CN113193569B - Capacitance energy control method with short-term frequency support and direct-current side oscillation suppression function - Google Patents
Capacitance energy control method with short-term frequency support and direct-current side oscillation suppression function Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
- H02J3/241—The oscillation concerning frequency
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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Abstract
The invention provides a capacitance energy control method with short-term frequency support and direct-current side oscillation suppression functions, which comprises the following steps: obtaining reference power command correction amount Delta W of capacitance energy control based on alternating-current side frequency deviation controlc1(ii) a Obtaining the correction quantity delta W on the direct current side by utilizing the control of the virtual capacitance energy on the direct current sidec2(ii) a Steady state power by versus MMC capacitor energy
Correcting to obtain reference power instruction for controlling capacitance energy
The MMC capacitor energy responds according to the frequency fluctuation of the alternating current side and the voltage of the direct current bus to participate in the absorption of oscillation energy, the control method directly draws energy from the MMC capacitor, the response to the frequency of the alternating current side and the oscillation of the direct current side is rapid, the functions of short-term frequency support and direct current side oscillation suppression can be achieved, and the advantage of MMC capacitor energy damping control is fully played.
Description
Technical Field
The invention belongs to the technical field of power grid oscillation control, particularly relates to a capacitance energy control method with short-term frequency support and direct-current side oscillation suppression, and particularly relates to a capacitance energy control method with short-term frequency support and direct-current side oscillation suppression functions.
Background
In the interaction of the converter and an external circuit such as a fan and a weak power grid, improper parameter setting of a controller can cause impedance mismatching between the converter and the external circuit due to overlarge output power of the converter and overlow power grid, so that subsynchronous oscillation occurs on an alternating current side or a direct current side, and the safe and stable operation of the power grid is influenced.
The existing method for suppressing subsynchronous oscillation on the alternating current side of the converter comprises the following steps: adding a series virtual resistor into an MMC alternating voltage controller; and adding additional damping control, introducing power damping synchronous control and the like into an outer ring of the sending end converter station. The implementation difference of the control in two levels and MMC topology is small, and the characteristics of MMC are not fully utilized.
The dynamic characteristics of a conventional dc grid are mainly determined by the gain of the droop control and the total stored energy of the capacitors in the dc grid. When the two-level converter is adopted, the total stored energy of the direct current power grid is determined by the direct current side capacitor of the converter, the dynamic characteristic of the direct current power grid is improved only by adjusting the gain of droop control, and the improvement effect on the stability of the direct current power grid is limited. When adopting Modularization Multilevel Converter (MMC), according to MMC's control strategy, its inside electric capacity energy and direct current side bus voltage decoupling zero, this part energy can be used for supporting direct current voltage, promotes direct current electric wire netting's stability.
The capacitance energy control of the MMC is divided into two types: one is internal energy balance control, which is used for controlling the difference value of the capacitance energy of the upper bridge arm and the lower bridge arm to realize the internal energy balance of the MMC, and the control can reduce the capacitance voltage fluctuation; the other is total capacitance energy control, which is used for controlling the sum of capacitance energy of an upper bridge arm and a lower bridge arm, namely the difference between the AC measurement and the DC side energy of the MMC. The total capacitance energy stored in the MMC can be controlled by controlling the zero sequence component of the circulating current, the circulating current fluctuation can be reduced by using the capacitance energy control, short-term frequency support of an alternating current side is provided, the stability of a direct current power grid is improved, and functions of inertia support, low-frequency oscillation inhibition and the like are provided. Capacitive energy control can provide redundant degrees of freedom for the MMC control system and provide damping energy in a short time scale. The internal energy balance control can effectively suppress the resonance peak in the MMC impedance, but the energy control only keeps the energy in the capacitor at a constant value, and the capacitance energy is not modulated. In order to fully utilize the rapidity and flexibility of MMC capacitor energy release and storage, further provide short-term frequency support for an alternating current side and improve the damping effect of energy control on direct current network oscillation, total capacitor energy control needs to be added into a control system. In the prior art, the total capacitance energy of the MMC is not generally modulated, and the problem that the MMC capacitance energy does not fully participate in short-term frequency support of an alternating current side and subsynchronous oscillation of a damping direct current power grid exists.
