CN113113920A - Dynamic oscillation suppression method based on modular multi-level-to-level converter topology - Google Patents
Dynamic oscillation suppression method based on modular multi-level-to-level converter topology Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
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- H02M7/539—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters with automatic control of output wave form or frequency
- H02M7/5395—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters with automatic control of output wave form or frequency by pulse-width modulation
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Abstract
The invention discloses a dynamic oscillation suppression method based on a modular multi-level flat-level current transformer topology. The method introduces an oscillation suppression angle, the switching conduction time of two continuous modules of an upper bridge arm or a lower bridge arm in each half of a sub-module switching period is correspondingly adjusted under the action of the oscillation suppression angle (a switching signal is turned on in advance or turned off in a delayed manner when the oscillation suppression angle is larger than zero, and a switching signal is turned off in advance or turned on in a delayed manner when the oscillation suppression angle is smaller than zero), pulse voltage generated in bridge arm inductance is generated by simultaneously turning on or simultaneously turning off sub-modules at corresponding positions of the upper sub-bridge arm and the lower sub-bridge arm, and therefore the circulation current change trend. According to the invention, the instruction value can be quickly tracked by the circulation by adjusting the oscillation suppression angle through the circulation closed loop, the oscillation generated by insufficient damping in the process of sudden load change of the modular multilevel DC transformer topology is eliminated, and the system stability is improved.
Description
Technical Field
The invention belongs to the technical field of high-voltage high-power electronics, and mainly relates to a dynamic oscillation suppression method based on a modular multilevel DC transformer topology.
Background
The series structure of a sub-module capacitor C, a bridge arm inductor L and a parasitic resistor R in the modular multilevel DC transformer can cause RLC series resonance, and the specific process is as follows: the sub-module capacitor charges the bridge arm inductor and generates magnetic energy stored in the inductor core; after a period of time, the bridge arm inductance in turn charges the capacitor, and the magnetic energy is converted into electric energy, and the steps are repeated. If the parasitic resistance in the bridge arm inductance is neglected, energy loss cannot be generated in the process of magnetic energy-electric energy repeated conversion, and a continuous equal-amplitude damping oscillation phenomenon is generated in the capacitance voltage and the circulating current. However, the parasitic resistance in the actual inductor cannot be ignored, and the process of magnetic energy-electric energy alternating generates energy loss in the parasitic resistance, so that under-damped oscillation with the amplitude decaying with time is generated in the capacitor voltage and the circulating current. In such a case, if the dc side voltage or the transmit power command changes, dynamic processes can exacerbate the under-damped oscillations in the circulating current and capacitance voltages, severely threatening the stable operation of the system.
Disclosure of Invention
The purpose of the invention is as follows: the invention provides a dynamic oscillation suppression method based on a modular multilevel DC transformer topology, aiming at the problems that RLC series resonance exists in the modular multilevel DC transformer, under-damped oscillation is generated under the condition of sudden load/power change, the operation stability of a system is influenced and the like.
The technical scheme is as follows: the invention discloses a dynamic oscillation suppression method based on a modular multi-level-to-straight-flow transformer topology, which comprises two groups of MMC bridge arms, wherein each group of MMC bridge arms respectively comprises an upper sub-bridge arm and a lower sub-bridge arm, the upper sub-bridge arm and the lower sub-bridge arm respectively comprise N sub-modules which are connected in series, and N is a positive integer. The dynamic oscillation suppression method comprises the following steps:
(1) constructing a closed-loop control system by taking the circulating current between the upper sub bridge arm and the lower sub bridge arm of each group of MMC bridge arms as a controlled quantity and taking an oscillation suppression angle as a control quantity, wherein the oscillation suppression angle is determined by the difference between the actual value and the given value of the circulating current;
(2) constructing a state space average model of the modular multilevel-straight-line current transformer by taking sub-module capacitance and circulation as state quantities, carrying out small-signal analysis, simultaneously considering the dynamics of an oscillation suppression angle, deducing a control-output transfer function of the closed-loop control system under the small-signal model through Laplace transformation, solving a characteristic root of the closed-loop control system according to the control-output transfer function and expressing a damping ratio; the damping ratio is made to be critical damping 1, and the gain of the closed-loop controller under the condition of optimal damping compensation is obtained;
(3) calculating a corresponding oscillation suppression angle according to the obtained closed-loop controller gain to serve as the optimal oscillation suppression angle;
(4) and controlling the switches of the submodules of the two groups of MMC bridge arms by taking the optimal oscillation suppression angle as a control quantity so as to control the change of the circulating current.
