of n submodule upper and lower brachium pontis capacitance voltage value of B phase and C phase,
Step 1 two: each mutually upper and lower brachium pontis capacitance voltage mean value of statistics, and by low pass filter filtering high-frequency fluctuation composition, each the mutually upper and lower brachium pontis capacitance voltage difference obtaining is sent into pi regulator and is regulated, and obtains the set-point Δ i of ac-side current side-play amount
a, Δ i
b, Δ i
c.
Present embodiment, current offset amount setting aspect, first detects upper and lower bridge arm capacitance voltage, after low pass filter, sends into subtracter, obtains upper and lower bridge arm capacitance voltage difference, sends into pi regulator and regulates, and obtains current offset amount set-point Δ i
a;
The A of take is example mutually, and each level voltage of detection sub-module supposes that upper brachium pontis n sub-module capacitance magnitude of voltage is respectively U
cP1, U
cP2..., U
cPn, lower brachium pontis n sub-module capacitance magnitude of voltage is respectively U
cN1, U
cN2..., U
cNn, statistics upper and lower bridge arm capacitance voltage mean value, and by low pass filter filtering high-frequency fluctuation composition.Capacitance voltage error to upper and lower bridge arm is sent into pi regulator, pi regulator output Δ i
abe electric current adjustment amount.I
* oAthe ac-side current reference quantity that the converter calculating mutually for A needs, Δ i
awith i
* oAstack, obtains new current reference value i
refA.
Embodiment three: present embodiment is described referring to Fig. 3, the difference of the modular multilevel brachium pontis capacitance voltage balance control method based on ac-side current compensation described in present embodiment and embodiment one or two is, the set-point Δ i of the AC three-phase current side-play amount obtaining according to step 1 described in step 4
a, Δ i
b, Δ i
cand the AC three-phase current reference value i of step 2 acquisition
refA, i
refB, i
refC, obtain residual voltage u
0process be,
By the set-point Δ i of AC three-phase current side-play amount
a, Δ i
b, Δ i
cand the AC three-phase current reference value i of step 2 acquisition
refA, i
refB, i
refC, obtain feedback current zero-sequence component (Δ i
a+ Δ i
b+ Δ i
c)/3 and reference current zero-sequence component (i
refA+ i
refB+ i
refC)/3,
And by feedback current zero-sequence component (Δ i
a+ Δ i
b+ Δ i
c)/3 and reference current zero-sequence component (i
refA+ i
refB+ i
refCpI control is carried out in)/3, obtains residual voltage u
0.
Embodiment four: the difference of the modular multilevel brachium pontis capacitance voltage balance control method based on ac-side current compensation described in present embodiment and embodiment three is, described low pass filter employing sliding window averaging method realization.
In present embodiment: because one-period inner module capacitance voltage exists fluctuation, and upper and lower bridge arm voltage fluctuation of capacitor opposite direction, this wave component is introduced controller can be affected the stability of controller and excessively regulate, for avoiding this phenomenon, need to, to the filtering of upper and lower bridge arm capacitance voltage, only extract the low frequency composition of capacitance voltage.Can adopt average value filtering method, design width is the sliding window of a primitive period, the impact of harmonic carcellation on control performance for this reason.