Disclosure of Invention
The invention aims to provide a capacitance energy control method with short-term frequency support and direct-current side oscillation suppression functions, so that rapidity and flexibility of MMC capacitance energy release and storage are fully utilized, and the running stability of an alternating-current and direct-current hybrid power grid is improved.
In order to achieve the purpose, the invention adopts the technical scheme that: obtaining a reference power instruction of Modular Multilevel Converter (MMC) capacitor energy control by using a damping controller based on energy control; the MMC capacitor serves as a buffer, so that the capacitor energy responds according to the voltage of the direct-current bus, and fully participates in the absorption of oscillation energy, and the effect of inhibiting the subsynchronous oscillation of the direct-current power grid is achieved; the output of the control object is a reference power instruction for MMC capacitor energy control, and the controlled system is a direct-current power grid containing an MMC.
The invention provides a capacitance energy control method with short-term frequency support and direct current side oscillation suppression functions, which is applied to a multi-terminal direct current network system, wherein the multi-terminal direct current network system comprises a current converter controlled by constant direct current voltage, and the capacitance energy control method comprises the following steps: obtaining reference power command correction amount Delta W of capacitance energy control based on alternating-current side frequency deviation controlc1Wherein the AC side frequency deviation control comprises measuring the AC bus voltage frequency connected with the converter, and filtering the high frequency component by a low pass filter LPF with a transfer function of GLPFThe filtered AC bus voltage frequency omega is usedPCCAnd a reference frequency
Calculating difference, and obtaining reference power command correction quantity delta W of capacitance energy control by PI control based on AC side frequency deviationc1As shown in formula (2):
obtaining the correction quantity delta W on the direct current side by utilizing the control of the virtual capacitance energy on the direct current sidec2(ii) a Steady state power by capacitive energy to converter
Correcting to obtain a reference power instruction for controlling capacitance energy of the converter
As shown in expression (1):
and the capacitance energy of the converter responds according to the frequency fluctuation of the alternating current side and the voltage of the direct current bus, and participates in the absorption of oscillation energy.
Further, the DC side virtual capacitor energy control comprises measuring the DC bus voltage u connected with the MMCdcBy adding a DC side dummy capacitor CvirTaking the derivative of the capacitor energy, and filtering out the high frequency component through a low pass filter LPF to obtain the correction quantity delta W on the direct current sidec2As shown in expression (3):
preferably, the MMC capacitor energy control comprises double closed-loop control, wherein the inner loop is a circulating current zero-sequence component idiff0And (3) controlling, wherein the PI dynamic equation is as follows: geic＝kpeic+kieicS, where k ispeicAnd kieicProportional control parameters and integral control parameters are respectively adopted; the outer ring is capacitance energy proportional control, and the proportional control parameter is kpec。
Preferably, the MMC capacitive energy outer loop control comprises applying capacitive energy WcAnd the capacitor voltage
Is expressed as shown in expression (4):
wherein, CarmThe bridge arm equivalent capacitance is MMC.
Further, adding second-order low-pass filtering in MMC capacitor energy control, wherein a dynamic equation is as follows:
wherein s is a complex parameter in the Laplace transform, omeganAnd ζ is the cut-off frequency and damping ratio ω of the filternAnd ζ is the cut-off frequency and damping ratio of the filter.
The invention has the beneficial effects that: the energy damping controller is used for controlling the capacitance energy of the MMC, and subsynchronous oscillation generated in a power grid at an AC side or a DC side of the MMC under the conditions that the power grid is weakened due to the increase of wind power and the change of the operation mode of the power grid can be inhibited. The alternating current side PCC node frequency is used as a feedback quantity to obtain capacitance energy correction quantity, and the maximum deviation value of the PCC node frequency relative to the reference frequency in the alternating current side oscillation can be restrained; the capacitance energy correction is obtained by using the virtual capacitance energy at the direct current side as the feedback quantity, voltage oscillation at the direct current side can be damped, and the phenomenon that the voltage of an MMC capacitor fluctuates violently is avoided.
In the capacitance energy control method with the short-term frequency support and direct-current side oscillation suppression functions, the MMC capacitor serves as a buffer, so that capacitance energy responds according to frequency fluctuation of an alternating-current side and voltage of the direct-current side, and the capacitance energy fully participates in absorption of oscillation energy. Structurally, the energy of the conventional damping controller mainly comes from the power supply side, and the energy is drawn in a manner which is not as fast as the energy is directly drawn from the capacitor. The requirement for the damping of the subsynchronous oscillation is relatively low in terms of the magnitude of energy, and the requirement for the rapidity is high, so that the capacitive energy damping control can sufficiently exert advantages in the short-term frequency support on the alternating current side and the oscillation suppression on the direct current side.