Further, in the step (1), the given value i of the circulation currentc*maCirculating current when the input power of the modular multilevel DC transformer topology is equal to the output power; wherein the input power is equal to the product of the circulating current and the topological direct-current side voltage, and the output power is equal to the square of the topological output voltage divided by the output resistance.
Further, in the step (2), the control-output transfer function is a polynomial obtained by taking an oscillation suppression angle as a denominator and a circulation current as a numerator; the characteristic root of the closed-loop control system is a pair of conjugate complex roots which are related to a damping ratio xi; damping ratio xi, number of submodules N and bridge arm self-coupling inductance LsAnd bridge arm mutual coupling inductance LmSum, submodule capacitor C, controller gain kpiAll are related.
Further, the control-output transfer function is expressed as:
wherein ,
wherein ,
the estimate of the circulating current is shown,
denotes the estimated oscillation suppression angle, s denotes the Laplace operator, Uca_sumRepresents the sum of the capacitance and voltage of all sub-modules in a group of MMC bridge arms, IcmaDenotes the circulation, RsShowing the parasitic resistance, delta, of the bridge arm inductancedRepresenting the quiescent operating point of the oscillation suppression angle.
The damping ratio is expressed as:
further, the step (4) specifically includes: sequencing the sub-modules in the upper and lower sub-bridge arms of the two groups of MMC bridge arms, sequentially selecting one sub-module from the upper and lower sub-bridge arms of the two groups of MMC bridge arms according to the sequencing in each half of the switching period of the sub-modules to form two groups of sub-module pairs, and enabling the optimal oscillation suppression angle delta to be optimald' controlling the selected two sub-module pairs as a control quantity such that the pulse edges of the selected sub-modules are imposed with the optimal oscillation suppression angle δd', the remaining sub-module pulse edges remain unchanged.
Further, in step (4), the optimum oscillation suppression angle δ is setd' controlling the selected two sub-module pairs as a control variable such that the pulse edges of the selected sub-modules are subjected to the optimal oscillation suppression angle δd', specifically includes: when deltadWhen negative, the selected sub-module is turned off and delta on the basis of 50% square wave signald' same angle, leading-in and delta at rising edgedThe same angle, resulting in an amplitude of U in the bridge arm inductance voltagesub＝Uca_sum/16, duty cycle 2 δd'/Pi positive pulse voltage, circulating current rises; wherein U isca_sumRepresenting the sum of the capacitance and voltage of all sub-modules in a group of MMC bridge arms; when deltad' is positive, so that the selected submodule is turned on at rising edge delay based on 50% square wave signaldAt the same angle, turn off early at the falling edge and deltadThe same angle, resulting in an amplitude of U in the bridge arm inductance voltagesubAnd the duty ratio is 2 deltadA negative pulse voltage of'/pi, the circulating current drops.
The working principle is as follows: and performing closed-loop control on the circulating current to obtain an optimal oscillation suppression angle of a control quantity, acting the optimal oscillation suppression angle on the pulse edges of the switching signals of the sub-modules of the upper sub-bridge arm and the lower sub-bridge arm to form an area in which the upper sub-bridge arm and the lower sub-bridge arm are simultaneously connected or disconnected, generating pulse voltage with positive or negative amplitude in bridge arm voltage to further control the circulating current change trend, and quickly tracking an instruction value in a dynamic state to achieve the purpose of suppressing oscillation.
Has the advantages that: compared with the prior art, the controlled quantity oscillation suppression angle in the circulating closed-loop system respectively acts on the rising edge or the falling edge of a group of 50% duty ratio switch pairs in the upper sub-bridge arm and the lower sub-bridge arm, the on-off time of the switch pulse is adjusted, and the problems that the inductance voltage stress is too high, the utilization rate of the primary voltage of the transformer is reduced, the switching loss is too high due to frequent switching action and the like caused by the fact that the oscillation suppression angle simultaneously acts on all sub-modules of the upper sub-bridge arm and the lower sub-bridge arm are solved.