Drawings
FIG. 1 is a schematic structural diagram of a four-terminal DC network system embedded with one embodiment of a capacitive energy control method with short-term frequency support and DC side oscillation suppression functions according to the invention;
FIG. 2 is a schematic structural diagram of an MMC according to an embodiment of the present invention;
FIG. 3 is a control block diagram of an MMC host controller according to an embodiment of the present invention;
FIG. 4 is a control block diagram of MMC ring current suppression according to an embodiment of the present invention;
FIG. 5 is a block diagram of MMC capacitor energy control according to an embodiment of the present invention;
FIG. 6 is a block diagram of an energy damping control in accordance with an embodiment of the present invention.
Detailed Description
Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.
The invention provides a capacitance energy control method with short-term frequency support and direct-current side oscillation suppression functions, which is applied to a multi-terminal direct-current network system, as shown in figure 1, wherein the multi-terminal direct-current network system is a four-terminal direct-current network system in figure 1. The four-terminal direct current network system comprises a first terminal 1, a second terminal 2, a third terminal 3 and a fourth terminal 4. The first end 1 is connected with the second end 2 through a first cable 5, the first end 1 is connected with the third end 3 through a second cable 6, the second end 2 is connected with the fourth end 4 through a third cable 7, and the third end 3 is connected with the fourth end 4 through a fourth cable 8. The first end 1 comprises an alternating current system positioned in a dashed line box, a direct current bus element, and an inverter MMC1 controlled by a constant direct current voltage arranged between the alternating current system and the direct current bus. In a preferred embodiment, the second terminal 2, the third terminal 3 and the fourth terminal 4 have substantially the same structure as the first terminal 1, except that a constant dc voltage control is applied to the inverter MMC1, for example, the voltage of the dc bus or dc bus components is controlled to a preset value such as 200 kv. The capacitive energy control method is realized by controlling the current converter.
Specifically, the capacitive energy control method of the present invention comprises: obtaining reference power command correction amount Delta W of capacitance energy control based on alternating-current side frequency deviation controlc1Wherein the AC-side frequency deviation control includes measuring the frequency ω of the AC bus voltage connected to the inverterPCCAnd filtering the high frequency component of the signal by a low pass filter LPF with a transfer function of GLPFThe filtered AC bus voltage frequency is compared with the reference frequency omegap*uCalculating difference, and obtaining reference power command correction quantity delta W of capacitance energy control by PI control based on AC side frequency deviationc1As shown in formula (2):
obtaining the correction quantity delta W on the direct current side by utilizing the control of the virtual capacitance energy on the direct current sidec2(ii) a Steady state power by capacitive energy to converter MMC1
Correcting to obtain a reference power instruction for controlling capacitance energy of the converter
As shown in expression (1):
the capacitance energy of the converter responds according to the frequency fluctuation of the alternating current side and the voltage of the direct current bus, and participates in the absorption of the oscillation energy.
The technical scheme of the invention is described by the following specific embodiments: as shown in fig. 1, the energy damping controller is added to a control system of MMC1 in a four-terminal dc power grid system, and when the energy damping controller is implemented, the energy damping controller comprises the following steps:
modeling of MMC and control system thereof
The structure block diagram of the MMC is shown in fig. 2. Under the condition of three-phase balance, the structure of the converter ensures that all electric quantities in the converter have symmetry, and the corresponding electric and control signal fundamental frequency components of the upper and lower bridge arms are opposite, and the direct current component and the secondary component are the same. As the number of the submodules of the MMC is large, the number of the levels of output voltage is large, the waveform quality is high, the harmonic content is low, and harmonic components of 3 times and above can be ignored in the modeling process. A mathematical model of the converter is established under a synchronous rotating coordinate system to convert three-phase alternating current into two-axis direct current, so that the model of the controller is simplified, the design of the controller is simplified, and the control effect is good. A mathematical model of the MMC is established by utilizing a dynamic phasor modeling method under a synchronous rotating coordinate system, so that the design of a subsequent control system is facilitated.