Drawings
FIG. 1 is a modular multilevel DC transformer topology;
FIG. 2 is a block diagram of closed loop control of the circulation loop;
FIG. 3 shows damping ratio xi and proportional controller gain k under closed-loop control of circulationpiAnd oscillation suppression angle quiescent operating point ΔdA relationship diagram of (1);
FIG. 4(a) and FIG. 4(b) show the optimal oscillation suppression angle under square-like wave modulation as positive (2δd'<Theta) and a bridge arm module conduction state diagram;
FIGS. 5(a) and 5(b) show the positive (delta) optimum oscillation suppression angle under square-like wave modulationd' > (N-1) theta) and a switch timing diagram and a bridge arm module conducting state diagram;
fig. 6(a), fig. 6(b), fig. 6(c), and fig. 6(d) are respectively dynamic response characteristics of capacitance voltage, circulating current, bridge arm inductance voltage, and output voltage of the sub-module of the MMDCT system without applying the dynamic oscillation suppression method of the present application;
FIG. 7 is a dynamic simulation waveform of MMDCT system variables under the dynamic oscillation suppression method of the present application when a load suddenly increases;
FIG. 8 is a dynamic simulation waveform of MMDCT system variables under the dynamic oscillation suppression method of the present application when a load suddenly decreases;
FIG. 9 is a dynamic simulation waveform of oscillation suppression angle and circulating current in five switching cycles after load dump using the dynamic oscillation suppression method of the present application;
FIG. 10 is a dynamic simulation waveform of oscillation suppression angle and circulating current in five switching cycles after a sudden load increase using the dynamic oscillation suppression method of the present application;
FIG. 11 shows switching signals S of arm 1 and arm 2 when the oscillation suppression angle is negativep1j and Sp2jBridge arm voltage u1 and u2Bridge arm inductance voltage uLaCirculating current icmaAnd bridge arm midpoint voltage uaNTiming diagram of (2).
Detailed Description
The modular multilevel dc transformer topology used in the present invention is shown in fig. 1. The primary side of the topology includes two sets of MMC bridge arms, namely a bridge arm set a and a bridge arm set b. Both ends of the bridge arm group a and the bridge arm group b are connected with a high-voltage direct-current side voltage UdcThe direct connection, the secondary side full-bridge structure is used for providing a low-voltage direct current port, and the primary side and the secondary side are coupled through a medium-frequency transformer. Each of the arm group a and the arm group b includes two upper and lower sub-arm connected in series (i.e., sub-arm 1 and sub-arm 2 in arm a, and sub-arm 3 and sub-arm 4 in arm b). The sub-bridge arms 1 to 4 respectively comprise N cascadedA half-bridge sub-module, wherein the upper and lower sub-bridge arms are self-coupled via respective bridge arm self-coupling inductors LsAnd two groups of bridge arms are connected, and ports a and b are respectively led out from the middle point of the self-coupling inductor and are used as a primary side high-frequency alternating current input port of the medium-voltage transformer. Wherein port a is through transformer leakage inductance LlkConnected to the intermediate frequency transformer. Leakage inductance L of transformerlkThe method is used for exchanging the power of the original secondary side. In addition, mutually coupled inductors L are equivalently arranged in the upper sub bridge arm and the lower sub bridge arm of each group of MMC bridge armsm。
Based on the above-mentioned modular multilevel dc transformer topology, a dynamic oscillation suppression method based on the modular multilevel dc transformer topology according to the present embodiment is described below.
First, as shown in fig. 2, taking the bridge arm group a as an example, the loop current i between the upper and lower sub-bridge arms 1,2cmaClosed-loop control is constructed for the controlled variable, which is defined as the oscillation suppression angle deltadAnd the device is used for dynamically adjusting the on and off time of the switching signal of the submodule.
Then, a modular multi-level flat-straight-flow transformer state space average model is constructed by taking the sub-module capacitance and the circulation current as state quantities, small signal analysis is carried out, meanwhile, the dynamic state of an oscillation suppression angle is considered, and a control-output transfer function G under the small signal model is deduced through Laplace transformationiδ(s)：
wherein ,
and wherein the one or more of the one,
the estimate of the circulating current is shown,
denotes the estimated oscillation suppression angle, s denotes the Laplace operator, Uca_sumRepresents the sum of the capacitance and voltage of all sub-modules in a group of MMC bridge arms, IcmaDenotes the circulation, RsRepresenting parasitic resistance, Δ, of bridge arm inductances in a set of MMC bridge armsdRepresenting the quiescent operating point of the oscillation suppression angle. The bridge arm inductance in one group of MMC bridge arms refers to self-coupling inductance L of upper and lower sub bridge arms in one group of MMC bridge armssInductance L mutually coupled with bridge armmSum, i.e. 2 (L)s+Lm)。
Solving a system characteristic root according to a characteristic equation of the closed-loop system and expressing a damping ratio xi:
the damping ratio xi is 1, and the gain k of the closed-loop controller under the condition of optimal damping compensation is obtainedpiThe formula (2) is as follows:
FIG. 3 shows damping ratio xi and proportional controller gain k under the closed-loop control of the circulating currentpiAnd oscillation suppression angle quiescent operating point ΔdA graph of the relationship (c).
According to the principle of power conservation, the voltage U of the circular current and the topological direct current sidedcThe product of (a) represents the input power, the topological output voltage UoIs divided by the output resistance RoRepresenting output power, so that a given value of circulating current can be found
Then detecting the actual value i of the circulating currentcmaWill give a given value
With the actual value icmaSubtracting to obtain a circulating current error value delta icmaThe oscillation suppression angle delta is obtained through the adjustment of a current inner loop proportional controllerd。
As previously mentioned, the control-output transfer function GiδThe term(s) is a polynomial obtained by taking the oscillation suppression angle as a denominator and the circulation flow as a numerator. The system characteristic root is a pair of conjugate complex roots:
where ξ denotes the damping ratio, ωnWhich is indicative of the natural oscillation frequency of the oscillator,
damping ratio xi and number of submodules N, bridge arm self-inductance and mutual inductance Ls+LmSub-module capacitance C, controller gain kpiAll are related. When other parameters are fixed, the larger the controller gain is, the larger the system damping ratio is, and the critical damping, i.e., ξ ═ 1, is an ideal damping state.
Designing a closed loop of the circulating current according to the gain of the closed-loop controller obtained by calculating the critical damping, thereby obtaining the optimal oscillation suppression angle delta corresponding to the critical dampingd', and suppressing the optimum oscillation angle deltadActing on the pulse edges of the switching signals of the upper sub-bridge arm sub-modules and the lower sub-bridge arm sub-modules of the two groups of MMC bridge arms as control quantities to enable the switching signals of the upper sub-bridge arm sub-modules and the lower sub-bridge arm sub-modules to form areas which are simultaneously conducted or simultaneously turned off, and generating pulse voltage in bridge arm inductive voltage, thereby controllingAnd the change of the circulating current inhibits dynamic damping oscillation. Wherein, the bridge arm inductance voltage refers to the voltage applied on all self-coupling inductances and mutual-coupling inductances of the upper and lower sub-bridge arms in a group of MMC bridge arms, namely 2 (L)s+Lm) The voltage of (c).
Specifically, the pulse edge of each corresponding submodule in the upper bridge arm and the lower bridge arm of each group of MMC bridge arms in each half of the submodule switching period is applied with the optimal oscillation suppression angle deltad', the remaining sub-module pulses remain unchanged. When the optimal oscillation suppression angle acts on the switching signal to enable the switching signal to be turned off in a delayed way at a falling edge and to be turned on in advance at a rising edge, the optimal oscillation suppression angle delta at the moment is definedd' is negative; when the optimum oscillation suppression angle deltad' when the switching signal is turned on in a delayed way at the rising edge and turned off in advance at the falling edge, the optimal oscillation suppression angle delta at the moment is definedd' is positive.