Considering the three-phase balance condition, the state space models of the electrical parts of the four converters in the static coordinate system are the same, as shown in equation (1):
wherein the content of the first and second substances,
and
total voltage of upper and lower bridge arm capacitors, idiffIs bridge arm circulating current, i is valve side alternating current, mpAnd mnIs the modulation factor, L, of the upper and lower bridge armsarm、CarmAnd RarmFor bridge arms or the likeEffective inductance, equivalent capacitance of bridge arm and equivalent resistance of bridge arm udcIs a DC side bus voltage uvIs the MMC ac side terminal voltage.
The modeling method based on the dynamic phasor converts a model of an MMC electric part from a static coordinate system to an xy synchronous rotating coordinate system to obtain an MMC 10-order dynamic phasor state space model in the rotating coordinate system, wherein the model is as shown in formula (2):
wherein
Subscripts x, y denote the values of the fundamental frequency component of the signal in the synchronous rotating coordinate system at an angular velocity of power frequency ω, and subscripts x2, y2 denote the values of the second harmonic component of the signal in the synchronous rotating coordinate system at an angular velocity of-2 ω.
The MMC control system adopts a classical PI control algorithm and is used for obtaining each component of an MMC modulation signal m through each state variable and each output voltage instruction value or each output power instruction value input into an electrical system. It is contemplated herein that the control system specifically includes capacitive energy control, a main controller, and circulation suppression control. When the alternating current side is an active system, the influence of a phase-locked loop needs to be taken into consideration, and at the moment, two coordinate systems exist in the system, namely a system coordinate system and an alternating current power grid are kept synchronous and are recorded as an xy coordinate system and a control coordinate system, wherein the control coordinate system and a PLL are kept synchronous and are recorded as a dq coordinate system. Let θ be the angular difference between the two coordinate systems, as the voltage uvFor example, the transformation relationship of the variables in two coordinates is shown in equation (3):
wherein G isdel1Representing the time delay, U, from the system coordinate system to the control coordinate systembaseIs a voltage reference value.
The equation (3) is linearized and combined with the dynamic equation of the phase-locked loop to obtain the equationvTransfer function to θ:
wherein, theta0Is the steady state value of θ; u shapevdIs uvxA steady state value in the controller; tf isPLL＝kppll+kipllThe/s is a transfer function representing the PI link dynamics of the PLL; k is a radical ofppllAnd kipllThe proportional coefficient and the integral coefficient of the PI link are respectively. Further, u can be obtained under two coordinate systemsvFrequency domain representation of the transform relationship:
the MMC has a main controller adopting double closed-loop control, an outer-loop control output voltage is an instruction value, an inner-loop control alternating current is a current reference value obtained by the outer-loop control, and a structural block diagram is shown in FIG. 3. Under the control of constant alternating voltage, the dynamic equation of the MMC main controller under the xy-axis coordinate is shown as the formula (6):
wherein k ispacAnd kiacProportional and integral coefficients of PI control, ZbaseIs an impedance reference value. Gdel2Representing the time delay from the control coordinate system to the system coordinate system.
The circulation current suppression controller of the MMC controls the secondary circulation current component in the bridge arm to be 0, and a control block diagram of the circulation current suppression controller is shown in fig. 4. The MMC circulation current inhibition is controlled as shown in a dynamic equation (7) under an xy-axis coordinate:
the energy control of the capacitor of the MMC can be realized by controlling the circulating current, and the control block diagram thereof is shown in fig. 5. The relationship of capacitance energy to capacitance voltage in the xy axis can be approximated as:
the energy controller controls the zero-sequence component of the circulating current, and adopts double closed-loop control, and the inner loop is the zero-sequence component i of the circulating currentdiff0And (3) controlling, wherein the PI dynamic equation is as follows: geic＝kpeic+kieicS; the outer ring is capacitance energy proportional control, and the proportional control parameter is kpec. In order to avoid the spike and instability caused by high-frequency components in the capacitance energy, a second-order low-pass filtering link, a dynamic equation of the link, is added in the capacitance energy measurementComprises the following steps:
wherein, ω isnAnd ζ is the cut-off frequency and damping ratio of the filter. The dynamic equation of the energy controller in the s-plane can be written as:
wherein, the first and the second end of the pipe are connected with each other,
in the above formula, the capacitance energy takes a per unit value and takes into account the sampling delay in the measurement of the capacitance voltage, IbaseAnd UdcnomRespectively a current reference value and a direct current voltage reference value.
(II) capacitive energy control with short term frequency support and DC side oscillation suppression
As shown in fig. 6, in the four-terminal dc power grid, the reference value of the capacitive energy of MMC1 is given by the output of the energy damping controller, and the reference values of the capacitive energy of the remaining three converters all take a constant value
The energy damping controller measures the AC bus voltage frequency omega connected with the MMCPCCAnd filtering the high frequency component of the signal by a low pass filter LPF with a transfer function of GLPFThe filtered AC bus voltage frequency is compared with the reference frequency
Calculating difference, and obtaining reference power command correction quantity delta W of capacitance energy control by PI control based on AC side frequency deviationc1As shown in formula (11); measuring a DC bus voltage u connected to an MMCdcBy adding a DC-side dummy capacitor CvirTaking the derivative of the capacitor energy, and filtering out its high frequency component by a low pass filter LPF to obtain the DC side correction quantity delta Wc2Expression (12); steady state power by versus MMC capacitor energy
Correcting to obtain reference power instruction for controlling capacitance energy
As shown in expression (13); in order to avoid overmodulation, an amplitude limiting link is added before an energy reference value, and a capacitor is protected.
The control method is also suitable for the direct current power grid oscillation damping control for reducing voltage fluctuation in various scenes.
The above-mentioned embodiments are merely illustrative of the preferred embodiments of the present invention, and do not limit the scope of the present invention, and various modifications and improvements made to the technical solution of the present invention by those skilled in the art without departing from the spirit of the present invention shall fall within the protection scope defined by the claims of the present invention.
Claims (4)
1. A capacitance energy control method with short-term frequency support and direct current side oscillation suppression functions is applied to a multi-terminal direct current network system and is characterized in that the multi-terminal direct current network system comprises a current converter controlled by constant direct current voltage, and capacitance energy is controlled by the current converterThe control method comprises the following steps: obtaining reference power command correction amount Delta W of capacitance energy control based on alternating-current side frequency deviation controlc1Wherein the AC side frequency deviation control comprises measuring the AC bus voltage frequency connected with the converter, and filtering the high frequency component by a low pass filter LPF with a transfer function of GLPFThe filtered AC bus voltage frequency omega is usedPCCAnd a reference frequency
Calculating difference, and obtaining reference power command correction quantity delta W of capacitance energy control by PI control based on AC side frequency deviationc1As shown in formula (2):
obtaining the correction quantity delta W on the direct current side by utilizing the control of the virtual capacitance energy on the direct current sidec2(ii) a Steady state power by capacitive energy to converter
Correcting to obtain reference power instruction for controlling capacitance energy of converter
As shown in expression (1):
the capacitance energy of the converter responds according to the frequency fluctuation of the alternating current side and the voltage of the direct current bus, and participates in the absorption of oscillation energy;
the DC side virtual capacitance energy control comprises measuring the DC bus voltage u connected with the MMCdcBy adding a DC side dummy capacitor CvirTaking the derivative of the capacitance energy and passing through a lowThe high frequency component of the LPF is filtered by the pass filter LPF to obtain the correction quantity delta W on the direct current sidec2As shown in expression (3):
2. the capacitive energy control method with short-term frequency support and direct-current side oscillation suppression functions according to claim 1, wherein the capacitive energy control comprises double closed-loop control, and an inner loop is a circulating current zero-sequence component idiff0And (3) controlling, wherein the PI dynamic equation is as follows: geic＝kpeic+kieicS, where k ispeicAnd kieicProportional control parameters and integral control parameters are respectively adopted; the outer ring is capacitance energy proportional control, and the proportional control parameter is kpec。
3. The method of claim 2, wherein the MMC capacitive energy outer loop control comprises W capacitive energycAnd the capacitor voltage
Is expressed as shown in expression (4):
wherein, CarmThe bridge arm equivalent capacitance is MMC.
4. The capacitive energy control method with short-term frequency support and direct-current side oscillation suppression function according to claim 3, further comprising adding second-order low-pass filtering to the MMC capacitive energy control, wherein a dynamic equation of the second-order low-pass filtering is as follows:
wherein s is a complex parameter in the Laplace transform, omeganAnd ζ is the cut-off frequency and damping ratio of the filter.
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