To reduce dv/dt, it is necessary to introduce a phase-shifting angle θ between the submodules, in order to verify that the phase-shifting angle does not affect the optimum oscillation suppression angle δdThe effect of the action on the circulating current is, for example, N-8, and fig. 4 and 5 show the timings of the optimum oscillation suppression angle 2 δ, respectivelyd'<θ and δd' > (N-1) theta indicates the conduction of the module. Under the square wave modulation, the variation quantity of the optimal oscillation suppression angle in one switching period to the circulation current is as follows:
in fig. 4(a), a solid gray line represents an upper arm switching signal, a solid black line represents a lower arm switching signal, and in fig. 4(b), a black square indicates a module input, and a white square indicates a module cut-off, so that a total of 18 switching modes, which are sequentially labeled as 1,2, …, and 18, are generated in one switching cycle. Each module output voltage is U assuming balanced capacitor voltages and no rippledc/8. The conduction numbers of the submodules in the modes 2 and 12 are 7, and the submodules are respectively conducted by 2 deltad', corresponding to the inductor voltage uL＝U dc8; the conduction number of the neutron modules in the rest 16 modes is 8, and the inductive voltage u L0. The amount of change in the circulation current can thus be expressed as:
when the oscillation suppression angle is large, the timing diagram of the upper and lower bridge arm switches and the schematic diagram of the bridge arm module conduction states are shown in fig. 5, in the first half of the sub-module switching period, the number of conduction modules in the mode 1 is 8, the inductance voltage u isL0; and 7 modules in the modes 2 to 9 are conducted, and the inductive voltage u is conducted at the momentL＝Udc/8, with mode 2 conduction δd', the modes 3 to 8 are respectively conducted with theta, and the mode 9 is conducted with deltad' -6 θ. In the second half of the sub-module switching period, the number of conducting modules in the mode 10 is 8, and the inductive voltage u isL0; and 7 modules in the modes 11 to 18 are conducted, and the inductive voltage u is generatedL＝Udc/8, with mode 11 conduction δd', conduction of modes 12 to 17 is theta, and conduction of mode 18 is deltad' -6 θ. The amount of ring flow change in this case can thus be expressed as:
at an optimum oscillation suppression angle deltadThe same conclusion can be drawn when' is negative, so that the phase-shift angle does not affect the optimum oscillation suppression angle δd' the amount of change in the circulating current generated at each switching period.
Optimum oscillation suppression angle deltadWhen the bridge arm inductance voltage is positive, the amplitude value generated in the bridge arm inductance voltage is a single submodule voltage Usub＝Uca_sum/16, duty cycle 2 δdThe positive pulse voltage of'/pi, the pulse voltage generated each time is respectively passed through a group of switch pairs (switch signal S in bridge arm 1) in upper and lower sub-bridge arms of each group of MMC bridge armp1jSwitching signal S in the sum bridge arm 2p2jOr switching signal S in bridge arm 3p3jAnd the switching signal S in the bridge arm 4p4jWhere j ═ 1 … 8) is formed in the region of more switches on the basis of the 50% square-wave signal, so that the circulating current rises; in a similar manner, the optimum oscillation suppression angle δdWhen the voltage is negative, the generated amplitude of the bridge arm inductance voltage is-UsubAnd the duty ratio is 2 deltadThe negative pulse voltage of'/pi, the pulse voltage produced each time is formed by the area of the switch pairs in the upper and lower sub-bridge arms which are more conductive on the basis of 50% square wave signals, so the circulating current is reduced.
FIG. 11 plots switching signal S for arm 1 and arm 2 when the optimum oscillation suppression angle is negativep1j,Sp2jBridge arm voltage u1,u2Bridge arm inductance voltage uLaCirculating current icmaAnd bridge arm midpoint voltage uaNTiming diagram of (2). As can be seen from the figure, the duty ratios of two consecutive modules of the upper arm or two consecutive modules of the lower arm change correspondingly under the action of the optimal oscillation suppression angle every half of the switching period (recorded as one action period) of the sub-module. Specifically, the method comprises the following steps: in the first action cycle, Sp11Advance deltadIs turned on and Sp18Delay deltad' off, Sp12～Sp17 and Sp21～Sp28The switching signal remains unchanged; in the second action cycle, Sp21Delay deltad' off and Sp22Advance deltad' on, Sp11～Sp18 and Sp23～Sp28The switching signal remains unchanged; in the third action cycle, Sp12Delay deltad' off and Sp13Advance deltad' on, Sp11、Sp14～Sp18 and Sp21～Sp28The switching signal remains unchanged; in the fourth action cycle, Sp23Delay deltad' off and Sp24Advance deltad' on, Sp11～Sp18 and Sp21、Sp22、Sp25～Sp28The switching signal remains unchanged. By analogy, in the eighth action period, Sp27Delay deltad' off and Sp28Advance deltad' on, Sp11～Sp18 and Sp21～Sp26The switching signal remains unchanged.
In eight action cycles, namely four switching cycles, the optimal oscillation suppression angle completes one round of action in 16 modules. Upper and lower bridge arm switch pair (S)p1j and Sp2j) The circulation current is continuously changed in two adjacent action periods. Viewed laterally, each submodule participates in the regulation of the circulating current only during one action period, and keeps the 50% duty cycle unchanged during the remaining seven action periods. Under the action of the negative optimal oscillation suppression angle, the upper bridge arm switch only adjusts the pulse edge time in the odd action period, so that the voltage of the bridge arm 1 can generate the amplitude value of U at the rising edge and the falling edge respectively in the odd action periodca_sum16 and a conduction time of deltadThe level of'; the lower bridge arm switch only adjusts the pulse edge time in the even number action period, so that the voltage of the bridge arm 2 can respectively generate the amplitude value of U on the rising edge and the falling edge in the even number action periodca_sum16 and a conduction time of deltadThe level of. The amplitude generated in the bridge arm inductance voltage is Uca_sum/16, duty cycle 2 δd' pi/pulse waveform, so that the bridge arm inductance voltage is-U ca_sum2 δ of/16dIn the region, the circulation presents a descending situation, a new steady state value can be reached after a plurality of action cycles, and a new circulation command value is tracked in time when the load is suddenly changed, so that damping oscillation in a dynamic process is restrained. Since the optimum oscillation suppression angle is applied to one group of switch pairs at a time and is conducted along the opposite direction of the pulse edge, the +/-U is generated in each sub-bridge arm voltageca_sumA 16 gap, so that the bridge arm midpoint output voltage uaNIn addition generate 7Uca_sum32 and-7Uca_sumTwo levels,/32.
Similarly, the action mechanism of the optimal oscillation suppression angle as positive is shown in the abstract figure 2, and is still analyzed by taking a half of the switching period of the sub-module as an action period. In the first action cycle, Sp21Advance deltad' off and Sp28Delay deltad' on, Sp22～Sp27 and Sp11～Sp18The switching signal remains unchanged; in the second action cycle, Sp11Delay deltadIs turned on and Sp12Advance deltad' off, Sp21～Sp28 and Sp13～Sp18Switching signalKeeping the same; in the third action cycle, Sp22Delay deltadIs turned on and Sp23Advance deltad' off, Sp21、Sp24～Sp28 and Sp11～Sp18The switching signal remains unchanged; in the fourth action cycle, Sp13Delay deltadIs turned on and Sp14Advance deltad' off, Sp21～Sp28 and Sp11、Sp12、Sp15～Sp18The switching signal remains unchanged … … and so on, during the eighth action period, Sp17Delay deltadIs turned on and Sp18Advance deltad' off, Sp21～Sp28 and Sp11～Sp16The switching signal remains unchanged.
Under the action of the forward optimal oscillation suppression angle, the upper bridge arm switch only adjusts the pulse edge time in an even number of action cycles, so that the voltage of the bridge arm 1 generates an amplitude of 7U at a rising edge and a falling edge respectively in the even number of action cyclesca_sum16 and a conduction time of deltadThe level of'; the lower bridge arm switch only adjusts the pulse edge time in the odd action period, so that the voltage of the bridge arm 2 can generate the amplitude of 7U respectively on the rising edge and the falling edge in the odd action periodca_sum16 and a conduction time of deltadThe level of. The amplitude generated in the bridge arm inductance voltage is Uca_sum/16, duty cycle 2 δdA pulse shape of'/pi, whereby the circulating current is 2 delta in this cased' the region assumes a rising situation and after a few action cycles can reach a new steady state value for damping dynamic damped oscillations. Output voltage u at the midpoint of the bridge armaNIn most cases generate 7Uca_sum32 and-7Uca_sumTwo levels,/32.
Fig. 6 is a dynamic response simulation waveform of the sub-module capacitance voltage, the circulating current, the bridge arm inductance voltage and the output voltage without adding the oscillation suppression method. The waveform has a very obvious under-damped oscillation phenomenon, the oscillation frequency is 739rad/s, and particularly after the load is suddenly changed, the oscillation is more obvious.
Closed loop control and output in the loopOutput voltage u of MMDCT system under voltage closed-loop control synergistic actionoPrimary and secondary side voltage uab,usCirculating current icmaPhase angle phi is shifted outward, and optimal oscillation suppression angle delta is improvedd' sudden increase in load (R)o＝5Ω→Ro2.5 Ω) and sudden load decrease (R)o＝2.5Ω→Ro5 Ω) are shown in fig. 7 and 8, respectively.
Under the closed-loop control of the external phase shift angle, the output voltage can track the instruction value in real time, is not influenced by the change of the load on the secondary side of the transformer, has good tracking performance and is stabilized at about 750V. Primary and secondary side voltage u of transformerab,usAre 4kV and 750V, respectively, the voltage ratio is equal to the transformer transformation ratio n-16: 3, no reactive power is present.
In fig. 7, when t is 0.2s, the output end load R isoSuddenly increased to R5 omegao2.5 Ω. Because the voltage on the direct current side reaches 4kV, the transmission power between the original secondary side can be adjusted only by a small value of an outward phase shift angle. The circulating current i can be calculated according to the conservation of powercmaThe load is increased from 14A to 28A at the time of 0.2s load surge, the dynamic process is only 0.15s, and the figure shows that the dynamic damping oscillation is better inhibited after closed-loop control, wherein the gain of the damping controller in the inner loop of the circular current is adjusted to be 0.0044. Further, during the increase in the circulation, the optimum oscillation suppression angle is kept positive, and the optimum oscillation suppression angle is returned to 0 when the circulation reaches the steady state. In fig. 8, when t is 0.2s, the output terminal is loaded with RoSudden increase of 2.5 Ω to RoWhen the phase angle of the external shift is increased from 1 degree to 1.5 degrees, the circulating current i meets the condition of power conservationcmaThe dynamic process continues for 0.1s, decreasing from 28A to 14A at 0.2s load ramp down, during which the optimum oscillation suppression angle remains negative, while the optimum oscillation suppression angle remains constant at 0 when the circulating current is at steady state.
To further verify the effect of the optimal oscillation suppression angle on the circulating current, fig. 9 and 10 show the circulating current i during the dynamic process of load dump and load dumpcmaAnd an optimum oscillation suppression angle deltad' simulation waveform in five switching cycles after the start of the dynamics. When the load is suddenly appliedWhen the current value is reduced, the new circulation instruction value is smaller than the actual circulation value, the closed-loop controller detects the difference value, an oscillation suppression angle smaller than zero is generated through closed-loop adjustment (figure 9(b)), and the oscillation suppression angle acts on 8 groups of switch pairs of the upper bridge arm and the lower bridge arm in turn in each half of the switching period of the sub-modules. When the load suddenly increases (fig. 10), the circulating current needs to quickly track the newly increased command value under the regulation action of the closed-loop controller, so that the forward oscillation suppression angle (fig. 10(b)) needs to act on each group of switch pairs to generate a region which is simultaneously turned off, and an amplitude value U is constructed in the bridge arm inductancedcA positive pulse voltage of/8. The optimal oscillation suppression angle acts on eight groups of switch pairs in turn under the regulation action of the closed-loop controller, so that the circulating current tracks a new instruction value.
Claims (6)
1. A dynamic oscillation suppression method based on a modular multi-level-to-level converter topology comprises two groups of MMC bridge arms, wherein each group of MMC bridge arm comprises an upper sub-bridge arm and a lower sub-bridge arm, the upper sub-bridge arm and the lower sub-bridge arm respectively comprise N sub-modules which are connected in series, and N is a positive integer; the dynamic oscillation suppression method is characterized by comprising the following steps:
(1) constructing a closed-loop control system by taking the circulating current between the upper sub bridge arm and the lower sub bridge arm of each group of MMC bridge arms as a controlled quantity and taking an oscillation suppression angle as a control quantity, wherein the oscillation suppression angle is determined by the difference between the actual value and the given value of the circulating current;
(2) constructing a state space average model of the modular multilevel straight-straight flow transformer by taking sub-module capacitance and circulation as state quantities, carrying out small signal analysis, simultaneously considering the dynamics of an oscillation suppression angle, deducing a control-output transfer function of the closed-loop control system under the small signal model through Laplace transformation, solving a characteristic root of the closed-loop control system according to the control-output transfer function and expressing a damping ratio; the damping ratio is made to be critical damping 1, and the gain of the closed-loop controller under the condition of optimal damping compensation is obtained;
(3) calculating a corresponding oscillation suppression angle according to the obtained closed-loop controller gain to serve as an optimal oscillation suppression angle;
(4) and controlling the switches of the submodules of the two groups of MMC bridge arms by taking the optimal oscillation suppression angle as a control quantity so as to control the change of the circulating current.
2. The dynamic oscillation suppression method based on the modular multi-level-to-flat-current transformer topology according to claim 1, characterized in that: in step (1), the given value i of the circulation currentc*maCirculating current when the input power of the modular multilevel DC transformer topology is equal to the output power; wherein the input power is equal to the product of the circulating current and the topological direct-current side voltage, and the output power is equal to the square of the topological output voltage divided by the output resistance.
3. The dynamic oscillation suppression method based on the modular multi-level-to-flat-current transformer topology according to claim 1, characterized in that: in the step (2), the control-output transfer function is a polynomial obtained by taking an oscillation suppression angle as a denominator and a circulation current as a numerator; the characteristic root of the closed-loop control system is a pair of conjugate complex roots which are related to a damping ratio xi; damping ratio xi, number N of submodules and self-coupling inductance L of bridge armsAnd bridge arm mutual coupling inductance LmSum, submodule capacitor C, controller gain kpiAll are related.
4. The method according to claim 3, wherein the control-output transfer function is expressed as:
wherein ,
wherein ,
the estimate of the circulating current is shown,
denotes the estimated oscillation suppression angle, s denotes the Laplace operator, Uca_sumRepresents the sum of the capacitance and voltage of all sub-modules in a group of MMC bridge arms, IcmaDenotes the circulation, RsShowing the parasitic resistance, delta, of the bridge arm inductancedRepresenting the quiescent operating point of the oscillation suppression angle.
The damping ratio is expressed as:
5. the dynamic oscillation suppression method based on the modular multi-level-to-flat-current transformer topology according to claim 1, wherein the step (4) specifically comprises:
in the upper and lower sub-bridge arms of the two groups of MMC bridge arms, the sub-modules are sequenced, one sub-module is selected from the upper and lower sub-bridge arms of the two groups of MMC bridge arms in each half of the switching period of the sub-modules in sequence according to the sequencing to form two groups of sub-module pairs, and the optimal oscillation suppression angle delta is adjustedd' controlling the selected two sub-module pairs as a control quantity such that the pulse edges of the selected sub-modules are imposed with the optimal oscillation suppression angle δd', the remaining sub-module pulse edges remain unchanged.
6. The dynamic oscillation suppression method based on the modular multilevel flat-level current transformer topology according to claim 5, characterized in that the optimal oscillation suppression angle δ is setd' controlling the selected two sub-module pairs as a control quantity such that the pulse edges of the selected sub-modules are subjected to the optimum oscillationOscillation suppression angle deltad', specifically includes:
when deltadWhen negative, the selected sub-module is turned off and delta on the basis of 50% square wave signaldAt the same angle, turn on earlier at rising edge and deltadThe same angle, resulting in an amplitude of U in the bridge arm inductance voltageca_sum/16, duty cycle 2 δd'/Pi positive pulse voltage, circulating current rises; wherein U isca_sumRepresenting the sum of the capacitance and voltage of all sub-modules in a group of MMC bridge arms;
when deltad' is positive, so that the selected submodule is turned on at rising edge delay based on 50% square wave signaldAt the same angle, turn off early at falling edge and deltadThe same angle, resulting in an amplitude of U in the bridge arm inductance voltageca_sum/16, duty cycle 2 δdA negative pulse voltage of'/pi, the circulating current drops.
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| CN110943639A (en) * | 2019-09-26 | 2020-03-31 | 南京师范大学 | Modular multilevel topology adjustable discontinuous modulation method based on double buck sub-modules |
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